EXTRATROPICAL CIRCULATION INDICES IN THE SOUTHERN HEMISPHERE BASED ON STATION DATA

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 19: (1999) EXTRATROPICAL CIRCULATION INDICES IN THE SOUTHERN HEMISPHERE BASED ON STATION DATA P.D. JONES a, *, M.J. SALINGER b and A.B. MULLAN b a Climatic Research Unit, Uni ersity of East Anglia, Norwich NR4 7TJ, UK b N.I.W.A., Auckland, New Zealand Recei ed 2 October 1998 Re ised 5 February 1999 Accepted 6 February 1999 ABSTRACT Zonal and meridional pressure gradient indices of the Southern Hemisphere (SH) circulation are analysed in the mid-to-high (35 65 S) latitude zone. The dearth of land regions, and hence long pressure records, means that these are restricted to the southern South American and New Zealand sectors. The Trans Polar Index (TPI) is the only large-scale station pressure based extratropical SH index that has been proposed, and is based on the normalised pressure difference between Hobart, Tasmania and Stanley, Falklands. This index is compared with variants which involve stations in the vicinity of New Zealand and southern South America. The index shows considerable year-to-year and some decadal-scale variability and is a measure of wavenumber 1 of the SH pressure field. Significant correlations (r 0.3 to 0.5) occur between the TPI and southern South American temperatures in the austral summer and autumn seasons. Similar size correlations of the opposite sign occur in New Zealand but only in the austral summer season. In New Zealand and southern South America, temperature series are strongly affected by the strength of the local meridional circulation (r values 0.4 to 0.7 over New Zealand depending on season and period and values of 0.2 to 0.4 for southern South America). In both regions there is no concomitant increase in northerly flow or decrease in southerly flow to explain the long-term increase in temperatures. The relationships are mostly at the interannual rather than the decadal and longer timescales. The decadal temperature rise, therefore, reflects a general warming of the Southern Ocean, rather than decadal-scale variations in the circulation. Copyright 1999 Royal Meteorological Society. KEY WORDS: Southern Hemisphere; New Zealand; austral summer 1. INTRODUCTION The two most well-known indices of the atmospheric circulation are the Southern Oscillation Index (SOI) and the North Atlantic Oscillation (NAO). They are formed by taking the difference between normalised pressures at two key sites, Tahiti and Darwin for the SOI, and an Azores and Icelandic station in the case of the NAO. The SOI influences most of the tropics and many midlatitude regions, causing characteristic patterns of temperature and precipitation anomalies when the oscillation is in one phase and vice versa (Ropelewski and Halpert, 1987, 1989). In a somewhat analogous manner the NAO, particularly during the winter and spring seasons, has a pronounced influence on temperature and precipitation patterns over much of Europe (Hurrell, 1995). The extratropics and higher latitudes of the Southern Hemisphere (SH) have been less studied, principally because of the relative shortness of some of the series but also partly because of the sparcity of the land regions. Regional indices have been developed, using key long stations, over some regions (e.g. New Zealand, Trenberth, 1976). Several similar indices have been developed in other mid-to-high latitude regions of the SH at the regional scale (Mayes, 1981). * Correspondence to: Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK. CCC /99/ $17.50 Copyright 1999 Royal Meteorological Society

2 1302 P.D. JONES ET AL. On the larger scale, the Trans Polar Index (TPI) has been proposed by Pittock (1980, 1984) and is the difference of normalised pressures at Hobart, Tasmania and Stanley, Falklands. Selecting stations on the opposite sides of Antarctica means that the TPI is a measure of the wavenumber one of the pressure field in the SH, the dominant mode in the hemisphere (Trenberth, 1980). The TPI will also be influenced by the interannual variability of the zonal westerlies (Rogers and van Loon, 1982). In a Principal Components (PC) analysis of surface pressure data over the SH (15 60 S), Karoly et al. (1996) showed that the TPI was correlated with one of the four leading PCs of a rotated PC analysis, using both raw and low-pass filtered data (r=0.70 for raw with RPC4 and r=0.66 for filtered data with RPC2 (filtered) over ). The purpose of this paper is to attempt to extend the pressure records from the two sites that make up the TPI, Hobart, Tasmania and Stanley, Falklands, and to consider circulation indices in the New Zealand and southern South American regions of the SH. Time series of all the developed indices are intercompared, and an assessment made of the strength of the relationships between the indices and temperatures over New Zealand and southern South America. 2. DATA This section includes details of the sources of all the pressure series used. The location of the sites is given in Figure Hobart Meteorological observations commenced at Hobart in 1841, with the monthly mean-sea-level pressure (MSLP) data for published by Watt (1936). These data were combined with MSLP values obtained from the Bureau of Meteorology, Melbourne for most of the period from 1894 to A few missing values after 1895 were infilled from data published by World Weather Records (WWR). Apart from missing values for February 1880 and from July 1880 to May 1881 and September 1882 and March May 1892, the record is complete from January 1841 to the present. Figure 2 shows the annual mean MSLP values for this 156-year period. Figure 1. Location map of the principal sites of monthly mean-sea-level pressure (MSLP) mentioned in the text

