Submonthly convective variability over South America and the South Atlantic convergence zone

摘要

Relationships between deep convection over South America and the atmospheric circulation are examined, with emphasis on submonthly variations of the South Atlantic convergence zone (SACZ) during austral summer. Outgoing longwave radiation (OLR) is used as a proxy for convection, while the associated circulation patterns are depicted by the National Centers for Environmental Prediction Reanalysis. Over South America and the adjacent oceans, OLR fluctuations with periods less than 90 days show maximum variance in the SACZ and over central South America during December–February. There is a local minimum in variance over the southern Amazon Basin, where mean convection is at a maximum. OLR spectra display several statistically relevant peaks corresponding to periods of less than 30 days over tropical South America, with the relative proportion of higher-frequency power increasing as the base grid point is moved to the southeast within the SACZ. Correlations between submonthly (2–30-day) OLR in the vicinity of the SACZ and 200-mb streamfunction reveal the preferred path of Rossby wave energy impinging on the SACZ from the midlatitudes of the Southern Hemisphere. Episodes of enhanced convection within the SACZ, indicated by negative OLR anomalies, occur at the leading edge of upper-level troughs propagating into the region. The corresponding pattern at 850 mb reveals that the disturbances are nearly equivalent barotropic west of South America but tilt westward with height in the region of the SACZ. Negative low-level temperature anomalies lie to the southwest of the convection. The results are consistent with baroclinic development along an associated cold front. Convection over the southwestern Amazon Basin on submonthly timescales is seen to progress into the region from the south. Upper-level anomalies, which at times may play a role in the initiation of the convection, move eastward and rapidly become decoupled from the convection. Low-level cold air along the eastern flank of the Andes appears linked to the convection as it moves northward. In contrast, convection over the southeastern Amazon is accompanied by disturbances moving into the area from the Atlantic, but there is little sign of a low-level temperature anomaly. In this case convection seems to result in cross-equatorial outflow into the North Atlantic, rather than be the result of forcing from the extratropics. The authors speculate that the relatively stable position of the SACZ is associated with a Rossby wave guide, which ultimately is related to the large-scale circulation driven by sources and sinks of diabatic heating. It also appears that the SACZ forms when the northwesterly flow associated with a low-level trough is able to tap moisture from the Amazon.