Seasonal anomalous rainfall in the central and eastern Amazon and associated anomalous oceanic and atmospheric patterns

Abstract

Previous papers have shown that the rainfall anomalies over the Amazon region result from the independent or combined effects of the eastern tropical Pacific and Atlantic Basins. Here, the monthly rainfall data of Itaituba, Manaus, Parintins, and Santarém for the 1931-1996 period has been used to classify its dry and rainy seasons into dry, very dry, wet, and very wet categories. The sea surface temperature (SST) and sea level pressure (SLP) patterns associated with these climate conditions in the central and eastern Amazon are discussed in terms of the relative influence of the tropical Atlantic and Pacific sectors, as well as their seasonal differences. These seasonal differences are, in part, determined by the seasonal phase locking of the El Niño-Southern Oscillation (ENSO) and Atlantic equatorial modes. The rainy season cases are mostly driven by the ENSO and the inter-hemispheric SST gradient mode in the tropical Atlantic, and the dry season cases, by the Atlantic equatorial mode (AEM). A new result here is the role of the inter-Pacific-Atlantic anomalous SST gradient mode on the rainfall anomalies. This mode acts concordantly with the Atlantic modes on the rainfall over the central and eastern Amazon, and is responsible for precipitation extreme cases during the dry season and for the very wet rainy season case. In other words, the intense inter-Pacific-Atlantic mode for these cases is the differential factor in relation to the moderate cases. The results here, especially in relation to the role of the inter-Pacific-Atlantic gradient in the central and eastern Amazon precipitation, have not been discussed before and should be taken into consideration in the diagnostic activities. © 2011 Royal Meteorological Society.