3 SOUTHERN HEMISPHERE CIRCULATION INDICES 1303 Figure 2. Annual MSLP values for Hobart, The smooth line on this and subsequent figures highlights variations on the decadal timescale using a 10-year Gaussian filter 2.2. Stanley The first observations taken in the Falkland Islands were at Port Louis (about 51 30S, 58 W) by Ross (1847) during April August Observations were then taken at Cape Pembroke Lighthouse (CPL, S, W, 21 m) since 1850, with occasional gaps in observations (Scott, 1871). The latter site is within 10 km of the island capital of Stanley. According to Brooks (1920), however, the absence of adequate supervision and instruction means that the records were of little value until the visit of the Scotia (Mossman, 1907) in January 1903 provided the necessary stimulus. Despite the above comments, monthly MSLP data from CPL from 1895 onwards were published by Brooks (1920) and used in WWR. The CPL site continued recording until In 1923, continuous meteorological recordings were taken at Stanley (51 42 S, W, 21 m), monthly MSLP data being published for by Pepper (1954) and in WWR from 1951 onwards. Earlier observations have been published in the Falklands Islands Blue Book (copy available at the U.K. Meteorological Office Library). This site continued operating until March From July 1986 onwards the new site at Mount Pleasant Airfield (51 49 S, W, 240 m) has operated. Missing values from March 1982 to June 1986 were infilled from Australian operational charts (Jones, 1991). A complete series for Stanley/CPL can be easily developed from Prior to this date, Brooks (1920) gives a number of sources of meteorological readings in Stanley between 1875 and MSLP values for May 1875 December 1877 were taken by F.E. Cobb and published by Marriott (1880). It is presumed (Brooks, 1920) that this series continued, but no records seem to have been preserved. Data for August 1882 September 1883 taken at Port Stanley have been published by von Danckleman (1885). Brooks (1920) also notes that observations were published in the Falkland Islands Gazette, but these could not be located. In Falkland Islands Blue Book, 5 months of these are available for January May 1891 taken by a Jesuit priest, Father M.L. Mignone. Finally, observations were found for September 1872 May 1873 taken in Port Stanley (Dach, 1884). Summaries of the CPL readings for , considered to be of little value by Brooks (1920), were published by Scott (1871) as monthly MSLP summaries. These are based on four observations per day at 04:00, 09:00, 15:00 and 20:00 h. Initial inspection indicates that they appear reasonable, but frequent missing observations are noted. The original daily records could not be located in the Meteorological Office archives and they have probably been lost, together with the presumed incomplete records for 1850 June 1859 and August Indeed, because of the comments by Brooks (1920), reiterating an earlier assessment by Mossman (1907) the records could easily have been discarded. Searches of other potential archives in the UK (Scott Polar Research Institute library in Cambridge and Trinity House) and in the Falkland Islands have proved fruitless. The various series have been pieced together with some corrections necessary. The record is complete from 1895 onwards. Figure 3 shows the annual mean MSLP values for all complete years available from New Zealand region Continuous meteorological recording began in New Zealand (NZ) in the 1850s. In this study, MSLP records from three sites are used, Auckland (records begin 1853), Dunedin (1852) and Chatham Island

