28 November 2014
The first signs that the high pressure control in the upper air over eastern Africa, is breaking up, start appearing on synoptic maps this week. For the past month, upper air high pressure control was evident over eastern South Africa, Mozambique, Tanzania and into Kenya, even beyond the equator. This upper air barrier has a major impact on the further development of thew season in Western Zambia, the southern Kongo, and the entire Angola. It is important to us as these three regions are the feeder of moisture into Namibian airspace, but they in turn, depend on the advection of moisture from the Indian Ocean, the ultimate source of precipitation for southern Africa. When a ridge (high pressure obstruction) is present in the key upper air levels, (25,000 feet and above), it acts as an effective barrier, blocking, or dissipating much of the oceanic moisture that must enter the atmosphere at this level. A blend of late winter / early summer conditions persisted over Namibia. The South Atlantic high pressure cell was anomalously far north. Being further north than usual, it is affected by warmer sea surface temperatures hence the core barometric pressure is only about 1020 mB but its position has a strong impact on surface conditions across the southern half of Namibia. For most of the week, the excessive heat over the interior was slightly diminished but anabatic compression, due to the airflow from the east, still lead to very hot temperatures over the interior around 16:00 every day. The further east, the higher the late-day temperatures. The same phenomenon also caused very hot conditions over the southern Namib, which is fairly typical for this time of year.
The juxtaposition between the South Atlantic high and its sibling in the southern Indian Ocean, coupled to the increasing heat over the southern African interior, creates on a daily basis, conditions conducive for a shallow, weak trough that starts in southern Angola, cuts across the north-eastern half of Namibia, and eventually grows somewhat stronger over eastern Botswana. Below this trough, some brief thundershowers develop almost every day, but these are very isolated. It is a feature of the season so far that the South Atlantic high often finds itself further north than usual. Its presence brings unseasonally cold nights, and it strips the lower levels below 15,000 feet of any moisture. In the middle layers, from 15,000 to 30,000 feet, good penetration occurs from the tropical sources of moisture, but the prominent zonal flow in the upper layers above 30,000 feet, from west to east, severely restricts the rainmaking capacity of the visible cloudy layer. Again this week, good penetration from Angola occurred on a daily basis, but only between 15,000 and 30,000 feet. The clouds show very little structure and convection is limited usually to below 35,000 feet. This produces rain but only for five minutes giving totals hardly exceeding 5mm.
Election day sees Namibian weather split along a weak convergence line from the Kunene region to Ariamsvlei. North-east of this line, conditions are hot with isolated thunder showers in the afternoon. West and south of this vaguely-defined line, moisture penetration is limited with only the Kunene region and the northern parts of Erongo, building up some cloud.
During the weekend, a vortex develops south of the Cape, followed by a much stronger South Atlantic high, reading about 1032 mB at its core. But as has often happened lately, the cyclonic rotation and the racing vertical velocity (rising air) inside the vortex, robs the high of its strength. Another vortex some 1000 km east of Madagascar does the same to the southern Indian high pressure cell, thereby reducing the high pressure impact over eastern Africa, but at the same time, weakening the anti-cyclonic circulation of the South Atlantic high as it passes the southern Cape. This anti-cyclonic rotation is the engine behind the air circulation over the southern African interior, and when it is weakened, the rest of the motor fails to ignite. Light showers east of the convergence line remain the order of the day. These conditions will persist throughout next week.