INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 21: 1705–1721 (2001) DOI: 10.1002/joc.691
INTERDECADAL PACIFIC OSCILLATION AND SOUTH PACIFIC CLIMATE M.J. SALINGER*, J.A. RENWICK and A.B. MULLAN National Institute of Water and Atmospheric Research, PO Box 109 695 Auckland, New Zealand Recei6ed 26 June 2000 Re6ised 2 May 2001 Accepted 8 May 2001
ABSTRACT The Interdecadal Pacific Oscillation (IPO) has been shown to be associated with decadal climate variability over parts of the Pacific Basin, and to modulate interannual El Nin˜o – Southern Oscillation (ENSO)-related climate variability over Australia. Three phases of the IPO have been identified during the 20th century: a positive phase (1922– 1944), a negative phase (1946–1977) and another positive phase (1978– 1998). Climate data are analysed for the two most recent periods to describe the influence of the IPO on decadal climate trends and interannual modulation of ENSO teleconnections throughout the South West Pacific region (from the equator to 55°S, and 150°E to 140°W). Data coverage was insufficient to include the earliest period in the analysis. Mean sea level pressure (SLP) in the region west of 170°W increased for the most recent positive IPO period, compared with the previous negative phase. SLP decreased to the east of 170°W, with generally more southerly quarter geostrophic flow over the region. Annual surface temperature increased significantly southwest of the South Pacific Convergence Zone (SPCZ) at a rate similar to the average Southern Hemisphere warming. Northwest of the SPCZ temperature increases were less, and northeast of the SPCZ more than the hemispheric warming in surface temperature. Increases of annual precipitation of 30% or more occurred northeast of the SPCZ, with smaller decreases to the southwest, associated with a movement in the mean location of the SPCZ northeastwards. The IPO modulates teleconnections with ENSO in a complex way, strengthening relationships in some areas and weakening them in others. For New Zealand, there is a consistent bias towards stronger teleconnections for the positive IPO period. These results demonstrate that the IPO is a significant source of climate variation on decadal time scales throughout the South West Pacific region, on a background which includes global mean surface temperature increases. The IPO also modulates interannual ENSO climate variability over the region. Copyright © 2001 Royal Meteorological Society. KEY WORDS: decadal
variability; ENSO; interannual variability; Interdecadal Pacific Oscillation; mean sea level pressure; precipitation; South West Pacific climate variation; temperature
1. INTRODUCTION Interannual climate variability over the Pacific is dominated by the El Nin˜o–Southern Oscillation (ENSO). This has the strongest sea surface temperature (SST) signals of one sign along the equator over the central and eastern Pacific and a boomerang-shaped pattern of weaker SST signals of opposite sign extending over the middle latitudes of both hemispheres in the North and South Pacific. Recently, ‘ENSO-like’ features in the climate system that operate on decadal to multidecadal time scales have been identified. This lower frequency SST variability is less confined to the equatorial belt in the central and eastern Pacific, and relatively more prominent over the extratropics, especially the North Pacific (Folland et al., 1999; Power et al., 1999). The corresponding sea level pressure (SLP) signature is also strongest over the North Pacific, and its wintertime counterpart in the middle troposphere more closely resembles the Pacific North American (PNA) pattern (Zhang et al., 1996; Livezey and Smith, 1999). * Correspondence to: National Institute of Water and Atmospheric Research, PO Box 109 695, Newmarket, Auckland, New Zealand; e-mail:
[email protected]
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The Pacific Decadal Oscillation (PDO) of Mantua et al. (1997), with lower frequency variations in the leading North Pacific SST pattern, may be related to the same Pacific-wide features, and parallels the dominant pattern of North Pacific SLP variability. The relationship is such that cooler-than-average SSTs occur during periods of lower-than-average SLP over the central North Pacific and vice versa. It has been argued that low-frequency variations in climate over the North Pacific are part of a basin-wide feature termed the Interdecadal Pacific Oscillation (IPO; Power et al., 1998, 1999; Allan, 2000; Folland et al., 1999). The time series of this feature is broadly similar to the interdecadal part of the North Pacific PDO index of Mantua et al. (1997). The IPO may be a Pacific-wide manifestation of the PDO, excluding subdecadal time scales, and seems to be part of a continuous spectrum of low frequency modulation of ENSO, and so may be partly stochastic. Indices of the IPO show three major phases this century as defined from Meteorological Office HadISST data (Rayner et al., 