4 1304 P.D. JONES ET AL. Figure 3. Annual MSLP values for Stanley, Falkland Islands, (1878). All three sites require extensive adjustment factors to produce homogeneous series for their entire lengths of record. The corrections were extremely complex at Dunedin where there were numerous site changes. Details of the methods used to make the adjustments and the adjustment factors themselves at all sites are given in Fuohy et al. (1992) and Rhoades and Salinger (1993). Auckland is complete apart for missing data during the whole of Dunedin has a few missing months in 1886, 1900 and 1916 and All data sources (e.g. WWR) contain completely missing data for Chatham Island during the 1910s and 1920s. Recently these data were located in Wellington and a near complete series has been developed. The new series has 13 missing months in total occurring in the years 1881, , 1908 and The reason for the missing data in the 1910s and 1920s seems to be simply due to the monthly averages not being calculated at the station or in the manuscripts stored in Wellington. Figure 4 shows annual mean MSLP values for Auckland, Dunedin and Chatham Island Southern South America/Antarctic Peninsula (SAAP) region Apart from the Stanley record, two long MSLP records were developed for this region from Punta Arenas (1885) in southern Chile and Orcadas (1903) on the South Orkney Islands to the northeast of the Antarctic Peninsula (Figure 1). The source of the Punta Arenas record is WWR with a few missing values infilled from sources at the Chilean Meteorological Service. Longer MSLP data for the Argentinian site of Ushuaia on Tierra del Fuego (back to the 1870s) exist, but the record is completely missing for many years in the late 19th and early 20th centuries. The principal source of MSLP data for Orcadas is WWR Figure 4. Annual MSLP values for three sites in New Zealand: Auckland ( ), Dunedin ( ) and Chatham Island ( )

5 SOUTHERN HEMISPHERE CIRCULATION INDICES 1305 Figure 5. Annual MSLP values for Punta Arenas ( ) and Orcadas ( ) supplemented by Schwerdtfeger et al. (1959). The Orcadas record contains considerable missing data after the early 1960s. This has been infilled by using MSLP data from the nearby site on Signy Island, which began in Figure 5 shows annual mean MSLP values for Punta Arenas and Orcadas Transpolar index (TPI) 3. CIRCULATION INDICES As mentioned earlier the TPI was first proposed by Pittock (1980, 1984). More recent analyses of the TPI have included Carleton (1989) and Villalba et al. (1997). Both have suggested that the choice of the two sites may not be the optimal locations but this conclusion is undoubtedly dependent on the period analysed. Villalba et al. (1997) developed an alternative index, the difference between the average of five sites over New Zealand (Wellington, Christchurch, Chatham Island, Hokitika and Dunedin) and three in the south-western Atlantic (Stanley and locations on South Georgia (Grytviken) and the South Orkney Islands (Orcadas) stations about 5 8 south and east of Stanley). Because of the potentially greater variability at one of the two sites compared to the other, the calculation of the original Pittock TPI has traditionally been formed using monthly normalised values at both Hobart and Stanley. The monthly means and S.D.s are based on the period (see Table I for the S.D.s). Figure 6(a) shows the TPI (Hobart Stanley) on a seasonal basis using all available years of record. The record shows considerable year-to-year and decadal-scale variability, with the latter particularly evident in the austral summer and autumn seasons. Table I. S.D.s (hpa) of monthly pressure data for the period Hobart Stanley Auckland Dunedin Chatham Island Punta Arenas Orcadas January February March April May June July August September October November December

6 1306 P.D. JONES ET AL. This record is compared with that produced by Villalba et al. (1997) (available for the years ) and with the author s modified version of their series. The latter is based on the average of normalised values of five New Zealand stations (Auckland, Wellington, Hokitika, Chatham Island and Dunedin) minus three southern South American locations (Stanley, Orcadas and Punta Arenas). The New Zealand sites used here almost certainly represent an improvement on the series used by Villalba et al. (1997), because of the recent improvements to homogeneity mentioned earlier. Figure 6(b) shows the author s version of the Villalba et al. (1997) series (TPI(V)). The three series (the original Villalba et al. (1997), VILL, series not shown) are highly correlated with one another (Table II) in all seasons. Correlations are shown for the full period of record ( ) and for three subperiods ( , and ) which were defined based on studies in New Zealand (see later). Although these periods may be inappropriate in southern South America, they were chosen to test the variability of any relationship and for completeness with other analyses in this study. Correlations tend to be higher when the TPI(V) series is involved. Correlations fall to insignificant values before 1950 in the austral summer season when the VILL series is involved. VILL differs markedly from TPI and TPI(V) during the 1920s and 1930s, but only during the austral summer. The probable Figure 6. Seasonal values of The Trans Polar Index (TPI) based on MSLP data from Hobart and Stanley. Definition of the index is discussed in the text. The base period is

