sign in sign up
<<
Home , Current (stream), Ocean current, Swell (ocean)

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. C4, PAGES 5037-5046, APRIL 15, 1986

Interocean Exchange of

ARNOLD L. GORDON

Lamont-DohertyGeolo•Tical Observatory of Columbia University,Palisades, New York

Formation of North Atlantic Deep Water (NADW) representsa transfer of upper layer water to abyssaldepths at a rate of 15 to 20 x 106 m3/s. NADW spreadsthroughout the Atlantic and is exported to the Indian and Pacific by the Antarctic Circumpolar and deep western boundary currents. Naturally, there must be a compensating flow of upper layer water toward the northern North Atlantic to feed NADW production. It is proposed that this return flow is accomplished primarily within the ocean's warm water thermocline layer. In this way the main thermoclinesof the ocean are linked as they participate in a thermohaline-drivenglobal scalecirculation cell associatedwith NADW formation. The path of the return flow of warm water is as follows: Pacific to Indian flow within the Indonesian , advection across the in the 10ø-15øS belt, southward transfer in the Mozambique , entry into the South Atlantic by a branch of the that does not complete the retroflection pattern, northward advection within the subtropical gyre of the South Atlantic (which on balance with the southward flux of colder North Atlantic Deep Water supports the northward oceanic heat flux characteristic of the South Atlantic), and cross-equatorial flow into the western North Atlantic. The magnitude of the return flow increasesalong its path as more NADW is incorporated into the upper layer of the ocean. Additionally, the characteristicsof the return flow are gradually altered by regional ocean-atmosphereinteraction and mixing processes.Within the Indonesian seasthere is evidence of strong vertical mixing acrossthe thermocline. The cold water route, Pacific to Atlantic transport of water within the Drake Passage,is of secondaryimportance, amounting to perhaps 25% of the warm water route transport. The continuity or vigor of the warm water route is vulnerable to change not only as the thermohaline forcing in the northern North Atlantic varies but also as the larger-scalewind-driven criculation factors vary. The interocean links within the Indonesian seas and at the Agulhas retroflection may be particularly responsiveto such variability. Changesin the warn: water route continuitymay in turn influenceformation characteristicsof NADW.

INTRODUCTION tern in the meridionalplane associatedwith the NADW for- Warm salty water spreadsinto the northern North Atlantic, mation is one of a negative [Stommel,1956; Reid, where it is cooled primarily by evaporation. Ironically, this is 1961; Worthington, 1981; Gordon and Piola, 1983]: upper a consequenceof its anomalously high relative to layer water movesto the north, while deeperwater movesto the atmosphere [Warren, 1983]. This in turn maintains rela- the south. This pattern is clearly seen in the results of the tively high and despite an abundance of pre- inverse for the using the Internation- cipitation. The cooled salty water sinks to the deep ocean, al GeophysicalYear (IGY) set of zonal hydrographicsections marking the formation of North Atlantic Deep Water [Roeromich,1980, 1983; Fu, 1981; Wunsch,1984; Roeromich and Wunsch,1985]. Comparison of the IGY data with recent (NADW) [Warren, 1981; Killworth, 1983]. The NADW from data setsindicates the suspectedstability of the thermohaline the two northern sites (Labrador and the Greenland Sea- circulation,though the distributionof the transportwithin the Norwegian Sea overflow), with a mean temperature and salini- various density strata does vary [Roemmichand Wunsch ty of approximately 2øC and 34.93%0(the northern component 1985]. defined by Broecker et al., [1976] and Broecker and Peng The choice for the separationbetween the two layers varies, [1982]), spreadsto the south within the deep western bound- but only slightly, among authors. Gordonand Piola [1983], ary current, being joined by the saltier outflow from the Medi- terranean Sea. The NADW water mass influences most of the noting that the salty characteristicof NADW is incorporated into the Antarctic circumpolarbelt below the ao density of global ocean [Reid and Lynn, 1971]. Warren [1981], reviewing the estimates of NADW formation rate, arrives at a number of 27.6 [Georgi, 1981], place water less dense than that value 14 Sv (1 Sv = 106 m3/s). Broecker[1979], using radiocarbon within the upper layer (this includesthe thermoclineand inter- data, suggestsa formation rate of greater than 20 Sv. The two mediatewater). The densityinterval from ao of 27.7 to if2 of northern components account for over 90% of the NADW 36.82divides the upperand lower layersin the inversemethod volume flux. approaches[Roemmich, 180; Roemmichand Wunsch,1985]. The process of NADW formation is self-perpetuating in McCartneyand Talley's[1984] separationbetween northward that as the surface layer water sinks and is exported south- and southward flow falls near 4øC, which coincides more or ward within the deep layer, more upper layer water is drawn lesswith the 27.7 ao. into the northern North Atlantic. This in turn drives the high Broecker and Peng [1982] in their discussionof the upper evaporation rates continuing the NADW formation process. layer feed for NADW point out that the upper layer water Whether a random event initialized the circulation or whether must have about the same nutrient as NADW, it is a responseto the changingdistribution of the land masses sincethere is no significantsource or sink of nutrients in the and associated large-scale circulation remains a tantalizing North Atlantic. In their Table 7-2 the nutrient concentrations problem in .The pat- for the various components of NADW are listed. Character- istic PO4, NO3, and SiO 2 concentrations are 1, 15, and 12 mol/kg, respectively.Inspection of the GEOSECS data [Bain- Copyright 1986 by the American Geophysical Union. bridge, 1981] indicatesthat within the central North Atlantic Paper number 6C0064. thesevalues are associatedwith temperaturesbetween 11ø and 0148-0227/86/006C-0064505.00 13øC and salinity of approximately 35.55%0,corresponding to

