Weather 22 May 2015
The synoptic map was dominated this week by an array of high pressure cells covering the southern Indian Ocean, the ocean south of Madagascar, the eastern half of the southern African sub-continent, and the South Atlantic, both its African side and its South American side.
Core pressure of these cells ranged between 1028 mB (Indian) to 1020 mB (South American). These are fairly normal early winter readings indicating that the summer is finally over and that, progressively over the next month, winter conditions will manifest more clearly.
The six high pressure cells were also more or less at the expected latitude in line with Saldanha Bay in South Africa. The cells east of the continent were slightly farther south while the South Atlantic high pressure cell was displaced slightly to the north. The remarkable feature though is that with such a continuous high pressure band, it acts as an effective block obstructing the northern displacement of the winter-type low pressure vortices that are the harbingers of cold fronts. Hence, while it was chilly in the mornings, the deep cold of the south was prevented to follow its normal passage to the north and the days quickly became pleasant.
Make no mistake, highs are cold and dense by nature so it is to be expected that they bring in colder conditions, but a high situated over land heats up fairly quickly when the sun rises, leading to day temperatures in the upper twenties, and in northern Namibia, even in the lower thirties. This daily heating is amplified in no small measure by the direction of airflow, and by the unique Namibian topography.
With a high situated over South Africa, the direction of the airflow is determined by the north/south displacement of its core. On its northern rim, the circulation, being anti-clockwise, comes from the Mozambican Channel, crosses the subcontinent from east to west traversing Mozambique, Zambia and Zimbabwe. As it moves over Botswana it follows the curve of the anti-cyclonic circulation, entering Namibia from the north-east.
This signature airflow on the surface is what bring in the chill but since the air has travelled a few thousand kilometres over land, it is already warmer and as the day progresses, its energy continues to increase. This is the pattern of the typical early-winter cold mornings but pleasant afternoons and it applies to most of Namibia excepting the coastal plains.
The South Atlantic high, back in its customary position some 2000 to 2500 kilometres offshore south-west of Lüderitz, provided the counter balance to the high over South Africa. However, between the two cores, a weak trough develops marked by lower pressures over south-eastern Angola and over Namibia’s Kunene Region. This is also a hallmark of early winter and this pressure differential between the lower pressure over the Skeleton Coast and the higher pressure over the interior creates the conditions for the very familiar Oosweer. This phenomenon is a form of the so-called bergwind, meaning the airflow is from the higher plateau to the lower coastal plain. As the air descends it compresses, releases energy and while it causes much wind and dust during the day, it settles when the sun sets, and it creates those balmy warm evenings. During Monday and Tuesday this was evident over the northern Namib.
The South Atlantic high approaches over the weekend and makes landfall at the Cape by around Saturday. It pushes a mild cold front ahead of it and colder conditions will be experienced in the Karas Region and the Namib part of the Hardap Region. As it slips past Cape Agulhas it weakens, and collapses so Saturday and Sunday again should be mild and pleasant with warm afternoons. The only exception remains the coastal plain where it will be windy (north-east) north of the Kuiseb and very windy (southerly) at Lüderitz and Oranjemund.
But with so many high pressure cores in succession, the next South Atlantic high arrives quickly bringing colder conditions to the south for next week. The pattern repeats itself with the airflow for the rest of the country first coming from the south-east, then backing to east and finally to the familiar north-east during next week. It is very much a repeat of the pattern described above.