Hydrology of Circumpolar Waters South of New Zealand

ISSN 2538-1016; 10

NE\\' ZEALAND

DEPARTMENT OF SCIENTIFIC AND I�"DC TRIAL RESEARCH

BULLETIN 143

HYDROLOGY OF CIRCUMPOLAR WATERS SOUTH OF NEW ZEALAND

by R. W. BURLING

New Zealand Oceanographic Institute Wellington

New Zealand Oceanographic Institute Memoir No. 10

1961

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ {

l'/,otogr(IJ>lt by T. J.loyd HM ZS Hawea in the Southern Ocean, December I 956.

Fronti1piece

DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH

BULLETIN 143

HYDROLOGY OF CIRCUMPOLAR W.Lt\.TERS SOUTH OF NEW ZEALAND

R. W. BURLING

New Zealand Oceanographic Institute

Wellington

New Zealand Oceanographic Institute Memoir No. 10

1961

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ FOREWORD lN December 1956 and January 1957, the N.Z. Oceanographic Institute carried out oceanographic observations in Antarctic and Subantarctic waters from the frigates HM ZS Pukaki and ffawea. This undertaking initiated a series of cruises to southern waters during the International Geophysical Year. During the voyage the principal objectives were hydrological observations, particularly in the region of the Antarctic Convergence and, on the return to New Zealand waters, in the region of the Subtropical Convergence. The sampling also included surface plankton tows and sediment and core sampling.

Some of the hydrological results ( 14C activities of bulk water samples from depth) have been interpreted and published elsewhere. Preliminary analysis of the results reported here has guided subsequent work by the Institute carried out in the Southern Ocean. Preliminary editing has been carried out by Dr D. E. Hurley and Mrs P. M. Cullen. The material has been finally edited for publication by Mr M. O'Connor, Information Bureau, D.S.LR.

J. W. BRODIE, Director, New Zealand Oceanographic Institute.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ CONTENTS Page No. INTRODUCTION 9 RESULTS OF THE Pukaki-Hawea CRUISES IN SOUTHERN WATERS Collection of Data JO Surface Characteristics .. . JO Ambiguities in the Construction of Isolines JO Bathythermograph Sections 11 Station Data ll REVIEW OF PREVIOUS WORK ON WATER MASSES AND THEIR BOUND- ARIES SOUTH OF NEW ZEALAND 14 HYDROLOGICAL FEATURES IN TH£ ANTARCTIC AND SUBANTARCTJC REGIONS Subsurface Waters 15 The Antarctic Convergence 16 Antarctic Surface Water .... I 9 Australasian Subantarctic Water and Circumpolar Subanlarctic Water 20 Warm Saline Water South of New Zealand 23 Australasian Subantarctic Front ...... 24 General Water Movements Over the Campbell Plateau 34 THE SUBTROPICAL CONVERGENCE REGION Introduction 25 Atlantic and Western Indian Oceans 26 East Indian Ocean and South of Australia . 27 East of New Zealand 28 West of New Zealand ...... 30 South of New Zealand ...... 33 Extent of the Subtropical Convergence Region 36 THE SOUTHLAND FRONT General Description 37 Water Movements near the Southland Front 38 A MIXING PROCESS AT THE SUBTROPICAL CONVERGENCE 42 EDDIES, DIVERGENCE, AND STREAMS An Eddy in Antarctic Waters 44 Divergence and the Antarctic Convergence 46 A Free Stream Current in Subantarctic Waters 49 CURRENTS IN SUBTROPICAL AND SUBANTARCTJC WATERS Tasman Current 51 Southland Current and Canterbury Current 5 J East Cape Current 52 The Circumpolar Current 52 Drift Currents 52 A Constricted Current 53 Bounty-Campbell Gyral . 54 Free Stream Current 54 Origin of the Southland Front 54 SUMMARY The Subtropical Convergence Region 56 Subantarctic Water 56 The Southland Current and Southland Front 56 Constricted Current 56 Australasian Subantarctic Front 57 Free Stream Current 57 Bounty-Campbell Gyral 57 �� Divergence 57 Antarctic Intermediate Current 57 Mixing Processes 57 ACKNOWLEDGMENTS 58 REFERENCES 59 APPENDIX A STATION DATA Pukaki Stations 62 Hawea Stations 64 INDEX 66

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ FIGURES Fig. No. Page No. 1. Tracks of Pukaki and Hawea, bathymetry, and station positions 12 2. Surface isotherms and isohalines 13 3. Temperature sections from bathythermograph observations north of Lat. 56°S 17 4. Temperature sections from bathythermograph observations south of Lat. 56°S 21 5. Temperature sections from bathythermograph observations between Lat. 65° S ° and Lat. 63 S 25 6. Temperature sections from bathythermograph observations between the Chatham Islands and Dunedin 27 7. Vertical section of salinity and density distributions between NZOI Stations C 10 and C 29 29 8. Vertical section of temperature distribution between NZOI Stations B 28 and B 36 31 9. Vertical section of salinity distribution between NZOI Stations B 28 and B 36 31 I 0. Vertical section of density between NZOI Stations B 28 and B 36 33 11. Vertical section of temperature and salinity between Macquarie and Auck- land Islands 35 .12. Water characteristics at constant depths across the Subtropical Convergence 35 13. Observed positions of the Subtropical Convergence Region 37 .14. T-S characteristics, NZOI Stations C 10 to C 29 . 39 15. T-S characteristics, Discovery 11 Stations. South of Atlantic and Western Indian Ocean, and Ob Stations south of Central Indian Ocean . 41 16. T-S characteristics of Discovery ll and Ob Stations. South of the Eastern Indian Ocean and Tasmania 43 17. T-S characteristics of Discovery II, Oh, and Derwent Hunter Stations in and south of the Tasman Sea and South of New Zealand 45 l 8. T-S characteristics of Pukaki and Discovery ll Stations in and south of the Tasman Sea and south of New Zealand 47 19. Comparison of the observed range of Subtropical Convergence Region with mean monthly isotherms for February and August 49

-PLATES Facing page Frontispiece. HM ZS Hawea in the Southern Ocean, December 1956 I. Bathythermograph observations from HM ZS Pukaki 18 2. Reading reversing thermometers on board HMNZS P11kaki 18 3. Bergs south of the Antarctic Convergence 34 4. Lowering the 14C Sampler, Station B 28 34

CHARTS J. (in folder) Currents in the Sovthern New Zealand Region.

INTRODUCTION

This paper provides a description and interpretation of hydrological data obtained by the New Zealand Oceanographic Institute during cruises on the RNZN frigates Pukaki and Hawea into Antarctic waters during December 1956 and January 1957. These cruises formed part of the Institute's contribution to the International Geophysical Year programme. Some results of this investigation suggested the more detailed study of waters in and close to the northern Subantarctic region, using also data from the RRS Discovery 11, the Russian RV Ob, and the Australian FRV Derwent Hunter. Hypotheses are then offered concerning the nature of currents in the Southern Ocean, south of New Zealand.

Hydrological observations carried out from Pukaki and Hawea were designed to sample sur face temperature and salinity over the regions traversed and temperatures to 250 m by bathy thermograph. During the return voyages serial temperatures and salinity observations to greater depths were made through the Antarctic and Subtropical Convergences and over the Campbell Plateau.

J. W. Brodie, H. M. Pantin, and R. W. Burling conducted the oceanographic work from Pukaki and R. P. Willis, that from Hawea.

RESULTS OF THE "PUKAKI"-"HAWEA" CRUISES IN SOUTHERN WATERS

From 17 to 19 December 1956 the frigates Island to the pack-ice, near Scott Island. Endea HMNZS Pukaki and HMNZS Hawea escorted the vour continued her southward voyage to McMur Royal Yacht Britannia, with H.R.H. the Duke of do Sound, while the two frigates returned to Edinburgh on board, from Lyttelton to Waitangi, New Zealand along the meridians 169° E (Pu Chatham Islands. The frigates left Waitangi on the 0 evening of 19 December and proceeded to Dune kaki) and 180. (Hawea) (fig. 1) Hawea returned din. They then accompanied HMNZS Endeavour to Wellington on 3 January 1957 and Pukaki to between 22 and 27 December from near Stewart Lyttelton on 4 January.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Cou.Ecr10N or DATA this instrument is given by Hamon as +0·04°/ 00, but the observed consistency of the measurements The temperature of sea water entering the en on the sub-standard suggests that they are correct gine-room intakes at a depth of 4 m, was con to ±0 030/ tinuously recorded by thermograph throughout 00 . the voyages on both frigates. The surface thermo Nine serial stations were worked on Pukaki graph readings on Pukaki were calibrated from at N.Z.O.I. Stations B 28 to B 36 (fig. 1). Ob reversing thermometer readings at nine stations. served temperatures and salinities and computed The average Hawea temperatures were then cor values of at are given in table 1. rected to agree with average Pukaki temperatures, Two water samples for 14C analysis were col measured while the ships were stationed less than lected from Pukaki. These have been reported 5 miles apart (see fig. 1). Sutiace isotherms are previously (Brodie and Burling, 1958; Rafter and shown in fig. 2. the data being supplemented Fergusson, 1958), and the relation of the HC south of Scott Island by a thermograph recording activity in these-samples to the general distribu from Endeavour during the southward journey. ° tion of observed 11C activities between 9 S and Hourly samples for salini�y analysis were also 67° S near New Zealand has been discussed else taken on both frigates. Those on Pukaki proved whe;·e (Burling and Garner, 1959). to be unsatisfactory because of a periodic recircu lation occurring in the cooling system, and were SURFACE CHARACTERISTIC'S discarded. The thermograph element was nearer the intake entrance and this recirculation had no Surface characteristics of the waters between measurable effect on the recorded temperatures. New Zealand and Scott Island, as found by Pu Surface isohalines, drawn from the Hawea data kaki and Hawea, are shown in fig. 2. Surface and from surface salinities measured at nine sta isohalines are drawn from samples obtained hourly tions worked on Pukaki, are also shown in fig. 2. along Hawea's tracks and from surface values at I). Bathythermograph observations were taken Pukaki hydrological stations (fig Dashed lines for isohalines (fig. 2) indicate the rela from both frigates (fig. 3, 4, 5, and 6). Surface Pukaki temperatures have been corrected from the ther tive uncertainty of their p::>sitions Isotherms are based on corrected thermograph recordings. Dur mograph readings, and an additional check on the bathythermograph used on Pukaki is given by ing the return journeys, significant warming was observed where the ships crossed their earlier nine hydrological stations worked during the re HMNZS turn journey. Comparisons with reversing thermo tracks. Observations from Pukaki indi cated no noticeable change where the tracks meter temperatures show that observations may B 35 and B 36 ( 12 days be corrected to within O·25°c. crossed between Stations later) but indicate warming of about O·5°c where Water samples for salinity analysis were col the tracks cross off Dunedin and Banks Peninsula lected at four depths from Hawea at each of the (14 and 18 days later). No account was taken of Stations C l to C 29 (fig. I; table 2). Salinity the latter difference in drawing the isotherms, but ° and at isolines for the northern section of this the 13 c closed isotherm was possibly absent on line of stations are shown in fig. 7. Surface sam the earlier occasion, with the 12°c isotherm ples at these stations were collected by bucket; extending farther north. salinities of this extreme surface water averaged On the Hawea tracks near the Chatham Islands about 0 03 (±O·O4)0fo and 0·02 ( +0·02)% · 0 0 at about 44° S, 180° E the warming was slightly above those at 4 m at the 10 southern and 19 less than l 0c (13 and 15 days later). On the northern stations respectively. These surface sam northward course a temperature of 14°c was ob ples were not collected on the hour as were served at the southern crossing of tracks. This those from 4 m below the surface via the engine warming may cause the isotherms as drawn, ex room intake; thus the above differences may be cept those for 14°c and 15°c, to be bent south due to differences in the time of sampling and ward along the eastern and western tracks, and to the measurement errors in each sample. the 16°, 17°, and 18°c isotherms to be displaced All salinity values reported in this paper have southward. However, the bending will be small been obtained using a conductivity meter (Ham south of 60° S where fewer than five days separate on, 1956) as a transfer instrument checked against the observations, and is probably significant only a sub-standard sample which was in turn checked north of 55° S along the 180° meridian (1O- against a Copenhagen standard. The accuracy of 130c) . The remaining isotherms shown along

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ the northward lrack are based on observed temper servations ansmg: (a) from instrumental errors atures. ( ±0·5°c); (b) from navigational uncertainty (+0·15° of latitude) ; (c) from real differences AMRIGUlTJES IN THE CONSTRUCTION OF lSOLINES in the east-west temperature distribution; and (d) from sampling at different times (the observa There are ambiguities in lhe manner in which tions were made at alternate hours from the any isotherm may be drawn when it is intersected two ships north of 61 ° S) or different time inter more than once along one or more tracks. Most vals (hourly on Pukaki, two-hourly on Hawea, difficulties in the present instance appear in the south of 61 ° S). southern region where the isotherms for 9°c and 7°c represent two ways in which the isolines may The two sections are similar. At only one or be drawn for similar distributions of the observed two points does the difference reach I 0c, e.g., surface temperatures. Nearly all isohalines for near 48° S between 100 and 220 m and between salinities less than 34-40 / 00 and all isotherms 63° S at 50-150 m. This is not obviously due to except those for 4°, 5°, and 10°c are topologically the effects of instrumental or position errors and ambiguous in a similar manner, and unless there particularly not to different sampling times. The is a definite reason for choosing a particular des position near 48° S is on a "front", or sharp cription of a feature, the choice of shape of parti horizontal gradient, in the subsurface temperature cular isolines must often be made arbitrarily. distribution. At 62° S there is an unusual cold Here it may be noted that certain features can be tongue, possibly associated with considerable interpreted: (a) as limited pockets (perhaps ed east-west temperature variation. The smaller scale dies) of water; (b) as tongues, which may have vertical fluctuations revealed by halving the either of lwo orientations near a given line; (c) sampling interval are particularly noticeable. as enlarged or extended pockets or tongues inter sected by tw0 ships' tracks but apparently not STATION DATA ° extending to the third (compare the 9 c and Salinity and CTt sections between Stations C 10 34·20/oo isolines); (d) as still more extended and C 29 (fig. 7) have been constructed from strip features (perhaps streams) intersected by all analyses of samples obtained at a few depths only three tracks (illuslrated by the 33·9%0 isohaline); at each station. The O"t values were computed or (e) as combinations of the above features. from bathythermograph temperatures at the sampling depths and are intended to show the BATHYTHERMOGRAPH SECTIONS trend of the isolines; there will be greater errors than in O"t values computed from standard station Vertical temperature sections along the three procedure. This figure illustrates in more detail tracks are shown in fig. 3, 4, 5, and 6. For com a region shown in fig. 3a; in fig. 7 the horizontal parison they are plotted according to latitude. scale has been doubled. Where there was a change of course longitudes are indicated at the foot of each diagram. Fig. 8, 9, and 10 are drawn from data obtained at reversing bottle Stations B 28 to B 36. Between Fig. 3b and 4b were drawn from Hawea Stations B 31 and B 32 the isolines are dashed observations and fig. 3c and 4c from Pukaki since there is good evidence for the temperature observations 5 miles to the west when both ships structure only in the upper 270 m. Lines are were following their parallel southward courses. dashed also near Stations B 34 and B 36 where the The surface reading on each bathythermograph slopes are unknown. Isolines of CTt in fig. 10 con was corrected from the thermograph reading on form with temperature and salinity values in the same ship, the mean of all thermograph fig. 8 and 9. readings from each ship having first been stand ardised. No further effort was made to standardise Temperature and salinity (T-S) characteristics temperatures in the sections, nor to standardise at Stations C 10 to C 29 are plotted in fig. 12 the relative positions. Thus, these sets of diagrams and 14 and those of Stations B 32 to B 36 in illustrate differences between the two sets of ob- fig 18.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Fig. 1: Chart showing Hawea and Pukaki tracks, N.Z.O.I. Station positions and contours of bot tom relief (depths in fathoms from U.S. Chart H.O. 2562) . The frigates Pukaki and Hawea steamed in company from Christchurch to Dunedin; Hawea was 5 miles east of Pukaki along track marked by double line; Hawea returned to New Zealand via the 180° meridian and Pukaki via the western track.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 'v0.

65°S

70 °S

0 ° 0 0 0 ° ° ° 0 140°E 145°E 150°E 155 °E 160°E 16s E 110 E 11s E 1so 11s w 110 w 165 W 160 w 1ss w ° SHIPS' TRACKS - ISOHALINES (¼o) ---� ISOTHERMS ( C) � Fig. 2: Surface distributions of: Isotherms (blue lines) ; from continuous thermograph recordings. Isohalines (red lines) ; hourly samples from engine room intake along solid line ships' tracks and from station positions (marked X) along Pukaki's track (dashed line) . 13

THEIR BOUNDARIES SOUTH OF NEW ZEALAND

The hydrology of the Southern Oceans has been in southerly regions. The temperature of this discussed by various authors (e.g., Deacon, 1937; layer increases northward from the edge of the Midttun and Natvig, 1957; Sverdrup, 1933). The pack-ice. During spring and summer, the surface following description is based on that of Deacon of this layer is warmed and the salinity is lowered

(1937); to less than 34·0%0 by the addition of fresh water from melting pack-ice and precipitation. In winter The region between the Antarctic Continent the surface water is cooled, and mixed downward and about 65° S latitude lies in the easterly wind through convection and wind action. Far south, zone; the region to the north lies in the westerly ice formation leads to an increase in salinity and wind belt of the "forties" and "fifties". The temperature of surface waters in the Pacific Ocean the consequently increased surface density further encourages mixing. is lowest near the edge of the ice-pack surround ing the Antarctic Continent and is only slightly Below Antarctic Surface Waters lies Deep Water warmer immediately to the north; a sudden sharp with higher temperature and salinity than winter temperature increase is however met somewhere Surface Water above, or the Bottom Water be between about 54° S and 62° S. The position of low. This Deep Water originates mainly in the this increase is known as the Antarctic Conver North Atlantic Ocean, where highly saline sur gence. To the south lies Antarctic Water. To the face water cools and sinks. Lt moves southward north, the warmer Subantarctic Region extends as the North Atlantic Deep Current and mixes to where its surface waters moving to the north on the way with high salinity Mediterranean and east meet even warmer Subtropical Water. water. Near 40°-50° S most of this Deep Water This occurs in a sometimes well defined, but moves eastward after mixing with water of slightly often indistinct, region known as the Subtropical lower temperature and salinity which has come Convergence, between 35° S and 47° S. through Drake Passage from the Pacific Ocean. Still further east it mixes with Indian Ocean Deep The main movement of the whole water mass Water which is partly derived from high salinity in the westerly wind system is towards the north Red Sea Water. east. Northward or southward motions, character istics of certain water masses which may be The Deep Water has two distinct layers. South identified by such properties as temperature and of the Antarctic Convergence the lower layer is salinity, are superimposed on the general north characterised by a salinity maximum and salini eastward motion. ties greater than 34·66%0, and rises steeply to wards the surface near the convergence. The upper In the easterly wind zone there is a component layer may be recognised by a temperature maxi of motion towards the west at the surface, often mum and has a salinity (34·5¾0 approximately) extending to depths of several hundred metres. greater than that in the Surface Water, but less than that in the lower Deep layer (34·66% There is ample evidence (e.g., Deacon, 1937; 0). Sverdrup et al, 1942), that the whole main body North of the convergence the upper layer does of flow is deflected lo the north on approaching not usually have a temperature maximum in the a ridge from the west, but to the south when the Tndian and Pacific Oceans, except near the Ant depth increases. arctic Convergence. Antarctic Bottom Water, characterised by lower South of the Antarctic Convergence, three main temperature and salinity than Deep Water, is water masses can be distinguished-Antarctic Sur formed mainly in the Weddell Sea by winter cool face Water, Deep, and Bottom Waters. North ing of high salinity Deep Water over the con of the Antarctic Convergence there is a further tinental shelf. This water moves partly north water mass, the Antarctic Intermediate Water, ward into the Atlantic Ocean and partly eastward which sinks near the Antarctic Convergence. in the Circumpolar Current. Some Bottom Water Tn winter almost the whole of the Antarctic is also formed south of the Indian Ocean (Sver Surface Water forms a homogeneous surface drup, et al, 1942) and perhaps .in the Ross Sea layer with temperatures near freezing point (Deacon, 1937). Water from the Ross Sea does ° ° ( - l ·8 to -1·9 c) and salinities of 34·0-34·5° / 00 not appear to contribute towards Bottom Water