1999): positive phases in 1922– 1944 and 1978–1998, and a negative phase between 1946 and 1977. Mantua et al. (1997) used the nomenclature Pacific Decadal Oscillation to describe essentially the same interdecadal fluctuations in North Pacific SST pattern. The IPO phases are also in good agreement with the North Pacific PDO index of Mantua et al. (1997), as well as the time scales inferred from a number of other climate indices around the Pacific (Minobe, 1997). When the IPO is in a positive phase, SST anomalies over the North Pacific are negative, as are anomalies near New Zealand, while SST anomalies over the tropical Pacific are positive. White and Cayan (1998) identify a similar pattern over the Pacific, although confined through filtering to the 15– 30-year time scale, with the bidecadal signal in the Pacific established earlier by Mann and Park (1996). Power et al. (1999) have shown that the two phases of the IPO strongly modulate year-to-year ENSO precipitation variability over Australia. During the positive phase, the ENSO signal over Australia is suppressed. Salinger and Mullan (1999) have shown that near bidecadal climate variations in New Zealand, partly identified in the temperature signal by Folland and Salinger (1995), are related to phases of the IPO or a SST pattern that is similar. Similarly, the phase of the IPO (or PDO) may also play a key role in modulating ENSO teleconnections across North America on interdecadal time scales (Gershunov and Barnett, 1998; Livezey and Smith, 1999). Now that reliable multi-decadal time series of SLP pressure, SST, surface temperature and precipitation exist, it is important to use these to define any regional modulation of climate. This paper will identify how the IPO affects decadal-scale patterns of variation in atmospheric circulation in the South West Pacific region and how these affect changes in sea surface and island surface air temperatures of the region. Finally, the IPO modulations of ENSO teleconnections between SLP, temperature and precipitation will be described. This analysis is a further step in a long-term investigation of climate variability of the South West Pacific, an area occupying 21 million km2.
2. DATA AND METHODS
2.1. Data Monthly mean data were obtained from the UK Meteorological Office (UKMO) Hadley Centre Ice and SST (HadISST) dataset version 1.0 covering the years 1931 through to 1998 (Rayner et al., 1999; see Rayner et al., 1996 and Folland et al., 1997 for discussion of previous versions of the dataset known as GISST). Points were sampled at 5° ×10° latitude/longitude resolution, at all longitudes in a latitude band between 50°S and 60°N. SST anomalies were calculated as differences from mean monthly fields over the years 1961–1990. Monthly mean sea level pressure (MSLP) data were taken from the UKMO Global MSLP (GMSLP) dataset (Basnett and Parker, 1997) for the same period as the SST data. For large-scale analyses, points were sampled at 5°× 10° latitude/longitude resolution, at all longitudes in a band between 65°S and 65°N. For local analyses, points were interpolated to a 2.5° × 2.5° resolution grid over the South West Pacific, as will be illustrated later. As with SST fields, MSLP anomalies were calculated as differences from mean Copyright © 2001 Royal Meteorological Society
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monthly fields over the years 1961– 1990. Some of the analyses described below were repeated using MSLP fields from the National Center for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis (Kalnay et al., 1996). The NCEP–NCAR fields were sampled and processed in the same way as GMSLP data, apart from the removal of a linear trend at each grid point (Hines and Bromwich 1999; Renwick and Revell 1999). Salinger et al. (1995) analysed trends in South Pacific air temperature and precipitation. Measurements in the South Pacific are restricted mainly to the western half of the region, where populated islands occur. Climate records from this region are largely from small island sites, many being tropical coral atolls free from urban influences. Monthly mean temperature and precipitation data were used from stations (Figure 1) in the South Pacific Historical Climate Data Network (Collen, 1992; Fouhy et al., 1992). The data used were those homogenized by the procedures of Rhoades and Salinger (1993) for 36 surface temperature and 58 precipitation sites throughout the South West Pacific region, which covered the most recent negative and positive IPO phases (1946–1998). A set of 15 temperature and 17 precipitation sites were used for New Zealand. Although many more sites were available, this study chose not to over-emphasize New Zealand in the South Pacific context. Precipitation data were converted to percentage of normal at each location, the normal period being 1961– 1990. For display purposes, station values were interpolated onto a 1°× 1° grid using a Delaunay triangulation and local cubic splines.
