A recent field campaign aimed at obtaining an improved temporal and spatial description of the tropospheric flow over central South America was essential for the validation, and improvement of, short-and long-term predictions in the region.

There are two pervasive modes of atmospheric variability in the Southern Hemisphere (SH) that influence circulation and rainfall anomalies over South America. They appear as leading empirical orthogonal functions (EOFs) of 500-hPa height or 200-hPa streamfunction anomalies and are found from intraseasonal to decadal time scales. Both patterns exhibit wave 3 hemispheric patterns in mid to high latitudes, and a well-defined wave train with large amplitude in the Pacific-South American (PSA) sector. Therefore, they are referred to as the PSA modes (PSA1 and PSA2).PSA1 is related to sea surface temperature anomalies (SSTAs) over the central and eastern Pacific at decadal scales, and it is the response to El Niñ o-Southern Oscillation (ENSO) in the interannual band. The associated rainfall summer pattern shows rainfall deficits over northeastern Brazil and enhanced rainfall over southeastern South America similar to rainfall anomalies during ENSO. PSA2 is associated with the quasi-biennial component of ENSO, with a period of 22-28 months and the strongest connections occur during the austral spring. The associated rainfall pattern shows a dipole pattern with anomalies out of phase between the South Atlantic Convergence Zone (SACZ) extending from central South America into the Atlantic and the subtropical plains centred at 35°S.These two modes are also apparent in tropical intraseasonal oscillations for both summer and winter. Eastward propagation of enhanced convection from the Indian Ocean through the western Pacific to the central Pacific is accompanied by a wave train that appears to originate in the convective regions. The positive PSA1 pattern is associated with enhanced convection over the Pacific from 150°E to the date line. The convection pattern associated with PSA2 is in quadrature with that of PSA1. Both PSA modes are influenced by the Madden Julian Oscillation and influence rainfall over South America.

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A reconstructed rainfall dataset, and satellite estimates are used to analyze interannual to decadal variability of austral summer precipitation over South America. Rotated empirical orthogonal function (REOF) analysis is applied to isolate dominant patterns of rainfall. Links of these patterns to sea surface temperature anomalies (SSTAs) are examined. The leading mode is related to El Niño-Southern Oscillation (ENSO), which explains 12% of the total variance. During warm ENSO events, the positive phase of this mode shows dry conditions over northern South America and wet conditions over the subtropical plains between 25Њ and 35ЊS. The situation reverses during cold events. The second REOF 2, which explains about 10.8% of the total variance, consists of positive loadings over northeast Brazil centered at 50ЊW near the equator and negative loadings over Colombia and the subtropical plains. For December-January-February (DJF), REOF 2 is influenced by tropical South Atlantic SSTAs through displacements of the intertropical convergence zone. Northeast Brazil receives most rainfall in March-April-May (MAM) and it is modulated by both the Atlantic SSTAs and ENSO. In the interannual frequency band, the North Atlantic Oscillation (NAO) has very limited influence on rainfall. On the decadal timescales, the NAO leads REOF 2 by three years. Latitudinal variations of tropical convection are through the joint contribution of REOF 2 and REOF 4. REOF 4 is similar to REOF 2, but centers are displaced about 10Њ south. When these two EOFs are both positive, central South America is wet. The amplitudes of REOF 2 and REOF 4 are small during the mid-1950s to the mid-1960s and they are out of phase from 1968 to 1970, periods with persistent dry conditions over the upper La Plata River basin.

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Analysis of 1 week's data in August 1960 shows significant diurnal variations in surface geostrophic wind over the south-central United States. The oscillation in the southerly component (V,) is driven by the response of the thermal wind t o the diurnal temperature cycle over sloping terrain. A smaller oscillation in U, derives from spatial variations in the amplitude of the diurnal pressure wave. The amplitude of the oscillation in V g is about 3 to 5 m see-1 a t the surface, decaying exponentially with height to near 0 a t ' 2 km. Examination of 11 yr of summertime rawinsonde data at Fort Worth, Tex., shows a very regular diurnal variation in boundary layer wind with maximum amplitude of about 3 m sec-* a t 600 m abovc the ground. This oscillation is forced by periodic variations in both eddy viscosity and geostrophic wind. Using a simplified model of the boundary layer, we obtain solutions for the diurnally periodic wind resulting from "reasonable" variations in eddy viscosity and "observed" variations in geostrophic wind.

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