PATTERNS OF CONVECTION IN THE TROPICAL PACIFIC AND THEIR INFLUENCE ON NEW ZEALAND WEATHER

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 22: (2002) Published online in Wiley InterScience ( DOI: /joc.737 PATTERNS OF CONVECTION IN THE TROPICAL PACIFIC AND THEIR INFLUENCE ON NEW ZEALAND WEATHER JOHN W. KIDSON and JAMES A. RENWICK* National Institute of Water and Atmospheric Research Ltd, Wellington, New Zealand Received 26 April 2001 Revised 28 September 2001 Accepted 3 October 2001 ABSTRACT Characteristic patterns of convection in the tropical Pacific Ocean have previously been inferred from analysis of outgoing longwave radiation (OLR), and associated with year-to-year variations in El Niño (EN) Southern Oscillation events. This study examines both the effects of these convection patterns on the New Zealand climate, and the more general influence of tropical convection on the New Zealand sector of the Southern Hemisphere. The Southern Hemisphere circulation, as a whole, is found to be most strongly influenced by equatorial convection near the Philippines, and in a broad band over the central Pacific. Where increased convection occurs west of 160 E, La Niña-like (LN) conditions prevail. When the anomalous convective activity is located near the dateline, in moderate EN conditions, SW flow prevails over New Zealand. This gives way to stronger WSW anomalies as the centre of convection is displaced further eastwards and a second centre of reduced convection becomes prominent west of the dateline in strong EN (EN+) events. The changes in wind regimes over the New Zealand region implied by the hemispheric 1000 hpa height fields are supported by mean sea-level pressure differences between a number of New Zealand and adjacent island stations. Indices of the zonal flow show a weak reduction in strength of the westerlies for LN OLR composites, and no apparent effects for EN composites, whereas EN+ conditions strongly favour above-normal westerlies. The meridional flow over New Zealand is skewed towards more frequent southerlies in both the EN and EN+ composites, whereas LN conditions favour northerly flow anomalies. A change is also observed in the frequency of New Zealand-area weather regimes. Enhanced convection centred on 5 S and east of the dateline, as found in the EN+ composites, leads to an increase in zonal regimes and a corresponding decrease in blocking regimes. The direct influence of tropical OLR variations on New Zealand temperature and precipitation has also been assessed. These indicate that the response is not simply one of degree. Different spatial anomaly patterns in the climatic elements result from the varying regional circulation patterns, and these need to be considered if present climate-forecasting schemes are to be improved. Copyright 2002 Royal Meteorological Society. KEY WORDS: ENSO; tropical convection; New Zealand climate 1. INTRODUCTION The recently completed TOGA experiment [e.g. see the comprehensive reviews by Trenberth et al. (1998) and Wallace et al. (1998)] has stimulated increasing interest and led to greater knowledge of coupled atmosphere ocean systems in the Pacific region and their effects on the circulation at higher latitudes. Particular emphasis has been placed on the El Niño (EN) Southern Oscillation (SO) phenomenon, leading to increased understanding of how the circulation at higher latitudes is influenced by Rossby waves originating from convection in the tropics. Modification of Rossby wave propagation by the mean flow at higher latitudes (e.g. Simmons et al., 1983; Sardeshmuk and Hoskins, 1988), interactions with extratropical weather systems (Kok and Opsteegh, 1985; Held et al., 1989; Hoerling and Ting 1994), and orographic influences (Nigam and DeWeaver, 1998), is * Correspondence to: James A. Renwick, NIWA, PO Box , Wellington, New Zealand. Copyright 2002 Royal Meteorological Society

2 152 J. W. KIDSON AND J. A. RENWICK thought to constrain the Northern Hemisphere response so that it is relatively insensitive to the location of the tropical forcing. However, with greater zonal symmetry in the Southern Hemisphere (SH), more variety might be expected in the mid-latitude response as the location of the tropical forcing changes. Indeed, a recent paper by Kidson et al. (2002) shows, through a combination of diagnostic and barotropic modelling studies, that it is possible to differentiate the EN response in the SH circulation, depending on the location of the principal areas of enhanced convection. Although variations in climatic elements may be inferred to some extent from changes in the prevailing wind regimes, we have also sought to establish direct relationships between tropical convection patterns and New Zealand temperatures and precipitation. Presently, statistical schemes applied in New Zealand for prediction of seasonal climate anomalies often use the SO index (SOI) as a predictor, along with a combination of sea surface temperature (SST) and atmospheric circulation patterns (Francis and Renwick, 1998; Renwick et al., 1999). Many of the latter also reflect elements of ENSO variability. Forecast skill is generally highest in the north, but there is a considerable gap between actual and potential predictability in all regions (Madden and Kidson, 1997; Madden et al., 1999). Although the SOI is indicative of the broad-scale pattern of convection in the tropical Pacific, it does not necessarily indicate variations between ENSO events that may be associated with significant changes in the extratropical response identified by Kidson et al. (2002). It is therefore desirable to establish whether better predictions of climate variations can be made, if additional information on the tropical convection patterns can be provided by outgoing longwave radiation (OLR) anomalies. In this paper we review briefly the large-scale circulation changes associated with the three ENSO-related OLR patterns found by Kidson et al. (2002; Section 3) and show how their convective anomalies relate to the locations where tropical convection has the greatest influence on the SH circulation (Section 4). In Section 5 we compare the local changes in wind flow over New Zealand with variations in the circulation indices defined by Trenberth (1976) and relate them to changes in frequencies of the weather regimes identified by Kidson (2000). Predictive relationships for temperature and precipitation over the six districts defined by Mullan (1998) are determined in Section 6, and we close with further discussion in Section DATA AND PROCESSING TECHNIQUES The principal sources of data used in this study are the OLR dataset compiled by NOAA, monthly mean 1000 hpa heights and 300 hpa stream functions from the National Centers for Environmental Protection (NCEP) National Center for Atmosphere Research (NCAR) reanalysis dataset, and monthly temperature and precipitation statistics from New Zealand climate stations OLR data The OLR data were obtained from twice-daily measurements from NOAA polar-orbiting satellites, which are checked for gross errors and interpolated in time and space (Liebmann and Smith, 1996). Preliminary results indicated significant discontinuities in calibration across the gap in this dataset during 1978 with the change in observing platform. Consequently, the analysis has been largely based on monthly mean data for the period from January 1979 to December 1999, with a subsequent extension of the series to February 2001 to provide greater cover of La Niña (LN) conditions. Given that a change in calibration is unlikely to affect the spatial patterns, we have, in some cases, used the data between January 1974 and March 1978 for compositing other variables. The subsequent processing of these data and identification of the principal modes of variability have been described by Kidson et al. (2002). The spatial distribution of the monthly variance suggested that the analysis be confined to the region between 70 E to 120 W and 20 N to20 S shown in Figure 1. In view of the skewed distribution of the time series of empirical orthogonal functions (EOFs) based on the monthly departures, cluster analysis was also undertaken by Kidson et al. (2002) to identify the most commonly occurring convection patterns, without the requirement for the principal positive and negative anomalies to occur in the same location. The cluster analysis was initially applied over the full area defined