Figure 1. Map of the South West Pacific region showing all station locations used (dots) and place names referred to in the text. The heavy dashed line indicates the approximate mean position of the SPCZ Copyright © 2001 Royal Meteorological Society
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2.2. Analysis techniques All data were initially at a monthly resolution, but for this analysis they were averaged to either annual means or 3-month seasonal means: March– May (MAM), June–August (JJA), September–November (SON) and December– February (DJF). Annual means were calculated in two ways, either as calendar years or as ‘ENSO years’ from May to the following April. All annual results presented here use calendar years, as ENSO-year results were generally very similar in form. For the coupled-mode analysis, a 7-year (29-season) low-pass filter was applied to the SLP and SST data series to remove variations on interannual time scales. A 61-point Lanczos filter with half-power point at 7.25 years (29 seasons) was applied (Duchon, 1979), requiring the loss of 3.5 years’ data at either end of the time series. The global mean SST was removed for each season, to remove the global mean temperature trend. The leading coupled modes of variation in the low-pass filtered data were taken from a singular value decomposition analysis (SVDA; Bretherton et al., 1992; Renwick and Wallace, 1995) of SST and MSLP anomalies. Results were compared with the leading empirical orthogonal function (EOF) modes calculated from each dataset separately. Annual and seasonal changes in mean SLP, surface air temperature and precipitation were identified between two of the three IPO periods as follows: (i) IPO negative (1946–1977); (ii) IPO positive (1978–1998). The prior positive phase of the IPO was not analysed for changes in temperature and precipitation because of insufficient data coverage. Finally, the modulation of ENSO teleconnections was examined by correlating the Southern Oscillation Index (SOI; Mullan, 1995) with surface air temperature and precipitation for the most recent negative and positive phases of the IPO. The SOI is derived from Bureau of Meteorology monthly mean SLP data for Tahiti and Darwin, which are then normalized using the so-called Troup method (Troup, 1965), which normalizes the Tahiti–Darwin difference to zero mean and unit standard deviation. The normalizing base period is 1941–1980. For a lot of results shown below, statistical significance has been estimated using a randomization approach. Mean differences in surface parameters (SLP, temperature, precipitation) or differences in the correlation with the SOI are compared between polarities of the IPO. For each such difference, the ‘positive’ IPO period was defined in turn as each possible contiguous block of years of the appropriate length, and the negative IPO period taken as the remaining years. Mean differences were calculated over each of the pairs of periods and percentiles of their distribution were calculated at each point (or station). An observed difference was considered to be statistically significant if its magnitude (absolute value) exceeded the 95th percentile of the magnitudes of the permutation-generated differences. The above approach was adopted in an attempt to retain low frequency (decadal-scale) structure in the data while still sampling the likely range of differences that might arise by chance. An approach of using individual years chosen at random to form ersatz positive and negative IPO periods was not adopted, as this would negate low frequency variability. Using experimentation with randomly overlapping periods (i.e. retaining contiguous blocks of years of the appropriate length but allowing the two randomly chosen sets of years to contain common years) extremes of the difference distribution were under-sampled. Classical approaches, such as the Student’s t-test, are also considered problematic with such strong low-frequency signals in the data. Given relatively short time series containing multi-decadal variability, it may not be possible to assess statistical significance accurately. However, the approach used here is at least indicative of areas of significant IPO-related variability.
