Variations of South America summer circulation on subseasonal time scales

The upper-tropospheric circulation over South America during austral summer presents some well-defined regional-scale systems such as the Bolivian high, a prominent warm-core anticyclone that establishes itself over the elevated Altiplano of the central Andes. Previous observational and modeling studies revealed that the position of the Bolivian high is primarily determined by Amazonian precipitation. Also it is documented that intraseasonal and interannual rainfall variability in the region of the Altiplano is closely linked to changes in intensity and position of the Bolivian high.

In this paper, upper-level summer circulation variability on subseasonal time scales is examined over South America, based on 14 years (1979-93) of European Centre for Medium Range Weather Forecasts reanalyses. The 300-hPa meridional wind perturbations, determined from a rotated extended empirical orthogonal function (REEOF) analysis, provide the baseline modes of atmospheric variability.

The dominant upper-level meridional wind fluctuation REEOF over tropical South America is characterized by a short-scale quasistationary wave pattern along the 25S associated with Bolivian high variations. A spectral analysis of the REEOF time series (PC) revealed significant signals on low-frequency (40-70 days) and high-frequency (15-25 days) intraseasonal time scales and also on synoptic time scales. Positive (negative) events of this mode were defined from the time when PC was greater (smaller) than 0.8 times of its own standard deviation, for more than 5 consecutive days, until the time when the next crossing of this threshold occurs. Composite fields associated with the extreme phases of this mode were computed averaging over all events. The average duration for both positive and negative events is around 8 days.

Positive events display an enhancement of the convection over subtropical Argentina, the Altiplano and also over eastern Brazil and surrounding oceans. Also, a strengthening of the South Pacific Convergence Zone, located further east, and an inhibited South Atlantic Convergence Zone (SACZ) are also evident. During this phase, the Bolivian high gets more intense and shifts further east of its climatological position while at middle latitudes, cold fronts move eastward, enhancing low-level northerly flow on the eastern slope of the Andes.

Negative events are associated with an intensification and southward extension of the SACZ and inhibited convection over subtropical plains and Altiplano. The Bolivian high is located further west and gets weaken. Successive frontal systems penetrate into tropical latitudes and remain quasistationary over southeastern Brazil. There, they contribute to tap moisture from the Amazon to SACZ region while they inhibit the southward flow of moisture on the eastern slope of the Andes.

Many authors have reported this summer precipitation seesaw pattern over tropical South America from the analysis of OLR or precipitation data. However, our results show that such precipitation variations are related with a clearly defined mode of variability of the upper-level circulation.

Changes in low-level circulation and synoptic-scale activity during both phases will be extensively discussed in the extended manuscript and during the meeting.