3 TROPICAL PACIFIC OSCILLATION Figure 1. Interannual variability in OLR, shown by standard deviation of monthly departures from the long-term mean between January 1979 and December The smaller rectangle outlines the area between 20 S, 20 N, 70 E and 120 W, used for cluster analysis of ENSO patterns above, but failed to resolve any differences in convection patterns between warm ENSO events. Analysis was then carried out for a more restricted area between 140 E to 140 W and 10 N to10 S, which contains most of the variance captured by the two leading EOFs. The clustering process started with the 252 sets of anomalies at each grid point and used the root-mean-square difference in OLR anomalies as a measure of similarity. A variation of the k-means procedure of MacQueen (1967) was adopted so that the individual monthly patterns could be reassigned to the closest cluster following each merge. A scree test based on the total within group variance (Kalkstein et al., 1987) suggested that ten or fewer clusters could be statistically significant. Analysis of the patterns and membership associated with each set of clusters led to the retention of six clusters, with membership ranging from 7 to 88 monthly means over the 21 year period January 1979 December NCEP NCAR reanalysis data We have made use of the NCEP NCAR dataset comprising 12 h analyses at 2.5 spatial resolution between January 1958 and December 1999 (Kalnay et al., 1996). Monthly mean 1000 hpa height and 300 hpa stream functions were obtained as described by Kidson (1999), who also provides more details of this dataset and some problems experienced with the SH analyses New Zealand climate data Temperature and rainfall data indices for New Zealand districts were obtained from the work of Mullan (1998). Mullan used rotated EOFs of monthly precipitation departures at individual stations to define the set of six homogeneous districts shown in Figure 2. The district names follow the convention of north (N), east (E) or south (S), followed by the island abbreviation (NI or SI). Their boundaries are largely determined, in a physical sense, by the interaction of the mountain chains extending along each island with the mean westerly flow (e.g. Sturman and Tapper, 1996). A similar analysis of temperature data resulted in three distinct districts comprising the North Island, the east coasts of both islands, and the west coast of the South Island (i.e. rainfall districts NNI and WNI, ENI and ESI, and WSI). The divisions between districts are not incompatible with those for the rainfall patterns, and we have chosen the same six districts for convenience. Indices were obtained for each district by averaging the temperature departures or the percentage of normal precipitation for each station contained in them. 3. PRINCIPAL PATTERNS OF OLR VARIABILITY The three principal OLR clusters obtained by Kidson et al. (2002) that are ENSO-related and of interest to this study are shown in Figure 3. These include moderate EN and strong EN (EN+) patterns, and a

4 154 J. W. KIDSON AND J. A. RENWICK NNI NSI WNI 2 ENI WSI ESI 0-50 Dm Dm Dm >150 Dm Figure 2. The six New Zealand climatic districts established by Mullan (1998) and the locations of the 78 climate stations from which the district averages are formed. These are superimposed on a relief map that shows the principal mountain ranges single LN pattern. The corresponding SST anomaly patterns presented by Kidson et al. (2002) for the ENSOrelated patterns show that the western boundary of the warm SST anomaly and the enhanced convection are displaced 20 further east for the EN+ cluster, compared with their location for the EN cluster. LN clusters have a similar SST anomaly pattern in the Pacific to that for the EN pattern, but with the sign reversed. The EN+ pattern typically represents the mature and decaying phases of a strong warm event, when the enhanced convection has progressed further east than normal, along with the western edge of the SST anomaly. Although associated with events where the SOI reaches large negative values, the EN+ and EN convection patterns cannot simply be identified from the SOI value, as mean values of this index for the EN, EN+ and LN clusters are 1.59, 0.89 and respectively. In December February (DJF) the mean value for EN+ of 2.5 exceeds that of 1.5 for EN, but its overall strength is less than for EN, due to contributions of 0.11 in June August (JJA) and during September November (SON). The composite anomaly patterns in the 1000 hpa height field for each of these tropical OLR clusters are shown in Figure 4. Over the South Pacific the EN and LN patterns are broadly similar, when the change in sign of the LN response is taken into consideration. Appreciable differences from these two patterns are seen in the EN+ composites. These give an anomalous flow over New Zealand from the west, compared with the SW or NE orientation of the EN and LN composites. The difference between the EN and EN+ patterns over the New Zealand region is also evident in the 300 hpa stream function anomalies [shown in Kidson et al. (2002)], which indicate moderate southwesterly anomalies for the EN composite and strong westerly anomalies for EN+. The difference in the EN+ pattern may be attributed both to the eastward displacement of the enhanced convection in the central Pacific and to the strong centre of reduced convection west of the dateline (Figure 3), as indicated by a linearized barotropic vorticity equation model in Kidson et al. (2002).