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ in the Pacific Sector, but merely to maintain low often be seen. Usually there is a nearly homo average temperatures in this region These temper geneous layer between the surface and 50 or atures are, however, not as low as those of Bot 80 m, and beneath it a layer with a salinity maxi tom Water found in the Indian and Atlantic mum which is sometimes very weak. This salinity sectors of the Antarctic. maximum is quite shallow near the Subtropical Convergence and it is often continuous through In the Subantarctic Region a vertical salinity the convergence, at a depth of about 75 m on profile often shows a low surface salinity (34·O- m 0 the north side, increasing rapidly to about 200 34·4 fo0), with a maximum below, and a mini in the Subantarctic Region and about 300- 500 m mum at still greater depth. The minimum is a further south. Less than 200 miles north of the characteristic of Antarctic Intermediate Waler, Antarctic Convergence this feature tends to weak which is formed by the sinking of a mixture of en and may even disappear. Deacon (1937) sug poorly saline Antarctic and Subantarctic surface gests that this lower layer of Subantarctic Water waters near the Antarctic Convergence. This Ant must move southward since it maintains its higher arctic Intermediate Water sinks and spreads north salinities in spite of mixing with the poorly saline ward in its general eastward motion along the fntermediale Water moving to the north below. surfaces of density of approximately

HYDROLOGICAL FEATURES

IN THE ANTARCTIC AND SUBANTARCTIC REGIONS

SUBSURFACE WATERS Water characteristics at the one latitude differ In general the distribution of subsurface water considerably with longitude and with time. South properties south of New Zealand, as found by of New Zealand the effecl on the eastward-flowing the present Pukaki-Hawea investigations (fig. 8, currents of the Macquarie - Balleny Ridge, the 9, and 10), is in accordance with those described channel between the Macquarie Islands and the by previous authors (e.g., Deacon, 1937). Campbell Plateau. and other topographic features (fig. 1) gives rise to differences of salinity in an At Station B 28 the water at 2,000 m has a ° east-west direction. Discovery Stations 2768-2771 temperature of O·26 c, indicating a mixture of (Anon., 1957) show that at 175° E, in November Bottom Water with some Deep Water. The lower 1950, the upper layer of Deep Water which had stratum of Deep Water has salinity values greater a temperature greater than 2°c penetrated south than 34·70fo (fig. 9) and the upper stratum has ° 0 ward beyond 63 S between 400 and 1,000 m. salinities between approximately 34·4 and 34·60/oo In the present section (169° E, fig. 8) and an in the Subantarctic Region (indicated in fig. 8 other near 163° E. (Discovery Stations 2201 and by the southward pointing tongue of warm water 2213, January- February 1938) this water was in the Antarctic zone). The Intermediate Water not present south of 62° S. This trend of iso is represented (fig. 9) by a typical downward and therms to the south-east, supports a general trend northward p'.)inting tongue of low salinity water suggested by Deacon (1937, fig. 22) and also between 55° S and 60° S; salinity minima at Sta based on rather meagre data. tions B 34 and B 36 on the western and north eastern slopes of the Campbell Plateau show its Changes of water characteristics with lime are extension to these regions. well illustrated by the difference in maximum

15

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ salinities in two sections through Deep Water in the at section is only slightly reduced and the between the Macquarie and Auckland Islands. 34· l, 34·2, and 34·3¾0 isohalines still lie close One is Section 14 of Deacon (1937) drawn from together through the cold tongue. with maximum Discovery data observed in June-July 19 32; the depths near 400, 600, and 800 m respectively. second is drawn from measurements from the Research Ship Ob during April, 1956 (Ob Sta T�IE ANTARCI'lC CONVERGENCE tions 56-77, Anon, 1958). Part of the Ob section is reproduced in fig. 11. Jn the Discovery section A "convergence" at the sea surface may be the maximum salinities were about 34·74°/ 00; in defined as a point or line towards which there is the Ob section maximum salinity was 34·820fo0. mean motion of surrounding surface water and These differences support the suggestion (Deacon, at which sinking occurs. In the major oceans 1937, p. 102) that higher salinities in Deep Water "convergences" are regions in which sinking of to the east of New Zealand may have been due surface water may occur and in which converging to fluctuations in the salinity of Deep Water pass water may be transferred by accelerated surface ing to the south of Australia from the Indian currents. The Antarctic Convergence usually ex Ocean. Wtist (1929) did not consider this possi tends through one or more degrees of latitude bility in attempting to explain salinities which with surface water moving towards it on the south were hjgher to the north of New Zealand than side, and sinking may occur throughout a zone farther south and found it necessary to postulate extending from slightly to the south to somewhat that high salipity water should sink from a surface north of the convergence region; the meridional source somewhere in the Pacific Ocean. The motion of surface waters on the north side, how present author considers Wtist's hypothesis is un ever, is still in dispute. The Antarctic Convergence necessary. is often marked by a steep meridional gradient The high salinities to the north may support of surface temperature but not of salinity. Stommel' s theory ( 1958) that east of New Zea The Antarctic Convergence is usually located land, water below 2,000 m flows northward, as by determining the centre of the zone where the part of a circulation system which is required by north-south surface temperature gradient is at a continuity to balance an upward transfer of mass maximum. For the present data this lies close throughout most of the oceans. to the 4°c isotherm (fig. 2). and agrees with the There is a sharp horizontal salinity gradient mean mid-ternperature for December-January between 52° 06' S and 52° 36' S along the Ob found by Mackintosh (1946). However the ob served position lies south of the mean positions section between the Macquarie and Auckland ls ° ° lands near longitude 162° 40' E (fig. 11). Surface plotted by Mackintosh (about 59 S near l 70 E, and 60° S near 180° E) which are based on ob salinity rises from 34·04 to 34·42°/ 00 a short dis tance to the north-east, and further increases to servations east and west of the present area and ° one observation within the area. From data pub 34·53%0 with another 0·6 decrease in latitude. The downward sloping tongue of low salinity is lished by Lyman ([958), the mid point of the convergence in this region is near 52° 40' S in such that isohalines for 34·1-34·4g00 extend to depths of about 220, 330, 560, and 1,080 m re the December to February period, or about 30 spectively. It is possible that the isohalines in nautical miles south of the mean of the three the Pukaki section (fig. 9) follow a similar dis positions shown. tribution and that a steep surface salinity gradi Mackintosh (1946) gives an alternative defi ent occurs near 55° S-56° S. nition for the location of the convergence-that position at which the temperature minimum sinks A steep gradient of surface salinity was also below 200 m. However, the present and earlier observed in the N.Z.O. T. investigations near 55° S data indicate that the position of the subsurface and 173° E (fig. 2) but was not observed on the temperature minimum may fluctuate beween posi Pukaki return voyage farther west where sample tions 1 °-2° of latitude south of the surface feature were taken only at stations. By constructing iso and up to 4° north of it (fig. 4a). halines with a steep front near this position, ( cor responding to that of the temperature tongue (fig. From Deacon (1937) it would appear that 8)), it is possible to minimise the vertical density winter water ju-;1 south of the convergence has tongue ( fig. 10). The general impression given a temperatur.e of about I 0c. In the present data, by such amended versions of fig. 9 and 10 re the I 0c isotherms (fig. 4a and 4d) slope rapidly mains the same: the intensity of the disturbance downward towards the north from about 100 m

16

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Fig. 3: Distributions of temperatures ( 0c) from bathythermograph observations, in sections (depth against latitude) along Hawea and Pukaki's !i- acks (fig. 1) north of latitude 56° S. The latitude of each observation is marked at !he !op of each diagram; the longitudes may be linearly interpolated between values entered beneath each diagram which show where there were appreciable changes of course. The horizontal scale varies in these diagrams. .. . . 51' 52' 53' 55'

,. (\ ,. (\ c.zq 8 2 24 22 20 10 lb 4 13 I II C.10- (N.Z.QL ,lotion ..-,9- IOO"E IIIO'E IOO"E (a) Bathythermograph observations from Hawea along northward track. The posttwns of shallow stations are indicated at the top and bottom of the diagram. The stippled area shows the position of a pocket of high salinity water. ° ° ° 500 51° 52° 53° 54 55 56 s

,,,,,

/' ,,,, 50m

100m

150m ' ' ' \ \ I I I I I 200m I I

70 250m 7 bO � • \ (b) Bathythermograph observations from Hawea along southward track (5 miles to east of Pukaki) . 17

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ° ° 47°5 48 51° s2° 53 ° 55 so• '-- -, Om-l-- - ..-..-½--,--r-'--r-,----,-....1...-,--,---r-'-r--r--ir r,--,---,-,..---,---,-,----t--,---r,-....,--,- ,---,-,r-Om

50m Som

100m 70 100m

150m ISOm

200m

250m 70 70 �-

00m 3 L----+-'-'-----'---l--'-__JL--+--..__.__+---'-_,_-+---'--'--'--t-....L.--'--+--'----'- -t-��--11 ° ° ° 5' ° lb9 2o'E lb'l0 18' 170018' 170°4b' 171 31' 171 4 172 20' 1n•oe' (c) Bathythermograph observations from Pukaki along southward track (5 miles to west of Hawea) .

D � I D t;; �- b" 0

�

1 m·oi' 110 ° 11>4• 12' 1 ·,� lo7 2l' 1o1•,0 i!,9"52' lb'l"Oi'E (d) Bathythermograph observations from P11kaki along the northward track.

18

Plate 2. Reading reversing thermometers on board HMNZS Pukaki

faring />. 18

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ but turn back southward at about 250 m beneath (which here extends to about 63 ° S) and summer the tongue of cold winter water; the most north warming of the water begins later erly position of this 1 °c tongue is close to the minimum ( <33·9 ), convergence. B::1thythermograph sections from Salinity values show a 0fo0 A other observations in this region ( e.g., three on tracks north of the convergence (fig 2). sections in Garner (1959), four sections in Lyman maximum shows up somewhat south of the con (1958)) during November to March, show a vergence on the two central tracks and appears to similar pattern with the 1 °c isotherm varying in be present just north of the convergence on the greatest depth from 180 m to more than 270 m. eastern track and just south of it on the western track. Because of the minimum to its north, the The maximum northward reach of the continu salinity maximum cannot be due to a southward ous l 0c isotherm lies no more than 2° of latitude movement of Subantarctic Surface Water. Dea north of the maximum surface temperature gradi con points out that the salinity maximum to the ent. Even where an anomalous situation apparently south is due to upward movement of highly saline occurs, such as near the cold tongue at 62° S Deep Water from beneath. These salinity maxima (fig. 4b and c) which is the deepest observed and minima are most distinct during summer and penetration of water colder than l 0c, or where late autumn. This may be explained by the addi a detached mass colder than 1 °c appears to the tion of seasonal melt-water which is carried north north (fig. 4d), this is still true for the data in ward in the Ekman layer to the position of the spected. No other detail of the temperature struc minimum. It should be noted that the present ture appears to agree so consistently with the posi data do not conform with the usual summer dis tion of the surface feature. It is therefore suggested tributions observed elsewhere, in that the salinity that where the surface gradient is not well defined, maximum lies much less than 4° of latitude or is obscured, and bathythermograph observa south of the convergence and even slightly north tions are available, the northward extension of of it in the east of the sector (fig. 2), and mini the continuous l 0c isotherm can be used to locate mum surface salinities lie to the north instead of the convergence between November and March being either at the convergence or between it and in this region. This question has not been pur the maximum. This anomaly may be due to the sued further for other regions nor for winter meridional position, to prolonged abnormal wea conditions; however, Midttun and Natvig (1957), ther conditions. or possibly to disturbances in indicate from Brategg data that the 1 °c isotherm the general water motion of a more temporary and the steepest surface temperature gradient lie nature. within 1 ° of latitude in the Pacific Ocean near 90°, 120°, and 150° W in December and January. North of Scott Island there is clearly a south This is supported by one section at 148° E in ward displacement of surface isohalines and prob Lyman (1958) . ably of isotherms. From less detailed observations, Mackintosh ( 1946), has drawn monthly surface isotherms which also indicate a southward bulge, ANTARCTIC SURFACE WATER but it is much less pronounced than the shape of the tongue shown here. The coldest Antarctic Surface Water was found Frequently there is a complete summer clear within the pack-ice belt just south of Scott Island ance of pack-ice along the 180° meridian, while (fig. 2). Both north and south of the belt the to the east and west the pack-ice remains quite open water is warmed by solar radiation. thick. This ice clearance and the su1face distribu Both surface salinities and temperatures (apart tions shown in fig. 2 suggest that there may be from some minor fluctuations) decrease with dis a divergence of surface waters along this longi tance southward to the point just south of Scott tude, perhap� due to the bottom topography Island at which the two frigates left Endeavour near Scott I. At this latitude the prevailing winds (fig. 1). This was about 6 miles inside loose are easterly and surface waters and pack-ice will mushy pack at the edge of the main pack-ice belt. drift westward. This drift has been observed Near the convergence, the surface is warmed at farther west (Wordie, 1921). If this motion ex the start of summer and the effect of precipita tended to a depth of a few hundred metres, it tion and melting and the northward motion of would be deflected southward on moving onto the Ekman layer is reflected in low salinity. Fur the shallower region near Scott Island. Deflec ther south in early summer, the salinity decrea�es tions of this nature would produce the south since insolation first melts the winter pack-ice, ward extensions of the surface contours found by

19

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Pukaki and Hawea near Scott Island, and the between about 100 m and 150 m but it then in forced upwelling would cause a clearance of pack creases again still farther south. Midttun and ice. Natvig's observed least depths of the minimum were about 80 m near 66° S, at 165° W to 175° W. In general. Pukaki and Hawea bathythermo The corresponding Pukaki and Hawea depth was graph sections (fig. 4) show the presence of about 100 m near latitudes 64 ° S-66° S. This is winter water near the temperature minimum as where the maximum rate of Deep Water upwelling far north as 62° S overlying warmer Deep Water occurs. and having summer-warmed water above. The depth of the minimum temperature increases in The presence of internal waves is indicated by all sections, from the south towards the Ant a wave-like structure through the winter · water; arctic Convergence, except where influenced by the structure is evident to some extent in the fluctuations due to internal waves and/or other bathythermograph sections south of 56° S, more disturbances. Features illustrating northward mo especially where the sampling interval is least tion and sinking of winter water and of the sur (fig. 4, 5). face layers are somewhat obscured by mixing; but the nature and extent of these motions and of summer warming is indicated by water colder AUSTRALASIAN SUBANTARCTIC WATER AND 0R than 2°c (shaded areas, fig. 4 and 5), and water CUMPOLAR SUBANTARCTIC WATER less than 0°c ( darker shading). Between the Subtropical Convergence Region The southernmost observation was made about and the Antarctic Convergence is the Subantarctic 6 miles inside loose mushy pack-ice. The coldest Region. Here, distinct types of Subantarctic Sur water at this position was only 10-15 m from the face Water are present above 600 m. Subantarctic surface and only very slight warming of winter Water south of the Atlantic and Western Indian water had taken place. It is usual, according to Oceans and the east of New Zealand is mostly earlier investigators (Deacon, 1937; Lyman, less saline than 34·50 / 00. In these regions, salini 1958), to observe almost isothermal water ties between 34·5 and 34·80/ 00 occupy only a ( - l ·5°c) beneath true pack-ice-which lies only narrow zone in the upper 200 or 300 m. This ° ci. few miles farther south of 68 S-to a depth of zone is usually not more than about 2° of latitude at least 50 m and more often to 100-200 m. The wide and lies just south of the Subtropical Con unusually shallow depth of the minimum at vergence. However, just south of the convergence 66° 30' S found by Pukaki and Hawea (fig. 4c) between 100° E and I 67° E in the Australasian supports the suggestion, put fo1ward to explain Region, water of this higher salinity, >34·50fo0, the summer dearance of ice-pack south of Scott is present to depths of 400--600 m. or more (fig. Island and the associated southward extension of 16, 17, and 18), and water with salinities in ° ° isohalines (fig. 2), that upwelling occurs near excess of 34·50fo0 extends as much as 7 -8 of the 180° meridian. latitude further south. Some observed positions of the boundary between the more saline and North of the pack-ice, in summer, the surface less saline types of Subantarctic Water are shown is warmed and the temperature decreases down in fig. 13; Chart 1 shows its positions between ward through the upper layer to nearly freezing ° ° 157 E and 180° E deduced from observations point (-1·8 to -1 ·9 c) at the temperature mini on Pukaki during summer (early January 1957) mum. The minimum increases northward to about and from Ob in autumn (April 1956). It is pro 1 °c near the convergence. Observed temperature posed that Subantarctic Water north of the minima in ,tll Pukaki and Hawea sections con boundary be called "Australasian Subantarctic form with mean values given by Deacon (1937). Water" since it exists mainly south of the Aus From observations during. the Brategg Expedi ° tralasian Region; and that the Subantarctic Water tion east of 175 W, and from Discovery stations to the south be called "Circumpolar Subantarctic to the west, Midttun and Natvig (1957), show Water". that in the Antarctic Region the depth to the Upper Deep Water is greatest ( >200 m) at the The southern boundary of Australasian Sub convergence. (The depth to Antarctic Upper antarctic Water is here called the Australasian Deep Water is defined as the depth to the greatest Subantarctic Front. The eastern boundary crosses vertical temperature gradient between the temper the Campbell Plateau between Campbell and ature minimum and the warmer Deep Water be Auckland Islands and continues northward to neath). To the south this depth decreases to wards New Zealand to meet the southern bound-

20

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ary of the Subtropical Convergence Region. This April 1951 along a triangular course between boundary is described later as the Southland Front. Dunedin, Campbell Island and a point near 50° S, 175° E, are similar to the Pukaki-Hawea isoha The Australasian Subantarctic Water thus in lines. (Positions of the 10°c and 11° c isotherms cludes Gamer's Campbell Plateau Water (Garner, may have been affected in the interval between in press) : it originates, as Garner suggested, observations by warming of water along the through a relatively greater southward transfer eastern Hawea track; if this effect were allowed of higher salinity water across the Subtropical for, these isotherms would then resemble more Convergence south of the Eastern Indian Ocean closely the ;,hape of the isohalines for 34·3 and and the Tasman Sea than elsewhere. Australasian 34·4°/00.) Subantarctic Water is usually colder than 8°c below 400----600 m or more, but warmer above, Particularly noticeable in the bathythermograph whereas Circumpolar Subantarctic Water is colder sections (fig. 4) is the variability of the tempera than 8°c below 200 m except for a very narrow ture structure over some six degrees of latitude zone near the Subtropical Convergence (fig. 3a, north of the Antarctic Convergence. This · is 8 and 11, and 13 to 18). shown by fluctuations in the relative depths and positions of the 4°, 5°, and 6°c isotherms. To Over the Campbell Plateau the position of the gether with the relative constancy of the 1 °c 34-30 / 00 isohaline shows that there is a marked isotherm farther south (and to some extent of south-eastward surface extension of Australasian those for 2°c and 3°c), this suggests that much Subantarctic Water. Isotherms drawn by Garner mixing takes place here down to greater depths (1959) from surface temperatures, observed in than were reached by the bathythermographs.

Fig. 4: Distributions of temperature ( °c)' from bathythermograph observations in sections along Hawea and Pukaki tracks south of latitude 56° S (Legend as in fig. 3). The light stipple indicates the northwards penetration of Antarctic Water and the darker stipple the main core of Winter Water.

° 1eo• 179°21:,'w 11q•21'w 178 35'E (a) Observations from Hawea (northward voyage) .

21

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 5o•5 60° 61° 63' 65° 67"$ Omf-,------,--,'--,-�,-,-.==--.---+-,,----+-.r--.----crl---.---.----;"!'i-,-,-r--.- �-rr.,,c-r-:i;rrr..,.-'T='T--:-r-:r:-,:,:".".''7:::7--:,-,

.'.,-l·S ______• 50m

JOOm

200

250m 5• 3• 2• � 4•

(b) Observations from Hawea (southward voyage-5 miles cast of P11kaki) .

,..--1-s•c c -1• -:,,..- /- IOm ' .' '' I

..(/ ,' 100• ! :' /' t_/

ISOm

llOm

&•,..• 5• 2• CJ" 4• 3• A I°

(c) Observations from P11kaki (southward voyagc-5miles west of Hawea) .

