ABSTRACT. Temperature and precipitation trends are described for newly homogenized historical climate data sets for the. South-west Pacific. Regions that ...
INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 15,285-302 (1995)
551.583.1q265.7)
CLIMATE TRENDS IN THE SOUTH-WEST PACIFIC M. J. SALINGER AND R. E. BASHER
Naiional lnsiiiute of Water and Atmospheric Research, P.O. Box 28 841, Auckland, New Zealand B. B. FITZHARRIS
Deparimeni of Geography, University of Oiago, P.O. Box 56, Dunedin, New Zealand J. E. HAY
Environmental Sciences, Universiiy of Auckland, Privaie Bag 92019, Auckland, New Zealand P. D. JONES
Climaiic Research Unit, Universiiy of East Anglia, Norwich NR4 7TJ, CJK I. P. MACVEIGH
Meieo France, B.P. 151 Noumea, New Caledonia
AND 1. SCHMIDELY-LELEU.
Meteo France, B.P. 6005 FAAA - Aeropori, French Polynesia
ABSTRACT Temperature and precipitation trends are described for newly homogenized historical climate data sets for the South-west Pacific. Regions that exhibit similar temperature and precipitation trends and variability are defined, and the temperature and precipitation time series aggregated according to these regions. Four temperature regions show distinctive trends: two regions south-west of the South Pacific Convergence Zone (SPCZ), which dispiay steady climate warming; two regions north-east of the SPCZ, which cooled during the 1970s, and warmed in the 1980s. Annual anomalies differ in response to the El Nifio-Southern Oscillation (ENSO) phenomena, depending on the region's position with respect to a pivotal line along the SPCZ. The climate warming apparent throughout much of the south-west Pacific comes from sites where there can be no question of any urban influence. Five main South-west Pacific precipitation regions show distinctive trends that are connected to the main climatological features. Four New Zealand precipitation subregions relate to the interaction of the main climatological features with local orography. Annual precipitation anomalies show marked variability and are also affected by ENSO in most regions. The pivotal line for the response of precipitation regions lies just to the north-east of the SPCZ. The ENSO relationships with precipitation appear consistent on both annual and interdecadal time-scales. From these climatic trends four climatic response regions are recognized in the South-west Pacific. KEY WORDS
South-west Pacific; climate trends; temperature; precipitation; New Zealand; El NiBo; Southern Oscillation
INTRODUCTION Model predictions of climate change arising from enhanced greenhouse gases (Houghton et al., 1992) have emphasized the importance of existing natural climate variations. Analysis of historical climate records is required to improve understanding of natural variability so that more realistic climate models can be developed and tested. These include phenomena that dominate global climate variability, such as the El Nifio-Southern Oscillation (ENSO). Historical climate records provide the context of natural variation CCC 0899-8418/95/030285-181 0 1995 by the Royal Meteorological Society
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against which anthropogenicallyforced climate change will have to be detected. The Supplementary Report to the IPCC Scientific Assessment (Houghton et al., 1992) has noted a real, but irregular, increase of global surface temperature since the late nineteenth century. Global mean surface temperatures have increased by 0.3-0.6"C, a finding that is consistent with new evidence of nineteenth century ocean temperatures (Parker and Folland, 1991). The Southern Hemisphere was marginally warmer than the Northern Hemisphere in the late nineteenth century, especially around 1880. Since then, the overall temperature increase of the Southern Hemisphere between the 20-year period 1881-1900 and the latest decade 1981-1990 has been 0.48"C (Folland et al., 1992). Our study deals with the South-west Pacific, a vast area of mainly ocean between 165"E to 145"W and 5"N to 55"s (21 million km2).The climates of the region are controlled by its oceanic nature and large-scale circulation features (Streten and Zillman, 1984; Zillman et al, 1989; Basher et al., 1992; Mullan, 1992), as shown in Figure 1. These include the trade wind regimes, the Hadley and Walker circulations, the seasonally varying, tropical convergence zones, the semi-permanent subtropical high-pressure belt and the zonal westerlies to the south. The Intertropical Convergence Zone (ITCZ) lies just north of the Equator, and the South Pacific Convergence Zone (SPCZ) lies diagonally from near the Solomon Islands to Samoa and beyond. These are zones of lower pressure, where converging, rising air produces cloud and