Relationship between Temperature and Circulation in Southeastern South America and its Influence from El Nino and La Nina Events.

Abstract

This paper explores the relationship between regional circulation anomalies and the interannual variability of surface temperature in Southern South America, east of the Andes (SSA). Two sets of monthly surface temperature for the 1963-1990 period were used; one was taken from the NCEP/NCAR reanalysis, and the other was built from the synoptic network records. Although these sets present some differences between them, the conclusions found are independent of the data set. Monthly and seasonal indices were developed to explore, through linear correlation, the connection between interannual temperature variability in SSA and some features of regional circulation. The intensification (reduction) of the west component of the wind at the upper troposphere over the subtropical latitudes of South America, and the northward (southward) shift of the maximum westerly wind over SSA tend to be associated with cold (warm) anomalies of surface temperature during practically every month throughout the year. At low levels, the enhancement (reduction) of the zonal geopotential gradient between the South Atlantic high and the Chaco low, representative of the northern component of the low-level flow, is generally associated with warm (cold) surface temperature anomalies. In SSA, monthly precipitation is positively correlated with the northern component of the geostrophic flow. Therefore the advection of temperature offsets the possible influence of precipitation, caused by the associated evaporation and cloudiness, on surface temperature. Hence, there are positive correlations between precipitation and temperature except for central Argentina in summer, and for southern Brazil in certain months. The fact that increased (decreased) precipitation accompanies enhanced (less) warm advection, contributes to moderate the interannual variability of surface temperature. The partial balance between the effects of warm advection and precipitation on surface temperature variability explains why, though ENSO phases have a considerable impact on precipitation and some influence on the regional circulation, their signals in the surface temperature are relatively modest. The exception is in JJA (0) in western SSA, for the most part a region with very scant precipitation during the austral winter, where the surface temperature anomalies are positive in El Ninno, and negative in La Ninna. The maximum anomalies coincide approximately with an enhancement (reduction) of the northern component of the flow, and of the warm advection at low levels in El Ninn o (La Ninna) events.