22

SOm

IOOm

200m

250m 5° 01 \ i

16qoo6'E 169°07' (d) Observations from Pukaki (northward voyage).

This agrees with the idea that the Antarctic Inter from the west around the south of the South mediate Current originates near the Antarctic Island, of water which is warmer and more saline Convergence through the sinking of mixed Sub than Subantarctic Water to the east and south. antarctic and Antarctic Surface Waters (Deacon. A similar intrusion wa<; described by Bary ( 1956) 1937). The rapid increase in salinity northward of and Garner ( 1959). The present observations the salinity minimum in Antarctic Intermediate show that this is most intense inshore and extends Water is due to mixing which probably occurs some distance up the east coast. Near Dunedin, quite near the surface. The cold tongue between it is cut off from Subtropical Water farther north 55° S and 56° S which appears in each Pukaki and in the warm saline tongue indicated by the I 5°c Hawea section (fig. 3) may be associated with this and 35·00/ 00 isolines, by cold, poorly saline water mixing process, but it is more Iikely that it repre- which has upwelled from shallow depths. The ents a stream of cold water. presence of the warm saline water cannot there fore be explained by an extension to the south WARM SALI NE WATER SOUTH OF NEW ZEALAND of Subtropical Water from north-east of Banks The general configuration of isolines just south Peninsula; it must arrive from the Tasman Sea of New Zealand (fig. 2) suggests an intrusion, west of New Zealand.

23

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Predominantly Subantarctic Water with tempera AUSTRALASIAN SUBANTARCTie FRONT ° tures less than 8 e is present at depths greater A sharp '·front" existed in April, 1956 between than 100 m over the southern and eastern portion Ob Stations 71, and 72 and 73 (fig. 11, 17): that of the Campbell Plateau (fig. 3b, 3c, and 3d; ° ° is, at the boundary of the two Subantarctic Water south of 50 30' S). Along the 180 meridian, Masses. This feature is defined by the close spac Subantarctic Water extends much further north 0 ° ° ° ing of the 34·3-34·6 /00 isohalines and the 7 e ward to the Subtropical Convergence in 44 -45 S. and 8°e isotherms. A similar front was present The convergence is clearly shown by the close ° ° between Ob Stations 98 and 99 (fig. 16) and be ness of the 8 e to 11 e isotherms and by a steep tween the pairs of Discovery Stations 2144 and surface gradient of the isohalines (fig. 2). 2145, 899 and 900. It is well developed in sec A north-eastward trend in subsurface isotherms tions including Ob Stations 70 to 77 and 92 to 100 over the plateau may be seen by comparing the (Moroshkin, 1958); it is Jess clearly developed western bathythermograph section (fig. 3d) with in sections drawn from Discovery data (Discovery those further east (fig. 3b and 3c). The 8°e iso Stations 869 to 926, Sect.::ms 9-13, Deacon 1937). therm for example appears over the plateau at [t is probable that this front also exists west of 250 m in the western section south of 50° S, 165° E near the southern boundary of Austral while on the eastern side, this isotherm is near asian Subantarctic Water, and that there is a steep 48° S; it apparently turns more sharply north south-north horizontal gradi�nt of characteristics ward in the distance (5 miles) between the two across it-roughly between salinities of 34'4-, eastern sections. The great vertical spacing of 34·5¾0 between 150 and 400 m, while tempera the isotherms between 49° S and 52° S (fig. 3d) ture increases 1 °c or more ( e.g., from less than appears to indicate intense vertical mixing. 7°e at Ob Station 99, to more than 9°e at Ob Station 98) between 150 m and 200 m. The Ob section (Moroshk:in, 1958; Anon. 1958) between Macquarie Island and the Auckland Is Observations from December to May in any lands (fig. 11) shows a temperature increase from year indicate that Australasian Subantarctic Water 7·9° to 9·5°e at a steep salinity gradient, while and its associated Front may extend much further the 8°e isotherm slopes downward to meet the south of the Subtropical Convergence in the Tas shelf at about 300 m. man Sea in early summer and late autumn than in mid winter. The seasonal spread of the observa A tongue of high salinity water extends out tions is too limited to make any positive asser wards and upwards from the steep slopes just tions regarding seasonal trends, but Australasian south-west of the Auckland Islands. Water with Subantarctic Water may exist through a narrower salinity values greater than 34·6°/ 00 lies between zone of latitude during winter. This implies either 200 m and 400 m at the slope, and at 100 m a decrease in the transfer of warmer, more saline near 52° 12' S at the tip of the tongue. This is water to the south during late autumn and winter, about 100 miles south of Pukaki Station B 34 at or an increased transfer of Circumpolar Subant which highly saline warm water is present between arctic Water to the north at this time; this latter 75 m and 250 m (fig. 8 and 9). This water contri transfer could be associated with an increasing butes to the formation of the Campbell Plateau movement of Antarctic Surface Water to the Water of Garner (in press). north and an increasing tendency for water just A remnant of this tongue is indicated by higher north of the Antarctic Convergence to sink as salinity values at Station B 32 much further east Intermediate Water with the approach of winter. and south, between 25 m and 235 m (table 1). The Australasian Subantarctic Front reaches to Both the Ob and Pukaki observations show that 1,000 m or more (fig. 11, 16, and 17). Circumpolar Subantarctic Water, with tempera ° tures below 8 e and salinities less than 34·5°/ 00, GENERAL WATER MOVEMENTS OVER THE CAMP· was present on each occasion over most of the BELL PLATEAU 0 Plateau roughly east of the 34·5 / 00 isohaline Australasian Subantarctic Water intrudes south shown in fig. 2. At Ob Station 76, mid-way be ward along the western edge of the plateau as tween Auckland and Stewart Islands, salinities far as the Auckland Islands. This rapidly mixes less than 34·40¾0 occurred at all depths above with colder, less saline water as it moves east 400 m. Water of much higher salinity was ob ward so that the characteristics at Stations B 32 served both to the north and in the tongue south and B 33 near Campbell Island are predominantly of this position. typical of Circumpolar Subantarctic Water fur-

24

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ther south or east of New Zealand. However, the Water in this tong ue moves eastward on the salinity is greater than at Ob Station 76 midway southern side of low salinity gyral water. Water between Auckland and Stewart Islands or at Sta in the subsurface, higher salinity tongue is derived tion B 36 farther north. These observations (parti from Australasian Subantarctic Water farther cularly at Ob Station 76 and Pukaki Station west. The much more saline water found at Sta B 36) imply, tion B 35 south of Stewart Island is typical of (a) north-westward flow of Subantarctic Water water observed within, or very close to, the deeper than 100 m over the south-east of Subtropical Convergence Zone. the Plateau, or (b) south-westward flow from the east of the South Island, or (c) eastwa-rd flow of poorly saline water from west of Station B 34 above the higher salin ity tongue. l•C o• The presence of such poorly saline water in June 1932, north-west of Station B 34 is shown by Discovery Station 923, but to account for the

observed distribution d uring January 1957, such IOQ,. low salinity water would have had to flow above _,. the water in the high salinity tongue (compare o• Station B 34) as a restricted stream between sta ,. tions during observations north of latitude 51 S 150m from both the Pukaki and the Ob. Since the° presence of &uch a stream is most improbable ./".�\ some other explanation must be so ught. It is 200 / \ 00 therefore p0stulated that an anticlockwise flow . . !! . l occurs in a gyral extending over the Bounty .· .. : Trough and the north-eastern area of the Camp 150 l50m bell Plateau. This feature is here named the , .....• · .. :- �\ ...... _ Bounty-Campbell Gyr al. �: . ..._.. /·,· The tongue of water of relatively high surface salinity projecting south-east of Campbell Island �....L----L..- -l--l--'---4--+---L..- �-'--+-----'"JOOm (fig. 2) must be due to mixing of Subantarctic Surface Water from both the south-west and north Fig 5: Distribution of temperature: (°c) from bathy east with water in the high salinity subsurface therm-0graph observations from Pukaki between 65° S, 178° E, and 63° S, 169° E. Stippled areas tongue extending around the edge of the Plateau. may be compared with those in fig. 4.

THE SUBTROPICAL CONVERGENCE REGION

INTRODUCTION The Subtropical Convergence extends through This section describes the Subtropical Con a region. Subtropical Surfa�e Water is present vergence and northern Subantarctic Water south to the north, and, according to Deacon (1937), of the Atlantic, Indian, and western PacificOcea ns Subantarctic Surface Water approaches it from with special reference to the Australian and New the south. There is usually a general sinking Zealand regions. Discovery I!, Ob, Derwent througho�t the region, particularly in winter. Hunter, and Pukaki-Hawea data have been used but it has not been practicable to discuss data The Subtropical Convergence is usually marked from other sources. A complete discussion using all by a steep salinity gradient and a much less pro available data (e.g., from Meteor, Gauss, Deutsch nounced temperature gradient. Where it is sub land, Galathea, D ana) could well yield much ject to coastal influences it may not exhibit the further information. usual convergent motions of surface waters. It is

25

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ therefore regarded here simply as a boundary 125. Those stations with salinities greater than region separating different water masses 34 8¾0 at 200 m are clearly in Subtropical Water; 847 and 1807 are evidently very close to East of New Zealand the Subtropical Conver Stations gence occupies a narrow band of latitude and the convergence region but their distinct differ separates Subantarctic and Subtropical Water. The ences from stations to the south show that they are upper and lower surface salinity values are predominantly Subtropical. Ob Station 125 is evi . 0 dently within the convergence region. At the roughly 35·0 and 34 5 / 00. West of New Zealand waters with surface salinities between these values northern staiions. the

26

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ within the thermocline (as at Station 673), but ments between the characteristic curves at ad the situation is confused near the convergence at jacent stations north and south of latitude 47° S Stations 847 and 1772. (e.g., between Ob Stations 98 and 99), it is clear that a steep north-south horizontal salinity gradi EAST INDIAN OCEA N AND SOUTH OF AUSTRALIA ent occurs about 7° south of the Subtropical Con Subantarctic Water in the sector of the Indian vergence. A similar steep gradient is apparent Ocean between 100° E and 147° E (fig. 16; at a number of positions betwe

° 0 ° ° ° 170° 174 11s• 17b 177 178 180 11s•w m•w

SOm

10

150

200m

250m

Fig. 6: Distribution of temperature ( °c) between Chatham Islands and Dunedin, from bathythcrmograph observations taken alternatively from Pukaki (positions marked by short line at top and bottom of diagram) and from Hawea (positions marked by Vs).

27

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ between 26·8 and 26 9, except at st�tions obviously considerations is shown in fig. 13 by the shaded within the Subtropical Convergence Region; at regions alongside the ship's tracks. This figure these the maximum salinity occurs mainly at illustrates how the convergence region may be shallower depths, and at lower

28

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ upwelling from quite shallow depths and it may Garner (I 953, 1954, 1959, and 1960) show this be partly due to run-off. On the evidence of variability. Pukaki-Hawea data, it is unlikely to be surface Rochford ( 1957) has suggested that there is a water of Subantarctic origin. considerable flow of water to the north-west Since the water in the Ekman layer tends to through Cook Strait, while Deacon ( 1937) postu move to the left of the wind direction, upwelling lated that Subantarctic Water from the west could be caused by prevailing north-easterly winds coast of New Zealand flows through to the east. north-east of Christchurch and north-westerlies These inferences were based on insufficient data. between Christchurch and Dunedin A band of More recently Garner (1959) has indicated that cold water off Dunedin, previously noted by Bary the west CO'!St surface water is of Subtropical ( 1956) , is a north-westward extension of Sub origin, having moved eastward from the East antarctic Water at 30-50 m. This moves shore Australian Current. A small amount of this flows ward and upwells to the surface on the inshore to the east coast through Cook Strait (Garner, 0 side of warmer water of higher salinity. The up 1960). The configuration of the 35·0 /00 isoha welled water mixes with warmer water and moves line (fig. 2) suggests that this was so during the northward along the coast. Upwelling fluctuates present observations. considerably with changes of wind and local By interpolating salinities (fig. 7) and using weather. The various distributions found by bathythermograph temperatures for the depths

4b0 47°5

34 · 25·6 2b·0 2b·2 2b-4 SOm 2b·b • • • • 2b 8 · • � 2b·C/ IOOm " --t- • / \ �---. / \ , ------1-\ -- ' ' \ ', \.' ', ' ' ISOm ' ' ' ' ' ' ...... -- '--...... ' ' ' ' ..... 2CX)m ' ' ' ' \ \ I I 2SOm I .--- / · �. 34 4 :-1 ..

24 22 20 18 27·0 lb 14 12 C.10

Fig 7: Distributions of salinity (red) and density (black) in the Hawea section between N.Z.O.L Stations C to and C 29. Samplin� positions are marked with a dot. Stipple indicates the position of a high-salinity pocket and the re�1on from which the water within it had been transferred (assuming that major trends of features are fairly homogeneous transverse to the section). Abscissa represents latitude and the horizontal scale varies according to the direction of the ship's track.

29

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ shown in the T-S diagram the characteristics of been warmed l-2°c more than is usual during the water masses along 180° to a depth of 250 m early summer; a distinct summer thermocline is can be demonstrated ( fig. 14). The curve for present (fig. 3, 6), resulting in long straight line each station passes through the sample values portions of the curves above 100 m for Stations given in table 2. The closeness of the stations C 10 to C 19 (fig. 14), and also for Station B 36 permits a very detailed salinity section; hence (fig. 18). characteristics through the convergence are well illustrated. The salinity maximum south of the converg ence lies beneath the thermocline (fig. 3a and 7), From HawPa sections (fig. 3a, 7), it is evident which is represented by the relatively great verti that Stations C 10 to C 23 lie within the Subant cal distance on the curves for Stations C 10 to arctic Region, although above 100 m the charac C 19 between 25 111 and 50 m (fig. 14). teristic curves at Stations C 19 to C 23 are dis The salini�y of Subantarctic Water at N.Z.O.1. turbed by mixing. This disturbance is illustrated Stations B 32. B 33, and B 36; at Discovery Sta by a tongue between 50 and lOO m near 9°c and tions 900 and 1681 (fig. 18); at Ob Stations 70, 34·7%0 (fig. 14). Stations C 10 to C 17 have salinity maxima between 150 and 250 m which 71 and 76 (fig. 17) ; and N.Z.O. r. Stations C 10 Charac do not appear on the diagram but occur within to C 17 (fig. 14), is less than 34·480/00 . teristics at Discovery Stations 2200 to 2216 which the region in:.:luding the 250 m characteristics; at greater depths lower salinities and temperatures are not plotted in any figure, but for which po si may be expe:.:ted (fig. 14 ). The characteristics at tions are shown (fig. 13) are similar to those for Stations C 28 and C 29 typify Subtropical Water Stations B33 and B36 (fig. 18). The region south of the just north of the convergence, anct show a Sub boundary between Australasian and tropical salinity maximum near 25 m. This maxi Circumpolar Subantarctic Water (fig. 13) and east of 168° mum is prominent in the Hawea section (fig. 7) E is thus occupied by Subantarctic Water with salinities at all depths lower than and apparently extends south over the steeply 4 5 although the boundary may fluctuate. sloping subsurface front between Stations C 23 3 · %0 and C 27. We may assume these two stations to be at the limits of the convergence region. WEST OF NEW ZEALAND As the station latitude decreases the character "West of South Island the water more than istics between 100 and 250 m progressively ap 100 miles off-shore has the temperature and sal proach those of Sverdrup's Western South Pacific inity of Subtropical Water and is no doubt de Central water mass (fig. 14). Generally, in this rived from the East Australian Current and from Subtropical Convergence Region the salinity above the easterly movement south of Australia", (Dea 100 111 is between 34·7 and 35·00 / 00 and the con, 1937, p. 62) and "The movements of Sub temperature is higher than that at the upper limit antarctic Water in the neighbourhood of New of the W.S.P.C. water mass. Zealand show on the whole a dose resemblance At Station C 25 (Lat. 43° 58' S) the density to those east and west of South America. As at 200 m is approximately CTt=26 8. At this level the West W:nd Drift approaches the west coast the density increases to the south but decreases of South Island, it divides into two branches just to the north. Similar properties hold for stations as it does off the west coast of Chile. Off each near the convergence elsewhere. coast the division takes place in about 44° S and one branch flows northwards, whilst the other turns 0 Water with salinity of less than 34·5 fo 0 lies to the south. In each region also. the southward only 60 mile., south of the convergence (fig. 7) , current flows towards the east round the southern above 100 m, and is present at the edge of the extremity of the land, and, joined by more water convergence I egion at greater depths. from the main easterly drift, tnrns northwards The extension of the shallow (25-40 m) Sub along the east coast" (Deacon, 1937, p. 54). tropical salinity maximum over the deeper con Garner (1959) has discussed in detail the Stations vergence features (shown by the tongue, known surface features of the Subtropical Con C 24 to C 29), is apparently a local feature which vergence near New Zealand and the various sug e has not been observed elsewhere and may b gestions and explanations offered by previous transient. authors for distributions of temperature, saliniy, Water in the upper 50 m just south of the con and fauna observed to the west and south of vergence to the east of New Zealand, may have New Zealand. This region, has been variously

30

/ ,,I 1/,, I/, ,,,// 7• I ,'f 1 __,, ; I /

// / / I I / b•,...... / / . 800 I I .. ,.. .. ,1 1•/ ;o.. __;_: __ _ / · I / ,,,,,.,---- . l()()On, / ...... -·· I _.. · ' . l',, CAMPBcll .. CAM?BELL I · PLATEAU PLATEAU I · I ·· ·· I ...--- · · J• / ··· 1 / ... i,CO,, · . . · 0-1• . . .. . , __ ,.,.. . . - j+ - ···· ...... - · ir loOO I n•. . I 1&;0. 2·5·

Fig. 8: Distribution of temperatures ( 0c) in the section along the Pukaki northward track. Sampling posi tions at N.Z.O.I. Stations B 28 to B 36 are shown (dots) ; at depths less than 250 m the isotherms are constructed from bathythermograph data. Some Imes are broken where the position or orientation o( isolines is ambiguous. Abscissa represents latitude and the horizont�l scale varies according to the direction o( the ship's track.

lil2 lill 53' 54° 55' 5'J' 57' 58' ' I I I I I I I I I 200m I ·�o,'I 34·2/ I 34 1\ I I · ' II \ I I 400m \ I ' I , _____ \ \' / ...... ____ .,.,, ,// BOO ,//

1200m CAMPBELL 14·4 CAMPBELL PLATEAU PLATEAU ,,- 1400m )4·5 ------

loOOm

------··----- ... 1600m -...... ', ', ··.,, _..J.....:.347 ....:.34 7 _..J..L.J lOOOm,�-�---'---...U...---'-'----'---..J..--_j_--"--�--..J..--L..- ....J_',:...· .:.:· L/ ..J..__ J.... .:.:· :.,:_· Fig 9: Distribution of salinity (0/oo) in the Pukaki section between N.Z.O.I. Stations B 28 and B 36. Abscissa represents latitude and the horizontll scale varies according to the direction of the ship's track.