rainfall: the variability of the ENS0 phenomenon (Rasmusson and Carpenter, 1982; McBride and Nicholls, 1983; van Loon and Shea, 1987) is an important determinant of the region's climate at the interannual time-scale. This arises from major shifts in the region's winds, convergence zones, tropical cyclone tracks (Revel1 and Goulter, 1986), precipitation (Ropelewski and Halpert, 1987, 1989) and temperature (Halpert and Ropelewski, 1992). Because of the significance of these major circulation features, we adopt the hypothesis that the region's long-term climate trends will be determined largely by the long-term variations in strength and position of these features. Of course, there may also be other, global-scale, factors that superimpose climate trends upon the region. Long-term trends in climate from the late nineteenth century have been described for the Australian and New Zealand regions by Coughlan (1979), Coughlan et al. (1990), Salinger and McGlone (1990), Plummer (1991), and Salinger et al. (1992a,b, 1993). Australian analyses show a general rise in annual average daily maximum temperatures. There is a distinct upward trend in mean annual temperature at coastal sites in south-east Australia. The most significant warming begins about 1950, when marked changes also take place in east Australian precipitation (Pittock, 1975; Nicholls and Lavery, 1992). Instrumental records from New Zealand dating from 1860 also show a clear increase in temperature over the past 130 years, with similar discontinuities around 1950.At this time, New Zealand temperatures warmed markedly, and precipitation increased in northern and eastern areas (Salinger et al., 1992a,b),and decreased in the south and west. Climate data records for the oceanic South-west Pacific are restricted largely to the tropical western half of the region, because this is where the populated island groups are located. Fortunately this is also where the most active climatological features lie. Zillman et ad. (1989) have documented temperature trends at four Pacific Island locations and rainfall trends at key long period stations. The data indicate a large interdecadal variability of rainfall in the region, particularly at low latitudes. Basher et al. (1992) further discuss this rainfall variability, which has been attributed to the influence of the Southern Oscillation (Nicholls and Wong, 1990).Stations in this area are largely from island sites, many being coral atolls and are free from urban influences.This makes South Pacific data of particularly high quality for trend studies. This paper is the culmination of a major long-term project to digitize and homogenize the historical climate records of the South-west Pacific (Salinger ez a/., 1992a,b). Comprehensive station histories were prepared (Collen, 1992; Fouhy et al., 1992). Long-term climate data sets of temperature and precipitation were assembled for selected stations in the Cook Islands, Fiji, French Polynesia, Kiribati, New Calendonia, Niue, Tokelau Islands, Tonga, Tuvalu, Vanuatu, and Western Samoa. These we added to established data sets for New Zealand and its outlying islands. Stations are grouped together into areas that depict similar temperature and precipitation trends. Annual trends are described for the aggregated records. New findings are discussed in the context of global and hemispheric trends, and in terms of overall change and variability in the main climatological features.
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DATA AND METHODS
Climate stations From newly homogenized historical climate data sets for the South-west Pacific (Collen, 1992; Fouhy
et a/., 1992), 37 stations are selected for analysis of temperature and 41 stations for the analysis of precipitation records. The stations are widespread (Figure 2 and Table I). For New Zealand we selected six sites with good quality records and no known anthropogenic influences. Many more could have been chosen, but we wanted to keep the density comparable to that elsewhere in the study area. Station histories are to be found in the monographs compiled by Collen (1992) and Fouhy et al. (1992).
Homogenization procedures Conrad (1944) defines homogeneity of a climatological series as follows: ‘A numerical series representing the variations of a climatological element is called “homogeneous” if the variations are caused by and only by variations of weather and climate’
The purpose of the homogenization process is to produce consistent and reliable temperature and precipitation series that are as complete as possible for each of the selected climate stations. The adjustment procedures described by Rhoades and Salinger (1993) are used to adjust the data to that of the latest site.
