Re Okoola

A diagnostic study of the eastern Africa monsoon circulation during the northern hemisphere spring season

The eastern Africa monsoons during the Northern Hemisphere spring (NHS) season are described based on composites derived from various rainfall anomaly scenarios. The years 1981/1984 were delineated as some of the recent wettest/driest years over this region during the NHS season. Wet/dry spells within these anomalous years were further selected using PCA T-mode analysis. Both the anomalous years and the wet/dry spells identified from these years were used to create wind composites that were used to describe the eastern Africa Monsoon circulation. Composite wind analyses showed the dominance of the westerlies/easterlies in the lower/upper troposphere and their migration from the Southern Hemisphere to the Northern Hemisphere. Analysis of the individual wettest/driest years showed that before the start of the equatorial eastern Africa (EEA) long-rains season, easterlies are dominant near the Equator and westerlies near 15°S in the lower troposphere, while at the end of the rainy season the westerlies were located to the north of the Equator, near 5°N, with easterlies on their lateral sides. However, during the EEA long-rains season westerly/easterly wind events occurred in alternation over the region. These westerly/easterly episodes were associated with wet/dry rainfall spells. Vertical sections of zonal wind component showed that the wind alignment during wet spells was similar to that generally observed in other monsoonal regions, with lower tropospheric westerlies overlain by upper tropospheric easterlies. But the reduced frequency of lower tropospheric westerlies is suggested to be due to the barrier effect of the north–south mountain chains which allow only the most intense westerlies to cross the mountains into the EEA region. Copyright

Midtropospheric Circulation Patterns Associated with Extreme Dry and Wet Episodes over Equatorial Eastern Africa during the Northern Hemisphere Spring

Abstract The anomaly circulation patterns of contrasting wet and dry episodes over the equatorial eastern Africa (EEA) region are studied using the European Centre for Medium-Range Weather Forecasts analyses. The study focuses on the Northern Hemisphere spring (NHS). Examination of composite synoptic-scale features indicates that cyclonic circulation over southern Madagascar in the midtroposphere forces southerly flow through the Mozambique Channel. The southerlies turn eastward on approaching latitude 10°S giving broad westerlies in the latitude belt 10°S to the equator. These southerlies followed by westerlies were associated with wet spells over the EEA region. Dry spells exhibit stronger than normal easterly wind flow over the EEA region. These dry spells are associated with a cyclonic circulation with a center near the point 25°S, 25°E and an anticyclonic circulation over Madagascar that causes northerly winds to blow through the Mozambique Channel. Thus, the southerlies act as a trigger for active c...

Spatial evolutions of the active convective patterns across the Equatorial Eastern Africa region during northern hemisphere spring season using Outgoing Longwave Radiation records

SummaryThis study has used low Outgoing Longwave Radiation (OLR) values to study the structure and evolution of the active convection across Equatorial Eastern Africa (EEA) region (5° N to 10° S, 28° E to 42° E) during the northern hemisphere spring season. This involved the examination of the map patterns and cross-sections of OLR data as derived from once-daily NOAAs Operational Polar Orbiting satellites within the period June 1974 to May 1991.The results from the study indicated that before March the mean ITCZ was active over the west Indian Ocean and Central Africa. The migration northwards of the zone of active ITCZ was associated with pre-season evolution patterns over the extratropics. The time-longitude cross-sections further indicated evidence that low OLR values were already occurring over central Africa to the west of 35° E before March. Such low OLR values penetrated to the east of 35° E in some occasions. Theresults from the study suggest that behind the surges of extratropical frontal systems strong meridional flow does occur and that these are associated with the advance of the ITCZ further northwards from extratropical regions of southern Africa. Then, active convection occurred over EEA region and this extended westwards to cover West Africa as well. The withdrawal of the ITCZ from the EEA region was however associated with the establishment of a centre of low OLR values to the southwest of Peninsula India.The results from the study further revealed that the years 1981/1984 has the lowest/highest mean OLR values in the region within the period 1974 to 1991. The time-latitude cross-sections of the anomalous years indicated that active convection crossed the EEA region from south to north of the equator early/late during the anomalous wet/dry years of 1981/1984. The number of pentads with low OLR were also more/less during 1981/1984 respectively.

Heavy Rainfall Associated with the July-August Season over the Central Rift Valley of Kenya

Two rainy seasons are recognized over Kenya. These are the long rainy season March to May (MAM) and the short rainy season of October to December (OND). During these seasons the whole country experiences rainfall. However, over the central Rift Valley, the second rainfall season seems to be missing. This is due to the escarpments over the region with the shift of the short rains such that the July to August rainfall becomes a dominant season. During this season, the heating zone has generally shifted to the north of the equator. Orographic rainfall is generally prevalent during the months of July and August. Orographic rainfall in the tropics is not common. It occurs mainly in July-August when the dynamic low lies over the region and the escarpments lift the westerly winds over the region bringing with them the moisture laden Congo air mass. They occur during the low pressure season with the plateau lifting the moist air (in the dynamic low) with the ITCZ far to the north of the equator. It is then observed that the moisture laden Congo air is lifted over the escarpments bringing rainfall over the region. Please cite this article as: Okoola R. E., Gitau W., Shilenje Z. W., Camberlin P., 2015, Heavy Rainfall Associated with the July-August Season over the Central Rift Valley of Kenya. J. Meteorol. Related. Sci. vol 8 2-11. http://dx.doi.org/10.20987/jmrs.2015.08.801 Journal of Meteorology and Related Sciences Volume 8