Relationship between water vapor sources and rainfall over southern South America.

It is analyzed the entrance of water vapor over the continent throughout the year and its relationship with precipitation. Daily data were obtained from the National Centers for Environment Prediction (NCEP) reanalysis for the period 1988 - 1992, over the area from 10° S to 65°S and between 90° W and 20° W. Specific humidity, was daily integrated for the surface - 700 hPa layer and then the monthly values were calculated. Water vapor transport and water vapor divergence were calculate for each level in this layer and later the daily and monthly vertical integration were calculated.

From early spring until late autumn there is a strong entrance of humidity over Paraguay and northern Argentina from the Amazon. This explains why spring and autumn precipitation in the subtropical region is strongly influenced by the beginning and ending of summer monsoon circulation. On the other hand the coastal area is more influenced by the entrance of water vapor from the Atlantic Ocean. There are two strong convergence nucleus: one extending NW-SE from the continent over the ocean between 20 and 30° S, where the South Atlantic Convergence Zone (SACZ) is generally located; the other is centered in 30° S near the border between Brazil and Uruguay. The entrance of water vapor from the Amazon basin follows the circulation pattern in 850 hPa, where a trough extends between 15° and 40°S with a NW - SE direction from the Amazon region to the Atlantic coast. The water vapor contribution from the Atlantic ocean is conducted by the anticyclonic circulation entering the continent between 10° and 20° S. A third convergence area extends over SE Argentina. At 40°S water vapor is advected both by the westerly eddies and from the northern sources described above.

In the coastal areas centered at 30 and 40°S rainfall presents high positive correlations with the Atlantic sea surface temperature during the summer months, i.e. warm (cold) temperatures lead to wet (dry) conditions.