CR 17:83-92 (2001)  -  doi:10.3354/cr017083

Intra-seasonal convective structure and evolution over tropical East Africa

Emmanuel J. Mpeta*, Mark R. Jury

Environmental Studies Dept. University of Zululand, KwaDlangezwa 3886, South Africa
*Permanent affiliation: Tanzanian Directorate of Meteorology, Dar es Salaam, Tanzania. E-mail:

ABSTRACT: Intra-seasonal convection oscillations over southwestern Tanzania during the December to February season are examined using 15 yr (1979 to 1994) of pentad Outgoing Longwave Radiation (OLR) data and kinematic/thermodynamic parameters from ECMWF data. Most significant spectral energy for an area-averaged OLR index is concentrated in periods of 16 to 33 d. Time-longitude diagrams of filtered OLR, and zonal wind anomalies at 850 hPa averaged over the 7.5 to 10°S latitude band for the period November to May reveal eastward (62%), westward (11%) and quasi-stationary convective (27%) features. Phase speeds for transient cases are in the range of 2 to 8 m s-1. In the eastward propagating case study, 1 to 15 January 1993, map sequences reveal that deep convection shifts northeastward as a southern mid-latitude trough couples with the ITCZ. Convective events over tropical east Africa are associated with an influx of northeasterly Indian monsoon flow followed by increased westerlies from the Guinea/Congo region. Eastward propagating OLR anomalies couple with zonal circulations, suggesting that a transient Walker cell emerges periodically from East Africa.

KEY WORDS: Intra-seasonal climate · East Africa

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