Namibia: Rainfall Year 2009 - Unevenly Eventful
John Olszewski, Namibia Economist in AllAfrica.com, 12 June 2009
Windhoek — Less than a year ago, the prospects for the new season offered some promise but also considerable doubt. In building an analysis, the interesting 50-year pulse provided the prospect of some rain but with drought spells and an overall below normal total at the close. The effect of climate change could not be estimated but could not be ignored.
The results provided a magnification of the prospect ranges.
The arrival of early rains during the spring months was to be viewed with gratitude but also with the misgiving of a departure with no sure prospect of a return. The "green Christmas and a bad year" coupling had to be borne in mind.
October and, more particularly, November provided some productive rain.
December, similar to 1959 in many ways, went dry.
Into January and the persisting heat and dry air brought mounting concern.
Meanwhile the Pacific Ocean, which had been dithering on the verge of a La Nina event, found itself on the positive side. The Australian Bureau of Meteorology pronounced its opinion that the Southern Oscillation feature was now in a La Nina mode. This was during the second week of January.
Active weather, the Inter-Tropical Convergence Zone (ITCZ), was there for all to see away to the east of Rundu and well marked at Katima.
During the second week, timing well with La Nina revealing itself, the ITCZ drifted westward. Rains ensued forthwith.
Season 2008 had provided a glimpse of what an ITCZ invasion could do when it enters our (northern) skies.
A very similar repetition was to follow.
Okatana Mission was to herald the onset: 170mm on the 18th opened the score. Ondangwa followed suit with a 5-day Wet Spell totalling almost 150mm simultaneously. Similar ranges were to be recorded across the northern parts with very little break for the next 7 weeks.
Such features as wettest day, duration of Wet Spell days and amounts recorded were equalled or improved and the contributions to the Rainy Spell ranges (consecutive days of 1mm or more) appeared en masse by our standards.
For the Owambo regions in particular, this was just too much water. Having broken records just a year earlier, the saturation levels of the Oshanas and the slightly higher ground in-between were quickly attained and exceeded: widespread flooding yet again.
The ITCZ presence was propagating very heavy and protracted falls across southern Angola. The resultant efundja led to even higher flooding levels. For the second year running, the Etosha pan was full to the brim and even, according to some sources, overflowing into its surrounds.
On the fringes of this ITCZ activity, Sitrusdal led the Rainy Spell parade with 18 consecutive days of rain in February and only 2 rainless days in that month. This group of days of rain extended to 38 consecutive days of rain. And this in a climate zone labelled arid!
Further afield, but also in February, Omaruru has a Rainy Spell of 12 days.
Even further afield, Usakos had an 8-day offering. In between, while not breaking records, the rains were considerable.
The south and westward extension reached the coast. Mowe Bay, Ganab, Walvis Bay and Gobabeb all could report unusual rains or numbers of days on which rain fell. Inland, patchiness emerged further away from the ITCZ core of activity. This dry zone was most notable across the eastern parts from about Tsumkwe southwards to well beyond Gobabis.
It was only as the ITCZ core weakened that effective rains began to fall across these thirsty areas.
The ITCZ had been held in place by a mid-level anticyclone above central Botswana.
The southward flow generated by the anticyclone did advect some moisture into the south. But, considering the standards further north, the overall picture left a feel of need unfulfilled.
The ITCZ drifted northward during March, La Nina weakened during April and the impact on these months of usually ample rainfall input was generally limited: 1959 shadow returning?
From this, do we dare assume that ITCZ presence is now to be anticipated with some form of regularity? Are northern floods to be a repetitive event? The eastern shortfall is easily identified, but is this to be a mix of wet and dry in such close juxtaposition? ITCZ range of influence maybe difficult to measure, but can the variability of an arid climate zone find some kind of boundary? With climate change, particularly varied circulation patterns, in evidence outlooks will be varied. Season 2009 will have made its mark in disappointment, dryness and record-breaking all in the same 6-month range. Variability suggests versatility, are these boundaries about to be widened?
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