31

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ described as predominantly Subantarctic Water antarctic Water. Rochford estimates that Tasman (Deacon, 1937, 1945), Subtropical Water (Turn Sea surface waters in August are 100% Sub bull, 1875; Garner, 1954, 1959; Bary 1956), and antarctic Water in the south and 50-75% in the as waters of mixed origin (Fleming 1944, 1952). north, but it would seem from the foregoing that his discounting of cooling effects on Subtropical Garner (1959) interpreted the Discovery data Water may not be justified. of June-July 1932 from the Tasman Sea differ ently from Deacon ( 1937). By choosing the 10°c Confusion in the terminology used to describe

and 34·7%0 isolines at the surface to define the water masses west and south of New Zealand its position during the Discovery observations in has arisen through sparseness of data and also June 1932 (fig. 12), he portrayed the Subtropical from the use. by most authors, of extreme sur Convergence much farther south than Deacon face propenies only. Ln this paper subsurface placed it. He suggested it Jay between latitudes properties are used to define the convergence 46° S-47° S across the Tasman an

J2

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ The positions of data observed between 100° E South of the boundary between the two types and 180° are shown in fig. 13. T-S characteristics of Subantarctic Water (fig. 13) salinities are less at the closely spaced Stations C 10 lo C 29 on the than 34·5%0; north of the boundary salinities 180° meridiai1 (fig. 14) illustrate a general pat are higher than 34·5¾0 at some depth in Sub tern of' detailed changes across the convergence. antarctic Wa(er. South of Australia this boundary T--S charact,�ristics in the AtJ;rntic Ocean and corresponds v:ith a sharp horizontal salinity gradi south of Australia (fig. 1 5 and 16) show that the ent. Over the Campbell Plateau to the south of Subtropical Convergence may be located there New Zealand, and further west, the region may by comparing water properties in the upper few fluctuate and a corresponding steep horizontal hundreds of metres. The characteristics of water gradient in salinity values does not always exist. south of the Tasman Sea and south of New Zea For example, there is no steep gradient along the land are illustrated in fig. 17 and 18; comparison Discovery track shown by Stations 921, 922, and with fig. 14-16 indicates the relation of station 923 (fig. 17); the salinity at the southern station positions in fig 13 to the Subtropical Converg (921) barely reached a maximum of 34·5°/ 00 ence at the times of observation. between 300 and 500 m and the most northern stations showed only slightly higher values at SOUTH OF NEW ZEALAND these depths, but there is a steep salinity gradient between Sta�ions 923 and 924 above 400 m. T-S characteristics east of 148° E are plotted Comparison of these Discovery Stations with in fig. 17 and 18. Data from Discovery Stations Pukaki Station B 34, Ob Stations 72 to 74, and 2209 to 2216 have not been used but the charac Discovery Station 1682 (February 1936) suggests teristics of Stations 2212 to 2216 resemble those that colder and more poorly saline water was of Ob Statioa 76 (fig. 17) and Pukaki Stations present between 100 and 400 m south-west of B 32, B 33, and B 36 (fig. 18). At Station 2209 New Zealand in June 1932 than on other occa the salinity is between 34-4 and 34·5%0 above sions. The maximum salinity value (34·50fo0) 700 m with a maximum of 34·5° / 00 and the at Discovery Station 2209 (T-S characteristics temperature is 7·5°c between 1 50 and 200 m. not plotted) suggests that the more saline Austra-

IDS • B.l4 B.33 B.32 BJI B.JO B.2'1

/ - / / I-- ,\ ' ' 11, .... ,\ ,,...-.,,.. / '1 \ \ - .,,,.,,,,/ /1,, \\ \ \ /,,.-,_ \ - 400m ,,, ,',I ,�-\ \ / I ' ------(> /II \ 27· \ , ' bOO"' ', /II, , / O , ,.-1.Y I I, I I _?l:0.- I I /,.. .,,.., / / ______21:) __ � / l / -2, 'V' / / / / ...J.1:l_ / /

CAMPBELL CAMl'MLL PLATEAU PLATEAU 21:.L 1400m 27-8

l&OOm

__ -21 ·B· -----_ - ::,,,__ 2000rn --'-,---�• '--·'-�--+--� ----'--,--- - r -,-- r--�� ---'r----Lr---',--r----1 Fig. JO: Distribution of density (ut) in the Pukaki section between N.Z.0.1. Stations B 28 and B 36. Abscissa represents latitude and the horizontal scale varies according to the direction of the ship's track.

33

4 Plate 4. Lowering the 1 c Sampler, Station B 28 a

facing p. 34

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ surface salinities were observed. There are at est. The Eastern Tasman Sea south of 38° S lies least two possible explanations of this salinity always within the westerly wind zone, with wind maximum. velocities tending to increase to the south ( e.g., McClintock, 1959). Since the surface water layer A salinity maximum would be produced if tends to move to the left of the wind direction Subantarctic Surface Water shallower than about it will have a northward tendency through the · 50 m were to move northward and mix with Subtropical Convergence Region. On the west much more highly saline Subtropical Water from coast of New Zealand rainfall is high and in greater depths. The surface layers would be heated creases to the south (reaching over 200 in. per at the more northerly positions. Alternatively a year in the south-west), but it is not known how low salinity surface layer could be produced in far off shore this high orographic rainfall extends. Subtropical Water and maintained by land run While the westerly winds in most sectors of the off and excess precipitation On available data Southern Ocean extend a few degrees to the it is not possible to decide between these two north of the convergence region, the orographic possibilities but the following points are of inter- effect on the rainfall is a local phenomenon. Be cause of low in-shore surface salinity, rainfall and land run-off are the more likely causes of a salinity maximum. It is possible that a combina 74 73 7S 70 tion of these, and other, factors may operate. The possibility of movements to the north-east through Cooic Strait should not be overlooked, although cu.-rent measurements (Olsson, 1955) 200 and drift card observations (Brodie, 19 60) showed m. mean currents and surface drifts in the opposite direction. 400

/ / / / 0 So Lnity ( /00) 600 34·2 ·4 ·b 8 l'.>O ·2 ,. / .," ,. / , I / ° lb' 0 / 2·5 800 3 / / / I , I / ,[ / 1000 / I/ /I I 20 ° 11 C 12 1200 r;,.1;)1 / I �I I I Unei of constant depth 1400 / I // / Lme1 of constont <1t ,.,. / Water moss boundary / Pocket of highly saline water 1,,1.,, .,, 1600 3/ .,, ' I ' I \ I ' 1800 '?;)1 \ I Fiu. 12: Schematic diagram showing water characteristics � at constant depths in a section along the 180' I meridian across the Subtropical Convergence I ( fig. 3a and 7). The thick continuous lines show th,� smoothed main trends in characteristics, at constant depths neglecting minor fluctuations. 0 Fig. ll: Distribution of temperature ( c full lines) and The stipp!cd region represents water within the salinity (O/oo, broken lines) in a section between high-salinity pocket, and the thin portions of Macquarie and Auckland Islands constructed th.: isobaths for 25 m and 50 m show distortiom from observations from the Russian ship Ob from the general trends in characteristics at (Moroshkin, 1958) ; station positions are shown these depths near the high-salinity pocket. (This in fig. 13. figure is deri,-ed from the T-S diagram. fig. 14.)

35 Sig. 3

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Whatever the reason for the formation of the EXTENT OF THE SUBTROPICAL CONVERGENCE Subtropical salinity maximum it is reasonable REGION to use water properties deeper than about 150 m The Subtropical Convergence Region deter to define the position of the convergence. In the mined from the above criteria is illustrated in Australian (fig. 16) and east New Zealand (fig. fig. 13 by the shaded region-neglecting possible 14) sectors u I at 200 m is greater than 26·8 in seasonal influences. East of New Zealand it cor Subantarctic Water and less than 26·8 in Sub responds with convergence positions from Pukaki tropical Water, and with only a few exceptions, Hawea data and its northern limit east of Cook this property conforms with other properties used Strait corresp::mds with that determined by Garner to define these water masses, which are separated (1954). West of New Zealand the region will by a region of water of salinity 34·7-35·0% , at 0 be narrower at any one time. depths less than 200 m. Jn the Subantarctic Region, the depth of the salinity maximum nearly The Subtr8pical Convergence Region and the always exceeds 150 m and is greater than the convergence determined by Deacon ( 1937) have depth of the summer thermocline. Using these been compared (fig. 19) with mean summer and criteria, stati-Jns in the Tasman Sea and south of winter isotherms, constructed from sea surface New Zealand which fall within the convergence temperatures observed on British ships from 1855 region or were not obviously in Subtropical or to 1939 (Meteorological Office, 1949). In some Subantarctic Water, are: Derwent Hunter Sta areas the data is inadequate and the positions of tions 56/55 and 53/56; Discovery Station 924; the isotherms are uncertain; particularly in the Ob Station 77 (fig. 17); and Discovery Stations eastern Tasman Sea between 41 ° S and 46° S. 1684 and 2820 (fig. 18). However, the monthly isotherms in the published charts, except for January are similar to those There are slight anomalies at some other sta for February and August (fig. 19). The January tions. For example, at Station B 34 in Subantarctic isotherms differ from the general pattern in the Water the density is low at 200 m (fig. 18). This eastern Tasman Sea--the 58° F, 60°r-, and 62°F may be due to a Tasman Sea influence on the (l4·4°c. l5·6°c, and l6·7°c) isoline� trend north northern higher salinity Subantarctic Water, while ward toward<; New Zealand east of longitude the characteristics of Station B 35 show that it 162° E inste:1d of southward, as in February. must be very near the southern edge of the con East of New Zealand the January isotherms have vergence region. a southward-pointing tongue with the axis about Even if Subantarctic Surface Water does move 100 miles off shore. northward in the Eastern Tasman Sea and is fn general, the trends of the northern and modified by heating and mixing, the water north southern lim;ts of the convergence region are of latitude 42° S has Subtropical characteristics: similar to the trend of the isotherms. West of (low density at 200 m, maximum salinity above Tasmania, Deacon's convergence positions are in good agreement with the convergence region de 35·0%0, and a salinity maximum coinciding with the thermocline) except at the anomalous Der fined above. The latter corresponds to surface ° ° went Hunter Station 6/ 54 and at shallow water temperatures of about 55 -60°F (12·8 -15·6°c) ° ° ° ° stations close in shore. Surface temperatures at February, and 50 -53 F (10·0 -1l ·7 c) in August. Derwent Hunter and Ob Stations in the eastern South of Tasmania (longitude 151 ° E) and Tasman are all above 15°c which agrees with south of New Zealand, the cold, poorly saline Deacon's criterion that Subtropical Water exceeds limit of the convergence region is associated with ° 14·5°c in summer. Discovery Stations 2819, 2821 surface temp�ratures about 1 c less than those (fig. 18), and 925 (fig. 17) have Subtropical south of Australia. East of New Zealand the characteristics although they must be quite near region appears to be associated with temperatures 0 ° the convergence region. about I c lower in winter and about 0·5 c warmer in summer than south of Australia West of New Discovery Stations 921 to 926 (June 1932) may Zealand, where the positions of both the isotherms indicate that the convergence region is farther and the convergence are most uncertain, the north in June than in other months. Compare, temperatures ranges of the convergence region for example, Discovery Station 924 with Stations apparently agree to with 0·5°c of those observed 2820 and 2821 (May 1951) and Stations 925 and south of Australia. 926 with Derwent Hunter Stations 4/57 to 9/57 These limits require confirmation but the ob (January 1957). served ranges agree qualitatively with Deacon's

36

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ criterion that north of the 14·5°c isotherm in At any given season it will occupy a less extensive summer and the ll·5 °c isotherm in winter the region than that represented by the shaded regions water is entirely Subtropical and is not mixed (fig. 13 and 19). Water within the convergence with Subantarctic Water. It is p::issible, as pre region has characteristics intermediate between viously noted, that the convergence region has those of Subtropical and Subantarctic Water different mean positions in summe1 and winter. masses.

THE SOUTHLAND FRONT

GENERAL DESCRIPTION Since the temperature and salinity ranges corres East of New Zealand is a distinctive front p::ind closely with those at the warm northern (Chart 1) associated with the southern edge of the edge of the Australasian Subantarctic Front it Subtropical Convergence. It is also the Southern might be conjectured that the two fronts are con limit of the narrow zone of Subantarctic Water tinuous. For reasons given later, it is believed that they are not dynamically associated, and (salinity>34·50fo0) in this region, and is thus the northern limit of Circump::>lar Subantarctic Water. that the Southland Front originates above 200 m Within the front, the 8 °-9°c isotherms, the not far west of the Auckland Jslands. Except where it originates it may conveniently be associ 34·5- 34·6%0 isohalines and the constant density surfaces (o-t approximately 26·8-26·9) slope ated with the southern limit of the Subtropical steeply down to the north from below the sum Convergence Region, from near the Auckland ° mer thermocline in Subantarctic Water from a Islands to approximately 175 E off Banks Pen depth of about 70 m. Because of its association insula. with the Southland Current, (Garner, 1960), it The posithn of the steep horizontal gradient is proposed to name this the "South/and Front". of surface salinities marking the Subtropical Con There, are however, no observations west of vergence Region on the 180° meridian coincides 164° E sufficiently concentrated to indicate its with a steep downward slope of isotherms towards existence or position east of the Auckland Islands. the north (fig. 2, 3a). This is sharply defined at

I SubTrOpic.olCorw.rqc"Ct I 0; 5CO\'C ry -OcocOI' IQ]7 Ob ,o•s JO"S �oionqWilp1 1Tocl1 t. Ct, ... ,,,turi H tu .._RQ119e of STC llcqlOl'I 0 PtJkoi.i (J Howco

2148

•o• ' ",," .,,,'" / 681 �, lY ...... ·•;y:.y I � .,/' ,o• ,co· ········· ········ m

��--'-'�-'-'-...... C�..L.....�.....L..��LJ__�'-'-'�-L.,l��_L_.,___,__,��

IOO'E 120• 1.;0-0 1�0· lbO' Fig. 13: Observed positions of the Subtropical Convergence Region and positions of Stations. Roman numerals show the month of observation. Short dotted lines indicate the observed southern boundary of Sub antarctic Water with salinity greater than 34· 50/oo; finer dotted lines show a possible seasonal varia tion of this boundary. For T-S characteristics at all stations on this diagram except Discovery II Stations 2209 to 2216 see fig. 14, 16, 17, and 18.

37

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ depths greater than about 50 m and is roughly tween 48° S and 50° S in the western sec bounded on the cold water side by the 8°c iso tion (fig. 3d) suggest that Pukaki's track therm. Below 50 m a sharp horizontal tempera between Stations B 34 and B 35 was only ture gradient also exists but it is absent at the a few miles to the north of the feature. surface. Where the summer thermocline is well (e) Ob Stations 76 and 77 in April 1956 (Mo developed south of the convergence, the 8°c iso therm lies near its deeper limit (fig 3). roshkin, 1958; Anon, 1958) near Stewart Island show the same feature in the same Similar features are present in other bathyther position (Station positions, fig. 13; T-S mograph sections (fig. 3, 6). curves, fig. 17). (a) Near 44° 40' S, 173° 20' E (fig. 3d) the Thus south and east of New Zealand a con sleep slope lies south of the surface features tinuous region of water with steeply sloping iso of the Subtropical Convergence where some lines of physical characteristics existed in Decem of these trend towards Cook Strait. Here, ber 1956 and · January 1957. Bathythermograph ° the 8 c isotherm slopes steeply from its data show that where the isopleths commence to near-horizontal disposition beneath the ther slope steeply downward from about 70 m beneath moclin�. at a position close to the 34·6%0 warmer and more saline water, surface salinities surface isohaline (fig. 2). ° are slightly greater than 34·6%0 near the 180 (b) In the east-west section from Chatham Is meridian anJ between 34·55 and 34·60¾0 else- • land to Dunedin (fig. 6) the 8°c isotherm where. This demarcates a boundary on the cold is similarly related to the summer thermo water side of the region, deeper than 70 m, sepa cline, md its steep slope again nearly coin rating Subantarctic Water with temperature below ° cides with the 34·6%0 surface isohaline (fig. 8 c and salinity below 34·5%0 (Circumpolar Sub 2). Moreover, from the relation of this antarctic Water) from warmer, more saline water. east-west section to the surface isohalines, it appears that the steep slope at the eastern East of B3.nks Peninsula this feature coincides end passes through that in the section along with the southern limit of the Subtropical Con 180° ( fig. 3a). Since the western isotherms vergence, as defined by observed surface charac between 44° 30' S and 48° S are obviously teristics. The northern limit of the convergence connected (fig. 3d), it is suggested that trends to the north towards Cook Strait. The the region of steep slope is continuous. subsurface feature trends to the south-west south This is supported by the position of the of New Zealand, to near Auckland Island. Be 8°c isotherm in other meridional temper tween the front and the New Zealand coast warm ature sections (fig. 3b, 3c, and 3d; near water, arriving from the west, i� not Subtropical . 50° 30' S), and by the relation of the steep Water, which has a higher salinity than 35·00/ 00 Near isothermal slopes to the 34·6% isohaline the coast it is typically Subtropical Con 0 vergence Water and elsewhere Australasian Sub (fig. 2). antarctic Water. (c) The isohalines and lines of equal cr1 values also slope downward near the same posi The subsurface feature has everywhere steeply tion on the 180° meridian (fig. 7), and sloping temperature, salinity and density isolines, the section shows that water beneath 8°c and associated sharp horizontal gradients at and 9°c is associated with salinities between depths greater than about 70 m. Since this feature 0 separates two water masses with different charac 34·5 and 34·6 fo0 and cr1 values between 26·85 and 26·95. Distributions of tempera teristics, it is proposed that it be called a "front".

ture, salinity, and cr1 (fig. 1, 8, 9, and 10), especiaily from Station B 36 on the cold WATER MOVEMENTS NEAR THE SOUTHLAND FRONT water side of this feature. suggest that the We may discuss the motion near the subsurface ° steeply sloping isotherms near 48 S and feature using Margules equation for a simple east of Stewart ls land ( fig. 3d), are associ two-layer system, separated by a surface of dis ated with similar salinities and densities. continuity (e.g., Proudman (1953) p. 59). If p Water properties near Station B 34 and at and ' ( '> p) are densities and v and v' are ° p p 50 30' S in the Pukaki western tempera components of horizontal velocity parallel to the ture section (fig. 3d) similarly agree. surface of �eparation in the upper and lower (d) The downward slope usually commences layers respectively, and if g (cn1/sec2 ) is the ac near a depth of 70 m and the isotherms be- celebration due to gravity, 'P the latitude, n the

38

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ earth's angular velocity, and 8 the angle of slope (increases), 8 is measured downward from a level of the surface from the horizontal. then surface to the left (right) of the current direction. ,/ ' 2 n sin If pV - v Thus, in the Tiawea section (fig. 7) between tan 8 = Stations C 10 and C 29, if the Subtropical Current g p' - r, is easterly, the current in the heavier Subantarctic For continuous density and velocity distribu Water has a smaller east-going velocity, is less tion, this becomes easterly and may even flow towards the west. 2nsin c,o 8(pv) If the lighter warmer water flows towards the tan 8 = - --- west then the colder, Jess saline water flows more g 8p rapidly westward. The convention of signs is such that in the It has been shown that the 8 ° and 9°c iso ( v) therms and the isohalines for 34·5 and 34·6% 8 p 0 southern hemisphere (

5(%.) 34·2 ·l ·5 ·b ·7 ·8 .q 350 ·3 ·4 17 .4 - · 2

lb 27 2 0 40 250 10 0 15 dq \ 3 B 12o6 1 d 1 0 1 6l(,. l \\ 14 lir� • "4- o\ llI ' � I ·J' io ..., \ l I I "(' I ' I I I I I I \· I I I ll I I I I I / I I I I I I I I I I I I \1 I I 'I 12 � t � /' E... ,! II " /I' l I 10

o . 0 m q . • . . 25 + + .. 50 X. . 100 D . 150 /J. ... 200 8 0 .250

Fig. 14: T-S characteristics at N.Z.O.I. Stations C 10 to C 29. The region characteristic of Sverdrup's Western South Pacific Central Water is bounded by the smooth curves denoted W.S.P.C. and lines of constant density (for ut = 26 · 8 and 26 · 9) are shown.

39

40

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ deep east of the cold, poorly saline pocket (fig. Since the u1 value at the surface of the pocket 6), and generally between 30 and 50 m on the is about 26·4, while that to the east and west is cold-water side of the Southland Front (fig. 3a, only about 26·0 there is no possibility that the 3d, and 6). In the Dunedin - Chatham Islands cold, poorly saline water is derived from river section (fig. 6), isotherms above 50 m are drawn outflow. to agree with thermograph data. These also indi The Southland Front swings north-east from cate that water at 40 m with a temperature o.f near Station B 34 and apparently lies over a l l ·5°c is continuous with that at the surface of bottom shallower than 1,000 m (fig. l), following the cold pocket and lies above the steep slope the 34·5 and 34·6%0 isohalines east of New Zea of the Front. This isotherms distribution and the land (fig. 2). Over most of the Campbell Plateau, low salinity within the pocket (34·33%0) strongly bottom depths beneath the Front are about 500- suggest an upwelling from depths of 30-50 m 700 m. From the Dunedin - Chatham Islands sec over the steep slope of the isotherms beneath. tion (fig. 6), in which the depth at the most

"

12 o. 2025

\ \ PotittOn 0.kO,,tf't of ITC. 1tn.n0- ·$ "W April bb3

'>· 5epturbct l30b l'>· oi• l

Fig. 15: T-S characteristics at Discovery JI stations south of the Atlantic and Western Indian Ocean and at Ob stations south of the Central Indian Ocean. Regions characteristic of Sverdrup's Western South Pacific Central Water (W.S.P.C.) and South Atlantic Central Water (S.A.C.) are shown for comparison. Figures on the station curves indicate depth in units of 100 metres.