5 TROPICAL PACIFIC OSCILLATION N EN 39 20S 20N EN S 20N LN 81 20S 20N SP 25 20S 80E 120E 160E 160W 120W Figure 3. Principal convection anomaly patterns derived from cluster analysis of monthly OLR variability by Kidson (2000). The number of cases is indicated to the right of the cluster name. Departures from the climatological mean monthly OLR are shown in Kelvin 4. INFLUENCE OF TROPICAL CONVECTION ON SH CIRCULATION In order to explore, in more detail, the influence of tropical convection on the SH circulation, correlations were obtained between OLR anomalies, averaged over 10 latitude by 20 longitude boxes, and a number of other fields. [As shown in Figure 3 and discussed by Kidson et al. (2002), enhanced convection in one part of the tropics is usually offset by reduced convection elsewhere. In particular, negative OLR anomalies near the Philippines are matched by positive anomalies over the central Pacific and vice versa, as seen in the first EOF of monthly OLR variability in Kidson et al. (2002).] The influence of the area-averaged OLR perturbations on the 1000 hpa circulation was measured by averaging the variance reduction (i.e. the square of the correlation coefficient between the monthly OLR for a particular box and geopotential height anomalies at each grid point over the hemisphere or globe). The resulting pattern, shown in Figure 5 for all months, indicates similar average variance reductions on both

6 156 J. W. KIDSON AND J. A. RENWICK EN EN+ 4 LN Figure 4. Composite departures in the SH 1000 hpa geopotential height field associated with the three ENSO-related clusters shown in Figure 3. (Months having a pattern correlation <0.5 with the cluster mean are not included in the composites)

7 TROPICAL PACIFIC OSCILLATION 157 Global S.H. 5 Figure 5. The mean percentage of global and SH 1000 hpa geopotential height variance explainable by the monthly OLR anomaly averaged over 10 latitude by 20 longitude boxes centred at the locations shown on each map global and SH grids. Two principal centres of forcing are found near the equator, at 120 E and between 160 and 180 W. The average variance reduction is least at 150 E, where the sign of the hemispheric response pattern changes. As discussed by Kidson et al. (2002), the Northern Hemisphere subtropical jet prevents poleward propagation of disturbances originating west of the dateline during the northern winter. This may account for the eastward displacement of the Pacific maximum in the global pattern in Figure 5, as the convection needs to occur further east in order to excite wave trains (such as the Pacific North American teleconnection pattern) that affect the north Pacific and North America. The correlation of the 1000 hpa height anomaly with the OLR box anomaly for the two principal centres is shown in Figure 6. These patterns are somewhat noisy, but are nearly identical apart from the reversal in sign. Since the convection centres at 180 and 120 E are close to the EN and LN anomaly centres shown in Figure 3, it would be expected that the patterns in Figure 6 are ENSO-related. This is confirmed at higher 5S, 120E 5S, 180E Figure 6. The correlation between monthly mean 1000 hpa height anomalies over the SH and monthly OLR anomalies for the 10 latitude by 20 longitude boxes centred on 5 S, 120 E and 5 S, 180 E. Shading is for clarity, but also highlights correlations > 0.3 significant at the 5% level

8 158 J. W. KIDSON AND J. A. RENWICK Figure 7. The leading mode of a CCA between OLR anomalies (top) and 1000 hpa height anomalies (bottom) for the period January 1978 April Contour plots are correlation maps (non-dimensional), using the time series of the leading OLR mode in both cases. The contour interval is 0.2 for OLR and 0.1 for 1000 hpa height. Negative contours are dashed. Time series amplitudes of both patterns are shown in the bottom panel; OLR solid and 1000 hpa height dashed. The squared covariance fraction (CCA analogue of explained variance ) is 57% and the correlation between the two time series (Tscorr) is latitudes by comparison with the EN and LN patterns in Figure 4, but at lower latitudes the magnitude of the height anomalies is reduced and we are unable to see the features revealed by the correlation maps. The response patterns shown in Figure 6 are also in good agreement with a canonical correlation analysis (CCA; Glahn, 1968) of monthly mean OLR and 1000 hpa height anomalies over the period January 1978 April Both fields were first normalized and truncated to retain only the variance contained in their leading 15 EOFs, to avoid over-fitting problems (Barnett and Preisendorfer, 1987). The leading two CCA mode pairs are shown in Figures 7 and 8. The leading mode pair has an OLR pattern similar in form to the EN LN pair shown in Figure 3, and the 1000 hpa response pattern matches well with Figure 6. The

