Precipitation trends in southeastern South America: relationship with ENSO phases and with low-level circulation

Rainfall trends in subtropical South America east of the Andes during the period 1960-1999 were studied in connection with El Nino-Southern Oscillation (ENSO) phases. In addition, monthly mean sea level pressure (SLP) fields from the National Center for Environmental Prediction reanalysis were used to look at the connection between rainfall trends and the interannual variability of the low-level atmospheric circulation. South of 22 degrees S, annual rainfall trends were positive everywhere, and statistically significant over part of western subtropical Argentina. Over most of the region, trends resulted mostly from the warm semester (October-March) trends with little contribution from the cold semester (April-September). Under certain conditions, linear precipitation trends of each ENSO phase add up to the total rainfall trend. In northeastern Argentina, southern Brazil and Paraguay, half of the annual rainfall trend came from El Nino phase with a minor contribution from La Nina and the neutral phase. However, in most of subtropical South America, east of the Andes, precipitation trends during the extreme phases of the ENSO constitute only a small part of the trends. A Principal Component Analysis (PCA) was applied to the monthly SLP fields of the austral warm semester. The mode that advects humid air from the Atlantic Ocean over most of Argentina and Uruguay has grown at the expense of the one that favors the flow from the tropical forest over the eastern part of the continent. South of 30 degrees S, annual rainfall trends took place predominantly during the neutral phase of the ENSO, and were related to the trends of the first two SLP leading modes. On the other hand, the annual trends were negative in the continental part of the South Atlantic Convergence Zone (SACZ), but with contributions from all ENSO phases.