41

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ western bathythermograph station is 610 m, it can the Tasman Sea after moving round the southern be seen that the warmer water deeper than 70 m coast. Similar warm surface water to the east of the lies along the outer edge of the steep continental p:>cket (fig. 2 and 6) must have the same origin, slope. Margules equation applied to these circum since southward movement of the more northern stances shows that this warm water will continue Subtropical Water contradicts the implications of to move to the north and later to the east around drift observations and deductions from Margules the Front faster than Subantarctic Water, paral equation. Thus, the surface water from the Tas lel with the contours of the sloping bottom (fig. man Sea has on this occasion split to take two 1, 2, and 4d). East of Banks Peninsula it merges .paths near Dunedin. The in-shore portion pro with warmer water from the north, which also duces high salinities off Dunedin and presumably swings eastward near this position. Since the mixes to the north with the colder upwelled water. warm, northward current is restricted to the This mixture may contribute to the low salinity narrow region between the Southland Front and cold water inside the southward-pointing tongue the sloping bottom. it may have a quite high north-east of Banks Peninsula (fig. 2). The off velocity. shore portion of this current must move to the north above and on the shore side of the sub It has been shown that near Dunedin, Subant surface Southland Front, swinging to the east arctic Water moved in shore at depths between between 44° S and 45° S and merging with the 30 and 50 m and has formed a pocket of upwelled south-eastward moving Subtropical Water of the colder, less-saline water. Warmer, saline water East Cape Current (Fleming, 1950) which also on the shore side of this pocket originates from swings to the east.

A MIXING PROCESS AT THE SUBTROPICAL CONVERGENCE

Water properties in the Hawea section along pocket is associated with a pronounced distortion the 180° meridian (fig. 7 and 12) illustrate a of isotherms and of lines of constant density on process by which the high salinity stratum fre the southern side of the Southland Front. At quently observed in Subantarctic Water may be Station C 23, between the pocket and the front, formed. Deacon (1937) and Sverdrup (1934) the isotherms (e.g., the 7 °c isotherm, fig. 3a) and have suggested alternative processes leading to isohalines deeper than 80 m protrude upwards to the formation of this stratum. The present dis shallower depths than at adjacent stations; this cussion is stimulated by the presence of a high feature is interpreted as an eddy. It is assumed salinity "pocket" near Station C 21. * that the general trend of characteristics in the section is representative of those in other north Between about 40 and 90 m, salinities at Sta south sections, to the east and west, but that tions C 20 and C 21 (table 2) are higher than those fluctuations such as the eddy and the high-salinity to the north and south at Stations C 19 and C 22. p:)ckets are more localised. (This maximum is shown in fig. 7 by the stippled re3ion of salinities gre:tter than 34·7%0, and in It has been shown (Montgomery, 1938) that the temperature section, fig. 3a.) This high-salinity surfaces of equal density, at, are those surfaces along which the energy required to cause inter *At Stations C I to C 29 sampling bottles without ther change of water pockets or mixing is least. Hence, mometers were attached at known intervals along the bathythermograph wire and sampling depths calculated from the distribution of lines of constant at in by proportional methods based on the greatest depth the Hawea section (fig. 7), it follows that in the reached by the bathythcrmograph. The error in the Subantarctic Water of salinity less than 34·5% , estimated greatest depth (about 250 m) is about ± 0 20 m. The temperature at each sampling depth was mixing occurs most readily horizontally. But if read from the bathythermograph trace and where the there is mixing across the front-which is repre vertical temperature gradient is great (fig. 3a) the errors may be large. However, the general configura sented by the steep isohaline slopes 34·5 and tion of isolincs in fig. 7 must be approximately correct. 34·6%0, and isolines of constant at of values of This app1ies in particular, to the steep isoline slopes 26·8-26·9-then Subantarctic Water from a given near 44 ° S, to the shapes of the salinity tongues, and to the density distribution in the cold water tongue depth mixes with water from greater depths on near Station C 20. the north side of the Front. If this type of mixing

42

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ is to occur. Subantarctic Water 40-50 m deep must This process is illustrated in another manner mix with water near 100 m deep north of the in fig. 12, which is derived from a T-S diagram Front. But Subantarctic Water deeper than about along the 180° meridian (fig. 14). This shows 80 m-below the thermocline-mixes most readily salinity and temperature properties along lines with water of the same density from depths greater of constant depth (heavy lines) , assuming that the than about 250 m. effects of the eddy and other more minor fluctua It is suggested that the high-salinity water with tions are absent. The lines conform roughly with in the pocket has been transferred to its position the observed characteristics between Stations C 10 at Stations C 20 and C 2L along surfaces of con (left-hand extremities of equidepth lines) and stant a1, by motions associated with the eddy. C 29 (right-hand extremities ). ln the region rep If the transfer has occurred with little or no resenting Subantarctic Water the equidepth lines modification of the water in the pocket then this are nearly parallel to the constant density lines; water must have come from depths of 250 m at but equidepth lines are inclined to the constant Station C 27 or from greater depths farther north al lines in the region of the Subtropical Converg (stippled in fig. 7). ence (at depths greater than about 30 m). This

s1r..i o .,,-"�o---.----,·'�--,c---�---f---r--..------.·•�-,"�---.---f---r--..----r-----:,-,�--, ,.

"

,,

Moyr

------gqJ-Qt» JuMRJZ

W-QO Mor iqJt, 1732-l

Zl•U-48 0.c RJ7 2,u-� .1o,i ,q:sg Ob --

Fig. 16: T-S characteristics at Discovery II and Ob stations between longitudes 100° E (south of Eastern Indian Ocean) and 147 °F (south of Tasmania). Station positions are shown in fig. 13. Regions characteristic of Sverdrup's Western South Pacific Central Water (W.S.P.C.) and South At lantic Central Water (S.A.C.) are shown for comparison. Figures on the station curves indicate depth in units of 100 metres.

43

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ inclination is greatest in t he Southland Front re near Stations C 23 to C 29 (fig. 7). This water may gion and small in t he Subtropical Water Region. mix with shallow Subantarctic Water of nearly Water masses which may exchange and mix most the same density The m ixture would tend to readily are represented along the constant

EDDIES, DIVERGENCE, AND STREAMS

AN EDDY IN ANTARCITC WATERS Thus the cold core cannot be due to upwelling. The coldest water near the convergence is usually A st riking feature of the distribution of temper about 1 °c (Deacon, 1937) and water colder than Converg ature southward t hrough the Antarctic 0°c is not usually present so close to the conver ence near longitude 176° E in the Pukaki and gence. Explanations of the associated tempera Hawea Sections (fig. 4b, 4c) is a pronounced core ture structure (fig. 2, 4) offer some interesting of cold water near latitude ° southern 62 S at the possibilities. Wexler (1959) has offered an al edge of the Antarctic Convergence. The cold ternative explanation to the one given below; this water w ithin this structure extends deeper than is considered later. similar cold water in t he cold layer of winter water farther south. Its absence in the east and west Fuglister and Worthington (1951) showed that sections (fig. 4a and 4d), indicates that it is the G ulf Stream in the Atlantic Ocean is not a limited in these directions. At the surface, it is smoothly flowing current, but t hat it meanders. represented by a limited area of low temperatures One of these meanders was observed to become near 62° S, 177° E (fig. 2). very distorted and to break off i nto a c yclonic eddy on the warm water, lower latitude side of In general, the temperature at t he southern the Stream limit of the Antarctic Convergence in summer in creases with depth from a m inimum value of lt is possible that a s imilar phenomenon has about 1 °c at 200 m to a maximum value of about led to the formation of the Antarctic cold core. 2°c in the upper Deep Water ( see also fig. 8). Antarctic Water may be considered as a layer of

44

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ cold, poorly saline water overlying the warmer, gion of greatest horizontal density changes which more saline upper layer of Deep Water. The lie beneath the Antarctic Convergence. depth of this surface layer (from the surface to Tf the eastward flow of higher stream velocities the sharp temperature gradient between the cold is subject to meandering, the Antarctic Conver est Winter Water and the warmer Deep Water gence will also meander. That this may be so is beneath) varies from about 100 m at the Antarc indicated here; however, more observations are tic Divergence to about 200 m near the Antarctic required to show that the southward deviation of Convergence a few degrees of latitude farther the convergence observed to the east is not a north. At this depth, as well as the sharp temper permanent local feature. (The position of the ature discontinuity, there is a vertical density convergence is known to vary with time.) gradient which slopes steeply downward to the Tt may be shown that, if a homogeneous water north beneath the convergence. Strongest geo layer overlies denser water beneath and if friction strophic currents are expected, and have been ob is negligible, the vorticity ( (which is twice the served (Sverdrup et al, 1942 p. 614), in the re- angular momentum) of a vertical column of very

,r,.J .,,!l" �·_.2,.�o>___�-+----i�- +----'?-�-�--+--t-· -.-"'-, ---i!-· ----!:"r0---:--,r-:;:;-;,rr--;-:--i'-::---:j;-----f--:n

I�

" "

,,,.,·Sl/%···O OS ,,

o'\

0.'<:0WH'f I .,.cm-c m,. M'•ICJl2

De'""-"' Hi.liar !i!if.>!i-!i7h,!i Mo7�!i !i� ()«iq� 1/'H-�7 Jo,ri_llm \ e/',7, qj'J1 .loft iC;l!,7

Fig. 17: T-S characteristics at Discovery II, Oh, and Derwent Hunter stations in and south of the Tasman Sea and south of New Zealand. Station positions are shown in fig. 13. R�gions characteristic of Sverdrup's Western South Pacific Central Water (W.S.P.C.) and South At lantic Central Water (S.A.C.) are shown for comparison. Figures on the station curves indicate depth in units of 100 metres.

45

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ small extent throughout the depth D of the layer, more northward extension to 63 ° S of cold winter is given by the equation (for example see Rossby, water in the Hawea section (fig. 4b), is interpre 1938). ted as the remnant of a northward deviation of f+t the current system. Such a meander would be 0 D = reflected in the surface contours which would :t ) differ from those between latitudes 60° S and d ( 65° S (fig. 2). The 4°c and 3°c isotherms, for where is the total derivative following the example, would bend to the south from the dt observed positions on the Pukaki western track, motion of the column and f = 2n sin cp is the then to the north around the cold pocket. It is Coriolis parameter: n is the Earth's angular velo unlikely that the 2°c isotherm would be distorted city and cp the latitude. The convention for the so far north because of its observed positions Southern Hemisphere is that cp and f are negative, near the east-west parts of the tracks. Thus the and that t is positive for anticlockwise or anti phenomenon wouid be represented by a sharp, cyclonic rotation and negative for cyclonic cir almost northward-pointing, tongue of cold water culation. This equation, expressing the constancy (3° and 4°c isotherms) enclosing a cold pocket of the potential vorticity, may be written: of less than 2°c at its northern tip. The Antarctic Dt + f + t dD Convergence would bend northward here and a = (- � ) pocket of warmer 3°c water would occur one dt dt D dt degree of latitude south of the convergence. The vorticity becomes more cyclonic (or less Near longitude 180° and latitude 44° S, south dt anticyclonic) if - is negative. Thus there is a of the Subtropical Convergence, there is another 7) dt unusual temperature structure (fig. 3a, 6, and cyclonic tendency if the column of water moves in which the isotherms are deflected upward near the thermocline. This anomaly is probably associ northwards ( 'P and f increasing) or if f + t is negative and the depth increases. Hence, if water ated with the mixing process at the convergence near the shallowest region of the Antarctic Surface previously described. Here attention is drawn Waters (near 65° S, depth 100 m, say, temper merely to its closeness to a steeply sloping density atures less than - 1 °c) moves to near 62° S in a surface, normal to which there is a sharp density meandering motion, or by other means, the vorti gradient and where high geostrophic currents may city will be decreased; that is, there is a cyclonic occur. These two phenomena, near the Antarctic tendency because of northward displacement. and Subtropical Convergences, could possibly both originate from vorticity introduced in regions The observed eddy occurred near the southern of high velocity shear, from current meanders, edge of the convergence, but is apparently sepa from fluctuations on the surfaces of sharp density rated from and extends deeper than water of simi changes, or from external processes acting at the lar low temperatures farther south. Suppose a surface. The first two �re the most likely initial northward meander has become distorted and causes; but, whatever the origins of the pheno cut off. This cut-off water would experience a mena, it is suggested that their highly developed cyclonic tendency also, if it moved to the new state-particularly at 62° S- is made possible by position without any change in depth of the con the large changes of effective depth which become stituent water. It would be surrounded by water possible when lighter water overlies a steeply of lower density at certain levels; and vertical sloping su rface of sharp density change. extension must occur downward to maintain equi librium. Alternatively, suppose that an eddy has formed in Antarctic Water by an unexplained DIVERGENCE AND T HE A NTARCTIC CONVERGENCE process (perhaps under frictional or wind influ Wexler (1959) has postulated that the region ence at the surface) and has been forced nearer known as the Antarctic Convergence is in fact the convergence where Antarctic Surface Water a region of divergence. He argues that the cold is deeper, the same effect would result. Either tongue at 62° S (fig. 4b and 4c) and two similar interpretation suggests the tendency to vorticity but much less pronounced features occurring in within the cold tongue at 62° S will be strongly two bathythermograph sections observed 42 days cyclonic compared with that further south. and 12 days earlier (near 60° 30' S, 172° E, and The first explanation, that the core is caused 61 ° 30' S, 175° E respectively), "can only be by meanders in eastward flow, is preferable if the explained by upwelling induced by horizontal

46

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ divergence of the surface layer." The source of south of the position of the "conventional" con these cold cores "must have been horizontal vergence and for a temperature maximum farther motion northward from the cold subsurface Ant south (fig. 2; Pukaki-Hawea central tracks). Simi arctic water ... a breaking-off of this water mass lar minima show up to some extent on six of the from its source region, and a strong vertical 15 "Discovery" sections in Deacon (1937). stretching of the water column to produce cooling Thermograph recordings on both Hawea and both in the surface layer and at depths below from the points where they turned due 600 ft." With this last statement the present Pukaki, author agrees. However, Wexler also argues that north on their return journeys, showed a steady the detailed temperature sections, "seem definitely increase in temperature as far as the convergence to point in the direction of a narrow band of region. (This was apart from minor fluctuations, of ° horizontal divergence encircling Antarctica, at a ± 0·5 c, normally associated with detailed oceanic position very close to that given by Mackintosh summer observations). Only one of four bathy for the Antarctic 'Convergence' ". This could ac thermograph sections across the Antarctic Con count for a surface temperature minimum just vergence published by Garner ( 1958) indicated

s(¼,) - i '--- · 17 r" 4...._ ---'"ro'-- -;---r---i-----.�---+----ir---i---'i --·i-·_ __, ,.,,o' ----i----r'--�l;___..:;·•----;·';__--,-....,

lb lbl!4 ? i j i ; 2!1Q OS

I i " I B.JO I

12

1081 o l \

Pvk.Qki --- !}I 110 &Jb Jot1 Jq)1

011COVUl ------ec:,1 -qoo Ju?1• Rl2 Feb ,q}b ---- lb&l-5 --2&19-21 "'°" t9SI

Fig. 18: T-S characteristics at Pukaki and Discovery II stations in and south of the Tasman Sea and south of New Zealand. Station positions are shown in fig. 13. R�gions characteristic of Sverdrup's Western South Pacific Central Water (W.S.P.C.) and South At /antic_ Central Water (S.A.C.) are shown for comparison. Figures on the station curves indicate depth m umts of JOOmetres.

47

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ the presence of a surface temperature minimum. formula Temperature minima exist at lhe surface in 11 f divH M = fpWo = /3My + curl ; bathythermograph sections south of lhe Pacific Ocean and in the Scotia Sea, listed by Wexler, where M is the vertically integrated momentum but in only three sections is there a well of the layer and My the northerly component, D the depth of the layer, pronounced cold core. One of these three is the W0 the vertical component of velocity (positive upwards) at depth D, f the Pukaki southward section of 62 ° S and 177° E (fig. 4c) ; the other two, at 60° 30' S, 172° E. and 8f ° ° Coriolis parameter, f3 = - (y positive north- 61 30' S. 175 E are from observations sufficiently . 8y near in place and time to have observed this wards) and ; the wind stress acting at the surface. phenomenon. It is suggested here that the deep The divergence is zero where cold core is not directly connected with Wexler's 8rx mean divergence; a more probable interpretation /3My = - of the core is that already given. However, the . ay frequent occurrence along a meridian of a surface in the Southern Ocean westerly wind zone in temperature minimum in the upper 50 m south of which M y (to the left of the wind direction) 'and the sharp southward decrease in temperature, is /3 are positive; this position is north of the region probably associated wilh the "Ekman divergence" of maximum westerlies. The westerly wind com in this region, suggested by Wexler. ponent decreases southward from a maximum in Wexler has computed the latitudes of the maxi the Subantarctic to a minimum in the zone of mum westerly winds around the Antarctic Con easterly winds near the Antarctic Continent. Thus, tinent from the mean meridional pressure gradi this whole region south of the maximum westerlies ents-between the longitudes 170° E and 177° E, is potentially one of divergence. The maximum the wind maximum lies between latitudes 52° S divergence occurs roughly where the meridional and 57° S-and remarks that the wind stress dis gradient of the zonal wind velocity is greatest, tribution "leads to a divergence south of the and it is reasonable to assume that here the depth latitude of the maximum winds and a convergence of the Ekman layer and of the upper layers of to the north". This follows from a computation Antarctic Water is least. Observations indicate of horizontal divergence, based on wind stress that the maximum divergence occurs beween relations (Koopmann, 1953; see equation below). westerly and easterly wind zones. ln discussing This shows that convergence should occur only Antarctic surface features, a number of sections were mentioned in which a salinity maximum north of the maximum mean westerly winds, ° ° which are far north of the position normally occurs between 3·5 and 4·5 of latitude south of regarded as the "Antarctic Convergence". He lhe convergence. The density distributions in these examines several possibilities "lo explain the sections indicate that this salinity maximum co existence of such a narrow circumpolar zone" of incides with the greatest density of summer sur upwelling coinciding with the temperature mini face waters, with the nearest approach to the mum. This is described as similar to phenomena surface of a given density surface, and also with observed in regions of coastal upwelling off South the closest approach to the surface of the top of ern California and the west coast of South Ameri the warm tongue in the upper layer of Deep ca, about 100 miles from the coast; in the present Water. This is strong evidence that the maximum instance the boundary is the edge of the ice-pack upwelling occurs here, and agrees with Deacon 's hypothesis (1937). in winter and the position of the convergence is maintained in summer by inertia. No attempt is North of lhis maximum upwelling the surface made in this paper to explain Wexler's narrow layers have a northward component of motion. zone of upwelling near the position usually de The water at any given_ depth will tend to sink scribed as the Antarctic Convergence but the fol beneath the lighter water to the north and to move lowing argument explains a general region of up along the isopycnal surfaces which slope gradually welling or "divergence" south of the Antarctic downward lo the north. However, there will not Convergence and the fact that sinking must occur necessarily be a net sinking of water even farther to form Intermediate Water in the region of hori north where the density surfaces slope steeply zontal divergence in the Ekman layer. downward, since there must be a balance between the dynamic process tending to cause upwelling Stommel ( 1958A) shows that horizontal div of deeper water, the tendency of denser water to ergence in the Ekman layer is given by the sink, and the rate of modification of the waters

48

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ by climatic influences and by m1xmg. The first gion and exchanges along the sloping surfaces two processes. acting in opposition, must do much of constant density, o-t, will be induced .. However to stimulate this mixing. the average vertical component of motion taken over a very long time at one place, will be zero. The argument suggests that, between the point The northern limit of the mixing region appears of maximum divergence and the region where the to be quite near the maximum westerly winds o-t surfaces commence to slopesteeply downwards, (near latitude 55° S), and south of both the line the net vertical motion is upwards, and that the separating Circumpolar and Australasian Sub water in the northward moving Ekman layer is antarctic Water (fig. 13) and the stream described further modified by warming and the density below. North of latitude 55° S the isotherms in changes so that it does not sink; the small de the bathythermogra ph sections (fig. 3) are much crease in density values, as Surface Water moves smoother. North of the maximum westerlies, the northward. must be maintained at a given level northward increase of convergence in the Ekman by mixing downwards to the bottom of the layer. layer will impose a downward motion at the North-south sections of o-t values (or of speci bottom of the Ekman layer, in agreement with fic volume anomalies) through the Subantarctic Deacon's hypothesis of the vertical circulation in Region sometimes show (fig. 10; also fig. 162 the Subantarctic Region. The northward increase in Sverdrup et al, 1942) that above 500 m the of convergence possibly also influences the posi isolines slope steeply downward to the north tion of the Subtropical Convergence. from near the Antarctic Convergence as far north ° S, the northern limit of the downward and as 55 A FREE STREAM CURRENT IN SUBANTARCTIC northward pointing low salinity tongue (fig. 9 and WATERS 11). This limit coincides with the northern limit of the region of intense mixing on the north side An interesting phenomenon is the tongue of of the convergence indicated by fluctuations in cold temperatures between latitudes 55° S and the isotherms above 250 m ( fig. 4). From these 56° S in each Pukaki-Hawea bathythermograph facts we infer that sinking of Antarctic Intermediate section ( fig. 3). Coinciding with this cold tongue Water occurs beneath a wide zone of intense there is a temperature minimum on each surface mixing on the north side of the Antarctic Con thermograph record (fig. 2). There were insuffi vergence. (This is rather different from Deacon's cient data (stations only) along Pukaki's western ( 1937) analogy of water pouring down the steep track to determine the detailed surface salinity slopes as if over a waterfall.) Mixing may be structure, but there was a salinity minimum (fig. enhanced by near equality in the mean balance 2) on the central Pukaki-Hawea track, and also between the tendency to sink due to buoyancy on the Hawea eastern track on the 180° meridian. and the tendency to upwell due to wind distribu Lt is suggested that the surface temperature and tion. At a given time different tendencies will salinity minima and the subsurface cold-water predominate in different pa rts of the mixing re- tongue are continuous from east to west and that

--__.:- FE6RUA.RY MEAN MONTHLY TEMPUATUP.ES 4SS•f .. 1s•c - - - - _ AUGUST MEAN MONTHLY TEHPERATUR(S :J(YS ,o•s S-4)009!>• '0�12 s• .....___. Sl.18-TROPICAl CONVERGENCE (OEACON) !)q()" t!)O- RANCE OF S.TC REGION bH 17 )" ...,.,.