3. RESULTS
3.1. SVDA The leading mode of a SVDA between low-pass filtered SST and SLP anomalies (Figure 2) captures the IPO, as described by Mantua et al. (1997), Zhang et al. (1997), Power et al. (1999) and others. The IPO exhibits a tight coupling between the sea surface and the atmosphere, with correlation above 0.9 between the SST and MSLP time series. The first SVDA mode shown here is similar to the first EOF of the SST Copyright © 2001 Royal Meteorological Society
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Figure 2. The leading mode of a SVDA between low-pass filtered seasonal average SST and MSLP anomalies, after removal of the global mean SST. The top panel shows the SST pattern as a homogeneous covariance map ‘‘HOM’’ (0.1°C contours). The middle panel shows the MSLP pattern as a heterogeneous covariance map ‘‘HET’’ (0.1-hPa contours). The bottom panel shows the normalized amplitude time series of both patterns (SST time series thickened). In both contour plots, negative contours are dashed and the zero contour is thickened. This mode accounts for 42% of the total squared covariance. The correlation between the time series is 0.95, and the two patterns account for 27 and 16% of the variance in their respective datasets
data. The main centre of action in the SST anomalies is in the north Pacific, with an opposing weaker centre just south of the equator in the eastern Pacific. There is also another weaker centre of action in the South West Pacific, which is in the same phase as the north Pacific centre of action. The pattern is reminiscent of the ENSO ‘horse shoe’, but concentrated in the northern extratropics. The matching mean SLP pattern shows an east– west seesaw at all latitudes, but again centred over the north Pacific. When pressures are below average over much of the Pacific, they tend to be above average over the rest of the globe and south Pacific west of the dateline. Near New Zealand, the mean SLP pattern implies a near-meridional anomalous surface geostrophic wind variation, similar in form to the ENSO-related pattern seen on interannual time scales (e.g. Gordon, 1986). The amplitude time series shown in Figure 2 illustrates the well-known polarity changes of the IPO from the positive to negative phase in the mid-1940s, and a return to the positive phase in the late 1970s. Copyright © 2001 Royal Meteorological Society
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3.2. IPO/SOI relationships Although the IPO and ENSO operate on different time scales, there are similarities in their expression in tropical Pacific SSTs, suggesting that the IPO exerts a modulating effect on ENSO and on ENSO teleconnections (Gershunov and Barnett 1998; Power et al., 1999). The correlation between the SST-based IPO time series shown in the bottom panel of Figure 2 and the seasonally averaged SOI is around − 0.3 over the period 1947– 1998, suggesting that the positive polarity of the IPO is associated on average with enhanced and more frequent El Nin˜ o events. When ‘El Nin˜ o’ months (defined here as 3-month running mean of the SOI B −0.9 within at least a 6-month sequence where the SOIB − 0.5) are used, both the frequency and intensity increase for the positive IPO phase. The frequency of El Nin˜ o months increase from 12% to 27% of all months, and the SOI intensifies from − 1.25 to − 1.82. The effects of the IPO upon ENSO teleconnections over the South Pacific have been explored using a series of correlation maps, taking the SOI as the reference time series.
3.3. Annual Differences in MSLP, temperature and precipitation between positive and negative phases of the IPO are shown in Figure 3 (positive minus negative). The SLP response is largely latitudinally symmetric, with higher pressures to the west of the dateline and lower pressures to the east during the positive phase of the IPO. Differences in SLP are significant at many locations in the sub-tropics and tropics but not in mid-latitudes. Associated with the pressure difference field is enhanced meridional (southerly) geostrophic flow during the positive IPO. Near New Zealand, however, the implied flow anomalies are more southwesterly than southerly. Changes in mean annual surface air temperatures between the two IPO periods show significant warming over much of the region. This is to be expected given the observed global warming trends over the period. According to Karl et al. (2000), a warming of 0.26°C has occurred since 1976, compared with no warming (cooling of 0.05°C) during the period 1946–1975. This would convert to a warming of approximately 0.3°C between the two IPO phases considered. The warming in the region south of 20°S and including most of New Zealand except eastern South Island, is consistent with global mean surface air temperature trends. However, in the northeast of the region increases in mean temperatures exceed this amount, and in the northwest there is less or no warming. This is consistent with the observed gradients in the SLP difference field, with enhanced southerly quarter flow, especially west of the dateline, but little gradient east of 160°W. The changes in annual precipitation from the negative to positive phase of the IPO show the northeast of the region significantly wetter during the positive phase (Figure 3). Precipitation in the extreme northeast increases by more than 30%. Mean annual precipitation totals range from 750 to over 2500 mm in this area, so this precipitation increase is relative to the annual amount. The line of zero change runs from Tuvalu, north of Samoa and then southeastwards to the Austral Islands. Annual precipitation shows smaller but significant decreases southwest of the zero line. The annual precipitation changes are consistent with increases in atmospheric pressure in the west and south of the region and decreases in the northeast of the region with a change to the positive IPO phase. Over New Zealand, precipitation changes are complicated by topography. Decreases occur over North Island, in agreement with the oceanic region further north. However, increases in precipitation exceeding 15% are found for the western