Origins of Convective Variability over Equatorial Southern Africa during Austral Summer

Abstract

Analysis of NOAA/NESS outgoing longwave radiation (OLR) over the greater Africa region reveals a large area of low OLR (5-degrees-20-degrees-S, 20-degrees-E-40-degrees-E) during three austral summers (November through February; 1982/ 83, 1983/84 and 1984/85). This low OLR area is consistent with the climatological rainy area and persistent convective activity. Using OLR as a proxy for synoptic and larger-scale cloudiness and convection, OLR standard deviations are computed for the three summers. Highest OLR variability is observed across Africa along 15-17-degrees-S, which is about five degrees latitude south of the OLR minimum. Based on the region of maximum OLR standard deviations and minimum mean OLR, a box-average OLR index is derived. Time series of the OLR index for November through February indicate large (+/- 40 W m-2), aperiodic OLR fluctuations within each of the three summers. Outgoing longwave radiation composites are constructed for periods of large OLR changes from negative OLR anomalies (wet conditions) to positive OLR anomalies (dry conditions). Although fluctuations are noncyclic in time, OLR composites reveal propagation of OLR from south of the Cape of Good Hope toward the northeast. The origin of these OLR fluctuations appears to be the Southern Hemisphere midlatitudes. This is consistent with OLR correlation maps derived for each season. However, a large portion of the OLR changes over equatorial southern Africa are of standing character. Circulation features associated with the large OLR fluctuations are analyzed by compositing NMC wind and temperature fields. It is found that east-northeastward propagation of midlatitude waves into the subtropical western Indian Ocean occurs prior to OLR decreases over equatorial southern Africa. Trough (ridge) intrusions into subtropical and tropical Africa from the southeast are associated with OLR decreases (increases). The wind circulation and divergence in these equatorward penetrating troughs is strongest in the upper troposphere (300 mb), temperature perturbations are largest at 500 mb, and the wave signature can be seen down to the low levels. A close examination of circulation features associated with one prominent OLR change indicates that individual events are similar to the composite average, however, they reveal greater temporal detail. The midlatitude upper level trough does not penetrate directly to equatorial Africa. Rather, the midlatitude trough merges with the Tropical Upper Tropospheric Trough (TUTT), which is a persistent feature at 300-200 mb over the southwest Indian Ocean. The TUTT is then instrumental in modifying circulation over equatorial southern Africa, which is favorable for OLR decreases over that region. Based on these results it appears that a major source of OLR/ convective variability over the rainy region of equatorial southern Africa during austral summer is associated with interaction between midlatitude wave disturbances embedded in the westerlies and the quasi-stationary tropical upper tropospheric trough in the vicinity of the southwest equatorial Indian Ocean.