9 TROPICAL PACIFIC OSCILLATION 159 Figure 8. As for Figure 7, but showing the second CCA mode. The squared covariance fraction (CCA analogue of explained variance ) is 10% and the correlation between the two time series (Tscorr) is time series of both patterns are strongly correlated (r = 0.95) and the mode accounts for 57% of the total squared covariance between the two fields. The time series are clearly dominated by interannual variability. The correlation between the leading OLR time series and the SOI is The second mode, through only accounting for 10% of the squared covariance, is again strongly coupled (time series correlation 0.85), and appears to capture the EN+ pattern. Although there is considerable interannual variability in the amplitude of the second mode, there is much more short time scale variability than in the leading mode. The correlation between the second mode OLR time series and the SOI is only EFFECTS ON THE NEW ZEALAND CIRCULATION The influence of tropical convection anomalies on the New Zealand regional circulation is determined here with the aid of indices obtained from mean sea-level (msl) pressure differences between ground

10 160 J. W. KIDSON AND J. A. RENWICK stations, and from the changes observed in the frequency of the daily weather regimes defined by Kidson (2000) Zonal and meridional flow indices Changes in the regional circulation may also be measured by a number of indices of zonal and meridional flow obtained as msl pressure differences between synoptic reporting stations shown in Figure 9. These were first defined by Trenberth (1976) and have more recently been extended by Mullan. The sense of the differences shown in Figure 9 is indicated by the arrowheads, and the corresponding geostrophic wind anomaly is directed 90 to the right of the arrow. Thus, in general terms, the meridional indices (M1 M3) measure the strength of flow directed from the south, whereas positive values of the zonal indices (Z1 Z3) indicate flow from the west. In the following analysis the monthly values of each index have been ranked into quintiles to allow for easier comparison and aggregation of monthly statistics. It has been assumed for significance testing that the changes from an even distribution in each quintile associated with each tropical convection pattern show a linear relationship or trend between the number in each quintile versus the quintile number (1 5). Given that there are only three degrees of freedom, a high value for the correlation coefficient is required to establish a significant relationship. The 90%, 95% and 99% confidence limits are 0.81, 0.88 and 0.96 respectively. The results are summarized in Table I and show that only one-third of the relationships tested exceed the 90% confidence limit. Those that do are generally easy to detect in the distributions shown in Figures 10 and 11, which apply to the period October June in order to show more clearly the differences between EN and EN+ events. The zonal index distributions in Figure 10 for LN composites are relatively small in magnitude Auckland Z3 Z1 Hobart M3 Hokitika M1 Invercargill Z2 Christchurch M2 Chatham Is. Campbell Is. Figure 9. Regional zonal and meridional indices derived from msl pressure differences between the points indicated. The sense of the pressure difference is shown by the arrows, with positive differences corresponding to flow to the right (i.e. southerly for the M indices and westerly for the Z indices) Table I. Correlation coefficients between numbers of occurrences of each quintile value and the quintile number for zonal and meridional wind indices near New Zealand. Those significant at the 90% and 95% levels are shown in bold and italics respectively. See text for further explanation Z1 Z2 Z3 M1 M2 M3 EN EN LN

11 TROPICAL PACIFIC OSCILLATION 161 % Frequency Z1 quintiles for OLR clusters, Oct-Jun 1974/ La Niña ModEN StrEN % Frequency % Frequency Z2 quintiles for OLR clusters, Oct-Jun 1974/ La Niña ModEN StrEN Z3 quintiles for OLR clusters, Oct-Jun 1974/ La Niña ModEN StrEN Figure 10. Distribution of monthly zonal index values associated with ENSO-related OLR clusters and fail to reach the 90% confidence level. The positive correlations between quintile number and frequency for EN+ are consistently positive, and changes in the quintile distribution are large for indices Z1 and Z3, representing conditions over New Zealand. EN conditions do not have a significant influence on the zonal index distributions. For EN the correlations for all three meridional indices (M1 M3) shown in Figure 11 are consistently positive; they are most significant for M2, which shows large changes in the meridional flow east of the South Island. M2 also shows a strong negative relationship for the LN class. EN+ cases show a significant correlation (at the 90% level) only for M1, i.e. the pressure difference between Hobart and the Chatham Islands. This possibly indicates some variability in the longitude of the anomalous meridional flow. These results provide further confirmation of the differences between EN and EN+ conditions seen in the circulation patterns in Figure 4. Both are associated with an increased southerly flow over New Zealand, but only EN+ is linked to stronger westerlies, and this dominates the anomalous flow pattern. We have not presented seasonal variations here because of the relatively short period of the record, but the analysis by Kidson et al. (2002) provides evidence that the summer and winter responses for the ENSO-related patterns over the SH are similar. This contrasts with the Northern Hemisphere, where summertime responses to forcing in the tropical Pacific are very weak Circulation regimes Kidson (2000) has defined three regimes characterizing the New Zealand regional circulation in terms of the monthly frequencies of 12 daily weather types. A regime is characterized by dominance of a subset of these