part of South Island, consistent with increased westerly (or southwesterly) airflow. SOI and SLP teleconnections show changes throughout the South West Pacific between the two IPO periods (Figures 4 and 5). Larger correlations generally mean greater anomalies at ENSO extremes, or more consistent anomalies. Generally, correlations between the SOI and SLP strengthen significantly in the Coral Sea and north Tasman Sea, and southeast of New Zealand. During El Nin˜ o (La Nin˜ a) events, SLP is lower (higher) in the southeast, and higher (lower) in the Coral Sea and Tasman Sea regions during the positive IPO phase. The line of zero correlation shifts east at most latitudes, and the gradient from negative to positive correlations increases, particularly at the latitude of New Zealand. If the calculations are performed for ‘ENSO years’ (May – April), the decrease in SLP correlation north and northeast of Copyright © 2001 Royal Meteorological Society
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Figure 3. Changes in SLP (20-Pa contours), mean annual surface temperature (hundredths of °C, 0.1°C contours) and annual precipitation (percent, 10% contours) between the last negative (1946 – 1977) and positive (1978 – 1998) of the IPO. Negative contours are dashed and the zero contour is thickened. Shading indicates where the differences are significant at the 95% level, based on Monte Carlo trials (see text for more information)
New Zealand is not evident, but elsewhere the pattern is similar in form and magnitude to Figure 5. In the subsequent discussion, correlations are described as strengthening if they have a magnitude of at least 90.1 in the negative IPO period and their magnitude increases by at least 0.1 (positive becomes more positive, negative more negative) in the positive IPO period. SOI and temperature teleconnections strengthen significantly in the latest positive IPO phase to the east of New Zealand and west of the dateline near the equator. Correlations weaken significantly in the northeast of the region. There is little significant change in SOI–temperature relationships elsewhere. Thus, El Nin˜ o (La Nin˜ a) events bring stronger negative (positive) temperature anomalies east of New Zealand and in western Kiribati. These changes are consistent with the strengthened Copyright © 2001 Royal Meteorological Society
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Figure 4. Correlation (percent) between the annual SOI and annual SLP (top), surface temperature (middle) and percentage of normal precipitation (bottom), for the most recent negative (left) and positive (right) phase of the IPO. Contour interval is 10%, negative contours are dashed and zero contour is thickened
southwest/northeast geostrophic flow anomalies in the New Zealand region during the positive phase of the IPO. Relationships between annual precipitation and the SOI demonstrate consistent changes with the phase of the IPO also. Correlations become significantly more negative near the dateline at 20°S, but generally more positive to the west, east and northeast. Over New Zealand, correlations become more positive in the north of the country and more negative in the far south, consistent with a strengthened SOI – SLP correlation. El Nin˜ o (La Nin˜ a) events are wetter (drier) in those areas with higher negative correlations with the SOI, and drier (wetter) in the areas of increased positive correlations. The modulation of SOI teleconnections and annual precipitation match those with mean SLP. The temperature and precipitation correlation changes exhibit much smaller scale spatial variation than those for SLP. Copyright © 2001 Royal Meteorological Society
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Figure 5. Annual mean differences in correlation (percent) with the SOI between the positive and negative phase of the IPO (positive− negative) for MSLP (top), mean annual surface temperature (middle), and annual precipitation (bottom). Shading indicates where the differences are significant at the 95% level, based on Monte Carlo trials (see text for more information). Contour interval is 10%, negative contours are dashed and the zero contour is thickened
3.4. Seasonal Mean IPO-related changes in SLP during MAM show strong rises over New Zealand and the south Tasman Sea, with falls over the far northeast of the region (Figure 6). The anomalous gradient is associated with southeasterly flow over much of the region, rather than the more southerly orientation seen in the annual mean. Mean surface air temperatures again show a general increase, greatest in the northeast. There is a significant increase in seasonal precipitation in the northeast, with the zero line trending southeast from 15°S in the west, to 20°S in the east. MAM precipitation shows small decreases over the remainder of the region, except over the South Island of New Zealand, where precipitation increases in western areas. Patterns of temperature and precipitation change are broadly similar to those in the annual mean. Copyright © 2001 Royal Meteorological Society
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Figure 6. Left column shows MAM changes in SLP (top), mean surface temperature (middle) and precipitation (bottom) between negative and positive IPO phases. Units, contouring and significance shading as in Figure 3. Right column shows MAM differences in correlation with the SOI between negative and positive IPO phases for SLP (top), mean surface temperature (middle), and annual precipitation (bottom). Units, contouring and significance shading as in Figure 5
During MAM, changes in teleconnections between SOI and SLP are similar to the annual pattern, with rises east of 170°W, and falls to the west. Correlations between temperature and the SOI are more negative north of 15° –20°S, with significantly more positive correlations throughout much of the central and southern South West Pacific. Thus, El Nin˜ o (La Nin˜ a) events are warmer (cooler) in the north and northeast, and cooler (warmer) in central and southern parts of the region, in the positive IPO phase. MAM precipitation teleconnections with the SOI are more positive throughout most of the region, although significance is patchy, making El Nin˜ o (La Nin˜ a) events generally drier (wetter) in many areas. Copyright © 2001 Royal Meteorological Society