40'

...._._...... ,__.__,_....__,__w_.LL.LL.L..L..'--'-'--'-'-'---'---'--'---'--'---'--'--'--'--'--'-....w.--'---'-__._:__,,'"'-'-....__,_�.::,,..�-'--'--'-'---._L.,c...... _ ...... _,�...... ,-'-''-'--'--""'---'---'---'-'-�.,_,.,,o•s • ° ' 120• '""' 1:,0• tf)Q 170 �O Fig. 19: Comparison of the observed range of the Subtropical Convergence Region with mean monthly sea surface isotherms for February and August. Isotherms from M.0. 516-see References. 49

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ they define a continuous zone of cold water flanked to explain strong narrow streams. Since the abso by warmer water, north and south. lute vorticity, f + t (where f is the Coriolis para The absence of sufficient subsurface salinity meter and ( the vorticity relative to the earth) is data through the Pukaki-Hawea sections prohibits conserved in a frictionless current of constant a full description of this feature, although the depth, then the absolute vorticity is changed when general temperature, salmity, and density distri ever the current has a meridional component. For example, a change in latitude from 48° 30' S butions (fig. 8, 9, and 10) suggest that steep ° gradients in the surfaces of constant density extend to 55 S causes a change (anticyclonic) in the vorticity of 10-s sec-1 for a straight down to at least 700 m north of the cold water relative + : zone, but to somewhat lesser depths to the south. non-curving current ( equals the rate of change If the deeper water beneath the zone is assumed of the current transverse to its direction (for anti to have an eastward component of velocity, then cyclonic vorticity the current increases towards the poles in an eastwards current). Thus if ( = 0 pressure gradients, transverse to the axis of the ° zone and consistent with the density distribution, at 48 30' S (i.e., no transverse velocity gradient) and a frictionless current is deflected to 55° S, then imply that the eastward current velocity along ° the north side of the zone increases as the depth the velocity in an eastwards current near 55 S decreases. Thus a strong eastward flow may be would increase towards the south by about l · l expected along the northern side of the zone. The m/sec (2·2 knots) per degree of latitude. density distribution, south of the zone implies It is evident that a deflection of the generally • that the eastward component of motion decreases eastwards Circumpolar Current through about 7° with decreasing depth, and that the gradient cur of latitude would tend to produce a current with rent may possibly be westward at the surface. If a high velocity shear, and consequently high there is a strong narrow eastward current along velocities are possible. In an enclosed ocean, the northern side, a much smaller eastward cur horizontal circulations must have a meridional rent, or a weak shallow western countercurrent, component at some places because of continuity, will be present along the southern side. The re and narrow streams of high velocity and high sultant current system is thus one of relatively relative vorticity are impressed by the earth's high shear. rotation_ on a system of frictionless flow. The northern side of the cold temperature zone In the Circumpolar Current near New Zealand may constitute a continuation of the Australasian the current is forced to acquire a high velocity by Subantarctic Front. constriction as it is forced (by continuity) to flow southwards past Auckland Islands. It is probable Fofonoff ( 1954) concluded that, in a free steady that mixing is large along the constricted current circulation in which the vertical component of and that friction will tend to reduce the velocity absolute vorticity (i.e., vorticity of earth's rota gradients across this current. However, the anti tion plus that relative to the earth) is constant cyclonic tendency in the forced southwards motion along a streamline, eastward currents in an en will tend to maintain the velocity gradient. As closed homogeneous ocean of constant depth must the stream passes the southern boundary of the occur as narrow streams of high velocity and Campbell Plateau the restrictions to the left of high relative vorticity. There cannot be slow, the constricted current are removed and the cur broad, eastward currents in such a system. rent is free to resume its eastwards course. The Stommel ( 1957) has suggested that the Circum frictional forces imposed by the restrictions will polar Current behaves largely as a closed circula be much reduced and a free stream will floweast tion, in which the eastern and western boundaries wards. are both near longitudes of the western South Atlantic where the average ocean depths, trans It is therefore suggested that the zone of mini mum surface temperatures and salinities near verse to the main current, are quite shallow; the ° southern and northern boundaries are the Ant 55 S, and the extended zone of cold temperatures arctic Continent and the Subtropical Convergence. beneath (fig. 3 and 8) are observed manifesta Thus, within the Circumpolar Current, where tions of an eastwards free stream ( of low friction) friction is negligible (i.e., there is free flow), it with high transverse shear and high velocities. is possible that Fofonoff's conclusions may apply. This stream flows in geostrophic balance with the density distribution on the north side of the zone. However, it is not necessary to postulate that The density distribution on the south side of the flow takes place in an enclosed ocean in order zone will be associated with the return to much

50

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ lower eastwards velocities to the south of the this region of great eastward mass transport in strong stream; if there is a weak shallow westwards the Southern Ocean. Here there are high hori countercurrent in this region it could be explained zontal gradients of water properties and strong as a flow associated with a narrow high velocity currents in a limited geographical region. Con jet stream (Rossby, 1936). centrated observations on the complicated dynami cal nature of ocean current flow are possible and The above hypotheses can well be tested in should be made.

CURRENTS IN SUBTROPICAL AND SUBANTARCTIC WATERS

The Tasman, Southland, Canterbury, and East Most of the water in the southern part of the Cape Currents have been described by previous Tasman Current must be forced to the south as authors (Halligan, 1921; Fleming, 1952, Brodie, it approaches New Zealand; typical Subtropical 1960; Garner. 1961). New ideas are offered below Convergence Region water will flow to the east, on each of these currents. south of New Zealand. This accounts for the surface salinities found south of Stewart Island Movements of Subtropical Water (northern part by Pukaki and Hawea and at N.Z.O.I. Station of Tasman Current, East Cape Current) and of B 35 and Ob Station 77; (fig. 13, 17, and 18). typical Convergence Region water (Southland and Canterbury Currents and southern part of Tasman Current) are shown in chart 1 (solid arrows). Probably both surface and subsurface SOUTHLAND CURRENT AND CANTERBURY CURRENT currents flow in the same direction north of the The Southland Current has been described by southern limit of the Subtropical Convergence. Garner (1961) as a surface current, containing Hence these arrows represent currents both at water from the Tasman Sea, which flows along and below the surface except, possibly, for the the southern coast of New Zealand and turns Canterbury Current which is probably confined north-east past Dunedin. East of the South Island, to the surface only. The width of the arrows in the Southland Current contains both Subtropical dicates an assessment of their strength based on Convergence water and an admixture of Austra the steepness of slopes of constant density sur lasian Subantarcti-; Water which moves north faces but not of their depth and may bear little east along the northern edge of the Southland relation to total transport. Front in the upper 200 m. The dynamic conditions along this front indicate that the Southland Cur rent reaches to the bottom at a depth of several TASMAN CURRENT hundreds of metres and is confined to a narrow The Tasman Current (Halligan, 1921) is de region between the Front and the steep slopes of rived from the East Australian Current as it turns the continental shelf. eastward towards the Central Tasman Sea. The Near its northern limits, water carried by the main body of the current lies in the Subtropical Southland Cµrrent deeper than 70 m-the mini Region and extends farther north than latitude mum depth of the front-will turn east to join 41 ° S; it contains Subtropical Water but not the Subtropical Water transpJrted southward in the deeper Antarctic Intermediate Water. The deeper East Cape Current. Both of these currents will water may move more to the north in the eastern be influenced by the bottom topJgraphy but their Tasman Sea, judging from the observed density eastward deflection will be reinforced by their distributions at Discovery Staticns 1820 a1�d 1821 confluence. (fig. I 3 and l 8). The extreme sou:hern p:ut of the Current consists of water within the Subtropi Tt has been shown that the p:>cket of pJorly cal Convergence Region to depJ1s of only abou· saline. cold surface water observed off Dundin 200 m: the colder. less saline water bene,th is (fig. 2) implies that the Southland Current splits p1rt of the Circump:Jlar Curren� system T_,e at the surface. The pocket is believed to hav.::: pre::ise gcograpltirnl scuthern limit d tile Ta�nn1 been caused by upwelling of ""1·cr whid1 had Current is not known, and may vary seasonally. m�>Ved shorewards from just above the thermo-

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This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ cline in Subantarctic Water (fig. 6). This upwelled THE C!RCU MPOLAR CURRENT water must have crossed the Southland Front. In the New Zealand sector of the Southern which dips downward from about 70 m in the Ocean the author believes that the Circumpolar thermocline and would have divided the shallow Current (Deacon, 1937) is comprised of a series waters of the Current. These shallow waters con of subsidiary currents, each of different dynamical tinue onwards into the Canterbury Current (Gar nature, the features of which depend principally ner, 1961) farther north. It is not certain that all on the local topography. These subsidiary currents of this water continues northward; part may turn are: east with the deeper subsurface current, but prob ably similar shoreward transfers of upwelled Sub (a) the Eknum, wind-driven current in the antarctic Water often contribute cold, poorly sa wind-mixed surface layers; line water to the Canterbury Current. (b) a constricted, highly frictional current in The inshore current flowing northward near which most of the flow is forced to the Cook Strait is a continuation of the Canterbury south as·Subantarctic Water in the Circum Current with a contribution derived from a mean polar Current approaches the Campbell south-eastward flow through Cook Strait (Brodie, Plateau; 1960). (c) the Bounty - Campbell Gyral, postulated as All of the above nearshore currents around an anticyclonic rotation north of the main New Zealand are stable; that is, they may persist current south-east of New Zealand; if all forces are removed other than those due (d) an eastward free stream current of high to restraints imposed by the sloping configuration anticyclonic vorticity which develops where of the bottom and the Coriolis mass acceleration. the eddy-friction stresses become unimport In the Southern Hemisphere, Coriolis accelera ant as the constricted current clears the tions tend to cause a moving particle to turn left steep western slopes of the Campbell Pla of the direction of motion; thus when wind stresses teau. and pressure gradients are absent, nearshore cur In the Subantarctic Region, the Circumpolar rents are unstable unless the leftwards tendency Current consists of Subantarctic Water, Antarctic is prevented by a land mass. Intermediate Water, Deep Water, and Bottom Water (Deacon, 1937); as the depth decreases EAST CAPE CURRENT the lower water masses are excluded. The open The East Cape Current (Fleming, 1952) trans broken arrows in Chart 1 represent a concept of ports Subtropical Water lying above Antarctic the mean motion of the Circumpolar Current, Intermediate Water as far south as the Subtropical between the bottom of the Ekman layer and the Convergence. The present author interprets this ocean bottom. The relative widths of the arrows current as a westward intensification (Stommel. indicate the apparent strength of each current at 1957) of part of a complicated, anticyclonic (anti depths where it is most significant and do not clockwise) circulation system, north of the Sub necessarily indicate total transport. The current tropical Convergence which extends an unknown strengths are roughly those indicated by horizontal distance eastward into the Pacific Ocean. Since density differences observed at successive stations. the Tasman and Coral Sea circulations are fed largely by the South Equatorial Current (Roch DRIFT CURRENTS ford, 1957), these circulations, including the East Double-headed arrows in Chart 1 represent Australian Current which is also a westward in the mean northward motion of Subantarctic Sur tensification, the Tasman Current and the East face Water to the left of the Westerly Winds. Cape Current, may all form part of the same This is in the upper wind-mixed layer which ex general circulation system which includes Sver tends to the summer thermocline if the latter is drup's Western South PacificCentral Water Mass. present. Movement in this layer has two com Sverdrup's water mass extends eastward to about ponents; the northward Ekman current due to 160° W and north to approximately 10° S. It is wind action, and a more or less eastward compon the present author's conception that the East ent consistent with gradient currents just beneath Australian and East Cape currents are dynamic the wind-mixed layer.* The mean motion in this ally similar to the Gulf Stream, the Kuro Siwo layer is thus directed roughly between north and and the Agulhas Current on the western shores east but the precise directions are not important of the other major oceans. here and the Ekman currents are represented as

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This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ a north-eastward current. In most areas the direc northern side of the constricted current since tion of maximum velocity in the West Wind Drift Australasian Subantarctic Water had not pene at the extreme surface will be less than 45° to trated to the position of N.Z.O. l. Station B 33 the left of the wind. Westerly winds decrease about 50 miles south-east of Auckland Island in north and south of a maximum and similar January, 1957. Moreover, the very low salinity changes are found in the Ekman component of at Ob Station 76, 100 miles south of Stewart Is Subantarctic Surface Waler. These changes are land and 100 miles north-east of Auckland Island important to the production of vertical motions suggests that Subantarctic Water must approach just beneath the Ekman layer. They are implied this position from the east. This eastward motion by the relative lengths of the continuous arrows may be part of a gyral. (Chart 1) Since the depth of water between Campbell and Auckland Islands is about 500 m it might A CoNSTRrcrno CURRENT have been expected that the Australasian Sub The Circumpolar Current approaches the New antarctic Water would extend to the east between Zealand Sector of the Southern Ocean from the these islands. If, however some Australasian Sub west. Near 159° E it will be at its most northerly antarctic Water flowsfrom just south of Auckland position after being deflected to the north as it Tsland to just north of Campbell Island, extremely approaches the Macquarie - Balleny Ridge. East rapid mixing of this higher salinity Subantarctic of 159° E it turns south as it moves into deeper Water with a greater quantity of lower salinity water to the east of Macquarie Island; this south water must take place (compare for example fig. ward deflection, however, is mainly enforced by 2 and fig. 13 and 18-N.Z.O.I. Stations B 32, the constriction imposed on the flow by the steep B 33, and B 34); but a south-eastward pointing western slopes of the Campbell Plateau. This has tongue near Campbell Island in the 34·3%0 sur least effect on the southern part of the current, face isohaline indicates that the highest salinity but the northern part is forced to flow to the water in the current was south of Campbell Island. south-east in a relatively narrow stream, as a Less saline water from south of the tongue could constricted current. not have crossed the current to reach the Island. Near 157° E. the eastward currents in Subant Thus, even at shallow depths, Australasian Sub arctic Water are not uniform at all latitudes but antarctic Water must flow south-east in the con must be stronger along the northern sides of the stricted current parallel with the edge of the Antarctic Convergence and Australasian Subant Campbell Plateau in approximately the same direc arctic Front than elsewhere. The Australasian tion as water beneath. It is therefore expected that Subantarctic Front is near 52° S, thus at 160° E Australasian Subantarctic Water will extend an water above 500 m in the strong constricted cur unknown distance along the constricted current, rent flowing along the slopes of the Campbell and that the boundary between Australasian and Plateau must be derived mainly from Australasian Circumpolar Waters will bend towards the south Subantarctic Water. To maintain the pressure east along each side of this current. The present gradient which is in balance with this current the indeterminate position of this boundary is indi Australasian Subantarctic Front must also turn cated by the parts of the boundary extending south-east along the current's southern edge. along each side of the constricted current in chart 1. Mixing with poorly saline Circumpolar Sub Since the observed southern limit of Austra antarctic Water will occur along the boundaries lasian Subantarctic Water (dotted line, fig. 13) of the current and eventually its "Australasian" turns to the north near the Auckland Islands and characteristics will be lost. A maximum of salinity apparently crosses the strongest part of the current, (3 ) · 4·500fo0 at 150 m at Discovery Station 2209, there must be much mixing along the front be south of Campbell Island. partly supports the tween the two different Subantarctic Water masses, above hypothesis. Such an extension of the highly beginning where the constriction first takes effect saline water will be markedly sensitive to any near 160° E. There must also be mixing on the fluctuations in the Circumpolar Current system. *At the bottom of the surface layer, where the Ekman Data used to define the Australasian Subant Current velocities arc very small, eddy friction is considered to be negligible. The present author main arctic Front indicate that it reaches a depth of at tains that the persistant action of frictional forces just least 1,000 m; that is, deep into the Antarctic beneath the layer, even though small, must impart the mean motion of the gradient currents to the wind Intermediate Current. The southward deflectionof mixed Ekman layer the current near 160° E must extend to Deep

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This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Water below 2,000 m and the dynamic balance motion and deeper Subantarctic Water has a requires a steep horizontal density gradient at southward component in the high salinity tongue these great depths; hence the front along the after sinking south of the Subtropical Conver southern side of the constricted current will ex genc�, Subantarctic Water within the gyral may tend to similar depths Ob Stations 71 to 73 show exchange and mix with that in the main Circum that constant density surfaces slope down to the polar Current to the south. This exchange could north at all depths greater than 100 m. If the level take place at the depth of the salinity maximum of the zero horizontal pressure gradient is beneath along the southern edge of the gyral between the front, or if the front extends to the bottom, Campbell and Bounty Islands. then both the pressure gradient across the front and the gradient current velocity in the constricted FREE STREAM CURRENT current will increase as the depth decreases. Hence The zone of low temperatures between 169 ° E the flow must take place along the front at all and 180° near. 55° 30' S, has earlier been inter depths greater than 100 m. preted as associated with a strong but narrow An eastward extension of quite shallow Austra eastward current along the north of the zone, and lasian Subantarctic Water onto the Campbell Plat a relatively small flow in the same direction or a eau in a direct eastward flow could be prevented weak shallow countercurrent along the south side. by the presence of much less saline water east The strong eastward free stream current is the and north-east of the Auckland Islands; this less type of eastward flow, which must exist on a · saline water must then persist against the inertia rotating earth, where friction is small and the of the water masses impinging from the west. water has been transported from other latitudes. However, if a "frontal steering" explanation for lt has been remarked that the southern slopes the shallower parts of the constricted current be of the Campbell Plateau east of 169 ° E must less correct, this would allow the persistence of exert some control on the eastward flowing cur saline water east of the Auckland Islands; "Steer rents. This implies that not all of the Subantarctic ing" is caused by extension to the surface of the Water in the Constricted Current is led into the strong deep pressure gradient on the southern side Free Stream Current system, but that some is of the constricted current. diverted north of the Free Stream Current along the steep slopes between Campbell Island and BOUNTY - CAM PBELL GYRAL the Antipodes Islands. This current need not be subject to the Fofonoff restriction since friction The Bounty - Campbell Gyral, postulated may not be sufficiently small. earlier, explains the presence of poorly saline water not far to the east and north-east of the Confinement of the eastward Circumpolar Cur Auckland lslands, and east of Stewart Island. rent flow into strong narrow streams, may occur wherever frictional forces are sufficiently small Circumpolar Subantarctic Water east and south and the water mass has been deflected north or of Stewart Island may move south-westward along south. Eddy frictional stresses are probably im ° the Front from east of 180 with an anticlockwise portant in the mixed water region (Deacon, 1937), rotation over the Bounty Trough and most of the which extends through several degrees of latitude north-eastern part of the Campbell Plateau. It has north of the Antarctic Convergence. Here, roughly been shown that warm water moves eastward on between 168° E and 180° and south of 56° S, the northern side of the Sou!hland Front; the extended free streams will not develop. However, difference in velocity across the front would free stream currents may occur in most other parts balance the pressure distribution. Without more of the Circump::>lar Current where top::>graphical information the existance of the anticyclonic features deflect the main flow. This possibility is Bounty-Campbell Gyral remains hypothetical, but neglected in chart I ( especially near longitude this direction of rotation is consistent with the 157° E), except for the Free Stream Current which maintenance of the Southland Front near the has been described and except where frictional east coast of the South Island and south of Stew forces are important. art Island; it also agrees with observed distribu tions of salinity, and in p1rticular, with the eastern ORIGIN OF THE SOUTHLAND FRONT limit of Australasian Subantarctic Water. The Sou!h!and Front may be s;r.1ply explained Since the mixed surface layer of Subantarctil: as a front which is developed in Subantarctic Surface Water has a northward component of Water near the Auckland Islands between branches