Figure 2
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Table I. Homogenized temperature and precipitation time series used in this study (locations shown in Figure 1). Data used was from start of record to 1990 ~
1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
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Location
Latitude
Longitude
Temperature
Precipitation
Tarawa Christmas Island Banaba Beru Nan u mea Funafuti Nukunonu Penrhyn Atuona Port Vila Rotuma Niulakita Udu Point Suva Apia Keppel Vava'u Alofi Pukapuka Rakahanga Aitutaki Papeete Takaroa Hereheretue Koumac Noumea Aneityum Ono-i-lau Nuku'alofa Raoul Island Rarotonga Tubuai Rapa New Plymouth Rotorua Gisborne Hokitika Lincoln Chatham Island lnvercargill Campbell Island
Ol"21'N 01 "59"
172"55'E 157'29'W 169"33'E 175"59E 176"07'E 179"12'E 171"55'W 158"03'W 139"02'W 168"18'E 177'03'E 179'3OE 179"59'E 178"26E 171"46W 173"46W 173Y9W 169"55'W 165'49W 16l"06'W 159"46W 149"37'W 145"02'W 144"57'W 164"16'E 166"02E 169'46E 178'43'W 175"12'W 17735'W 159"49W 149'29W 144'2OW 174"1 1'E 176"19E 178"WE 17039E 172"28'E 176'34W 168"2OE 169'09E
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09/1946 12/1951-1 2/1963, 12/ 196609/19O4-11/1941, 05/194701/194501/1946 07/1927-07/1932, 0 1/194101/194802/19370 1/196 101/195301/19 1211/1941-05/1945, 01/194501/1946 0 1/188401/189& 0 1/194701/194712/1905-O5/1971, 06/1976 11/192910/194101/19140 I/ 191901/195201/196201/19500 1/190301/195201/194310/194408/193701.189901/196201/195103/1862-11/1880, 01/1894
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Selection of coherent regions
To summarize the temporal climate trends over such a vast area of the Pacific, we have found it desirable to define areas that share similar climate anomalies. Salinger (1981) has previously described coherent response areas for New Zealand. The principal technique used to divide the South-west Pacific into regions of similar trends for temperature and precipitation was cluster analysis. Annual values of temperature and precipitation data for the period 1951-1990 were scaled to a mean of zero and standard deviation of one. A hierarchical agglomerative clustering method was used (Willmott, 1978). This procedure begins with all stations as separate groups.
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The two stations that are nearest by the distance measure are combined to form a larger cluster. The distance matrix calculation is based on normalized Euclidean distances, that is root sum-of-squares of differences. The joining continues until all the stations have been grouped into clusters. A dendrogram delineates the level of association at which stations are grouped. An appropriate number of temperature and precipitation regions was decided by visual inspection of dendrograms to identify the stage at which large numbers of smaller groupings fused into a few comparatively larger ones. Plots of the agglomeration coefficientson semi-logarithmicpaper also identified the first significant point of inflection (Willmott, 1978). Confirmation of the preceding groupings was sought using rotated principal component analysis on the annual temperature and precipitation correlation matrices. Principal component analysis has been widely used in climatological studies (Craddock and Flood, 1969; Kidson, 1975a,b; Salinger, 1980a,b).In this case, the investigation was not directed towards accounting for the maximum possible variance of temperature and precipitation by a small number of components, as would occur by using the covariance matrix. Rather, the correlation matrix was used to show components that distinguished between groups of highly intercorrelated stations. This was achieved by orthogonal rotation of the principal components using the varimax criterion. The number of principal components retained for interpretation is a function of their explained variance and hence, their eigenvalues. The simple scree test of Cattell (1966) determines the eigenvalue associated with the point of inflection of a plot of eigenvalues from largest to smallest. The rank order of the eigenvalue associated with the point of inflection is considered to be an estimate of the number of components to retain. In addition, the number of principal components that pushes the sum of proportionate contributions from each eigenvalue over 75 per cent of the variance is a useful guide in deciding the number of principal components to retain. Correlations with the Southern Oscillation Index (SOI) were also used to validate the groupings. Station annual temperature and precipitation data are correlated over the period 1951-1990, or less for shorter records, with the SOI. The SO1 is the normalized pressure difference between Tahiti and Darwin. It is a principal indicator of the state of the Southern Oscillation. The SO1 is that of Troup (1967). Time series analysis
The 'single-site' time series in temperature and precipitation are displayed as differences from the means for the reference period 1951-1980. This period was selected as it is a common period of the record for all stations and is the period used in the IPCC scientific assessments (Houghton et al., 1990,1992).All stations in a region are weighted equally in anomaly form from a common mean. The temperature anomalies are expressed as departures ("C) from the mean for the reference period. Precipitation anomalies are calculated as the ratio of each value to the mean for the reference period. The 'single-site' data series are combined to form time series of annual anomalies for each of the emergent temperature and precipitation regions defined by the clustering techniques. These regional time series and annual anomalies are produced only when there were at least two single-site data series. Regional anomalies are smoothed using a selected gaussian filter (Jones et al., 1986a,b). This suppresses periods of less than 10 years, in order to facilitate the detection of longer term trends, and gives more weight to the central rather than the end values of the period being smoothed. TEMPERATURE The cluster analysis methodology yielded four consistent regions that behave coherently with respect to temperature anomalies (Figure 3). These regions were confirmed by the principal component analysis. Each group also shows coherent and strong relationships with the SO1 (Figure 4). Region TI (South-east Trades region)