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SLP changes during JJA (Figure 7) again show an east–west pattern, with anomalous south or southeast gradients over the region. The pattern of JJA mean temperature changes from the negative to positive phase of the IPO is similar to the annual but smaller in magnitude. Seasonal temperature changes are similar to the global warming average throughout much of the central and southern part of the region, but higher in the northeast. In the northwest, there have been some significant decreases in JJA mean temperatures. For JJA precipitation, seasonal increases have occurred in the far north, and especially in the northeast, with decreases in the Coral Sea/Fiji region. Taking differences in SOI– SLP correlations for JJA between the two IPO phases produces a latitudinally banded structure, with more positive correlations north of 15°S and south of 50°S, but much more strongly negative in the 20° – 45°S band (Figure 7). For temperature, changes in correlation are
Figure 7. As in Figure 6 but for JJA Copyright © 2001 Royal Meteorological Society
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generally not significant. However, the general pattern suggests El Nin˜ o (La Nin˜ a) events produce cooler (warmer) anomalies in the north and northeast of the region and also over the North Island of New Zealand. In this southern tropical dry season, SOI–JJA precipitation correlations become more positive over much of the east (drier El Nin˜ os, with cooler conditions and stronger easterly trades) and more negative in the northwest quadrant. Over most of New Zealand, the SOI–precipitation correlations become more positive, indicating El Nin˜ o events have become drier, along with cooler and more anticyclonic conditions. During SON, there is a strong and marginally significant decrease in SLP over and east of New Zealand, with increases over Australia (Figure 8). This results in a strong anomalous southwesterly gradient west of the dateline. Pressures are generally lower in the northeast during the positive IPO, but
Figure 8. As in Figure 6 but for SON Copyright © 2001 Royal Meteorological Society
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pressure gradient changes are weak. The warming in surface mean air temperatures from the negative to positive phase of the IPO is again strongest in the northeast, where associated surface wind changes are smallest. There is general warming throughout the rest of the South West Pacific, but this is similar to the magnitude of warming in global temperatures. The zero line of precipitation change from west to east runs from 10° to 20°S across the region. There are very large increases in SON precipitation in the north and northeast, of over 70%. There are also significant precipitation decreases from Vanuatu to Fiji. The precipitation decreases are consistent with pressure rises over the Coral Sea, but large increases in the northeast do not appear to be directly related to SLP. In the positive IPO period, positive correlations between SLP and SOI become more positive over much of the region during SON (Figure 8). Teleconnections between temperature and the SOI become significantly stronger north of about 10°S. Thus, El Nin˜ o (La Nin˜ a) events show increased and more consistent cool (warm) temperature anomalies in these regions. For SON precipitation, SOI correlations weaken in some areas (negative correlations become more positive and positive correlations more negative). Over the South Island of New Zealand, there has been a substantial change in the SOI –temperature and SOI– precipitation correlations, consistent with the changing orientation of the SOI –SLP correlation. Thus, El Nin˜ os become cooler (increased positive SOI–temperature correlation) and wetter in the east but reversed the correlations to become drier on the west coast. During DJF, SLP again tends to be (insignificantly) lower southeast of New Zealand in the positive phase of the IPO, and is significantly higher over the Coral Sea (Figure 9). The DJF changes in surface mean temperature are the largest for any season for the positive phase of the IPO. In the northeast of the region increases are between 0.8 and 1.0°C, while throughout the rest of the region temperature increases amount to 0.2–0.4°C. Precipitation changes for DJF show increases in excess of 50% in the northeast of the region. The zero change line trends from near 5°S in the west to 25°S in the east. Southwest of this there are small decreases in seasonal precipitation. For New Zealand, this summer season shows the largest increases in precipitation over western and southern South Island. DJF relationships between SLP and the SOI show a strengthening (positive correlations become more positive, and negative correlations more negative) to the west, north and east of New Zealand (Figure 9). Significantly stronger positive correlations between DJF temperature and the SOI occur to the east and northeast of New Zealand. Hence El Nin˜ o (La Nin˜ a) events have become cooler (warmer) in these areas. DJF precipitation teleconnections are more strongly negative with the SOI in the north and northeast and more strongly positive throughout a large central zone for the positive IPO period. The most significant change appears to be the increase in correlation near the Cook Islands and northern French Polynesia. This translates to El Nin˜ o (La Nin˜ a) episodes becoming wetter (drier) in parts of the north, and drier (wetter) between 20° and 30°S north of New Zealand. For New Zealand, SOI–precipitation correlations have changed from weak non-significant values in the negative IPO phase to strongly positive in the northeastern half of the North Island during the positive IPO phase, indicating El Nin˜ o (La Nin˜ a) episodes have shown a dry (wet) anomaly there since the 1977 phase shift. This is consistent with SOI – SLP changes.