5037 5038 GORDON: INTEROCEAN EXCHANGE OF THERMOCLINE WATER

a ao of 27.0. Thus the feed water is derived from within the lation pattern [Stommel,1957] as some South Atlantic ther- main thermocline, well above the Antarctic Intermediate mocline water is transferredinto the North Atlantic, it is pos- Water (AAIW) layer. sible to determine the mean temperature of the northward The magnitude of the thermohalinecirculation for the At- moving upper layer water across30øS. lantic Ocean has been estimated by various means. The in- The mass and heat flux equation across 30øS are verse method approach of Roemmich and Wunsch [1985] Mass yields for the IGY and 1981 transatlantic sectionsat 24ø and 36øN an average of 17 Sv for the northward flow of upper V• = [V•z + I/n] -- V•s (la) layer water (surfacecomponent of 11.3 Sv and intermediate water component of 5.8 Sv), which balances the southward Heat flowing deep water (19.9 Sv, 2.9 Sv of which is returningAnt- FiTi = [ Vbzrbz --[- [7n Tn] --[-Q œ (lb) arctic ). This estimate is close to the 15- to 16-Sv valuesgiven by Roemmich[1980] and Hall and Bryden where V• is the mass flux of the northward moving water, with [-1982]. These estimatesdo not take into account the Pacific a temperature of T•, within the interior of the South Atlantic; to Atlantic transfer of about 1 Sv in the Bering [Coach- V•z is the mass flux of the within the upper man et al., 1975], so the southward deep flow is expectedto be layer, with a transport averaged temperature of T•z; Vnis the slightly above the northward upper layer flow. Box models mass flux of NADW across30øS, with a temperature of Tn; basedon temperatureand salinity yield similar results.Gordon is the massflux of waterthrough the BeringStrait; and Qs is and Piola [1983] find that gradual increase in salinity of the the northward oceanic heat flux across 30øS. northward flowing Atlantic Ocean upper layer water can be A production rate of 20 Sv for NADW, with uniform up- justified with the Baumgartnerand Reichel [1975] estimatesof welling north of 30øS,yields 16 Sv for Vn; Tn is taken as 2øC. fresh water exchangewith the atmosphereif the magnitude of V•.,is taken as 1.5 Sv [Coachmanet al., 1979]. The transport of the thermohaline circulation cell is near 20 Sv. McCartney and the Brazil Current in the upper layer increasesfrom approxi- Talley [1984] investigatethe thermal field, finding southward mately 6.5 Sv across 19ø-23øSto 17.1 Sv across 38øS [Gordon flow across 50øN of 14.1 Sv of cold water. Their feed of upper and Greengrove,1986]. An intermediate value of 10 Sv is taken layer water into the NADW formation regionis about 11.5øC, for V•=. The combined transport of the Brazil Current and similar to the temperature horizon necessaryto meet the nu- NADW flow across 30øS is 26 Sv, which approximately bal- trient requirements. ances the Sverdrup interior transport across 30øS, given as 30 As NADW spreadsinto the South Atlantic and then east- Sv by Hellerman and Rosenstein[1983]. The value used for T•z ward with the Antarctic Circumpolar Current into the Indian is 18øC, as was determined from the distribution of the meridi- and Pacific oceans,the negative estuary circulation pattern is onal component of the volume transport in temperature- expectedto extend out of the North Atlantic into the rest of salinity space given by Miranda and Castro Filho [1981] for the world ocean [see Broecker and Peng, 1982, Figures 1-13; the Brazil Current at 19øS.Using the Hastenrath[1982] Qœ, Piola and Gordon, 1984]. The objective of this paper is to 50% of Q•, and 5% of Q•, equations(la) and (lb) are solved, explore the general form of this global scale thermohaline resulting in a T• of 15.4øC, 12.0øC and 9øC, respectively(Figure circulation cell and demonstrate support for the hypothesis 1). that upper layer return flow is accomplishedprimarily within The warmest water in the Drake Passage is about 8øC the main of the ocean. [Gordon and Molinelli, 1982-1,and the volumetric mode of the The NADW upwells within the world ocean, returning AAIW/SAMW in the southwestAtlantic, which can be traced water to the upper layer within the Antarctic region and into into the South Atlantic, is 3.5øC [Georgi, 1979]. Thus even in the thermocline. Antarctic and thermocline may be the extreme case in which the Hastenrath heat flux value is an coupled in that deep water upwelling around con- order of magnitude too high, the cold water route cannot be tributes to the formation of Antarctic Intermediate Water, the sole supplier of upper layer return to the NADW pro- which then spreads below the thermocline and upwells into ductionregion. Assuming that the HastenrathQœ is roughly the thermocline. There are two routes by which the upper correct, the bulk of the upper layer return flow must reside layer water can return to the Atlantic Ocean, though they are within the thermocline.The only source for such water south not mutually exclusive:the cold water route within the Drake of 30øS is the Indian Ocean thermocline water within the Passage, in which AAIW and Subantarctic Mode Water Agulhas Current. (SAMW) passinto the South Atlantic [Georgi, 1979; Piola and The proposed global scale warm water route associated Georgi, 1982; McCartney, 1977], and the warm water route, in with thermohaline-drivencirculation cell is as follows (Figures which Indian Ocean thermocline water is introduced to the 2a, 2b, and 2c): The upwelling NADW returns water to the South Atlantic south of Africa (Gordon, 1985). It is proposed thermocline which was lost during production of NADW• The that the warm water route is the more important. primary site of upwelling is the , where the upwelled NADW eventually flows below the thermocline and WARM WATER ROUTE enters the thermocline as AAIW. The thermocline water is As was mentioned above, most of the northward flow returned to the North Atlantic by the following route: within the upper layer in the North Atlantic resides in the 1. The NADW introduced into the Pacific Ocean is trans- warmer water above the intermediate stratum [Broecker and ferred as North Pacific Central (thermocline) Water to the Pen.q, 1982: Roemmichand Wunsch, 1985]. A simple demon- Indian Ocean through the Indonesian seas. stration that this is also the case in the South Atlantic can be 2. The Pacific water crosses the Indian Ocean in the 10 ø- made on the basis of the equatorward oceanic heat flux 15øSlatitude belt, incorporating the saltier Indian Ocean ther- characteristic of the South Atlantic. Hastenrath [1982] deter- mocline water. minesthat 69 x 1013W passto the north across30øS. Invok- 3. The mix of Pacific Ocean and Indian Ocean water ing the concept that the -driven Brazil Current within the passessouthward within the Mozambique Channel, supplying thermocline is weakened by the thermohaline-driven circu- a small component of the Agulhas Current transport. GORDON:INTEROCEAN EXCHANGE OF THERMOCLINEWATER 5039

for VeZ= 10x 106 rn3 sec TSZ = 18ø C

__ =2oC

O•VV I o• Qf-345x10wß RAKEPASSAGE .... I Of:34'5xlOlaW •_WATER I ]AAIWI I IREASONABLEI I•.• FORI •v.• 10 20 30 40 50 60 70 8O VN30os (10 6m3/sec) Fig. 1. Relationshipof the temperatureof the waterflowing to the north across30øS (T•, seeequation (1)) as is requiredto balancethe component of NADW advectedsouthward across 30øS (•, seeequation (1)). The relationis given for threevalues of Q/(the northwardheat flux across30øS): the valuegiven by Hastenrath[1982], half of that value,and 5% of that value.See equations (la) and (lb) for explanationof other values.