This region comprises a large part of the South-west Pacific around 20°S, which includes New Caledonia, Vanuatu, Fiji, northern Tonga, Niue, the southern Cook Islands, and the south-western islands
CLIMATE IN SOUTH-WEST PACIFIC
29 1
Figure 3. Coherent regions of temperature trends and variability in the South Pacific. See text for details on the regions
of French Polynesia. All stations are located in the subtropics to the south-west of the SPCZ in the South-east Trade Wind belt. Annual temperature anomalies are strongly related to the phase of the Southern Oscillation. The aggregated record for the region is positively correlated with the SO1 (t = +0.42) for the period 1951-1990, i.e. low temperature anomalies tend to occur during El Niiio events and higher temperature anomalies during anti-El Niiio events. Temperatures in this region trend upwards with an increase throughout the period of record (Figure 5). The temperature increase between decades 1911-1920 and 1981-1990 amounts to 0.9"C. The decade
Figure 4. Annual correlation (*loo) between temperature and the southern Oscillation Index. The maximum number of pairs were 40, and the minimum number 24. All stations within isopleths < -35 or >35 are at least significant at the 5 per cent level
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1911-1920 was the coolest and the decade 1980-1990 the warmest. Temperatures increased rapidly between 1920 and 1930 and again after 1980. Despite the two strong El Niiio events of the 1980s, and a tendency for cooling during El Niiio events in this region, the decade 1981-1990 is the warmest of the entire record, +0.2"C higher than the 1951-1980 reference period. Region T2 (central PaciJic divergence region)
This northern wedge-shaped region includes Samoa, Tokelau, eastern Kiribati, northern Cook Islands and remaining areas of French Polynesia. All stations are located between the ITCZ and SPCZ in an area of divergent easterly winds. Annual anomalies are negatively correlated with the SO1 (r = -0.69), so that there is a tendency for El Niiio years to be marked by higher temperatures and anti-El Niiio years by lower temperatures. The aggregated records (Figure 5) show a decrease in mean temperature from the 1940s to 1970 by 0.2"C, followed by a rapid increase. The decade 1981-1990 is the warmest on record, and is 0*.5"Cwarmer than the 1951-1980 reference period. Region T3 (New Zealand region)
This region comprises a large southern area, including Campbell Island (53"s) in the south, New Zealand, Chatham Island (44"S, 176"W) to the east, and as far north as Nuku'alofa (21"s) in southern Tonga. Most stations are influenced by the southern latitude westerlies and subtropical anticyclonic belt. Annual temperature anomalies are positively correlated with the SO1 (r = +059), i.e. with El Niiio years marked by below average temperature and anti-El Niiio years by above average temperature. This is the only region in the South Pacific where aggregated records (two or more) extend back to the nineteenth century. Annual data (Figure 5) show an increase over the entire 130-year record. The magnitude of the warming between the decades of 1861-1870 and 1981-1990 is 1.1"C. 1901-1910 is the coldest decade (-08°C below the reference mean), whereas 1981-1990 is the warmest on record (0.3"C above the reference mean). Temperatures show only a small increase between the 1860s and the 1940s, but then rise sharply. The temperature increase between the decades 1941-1950 and 1951-1960 amounts to 0.5"C. Again the last decade, despite the strong inverse relationship with El Niiio years, is the warmest of the record. For this region both 882-1983 and 19861987 are cooler than the smoothed trend, but the latter is warmer than the 1951-1980 average. Region T4 (region of meeting between ITCZ and SPCZ)
The final region comprises the area of western Kiribati and Tuvalu, close to where the SPCZ merges with the ITCZ. Most stations are on atolls. This region shows warm temperature anomalies in the El Niiio phase and cold temperature anomalies in the anti-El Niiio phase of the Southern Oscillation. However, the association with the SO1 is not so strong (r = -0.39). The aggregated temperature series for Region T4 show little long-term trend (Figure 5). However, there are decadal-scale variations that are very similar to those of the adjacent Region T2. Years before 1960 and after 1980 are slightly warmer (by 0.2"C) and years around 1970 are slightly cooler (by 0.2"C) than the 1951-1980 reference period. PRECIPITATION Five regions of the South Pacific that have similar precipitation trends and variability as determined by the cluster analysis methodology are shown in Figure 6. The number of principal component groups retained depends on the decision criteria. The simple scree test retained four groups, but when utilizing the 75 per cent variance rule nine groups are obtained. Five groups were yielded by cluster analysis. The clustering placed New Zealand stations into four of the five groups. The New Zealand regions are considered separately