4. DISCUSSION/CONCLUSIONS The analysis presented in this paper has described changes in SLP, temperature and precipitation associated with the transition from the most recent negative to positive phase of the IPO. The mean changes in climatic elements are substantial and often statistically significant. However, they must not be considered solely as an IPO response, since they occur within a background of longer-term trends, this being particularly the case for temperature. Modulation of teleconnections with the SOI has also been described. There is considerable evidence of IPO modulation of ENSO teleconnections, although patterns of change are complex and significance is generally lower than for mean changes in the parameters themselves. With the change to the positive phase of the IPO, SOI–climate correlations have strengthened in some areas but have weakened in other areas. The pattern of change in correlation shows little clear Copyright © 2001 Royal Meteorological Society
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Figure 9. As in Figure 6 but for DJF
spatial signal in many cases. However, in the region of New Zealand there is a consistent change towards stronger teleconnections with the SOI during the more recent positive IPO phase. Salinger et al. (1995, 1996) found that annual surface air temperatures increased between 0.2 and 0.6°C up to 1991 throughout most of the region in the period 1951–1991. For those regions northeast of the South Pacific Convergence Zone (SPCZ) most of the warming occurred since the mid-1970s, while regions to the southwest show steady warming throughout the period. Folland et al. (1997) noted similar trends in annual South Pacific island and ocean surface temperatures, and found that a major discontinuity in decadal-scale temperature trends occurred across the SPCZ. Salinger et al. (1995) noted that the SPCZ represented a pivotal line for decadal-scale changes in precipitation, with increases in annual totals in those areas to the northeast and decreases to the southwest after the mid-1970s. Copyright © 2001 Royal Meteorological Society
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The mid-1970s is a pivotal time for New Zealand circulation, temperature and precipitation trends also. Salinger and Mullan (1999) find that there is strengthened west to southwest circulation over New Zealand in the period 1976– 1994, compared with the 1951– 1975 period. In spite of the more southerly flow, regional temperatures show little change with the earlier period. However, annual precipitation decreased in the north of the North Island, and increased in the north, west, south and southeast of the South Island. This analysis shows that the IPO represents a major source of decadal climate variability in the South West Pacific. With a change from the negative to positive phase, mean SLP has increased in the region west of the dateline, and decreased to the east, consistent with changes found by Beucher (1997). Increases in mean SLP to the west are strongest in the MAM 3-month period, but are apparent at all times of the year. Thus, there is more prevalent southwesterly geostrophic flow over the New Zealand region, with more southerly flow in the South West Pacific. Although annual mean temperature increases generally between the two periods, no warming is seen in parts of western Kiribati where southerly geostrophic flow increases are largest, and the temperature increases in excess of 0.6°C occur in northern French Polynesia, where small geostrophic flow changes occur. The temperature changes between the two IPO periods are most accentuated in DJF. For New Zealand there is little seasonal variation in the warming between the IPO periods. The precipitation changes between the two periods relate well with the areas of increased and decreased mean SLP between the two IPO periods. Increases in annual precipitation of 30% or more occur in the northeast of the region in some seasons, with decreases southwest of a line from 7°S in the west to 22°S in the east. The largest seasonal increases in annual precipitation in the northeast occur in the SON and DJF periods. For New Zealand, the largest seasonal changes are increased precipitation in the west and south of the South Island in DJF, and reduced precipitation in the northern North Island in MAM. Power et al. (1998, 1999) have demonstrated modulation of ENSO teleconnections for Australia. For the positive IPO phase, interannual relationships between ENSO and Australian precipitation and temperature weaken. The results here also show that the IPO alters teleconnections with the SOI in the South West Pacific (Figure 4), but there is no clear spatial signature in