12 162 J. W. KIDSON AND J. A. RENWICK % Frequency % Frequency M1 quintiles for OLR clusters, Oct-Jun1974/ La Niña ModEN StrEN M2 quintiles for OLR clusters, Oct-Jun 1974/ La Niña ModEN StrEN % Frequency M3 quintiles for OLR clusters, Oct-Jun 1974/ La Niña ModEN StrEN Figure 11. Distribution of monthly meridional index values associated with ENSO-related OLR clusters daily weather types over a period of time, and by the associated departure patterns in the climatic elements. For the New Zealand region between S and 160 E-175 W, cluster analysis led to the definition of three regimes: trough (T) frequent troughs crossing New Zealand; blocking (B) anticyclones prominent to the south of the region; and zonal (Z) highs in the north and strong zonal flow in the south. T regimes, which are uncommon in autumn, lead to above-normal precipitation over the whole country and cooler temperatures in the west. B regimes are more frequent in summer and autumn and are associated with above-normal temperatures, more precipitation in the northeast and less precipitation in the southwest. Z regimes are less common in summer and bring milder conditions in the south of the country and below-normal precipitation in the northeast. The sensitivity of these circulation regimes to the location of tropical convection was measured by the change in their frequency of occurrence in composites where the 10 latitude 20 longitude OLR box anomaly was more than one standard deviation (1σ ) below or above its long-term mean. The signs of the results in Figure 12 indicate the effects of stronger convection (negative OLR anomalies) at each geographical location on the relative frequency of each regime type. The B regime becomes more frequent with enhanced convection in the northwest quadrant, whereas Z regimes are favoured by enhanced convection east of the dateline and just south of the equator. The occurrence of the T regime shows little dependence on the location of the tropical convection, except over the Indian Ocean, and, for the most part, compensating changes occur

13 TROPICAL PACIFIC OSCILLATION 163 T Regime B Regime Z Regime Figure 12. Percentage change in the frequency of New Zealand T, B, and Z regimes (Kidson, 2000), associated with a change from OLR anomalies of >1σ to < 1σ, corresponding to stronger convection at the locations shown on the maps Regime frequencies, Oct - Jun 1974/2000 T B Z % Frequency La Niña Moderate Strong SPCZ Mean Figure 13. Frequencies of monthly weather regimes (%) for the ENSO-related OLR classes shown in Figure 3. These have been computed for the peak months for ENSO events (October June) in the frequency of B and Z regimes. As the mean frequency of each regime type is in the range of 30 35%, there is an approximate ±50% change in the frequency of the B and Z regimes, relative to their overall means, for significant variations in convection over the central Pacific. The areas of greatest influence on the regime frequency in Figure 12 are seen to coincide with the locations of the principal convective anomalies for the ENSO-related patterns in Figure 3. Although changes in convection over the Philippines associated with the EN and LN patterns will alter the relative frequency of Z and B regimes, the EN and LN centres near the dateline are not far enough east to exert a strong influence on the regime frequencies. Both EN+ centres are optimally located to increase the frequency of the Z regime, and the enhanced (eastern) centre also acts to decrease the frequency of the B regime. The overall changes in regime frequency associated with the three ENSO-related convection patterns in Figure 3 are shown in Figure 13 for the peak EN season, October March. Although the results should be

14 164 J. W. KIDSON AND J. A. RENWICK interpreted with some caution, owing to the small sample size, the strong increase in Z regimes and reduction in B regimes associated with EN+ events are likely to be significant. These changes are consistent with Figure 12, and with the results of Kidson (2000), who showed that over the whole year a change from low (EN) to high (LN) SOI values was associated with an increase in B regimes relative to Z regimes. 6. LINKS TO NEW ZEALAND PRECIPITATION AND RAINFALL The connections between tropical OLR variations and the monthly temperature and precipitation anomalies are examined here for the six New Zealand districts shown in Figure 2. Initially, correlations were obtained between mean temperature and precipitation for each district over varying periods from 1 to 12 months to establish the direct effect of tropical OLR variations on these climatic elements. The patterns were generally similar, but the strength of the correlations increased with the averaging period and the results are presented here for overlapping 3 month periods without regard to season. The correlations between 3 month OLR anomalies, again averaged over 10 latitude 20 longitude boxes, and 3 month temperature anomalies for each district are shown in Figure 14. The patterns for each district are quite similar, as we might expect, given that previous results show that the first temperature EOF for New Zealand stations accounts for a large proportion of the variance, with similar departures over the whole country (Salinger, 1980; Kidson and Gordon, 1986; Madden and Kidson, 1997). The tropical forcing pattern that produces warm anomalies over all districts has reduced convection near the equator between approximately 170 E and 150 W and a tendency for stronger convection near the longitude of the Philippines. This implies that an LN-like OLR pattern leads to warmer conditions over New Zealand and a moderate EN pattern produces cooling, in line with previous relationships found for the SOI (e.g. Gordon, 1986; Mullan, 1995). The principal centres of the convective anomalies for the EN+ pattern (Figure 3) do not map very well on to the optimum forcing patterns in Figure 14, and their effect on New Zealand temperatures should be smaller. The magnitude of the correlations varies somewhat between districts. On a 3 month seasonal basis, OLR variations in the central Pacific can account for 22% of the variance in WNI temperatures, but only 7% of the variance in ESI temperatures. On an annual basis the patterns are similar, but peak correlation values in the Pacific exceed 0.60 for the four northern districts and 0.40 for the western and eastern South Island. The corresponding correlation patterns for rainfall are shown in Figure 15. This time there are much larger differences between the patterns for each district, though again the four northern district patterns show some common features. The relationship is strongest for the north of the North Island, where relationships with the SOI are also strong, particularly during spring (Mullan, 1995). The pattern is quite similar to that for the temperature departures shown in Figure 14. As noted above, the EN and LN convection patterns map onto it reasonably well, and the area of increased convection in the Pacific associated with EN+ also has an effect on NNI precipitation. EN and EN+ conditions, therefore, bring less rain to the north of the country whereas LN brings more. This is true also of the WNI, ENI and NSI districts, but the relationships are quite weak and may have little practical value. There is no significant relationship between OLR anomalies and rainfall over the west of the South Island. Over the east of the South Island, the pattern is similar to that for the eastern North Island, with the strongest correlations occurring near 150 E, a little north of the equator, and the location of the reduced convection associated with EN+ patterns. Again, on an annual basis, the relationships between tropical OLR anomalies and rainfall are much stronger. The NNI, WNI and NSI districts have similar patterns, as do ENI and ESI. On this time scale the WSI rainfall is related to increased convection in a band across the Pacific north of the equator, perhaps indicating a stronger Hadley circulation and increased zonal westerlies. Variance reductions obtained for convection in favoured locations range from 16 to 36% Quintile distributions of temperature and precipitation As for the zonal and meridional wind indices, it was found convenient to express the distribution of temperature and precipitation anomalies for each district in terms of five quintiles, which, taken over the full