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This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ of different curre!)ts. As Australasian Subantarc densities which merge there. The motions and tic Water approaches the Auckland Islands, from characteristics of these water masses will depend the west, denser Circumpolar Subantarctic Water on factors operating in widely different locations. lies to the east. In a gradient current, the density Australasian Subantarctic Water arriving near distribution is such that in the Southern Hemis the Campbell Plateau is influenced by the climates phere lighter water lies to the left of the direction of all the oceans to the west, particularly the of the current. To maintain dynamic balance, Eastern Indian Ocean and Tasman Sea. The Australasian Subantarctic Water which is not as Circumpolar Subantarctic Water east of New deep as the bottom over the northern Campbell Zealand is partly influenced by conditions near the Plateau, must turn eastward before it reaches a Subtropical Convergence for an unknown dis point where the denser Circumpolar Subantarctic tance to the east in the Pacific Ocean, and partly Water lies to its left. A pressure gradient is required by the general circulation of Circumpolar Sub to balance the eastward flow in the Australasian antarctic Water in higher southern latitudes. Thus, Subantarctic Water relative to the motion of the the general equilibrium near the Campbell Plateau Circumpolar Subantarctic Water. This is provided is subject to variations in oceanic· and meteoro by the development of a front near the Auckland logical conditions a great distance away. This Islands. The eastward flowing Australasian Sub suggests that some features may have large sea antarctic Water above 200 m is shown in Chart sonal and longer term variations. For example, l by the broken arrow sweeping round the south there will be fluctuations in the eastern limits of ern edge of the Subtropical Convergence Region. Australasian Subantarctic Water; in character The Circumpolar Current at depths of more than istics, particularly salinity, within the Constricted 200 m must continue to the south and contribute Current, over the western Campbell Plateau, to the Constricted Current. Where the Southland and in the currents and water masses associated Front is developed between Stewart and the Auck with the Southland Front south of Stewart Island. land Islands, it is not sharp and it appears to come to the surface (fig. 4d); farther north it is East of New Zealand, the continuation of the a subsurface feature with steep gradients occurring Southland Front is closely associated with the only at depths greater than 70 m. Subtropical Convergence, and its dynamic equi librium depends more directly on influences within The presence of poorly saline Circumpolar Sub the Western South Pacific (e.g., a balance between antarctic Water about 100 miles south-east of the water in the East Cape Current and the Circum Auckland Islands has already been explained as polar Subantarctic Water). Its geographical posi a westward movement along the Front east of New Zealand. Its persistence is attributed to its tion should be more static; it is probably being steered in the upper layers by deeperwater most sensitive to seasonal changes in the sub in the Constricted Current south of the Auckland tropical currents. However, just south of New Islands through vertical continuity of the pressure Zealand, near Stewart Island, salinities and tem gradients which are associated with the Front to peratures at a given locality could vary between the south. The Southland Front must then develop those of Circumpolar Subantarctic Water and near the Auckland Islands to the north-east, in those of Subtropical Water with regular seasonal such a way that a dynamic equilibrium is main variations superimposed on any significant non tained between the water masses of different seasonal fluctuations.

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The author's generalised interpretation of water arctic Water, between 100° E and 167° E; else masses, currents, fronts, and regions separating where, water of similar properties occurs only in water masses in the southern New Zealand Region a narrow zone, 2°-4° of latitude wide, south of is shown in Chart I. The positions of the fronts, the Subtropical Convergence. Australasian Sub east of 165° E, are derived mainly from the Pu antarctic Water probably originates mainly south kaki data December 1956 to January 1957, and of the Central and Eastern Indian Oceans. the Ob data April 1956. The fronts west of 165° E and the Subtropical Convergence Region are de THE SOUTHLAND CURRENT AND SOUTHLAND FRONT duced from pre-1958 data ( fig. 13). The arrows The Soutliland Current is shown to extend are a schematic representation of the most prob several hundred metres deep between the South able system of current circulations which may /and Front and the Continental slope of south determine, or be determined by, the water charac eastern New Zealand. The front develops near teristics; the main currents and their boundaries the Auckland Islands where the pressure gradient obviously also depend on the bottom topography. across it maintains dynamic equilibrium between In general the features illustrated will be mo,e the two types of Subantarctic Water which ap complicated at any one time than depicted and proach this region from opposite directions. The there may also be seasonal variations. Southland Current originates south-west of Stew- art Island north of the Front and is mainly water THE SunTROPlCAL CONVERGENCE REGION from the Subtropical Convergence Region with The Subtropical Convergence Region is located some Australasian Subantarctic Water. East and north-east of Stewart Island the Southland Front by water properties at depths less than 200 m. At is a subsurface feature in which surfaces of these depths the salinity ranges from 34·7-35·0 / ° oo constant temperature (8°-9°c); salinity (34·5- and the density (ut) is less than 26·8. Subtropical 34·60f 0), and density (ud (26·8-26·9 slope Waler is usually warmer and more saline at these o ) steeply downward from the base of the Subant depths, and less dense at 200 m. The density (ut) arctic Water thermocline, at 70 m, beneath the at 200 m in Subantarctic Waler is greater than warmer and more saline Southland Current. The 26-8. The Subtropical Convergence Region prob Southland Current is joined by Circumpolar Sub ably extends farther to the north in summer than antarctic Water which moves shorewards 30-50 m winter; and its width may also vary. It is broad above the subsurface Southland Front and upwells near the west of New Zealand where the currents off Dunedin. Below 70 m this current follows are greatly influenced by the land mass, but nar along the front as it turns eastward south of rower and better defined offshore to the east of Banks Peninsula. Shallower water above 70 m New Zealand. Surface water west of New Zealand, continuous mainly northward along the coast as north of 42° S, is warmer than that typical of the Canterbury Current. the Subtropical Convergence Region, and is often poorly saline, although, deep::r than 50 m its sal CONSl RIC1 ED CURREN'! inities are greater than 35·00 /00 . These low surface salinities are probably caused by freshwater run A Constricted Current flows south along the off from the land, although surface northward steep western slopes of the Campbell Plateau movement may possibly take in the Eastern Tas where the Circumpolar Current impinges from man Sea. the west. lt includes water masses at all depths below the upper wind-mixed layer to the bottom and its existence is deduced from salinity distri SUBANTARCTIC WATER bution. Subantarctic Water above 600 m is be Two distinct types of Subantarctic Water occur lieved to be "steered" along the current and pre south of the Eastern Indian Ocean, the Austra vented from flowing over the Plateau by pressure lian Continent, and the Tasman Sea. Southern gradients built up at deeper levels. Strong mix Circumpolar Subantarctic Water, of salinity be ing occurs each side of the current where the low 34·50/0 0, is present through most Circumpolar Australasian Subantarctic Water loses its identity, Regions. Warmer, more saline Australasian and friction must be an imp:lrtant process. How Subantarctic Water, with salinities sometimes ever, highly saline water may be carried far >34·50fo0, occupies a broad zone between the around the southern edge of the Plateau in the Convergence Region and Circumpolar Subant- narrow constricted and free stream currents.

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This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ AUSTRALASIAN SUBANTARCTIC FRONT DIVERGENCE A laterally continuous steep gradient of prop In the Southern Ocean there is a balance be erties, the Australasian Subantarctic Front exists tween the upwards tendency of water motion at near the southern boundary of Australasian Sub the bottom of the wind-mixed Ekman layer and antarctic Water and extends to depths of at least the downward tendency of cold ·Antarctic Surface 1,000 m. This front swings southward along the Water moving north in the Ekman layer to sink southern edge of the constricted current west of beneath warmer, lighter water to the north. It the Auckland Islands and here probably reaches is postul ated that there is divergence (associated the bottom; surfaces of constant water properties with a mean upwards motion) south of the Ant slope steeply downwards to the north. The Aus arctic Convergence and a small degree of con tralasian Subantarctic Front must be distinct from vergence to the north where the two tendencies the Southland Front to allow continuity of flow are nearly equal. The region of small convergence in the Constricted Current. extends northward through several degrees of latitude in mixed waters (Deacon, 1933). FREE STREAM CURRENT A Free Stream Current moves as a· strong narrow eastward flow of strong shear along the ANTARCTIC INTERMEDIATE CURRENT northern side of a zone of low temperature water south of the Campbell Plateau near 56° S. Either Mixed water sinks as Antarctic Intermediate a weak eastward flow or a countercurrent exists Water throughout the whole of the region of along the southern side. The Free Stream is devel mean convergence. This contrasts with the pre oped where frictional and boundary constraints vious conception that the Antarctic Intermediate acting on the Constricted Current are removed. Water sinks from the immediate vicinity of the Antarctic Convergence. The "balance" concept has some points in common with Wexler's hypo BOUNTY - CAM PBELL GYRAL thesis (1959) that divergence occurs at the Ant The observed salinity distribution requires a arctic Convergence. westward flow of Circumpolar Subantarctic Water over some part of the Campbell Plateau north of Campbell Island. The circulation which is dynami MIXING PROCESSES cally most consistent with the salinity distribution is an anticyclonic rotation, the Bounty - Campbell An unusual pocket of high salinity water was Gyrai, extending over the Bounty Trough and observed between 50-90 m in Subantarctic Water northern Campbell Plateau. near the Subtropical Convergence. A cold-water tongue (interpreted as an eddy) was found be Circumpolar Subantarctic Water in the Bounty - tween the pocket and a steep subsurface front Campbell Gyral must be influenced by factors along the southern edge of the convergence. The operating chiefly in the Western Pacific Ocean pocket is interpreted as water which has moved east of New Zealand but the properties of Aus- along a constant density surface from a deeper • tralasian Subantarctic Water depend on processes level north of the subsurface front. This transfer occurring to the west of New Zealand, mainly in leads to the exchange of waters with markedly the Eastern Indian Ocean. Thus the balance be different characteristics and because of the non tween these water masses south of New Zealand linear dependence of density on temperature and and consequently the position of their boundaries salinity the mixture of the exchanged waters will depends on factors operating in widely separated sink to greater density levels. Similar transfers areas. along surfaces of constant (p:Jtential) density oc curring near any frontal region or near boundaries EDDIES between water masses, will provide a mechanism Eddies were observed near 6 2° S, 176° E and inducing downwards transfers across surfaces of 45° S, 180° E just south of the Antarctic and constant density. Near the Subtropical Conver Subtropical Convergences, respectively; their high gence the process may contribute to the forma cyclonic vorticity is an effect either of high-velocity tion of the highly saline layer beneath the Sub shear or current meanders. antarctic Surface Water.

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The author's thanks are expressed to the New stitute of Oceanology, Moscow) for access prior Zealand Naval Board for their generous co to publication to the data from the Ob cruise operation in providing facilities to carry out between Antarctica and New Zealand and to oce1.nograp'1ic work during the Antarctic cruises Dr Harry Wexler (U.S. Weather Bureau) for of HMNZS Pukaki and Hawea. access to the manuscript of his paper prior to The successful comp!etion of the planned opera publication; to Mr C. T. Webb (Chief Carto tions was materially due to the active assistance grapher, N.Z. Geological Survey, D.S.I.R.) and given by Captain R. T. Hale (HMNZS Pukaki), his staff for preparation of figures for publication; Lieutenant Commander W. G. Brown (HMNZS to Miss B. Krebs for assistance with references, Hawea), and their ships' companies. and to Mr D. M. Garner, Mr J. W. Brodie, and Mr N. M. Ridgway for helpful criticism and Thanks are also due to Professor V. Kort (In- suggestions.

58

ANON. 1932: Discovery Investigations Station 1945: Water Circulation and Surface List 1929-1931. Sta. 300-700. Discovery Rep. 4: B:,undaries in the Oceans. Quart. J. ray. met. 1-65. Soc. 7 I: 11-25. --1941 : Discovery [nvestigations S ation FERGUSSON, G. J. 1958: Reduction of Atmo List 1931-1933. Sta. 701-1184. Ibid. 21: 1-226. spheric Radio-carbon Concentration by Fossil -- 1942: Discovery fnvestigations Station Fuel Carbon Dioxide and the Mean Life of List 1933-1935. Sla. 1185-1589. lb!d. 22: 1-196. Carb�n Dioxide in the Atmosphere. Proc. roy. Soc. A, 243: 561-74. --- 1945: Discovery Investigations Station List 1935-1937. Sta 1590-2072. Ibid. 23: 1-196. FLEMING, C. A. 1944: Molluscan Evidence of Pliocene Climatic Change in New Zealand. --1947: Discovery Investigations Station Trans. ray. Soc. N.Z. 74: 207-20. List 1937-1939. Sta. 2073- 2652. Ibid. 24: 198- 422. -- 1950:_ Some South Pacific Sea-birds Logs. The Emu 49 (3): 169-88. --- 1957: Discovery Investigations S'.alion - --- 1952: The Seas Between. in "The Ant List 1950,...1951. Sta. 2653-2911. Ibid. 28: 300- 98 arctic Today", Ed. F. A. Simpson, pp. 102-26. N.Z. Antarctic Society and A. H. and A. W. --- 1958: Rezul'taty Nabliudenii. In. Trudy Reed, Wellington. Komp. Antarkt Eksp. Akad. Nauk. SSSR. FoFONOFF, N. P. 1954 : Steady Flow in a Friction Gidrol. gidrokh. geol. i biol. issled; dizel' elek less Homogeneous Ocean. mar. Res. 13: trakhode 'Ob' 1955-56. 23-145. J. 254-62. BARY, B. M. 1956: Notes on Ecology, Syste -- - 1956: Some Properties of Sea Water In matics and Development of some Mysidacea fluencing the Formation of Antarctic Bottom and Euphausiacea (Crustacea) from New Zea Water. Deep-Sea Res. 4: 32-5. land. Pacif. Sci. JO (4) : 431. FuGUSTF:R, F. C.; WORTHINGTON, L. V. 195 l: BRODIE, J. W. 1960: Coastal Surface Currents Some Results of a Multiple Ship Survey of the Around New Zealand. N.Z. J. Geo!. Geophys. Gulf Stream. Tel/us, 3: 1-14. 3 (2) : 235-52. GARNER, D. M. 1953: Physical Characteristics of --- BURLING, R. W. 1958: Age Determina Inshore Surface Waters between Cook Strait tions of Southern Ocean Waters. Nature, Lond. and Banks Peninsula, New Zealand. N.Z. J. 181: 107-8. Sci. Tech. B. 35: 239-46. BURLING, R. W.: GARNER, D. M. 1959: A Sec - --1954: Sea Surface Temperature in the ° tion of 14C Activities of Sea Water Between 9 S South-west Pacific Ocean from 1949 to 1952. ° and 66 S in the South-west Pacific Ocean. Ibid. B. 36: 285- 303. N.Z. J. Geol. Geophys 2 (4): 799-824. --- 1957: Hydrology of Chatham Rise, in C.S.I.R.O. Ausr. 1957: Onshore and Oceanic "General Account of the Chatham Islands 1954 Hydrological Investigations in Eastern and Expedition", by G. A. Knox. N.Z. Dep. sci. South-western Australia, 1955. C.S.I.R.0. industr. Res. Bull. 122 (N.Z. Oceanogr. Inst. Aust. Div. of Fish. and Ocean. Oceanof?r. Sta. Mem. 2) : 18-27. List. 27: 82. - - - 1958: The Antarctic Convergence South -- 1959: Hydrological Investigations from of New Zealand. N.Z. J. Geol. Geophys. I: FRV. Derwent Hunter, 1957. Ibid. 37: 3-10. 577-94. DEACON, G. E. R. 1933: A General Account of -- 1959: The Subtropical Convergence in the Hydrology of the South Atlantic Ocean. New Zealand Surface Waters. Ibid. 315-37. Discovery Rep. 7: 171-238. -- 1961: Hydrology of New Zealand Coastal -- 1937: The Hydrology of the Southern Waters 1955. N.Z. Dept. Sci. lndustr. Res. Bull. Ocean: lbid. 15: 1-123. 138 (N.Z. Oceanogr. Inst. Mem. 8).

59

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ --- (in press) : Analysis of Hydrological PRIVETT, D. W. 1958: The Exchange of Heat Observations in the New Zealand Region 1874- Across the Sea Surface. Marine Observer, 28: 1955. N.Z. Dept. sci. industr. Res. Bull. 23-8. (N.Z. Oceanogr. Inst. Mem. 9). PROUDMAN, J. 1953: "Dynamical Oceanography." HALLIGAN, G. H. 1921: The Ocean Currents Methuen, London. Around Australia. /. roy. Soc. N.S.W. 55: 188. RAFTER, T. A. : FERGUSON, G. J. 1958: Atmo HAMON, B. V. 1956: A Portable Temperature spheric Radiocarbon as a Tracer in Geophysical Chlorinity Bridge for Sea Water Analysis. J. Circulation Problems; Paper read at Second sci. lnstrum. 33: 329. United Nations International Conference on the Peaceful Uses of Atomic Energy. 15 /P/2128. JACOBS, W. C. 195 1: The Energy Exchange be tween Sea and Atmosphere and Some of its ROCHFORD, D. J. 1957: The Identification and Consequences. Bull. Scripps lnstn. Oceanogr., Nomenclature of the Surface Water Masses in La Jolla. 6: 27-122. the Tasman Sea (Data to the end of 1954). Aust. J. Mar. & Freshwater Res. 8: 369-413. KOOPMANN, G. 1953: Entstehung und Verbrei tung von Divergenzen in der Overflli.chennahen --- 1959: The Primary ExternalWater Masses Wasserbewegung der Antarktischen Gewasser. of the Tasman and Coral Seas. C.S.l.R.O., Deut. Hydrog r. Zeitschrif t Ezgiinzungsheft 2, Aust. Div. of Fish. and Oceanogr., Tech. Pap. Deut. Hydrogr. Inst., Hamburg. No. 7. LYMAN, J. 1958: The U.S. Navy lnternational RossBY, C. G. 1936: Dynamics of Steady Ocean Geophysical Year, Antarctic Program in Ocean Currents in the Light of Experimental Fluid ography. The Intern. hydrogr. Rev. 35 (2): Mechanks. Pap. Phys. Oceanogr. Meteor., 111-26. Mass. Inst. Tech. & Woods Hole. Oceanogr. lnstn. 5 (/): 1-43. MACKINTOSH, N. A. 1946: The Antarctic Con vergence and the Distribution of Surface --1938: On the Mutual Adjustment of Pres Temperatures in Antarctic Waters. Discovery sure and Velocity Distributions in Certain Rep. 23: 179-212. Simple Current Systems, U. J. mar. Res. /: 239-63. McLI NTOCK, A. H. (Ed), 1959 : "A Descriptive Atlas of New Zealand." Government Printer, --- 195 1: On the Vertical and Horizontal Wellington, New Zealand. Concentration of Momentum in Air and Ocean Currents. Tel/us, 3: 15-27. MARINE BRANCH METEOROLOGICAL OFFICE, 1949 : "Monthly Sea Surface Temperatures of Austral STOM MEL, H. 1957: A Survey of Ocean Current ian and New Zealand Waters. M.O. 516." Theory. Deep-Sea Res. 4 (3) : 149-84. H.M.S.O. London. -- 1958a: "The Gulf Stream." University MIDTIUN, L.; NATVIG, J. 1957: Pacific Antarctic of California Press, Berkeley. Waters, Scientific Results of the Brategg Ex --- 1958b: The Abyssal Circulation. Deep pedition, 47-48. No. 3. (Pub!. Christensens Sea Res. 5 (1): 80-2. Hva!fangsttnus, No. 20. ) SuEss, H. E. 1953: Natural Radiocarbon and MONTGOMERY, R. B. 1938: Circulation in Upper the Rate of Exchange of Carbon Dioxide be Layers of Southern North Atlantic Deduced tween the Atmosphere and the Sea: Nuclear with Use of Isentropic Analysis. Pap. Phys. Processes in Geologic Settings: National Re Oceanogr. and Meteor., M.l.T. and W.H.0.1. search Council, Washington. 6 (2) : 1-55. --- 1955: Radiocarbon Concentration m MoROSHKIN, K V. 1958: Gidrologicheskie Modern Wood. Science, 122: 415-6. . Nabliudenia. Trudy. Komp. Antarkt. Eksp. SVERDRUP, H. U. 1933: On Vertical Circulation Akad. Nauk. SSSR. Opisanie Eksp. na dizel' in the Ocean due to the Action of Wind with elektrokhode 'Ob' 1955-56. 52-68. Application to Conditions within the Antarctic NEW ZEALAND MARINE DEP. 1946: N.Z. Pilot. Circumpolar Current. Discovery Rep. 7: I 39- Wellington. 70. OLSSON, B. H. 1955: The Electrical Effects of -- - 19 34: Wie Ensteht die Antarktische Kon Tidal Streams in Cook Strait, New Zealand. vergenz? Ann. Hydrog. Mar. Meteorol. 8: Deep-Sea Res. 2 (3): 204-12. 315-7.

