Circulation anomalies associated with tropical-temperate troughs in southern Africa and the south west Indian Ocean

Daily rainfall variability over southern Africa (SA) and the southwest Indi an Ocean (SWIO) during the austral summer months has recently been describe d objectively for the first time, using newly derived satellite products. T he principle mode of variability in all months is a dipole structure with b ands of rainfall orientated northwest to southeast across the region. These represent the location of cloud bands associated with tropical temperate t roughs (TTT). This study objectively identifies major TTT events during Nov ember to February, and on the basis of composites of NCEP reanalysis data d escribes the associated atmospheric structure. The two phases of the rainfa ll dipole are associated with markedly contrasting circulation patterns. Th ere are also pronounced intra-seasonal variations. In early summer the posi tion of the temperate trough and TTT cloud band alternates between the SWIO and southwest Atlantic. In late summer the major TTT axis lies preferentia lly over the SWIO, associated with an eastward displacement in the Indian O cean high. In all months, positive events, in which the TTT cloud band lies primarily over the SWIO, are associated with large-scale moisture flux ano malies, in which convergent fluxes form a pronounced poleward flux along th e cloud band. This suggests that TTT events are a major mechanism of polewa rd transfer of energy and momentum. Moisture transport occurs along three p rinciple paths: (1) the northern or central Indian Ocean (where anomalous f luxes extend eastward to the Maritime Continent), (2) south equatorial Afri ca and the equatorial Atlantic, (3) from the south within a cyclonic flow a round the tropical-temperate trough. The relative importance of (2) is grea test in late summer. Thus, synoptic scale TTT events over SA/SWIO often res ult from large-scale planetary circulation patterns. Hovmoeller plots show that TTT development coincides with enhanced tropical convection between 10 degrees-30 degrees E (itself exhibiting periodicity of around 5 days), and often with convergence of eastward and westward propagating convection aro und 40 degrees E. Harmonic analysis of 200 hPa geopotential anomalies show that TTT features are forced by a specific zonally asymmetric wave pattern, with wave 5 dominant or significant in all months except February when qua sistationary waves 1, 2 and 3 dominate. These findings illustrate the impor tance of tropical and extratropical dynamics in understanding TTT events. F inally, it is suggested that in November-January TTT rainfall over SA/SWIO may be in phase with similar rainfall dipole structures observed in the Sou th Pacific and South Atlantic convergence zones.