What the El Nino Southern Oscillation means for us
El Nino and its driving force, the Southern Oscillation, are back in the news. Not because of their presence, but because of a possible resurgence. Should this occur, what would it mean for us?
The entire Namibia is climatically labelled as an arid country. As per this description, Namibia is the driest country in the maritime, southern hemisphere. Aridity means the deserts of the world will be found within this geographical range.
But Namibia has an identifiable rainfall season across “summer to autumn” – from January, February, March and with April as a useful addition in cooler conditions. Across the country, these four months would provide over 80% of the recorded rainfall totals, with statistics supporting those expectations across the individual stations’ rainfall history. So when any comment is made regarding weather/climate prospects for a given year, the focus will be on January to April first and foremost.
The remainder of the year has little practical meaning. This emphasizes a focus on both the overall farming/agricultural world and those who would have to support it should spasmodic rains or drought prevail.
With just over a century’s worth of daily data on hand, there is the realisation that ENSO-dominated years are drought-prone with heavier falls limited to a premium, wetter events similarly restricted or absent, the January to April range beleaguered.
The two most recent ENSO events, 2007 and 2010 for short, without the individual month range of influence, were both in the dry range. Climate change did see the northern quarter of Namibia enjoy a more ordered wet January to April range in 2010 but for the remainder (usually drier in any case) of the country, 2010 matched the dry scenario.
But elsewhere, 2010 gained another, differing aspect.
2010 saw an ENSO pattern at variance from the usual. Pressures did decrease at Tahiti, but Darwin lay for months beneath the Inter-Tropical Convergence Zone. With global warming, the southern hemisphere high pressure belt lay some 10 degrees further south. There has been little change from 2008 until our present 2012 with high barometer values equally consistent. This departure from normal was frequently remarked by the Australian commentators. Australia’s calendar 2010 saw widespread rains, breaking a 6-year dry to drought-ridden sequence, also from April La Nina trends were appearing, but not in control, in the Pacific. Their move currently is to dub 2010 as La Nina year, calendar year but that is based on their local conditions in that particular year.
For Namibia, 2010 is in the ENSO stable acknowledging the drier January to April records from central and southern Namibia. The danger of reclassifying 2010 lies not in the actual rainfall but in the perception, hence in forecasts based on the weather experience of that year.
Medium term forecasts are now generally in agreement that by the second half of this year, conditions in the Pacific Ocean will have returned to ENSO neutral. The implication is that there is a 50/50 chance for the Pacific to switch to El Nino by the end of the year. El Nino means drought for southern Africa but if the definition does not comfortably fit our local conditions it might as well just mean an average year.
Currently La Nina, although weaker than last year, is still in control as witnessed by the prominent north-south-north airflow. For us the remainder of the season will not be above average but as long as ENSO conditions are not neutral (yet), rainfall can continue up to the beginning of May.