TEMPORAL AND SPATIAL VARIABILITY OF DAILY INTENSITY RAINFALL AND IT RELATION WITH SURFACE ATMOSPHERIC CIRCULATION IN SOUTHEASTERN SOUTH AMERICA

FEDERICO ARIEL ROBLEDO; OLGA C. PENALBA

Melbourne

Conferencia; 9th International Conference on Southern Hemisphere Meteorology and Oceanography.(9ICSHMO); 2009

American meteorological Society

The objectives of this paper are to identify the different spatial patterns and temporal variability of the intensity of extreme rainfall in on La Plata Basin and their relationship with atmospheric circulation. Two data bases were used: a) monthly fields of 1000 hPa geopotential height in the domain 20°S and 40ºS and 67,5ºW and 2,5ºW from NCEP Reanalysis and b) high quality daily rainfall for 40 stations, covering parts of Argentina, Uruguay, Paraguay and southern Brazil. The 1978-2006 period is taken as the longest common period between both data bases. The seasonal index of daily intensity of extreme rainfall (DIER index, rainfall greater than 75th daily percentile) and seasonal 1000 hPa geopotential height were calculated. Only during autumn, DIER index evidences positive significant trend in the whole region. Spatial patterns of both climatic variables were examined using Principal Component (PC) analysis. For DIER index, the first 4 PCs account for 70% of the total variance while for the 1000 hPa geopotential height, the first 3 PCs explain more than 90% of the total variance. In autumn, the four spatial pattern (PC4) of DIER index increases during El Niño years. For PC analysis of 1000 hPa geopotential height, autumn circulation shows an alternate combination of the first (summer) and second (winter) spatial patterns. However, in El Niño years, the third pattern (PC3) is more intense during autumn, indicating a shift to the east of the South Atlantic High pressure respective to the summer and winter circulation. The main result of the joint variability between circulation patterns and DIER index is during autumn, in the El Niño years, evidencing a positive correlation between PC3 of geopotential height and the PC4 of DIER index. This result explains part of the DIER autumn positives trend.