Rikus Grobler | Jun 20, 2017 | 0
Weather 03 June 2016
For the first time in just over a year the Southern Oscillation Index of the Australian Bureau of Meteorology has turned positive. This index measures the pressure differential between Darwin in Australia and Tahiti in the Pacific Ocean. When the index is negative, the differential is very small, as has been the case during the El Nino phase. When it turns positive, it indicates that there is a bigger difference between the sea surface temperatures around Tahiti and the Pacific Ocean at northern Australia. This is indeed the case. The equatorial Pacific in the so-called Nino 3 zone has rapidly shifted from an El Nino to a La Nina stance with sea surface temperatures in this area already more than half a degree Celsius cooler than what is considered “normal”. If this trend continues for a six months period (five consecutive overlapping three-month readings), then the La Nina is official. By all current indications, the La Nina will be quite strong since the deviation in the subsurface temperatures is as much as minus four degrees Celsius over a very large area. The local weather remains under high pressure control as can be seen in the striations at high altitude. In the upper air, the zonal flow from west to east is much enhanced. The so-called jetstreams are relatively strong, blowing at more than 200 km/h but this is only above 30,000 feet. The cold nights are a further indication that high pressure controls the surface conditions as well. Early in the week, the southern Indian high pressure cell lay south of Madagascar with a very strong core reading of 1036 mB while the South Atlantic high was subdued, reading only 1024 mB with the core far out to sea, more than 2000 km. Between the two highs, a well-demarcated low pressure system developed, advecting warmer air from the north (Angola) across Namibia into South Africa. As the week progressed, the band of low pressure developed into a proper trough stretching all the way from the Kunene river to the Orange river, bringing some cloudiness to the eastern half of the Karas region. This system developed to such an extent that the Namibia Meteorological Service expected slight precipitation in the south-east on Wednesday and Thursday, however, no confirmed report was received of either light rain or snow.
Meanwhile, the outer rim of the approaching South Atlantic high moved across South Africa and over the SA highveld. At the same time, the southern Indian high shifted to the east, losing much of its ability to impact the sub-continent from the east. The result was warmer weather in general over the whole Namibia. By Thursday evening, the core of the South Atlantic high was very close to the continent while the entire subcontinent was under mild high pressure control measuring about 1020 mB. It was only in Namibia’s north-western corner (Kaokoveld) where lower pressure was present but even here it measured 1016 mB, so still on the high side of the spectrum. By Friday, the southern Indian high has moved off our radar and the South Atlantic high has made landfall in the western Cape.
The weekend starts with most of the sub-continent engulfed by the high pressure system while the low pressure system along the northern Namib develops to the south but remains confined to the coastal plain. By Sunday the core of the high pressure cell reads around 1024 mB, indicating cold conditions over South Africa and the southern half of Botswana. This isobar misses the Karasburg district by a whisker. The low pressure system over the Kunene and northern Erongo regions grows in intensity and a marked pressure differential between east (Botswana border) and west (Atlantic Ocean) develops. This is a very typical winter pattern for Namibia. By Monday next week, cold air driven by the high over South Africa and Botswana, starts having an impact on local weather. The eastern quadrant, and the Karas region will experience its first proper winter cold, first from the south-east and by Tuesday from the east.
The presence of the high prevent the southward development of the coastal low, restricting it to the northern Namib north of Hentiesbaai. The pressure differential is small, resulting in only moderate winds over the northern Namib with only weak Oosweer possible.
Two forecasts indicate the presence of some moisture over the interior as from Tuesday evening but the bias is over the southern Namib and the western part of the Karas region. It may just bring Aus its first snow.