7 SOUTHERN HEMISPHERE CIRCULATION INDICES 1307 Figure 6 (Continued) cause of this is improvements to the New Zealand MSLP data, as the South American data is believed to be the same as used earlier, even allowing for the change of Grytviken with Punta Arenas in the TPI(V) series used here. In the Villalba et al. (1997) reconstruction based on tree-ring information from Tierra del Fuego and New Zealand, the poorest part of the calibration (their Figure 6) was during this time. Their reconstruction prior to 1895 shows negative values of the index during much of the nineteenth century and positive values from 1745 to Negative values imply lower pressure over New Zealand and higher pressure over southern South America and vice versa. Annual TPI values for all three versions were low in the austral spring and summer seasons of the 1920s and very low in the austral winters and summers of the 1940s, with an increase to their most positive values during the period Since 1978, values have declined. On average summer trends tend to reflect the annual trends, but are more marked. These decadal trends coincide with stronger westerly and southwesterly flow in the New Zealand region in the 1940s (Salinger and Mullan, 1999). The period shows more positive values caused by lower pressures in the South American sector. Subsequently, TPI values have declined, with particularly low austral winter values following the Mt. Pinatubo volcanic eruption in 1991 (for a discussion of the circulation effects in the New Zealand sector, see Salinger (1998)).

8 1308 P.D. JONES ET AL. Table II. Correlations between the three TPI series by seasons over various periods a b b b Winter (JJA) TPI vs. TPI(V) TPI vs. VILL VILL vs. TPI(V) Spring (SON) TPI vs. TPI(V) TPI vs. VILL VILL vs. TPI(V) Summer (DJF) TPI vs. TPI(V) TPI vs. VILL 0.21* 0.18* VILL vs. TPI(V) 0.23* 0.14* Autumn (MAM) TPI vs. TPI(V) TPI vs. VILL VILL vs. TPI(V) a All correlations are significant at the 95% level apart from those starred. In the assessment of significance, the effective number of independent samples was estimated using the autocorrelation of the two series (see also the approach of Ebisuzaki (1997)). b The VILL series is only available for New Zealand region The most comprehensive study of circulation indices in this region is that of Trenberth (1976), who defined several zonal and meridional indices. Here two of these are adapted, a zonal index (ZNZ) defined as the pressure difference between Auckland and Dunedin and a meridional index (MNZ) defined by the pressure difference between Hobart and Chatham Island. Both indices are calculated by using MSLP anomalies from the 1951 to 1980 period, the average values of the indices over this period being zero. The normalising step used for the TPI has not traditionally been made (Trenberth, 1976) as the seasonal cycle of variability between the two locations is similar (see Table I for the S.D.s of the key stations). Figure 7 shows seasonal values of ZNZ for and Figure 8 seasonal values for MNZ for ZNZ is a measure of the zonal westerlies between approximately 36 and 46 S while MNZ measures meridionality across the Tasman Sea and New Zealand (147 E 175 W). Defining indices in this way means that their variability is related to the distances between the stations. Normalisation would enable direct comparisons of variability to be made, but the authors have followed the original Trenberth (1976) method. This fact should be borne in mind when considering these series and with comparisons with TPI. In Figure 7 (ZNZ) stronger westerlies are positive and weaker westerlies negative relative to the period. For ZNZ, a 1 hpa difference between stations at these latitudes corresponds to a geostrophic westerly difference of about 0.8 m s 1. All seasons before about 1880 indicate weaker westerlies, an indication that may be erroneous if the necessary adjustments to the MSLP series for Auckland and/or Dunedin are inadequate. Inspection of Figure 4 and additional, but less complete, MSLP series for Wellington, Hokitika, Christchurch and New Plymouth gives some cause for concern that the MSLP data for Auckland may be slightly low and Dunedin slightly high before about These additional sources are less complete than either Auckland or Dunedin, thus it is difficult to draw any firm conclusions whether all the necessary adjustments to ensure homogeneity have been made. For this reason, discussion of the variations in Figure 7 is restricted to periods since about Since 1880, both interannual and interdecadal variability are lower during the austral autumn and winter seasons than for summer and spring. This zonal index shows some decadal-scale variability.

