Guest Contributor | Sep 24, 2020 | 0
Fortnightly weather overview and short-term outlook to Wednesday 02 October 2019
Visual 1: Statistical rainfall probability for Africa for the October, November, December season.
Source: International Research Institute for Climate and Society, Earth Institute, Columbia University
Visual 2: Sea Surface Temperature and 2-meter Air Temperature
Source: GrADS COLA/IGES, George Mason University
After another uneventful week last week, conditions visibly changed during the previous weekend, leading to extensive mid-level cloud in a broad convergence zone running from the central coast through the interior, into the Kalahari and beyond into central South Africa.
The clouds over the interior and the few drops at Walvis Bay and Windhoek originated from the anti-cyclonic system that circulated over Angola. On the other hand, the overcast and wet conditions in the South, was the spill-over of the frontal system that crossed the southern Cape.
The convergence zone was formed where the southern and the northern systems met, and it was the weak convection in this zone that produced the scattered drops in many areas in the interior.
This week’s visuals both convey negative messages. The first is from the University of Columbia in the US, showing their outlook for the upcoming first half of the season. It is not good for us and basically expects the drought to continue at least until the end of the year. We will have to wait and see how accurate they are but their work is typically based on a very large number of satellite observations, and on very powerful algorithms. Their take on the Namibian season can not be ignored.
The second visual shows very accurately why we had a drought and why it persists. There is much discussion about the so-called Indian Ocean Dipole but very little consensus on what the implications are for Namibia’s weather. This phenomenon is a coupled weather event in the sense that what happens in the western Indian Ocean has a direct, measurable effect on what happens in the eastern Indian Ocean and vice versa. Unfortunately, the science is driven by the Australian Bureau of Meteorology and their focus is Australia, not Africa.
Still, the visual shows that the sea surface temperatures directly north-east of Madagascar is relatively cool (encircled in blue) while about 4000 km to the north-west, there is a clearly visible concentration of warmer water, as shown by the dark red spot.
When the Indian Ocean Dipole is not well-defined, or absent, which it frequently is, then conditions for rainfall in East Africa are negative. This colder area also inhibits the operation of the so-called Indian Ocean Transport which is the mechanism that advects warm, moist air into central Africa from where it is driven across Angola and Zambia into Namibian airspace. This inability of the Indian Ocean Transport to function has been visible for the whole year so far.
The only positive aspect from this week’s conditions is that the general windiness over the interior, is a sign that the rim of the South Atlantic high pressure cell, as it crossed the southern Cape, is active on both its leading and its trailing edges. It is only during the past month that this mechanism has started operating properly again.
On the radar
The weather stance is in a typical late-winter transitory phase but it will be another four weeks before a more perceptible picture starts to emerge. The question of uncertainty remains since there is little agreement between various agencies what the season’s most probable outlook is.
Another weak frontal system with a northward extension crosses the Cape during the weekend. This bring much instability to the atmosphere roughly between 15,000 and 35,000 feet.
On the surface it will be experienced as lower pressure conditions with continued windiness, especially in the South. It is not expected to produce Oosweer conditions at the coast.
Next week Monday, the next approaching South Atlantic high has a brief impact over the two southern regions with cooler conditions shifting to the Botswana border on Tuesday.
For the rest of the week, quiet conditions are expected, generally with the absence of wind. The areas north of Grootfontein and Etosha will be very hot.
A weak mid-level trough from Angola through the interior into South Africa will bring in cloudiness and even some proper clouds but no rainfall is forecast.