Interannual and interdecadal variability of the South Atlantic convergencezone

Interannual variations of the summertime (January-March) atmospheric circul ation over subtropical South America are examined during the period 1958-97 using the National Centers for Environmental Prediction-National Center fo r Atmospheric Research reanalysis data. It is found from an empirical ortho gonal function analysis that an anomalous upper-tropospheric large-scale st ationary eddy in the lee of the Andes tends to accompany a dipole in anomal ous vertical motion. An anomalous cyclonic (anticyclonic) eddy accompanies an intensified (diffuse) South Atlantic convergence zone (SACZ), with anoma lous descent (ascent) to the southwest. The cold-core equivalent barotropic vertical structure of the anomalous cyclonic eddy and the 200-hPa vorticit y balance are both characteristic of a stationary Rossby wave; the tendency for the eddy to be advected downstream by the mean westerlies is compensat ed by meridional advection of planetary vorticity and stretching associated with vertical motion. The anomalous cyclonic flow at low levels reinforces the thermally direct circulation associated with the SACZ. A weak funnelin g of submonthly Rossby wave activity into this descent region is also ident ified. The interannual time series of the eddy is significantly correlated with no rth-south dipolar sea surface temperature (SST) anomalies over the southwes t Atlantic; one standard deviation 200-hPa wind speed anomalies of up to 5 m s(-1) are accompanied by SST anomalies of up to 0.3 degrees C. A near-cyc lic 15-yr component is identified, which the authors corroborate from indep endent analyses of southwest Atlantic SSTs and river hows; both are found t o exhibit very similar oscillatory components. When the SACZ is intensified , the Parana and Paraguay rivers in southern Brazil tend to swell, while th e Uruguay and Negro rivers to the south tend to ebb; this north-south contr ast in streamflow anomalies is most marked on the interdecadal timescale.