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Figure 6. Coherent regions of precipitation trends and variability in the South Pacific and New Zealand. See text for details on the regions
because the interaction between local orography and regional circulation plays a major role in delimiting the precipitation response (Salinger, 1980a). All the groupings were identified in the principal component analysis, and areas, apart from Pacific Region P3 and New Zealand Region NZP3, show strong relationships with the SO1 (Figure 7). South Pacific Region PI (central Pacific divergence zone). This northern region includes Nanumea and Funafuti in Tuvalu, Tokelau, northern Cook Islands, and the Tuamotu and Marquesas groups of French Polynesia. All these stations are located in the wedged-shaped region between the SPCZ and the ITCZ in the region of divergent easterly winds. Annual precipitation anomalies are strongly related to the phase of the Southern Oscillation. The aggregated records for this region are inversely correlated with the SO1 (r = -0.67) for the 1951-1990 period. El Niiio years are wetter than average, and anti-El Niiio years are drier than average. The aggregated precipitation series for Region P1 is shown in Figure 8. This shows a small decline in annual precipitation of about 10 per cent between 1940 and 1975. However, by the 1980s, precipitation increases by the order of 40 per cent. The 1980s in this region are the wettest on record, with the highest maximum annual values, and only one year below the 1951-1980 mean annual precipitation. This is due to two large El Niiio events during the decade. Region P2 (SPCZ region). This region lies to the south of Region P1 and comprises Niulakita in Tuvalu, Rotuma (the northernmost island of Fiji island group), Samoa, and the Southern Cook Islands. The stations in Region P2 are located either under or close to the SPCZ. Annual precipitation anomalies are also correlated to the SO1 (r =058), but in the opposite way to anomalies in Region P1. Wetter years occur when the index is positive (anti-El Niiio conditions) and drier years occur during El Niiio episodes. Precipitation records in this area extend back to 1900 (Figure 8). Annual data show the decade 1901-1910 to be about 15 per cent drier than the 1951-1980 reference period. Precipitation increases to a maximum
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in the early 1920s (15 per cent above the reference period). Precipitation during the decade 1981-1990 is 10 per cent below the 1951-1980 average. Region P3 (eastern end of SPCZ). This is the most eastern region of the study area and includes the Society and Austral Island groups of French Polynesia at the eastern end of the SPCZ. Unlike many of the other regions, and despite its dependence on the SPCZ, there are no strong relationships with the Southern Oscillation. There is little correlation between the SO1 and aggregated precipitation anomaly series (r = -004). The aggregated precipitation time series show large oscillations of precipitation on a decadal time scale (Figure 8). Region P4 (subtropical region). This region occupies the largest part of the study area, including northeast New Caledonia, Vanuatu, most of Fiji, Tonga, and Raoul Island to the northeast of New Zealand. It lies to the south-west of the SPCZ and is affected by the migratory anticyclones of the subtropical high-pressure belt. Annual precipitation anomalies are strongly correlated with the SO1 (r = +0.78). El Niiio years are wetter and anti-El Niiio years are drier than average in Region P4. Annual precipitation time series since the decade 1911-1920 show a general decrease over the period of record, especially after about 1980 (Figure 8). Years were wettest in the 1920s. The decade 1981-1990 is the driest, with 10 per cent less than the 1951-1980 average. Region P5 (region of meeting between SPCZ and ITCZ). Region P5 includes western Kiribati, and lies under the area where the SPCZ and ITCZ merge. The main characteristics are the dominance of the Southern Oscillation in determining annual precipitation anomalies (r = -0.80), and the resulting extreme interannual variability of precipitation. There is a noticeable trend of increasing precipitation, which is not statistically significant because individual years are highly variable. New Zealand. The four New Zealand precipitation regions (Figure 6) are mainly separated by the country's main axial mountain ranges. Annual anomalies in western areas of the North Island (region NZP1) are positively correlated with the SO1 (r = +0.43) over the period 1951-1990, but show no definite trends
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Figure 9. Aggregated precipitation trends in the New Zealand area for precipitation regions NZP1-NZP4, expressed as anomalies from the 1951-1980 period. The thicker solid line highlights variations on decadal-to-longer time-scales. The regions are displayed geographically from north to south, with the longer extending records on the left
