Linkages between Summer Rainfall Variability over South America and Sea Surface Temperature Anomalies

Abstract

A reconstructed rainfall dataset, and satellite estimates are used to analyze interannual to decadal variability of austral summer precipitation over South America. Rotated empirical orthogonal function (REOF) analysis is applied to isolate dominant patterns of rainfall. Links of these patterns to sea surface temperature anomalies (SSTAs) are examined. The leading mode is related to El Niño-Southern Oscillation (ENSO), which explains 12% of the total variance. During warm ENSO events, the positive phase of this mode shows dry conditions over northern South America and wet conditions over the subtropical plains between 25Њ and 35ЊS. The situation reverses during cold events. The second REOF 2, which explains about 10.8% of the total variance, consists of positive loadings over northeast Brazil centered at 50ЊW near the equator and negative loadings over Colombia and the subtropical plains. For December-January-February (DJF), REOF 2 is influenced by tropical South Atlantic SSTAs through displacements of the intertropical convergence zone. Northeast Brazil receives most rainfall in March-April-May (MAM) and it is modulated by both the Atlantic SSTAs and ENSO. In the interannual frequency band, the North Atlantic Oscillation (NAO) has very limited influence on rainfall. On the decadal timescales, the NAO leads REOF 2 by three years. Latitudinal variations of tropical convection are through the joint contribution of REOF 2 and REOF 4. REOF 4 is similar to REOF 2, but centers are displaced about 10Њ south. When these two EOFs are both positive, central South America is wet. The amplitudes of REOF 2 and REOF 4 are small during the mid-1950s to the mid-1960s and they are out of phase from 1968 to 1970, periods with persistent dry conditions over the upper La Plata River basin.

The impact of the subtropical South Atlantic SST on South American precipitation

“…Moreover, during Pacific warm events, the enhanced subtropical jet acts like a barrier, inhibiting frontal systems to go northward over the continent, and confining them to the south. Paegle and Mo (2002) suggested that the ENSOrelated PSA wave train increases the pressure gradient between the Chaco low and the Atlantic subtropical high, contributing to an increased supply of moisture and enhanced precipitation over eastern Brazil.…”

“…Kalnay et al, 1986;Garreaud and Battisti, 1999;Mo, 2000;Cavalcanti, 2000;Paegle and Mo, 2002). In general, the anomalous warm water associated with ENSO events induces changes in the Hadley and Walker circulations, generating Rossby wave trains that affect the South American rainfall (Grimm, 2003).…”

Inter-ocean fluxes south of Africa in an eddy-permitting model

“…Because the South Atlantic plays a key role in the energy transport towards the North Atlantic and influences the climate over South America, a better understanding of the basin-scale SST variability is required. Several studies (Paegle and Mo, 2002, and references therein) have diagnosed strong links between rainfall variability over South America and SST conditions in the South Atlantic. Recently, Robertson et al (2002) examined the atmospheric response to oceanic anomalies in the tropics and subtropics of the South Atlantic based on AGCM simulations and found strong, statistically significant signals on the atmospheric lowlevel circulation and precipitation on interannual time scales.…”

The South Atlantic dipole and variations in the characteristics of the South American Monsoon in the WCRP-CMIP3 multi-model simulations

“…During ENSO warm phase convection intensifies over western tropical South America with consequent suppression over eastern tropical South America (e.g. Zhou and Lau 2001;Paegle and Mo 2002). Grimm et al (2007) showed that during the austral spring El Niño events induce subsidence over the Amazon and eastern Brazil, decreasing moisture convergence and precipitation in these regions.…”