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Climatological Seesaws in the Southwest Pacific

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Climatological Seesaws in the Southwest Pacific Weather and Climate (1993) 13: 9-21 9 CLIMATOLOGICAL SEESAWS IN THE SOUTHWEST PACIFIC John Hayl Jim Salinger2 Blair Fitzharris3 and Reid Basher2 1 Environmental Science, University of Auckland, Auckland, New Zealand Postal Address: Environmental Science, University of Auckland, Private Bag 92019, Auckland, New Zealand Telephone: 649-373-7599 (Extn 8437) Facsimile: 649-373-7470 2 National Institute of Water and Atmospheric Research, Wellington, New Zealand 3 Geography Department, University of Otago, Dunedin, New Zealand ABSTRACT The analyses reported in this paper are based on an unprecedented 80 year record of reconstructed grid-point sea-level pressure data which are used to derive composite pressure anomaly fields for the Southwest Pacific for extreme phases of the Southern Oscillation. These show centres of action to the east and west of New Zealand. However, it is the intervening pivotal region that includes New Zealand which experiences a strong response — anomalous south to southwest flow when the Southern Oscillation Index (SOI) is negative and north to northeasterly flow when the Index is positive. These contrasting responses are reflected in the spatial distribution of rainfall anomalies for New Zealand. A more complex response occurs in another pivotal region in the vicinity of the Southern Cook Islands. The South Pacific Convergence Zone is displaced northward and southward in association with extreme phases of the Southern Oscillation resulting in large rainfall anomalies in the areas which are under its influence only in such conditions. However, the effective finite width of the South Pacific Convergence Zone and its limited north/south displacement mean that there are some locations in the pivotal region which are always influenced by the convergence zone. Consequently these do not exhibit the same high degree of sensitivity to variations in the SOL Such contrasting responses are demonstrated using a recently compiled rainfall archive for the Southwest Pacific. INTRODUCTION pressure anomalies for two widely separated locations (Sydney and Buenos Aires) were first As Rasmusson and Carpenter (1982) note, described by Hildebrandsson (1897). Lockyer well developed out-of-phase relationships of and Lockyer (1902a, 1902b, 1904) subsequently 10 Climatological Seesaws confirmed these findings, showed that such The present study uses an unprecedented 80 oscillations were almost world-wide and de- year record of reconstructed grid-point sea- scribed them as 'barometric seesaws'. It was level pressure data (Jones, 1991) for the Aus- some 20 years later that Sir Gilbert Walker tralasian region (Fig. 1) to characterize the named one of these seesaws the Southern atmospheric circulation patterns associated Oscillation (SO) and described the key fea- with extreme Southern Oscillation phases. In tures of its surface pressure, temperature and a spectral analysis of the differences in sea- precipitation variations over the Indian and level pressures for stations known to reflect southern Pacific Oceans (Walker and Bliss, the main characteristics of the SO, Trenberth 1932; 1937). (1976) showed that most of the variance oc- Associated with the SO are 'centres of ac- curred in periodicities between 2 and 10 years. tion' — defined as semi-permanent highs and Thus studies of SO and the related El Nino lows that appear on charts of mean sea-level phenomenon require an extended period of pressure — which principally lie over Indone- record in order to reveal the key features. sia and in the South Pacific anticyclone and The ability to analyse regional pressure are connected by an equatorial east/west cir- patterns extending back to 1911 provides an culation called the Walker Circulation opportunity to include a larger number of (Trenberth, 1976). Despite their name, studies significant Southern Oscillation events than such as those by Gordon (1986) and was possible for earlier investigations of re- Rasmusson and Carpenter (1982) have shown gional circulation patterns in the Southwest that some of the more significant climatologi- Pacific (e.g. Rasmusson and Carpenter, 1982; cal variations occur not at these extremities Trenberth, 1976; Kiladis and van Loon, 1988; of the seesaw, but in the intervening pivotal Gordon, 1986; Allan and Pariwono, 1990). Al- region. While the centres of action can also be lan (1988) notes that coherent evidence of identified by their well-developed Southern Oscillation related responses in the teleconnections with other centres (Trenberth, Australian region can be found only when 1976) the pivotal regions where significant extreme phases of the Southern Oscillation climatological variations occur are Index (SOT) are examined. The present analy- characterized by statistically non-significant sis focuses on such events by examining both correlations (Gordon, 1986). Similar paradoxi- El Nino and La Nina events. cal statistical patterns of sensitivity to pres- The objective of this study is