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(Figure 9). Eastern areas of the North Island (region NZP2) also are positively correlated with the SO1 (r = +0*41);this region was drier in the 1980s. Precipitation in the south-west of New Zealand and Campbell Island (region NZP3) is dominated by the strength of the southern westerfies. The Southern Oscillation has an influence on annual precipitation anomalies, but in the opposite sense to regions NZPl and NZP2. The association with the SO1 is also weaker than in other regions (r = -0.33). There was a slight increase in precipitation during the 1980s. The east of the South Island and Chatham Islands comprised region NZP4. Annual precipitation anomalies have no association with the SO1 (r = +0.03). Precipitation time series show a drying trend to the decade 1981-90. Detailed trends in the New Zealand precipitation regions have been described elsewhere (Salinger et al., 1992b). CLIMATE REGIONS The combination of air temperature and precipitation trends leads to four South-west Pacific climate response regions being recognized (Figure 10). These regions can be related to dominant climatological features and their movement, as identified by Hay et al. (1993). Key features include the positions of convergence zones, the latitude of the subtropical high, and direction of airflow over New Zealand orography. (i) The wedged-shaped region (region C1) of divergent easterly winds between the two convergence zones (ITCZ and SPCZ) covering central and western Kiribati has largely coherent temperature and precipitation trends. This comprises most of temperature region T2 and all of precipitation region P1, and the northern part of P3. Mean annual island surface air temperatures show a decreasing trend from the 1940s to the mid-l970s, followed by a rapid increase through the 1980s. Within this region, El Niiio years are warmer and anti-El Niiio years cooler than the long-term trend. Precipitation trends are largely consistent, although there are no records prior to 1920. Many stations in this region became slightly drier after 1950, with annual amounts decreasing by about 5 per cent, and became wetter after 1975, with annual amounts increasing by over 10 per cent. El Niiio years are wetter and anti-El Niiio years drier. (ii) The second climate region (C2) consists of the north-west region, where the two convergence zones merge over western Kiribati. It includes most of temperature region T4 and precipitation region P5. Although it is climatically distinct from the wedge zone of the divergent easterlies, our study shows that it has very similar trend behaviour. Mean annual island temperatures similarly decrease to the early 1970s, and increase thereafter through the 1980s. This 1980s increase is less marked than that for the zone of divergent easterlies. Temperatures in El Niiio years are above average, and in anti-El Niiio years are below average. The precipitation trends and their El Niiio anomalies are also broadly similar. Stations in this region received about 10 per cent less than the 1951-80 average up to 1970, then after 1975 received about 15-20 per cent above average. El Niiio years are wetter and anti-El Niiio years drier than average. (iii) The third climate region (C3) comprises the areas in the vicinity of the SPCZ, which includes most, but not all, of temperature region T1 and precipitation region P2, and parts of adjacent regions. We shall take just T1 and P2 as the representative regions. These show that temperatures have a steady small upward trend for 1910-1990, with a hiatus of low trend from 1930 to 1970. The precipitation series has relatively small variability and trend. The decadal-scale variations for 1940- 1990 are somewhat opposite to those in the divergent easterlies zone to the north. The SO1 relationships are weak. (iv) The fourth climate region (C4) is the large area that lies south-west of the SPCZ. It comprises the southern part of temperature region T1, the northern part of temperature region T3 and all of precipitation region P4. The dominant climatological features are the migratory subtropical anticyclones and trade winds on their northern flanks. The annual temperature data show a marked increasing trend since records began for both temperature regions T1 and T3, with the warmest years on record occurring in the decade 1981-1990. Lower temperatures occur during El Niiio events and higher temperatures during anti-El Niiio events, but the effect of the strong El Niiios of the 1980s is
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Figure 10. South-west Pacific climate response regions (see text for details). CI, divergence easterlies; C2, meeting of ITCZ and SPCZ; C3, South Pacific Convergence Zone; C4, subtropical high and trades
not evident. Presumably the lower temperature anomalies in El Niiio years are swamped by the long-term increasing trend from other causes. The area of consistent precipitation is smaller, and lies further to the south-west of the SPCZ (precipitation region P4). This region is distinctive in that the decade 1981-1990 was markedly drier. In part this is due to the strong control of annual precipitation by the Southern Oscillation. New Zealand (part of temperature region T3 and precipitation regions (NZPl-NZP4) shows different trends and relationships with the El Niiio-Southern Oscillation phenomenon. Temperature trends are consistent with trends in the other parts of region T3. However, precipitation trends and variability are different owing to the interaction of regional circulation and its changes with orography, particularly over the two main islands. Salinger (1981) associated precipitation changes between two periods 1951-1975 and 1921-1950 with a weakening of the prevailing west to south-west circulation and more frequent blocking anticyclones east of New Zealand, the latter also noted by Trenberth (1976). For the period 1951-1975 these regional circulation changes gave increased precipitation in region NZP1, and close to average precipitation in region NZP3. Similarly, precipitation changes after 1980 can be explained as a consequence of interaction of regional circulation with the orography. More frequent westerlies made region NZP3 wetter, but areas in the lee of mountains and closer to the subtropical high-pressure belt (regions NZP2 and NZP4) became drier. DISCUSSION AND CONCLUSIONS The results presented here demonstrate that there are relatively coherent climatic response regions within the South-west Pacific. This suggests a hypothesis that climate trends in the South-west Pacific may be influenced by changes in the strength and position of the major climatological features affecting the region. There are two clear regions, C1 and C4; the two others (C2 and C3) are where convergence zones influence and interact with climate. However, there are also other as yet unexplained features, including the apparently inexorable increase in temperature in the past two decades. There are two trend features to be noted: a temperature increase trend beginning about 1950 south-west of the SPCZ and in New Zealand, and a