15 TROPICAL PACIFIC OSCILLATION 165 NNI WNI ENI NSI WSI ESI Figure 14. Correlations between 3 month mean OLR anomalies evaluated over boxes and temperature anomalies for the New Zealand districts defined in Figure 2 dataset, would place 20% of months in each class. This makes it easy to compare and concatenate data for different months or seasons, and also provides a basis for issuing probability forecasts, given a particular tropical convection pattern. The significance of the relationships is again assessed by correlating the counts in each quintile with the quintile number. The correlation coefficient, shown in Tables II and III, provides an indication of the

16 166 J. W. KIDSON AND J. A. RENWICK NNI WNI ENI NSI WSI ESI Figure 15. Correlations between 3 month mean OLR anomalies evaluated over boxes and precipitation anomalies for the New Zealand districts defined in Figure 2 significance of the assumed linear relationship. The magnitude of the effect is provided by linear regression estimates of the percentage of months falling in each quintile, which is also shown in Tables II and III as the estimated difference in the percentages estimated for quintile 5 and quintile 1. This provides an indication of the practical significance of the relationship for climate forecasting. We note that, if the distribution were to

17 TROPICAL PACIFIC OSCILLATION 167 be divided into terciles, the corresponding difference between the above normal and below normal terciles would be 80% of the difference between quintiles 1 and 5. In Tables II and III the values are printed in bold type if the associated correlation coefficient lies in the top or bottom 5% of the Fisher Z distribution or if the estimated quintile differences, assessed using Table II. Departures of district monthly mean temperature anomaly ( C) and skewness of quintile distribution expressed as the difference in percent between regression estimates of quintiles 1 and 5 (i.e. ˆQ 5 ˆQ 1 ), for the moderate EN, EN+, and LN clusters. Numbers indicated by bold type have associated significance levels lying in the top or bottom 5% or their respective distributions. See Figure 2 for the definition of the district names (updated to 2/2001) NNI WNI ENI NSI WSI ESI Mean Skew Mean Skew Mean Skew Mean Skew Mean Skew Mean Skew Year EN EN LN DJF EN EN LN MAM EN EN LN JJA EN EN LN SON EN EN LN Table III. Departures of district monthly mean rainfall anomaly (%) and skewness of quintile distribution expressed as the difference in percent between regression estimates of quintiles 1 and 5 (i.e. ˆQ 5 ˆQ 1 ), for the moderate EN, EN+, and LN clusters. Numbers indicated by bold type have associated significance levels lying in the top or bottom 5% or their respective distributions. See Figure 2 for the definition of the district names (updated to 2/2001) NNI WNI ENI NSI WSI ESI Mean Skew Mean Skew Mean Skew Mean Skew Mean Skew Mean Skew Year EN EN LN DJF EN EN LN MAM EN EN LN JJA EN EN LN SON EN EN LN

18 168 J. W. KIDSON AND J. A. RENWICK the t-test, lie in the top or bottom 5% of the range. Although the number of samples is relatively small, the class mean departures and skewness of the quintile distribution show generally similar patterns of variation across districts and seasons. Most significant temperature departures in Table II occur during the winter (JJA) and spring (SON) and they are more common in the North Island. None was found during summer (DJF) or autumn (March May: MAM). A difference in response to EN and EN+ classes is evident, in that temperature departures with different signs outnumber those with the same sign by nearly three to one. By a similar margin, the EN+ departures more commonly have the same sign as those for LN. The significant precipitation departures in Table III are spread more evenly throughout the year, but none occurs in autumn (MAM). There are no strong relationships with the OLR ENSO classes for the ESI district. The number of EN and EN+ departures with different signs only slightly exceeds the number with the same sign, and EN+ departures are different for those in the LN class in 14 of 24 cases. Mean Temperature departures DJF MAM JJA SON Moderate El Niño Strong El Niño La Niña < -2σ < -1σ > 1σ > 2σ Figure 16. Mean temperature departures for New Zealand districts in individual seasons from 1974 to 1999 associated with the moderate EN, EN+, and LN OLR composites in Figure 3

19 TROPICAL PACIFIC OSCILLATION 169 Mean Precipitation departures DJF MAM JJA SON Moderate El Niño Strong El Niño La Niña < -2σ < -1σ > 1σ > 2σ Figure 17. Mean rainfall departures for New Zealand districts in individual seasons from 1974 to 1999 associated with the moderate EN, EN+, and LN OLR composites in Figure 3 The spatial patterns of influence of the EN, EN+ and LN classes over January 1974 February 2001 (except for April December 1978) on New Zealand temperatures and precipitation are shown in Figures 16 and 17. The shading indicates the significance of the seasonal departures from the mean, with t-values divided into ranges of 1 2σ ( above and below ) and more than 2σ ( much above and much below ) from the seasonal normal. For EN convection patterns, temperatures are normal or below normal in all districts, with no significant departures over summer. For the EN+ cluster, however, the departures are normal or above in all districts except WNI in summer. The effects are greatest in winter, where the stronger westerlies evidently keep the temperature above normal in all districts except ESI. The mean departure over North Island districts during winter is C. LN conditions are associated with weak departures in all districts, except for the North Island during autumn, and for the entire country during spring, when the mean departure over the whole country is +0.3 C.

