ALTERNATING WET AND DRY CONDITIONS OVER SOUTH-AMERICA DURING SUMMER

Time series of outgoing longwave radiation (OLR) fields and various gr idded reanalysis products are used to identify and describe periods wi th abundant and deficient rainfall over South America during summer. E mpirical orthogonal function analyses of OLR anomalies filtered to ret ain variations longer than 10 days reveal a meridional seesaw of dry a nd wet conditions over tropical and subtropical South America. It appe ars that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of S outh America. In contrast, when the SACZ weakens, precipitation over t hese plains is abundant. These results are in agreement with those of Kousky and Casarin. This seesaw pattern appears to be a regional compo nent of a larger-scale system, possibly related to the 30-60-day oscil lation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central a nd eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico , and in the ITCZ over the North Atlantic. In the opposite phase there is a strong influx of moisture from the Tropics into central Argentin a and southern Brazil. The moisture influx is enhanced by a strong low -level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs ov er the Great Plains of North America.