04 April 2014
A seasonal anomaly, very visible on sea-surface-temperature maps, is the large variance in temperature over the south-western Indian Ocean, most notably almost directly south of Madagascar.
While the sea-surface temperature in the upper half of the Mozambican Channel and the adjacent Indian Ocean (close to the continent) has been anomalously high since about the end of January, the ocean roughly 1000 km south of Madagascar has been markedly cooler than normal. The zone where the warmer and the colder oceans meet is clearly visible on satellite images based on refraction, i.e. difference in wavelengths due to difference in energy.
This feature has a marked influence on our local weather, especially over the past month or so, during which a fairly regular rain-bearing sequence of patterns has manifested itself over southern Africa. These conditions seem set to continue for a while, or for at least for as long as the stark difference in sea-surface temperatures prevails over adjacent sections of the Indian Ocean.
A marked feature on the synoptic chart earlier this week was the rather slow progression of the South Atlantic High. This gives the warmer water in the Mozambican Channel the opportunity to warm the air above it, leading to a prominent cyclonic circulation between Madagascar and the continent. This low-pressure area acts like a scoop, picking up vast amounts of moisture and dumping it into a zonal circulation crossing southern Africa from east to west. This zonal circulation is driven by the secondary high-pressure system south of Madagascar, which is just as prominent due to the colder-than-usual ocean temperatures in that area.
What happens is that the South Atlantic High maintains its strength as it slips past Cape Agulhas, bolstered by the cold water. Normally, the high-pressure system will weaken and dissipate as it crosses warmer water in the Agulhas current, but this has not been the case for a while. Since the high-pressure core stays strong, it exerts an influence for thousands of kilometres in a northerly direction, meeting the tropical zonal flow from the Indian Ocean and driving this in a very large curve across northern Mozambique, Zambia, the DRC and Angola. From there, it only requires the typical anti-cyclonic circulation over southern Africa to advect the moisture south across Namibia. This sequence has repeated itself at least six times over three months, every time bringing a few days of good, widespread precipitation. This is happening again.
But once the high pressure system is over eastern South Africa, it leans back (to the west) over the continent, thereby restricting convection in the upper air, leading to reduced rainfall. This is also happening again.
The South Atlantic High Pressure cell slowly migrates around the Cape and extends its grip on the eastern half of the sub-continent. On the downside of this circulation, a by-now signatory trough (low-pressure band) extrudes from Angola over Namibia, first towards the west, and then later towards the south.
This convergence zone will start its work over the weekend, leading to widespread light rain, slowly shifting towards the east, bringing heavier rain across the interior from the Kunene in the north-west to the Kalahari in the south-east. The high-pressure influence in the upper air, above 45,000 feet, remains strong, indicating that single showers will seldom exceed 10mm.