Interannual Variability of the South Atlantic High and rainfall in Southeastern South America during summer months Inés Camilloni 1, 2, Moira Doyle 1 and Vicente Barros 1, 3 1 Dto. Ciencias de la Atmósfera y los Océanos. Universidad de Buenos Aires. 2 CIMA (CONICET-UBA) 3 CONICET ABSTRACT Monthly SLP patterns over Southeastern South America were analyzed to explore nearsurface circulation changes and their connection with the observed positive rainfall trends during the last 40 years. Principal component analysis was performed on monthly SLP means obtained from the NCEP/NCAR reanalysis. The first (PC1) and second (PC2) spatial patterns account for almost 90% of the total variance. PC1 represents the winter fields dominated by the South Atlantic High (SAH) in its northernmost position while PC2 is the typical summer field with the SAH displaced to the south and the presence of the northwestern Argentinean Low. The corresponding temporal series FL1 shows a negative linear trend and FL2 a positive one indicating a change in the annual SLP cycle over the region. This analysis confirms the results obtained in a previous work showing a southward shift of the western border of the SAH since 1950. Summer correlation fields were calculated between rainfall anomalies and the first two SLP principal components. Statistically significant negative correlations are found in the eastern region in the band 25ºSºS indicating a reduction in the rainfall amounts when the SAH displaces southward (PC2) while positive correlations (positive rainfall anomalies) are found in the continental South Atlantic Convergence Zone (SACZ) region and in the western band of Argentina between 30ºS and 40ºS. Almost the opposite correlation fields are found when considering PC1 (the SAH in its northernmost location): an increase in the rainfall amounts over the region comprising the east of Argentina, Uruguay and south Brazil and negative rainfall anomalies in the SACZ region and in the west of Argentina. Larger significant correlations are found when considering in the analysis only those years with the FL1 and FL2 values larger than one sigma indicating the strong association between the position of the SAH and regional rainfall anomalies. This result is quite relevant to prepare the future regional climate scenarios as at least four global climate models (HADCM3, ECHAM4/OPYC3, GFDL-R30 and CSIRO-mk2) agree to show that the southward displacement tendency of the western border of the SAH will continue in the future at least until 2099 (Barros et. al, 2003). Under this circulation scenario, it can be
expected positive rainfall trends in the SACZ region and the west of Argentina and negative trends in the eastern region of South America between 22.5ºS and 32.5ºS during the next decades. Introduction Most of tropical and subtropical South America receives more than 50% of the total annual precipitation in the austral summer season in the form of convective rain (Figueroa et al. 1995, Gandu and Silva Dias 1998). Nevertheless, the interannual variability of summer precipitation is large, with interannual standard deviations of monthly means at individual stations often greater than half the monthly average (Barros et al. 2000). The interannual rainfall variability in subtropical South America is related to the ENSO (El Niño/Southern Oscillation) phenomenon (e.g., Ropelewski and Halpert 1987, 1996; Grimm et al. 2000), with enhanced precipitation during the warm phase. Other authors related the interannual variability of precipitation in this region with the tropical convection in central Brazil (González and Barros 1998) and the sea surface temperature (SST) variability of the south Atlantic and Pacific oceans (Diaz et al. 1998, Barros et al. 2000, Barros and Silvestri 2002). Zhou and Lau (2001) conducted a diagnostic study of the interannual and decadal variability of summer rainfall over South America (SA) and identified the ENSO influence at the interannual time scale associated to an enhancement of the South American summer monsoon (Zhou and Lau, 1998) in response to an El Niño anomaly. The influence of SST anomalies in the Atlantic Ocean on precipitation in southeastern South America has been examined in recent years. For example, Díaz et al. (1998) investigated the annual cycle of precipitation in Uruguay and Southern Brazil and found links between its anomalies and those in SST in the southwestern Atlantic Ocean. Díaz et al. (1998) results support the existence of relationships between wet/dry rainfall anomalies in the northern sector of Uruguay and southern Brazil and warm/cold SST anomalies in the South Atlantic convergence zone (SACZ) region and the equatorial Atlantic in the November-February period. Barros et al. (2000) found that, during summer and principally in January, Southeastern South America rainfall is related to both the intensity and position of the SACZ as well as to the SST of the neighboring Atlantic Ocean. Doyle and Barros (2002) analyzed the relation between the midsummer low-level circulation and precipitation in Southeastern South America and the SST anomalies in the South Atlantic. They found that the strongest relation between precipitation and SST is determined by the anomalies in the Atlantic region define by 20ºSºS and 30ºW-50ºW. Recently, Paegle and Mo (2002) analyzed the linkages between summer rainfall variability over SA and global SST anomalies but in contrast to the studies mentioned previously the focus of their analysis are the links between SST and continental rainfall modes rather than regional patterns of precipitation.