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systematic change in temperature and precipitation trends throughout the entire South-west Pacific around the mid-1970s. The four coherent zones of climate trends, each different, are strongly related to the major climatological features of the South-west Pacific, Region C1 lies between the ITCZ and SPCZ and is dominated by divergent easterly winds from the semi-permanent anticyclones of the south-east Pacific; region C2 occurs where the ITCZ and SPCZ merge; region C3 occurs in the vicinity of the SPCZ, whose behaviour is an important influence on the observed trends; and region C4 lies totally within the zone of migratory anticyclones of the South-west Pacific and trade winds on their northern flanks. Ropelewski and Halpert (1987, 1989) determined three broad areas of consistent ENSO related precipitation anomalies and one area (Halpert and Ropelewski, 1992) of ENSO related temperature anomalies in the tropical South Pacific. The precipitation areas are the central Pacific area ( 10DN-5"S),the south-central Pacific area (5"-12"S) and the Fiji-New Caledonia area (15"-23"S). These areas are broadly consistent with our climate raions. The results confirm that shorter term variability is largely controlled by the Southern Oscillation phenomenon, consistent with relationships established in other studies (Rasmusson and Carpenter, 1982; van Loon and Shea, 1987; Halpert and Ropelewski, 1992). Interannual temperature anomalies about the trend are generally opposite between the temperature regions T2 and T4 compared with the regions T1 and T3. Similarly, mean annual precipitation anomalies show marked interannual variability, and are also largely controlled by the El Niiio-Southern Oscillation system. The results here confirm relationships uncovered in earlier studies (Ropelewski and Halpert, 1987,1989).Our study also shows that ENSO events are not the only factor in observed temperature trends on longer time-scales. On decadal time-scales ENSO influences have not affected the warming trend in regions T1 and T3, which is particularly evident after 1950. The changes observed after 1950 coincide with other climate trends south-west of the SPCZ (climate regions 3 and 4) and in the adjacent Australian region. Pittock (1975) and Nicholls and Lavery (1992) all found that summer rainfall over much of eastern Australia increased abruptly around 1950, at the same time as an upward trend in mean annual temperature occurred in south-east Australia (Plummer, 1991). Trenberth (1976)notes more anticyclones to the east of New Zealand and fewer in the Australia and Tasman Sea area for the period 1951-1975, at the same time as the Southern Oscillation was more frequently in the anti-El Niiio mode. Fitzharris and Hay (1991) report this period to be one when the subtropical high-pressure belt in the New Zealand region was located further south. Khatep and Fitzharris (1984) also show that during wet periods in the New Zealand region the subtropical easterlies extend further poleward. These climate trends are consistent with a general weakening of the westerly circulation in the New Zealand region and more cyclonic activity east of Australia. The changes in temperature and precipitation throughout the entire South-west Pacific in the mid-1970s manifests itself in different ways in the different climate zones. Both the wedge-shaped region between the ITCZ and SPCZ of divergent easterlies, and the area where the ITCZ and SPCZ merge, became warmer and wetter. Prior to 1970, the temperatures in these two regions actually showed a decreasing trend. There is a suggestion here that there may be two processes at work; firstly one causing opposite trends in the northern regions and southern regions (as evident before 1970) and secondly, another overriding the first and causing increasing trend in temperature everywhere (as evident after 1970). It is interesting that the post-1970 trends in the northern (T2 and T4) regions are comparable with those that have been experienced in the southern regions (T1 and T3) for the last 90 years. At the same time the large subtropical region and zone of south-east trade winds became warmer and drier. More frequent El Niiio events have also occurred. The El Niiio pattern results in a more eastward location of the SPCZ (Hay et al., 1993), causing higher precipitation in the zone of divergent easterlies, and lower precipitation south-west of the SPCZ. These movements would account for the temperature and precipitation trends observed in the four climate regions and over New Zealand. In New Zealand, South Island areas exposed to the west became wetter, and the North Island and eastern areas of the South Island became drier. These trends confirm the changes seen in circulation and ENSO frequency since the mid-1970s. Over New Zealand (Gordon, 1985) eastern areas are drier and south western areas wetter.