4. A branch of the Agulhas Current flows into the South than approximately 500 m, flow is more substantial and is Atlantic and doesnot participatein the Agulhasretroflection, directed into the Indian Ocean. This water, derived from the which returnsmost of the Agulhaswater to the Indian Ocean. , contains subtropical salinity maximum 5. The warm water passesnorthward with the South At- and intermediate salinity minimum water massesof the North lantic subtropicalgyre, crossing the equatorto enterthe upper Pacific. "Thus in all layers with the exception of that of the layer of the North Atlantic. minimum, movements from the Pacific to the Indian Assuming a 20-Sv production rate of NADW with uniform Ocean predominate"(page 114 of the Wyrtki [1961] study). upwelling into the world ocean (thus the Pacific receives 50% Water massanalysis using the gridded Levitus [1982] data of the deep water, with the Atlantic and Indian oceanssplit- set(Figures 3, 4, and 5), supportsthe resultsof Wyrtki [1961]: ting the rest equally [Piola and Gordon,1984]) and assuming The water within the BandaSea thermocline(10ø-20øC layer) that the cold water route is insignificant,it is possibleto place is similar to that of the North Pacific Central (thermocline) some numbers on the magnitude of the warm water route Water, with only minor requirements for additional fresh (Figure 2a). The assumptionof uniformupwelling (introduced water (Figure 4). The Banda Sea thermocline water enters the by Stornrneland Arons[1960]) may not be strictlyadhered to. Indian Ocean primarily in the passagesadjacent to Timor For example,if a larger proportion of NADW upwellsin the (Figure 3). The low-salinity thermoclinewithin the east- Atlantic, the transport valuesgiven for the Pacific-Indianand ern tropical Indian Ocean (Figure 3) is similar to the Banda Indian-Atlantic transfer would be reduced. Hence these num- Sea/North Pacific thermocline water (Figure 5). It spreads bersare not overly significant,since the assumptionsare not within the across the full width of likely to be strictly followed,but they do provide some"ball the Indian Ocean, as will be discussedbelow. Furthermore, it park" estimateswhich can be comparedwith other, indepen- is noted that within the 10ø-20øClayer of the Indonesianseas, dent determinations. the vertical mixing (cross-isopycnal)coefficient must be large The followingsection attempts to showsupporting evidence (greaterthan 3 x 10-4 me/s)to accountfor the modification for key links of the proposedinterocean warm water route. of the North Pacific thermoclinetemperature-salinity struc- ture. The subtropical S maximum of the North Pacific ther- EVIDENCE FOR THE WARM WATER ROUTE mocline is destroyedas an isohaline thermocline forms in the Banda Sea (Figure 4). The thermohalinestructure is not con- lndonesian Through Flow ducive to salt finger activity. The mixing is more likely a The most comprehensivephysical oceanographic work on product of interaction of the circulation with the irregular the Indonesian seas is that of Wyrtki [1957, 1961]. On the topographyof the region. basis of water mass analysis he shows that Pacific water Intense vertical mixing implies substantialdownward heat spreads into the Indian Ocean down to depths of 1000 m. flux within the Banda Sea thermocline.The vertical temper- Replacement of the water filling the deep basin of the Banda ature gradient is 10øC in 150 m (area 23 on Plate 205 of Sea by sill overflow is an active feature within the Indonesian Wyrtki [1971]); a verticalmixing coefficientof 4 x 10-4 me/s seas. The displaced deep water is responsiblefor the low- yieldsdownward heat flux of 110 W/m e. The atmosphereto salinity Banda Intermediate Water of the Indian Ocean oceanheat exchangefor the region is approximately90 W/m e [Rochford,1966]. Above the 1000-mflow, Wyrtki tracessome [Talley, 1984]. Thus as the water flows through the Indones- eastwardtransfer, primarily by lateral mixing, of the Indian ian seas,the upper layers of the thermocline would remain at 02 minimum layer into the Banda Sea. At depths shallower the same temperatureor perhapscool slightly, even though 5040 GORDON' INTEROCEAN EXCHANGE OF THERMOCLINE WATER

180øW 120 ø 60 ø 0 ø 60 ø 120 ø 180 ø E

}øN (•) UPWELLING • SINKING

20"

0 o

b

b7 i

\ \

/b' DEEP WATERFLOW

.•. '? )o S "COLD"WATER TRANSFER INTO ATLANTIC OCEAN "WARM"UPPERLAYER FLOW Fig. 2a. Global structure of the thermohalinecirculation cell associatedwith NADW production. The warm water route, shownby the solid arrows, marks the proposedpath for return of upper layer water to the northern North Atlantic as is requiredto maintaincontinuity with the formationand exportof NADW. The circledvalues are volumeflux in 106 m3/swhich are expectedfor uniformupwelling of NADW with a productionrate of 20 x 106m3/s. These values assume that the return within the cold water route, via the Drake Passage,is of minor significance. the atmosphereto ocean heat flux is large, while the lower 0.5, 5, and 5 mol/kg near 18øC yield to 2.0, 25, 30 mol/kg near thermocline would warm. This effect probably accountsfor the 10øC isotherm for PO,, NO3, and SiO2, respectively,a the decreaseof surfacewater temperaturefrom over 28øC in factor of 5 increase. The breakdown of the nutrient-cline and the westerntropical Pacific to less than 28øC in the eastern elevated nutrient concentrations in the upper layer of the tropical Indian Oceanreceiving the through flow (for example, Banda Sea thermocline is likely to be a consequenceof the see the surface temperature maps of Pickard and Emery large vertical mixing coefficient within the Indonesian seas. [ 1982]) despitestrong atmospheric heating of the ocean. The high nutrient levels characteristic of the Indian Ocean Wyrtki [1971] showsthat the nutrient concentrationsin the thermocline north of 15øSmay be derived at least in part from Banda Sea on the 25.0ao surface (near the 18øC isotherm at the vigorous mixing within the Indonesian seas. 150 m) are 1.0, 15, and 23 mol/kg for PO4, NO3, and SiOn_, The North Pacific origin for the water flow into the Indian respectively. Closer to the base of the thermocline, at the Ocean [Wyrtki, 1961] suggeststhat the NADW contribution 26.6-ao surface(near the 10øC isotherm at 350 m), nutrients to the North Pacific thermocline is the chief supplier for this are higher by somewhatless than a factor of 2: 1.8, 30, and 35 link of the warm water route. The NADW entering the more mol/kg for PO4, NO3, and SiOn_,respectively. Within the Pa- saline South Pacific thermocline may add some water to the cific Ocean's , which is the source flow through, (associatedwith equatorial upwelling and ad- for the Mindanao Current [Tsuchiya, 1968] feeding the trans- vection) along the north of New Guinea (Figure 3), but fer to the Indian Ocean, there is a strong nutrient-cline across it appears to have a secondary impact on the Banda Sea the 15ø to 20øC layer [Wyrtki and Kilonsky, 1984]. Values of salinity. This leads to the question, where does most of the

z

HEAT HEAT u3u3 HEAT HEAT z ,,, OU3 zO • It•H20 ea

NADW.e--SALTIER o FOEaATION__ • • , •SALTIER• <• • __ • ATLANTIC• ••• AAIWl • INOIANTHERMOCLINE• I PACIFIC AAIW I I TO ATLANTIC ,cc •> ACC I I Fig. 2b. Schematicrepresentation of the thermohaline circulation cell associatedwith NADW production and the warm water route along line a-a' shown in Figure 2a. The upper layer water within the main thermocline begins its passageto the North Atlantic in the Pacific as low-salinity water. It enters the Indian Ocean via the Indonesian seas, where its salinity and volume flux increaseby excessevaporation and further upwelling of NADW, respectively.The thermocline water enters the Atlantic south of Africa and spreads to the northern Atlantic, continuing to increase in salinity and volume flux. GORDON: INTEROCEAN EXCHANGE OF THERMOCLINE WATER 5041

HzO HEAT HzO HEAT thermocline returns to the Atlantic by way of the Drake Pas- sage. However, there is likely to be additional Pacific to l Indian transport which balanceseastward transfer of (cooled) thermocline water south of Australia. The value presented in -- tTHER MOC•LIN•//(--. f Figure 2a representsonly that component involved in the global scaleNADW-thermocline circulation cell.

