11 April 2014
What Happened?
Two high pressure systems, one west of the continent and another far to the south-east, made their combined influence felt during this week. The high pressure system west of the continent, the so-called South Atlantic High Pressure cell is always present somewhere between South America and Africa.
It follows a regular, pre-determined migration from west to east meaning every week or so, as it approaches the African continent, it has a marked influence on Namibia’s weather. As it slips past Cape Agulhas, it encounters warmer water in the Agulhas current, thereby losing some of its strength, becoming a secondary high pressure cell, before continuing its relentless journey to the east across the southern Indian Ocean.
When the approaching South Atlantic High and the secondary high are situated as they were this week, the airspace above southern Africa is impacted on the surface from the west and in the upper air levels, from the east. We experience it as cold air coming in, first from the south-west, then south, then some days later, the south-east.
Meanwhile, the secondary high leans back over the continent leading to higher density, colder air present in the upper ail levels above 45,000 feet.
In between, in the middle layers, is some space for zonal intrusion from the north-east, which we saw as a fairly prominent but shallow layer of clouds which tend to disappear as each day ends and the temperature drops.
This was the typical pattern for the week.
The South Atlantic High caused strong winds from the south across South Africa’s Northern Cape Province, flowing into and across Namibia manifesting itself as rather cold evenings and nights. This impact was felt as far north as Otjiwarongo.
But with still more than 11 hours and 30 minutes of sunshine per day, surface heating continues during the day, leading to an anti-cyclonic circulation over the interior, bringing in some moisture from Zambia.
wThe daytime heat leads to good cloud formation but restricted in its convection by the very cold, dry, dense layer above, originating from the secondary high. This restricts rainfall generally as witnessed by the precipitation figures recorded for the week.
What’s coming?
That there is still the potential for autumn rain is clearly indicated by all forecast models, but that this will be limited and scattered, is just as much part of the expected pattern.
During this weekend, the strong southerly flow backs to the south-east and later due east as the South Atlantic High passes the Cape, morphing into the secondary high south of Madagascar.
Sea surface temperatures in the Mozambican Channel remain elevated indicating a ready ability to capture moisture. About 1000 km south of Madagascar, sea surface temperatures are much colder than normal, providing the substrate for the secondary high to remain with some strength.
This has the ability to drive the zonal flow across northern Mozambique and Zambia, meaning there is still a window of expectation for late rain for Namibia, albeit very limited. Kaokoland and Damaraland may be particularly fortunate, getting more late rain than the rest of the interior.
