Understanding Weather – not predicting – 8 June 2012
Much attention has been drawn to the majesty of the British Diamond Jubilee while another form of majesty has occurred on our doorstep: that of three dimensional weather. The scale of attraction is equally broad, not only in the southern hemisphere, but embracing the northern also.
The focus is on the Pacific Ocean: almost half the width of the planet. For some years now, the climate scene across the central, Equatorial Pacific, a mere 10degrees of latitude, has ranged between the given hallmarks of El Nino/Southern (negative) Oscillation (ENSO) and La Nina which should be labelled LaNSO, to emphasize its positive deflection. The normal range has been swept aside amid these fluctuations. But the majesty of the considerable impacts on the synoptic patterns must attract attention. A major air flow feed to the equatorial zone are the trade winds which is driven by the core strengths of the sub-tropical High Pressure Belt.
By normal standards, this is located along the 30o latitude, at least until climate change entered the scene. For much of the new millenium, we have seen a ready ability for this surface core to appear easily along 40oS (or N) and unashamedly close to the 50o’s. While increasing the distance of the trade wind flow, there has been very little diminution of the ability of the South-easterly Trade to cool the equatorial belt. This is La Nina at work. But with the southward drift of the high pressure core there has been an equally ready ability to extend anticyclonic flows deep into the even higher latitudes. This brings warm, subtropical air far to the south so exciting a warmer then normal inflow into the polar front vortex, meaning excessively lower pressures compared to the normal scale.
Ahead of the cold fronts, the ability to draw moist, maritime, semi-polar air northward into the sub-tropics has proceeded consistently. This north-south-north flow is a hallmark of La Nina’s control.
Yet across this vast ocean belt, observations indicate a weakened level of cooling. Recalling a similar scene during late 2009 and early 2010, a weak trough line could be tracked along some 10oS, vaguely west to east, so restricting the northern extent of the polar air flow. While not yet fully identified, this potential may loom.
By mid-week colder air advanced well into Namibia and, as the flow curved around descending the escarpment it brought heat to the Namib. This flow also brought cloud to the Kavango.
The warm northerly flow develops an intense vortex south of the Cape. Cloud and rain are likely for the far south, for a few hours. By Saturday this rainy weather has gone, in its rear a cold inflow penetrates across the whole interior for some 24 hours with real frost potential. The next front is less dramatic. By Monday an easterly flow takes precedence, with “oosweer” again for much of the Namib. This air flow brings cloud development into our northeast, at least. The majesty of towering columns of ascending and descending airmasses virtually next door to each other remains at a distance from our shores. With winter on hand, cold looms on a nightly basis and cold daytimes for the south until midweek.