Trans-Indian Ocean NADW S. MAX. The low-salinity thermoclinewithin the westward flowing Fig. 2c. Schematic representation of the paths followed as South Equatorial Current in the 10ø to 15øSlatitude belt of NADW upwells,becoming incorporated into the thermocline,along the Indian Ocean (Figure 3) has temperature-salinitycharac- line b-b' shownin Figure 2a. The main upwellingis expectedto occur teristics similar to those of Banda Sea water emerging from in the Antarctic region, with eventual transfer to the thermoclineas Antarctic Intermediate Water. the Timor Sea region (Figures 5 and 6; also see Wyrtki [1971], Plates 223, 231, and 237 for salinity on isopycnalsur- faces within the thermocline and Plates 364 to 369 and 394 for NADW enteringthe South Pacific thermoclinego ? This is not dynamic topography), attesting to its Pacific origin. The the main topic of this paper, but it is suggestedthat it may characteristic of this feature stands in sharp contrast to the feed the cold water route via the Drake Passage. Under the more saline thermocline of the south Indian Ocean and Ara- assumptionof uniform NADW upwelling,this would account bian Sea; a lesser contrast is seen with the of Bengal. for a maximum of 25% of the total return flow into the North Inspection of the evolution of the temperature-salinityrelation Atlantic Ocean. in the downstream direction shows a steady increaseof salini- There has been some work addressingthe magnitude of the ty above the 10øC isotherm (Figures 3 and 6). The salinity Pacific to Indian Ocean transfer. Wyrtki [1961] determines increasesby approximately 0.5%0throughout the thermocline the currentvelocity from differencesin dynamicheights. South before reaching the Somali Basin in the western Indian Ocean. of Timor Island (Timor Sea) a single station pair indicates a It is likely that the salt is introduced by lateral (isopycnal) flux of 6.4 Sv. This flow is accomplishedwithin the upper 300 mixing with the neighboring thermoclines.The salinity levels m. However, in his summary of transports for the upper 150 within the neighboring thermoclines are maintained by the to 200 m (his Table 12, p. 136), the resultant flow from the regional excessof evaporation over precipitation [Baumgart- Indonesian to the Indian Ocean rangesfrom a low of 1 net and Reichel, 1975]. In addition, NADW incorporated into Sv in the December to February period to 2.5 Sv in August, the Indian Ocean thermocline,via the AAIW route or directly with an annual average of 1.7 Sv. (Figure 2c), would be expectedto the transport of the Recently there have been a rash of estimates (based on a warm water return flow during transit of the Indian Ocean. variety of methods)of the Pacific to Indian transfer of water, The nutrient concentrations within the Indian Ocean ther- all of which are larger than Wyrtki's estimate (Table 1). The mocline north of 15øSare significantlyabove those within the averageof all estimatesis 9.2 Sv into the Indian Ocean. south Indian Ocean thermocline [Spenceret al., 1982]. As was Clearly, there is agreement that the water flow is toward the mentioned above, similar levels are observed in the Banda Indian Ocean, but there is a wide range of estimates as to its Sea, and it is suspectedthat the intense vertical mixing within magnitude. The value proposed in this paper (8.5 Sv, Figure the Indonesian seas may, at least in part, be responsible.This 2a) is close to the average estimate. The values would be may also be true for the more or lessisohaline thermocline of halved to about 4 Sv if the NADW entering the South Pacific the northern Indian Ocean. Thus it is possiblethat the Pacific

20øE 40 ø 60 ø 80 ø I00 ø 120 ø 140 ø 160 ø E 20øN ' ?•!.'"':. ' ::•' ' ' '•. ' .':;" , '/ ' :; CZ"-" •'L-•, .... 20øN

..,,N•... / •--•/ / •. -• •- ., • .-•.. ;• .-,?• • • ._•

0 o 0 o

20 ø 20 ø

.) .---...... X::: C s w - :/z • -•.•--' • •-•...... • ..... •6'c '-. •:• •.?:] • - . ./, ...... -- ...... •- .... • ...... ' • • • ••••• •... ..•I:;.•...... 20 oC>- ...... • ...... •[R• •• • 10-20•OLgYER 40øS I 40I • I •10• I 8 • • I I00I • I 120I • I 140I • I 160•EI Fig. 3. Average salinity in the 10ø to 20øC layer of the main thermocline,prepared from the Levitus [1982] data set. Intersections of the 20øC isotherm and the surface and of the 10øC isotherm and the sea floor are shown as thick dashed lines. NPCW, SPCW, NICW, and SICW are the central or thermocline water of the North Pacific, South Pacific, north Indian, and south Indian oceans,respectively. 5042 GORDON' INTEROCEAN EXCHANGE OF THERMOCLINE WATER

3O 30 ] ] ] ] I i [ ] [ I ! ] I ] I [ ] i [ i [ , I • ] [ [ [ ] ] [ t ] [ ] [ [

25 25- • '\ ',, - \

aJ 2o

D

/ / /

/ • 10 - WESTERN TROPICAL PACIFIC OCEAN • INOI•N o

HALMAHERA SEA CAPECUVIER TOKELAU 5 - NICOBAR BASIN • - BISMARCK SEA JAVA TRENCH NPCW ..... BANDA SEA BANDA SEA iI

0 I [ I I I I I I I ] I I I I ] I i i 32 33 34 35 36 32 • 34 35 36

SALINITY %0 SALINITY •/• Fig. 4. The potential temperature-salinity relationship for various Fig. 5. The potential temperature-salinity relationship for various seas or regions of the western tropical Pacific Ocean and Banda Sea, seasor regions of the eastern tropical Indian Ocean from the Levitus from the Levitus data set. The Banda Sea curve is similar to the data set. The water within the low-salinity thcrmoclinc along the 10ø thermocline characteristics of the North Pacific. It is possible to "pro- to 1SøSlatitude belt (see Figure 3) is similar to that of the Banda Sea duce" the Banda Sea curve from the North Pacific curve with strong and is much more saline than that of the Indian Ocean to the south vertical (cross-isopycnal)mixing and with the addition of slight and north. This similarity supports the Banda Sea (North Pacific) amount of fresh water (of the order of 10 cm). To produce the Banda origin of the low-salinityfeature. Sea curve from South Pacific thermocline water requires far more vigorous mixing and the addition of nearly 10 m of fresh water, a condition believedimprobably for realistic residencetime of through flow water within the Indonesian seas. L. Gordon et al., manuscriptin preparation, 1986] showsthe inshore segmentof the Agulhas Current is marked by reduced salinity within the thermocline at temperaturesabove 14øC. inflow to the Indian Ocean is an important determiner of the The temperature-salinity relationship of the feature is nearly thermocline water properties of the northern Indian Ocean identical to that found in the Mozambique Channel, while the and that it feeds the southward flux via the Mozambique temperaure-salinity curve, 10 to 20 km further offshore Channel. matchesthat of the East (Figure 7). This low-salinity thermocline feature along the inshore side of the Mozambique Channel The low-salinity thermocline characteristic of the Somali TABLE 1. Estimates of Pacific to Indian Ocean Transport Within Basin can be traced to spread southward within the Mozam- the Indonesian Seas bique Channel (Figure 7; also see Wyrtki [1971, Plates 223, 231, and 237]), but the signal is weak. Saetre and da Silva Who What, Sv How [1984] conclude that the circulation within the Mozambique Channel is dominated by anticyclonic gyres with only minor Wyrtki [1961] 1.7 dynamic calculations Cox [ 1975] 18 general circulation through flow. They call into question the older concept that model (experiment the Mozambique Channel is the main supplier of the Agulhas III); Current. Rather, the bulk of the Agulhas Current seemsto be around Australia associated with a strong anticyclonic gyre situated west of equated to zero Godfrey and Golding [1981] 10 mass transport in 45øE within the 30ø to 40øS belt of the Indian Ocean [Wyrtki, the eastern Indian 1971, Plate 394]; however, some input from the Mozambique Ocean Channel and is likely [Wyrtki, Piola and Gordon[1984] 14 freshwater box 1971, Plates 364 to 369; Harris, 1972; Lutjeharms, 1976]. model of the Harris [1972] estimates 10-Sv flows poleward through the Pacific and Indian oceans Mozambique Channel. Fu's [1986] inverse method Fine [1985] 5.1 tritium box model for six hydrographic sectionsin the Indian Ocean indicates (upper 300 m) southward transport in the Mozambique Channel of 6 Sv, Fu [ 1986] 6.6 inverse method similar to his value for the Indonesiansea through flow (Table applied to six Indian Ocean 1). sections A hydrographic section adjacent to Durban obtained from the R/V Meiring Naude in October 1983 [Grundlingh,1986; A. 1 Sv = 106 m3/s. GORDON' INTEROCEAN EXCHANGE OF THERMOCLINE WATER 5043

