Coen Welsh | Nov 14, 2017 | 0
Weather 26 February 2016
During the course of the week the pivot of the anti-cyclonic circulation over the sub-continent has been shifting up and down the Sashi between Botswana and Zimbabwe. This circulation has been particularly strong for the entire week.
This anti-cyclonic circulation formed a low pressure system over Namibia covering the area north and north-east of a fairly prominent convergence line running more or less from Hentiesbaai through Rehoboth and Koes in the south-east. A trough with considerable depth was present in the mid-levels reaching into the upper levels up to about 45,000 feet. As is typical, the airflow in the trough was from southern Angola, through Namibia and Botswana, and into South Africa. It brought in considerable moisture covering most of southern Africa in clouds except for the Karoo area which includes southern Namibia and the west-central area of South Africa.
Surprisingly, the so-called Inter-Tropical Convergence Zone has shifted south by a considerable margin. In the second half of summer, it usually cuts through Zimbabwe but for the past week it has moved towards the Limpopo Valley between Zimbabwe and South Africa. In the western section, Namibia, it was back on the Owamboland Angola border and it was the feeder source of the moisture that penetrated from Angola. By the end of this week, it was expected to move even further south thereby improving the conditions for precipitation.
The southern Indian high pressure cell sat immediately south of Madagascar. The zonal flow from east to west on its northern rim contributed much to drive the anti-cyclonic circulation over the sub-continent. Over the Atlantic Ocean, the core of the South Atlantic high collapsed as the week progressed, reading a rather meek 1018 mB by Friday. The 1016 mB isobar remained offshore with only a weak high pressure presence over Namibia’s south-western quadrant during the nights. During the days, however, as the sun’s energy heated up the interior, any high pressure presence quickly dissipated leading to lower pressure along the coastal plain. With the strong circulation from the north, diabatic compression, as the air descended from the interior plateau off the escarpment onto the coastal plain, very high temperatures were experienced over the southern Namib.
The Southern Oscillation Index of the Australian Bureau of Meteorology, while still strongly negative, has made one of its signature spikes (less negative). Often, after one of these pronounced spikes, Namibia gets rain for four to seven days. It seems this is what is happening this week, and it is set to continue, intermittently, for the weekend and into next week.
A succession of weak frontal systems has developed south of the weak South Atlantic high. These are approaching the Western Cape slowly and should have some impact on the development of rain over Namibia during the next few days. Ahead of each frontal system, the airflow is from north to south, enhancing the existing direction of the mid-level trough, which is also from north to south. In practical terms, it means there is a reasonable chance of low pressure conditions developing from the surface through the mid-levels and into the upper levels up to almost 60,000 feet. When that happens, rainfall conditions in Namibia are ideal.
The other element to watch is the southern limit of the Inter-Tropical Convergence Zone. It is expected to move south by as much as 400 km. That will bring it into northern Namibia and northern Botswana. It is a ready source of moisture and as the expected trough develops, it also expands towards the west. This if typical for late summer and it is also the reason why March and April are the two most important rainfall months for the South. There are many atmospheric indicators which have turned positive during the past two months, giving us, particularly the South, a rational expectation of improved late-season conditions. This may even carry on until early May.