the teleconnection changes. In contrast with Australia, associations become stronger in some parts of the region. In the positive phase of the IPO, relationships between the SOI and SLP strengthen across much of the Tasman Sea at 35°S, and to the southeast of New Zealand, with positive correlations becoming more positive and negative correlations more negative. Largest changes to these ENSO teleconnections occur in the DJF season. Surface temperature and SOI relationships strengthen, on an annual time scale, to the east of New Zealand and around western Kiribati (near the equator), but weaken north of New Zealand. The strengthening east of New Zealand and around the equator is observed for all four seasons to some degree. With annual precipitation, there is a strengthening of teleconnections in the zone 20° –30°S, and over southern New Zealand. Areas north of 20°S tend to show a weakening of associations. The decadal changes induced by the IPO in the South West Pacific region originate in the changes in atmospheric mass between the western and eastern parts of the region. These changes imply a displacement northeast of the SPCZ. The SPCZ extends southeast from the Intertropical Convergence Zone (ITCZ) at 5°S to lie between Fiji and Samoa, and southern Cook Islands and French Polynesia. Areas to the northeast of the SPCZ all show increases in annual precipitation, consistent with more convective activity in the northeast and a movement of the mean location of the SPCZ further north and east. The increase in MSLP in the region west of 170°W implies a strengthening of the migratory subtropical anticyclonic belt located between 30° and 35°S. All this area shows a decrease in annual precipitation. Finally, the southern westerlies to the south and east of New Zealand have strengthened (Salinger and Mullan, 1999). The IPO is a significant feature causing climate variability in the South West Pacific, modulating climates on the decadal time scale. The IPO changes identified here include a background of global warming, and both these phenomena provide the background to interannual variability caused by ENSO. Global mean surface temperatures have increased by about 0.6°C this century (Nicholls et al., 1996), a finding that is consistent with evidence from 19th century ocean temperatures (Parker and Folland, 1991). Copyright © 2001 Royal Meteorological Society
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There are two break points in analysis of global and hemispheric warming trends. These are at 1945 and 1976. The overall temperature increase of the Southern Hemisphere for the period 1946–1975 has been 0.06°C, and for the period 1976– 1998 has been 0.14°C (Karl et al., 2000). Folland et al. (1997) have compared homogenized series of mean air temperature averaged over the South West Pacific with high quality marine temperature data. A warming in all three data series (surface air temperature, sea surface temperature, night marine air temperature) of 0.6– 0.8°C is detected since 1910 throughout a large part of the region south west of the SPCZ. Decadal temperature changes are only seen widely to the north east of the SPCZ since the mid-1970s. This study shows that the temperature increase between the two IPO periods (1946– 1977 and 1978– 1998), which closely matches the break points in global temperature increases, is between 0.2 and 0.4°C in much of the area southwest of the SPCZ. This trend is consistent with hemispheric increases in mean surface air temperatures, despite the change to more southerly quarter geostrophic flow in the positive phase of the IPO. In contrast, there is little or no increase in temperature to the north of the SPCZ in the northwest, where southerly quarter geostrophic flow changes are strongest. A greater increase in the northeast of the region is observed where very weak southerly geostrophic flow changes occur. In the changes from the earlier IPO period from the positive to negative phase around 1946, the largest temperature increases occur southwest of the SPCZ, when northerly quarter geostrophic flow became more enhanced, with minimal warming to the northeast (Salinger et al., 1996). The IPO thus modulates regional temperature variability on a decadal time scale including the background of global warming trends. This paper has also demonstrated that the IPO modulates decadal precipitation trends, and it modulates the interannual response of temperature and precipitation to ENSO over the South West Pacific region. The most recent phase of the IPO has enhanced teleconnections with ENSO in some parts, and weakens them in other parts of the region. ACKNOWLEDGEMENTS
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