Agulhas Current is also seen in the 1973 Meiring Naude sec- 3O tion off Durban used by Pearce [1977]. Thus the water mass composition indicates that the low-salinity Mozambique Channel input is confined to the inshore segment of the Agulhas (this is also seen in the Wyrtki [1971] plates). Sec- 25 tions of the 1983 Meiring Naude data set further to the south, and those of the R/V Knorr obtained in November and De- cember 1983 in the Agulhas retroflection [Gordon, 1985; Huber et al., 1985; A. L. Gordon et al., manuscriptin prep- 20- aration, 1986] reveal that the Mozambique Channel signature can be traced to the south, but it becomes more diffuse as mixing with the more salineoffshore components occurs. Grundlingh[1984] reports the presenceof an intrusion of Red Sea water of 5øC near 1400 m in the southern end of the Mozambique Channel. He concludesthat the Mozambique Channel closurebelow the 27.5-o,surface as suggestedby Lut- 10- jeharrns [1976] is not complete. It is suggestedthat the Mozambique Channel water, while a minor component of the Agulhas Current volume flux, does WESTERN INDIAN OCEAN inject specific water mass characteristics derived from the SOMALI BASIN Somali Basin into the Agulhas, and it is feasiblethat the level MOZAMBIQUE CHANNEL ..... MADAGASCAR BASIN of transport required by the warm water route (Figure 2a) can MASCARENE BASIN be supported. 0 Agulhas Leakage •2

The Agulhas Current is the most energeticwestern bound- SALINITY %o ary current of the southernhemisphere; volume transport esti- Fig. 7. The potential temperature-salinity relationship along the matesvary from 44 Sv [Toole and Raymet, 1985] to the range western South Indian Ocean, from the Levitus data set. The Madaga- 62-75 Sv [Grundlingh,1980] as the current passesadjacent to scar Basin represents the East Madagascar Current regime, and the Mascarene Basin represents the anticyclonic circulation gyre of the Durban, with 80% occurring in the upper 1000 m [Grund- southwest Indian Ocean. The two fine dotted lines are from stations lingh, 1980]. within the Agulhas Current (data were obtained from the R/V Meir- The Agulhas Current turns westward as its path follows the ing Naude in October 1983) adjacent to Durban. The station from the inshore segment of the current is nearly identical to the water within the Mozambique Channel, whereas the characteristicsof the outer 3O segmentof the Agulhas Current are closer to those of the East Mada- gascar Current, suggestingthat the Mozambique input is confined to the inshore side of the Agulhas Current.

25 southern terminus of the African continent, separating from the margin near 22øE. After separationit executesan abrupt anticyclonic turn to the east in what is referred to as the Agulhas retroflection [Bang, 1970]. The dynamics of the re- troflection are addressedby de Ruijter [1982] and Ou and de Ruijter [1985]. Not all of the Agulhas water participates in the retroflection. Using the Knorr November-December 1983 hy-

15- drographic data set, Gordon [1985] on the basis of water mass characteristicsand geostrophicflow shows the presenceof iso- lated rings of Agulhas water west of the retroflection (also see Duncan [1968] and Olson and Evans [1985]). Between the ring 10- centers and the African mainland, is an Agulhas Current branch, carrying 14 Sv, directed into the South Atlantic Ocean. The 1983 situation is remarkably similar to that found INDIAN OCEAN, 10øS in Mach 1969, when a synoptic data set indicates that 5 Sv of 5- SOMALI BASIN Agulhas water enters the South Atlantic [Harris and Van For- tOøS, 66øE eest, 1978], and to the pattern shown by Dietrich [1935]...... 10os, 80øE IOøS, 96øE Model studies also depict linkage between the subtropical gyres of the South Atlantic and Indian oceans [Veronis, 1973; 0 de Ruijter and Boudra, 1985]. 32 33 34 35 36 The Mozambique Channel input to the Agulhas system SALINITY carries the high-nutrient characteristic of the Indian Ocean Fig. 6. The potential temperature-salinityrelationship for the north of 15øS.This is seenin density surfacemaps of Wyrtki low-salinity thermocline along the 10ø to 15øS latitude belt of the [1971] as nutrient rich water spreads poleward along the Indian Ocean (seeFigure 3), from the Levitus data set. The isohaline westernboundary. The hydrographic data obtained within the character is maintained as the salinity increasesby approximately 0.5%0on proceedingwestward with the South Equatorial Current Agulhas Current axis south of Africa from aboard the Knorr from the eastern Indian Ocean to the western margin, within the in November and December 1983 [Huber et al., 1985] reveal Somali Basin. nutrient levels similar to the level required to match the feed 5044 GORDON: INTEROCEAN EXCHANGE OF THERMOCLINE WATER water within the North Atlantic [Broecker and Peng, 1982]. as it incorporates NADW upwelling into the Indian Ocean The potential temperature nutrient relation for the southeast thermocline. Warm water transfer into the Atlantic is accom- Atlantic thermocline is quite similiar to that within the west- plished as a branch of the Agulhas Current enters the South ern and central South Atlantic (GEOSECS data of Bainbridge Atlantic rather that curl back to the Indian Ocean as part of [1981]). This may be due to strong exchange between the the Agulhas retroflection. The introduction of warm water South Atlantic and Indian Oceans, so "fresh" introduction of into the poleward eastern corner of the South Atlantic ther- Indian water into the South Atlantic may not contrast moclineis responsiblefor the unique equatorwardheat flux of strongly with the background. Introduction of high silica con- the South Atlantic. Upper layer water is carried to the north centrations from the Indian Ocean water into the South At- in the Atlantic Ocean within the two subtropicalgyres. Trans- lantic on the 27.1-•o surface is inferred by Kawase and Sar- fer across the equator is accomplishedwithin the complex miento [ 1985]. vertical and horizontal circulation pattern of the region. The upper layer water is further modified in the