9 SOUTHERN HEMISPHERE CIRCULATION INDICES 1309 Easterlies were more predominant before 1900 and in the 1910s particularly in winters and springs of the late nineteenth century. Easterlies were also more dominant in the 1930s. Stronger westerlies occurred in the 1940s, particularly in summer. The period 1951 to the mid-1970s is one of neither strong westerly nor easterly flow. The period since the mid-1970s is when westerly circulation across New Zealand is the strongest on record, and has occurred in all seasons except spring In Figure 8 (MNZ), positive values indicate stronger southerly flow anomalies over the Tasman Sea and New Zealand with negative values implying stronger northerly flow anomalies. Meridional flow anomalies show some quite distinct periodicities. Generally the period from the 1880s until 1950 is one of more marked southerly circulation anomalies in the New Zealand region, apart from a few years near 1895, 1907, 1917 and The period from is marked by more prevalent northerly circulation anomalies in most seasons, except winter, before a return to stronger southerly flow anomalies subsequently. These periodicities relate very well to shifts noted by Salinger and Mullan (1999) in New Zealand climate. They noted the period from 1930 to 1950 was one of more southwesterly airflow anomalies over New Zealand. The years are marked by slightly more easterly and northeasterly airflow anomalies, before a change to stronger westerly and southwesterly anomalous flow over the country. Figure 7. Seasonal values of the zonal pressure index over New Zealand (ZNZ) based on the MSLP anomaly difference between Auckland and Dunedin. The base period for the anomalies is

10 1310 P.D. JONES ET AL. Figure 8. Seasonal values of the meridional pressure index over the Tasman Sea and New Zealand (MNZ) based on the MSLP anomaly difference between Hobart and Chatham Island. The base period for the anomalies is These changes have caused marked precipitation variations because of the interaction of the circulation with the southwest northeast trending axial mountain ranges. The strength of meridional flow would be expected to correlate with temperatures. Seasonal values of the temperature series (NZtemp, Salinger (1980), updated) are shown in Figure 9. Two major changes in NZtemp have been identified in many earlier studies (see references in Salinger and Mullan (1999)). Correlations are, therefore, calculated over four periods, , , and The three subperiods cover a period of slightly warming (during which the average NZtemp= 0.56 C), strong warming (NZtemp= 0.08 C) and little change (NZtemp=0.09 C), respectively (see Table IIIa and Figure 9). Correlations (Table IIIb) between NZtemp and MNZ are strongest in the and periods. It is clear from Figures 8 and 9 that MNZ only explains a small part of the long-term rise in temperatures, the weaker correlations over the period attesting to this. Meridional flow (MNZ) has been more southerly in the recent period, particularly compared to but temperatures in all seasons have been warmer than in Sea surface temperatures to the south and southwest of New Zealand have been warming over the twentieth century (Jones and Allan, 1999), bringing less cold but stronger airflow than earlier in the century. Some of the strong warming during was partly due to the prevalence of northerly airstreams.

11 SOUTHERN HEMISPHERE CIRCULATION INDICES 1311 The changes in meridional flow have impacted precipitation patterns across New Zealand (Salinger and Mullan, 1999). During the period , there was an increase in precipitation in the north and east of the North Island, and north of the South Island. At the same time, precipitation decreases occurred in the west and south of the South Island. Since 1976, precipitation has decreased in the north of the North Island, whilst it has increased in the west and south of the South Island, in response to the increased westerly circulation across New Zealand. For precipitation, the strength of the zonal flow is important but ZNZ is only weakly correlated with NZtemp. The strongest correlations (r ) occur during the austral winter when stronger zonal flow leads to milder winters. Across New Zealand, correlations between ZNZ and local temperature are greater on the eastern side of the islands because of the föhn effect induced by the mountain ranges. Seasonally, correlations on the western coasts will be stronger in summer and winter, with higher values of ZNZ implying colder and warmer conditions, respectively. Correlations between NZtemp and the three TPI series are weak and extremely variable from season to season and period to period. The strongest correlations are in the summer season (r depending on period) when positive values of the TPI indicate warmer temperatures. This is the relationship that enabled Villalba et al. (1997) to reconstruct the TPI from southern New Zealand tree-ring series. Correlations between TPI and the two circulation indices over New Zealand (ZNZ and MNZ) are also Figure 9. Seasonal values of average New Zealand temperature (Salinger (1980) updated). The base period for the anomalies is

