Interannual Variability of the South Atlantic High and rainfall in
Southeastern South America during summer months
Inés Camilloni1, 2 , Moira Doyle1 and Vicente Barros1, 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 25ºS-35ºS 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 20ºS-30ºS 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 25ºS-35ºS, 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.
-20
-20
-25
-25
-30
-30
-35
-35
-40
-40
-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
FL1
Lineal (FL1)
98
19
94
96
19
90
92
19
19
19
88
19
86
19
84
19
80
82
19
78
FL2
19
19
76
19
74
19
72
19
68
66
64
70
19
19
19
19
19
62
0
Lineal (FL2)
Figure 2: Summer SLP factor loadings (FL) 1 and 2 with their linear trends.
a)
-10
-15
-20
-25
-30
-35
-40
-75
-70
-65
-60
-55
-50
-45
-40
-70
-65
-60
-55
-50
-45
-40
b)
-10
-15
-20
-25
-30
-35
-40
-75
Figure 3: Linear Correlations of summer rainfall with a) FL1 and b) FL2
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