Midsummer Low-Level Circulation and Precipitation in Subtropical South America and Related Sea Surface Temperature Anomalies in the South Atlantic

Abstract

The midsummer interannual variability of the low-level tropospheric circulation and of the precipitation field in subtropical South America (SA) associated to the sea surface temperature (SST) anomalies in the western subtropical South Atlantic Ocean (WSSA) is investigated using reanalyses, regional precipitation datasets, and monthly SSTs. The region of the WSSA where SST has the strongest relation with precipitation in subtropical SA was identified using canonical correlation analysis. This region extends from 20° to 30°S and from 30° to 50°W. Composites corresponding to extreme SSTs in this area show two well-differentiated patterns in the low-level circulation and in the precipitation fields. In the composite, corresponding to the more positive SST anomalies in this area, the mainstream of the low-level flow and of the moisture transport from the continental low latitudes starts to follow a southeastward direction at 10°S, and converges with the west flow at 35°S over the Atlantic Ocean. On the other hand, in the composite corresponding to the more negative SST anomalies, the low-level flow and the moisture transport from the continental low latitudes turn eastward toward the South Atlantic convergence zone (SACZ) at about 20°S, converging with the flow from the north driven by the South Atlantic high. In this composite, there is an anticyclonic circulation with a westward flow between 25° and 35°S, which turns southward after reaching the proximity of the Andes Mountains. In the composite of the more positive anomalies, there are two regional maximums in the precipitation field. One maximum stretches along the continental extension of a southwardly displaced SACZ and another is centered at about 30°S and 55°W, in the path of the main stream of the low-level moisture transport. In the other composite, there is only one regional maximum in precipitation, which coincides with the continental extension of the SACZ shifted northward of its mean position, and with a relative minimum in northeastern Argentina and southern Brazil. In this composite, in western Argentina, there are positive anomalies in the precipitation field favored by the transport of moisture from the Atlantic Ocean. The low-level patterns of the monthly composites, corresponding to the more positive and more negative SST anomalies in the WSSA, are similar to the respective patterns of each of the phases of the active and inactive SACZ. This follows from the prevalence, according to the SST in the WSSA, of one or the other of the low-level patterns associated to the seesaw of the SACZ. A positive feedback between positive (negative) SST anomalies and weak (intense) SACZ activity might enhance the low-level circulation pattern associated with the SACZ seesaw.