Overview for the Week and 5-day outlook to Wednesday 11 March 2018
Visual: 30-Day Moving Southern Oscillation Index
Source: Australian Bureau of Meteorology http://poama.bom.gov.au/climate/enso/
The Southern Oscillation Index maintained by the Australian Bureau of Meteorology broke through the 10 index point level last week, this week moving further up to Friday’s 12.3 value. This is for the so-called 30-day moving average which expresses an intensification (positive) or weakening (negative) of the difference in surface pressure between Darwin in Australia and Tahiti, some 1000 km to the east in the western Equatorial Pacific. This difference is then compared to the values of a month ago to determine the movement and derive at an index value.
Although the 90-day moving average is more subdued, reading only +4, the 30-day value is historically a leading indicator for short-term conditions over the western half of southern Africa. When the SOI index moves down, there is a noticeable dry spell over Namibia for the next two weeks, and when it moves up, usually some precipitation results in the following two weeks.
The relatively high positive value of the 30-day index is another encouraging sign that there is still much life in the current rainfall season. This is corroborated by other atmospheric factors, most notably the active state of the so-called Indian Ocean Transport.
One of the main impediments to local precipitation during November, December, January and February, was the persistent presence of high pressure control over East Africa. Since most of Namibia’s rainfall comes from the Indian Ocean, when the conveyor mechanism across Tanzania and into central Africa is broken or obstructed, it results in lower moisture levels in the countries north of Namibia. Even if there is enhanced anti-cyclonic circulation over the rest of the sub-continent, it will not rain in Namibia if the source of moisture over Angola and Zambia is depleted.
This is what happened during the first and middle parts of this season. It is only in the last month that the Indian Ocean Transport has come to life (after Tropical Storm Dumazile) with surface wind charts showing a strong zonal flow from east to west just north of Madagascar. These positive conditions have continued into this week.
Another assisting factor during the week was the relative strength of the southern Indian high pressure cell (1028 mB) versus the relative weakness of the South Atlantic high (1016 mB). The northern rim of the southern Indian high drives the Indian Ocean Transport enhancing the influx of moisture over central Africa. The South Atlantic high opposes this system, thus when it is weak, it allows space for the continental anti-cyclonic circulation to advect moisture from Angola into Namibia.
This was further amplified by a very strong vortex some 2000 km south of Cape Agulhas, at the southern end of the cold front that is moving over Cape Town this Friday, 06 April. Ahead of the cold front is a prominent trough, from north to south, which also helped drawing in moisture from Angola.
Behind the cold front, surface conditions are controlled by the approaching South Atlantic high. This leads to suppressed convection over the southern half of Namibia but only for Saturday. Rainfall conditions for the northern half remain positive.
By Sunday there is a major change in the overall synoptic pattern. With the leading rim of the South Atlantic high now past Cape Agulhas, its effect is felt more over the eastern half of South Africa.
Over Namibia the strong north-easterly to northerly airflow resumes during Sunday leading to extensive cloudiness that may even penetrate the southern Namib. The Indian Ocean Transport remains active and strong, bringing elevated levels of moisture to the southern DRC and the whole of Angola.
These conditions remain in place during Monday and Tuesday but by Tuesday night clearance from the west is expected. The forecast pattern is very much a repeat of last week’s conditions.
As from Wednesday, the synoptic pattern is expected to revert to a more conventional stance, dividing Namibia again into a northern and southern half. Conditions over the northern half are conducive for rain while the southern half should see clear skies or limited cloud cover.
However, the feature to watch is the budding low pressure system in the northern Mozambican Channel. This has all the potential to develop into a tropical depression which will feed into the Indian Ocean Transport. If this happens, the current outlook will be wrong and the rainfall window may just continue for the whole of next week and into the following weekend.