20 170 J. W. KIDSON AND J. A. RENWICK Few of the rainfall departures for EN composites are significant. The main exception is the winter season (JJA) when there is a tendency for dry conditions, particularly in the north of each island. The effects of EN+ conditions are much more noticeable, with a tendency for a contrast between wetter conditions in the southwest and drier conditions in the northeast. In summer, drier conditions are observed in the north and east of both islands, whereas in spring above-normal rainfalls occur in the west of the country. LN conditions bring above-normal rain in the north and east of the North Island over summer and a contrast over the South Island in winter, but otherwise their effect on precipitation is small SOI climatology The influence of the SO on New Zealand weather has been described by various workers, starting with Gordon (1986), and the most complete description comes from the work of Mullan (1995, 1996), who found correlations of the SOI with station values of precipitation and temperature, using a dataset that commenced in The character of the SOI variations has changed over the last four decades, with a climate shift involving a change to more frequent EN conditions detected around 1978 (Salinger and Mullan, 1999). This has been linked to a change in phase of the interdecadal Pacific oscillation (IPO) that occurred around the same time. Longer-term changes in the response of the New Zealand climate to ENSO variations may be expected, and Mullan (1995) has previously documented changes occurring prior to Consequently, we have analysed these relationships for the periods , , and (matching the availability of the OLR data) to look for consistent patterns in the seasonal temperature and precipitation Mean Temperature departures - 74/01 DJF MAM JJA SON SOI < -1 σ (El Niño) SOI > 1σ (La Niña) < -2σ < -1σ > 1σ > 2σ Figure 18. Mean temperature departures for New Zealand districts in individual seasons from 1974 to 2000 associated with SOI < 1σ (EN) and >1σ (LN)

21 TROPICAL PACIFIC OSCILLATION 171 Mean Precipitation departures - 74/01 DJF MAM JJA SON SOI < -1σ (El Niño) SOI > 1σ (La Niña) < -2σ < -1σ > 1σ > 2σ Figure 19. Mean rainfall departures for New Zealand districts in individual seasons from 1974 to 2000 associated with SOI < 1σ (EN) and >1σ (LN) departures for extreme SOI values. Although we show only the January 1974 February 2001 patterns in Figures 18 and 19, the following general conclusions were drawn Temperature. For all analysis periods and seasons, significant negative departures in temperature are associated with EN conditions (SOI < 1σ). These tend to be largest in the North Island and weakest in summer. LN conditions bring above-normal temperatures in all districts and seasons, though the effects are small in summer and less consistent in the west and east of the South Island Precipitation. Significant negative departures (between 1σ and 2σ ) may be found over the North Island for SOI < 1σ in all seasons. These occur most often in the north of the island, and the overall relationships are weakest during the summer months. No significant deviations are observed for South Island districts, regardless of the season. For LN conditions (SOI > 1σ) the response is mixed. In summer there is a tendency for drier conditions, except in the east of the North Island. For the other seasons there is a tendency for above-normal precipitation, except for the east coasts of both islands, which are either near or below normal Comparison with OLR classes. The patterns of New Zealand temperature and precipitation departures associated with the OLR classes may be compared with those for departures in the SOI exceeding 1σ over the same period, , in Figures 18 and 19. Some differences in detail are obvious, and these may be due either to sampling fluctuations or to variations in the location of tropical forcing, which are not captured by the SOI.

22 172 J. W. KIDSON AND J. A. RENWICK It can be seen from Figures 17 and 19 that EN and negative SOI rainfall composites generally favour drier conditions in all seasons. The increase in spring rainfall in the eastern North Island and the lack of any significant response in autumn for EN is inconsistent with the negative SOI composites. Compared with the EN patterns, the EN+ rainfall anomalies are more strongly negative during the summer months, whereas other seasons show evidence of above-normal rainfall not found in the EN composites. In general, these indicate gradients from wetter conditions in the west to drier conditions in the east, with rainfalls generally above normal in spring. The LN patterns go against the consistent positive SOI pattern of below-average rainfall in summer and drier conditions in the east of the South Island in winter, and are weak in autumn and spring. Greater agreement between the OLR and SOI-related patterns is shown in the seasonal temperature anomalies. The EN and negative SOI patterns generally match quite well, though the EN departures tend to be less significant. The EN+ departures, in most cases, show warmer conditions, in contrast to those for EN and negative SOI. The LN patterns mostly have the same sign as those for positive SOI, but, with the exception of the spring, have less significant departures. Other comparisons are possible, given known patterns of correlation between the six-district temperature and precipitation anomalies and variations in the circulation indices described in Section 5. As for the SOIrelated patterns, seasonal comparisons show frequent divergence from the anomaly patterns associated with increased meridional flow for EN and EN+, increased zonal flow for EN+, and a preference for Z over B regimes for the EN+ cluster. These differences are greatest for the EN+ cluster, where the patterns may have been distorted by the abnormally warm conditions during late 1997 and If this period is removed from the composites, cold conditions (< 1σ) are experienced in all districts during DJF, no significant departures are found in MAM or SON, and warm conditions remain over most of the country during JJA. These are in better agreement with the general relationships, that increased westerlies, as measured by the Z1 and Z3 indices, tend to bring cooler conditions in summer and milder conditions in winter and spring (except for WSI). A change from B to Z circulation regimes, associated with the EN+ cluster, should also be associated with cooler conditions over most districts in summer, and milder conditions over the south and east of the South Island during winter. 7. DISCUSSION AND CONCLUSIONS Following identification of three characteristic tropical convection patterns linked to the ENSO phenomenon and their influence on the SH circulation (Kidson et al., 2002), a more general study of the influence of tropical convection on the New Zealand climate has been undertaken. Correlations between spatially averaged tropical OLR anomalies and departures from normal in the SH 1000 hpa circulation reveal that the hemispheric circulation is most strongly affected by OLR variations centred near the equator at 120 E and close to the dateline. The hemispheric anomaly patterns induced by convection at these two locations are similar, but of the opposite sign, and generally agree well with the SH forcing associated with the LN and EN OLR clusters. This is to be expected, as the main centres of convective anomalies in these OLR clusters are found near to optimum locations for forcing of the SH circulation. Over the New Zealand region, enhanced convection in the central Pacific east of the dateline and south of the equator increases the frequency of Z regimes at the expense of B regimes [see Kidson (2000)]. The opposite effect is found when the enhanced convection is centred to the north of Australia above the equator. The influence of moderate EN events on the frequency of Z regimes is, consequently, small compared with that for the strong EN+ events, where the enhanced convection is located further east. Regional msl pressure differences also show that EN conditions are associated with a weak anomalous SW gradient over New Zealand, whereas EN+ convection patterns are associated with stronger anomalies from a more westerly direction. The strength of westerly winds over New Zealand is apparently unaffected during EN OLR composites, whereas there is a weak reduction for LN composites. EN+ conditions strongly favour above-normal westerlies, which dominate the southerly component associated with both EN+ and EN classes. LN composites are associated with anomalous northerly conditions. Positive relationships have also been found between tropical convective anomalies and departures in temperature and precipitation for New Zealand districts. Higher temperatures in all districts are associated with

