Weather 28 November 2016
The meteorological literature presents a difference of opinion regarding the so-called Inter-Tropical Convergence Zone (ITCZ). In very simple terms, this is the zone around the globe from more or less 10°N to 10°S that straddles the equator and where the south-easterly trade winds of the southern hemisphere meet the north-easterly trade winds of the northern hemisphere. This zone is popularly known as the doldrums.
But in real weather it is not so simple. It has been observed in the seventies by measuring wind velocities and anomalies, that the ITCZ may have a northern and a southern limit which may vary from 25°N to 25°S. The same researh showed that the ITCZ is relatively stable and stationary where it crosses oceans but may expand and wander widely over land since the day/night and summer/winter temperature and pressure differentials, are much more enhanced over land due to the sun’s energy.
For instance, in West Africa, a narrow ITCZ is typically north of the equator and it plays a major role in rainfall. Over East Africa, the ITCZ is much broader and dispersed, and only appears visibly during summer. It also plays a major role in the rainfall over central and southern Africa. Generally it constitutes the active band within which moisture from the Indian Ocean is conveyed across the continent into central Africa, and from there in a very wide curve to southern Africa. When the ITCZ is absent of fragmented, this conveyor system is impeded or interrupted, and the moisture that reaches central Africa is reduced. Then, only what is left after it has precipitated over central Africa, reaches southern Africa.
This week was much a repeat of the previous week. The ITCZ over East Africa is still fragmented. However, it has developed somewhat just north of Madagascar but it does not yet continues onto the continent. In the west, the ITCZ’s southern boundary is now lying across central Angola, with a southerly extension happening in the mid-levels, every three to four days. For the southern African sub-continent, the week presented a fairly typical early summer synoptic progression but full summer conditions have not set in yet. This is witnessed by the continued influence of the South Atlantic high pressure cell as its outer rim impacts the south-western quadrant of Namibia. From the other direction, the north-east comes the daily onslaught of lower pressure conditions, witnessed on the ground by the extremely hot afternoons. There is high pressure control on the surface as can be seen from the barometric readings at Walvis Bay and Lüderitz, while the pressure along the Angolan border and up to Katima are much reduced, going as low as 1004mB during the afternoon. The system from the north, being warmer and buoyant, tends to slip over the system form the south being colder and denser. This forms a convergence line at the mid-levels, typically between 16,000 and 20,000 feet. It is only in this zone that convection takes place and where clouds are formed. This is the reason for the signature intrusion of moisture from Angola which we see as clouds and which may even produce some rain as we saw this week over the north-eastern quadrant and even in Windhoek.
A frontal system driven by the approaching South Atlantic high passes the Cape on Friday. It brings the high’s core closer to the continent During the weekend, the core slips around the southern Cape and by Monday it sits just south-west of Madagascar. This allows a three-day window for the lower pressure conditions over Zambia, Botswana, Namibia and Angola, to develop in intensity, and to move towards the south.
This will again develop into a mid-level trough, bringing cloudiness to the Namibian interior up to the Orange River.
However, higher pressure (1016mB) reigns offshore so there will be a relatively strong pressure differential between the coastal plain and the Namibian and Botswana interior. This indicates very windy conditions along the escarpment. This windy zone is the convergenze zone meaning for the duration of the weekend and the whole of next week, there is a small chance of rainfall for the entire Namibia excluding the coastal plain and the western half of the Karas region.