12 1312 P.D. JONES ET AL. Table IIIa. Average seasonal values of New Zealand temperature (NZtemp) and MNZ for different time periods a Winter (JJA) NZtemp MNZ Spring (SON) NZtemp MNZ Summer (DJF) NZtemp MNZ Autumn (MAM) NZtemp MNZ a Temperature values in C with respect to and MNZ in hpa relative to relatively weak. TPI correlates with MNZ (r ) depending on season and period, while ZNZ is weakly negatively correlated (r 0.2 to 0.3) Southern South America/Antarctic Peninsula Circulation indices in this region have been less extensively studied than in New Zealand. Mayes (1981) developed some indices and other work has been undertaken by Rogers and van Loon (1982), Rogers (1983) and Jones (1991) as part of large-scale Southern Hemispheric studies. Here, a zonal and meridional index for the region are developed, which are refered to as ZSAAP and MSAAP, respectively. ZSAAP is defined as the anomalous pressure difference between Stanley and Orcadas, measuring the westerly wind strength between 51 and 60 S in the W zone. Table I also gives the S.D.s for the key sites in this region. MSAAP is defined as the anomalous pressure difference between Punta Arenas and Stanley, measuring meridional flow in the W region. As with the NZ series, the anomalies are defined with respect to the period. Figures 10 and 11 show seasonal values of ZSAAP and MSAAP, respectively. The zonal pressure difference series exhibits considerably more variability both interannually and decadally than MSAAP, even allowing for the anomalous values of MSAAP in winter and spring The location of the stations should be borne in mind here. ZSAAP would be expected to have more variability than MSAAP, because the stations are further apart. On an annual basis, longer timescale variations of ZSAAP are similar to those apparent in the winter season, although at certain times there are clearly differences between the seasons. Annually, westerlies were at their strongest during the 1950s and early 1960s and to a slightly lesser degree since the mid-1970s. Westerlies were weakest during the period before 1950, especially the 1910s, the mid-1920s and the mid-1930s and the period. Meridional flow (MSAAP) indicates stronger southerly flow Table IIIb. Seasonal correlations between average New Zealand temperatures and MNZ for different time periods Winter (JJA) Spring (SON) Summer (DJF) Autumn (MAM) All correlations significant at the 95% level, after allowing for serial correlation in the two series.

13 SOUTHERN HEMISPHERE CIRCULATION INDICES 1313 Figure 10. Seasonal values of the zonal pressure index between southern South America and the Antarctic Peninsula (ZSAAP) based on the MSLP anomaly difference between Stanley and Orcadas. The base period for the anomalies in (positive values) during the 1920s, and since about Northerly flow was strongest during the 1900s, the late 1910s, the early 1930s and the 1980s. To assess the importance of the circulation on temperature variability in this region, a temperature series for this region (45 60 S, W) is developed from the Jones et al. (1999) dataset. This series is shown seasonally from 1895 in Figure 12. The grid-box data used here includes anomalies of land stations and sea surface temperatures. The series exhibits century timescale warming of about 0.8 C with marked but short warm episodes during the late 1910s and the early 1940s. Average seasonal temperature values for the same four time periods used before are given in Table IVa. In Table IVb, correlations between MSAAP and the temperature series (SSAtemp) are significant in most seasons and periods, but are markedly lower than in New Zealand, possibly because of the inclusion of sea surface temperature data. Correlations are strongest for the period and weakest for As with the New Zealand region, changes in meridional wind strength do not explain the long-term increase in temperature, particularly the rise since the mid-1970s. In the austral summer and autumn seasons significant correlations (r depending on the period) between SSAtemp and the three TPI series are evident. Negative values of the TPI (higher pressure in southern South America) lead to warmer temperatures and vice versa. This is the correlation

