Understanding Weather – not predicting – 24 August2012
Again the northern half of the country remained off the central weather stage while the southern half saw brisk activity. Two major troughs, low pressure in action, sharpened west of Cape Town both weakening while advancing east then slid away leaving us unscathed. The South did see colder air briefly intrude but the rapid departure of (both) troughs gave scant opportunity for such intrusions to advance further north.
During the week, the Indian Ocean high pressure system which is mostly inactive in winter showed the first signs of its early summer revival. Moist air penetrated the Caprivi and Okavango regions but in limited intensity. It provided a source from which moist inflow proceeds inland, into the broad Limpopo and Zambezi valleys. Such moist inflow is essential to our savannah climate. This last week’s inflow saw clouds across the mid-continent reaching as far south as our northern limits. It would seem to have fed an early season upper air trough forming above Zambia.
A week of pleasantly mild to warm weather for the northern half, the south saw a ready ability to return to warmth, as a more northerly airflow returned.
Typical late winter? Not quite. The absence of the regular August winds and their driving synoptic patterns should not go unnoticed.
So just what are we looking at? The obvious excuse of climate change is a quick response, but the implications go deeper.
This brings up the situation evolving in the mid-Pacific: the playground of the two oscillations: positive and negative. Positive La Nina has declined, but the negative El Nino dawdles. The elements loom but, perhaps to follow the pattern of the last months of 2009, the full development being the “regular synopsis” during such events appears uncertain, The La Nina stance is at this stage still not entirely dead with persistent north-south orientation of the various anticyclones. Their surface level strengths, i.e. barometric pressures, show scant variation.
The pundits note El Nino style deviation regarding cloudiness and rainfall with sea surface temperatures warming. The cause is ascribed to weakening Trade Winds, yet with pressure cores well above 1030hPa, such weakened air flows require a more profound understanding. Namibia’s ENSO stance historically sharpens the rainfall variability during our rainfall season. The impact may be earlier rains of limited intensities and potential earlier seasonal activity for most of the country. The last ENSO event affecting our 2010 season (January to April) bore several of these expectations, yet the intrusion (regularly) of Congo air saw scattered rains for much of the country.
By Saturday, yet another deep trough collapses, departing southeast around the Cape. Rain is possible over our southwest. A brief wind swing across the central parts reverts to a northerly control during Sunday. The next trough appears by mid-week, the distant outlook favours its collapse too. Otherwise occasional cloudiness occurs amid the generally warm daytimes. The weather scene is on the cusp of spring. Temperatures below 6oC at night, are highly unlikely while daytime temperatures should range from the low twenties in the South to the high twenties or even low thirties in the North. Apart from a short intrusion by Tuesday, August winds should remain absent.