Atlantic by excess Atlantic evaporation, and it incorporates additional NADW. The Indian Ocean water introduced into the South Atlantic While additional field work is needed within various sites of as part of the warm water route would be carried to the north the proposed circulation pattern to arrive at better determi- by the (the warm is driven nation of transport values, available information supports the offshoreby the coastal upwelling process).The South Atlantic hypothesis that the flow compensating the production and subtropicalgyre is unique in that it receivesheat at the pole- export from the North Atlantic of NADW is accomplishedby ward eastern corner. The anomalous oceanic heat loss of 10øS linkage of the warm water or thermoclinelayers of the ocean. in the easternSouth Atlantic (but west of the Benguelaupwell- The time scale of the circulation pattern is likely to be ing regime) reported by Bunker [1980] may be a product of O(1000) years within the deep water [Broecker and Peng, the introduction of warm water from the south. 1982], but the return flow would represent a shorter time The path of the northward flowing warm water in the At- scale,as it is concentratedwithin the upper layer and along a lantic is presumably incorporated within the large-scale gyre more confined path. The approximate time for thermocline circulation, passing northward in the western limits of the water to be carried from the central tropical Pacific along the equatorial region and within the western boundary of the 40,000-km-long warm water route to the North Atlantic is North Atlantic. The path acrossthe equator is unclear, as the only 13 to 130 years, using 0.10-m/s and 0.01-m/s character- upper layer return flow is likely to become involved with the istic velocity, respectively. active mixing regime and complicated vertical and zonal There is increasing evidence that surface water character- circulation pattern characteristic of the equatorial zone. isticsin the North Atlantic and the rate of NADW production Kawase and Sarrniento [1985] show evidence for significant (including its specificcharacteristics and changesof the rela- upward flux of nutrients acrossisopycnal surfaceswithin the tive contribution from each of the major sites) vary on the Atlantic equatorial zone induced by mixing processes.Cross- decadal time scale [-Lazier, 1973, 1980; Clarke, 1985; S•vift, equatorial transfer of masswithin the upper layer is supported 1984; Roemrnichand Wunsch, 1985]. There is also evidence by inverse solution for sets of zonal hydrographic sections that deep water geochemical tracers have varied in phase with across the Atlantic. Roernrnich[1983] determines the upper variations in North Atlantic surface water on layer northward flux acrossthe equatorial zone accomplished time scales of tens of thousands of years, marking glacial- by geostrophicand for the IGY sectionsat interglacial oscillations [Ruddirnan and Mcintyre, 1981; Rud- 24øS, 8øS, 8øN, and 24øN as 31.1 Sv, 32.7 Sv, 5.1 Sv, and 18.6 diman, 1985; Mix and Fairbanks, 1985]. Positive correlation of Sv respectively(from his Figure 3). NADW production rate and North Atlantic surface water Molinariet al. [1985]find that the average temperaturesfollows from the argument presentedby Warren transport from April 1982 to August 1983 is 30.6 Sv. The [1983]: the removal allowing deep convection is transport of the Florida Current through the Florida induced in part by high evaporation rates sustainedby warm would thus be composed of about 50% thermohaline and surfacewater temperaturesrelative to the atmosphere. 50% wind-driven circulation. There is the question of the There are many causesfor variability involving the similarity of this transport to the wind-driven Sverdrup trans- coupledocean-atmosphere system with numerouspositive and port function over the interior North Atlantic [Leetrnaa et al., negative feedbacks. The thermohaline circulation cell de- 1977], but as was pointed out by Roemmichand Wunsch scribed in the paper may play some role in that the cell may [1985], the western boundary flow need not match the Sver- be weakened, strengthened or in other ways altered, by vari- drup interior transport, as nonlinear conditions may extend ations in the wind driven circulation. This would change inter- into the interior region. ocean exchange of thermocline water and hence the global In summary, it is proposedthat the upper layer water flow- heat and fresh water fluxes and perhaps influencing the pro- ing northward in the Atlantic Ocean required to feed NADW duction rate of characteristics of NADW. The two inter-ocean production is derived primarily from the thermoclineand that links may be vulnerable to change. it is drawn from each ocean, along what is referred to as the warm water route. North Pacific thermocline water enters the The Agulhas Link Indian Ocean via the Indonesian seas,where strong vertical Variation of the Agulhas Current flow into the Atlantic has mixing induces a nearly isohaline condition while carrying been discussedby de Ruijter and Boudra [1985] usinga wind- heat downward and nutrients upward. The Pacific water, driven nonlinear barotropic model. While increasing the marked by low salinity, is transported across the Indian Rossbynumber reducesthe connectionbetween the subtropi- Ocean within the South Equatorial Current. The excesswater cal gyresof the two oceans,changes also are forcedby altering introduced to the Indian Ocean by advection through the the wind field: as the zero wind stresscurl (maximum west- Indonesian seasis transferredto the Agulhas Current via the erlies)moves further south of Africa, exchangebetween the Mozambique Channel. The return water becomes saltier two subtropicalgyres increases. Ou and de Ruijter [1985] find during its transit of the Indian Ocean owing to the regional that the Agulhasseparation from the continentalmargin be- excessevaporation, and in addition its volume flux increases comesless complete (i.e., more water flows into the Atlantic) GORDON: INTEROCEAN EXCHANGE OF THERMOCLINE WATER 5045

