Southern African Monsoon

. Rarely mentioned in literature is the Southern African Monsoon (SAM), an area covering 80 longitudes (0~90E) and 25 latitudes (30S~5S). Small in size nevertheless a system endowed with all monsoon characters, the SAM has its major convective zone divided by Karahari Desert and Madagascar island, resulted a monsoon trough a string of three surface lows, which are Angola low, Mozambique Channel low and a narrow oceanic ITCZ stretching from the northeastern tip of Madagascar to the middle of Indian Ocean. In south a ridge extends from the mighty South Indian Ocean high and lands in Durban, occupying a coastal zone from 35S to 20S. This is the oceanic component of monsoon system in lower atmosphere. In north the Congo Air Boundary goes through the seasonal excursion, as the same deed by the well-known Somali jet. They comprise the conveyor belt to send heat and moisture to the monsoon trough on land. The upper troposphere is dominated by a thermal high upon Victoria Falls, a very compact high-pressure system almost diminutive in comparison with South Asian High of Indian monsoon.

The rain bearing systems include the easterly waves embedded in the southern flank of ITCZ and middle latitude front/cyclones from further south, which, in turn, are affected by the intensity and location of the Marion island blocking and Gough island high on either side of southern Africa. The tropical cyclones and Madden-Julian oscillation also intrude or modulate the weather events.

The rainy season of SAM starts from the south rim of Congo Basin on early October, a result from the seasonal migration from the perennial rain belts in tropical Africa. A few weeks later the Lesotho area, an area around the southeastern mountains of the Cape, also enters the rainy season. The next onset episode befalls in the central valley of Madagascar at the beginning of November. In the meantime the ITCZ shifts from 5~10S to 12~18S, since September. The center of the permanent South Indian Ocean High (around 30S) has moved from austic winter position; 100~110E, to austic summer position; 70~80E. The northeast trade wind (called the bat furan wind in Arabian) from north land mass will be confluent with the southwest trade wind in the middle of the Indian Ocean, to form the oceanic ITCZ. At the end of November the SAM suddenly explodes into a broad area and rainy season will stand for four and half months, then it stops as briskly as it starts, first in continent then in ocean.

The onset events are phase-locked with Julian Calendar very tightly in continental interior centered around Zambia (15S, 30E). The standard deviations of onset dates could be as low as two weeks and the changes between rainy and dry season are most distinctive on the west shore of Lake Malawi and Mozambique Channel. All above results are quite consistent based on separate studies of NCEP, EC Reanalysis and CMAP (pentad resolution) data set