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Within the South-west Pacific, the SPCZ represents a major discontinuity for decadal-scale climate trends. The regions to the north-east of the SPCZ cooled until the mid-l970s, then warmed. In contrast, the regions to the south-west of the SPCZ have steadily warmed. The SPCZ represents a pivotal line for the shorter term ENS0 anomalies also, and these anomalies alternate across the SPCZ. To the east and the north-east, warm anomalies occur in El Niiio events and cold anomalies in anti-El Niiio’s. South-west of the SPCZ, El Niiio events give cold anomalies about the trend and anti-El Niiio events give warm temperature anomalies. In regions to the north-east of the SPCZ and where the SPCZ and ITCZ merge (precipitation regions P1 and P5), wetter anomalies occur in El Niiio events and drier anomalies in anti-El Niiio’s. In the area occupied by the SPCZ and to the south-west (precipitation regions P2 and P4), El Niiio events tend to produce drier anomalies and anti-El Niiio events wetter anomalies. This pattern closely matches the South-west Pacific ‘see-saw’ in mean annual precipitation described by Hay et al., (1993). It is tempting to link the climatic warming that we have documented for the most recent decades, 1971-1990, to the enhanced greenhouse effect (Houghion et al., 1992).The trends over these years are about 0~2°Cper decade throughout the South-west Pacific region, and the most recent decade, 1981-1990, is the warmest on record. We are confident that the trend is real, as the data came from island locations where there can be no suggestion of urban influences. Of particular interest is that the temperature increase overrides the cooler climate anomalies expected in the region south-west of the SPCZ as a result of the more frequent El Niiio events during the period. There is also the suggestion that the warming after about 1970 represents a different process compared to that apparent up to that date. Of course, by itself, our evidence can only hint at the possibility of enhanced greenhouse effect. In summary, this study has had as its primary goals the careful development of the best possible temperature and rainfall data sets for the South-west Pacific region, and the documentation of the main patterns of long-term trends in the data. These confirm the relationships of the trends to the major meteorological features of the region and to the El Niiio phenomenon, and they offer a tantalizing view of the possible processes that might be at work in the region at the decadal scale, including the enhanced greenhouse effect. There is clearly much to be done in the way of more quantitative analysis and dynamical study in order to unravel these processes. In the meantime, the study underlines the importance of maintaining high quality observing programmes in the region to secure the long-term data sets for future trend detection. ACKNOWLEDGEMENTS
This research was supported by the New Zealand Foundation for Research, Science and Technology, grant 92-MET-33-532. The work was part of the collaborative programme ‘Refining the climate record of the South Pacific’, with support from Meteo France in Toulouse, New Caledonia, and French Polynesia. Ludovic Magnouloux and Nathalie Rouchy of the Ecole Nationale de la Meteorologie homogenized the climate records from New Caledonia and French Polynesia. The ANZUK Tripartite provided partial travel support. C. S. Thompson and X. Zheng provided useful comments in the manuscript preparation phase. REFERENCES Basher, R. E., Collen, B., Fitzharris, B., Hay, J., Mullan, B. and Salinger, J. 1992. Preliminary Studies for south Pacific Climate Change, New Zealand Meteorological Service, Wellington, 70 pp. Cattell, R. B. 1966. ‘The scree test for the number of PCs’, Mulf. Behau. Res., 1, 245276. Collen, B. 1992. South Pacific Historical Climate Network. Climate Station Histories. Part I : Southwest Pacific Region, New Zealand Meteorological Service, Wellington, I 1 3 pp. Conrad, V. 1944. Methods in Climatology, Oxford University Press, Oxford. Coughlan, M. J. 1979. ‘Recent variations in annual-mean maximum temperatures over Australia’, Q. J. R. Meteorol. Soc., 105, 707-7 19. Coughlan. M. J., Tapp, R. and Kinninmonth, W. R. 1990. Trends in Australian Temperature Records. Draft contribution to IPCC, WGI, Section 7. Craddock, J. M. and Flood, C. R. 1969. ‘Eigenvectorsfor representing the 500 mb geopotential surface over the Northern Hemisphere’, Q. J. R. Meteorol. Soc., 95,576593. Fitzharris, B. B. and Hay, J. E. 1991. ‘Atmosphericcirculation changes in the South West Pacific 191 1-1985 and their effect on glacier behaviour’, Weath. Clim. 11, 52-53.
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