as the Agulhas transport decreases.Thus the presence of Acknowledgments.The concept presentedin this paper was the strong seasonal variation of the over the South basis of a number of oral presentationsduring 1985; helpful com- ments were receivedfrom many people, including K. Wyrtki, T. Taka- Atlantic-Indian ocean basin as reported by Hellerman and hashi, G. Reverdin, W. Ruddiman, R. Fairbanks, and B. Huber. The Rosenstein[1983] (i.e., southward movement of the zero wind work was supported by the Office of Naval Research contract N000- stresscurl in the summer),may be expectedto have an impact 14-84-C-0132. Lamont-Doherty Geological Observatory contribution on the South Atlantic circulation and . 3962. Van Loon and Rogers [1984] show that much of the - al wind variation along 40øS is in the secondharmonic, with REFERENCES maximum wind at the time of each solsticeand with signifi- cant interannual variations: weak in 1958 and 1978 and Bainbridge, A. E., GEOSECS Atlantic Expedition, vol. 1., Hydro- strong in 1957 and 1979 (the four years included in their graphic Data, 1972-1973, 121 pp., National Science Foundation, U.S. Government Printing Office, Washington, D.C., 1981. study).Interannual variations in the south Indian Ocean may Bang, N. D., Dynamic interpretations of a detailed surface temper- be linked to the Southern Oscillation in that the air ature chart of the Agulhas Current retroflexion and fragmentation pressure along the westerliesover the South Indian Ocean area, S. Afr. Geogr.d., 52, 67-76, 1970. showsnegative correlation with air pressureat Darwin, which Baumgartner, A., and E. Reichel, The World Water Balance: Mean is representativeof changesover much of the central and east- Annual Global, Continentaland Maritime Precipitation,Evaporation and Run-off, 179 pp., Elsevier,New York, 1975. ern Indian Ocean [Rasmusson,1985]. As the wind stresscurl Broecker, W., A revised estimate for the radiocarbon age of North over the Indian Ocean changes the Agulhas transport and Atlantic Deep Water, d. Geophys.Res., 84, 3218-3266, 1979. retroflection pattern, its branching into the South Atlantic re- Broecker, W. S., and T-H. Peng, Tracers in the Sea, 690 pp., Lamont- sponds[de Ruijter and Boudra, 1985; Ou and de Ruijter, 1985], Doherty Geological Observatory, Palisades,N.Y., 1982. Broecker,W. S., T. Takahashi, and T. Li, of the Central affectingthe heat/water flux into the South Atlantic. Atlantic, I, The two-degreediscontinuity, Res., 23, 1083- On the glacial-interglacial scale, variation of the warm 1104, 1976. water link around the southern tip of Africa may also have Bunker, A. F., Trends of variables and energy fluxes over the Atlantic occurred. Prell et al. [1980], in their sediment core study of Ocean from 1948 to 1972, Mon. Weather Rev., 108(6), 720-732, 1980. Indian Ocean surfacecirculation 18,000 years ago (during the Clarke, R. A., Temporal and spatial scales of Labrador Sea water height of the last glacial period), found that the position of the formation, NASA Conf. Publ., 2367, 7-11, 1985. subtropical convergencewas about 2ø further north than the Coachman, L. K., K. Aagaard, and R. B. Tripp, Bering Strait: The present position and that the Agulhas Current was composed RegionalPhysical Oceanography,172 pp., University of Washington of cooler water. In this case, less warm Indian Ocean water Press, 1975. Cox, M.D., A baroclinic numerical model of the world ocean: Pre- would flow into the South Atlantic during the glacial periods. liminary results,in Numerical Models of OceanCirculation, 364 pp., This would alter the thermal character of the Atlantic thermo- National Academy Press,Washington, D.C., 1975. cline relative to present conditions, weaking the negative es- de Ruijter, W., Asymptotic analysisof the Agulhas Current, Rev. Pure turary thermohalin½circulation cell. Appl. Geophys.,12, 361-373, 1982. de Ruijter, W. P.M., and D. B. Boudra, The wind-driven circulation The lndonesian Sea Link in the South Atlantic-Indian Ocean, I, Numerical experimentsin a one-layer model, Deep Sea Res., 32, 557-574, 1985. Another sectionof the warm water route that may be sus- Dietrich, G., Veroeff. der Int. Komm.ruer der Meer, 27, p. 1, Univ. of ceptibleto variability is the Pacific to Indian exchange.The Berlin, German Democratic Republic, 1935. Duncan, C. P., An in the subtropicalconvergence southwest of tropical Pacific surface water and that within the Indonesian South Africa, d. Geophys.Res., 73(2), 531-534, 1968. seas representa large reservoir of heat and relatively fresh Fine, R. A., Direct evidenceusing tritium data for throughflow from water. Its transferinto the Indian Ocean could representa the Pacific into the Indian Ocean, , 315, 478-480, 1985. significantinterocean flux of heat and fresh water. The trans- Fu, L., The general circulation and meridional heat transport of the South Atlantic determinedby inversemethods, d. Phys. Oceanogr., port of three primary water masslayers across30øS for the 11, 1171-1193, 1981. world ocean as presentedby Storereel[1980] is very large, Fu, L., Mass, heat and freshwater fluxes in the south Indian Ocean, J. unreasonably so when compared with residencetime esti- Phys. Oceanogr., in press, 1986. mates.These transport values are significantlyaltered towards Georgi, D. T., Modal properties of Antarctic Intermediate Water in the southeast Pacific and the South Atlantic, J. Phys Oceanogr.,9, more "acceptable"levels when the Indonesian sea link is in- 456-468, 1979. cluded[Piola and Gordon,1985]. Georgi, D. T., On the relationshipbetween the large-scaleproperty Significantvariability of the through flow transportsis an- variations and fine structure in the Circumpolar Deep Water, d. ticipated on a variety of time scales,including the seasonal Geophys.Res., 86, 6556-6566, 1981. cycle forced by the regional monsoon wind system and in- Godfrey, J. S., and T. J. Golding, The Sverdrup relation in the Indian Ocean, and the effect of Pacific-Indian Ocean through flow on terannual variations associated with the Southern Oscillation. Indian Ocean circulation and on the , J. The presenceof strong seasonalityis pointed out by Wyrtki Phys. Oceanogr.,11, 771-779, 1981. [1961], who notes: "...the close interrelation between the Gordon, A. L., Indian-Atlantic transfer of thermocline water at the [monsoonal-driven]circulation at the surface[including up- Agulhas retroflection, Science,227, 1030-1033, 1985. Gordon, A. L., and C. L. Greengrove, Geostrophic circulation of the wellingdistribution] and in the depth of the water exchange Brazil-Falkland ,Deep Sea Res.,in press,1986. betwen the Pacific and Indian Ocean.... "However, interan- Gordon, A., and E. Molinelli, Southern Ocean Atlas: Thermohaline nual changes in sea level between the Pacific and Indian and ChemicalDistribution and the Atlas Data Set, 34 pp., 233 plates, oceans,which would be a factor in driving the interocean Columbia University Press,New York, 1982. exchange,appear to be small (K. Wyrtki, personalcommuni- Gordon, A. L., and A. R. Piola, Atlantic Ocean upper layer salinity budget, d. Phys. Oceanogr.,13, 1293-1300, 1983. cation, 1985). A relation betweensea surfacetemperature in Grundlingh, M. L., On the volume transport of the Agulhas Current, the Indon½sianseas and SouthernOscillation-E1 Nifio events, Deep Sea Res., Part A, 27, 557-563, 1980. with a few months' lead of the temperatureanomalies, has Grundlingh, M. L., Occurrence of Red Sea water in the southwestel. been reported [Nicholls, 1984]. This relation may be a mani- Indian Ocean, 1981,J. Phys.Oceanogr., 15, 207-212, 1984. festationof larger-scalevariability, as the heat content of the Grundlingh, M. L., Features of the northern Agulhas Current in , 1983, S. Aft. d. Sci., in press,1986. tropical Pacific is observedto graduallyincrease prior to an Hall, M. M., and H. L. Bryden, Direct estimatesand mechanismsof E1Nifio event [Wyrtki, 1985a,b]. ocean heat transport, Deep Sea Res.,29, 339-359, 1982. 5046 GORDON: INTEROCEAN EXCHANGE OF THERMOCLINE WATER