14 1314 P.D. JONES ET AL. that enabled Villalba et al. (1997) to reconstruct the TPI from temperature sensitive trees in Tierra del Fuego. Correlations between the TPI and MSAAP and ZSAAP are slightly weaker than their counterparts in the New Zealand sector. MSAAP is inversely correlated with TPI (r 0.3 to 0.4) with ZSAAP correlations rarely reaching significant levels. Correlations between the meridional and zonal pressure indices over New Zealand and southern South America are very weak and barely reach statistical significance in some seasons for some periods. 4. CONCLUSIONS The authors have extended and developed a number of regional circulation indices in the New Zealand and southern South American sectors of the SH. In both sectors, average temperature series for the two areas are very significant (New Zealand, r 0.4 to 0.7 depending on the period and season) and significantly (southern South America, r 0.2 to 0.4) related to meridional pressure indices. Assessment over different time periods indicates the relationships are principally on interannual, rather than longer, timescales. Meridional gradients are unable to explain the long-term increase in temperature. Figure 11. Seasonal values of the meridional pressure index over the extreme tip of southern South America (MSAAP) based on the MSLP anomaly difference between Punta Arenas and Stanley. The base period for the anomalies is

15 SOUTHERN HEMISPHERE CIRCULATION INDICES 1315 Figure 12. Seasonal values of average temperature for southern South America (SSAtemp, S, W from the Jones et al. (1999) dataset). The base period for the anomalies is In both regions meridional flow has been more southerly recently (over New Zealand especially since 1976 and southern South America since 1988), yet annual temperatures are higher for the period by 0.52 and 0.28 C, respectively, than for the period Southerlies in both regions are, therefore less cold and/or northerlies warmer, a result of warmer sea surface temperatures over the mid-to-high latitude Southern Oceans. Folland and Salinger (1995) note that nearby ocean surface temperatures warmed by about 0.7 C between 1871 and 1993 in the New Zealand region. In both regions, zonal pressure series have also been developed. These series are only weakly correlated with the temperature series. In New Zealand, Salinger and Mullan (1999) show strong relationships between more local regional precipitation series and the zonal pressure index over New Zealand (ZNZ). These relationships were not investigated in this paper. It is expected that similar strong relationships will exist over southern South America. Pittock (1980, 1984) proposed a large-scale index (the Trans Polar Index, TPI) of the SH circulation, which he defined as the normalised pressure difference between Hobart, Tasmania and Stanley, Falklands. The authors have updated and compared this series with variants involving other MSLP stations in New

16 1316 P.D. JONES ET AL. Table IVa. Average seasonal values of southern South America temperature (SSAtemp) and MSAAP for different time periods a Winter (JJA) SSAtemp MSAAP Spring (SON) SSAtemp MSAAP Summer (DJF) SSAtemp MSAAP Autumn (MAM) SSAtemp MSAAP a Temperature values in C with respect to and MSAAP in hpa relative to Zealand and southern South America. It is shown that positive values of the TPI relate to warmer temperatures over New Zealand (only in the summer) and cooler temperatures over southern South America (all seasons) and vice versa. TPI temperature relationships are much weaker than the local meridional gradient relationships over New Zealand but of a similar magnitude to those over southern South America. The decadal and longer timescale trends in TPI show some movement in the displacement of wavenumber one around the SH. Troughing (low pressure) was more frequent in the New Zealand region in the 1920s and at a maximum in the 1940s. Anticyclonicity was favoured from the late 1950s to 1976, with troughing in the South American sector. Troughing was again apparent in the New Zealand sector in the 1990s. The more frequent ridges near New Zealand in the middle period are consistent with the strong warming observed during this period. However, no temperature decrease is seen in the last period when more frequent troughs occurred. The troughs of the 1990s near New Zealand relate well to the circulation pattern induced by climate forcing induced by explosive volcanic eruptions (Salinger, 1998). The TPI series is complete from Hobart MSLP data are only missing for a couple of years in the early 1880s and extend back to Although a barometer was in use near Stanley around this time and records appear to have been taken for much of the period, most of the pre-1895 records appear to have been discarded. Table IVb. Seasonal correlations between average temperatures over southern South America (45 60 S, W) and MSAAP for different time periods Winter (JJA) * Spring (SON) 0.18* 0.18* * Summer (DJF) * * Autumn (MAM) * All correlations significant at the 95% level, after allowing for serial correlation, except for those with the asterisk.

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