Interdecadal rainfall variability in subtropical South America and its relationship with tropical Pacific SST

during fall. During the last decade there has been an increased interest on interdecadal climate variability studies of the ocean-atmosphere system around the globe, especially over the North Pacific and North America. However, few works have focused on the Southern Hemisphere.

In the present study the interdecadal rainfall variability over subtropical South America (25°S-40°S) is analyzed by means of principal component analysis, using 79 rainfall stations with monthly records from 1918 to 1989. Furthermore, the 6-yr high-pass filtered Cold Tongue index (CT*) and the residual of the global principal component of SST (GR) are used to study the seasonality of the relationship between SST and rainfall on interannual and interdecadal time scales, respectively.

Rainfall time series were normalized and smoothed by a 6-yr low-pass filter. Principal component analysis shows that the first mode (30% of the total variance) presents a coherent signal over the southeastern South America (eastern Argentina and Uruguay) with a positive trend beginning in the early 50's. The second mode (19% of the total variance) shows a coherent signal over central Chile (30°S-36°S) and southern Brazil. In this case, the principal component shows a negative trend up to the late 60's, that reverts after the early 70's.

Seasonality of the relationship between tropical Pacific SST and rainfall variability at interannual and interdecadal time scales is analyzed using correlation between rainfall (on a bimonthly basis) and CT* and GR, respectively. This analysis shows a similarity between the correlation maps for both ENSO-cycle (interannual variability) and interdecadal variability. That is the interdecadal rainfall variability shows an ENSO-like pattern. This relationship is of the warm-wet/cold-dry type in southeastern South America (southern Brazil, Paraguay, Uruguay and eastern Argentina) during spring, moving westward from spring to early summer. A similar relationship appears during late winter in subtropical Chile and southern Brazil. In Chile a southward propagation of the area with significant correlation is observed from winter to spring. Finally, a significant positive correlation on the interdecadal time scale appears exclusively over the southern Brazil and Uruguay