Principal modes of interannual and decadal variability of summer rainfall over South America

Abstract

Using the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) product together with the Goddard Earth Observing System (GEOS) reanalysis and the National Center for Environmental Prediction (NCEP) sea‐surface temperature (SST) data, we have conducted a diagnostic study of the interannual and decadal scale variability of principal modes of summer rainfall over South America for the period 1979–1995. By filtering the annual and short (<12 months) time‐scale variations, results of empirical orthogonal function analysis show three leading modes of rainfall variation identified with interannual, decadal and long‐term variability. Together, these modes explain more than half the total variance of the filtered data. The first mode is highly correlated with El Niño–Southern Oscillation (ENSO), showing a regional rainfall anomaly pattern largely consistent with previous results. This mode captures the summer season interannual variability, not only the Northeast Brazil drought but also its connection with excessive rainfall over Southern Brazil and the Ecuador coast in El Niño years. Another distinctive feature is the strengthening of the low‐level flow along the eastern foothills of the eastern Andes, signifying an enhancement of the South American summer monsoon in response to an El Niño anomaly. The decadal variation displays a meridional shift of the Inter‐Tropical Convergence Zone (ITCZ), which is tied to the anomalous cross‐equatorial SST gradient over the Atlantic and the eastern Pacific. Associated with this mode is a large‐scale mass swing between polar regions and the mid‐latitudes. Over the South Atlantic and the South Pacific, the anomalous subtropical high and the associated anomalous surface wind are dynamically consistent with the distribution of local SST anomalies, suggesting the importance of atmospheric forcing at the decadal time scale. The long‐term variation shows that since 1980 there has been a decrease of rainfall from the northwest coast to the southeast subtropical region and a southwards shift of the Atlantic ITCZ, leading to increased rainfall over northern and eastern Brazil. Possible links of this mode to the observed SST warming trend over the subtropical South Atlantic and to the interdecadal SST variation over the extratropical North Atlantic are discussed. Copyright © 2001 Royal Meteorological Society