60

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ -- JOHNSON, M. W.; FLEMING, R. H. 1942: WEXLER, H. 1959: Atmosphere and the Sea in 'The Oceans." Prentice Hall, New York. Motion. In Bolin, B. (Ed.) "Scientific Contri TURNBULL, T. 1875: The Haast Wreck and butions to the Rossby Memorial volume. Ocean Currents. Trans. Proc. N.Z. Inst. 8: New York. Rockefeller Inst. Press. in assoc. 446-50. with Oxford Cniv. Press: pp. 107-20. UNITED STATES NAVY HYDROGRAPHIC OFFICE, WORDIE, J. M. 1921: The Ross Sea drift of the 1956: Report on Operation Deep Freeze I. Aurora in 1915-16. Geogr. l. 58: 219-24. U.S.N.H.O. - Tech. Rep. TR-33. (Formerly H.O. 16331-1). WiisT, G. 1929: Schichtung und Tiefenzirkulation -- 1957: Operation Deep Freeze II, 1956-7. des Pazifischen Ozeans. Verofjentl. Inst. Meer Ibid. TR-29 eskunde, Berlin, pp. l-{54.

61

(1) PUKAKI STATIONS

NZOI Station Date Time Latitude Longitude Depth Temp. Salinity ut No. I (m) (oc) (0/oo) g/ml i Dec. 1956 B 28 29 0745-1200h 63° 50' s 112° 00' B 0 I .61 33.46 26.78 10 1.20 33.48 26.83 30 -0.68 33.64 27.08 55 -1 .63 33.73 27.18 80 -J .67 33.87 27.29 105 -t.61 34.04 27.43 130 34.36 160 I.23 (34. 56)* (27. 70) 200 I .49 14 .51 27.64 300 I.58 34.54 27.66 395 I.57 34 .63 27 .73 490 I .40 (34 . 38) (27 . 55) 590 I .39 34.70 27.79 790 1 .24 34.74 27. 84 990 0.96 (34. 67) (27. 80) 1,190 0.93 (34 .56) (27. 72) 1,490 0.42 34.69 27.85 1,990 0.26 ° B 29 30 0230-0430 h 61 ° 56' s 169 00' E 0 3.32 33.82 26 .94 32 1.22 34.04 27. 12 55 1.01 33.98 27 .25 80 0.56 34.04 27.32 130 -0.22 34.0'i 27.38 180 o. 16 34.18 27 .46 280 I.77 34.51 27.62 630 I .92 34.60 27.68 1.030 I .65 34.69 27.76 ° ° B 30 30 1600-1 700 h 59 58' S 169 07' E 0 6.95 33.82 26.52 25 6.72 33.82 26.55 75 3.75 33.87 26 .93 150 3.28 34.00 27.09 295 3. I I 34.18 27.?4 585 2.68 34.40 27.46 985 2.27 34.60 27.66 1.490 2.07 34.65 27 .71 1,950 I.75 34.69 27 .77 ° B 31 31 040

Jan 1957 ° B 32 1 0500-0800 h 53° 38' S 169 52' E 0 10.70 34 .34 26.34 10 10.41 34.31 26.36 25 10.05 34 .40 26 .50 50 8.65 34.36 26.70 75 8. 10 34.38 26 .80 145 7.66 34.36 26.84 235 7.61 34.38 26.87 375 7. 15 34.34 26.91 545 6.92 34.33 26.93 690 6.55 34.34 26.98 62

ZOI Station Date Time Latitude Longitude Depth Temp.° Salinity at 0. (m) ( c) (0/oo) g/ml

Jan. 1957 B 33 1-2 2300--0215 h 52' 00' S 167° 30' E 0 9.91 34.05 (26 .40) 10 10.82 34.38 26 .35 25 10.64 34.40 26 .40 50 10.32 34.42 26.47 80 9. 18 34.42 26.66 121 8.52 34.47 26 .80 180 7.80 34.42 26.87 260 7.63 34.42 26.90 435 7.34 34.40 26.93 625 6.75 34.36 26.97 ° B 34 2 1240-1615 h 50° 20' S 164 12' E 0 12.0S 34.51 26.22 10 11.95 34.54 29.27 25 11.66 34.58 26 .35 50 11.33 (34 . 38) (26. 26) 75 10.35 34.60 26.60 100 10.28 34.61 26.62 150 9.84 34.65 26.73 245 9. 14 34.60 26.80 390 8.42 34.51 26.85 670 7.24 34.42 26.96 965 4.79 34. 33 27.1 9 1,4,0 3.03 34.45 27.47 1,950 2.38 34.61 27.65 2,450 2.00 34.69 27.75 B 35 3 0400--0600 h 48° 28' s 167° 23' E 0 12.32 34.76 26.37 10 12.32 34.69 26.32 20 12.30 30 12.30 34.69 26.32 50 12.35 34.67 26 .29 75 11.99 34.78 26.45 100 11.94 34.72 26.40 130 11.98 34.78 26.45 ° B 36 3 1630-1900 h 47 ° 23' S 170 35' E 0 12.71 34.29 25.92 12 12.65 34.31 25.95 2) 12.68 34.34 25 .97 50 8.90 34.29 26.60 75 7.56 (34 .40) (26 . 89) 95 7.41 34.33 26.86 140 7.30 34. 33 26.88 180 7.37 34.36 26.90 270 7 .30 34.38 26.92 460 6.61 34.34 26.98 650 5.60 34.25 27.03 940 3.86 34.27 26 .24 1,230 2.64 34.34 26.42

*Salinity values in brackets are anomalous and have been neglected in the present consideration.

NZOI Depth Temp. Salinity at Station Date Time Latitude Longitude ° No. (m) ( c) (0/oo) g/ml

Dec. 1956 ° Cl 28 2145 63° 42' s 179 21' W 0 3.0 33.91 27.04 44 I . l 34 .05 27.30 88 0.1 33.96 27.29 ° C2 29 0240 62° 53' s 179 22' W 0 4.8 33.91 26.85 43 2.4 33.95 27. 13 85 1.3 33.98 27 .23 256 1.5 34.23 27.42 ° ° C3 29 0400 62 43' S 179 22' W 0 4.7 33.91 26.86 44 2.5 33.96 27 .12 91 I .7 33.98 27.20 272 1.2 34.25 27 .45 ° ° C4 29 0515 62 32' s 179 22' W 0 4.9 33.87 26 .81 46 3.0 33.95 27.07 91 1.9 . 33.96 27 .17 274 1.3 34.22 27.42 ° CS 29 0625 62° 22' s 179 23' W 0 5.3 33.89 26.78 45 3. I 33.91 27 .03 90 I .9 34 .02 27.22 268 1.3 34 .22 27.42 0 ° C6 29 0740 62° 13' S 179 23' W 0 5.3 33.87 26.76 45 3.2 33.91 27.02 89 1 .9 33.95 27. 16 268 l .3 34.16 27 .37 ° ° C 7 29 0855 62 03' s 179 23' W 0 5.2 33.91 26 .81 44 3.1 33.98 27 .-09 88 I .9 33.95 27.16 262 1.3 34 .13 27.35 ° ° CS 29 1010 61 52' s 179 23' W 0 5.5 33 .89 26.76 44 3.2 33.91 27.02 88 2.0 33.93 27. 14 262 1.4 34 .14 27 .35 ° ° C9 29 1330 61 12' s 179 29' W 0 5.6 33.89 26.75 42 3.2 33.91 27 .02 83 2.0 33.95 27.16 250 1 .6 34 .]1 27 .31 Jan. 1957 ° ° C 10 I 1815 46 52' s 179 50' E 0 13.8 34.29 25.70 68 8.0 34.27 26 .73 ° ° CII 2030 46 33' s 180 oo· E 0 . 14. 1 34.29 25.64 57 8.5 34.36 26.72 98 7 .3 34 .38 26.92 262 6.7 34.43 27.04 ° ° C12 2230 46 14' s 180 oo' E 0 14.4 34.36 25 .63 60 8.0 34.34 26.78 103 7.1 34.29 26 .88 ° ° C 13 2 0030 45 57' S 179 58' W 0 14.4 34.36 25.63 59 8.0 34. 38 26 .81 268 6. 7 34.42 27.04 ° ° C 14 2 0230 45 47' S 179 53' W 0 14.4 34.40 25.66 60 7 .9 34.36 26 .81 274 6.7 34 .40 27.02 ° ° C 15 2 0330 45 40' S 179 49' W 0 14.4 34.43 25.68 44 10.0 34.45 26.54 89 7.7 34.36 26 .84 266 6.8 34 .42 27-02

64

NZOI Station Date Time Latitude Longitude Depth Temp. Salinity ut (oc) No. (m) (0/oo) g/ml

Jan. 1957 ° ° C 16 2 0430 45 3l' S 179 51' w 0 14.4 34.42 25.68 66 7.7 34.40 26.87 108 7.5 34.33 26.86 274 6.8 34.40 27.01 ° ° C 17 2 0530 45 2l'S 179 54' w 0 14.4 34.42 25 .68 66 8.0 34.33 26.70 108 7.3 34.38 26.92 27tJ., 6.9 34.40 26.98 ° ° C 18 2 0700 45 04' s 179 56' w 0 ]4.5 34.49 25 .71 268 6.9 34.38 26.97 ° C 19 2 0830 44° 52' 179 59' w 0 14.9 34.52 25 .64 s 44 9.9 34.49 26 .60 88 8.5 34.60 26 .90 262 7 .1 34 .38 26.94 ° C20 2 0945 44° 44' 180 00' E 0 15.3 34.72 25.70 s 43 9.6 34.69 26.80 85 9. I 34.69 26.88 91 9.0 34.61 26.83 256 7.3 34.45 26.98 C21 . 2 1110 44° 37' 179° 59' E () 15.5 34.67 25.62 s 44 10.6 34.69 26.63 88 9. 1 34 .76 26.94 262 7.6 34.45 26 .93 C22 2 1235 44° 28' 179° 58' E 0 15.3 34.63 25.64 s 41 12.8 34.63 26.16 83 9.1 34.58 26.79 250 7.3 34.40 26 .94 C23 2 1350 44° 12' S 179° 59' E 0 15. 3 34.69 25 .68 63 11.3 34.70 26 .51 104 7.2 34.42 26.96 266 6.9 34.36 26 .96 C24 2 1510 44° 05' 180° 00' E 0 !5.8 34.79 25 .67 s 39 14.6 34.74 25.88 77 10.5 34.63 26.60 237. 7.4- 34.47 26.98 C25 2 1625 43° 58' 180° 00' E 0 15.8 34.88 25.72 s 34 15.3 35.03 25 .94 69 12.0 34.88 26.52 208 9.3 34.63 26.80 C26 2 1740 43° 52' 180° oo' E 0 15.9 34.94 25.74 s 40 15.2 34.92 25 .88 79 12.0 34.96 26.59 238 9. 1 34.63 26.83 C27 2 1850 43° 41' s 180° OO' E 0 15.8 34.88 25.72 43 15. 1 34.87 25.87 86 12.3 34.92 26.50 C28 2 2000 43° 29' 180° OO' E 0 16.5 35 .16 25 .77 s 37 16.1 35.23 25.92 73 12.7 35.05 26.54 220 10.5 34.88 26.79 C29 2 2100 43° 20' 179° 57' E 0 16.7 3).23 7. ). 78 s 8�- 12.5 35.05 7.6 .56 127. 11 .9 1) .08 ?F..70 274 9.9 34.79 26 .82 ------65

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ INDEX Agulhas Current, 52 Hale, Captain R. T., 58 Antarctic Bottom Water, 14, 15. 42 llawea, RNZN, 9 Antarctic Continent, 14, 48, 50 Antarctic Convergence, 9, 14, 15, 16. 21, 23, 24, 44, 45. Indian Ocean. 14. 16, 20. 21, 25, 26, 27, 28, 34. 55, 46, 47, 48, 4� 53, 54, 57 56, 57 Antarctic lntermediate Current, 23. 26, 53 Indian Ocean Central Water, 2fi Antarctic fntermediate Water, 14, 15, 24, 48, 49, 51, lndian Ocean Deep Water, 14 52, 57 International Geophysical Year, 9 Antarctic Region, 20 Antarctic Surface Water, 14, 15, 17, 21, 24. 44. 46, Kort, Professor V .. 58 48, 57 Kura Siwo current, 52 Antarctic Upper Deep Water, 20 Antipodes Islands, 54 Lyttelton, 9 Atlantic Ocean, 14, 20, 25, 26, 27, 28, 33, 44, 50 Auckland Islands, 16, 20, 24, 25. 27, 34, 37. 38, 50, 53, Macquarie-Balleny Ridge, 15, 53 54, 55, 56 Macquarie Island. 15, 16, 24, 27, 40, 53 Australasian Region, 20 Margules Equation, 38, 40 Australasian Subantarctic Front, 20, 24, 37, 50, 53. 57 McMurdo Sound, 9 Australasian Subantarctic Water. 20, 21. 24, 25, 30, 34, Mediterranean Water, 14 38, 49, 51, 53, 54, 55, 56 Meteor, 25 Australia, 16, 27, 30, 34, 36 North Atlantic Deep Current, 14 Banks Peninsula, I0, 23, 28, 37, 38, 40, 42, 56 North Island, 28 Bounty-Campbell Gyral. 25. 40, 52, 54, 57 New Zealand Navai Board, 58 Bounty Trough, 25, 54, 57 Braten, 17, 20 Ob, RV, 9 !Jrittania, RY, 9 Pacific Ocean, 14. 16, 17, 48, 52, 55, 57 Brown, Lieutenant Commander, W. G., 58 Pukaki, RNZN, 9 4 l C activities, 10 Red Sea Water, 14 Campbell Island, 20, 21, 24, 25, 53, 54 Ross Sea, 14 Campbell Plateau, 9, 15, 20, 21, 24, 25, 33, 40, 41, 50, 52, 53, 54, 55, 56, 57 Scotia Sea, 48 Campbell Plateau Water, 20, 24, 40 Scott Island, 9, JO, 17, 20 Canterbury Current, 51, 52, 56 South Africa, 26 Chatham Islands, 10, 28, 38, 40, 41 South America, 30, 48 Chile, 30 South Atlantic Central Waler, 26 Christchurch, 29 South Equatorial Current, 52 Circumpolar Current, 14, 40, 50, 51, 52, 53, 54, 55, 56 South Equatorial Water Mass, 32 Circumpolar Subantarctic Water, 20, 21, 24. 30, 34, 37, South Island, 23, 25, 30, 40, 51, 54 38, 40, 4� 54, 55, 56, 57 Southern California, 48 Constricted Current, 52, 53, 54, 55, 56, 57 Southern Ocean, 9, 35, 48, 50, 52, 53. 57 Convergence, defined, 16 Southland Current, 40. 51. 56 Cook Strait. 28, 35, 36, 38, 52 Southland Front. 21, 37, 40, 41. 42, 44, 51, 52, 54, 55, 56 Coral Sea, 32, 52 Stewart Island, 9, 24, 25, 32, 18, 51, 53, 54, 55, 56 Coriolis Parameter. 46, 48, 50, 52 Subantarctic Region, 14. 15, 20, 30, 49 Subantarclic Surface Water, 15, passim Dana, R. V. 25 Subtropical Convergence, 9, 14. 15. 21, 24. 25. 26, 28, Deep Water, 14, 15, 16, 17. 20. 44, 45, 48, 52. 53 30, 32, 33, 37, 38, 43, 46, 50, 51, 52, 54, 55, 57 Derwent Hunter, FRV, 9 Subtropical Convergence Region, 20, 2 I, 25, 28, 30, 32, Deutsrhland, 25 34, 35, 36, 37, 55. 56 Discovery TI, RRS, 9 Subtropical Current, 39 Divergence, I 7, 48, 57 Subtropical Region, 51 Drake Passage, 14 Subtropical Surface Water, 14, 23, 25, 26, 28, 30, 32, 35, Drift Currents, 52 36, 37. 42, 51, 52. 55 Dunedin, 9, I 0, 21, 23, 29, 38, 40, 41, 42, 51, 56 Tasman Current. 51, 52 East Australian Current. 29, 30, 51, 52 Tasman Sea, 21. 23. 24, 32, 33. 34, 35, 36, 42, 51, 52, East Cape Current. 51, 52, 55 55, 56 Eddies, 11, 46, 57 Tasmania, 28. 34. 36 Ekman layer, 15, 17. 29. 48, 52, 53, 57 Endeovour, HMNZS. 9, 10, 17 Waitangi. 9 Wccldcll Sea. 14 Free Stream C::tirrent. 49. 52, 54. 57 W..:llinglon, 9 West Wind Drift. 30. 40. 53 Galathea, H.D M.S.. 25 Westerly Winds, 52 Gauss, RV, 25 Western South Pacific Central Water, 26. 30, 32. 52 Gulf Stream. 44. 52 W�\kr. Dr Harry. 58 R . f.. 01\"I':'.\ GOVER:--�!Ei\"T PRJ'.\TER. \\TI LJ.'" GTO.'". '.\F.\\' 7,F\L\'I [). J%1

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REFER.ENCES ReFER.ENCES Flt.ON'TS 1-.�111.,,,_.t ,-...�, Nonh ,o( Swt-uopi,cal Con,uac- Rq:i°" _....,,...,, lAIICt,..o.,..., ud-,-1fllT..-O.,..., rr ..rc::w-,.,.,...,, 1.. :•,•·• .. •t> C•_ ....,..._ ..__ ..ew.aW A.--S..�F.- llllllll\\11111111 .s.,..hall,_,,Jr-c..,.-,1•-*•I �c.,.,... 1-«-.. ...,-.1 . WATEll MASS IOUNDARY ,-..,,c.- NEW ZEALAND OCEANOGRAPHIC INSTITUTE A""""'-IIS..-iw.,,,;Wws WELLINGTON s��ntarc1ic �poa t...ir..11 > "4-,•1• ..1.- ....u...._, ..... ""-l"'L Scur l:))M,o;,lOIL.&LI!' o,_,,dnl',,.,,,,__...,_,....,.._ _ �i., .. ; ; c-...... _a... 1�-.arncw,.., uaa-,,, < M 1'1,,l•-IMnliok. �c-,� 0-.. ,..... _, _, .c...,.t,dl c.1n1.. __ ..., ISOBATllS CONV£RVENCE5

bi,...JS..� C--.r-A..- (•, < lll·IMD,_;..,.;,, J.1-1 ..JJ 0•1,� < :001"� 1 , ��.." .�:� ,1..., ;::i,-.tc .-...,.."n"N.IS:-.:::'��::...... ,. J,,:(�� ,oJ .:::��·l,-:,n11,U ll..,,::t=<11,,��·" !WO .. , . I ; 1 .. "' This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. /IQJ ll,lll To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/