Harris, T. F. W., Sources of the Agulhas Current in the spring of the tropical Atlantic Ocean, J. Phys. Oceanogr., 13, 1534-1539, 1964, Deep Sea Res., 19, 633-650, 1972. 1983. Harris, T. F. W., and D. Van Foreest,The AgulhasCurrent in March Roemmich, D., and C. Wunsch, Two transatlantic sections: Meridio- 1969, Deep Sea Res.,25, 549-561, 1978. nal circulation and heat flux in the subtropical North Atlantic Hastenrath, S., On meridional heat transportsin the world ocean, J. Ocean, Deep Sea Res.,32, 619-664, 1985. Phys.Oceanogr., 12, 922-927, 1982. Ruddiman, W. F., Quarternary North Atlantic surface paleoceano- Hellerman, S., and M. Rosenstein,Normal monthly wind stressover graphy in regionsof potential deep-water formation, NASA Conf the world ocean with error estimates,J. Phys. Oceanogr.,13, 1093- Publ., 2367, 28-31, 1985. 1104, 1983. Ruddiman, W. F., and A. Mcintyre, The North Atlantic ocean during Huber, B., D. Camp, W. Haines, S. Rennie, and A. Gordon, Agulhas the late deglaciation, Palaeogeogr., Palaeoclimatol., Palaeoecol.,35, retroflection cruise, November-December 1982, A data report, 145-214, 1981. Tech. Rep. 85-4, Lamont-Doherty Geol. Observ.,Palisades, N.Y., Ruddiman, W. F., and A. Mcintyre, Ice-age thermal response and 1985. climatic role of the surface Atlantic Ocean, 40øN to 63øN, Geol. Kawase, N., and J. L. Sarmiento, Nutrients in the Atlantic thermo- Soc. Am. Bull., 95, 381-396, 1984. cline, J. Geophys. Res., 90, 8961-8979, 1985. Saetre, R., and A. J. da Silva, The circulation of the Mozambique Killworth, P. D., Deep convection in the world ocean, Rev. Geophys., Channel, Deep Sea Res., 31, 485-508, 1984. 21(1), 1-26, 1983. Spencer, D., W. Broecker, H. Craig, and R. Weiss, GEOSECS Indian Lazier, J. R. N., The renewal of Labrador Sea water, Deep Sea Res., Ocean Expedition, 140 pp., U.S. Government Printing Office, Wash- 20, 341-353, 1973. ington, D.C., 1982. Lazier, J. R. N., Oceanographic conditions at O. W. S. Bravo, 1964- Stommel, H., A survey of theory, Deep Sea Res., 4, 1074, Atmos. Ocean., 18, 227-238, 1980. 149-184, 1957. Leetma, A., P. Niiler, and H. Stommel, Does the Sverdrup relation Stommel, H., Asymmetry of interoceanicfresh-water and heat fluxes, account for the mid-Atlantic circulation?, J. Mar. Res., 34, 1-10, Proc. Natl. Acad. Sci. U.S.A., 77, 2377-2381, 1980. 1977. Stommel, H., and A. Arons, On the abyssal circulation of the world Levitus, S., Climatological atlas of the world ocean,NOAA Prof Pap. ocean, I, Stationary planetary thru patterns on a sphere,Deep Sea 13, 173 pp., U.S. Government Printing Office, Washington, D.C., Res.,6(2), 140-154, 1960. 1982. Swift, J. H., A recent T-S shift in the deep water of the northern Lutjeharms, J. R. E., The Agulhas Current system during the north- North Atlantic, in Climate Processesand Climate Sensitivity,- east monsoon season,J. Phys. Oceanogr.,6, 665-670, 1976. phys. Monogr. Set., vol 29, edited by J. E. Hansen and T. Taka- McCartney, M., Subantarctic Mode Water, in A Voyage of Discovery hashi, pp. 39-47, AGU, Washington, D.C., 1984. (Suppl. to Deep Sea Res.),edited by M. Angel, pp. 103-119, Perga- Talley, L. D., Meridional heat transport in the Pacific Ocean, J. Phys. mon, New York, 1977. Oceanogr.,14(2), 231-241, 1984. McCartney, M. S., and L. D. Talley, Warm-to-cold water conversion Toole, J., and M. Raymer, Heat and fresh water budgetsof the Indian in the northern Atlantic Ocean, J. Phys. Oceanogr., 14, 922-935, Ocean--revisited, Deep Sea Res., 32, 917-928, 1985. 1984. Tsuchiya, M., Upper Waters of the Intertropical Pacific Ocean, Johns Miranda, L. B., and B. M. Castro Filho, Geostrophicflow conditions Hopkins Oceanogr.Set. 4, 50 pp., Johns Hopkins Univ., Baltimore, of the Brazil Current at 19øS,Cienc. Interam., 22, 44-48, 1981. Md., 1968. Mix, A. C., and R. G. Fairbanks, North Atlantic surface-ocean con- Van Loon, H., and J. C. Rogers, Interannual variations in the half- trol of Pleistocenedeep-ocean circulation, Planet. Sci. Lett., yearly cycle of pressuregradients and zonal wind at sea level in the 73, 231-243, 1985. southern hemisphere,Tellus, 36A, 76-86, 1983. Molinari, R. L., W. D. Wilson, and K. Leaman, Volume heat trans- Veronis, G., Model of world ocean circulation, I, Wind-driven, two ports of the Florida Current: April 1982 through August 1983, layer, J. Mar. Res.,31, 228-288, 1973. Science, 227, 295-297, 1985. Warren, B. A., Deep circulation of the world ocean, in Evolution of Nicholls, N., The Southern Oscillation and Indonesian sea-surface ,Scientific Surveys in Honor of Henry Stom- temperature, Mon. Weather Rev., 112, 424-432, 1984. mel, edited by B. A. Warren and C. Wunsch,pp. 6-41, MIT Press, Olson, D. B., and R. H. Evans, Rings of the Agulhas Current, Deep Cambridge,Mass., 1981. Sea Res., in press, 1985. Warren, B. A., Why is no deep water formed in the North Pacific?J. Ou, H. W., and W. de Ruijter, Separation of an inertial boundary Mar. Res., 41, 327-347, 1983. current from an irregular coastline, J. Phys. Oceanogr.,in press, Worthington, L. V., The water masses of the world ocean: Some 1985. resultsof a fine-scalecensus, in Evolutionof PhysicalOceanography, Pearce, A. F., Some features of the upper 500 m of the Agulhas Scientific Surveys in Honor of , edited by B. A. Current, J. Mar. Res., 35(4), 731-751, 1977. Warren and C. Wunsch, pp. 42-69, MIT Press,Cambridge, Mass., Pickard G., and W. Emery, Descriptive Physical Oceanography,4th 1981. ed., 249 pp., Pergamon,New York, 1982. Wunsch, C., An eclectic Atlantic ocean circulation model, 1, The me- Piola, A., and D. Georgi, Circumpolar propertiesof Antarctic Inter- ridional flux of heat, d. Phys. Oceanogr., 14, 1712-1733, 1984. mediate Water and Subantarctic Mode Water, Deep Sea Res., 29, Wyrtki, K., The water exchange between the Pacific and the Indian 687-711, 1982. Ocean in relation to upwelling processes,in Proc. Pac. Sci. Congr. Piola, A. R., and A. L. Gordon, Pacific and Indian Ocean upper-layer 9th, 16, 61-66, 1957. salinity budget,d. Phys.Oceanogr., 14, 747-753, 1984. Wyrtki, K., Physical oceanography of the Southeast Asian waters, Piola, A., and A. Gordon, Note on oceanic heat and freshwater fluxes Scientific Results of Maritime Investigationsof the South China at 30øS, paper presentedat the IAMAP/IAPSO Joint Assembly, Sea and of Thailand 1959-1961, NAGA Rep. 2, 195 pp., Int. Assoc.of Meteorol. and Atmos. Phys./Int. Assoc.for Phys. Sci. Scripps Inst. of Oceanor., La Jolla, Calif., 1961. of the Oceans,Honolulu, Hawaii, Aug. 5-6, 1985. Wyrtki, K., OceanographicAtlas of the International Indian Ocean Prell, W., W. Hutson, D. Williams, A. B6, K. Geitzenauer, and B. Expedition,531 pp., National ScienceFoundation, Washington,D. Molfino, Surface circulation of the Indian Ocean during the last C., 1971. glacial maximum, approximately 18,000 yr. B.P., Quat. Res., 14, Wyrtki, K., Water displacementsin the equatorial Pacific during the 309-336, 1980. 1982-83 E1 Nifio, paper presentedat the abs. IAMAP-IAPSO Joint Rasmusson, E., El Nifio and variations in climate, Amer. Scient., 73, Assembly,Int. Assoc.of Meteorol. and Atmos. Phys./Int. Assoc.for 168-177, 1985. Phys.Sci. of the Oceans,Honolulu, Hawaii, Aug. 5-16, 1985a. Reid, J. L., Jr., On the temperature, salinity and density differences Wyrtki, K., Water displacementsin the Pacific and the genesisof E1 between the Atlantic and Pacific oceans in the upper kilometre, Nifio cycles,J. Geophys.Res., 90(C4), 7129-7132, 1985b. Deep Sea Res., 7, 265-275, 1961. Wyrtki, K., and B. Kilonsky, Mean water current structureduring the Reid, J., and R. Lynn, On the influence of the Norwegian-Greenland Hawaii-to-Tahiti shuttleexperiment, J. Phys. Oceanogr.,•4(2), 242- and Weddell Seasupon the bottom waters of the Indian and Pacif- 254, 1984. ic oceans,Deep Sea Res., 18, 1063-1088, 1971. Rochford, D., Distribution of Banda Intermediate Water in the A. L. Gordon, Lamont-Doherty GeologicalObservatory of Colum- Indian Ocean, Aust. J. Mar. Freshwater Res., 17, 61-76, 1966. bia University, Palisades,NY 10964. Roemmich, D., Estimation of meridional heat flux in the North At- lantic by inversemethods, J. Phys. Oceanogr.,10, 1972-1983, 1980. (Received November 11, 1985; Roemmich, D., The balance of geostrophicand Ekman tansports in acceptedJanuary 10, 1986.)