The latitude of the axis of the maximum sea level pressure (SLP) in the southwestern border of the SAH varies throughout the year. In winter this axis reaches the South American coast at 27 S. On the other hand, during summer there are two axes of maximum pressure, one reaching the coast at about 17 S and the other one near 37 S. The annual cycle of climate in eastern SA is related to this seasonal shift of the SLP field. For instance, in the band between 30 and 35 S, the mean zonal component of the surface wind changes from west in winter to east in summer. In the case of precipitation over eastern SA, it is caused by the convergence of the water vapor brought by the low-level atmospheric circulation that comes either from the tropical continent or from the tropical Atlantic (Wang and Paegle 1996, Doyle and Barros 2002). In both cases this circulation is under the influence of the western border of the SAH, and therefore, trends in the mean latitude of this system might be related to regional climate changes. The objective of the present study is to explore how the interannual variability of the position of the SAH affects rainfall variability in Southeastern South America during summer. Results Monthly SLP patterns over Southeastern South America were calculated and analyzed to explore near-surface circulation changes during the last 40 years. Principal component (PC) analysis was performed on monthly SLP means obtained from the NCEP/NCAR reanalysis. The first (PC1) and second (PC2) spatial patterns (figure 1) account for almost 90% of the total variance (49.1% and 38.3% respectively). PC1 represents the winter fields dominated by the SAH in its northernmost position while PC2 is the typical summer field with the SAH displaced to the south and the presence of the northwestern Argentinean Low. FL1 shows a negative linear trend and FL2 a positive one indicating a change in the annual SLP cycle over the region (figure 2). This analysis confirms the results obtained in previous works (Camilloni 1999, Barros et al. 2003) showing a southward shift of the western border of the SAH since 1950. Total monthly precipitation data from stations between 10ºS and 40ºS, available from National Weather Services, ANEEL and NCDC, between 1961 and 1999, were gridded into 3º by 3º boxes. Summer correlation fields were calculated between rainfall anomalies and the first two SLP principal components (figure 3). This season presents negative correlations in the eastern region of the band 25ºSºS, indicating a reduction in the rainfall amounts when the SAH displaces southward (PC2), while positive correlations (increased rainfall) are found in the continental South Atlantic Convergence Zone (SACZ) region and in the western band of Argentina between 30ºS and 40ºS. Almost the opposite correlation fields are found when PC1 is considered (the SAH in its northernmost location): an increase in the rainfall amounts over the region comprising the east of Argentina, Uruguay and south Brazil and negative rainfall anomalies in the SACZ region and in the west of Argentina. Larger significant correlations are found when
considering in the analysis only those years with the FL1 and FL2 values larger than one sigma indicating the strong association between the position of the SAH and regional rainfall anomalies. -45-45 -65-60 -55-50 -45-65 -60-55 -50-45 Figure 1: Summer SLP Principal Components 1 (left) and 2 (right) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 FL1 FL2 Lineal (FL1) Lineal (FL2) Figure 2: Summer SLP factor loadings (FL) 1 and 2 with their linear trends.
a) -10-15 -75-70 -65-60 -55-50 -45 b) -10-15 -75-70 -65-60 -55-50 -45 Figure 3: Linear Correlations of summer rainfall with a) FL1 and b) FL2
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