Understanding Weather – Not Predicting – 24 Nov 2011
Our “big weather world” is the southern Hemisphere with particular emphasis on the oceans. Vast oceans are the big bases above which key atmospheric pressure patterns form, move, weaken in tandem with one another so giving rise to an incessant wave of motion around the hemisphere.
For several seasons we have seen the main engine, the so-called High-pressure Belt from which these pressure patterns function, close to the 40oS latitude. Their Trade Wind flow maintains the Equatorial Rainfall belt in so doing. But while high barometric pressures of 1040 are on record for the high pressure cores, so are much lower values like 1016 to 1020. This last week gave two interesting examples.
South Pacific trade winds kept La Nina in positive oscillation, active along the equator. But weaker pressures (hence weaker outflow) have seen a warming in the far eastern Pacific. This is not El Nino, negative oscillation, but also not a true La Nina.
In the South Atlantic a similar pattern has held sway along 30oS. So only limited reactivation of Indian Ocean anticyclonic circulation occurs. The trade winds are limited with the result that less moisture flows to the heat-low area present over the subcontinent this time of the year. This heat-low is an almost constant feature of summer of much of Botswana, Namibia, western Zambia and southern Angola. In the early season, it causes the excessive heat of December, but a month later, it is the main conduit to draw moisture across Namibia from the equatorial zones over central Africa.
The tropical cyclones which have lately developed over the Indian Ocean, restricts this process.
Yet Namibia has still had quite rainy weather. Equatorial activity with the Inter-Tropical Convergence Zone (ITCZ) as the mainspring, has ensured the presence of moist Congo air across our northern borders. A surface trough running al the way from Tanzania to Namibia’s interior ensured that this moist air penetrated our skies in limited quantities. So in spite of unfavourable patterns there was tropical-type weather, with another weak lower level trough above the coast supporting a northerly flow across Namibia.
But isn’t this like a “normal” February? Yes, but with the difference that this has been “in spite of” rather than “because of”. The interpretation which appears is that otherwise the ITCZ would have limited ability to impart its moist air into our airspace meaning limited rain at best or even widespread drought similar to the early 60’s.
An equatorial wave drifts along the northern border while a cold front sweeps across far south of the Cape. During the weekend, anticyclonic development builds west of Marion Island (45S) and advances north-east to begin a noticeable inflow into the mid and western subcontinent. Across southern Angola, a drift westward takes the helpful circulation and its attendant southward airflow on to the perimeter of our weather scene. Because of the anomalously warm western Indian Ocean surface temperatures, a new Cyclone forms and after some south-westward drift is expected to move south (east of Madagascar) and be dragged into the west-wind flow around 35oS, but it is close enough to absorb some of the maritime inflow otherwise destined for the southern African heat low pressure area. An intervening anticyclonic mid-level core appears and will provide a northerly drift across Namibia. The prospect of showers remains for much of the next week for much of Namibia. The main axis of the system develops over the western escarpment, and as it develops towards the south, it slowly drifts west, hence much improved rain prospects for the Kaokoveld and Damaraland.