to describe the sure anomalies have also been described by dramatic nature of the circulation and related Fitzharris and Bakkehoi (1986). changes occurring in the pivotal zone of the Physically the relationships are far from seesaw in response to changes in the centres paradoxical — the pressure oscillations, or of action, and hence competing with them for barometric seesaws, are associated with sig- climatological significance. Recently pre- nificant reversals in the atmospheric circula- pared archival data for the South Pacific tion anomalies. For the Pacific region, (Table 1) are used to identify some of the Trenberth and Shea (1987) show pivotal re- climatological consequences of the gions occurring in the vicinity of Rarotonga documented changes in atmospheric for the major centres of action (Indonesia and circulation. tropical Southeast Pacific Ocean) and over New Zealand for one of the several secondary DATA centres. These two areas should therefore ex- perience the most marked reversals in atmo- Southern Oscillation Index (SOI) spheric circulation anomalies as the standing oscillation itself changes sign. The apparent The monthly SOI data (normalised pressure paradox is that, in some situations, locations differences between Tahiti and Darwin) used near the pivot of the seesaw, rather than at here cover the period 1911 to 1989. Mullan the so-called centres of action, have the great- (1989) has described their derivation. Parker's est variability in climatological conditions. (1983) monthly values for the period January Substantial changes in the direction of the 1935 to March 1983 were updated using stand- anomalous flow will manifest as major modi- ard six-hourly pressure differences between fications to precipitation and other atmo- Tahiti and Darwin. These values were normal- spheric patterns. ised against a base period of 1941-80 to form Climatological Seesaws 11 TABLE 1: LIST OF RAINFALL STATIONS FOR THE Nuku'alofa, Tonga 1944 TROPICAL SOUTHWEST PACIFIC USED IN THE Alofi, Niue 1905 PRESENT STUDY Puka Puka, Cook Islands 1929 Rakahanga, Cook Islands 1941 Station Name Start Year Penrhyn, Cook Islands 1937 Palmerston, Cook Islands 1940 Buteriteri, Kiribati 1945 Aitutaki, Cook Islands 1914 Tarawa, Kiribati 1947 Mauke, Cook Islands 1929 Banaba, Kiribati 1953 Rarotonga, Cook Islands 1927 Beru, Kiribati 1945 Mangaia, Cook Islands 1914 Nanumea, Tuvalu 1946 Nui, Tuvalu 1946 the Troup Index (McBride and Nicholls, 1983). Funafuti, Tuvalu 1941 The SOI time series was extended back to Niulakita, Tuvalu 1947 Nadi, Fiji 1942 1851 (only data back to 1911 were used in the Atafu, Tokelau 1949 present study) using seasonal Index values Nukunono, Tokelau 1944 published by Wright (1975, 1989). Monthly val- Apia, Western Samoa 1890 ues of the Index were calculated from these Keppel, Tonga 1942 Vava'u, Tonga 1947 seasonal seasonal data using regression qua- 120E 140•E 160•E Ito' ISO•VI ILVW I Hawalan is. 20'N r- NORTH EAST TRADES •sx•c3 • Marshall Is 1 irr r1-0 "Tr i rL.-673 tri Fro-g ireg .6-NI Nauru t TERL V—M-073-/-0N Kiribati Lino Is. NewPpuaulneaN • DIVERGENT • , Tuvalu Tokelau E717i G • sou A L V r G T Scl amr.c'e 'EASTERLIES 0 Vanuatu Fiji ..crs.. •••. ONL 20•S AUSTRALIA H H AD'S TRAvELL TROUGHS DISTURBED 0 I OW km 60'S 1 1 Fig. L Typical circulation features in the Southwest Pacific for summer and (next page) winter. Also shown are the areas analysed using the gridded pressure data and the tropical rainfall data. After Steiner (1980). 12 Climatological Seesaws 120'E 140"E 180* 160'W 140W • 11 Hawalan Is. • 20•14 7— 1 • :s Mariana Is. • i▪ N• tER F-R-6 Tqz.74-17,,,,4,......marzhall Is. ... Ct371-1/ERGENCE • • Kiribati Ins Is. Solomon is. 01%,juvalti Tokela4 I DIVERGENT '."•!e.\ • • Ade- .IEAST•ERLIES • A%. • SOUTHIEAST.-ADES ••••• 4, cSarta... //P./ • %. Cook Is • Vanuatu \.t %.• "..• • ivle %. • , -, • 20'S Nip .;.34t • .E/i, Q• %. Tahiti NovN. % Caledonia s' Tonga S. S. S. ' o4, S. S. S. H THAVEIAANG \ ANTIC'YCLO AND TROUGHS \ 40'S H —NV NEW ZEALAND DISTURBED O MOO km 60'S 1 tions relating the monthly Tahiti-Darwin pres- Southwest Pacific have long pressure time sure difference to Wright's Index values for series, with some extending back to the mid- the surrounding three seasons. The period dle of the 19th Century. Gridded pressure data 1935-1974 was used to generate the coeffi- were derived following the methods described cients of the regression equations. by Jones (1987) and Jones and Wigley (1988). Ropelewski and Jones (1987) have discussed Their technique uses principal components re- the strengths and weaknesses of this and other gression to develop a series of equations relat- forms of the SO!. ing grid point mean sea level pressure values to contemporaneous pressures observed at in- Gridded Sea Level Pressure Data dividual stations. Pressures were reconstruct- ed for 1911 on, when air pressure was Jones (1991) describes the homogenization measured at 17 stations. The grid provides 65 procedures used to assess the observed sea points in an area between 20 and 60 S latitude level pressure data that form the basis of the and 100 E and 140 W longitude.
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