23 TROPICAL PACIFIC OSCILLATION 173 reduced convection in the intertropical convergence zone and, less clearly, in the South Pacific convergence zone. Lower temperatures are associated with increased convection to the south of the Philippines. Both these effects are present in LN conditions, which generally favour milder temperatures. Precipitation correlation patterns show more variation between districts, but in the north and east of both islands the rainfall is increased by OLR anomalies similar to those producing higher temperatures. Mean temperature and precipitation departure patterns for the three ENSO-related OLR classes have also been prepared for individual seasons and related to those composited for extremes of the traditional SOI. In general, the agreement is better for temperatures with a tendency for below-normal values in both EN and negative SOI composites. In contrast, the EN+ composites tend to be associated with above-normal temperatures. This is due, in part, to the inclusion of the exceptionally warm period; when this is excluded, the results are closer to what we would expect from the changes in circulation patterns. For precipitation the correspondence is poor, particularly for LN and positive SOI composites. Below-normal values are observed for both EN and negative SOI composites, but the EN+ composites do give some indication of the commonly accepted EN pattern of wetter conditions in the southwest and drier conditions in the east. The relationships derived above give some indication of how it may be possible to increase local climate forecasting skill, but this study has stopped short of developing operational relationships. These will need to take account of the strength of the tropical OLR anomalies, as well as of their locations, in order to allow for changes in the circulation patterns, and improve on techniques based principally on the state of the SOI. There are still reservations concerning the relatively short length of the OLR record, which contains only two EN+ episodes and In the latter period, New Zealand temperatures were anomalously high and a longer period of record will be needed to establish whether this is a consistent feature of forcing by the EN+ OLR pattern. Nevertheless, the study highlights the variations in the SH and New Zealand regional circulation that result from changes in tropical convection patterns, and points to a need to monitor these in future seasonal forecasting schemes. ACKNOWLEDGMENTS Thanks are due to Dr Brett Mullan for supplying data series used in this analysis, and his comments on the initial draft of this manuscript. We also thank an anonymous reviewer for suggestions that have improved the clarity of the presentation. This research was funded by the New Zealand Foundation for Research, Science and Technology, through contracts CO1628 and C01X0030. REFERENCES Barnett TP, Preisendorfer RW Origins and levels of monthly and seasonal forecast skill for United States surface air temperatures determined by canonical correlation analysis. Monthly Weather Review 115: Francis RICC, Renwick JA A regression-based assessment of the predictability of New Zealand climate anomalies. Theoretical and Applied Climatology 60: Glahn HR Canonical correlation and its relationship to discriminant analysis and multiple regression. Journal of the Atmospheric Sciences 25: Gordon ND The southern oscillation and New Zealand weather. Monthly Weather Review 114: Held IM, Lyons SW, Nigam S Transients and the extratropical response to El Niño. Journal of the Atmospheric Sciences 46: Hoerling MP, Ting M Organization of extratropical transients during El Niño. Journal of Climate 7: Kalkstein LS, Tan G, Skindlov JA An evaluation of three clustering procedures for use in synoptic climatological classification. Journal of Climate and Applied Meteorology 26: Kalnay E, Kanamitsu M, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woollen J, Zhu Y, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo KC, Ropelewski C, Wang J, Leetmaa A, Reynolds R, Jenne R, Joseph D The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77: Kidson JW Principal modes of Southern Hemisphere low-frequency variability obtained from NCEP-NCAR reanalyses. Journal of Climate 12: Kidson JW An analysis of New Zealand synoptic types and their use in defining weather regimes. International Journal of Climatology 20: Kidson JW, Gordon ND Interannual variations in New Zealand temperature and precipitation patterns. New Zealand Journal of Geology and Geophysics 29: