25 April 2014
After some weeks of zonal airflow indicated on the local synoptic map, the latter part of this week saw a return to the typical early-winter pattern of strong north-south-north flows.
A well-marked cold front crossed the southern Cape early in the week, pushing a (low pressure) trough ahead of it towards the east. A cold front is by definition the estimated position of the interactive zone between a low pressure col and a high pressure ridge behind it. Ahead of the cold front, air flows from north to south, i.e. from warmer (wetter) to colder (drier) environs. Immediately behind the cold front, airflow is in the opposite direction from south to north, bringing in copious volumes of cold antarctic air. This is exactly what transpired during the week, and by Thursday morning, the noticeable drop in nighttime temperatures have made its mark up to about Otjiwarongo.
The cold front sits on the leading (eastern) rim of the South Atlantic high pressure cell. As it slowly rotates anti-clockwise, so it also migrates from west to east. This is a very powerful engine driving weather conditions across southern Africa with its impact seen right up to the tropical zone. Because of its immense size, thousands of kilometres across, the high pressure cell grabs cold antarctic air very far south in the southern oceans bordering the antarctic circle. This cold air it pushes north creating the prominent south winds in winter which bring in the cold.
As the high pressure cell drifts across the southern extremities of the continent, the same circular motion now turns into a conveyor (on its western rim) creating a low pressure trough that sometimes extend from central Africa across the sub-continent to far south of the continent. As the season turns and winter approaches, the core of the high pressure cell grows in strength, this week posting a typical winter value of 1024 mB. Stronger high pressure cells dominate local weather conditions by pushing the opposing zonal flow from the Indian Ocean away to the east.
The little rain we experienced came in the wake of the (low pressure) trough but its general position was too far east to have any marked impact on local precipitation. But the high pressure cell extended sufficiently north for us to experience a colder week and to give us the first taste of the regular three days cold, four days mild pattern during winter.
Only Kunene, Owamboland, the Okavango and the Zambezi areas received late summer rains as the trough shifted from west to east.
The synoptic map on Thursday displayed a very typical early winter pattern. The core of the South Atlantic high pressure cell runs on its designated latitude, with a regular advection of cold air from the south.
This weekend probably offers the very last opportunity for rainfall albeit limited in intensity. But as the high pressure cell migrates to the east, due to its strength, it carries the possibility of pushing up far north, even to Tanzania, from where it has the potential to drive zonal flow from east to west across the continent.
However, the next cold front is already in the wings, and the northerly flow of cold air will turn to a easterly flow over the weekend becoming markedly north-easterly, then northerly, during next week. Another trough develops just inshore from southern Angola to about Walvis Bay but without the required moisture to bring any rain. But it does set the stage for the first Oosweer conditions at the coast which are expected to materialise by about Wednesday.