Weather 11 December 2015
The rigid control of Namibia’s weather by the South Atlantic high pressure cell was amply demonstrated this week.
In the wake of the departing high, the signature mid-level (low pressure) trough developed from southern Angola across Namibia from north to south, and into the Northern Cape interior. This system dominated local conditions over the weekend, continuing into Monday and Tuesday with a mid-level presence remaining until the end of the week.
On Monday, the absence of high pressure control on the surface was witnessed by the cloud base that has descended to around 9000 feet asl. This created an effective conveyor for tropical air from the Congo to traverse southern Angola and enter Namibian airspace during Sunday night. The result was the widespread and substantial falls that occurred over three quarters of the interior plateau and the northern Namib.
But by Tuesday evening, the outer rim of the next approaching South Atlantic high was close enough to exert a visible influence on surface conditions west and south of the so-called convergence line.
As the austral summer nears the solstice, sea surface temperatures in the southern hemisphere respond by getting warmer. This increase in water temperature has a measurable impact on the air immediately above the water. Over time, about three months, the warmer (or less cold) surface air has an impact on the higher levels meaning that the typically winter-style high pressure cells reduce in intensity and their cores may be displaced slightly southward.
At the beginning of this week, the South Atlantic high pressure cell was more or less in its customary place midway between South America and Africa.
However, as the week progressed, the core collapsed with the result that only a very weak (1016mB) cell remained, but at the same time it was dispersed over a vast stretch of ocean reaching all the way from close to the South American coastline to the edge of the African continent. Within the flattened, reduced cell, quiet and stable conditions were the norm. In Namibia it was indicated by the absence of strong westerly and south-westerly wind on the coast, both north and south of the Kuiseb vallley.
It is this weak South Atlantic high that allowed the anti-cyclonic system situated over the southern African interior to advect copious amounts of moisture on its western rim from the Congo to Angola and then into Namibia.
Another important feature of the week’s weather was the presence of moisture in all three levels, from surface up to 45,000 feet. The mid-level conveyor system usually remains active in summer despite the surface influence of the high, but when the high is as weak as this week, then moisture penetrates in all level, from around 9000 feet asl to as high as 45,000 feet. In the very thick atmosphere, convection is enhanced and good rains occur. When this system covers a very large expanse of land, it bring widespread rains.
The core of the South Atlantic high regains some of its lost strength as the continental obstruction forces it on a trajectory that is deflected to the south. The cloud cover starts breaking up during Friday with only the Kavango and Zambezi regions indicated for continued rain. Temperatures in these two areas will remain elevated as it lies close to the core of the strong anti-cyclonic continental circulation.
Watch out for the tropical depression some 3000 km east of Madagascar. At this stage is relatively far but at 979 mB it is already strong enough to put unlimited quantities of moisture in the upper air from where it can be readily advected back to the continent by the zonal (east to west) upper airflow. It also reduced the strength of the southern Indian high, making room for the approaching South Atlantic high to pass quickly south of the continent.
The Karas region, dry for most of this past week, shows the first signs of clearing up, gradually followed by the rest of the country during Saturday and Sunday. Next week Monday and Tuesday will be mostly clear with a weak intrusion of moisture in the mid-levels, but it will only be towards the end of next week, once the high has departed, that another 4-day rain opportunity presents itself again.