As the Pacific sea surface temperature anomaly weakened from its strong positive number scientists became less confident about future trends with forecasts subject to more than the normal variation month to month. On 8 December the International Research Institute for Climate and Society at Columbia University’s Earth institute (IRI) reported that the anomaly was -0.5°C, representing the threshold for a weak La Niña. The following graph illustrates their prediction that a strong La Niña will not develop and that the likely outlook well into 2017 is for a neutral situation (as defined by deviations from the normal of less than 0.5°). This corresponds to historical data indicating that anomalies either way tend to weaken after our southern hemisphere summer. In fact they predicted that by the May/June/July 2017 forecast period the probability for a El Niño event will surpass that for La Niña (23 v 18% but with neutral still way ahead at 59%). The graph below illustrates this forecast. This, together with the other factors that they take into account1, nevertheless translates to a 40 to 70% chance of South African summer temperatures being significantly above normal especially for the earlier forecast periods but tapering off in the March – May 2017 period. Interestingly IRI have only one part of Africa highlighted as having a moderately or enhanced probability of significantly lower or higher than normal rainfall, and that is for Zimbabwe (moderately enhanced probability – 45 to 50% – of above average rainfall) and then only for the December through February element of the forecast. This is in contrast to the temperature forecast where a long list of African countries is included in the high temperature probabilities.
1 The other factors taken into account include tropical Indian Ocean forecasts, global atmospheric general circulation model predictions and sources such as NASA. It is interesting to note that NASA recently launched a geostationary satellite to provide more frequent and higher resolution images of a range of weather patterns with the aim of improving long term forecasts, watches, warnings etc. Perhaps with an eye on Donald Trump, popular US weatherman Al Roker said on NASA tv “We want to make American weather great again”. Just as with support for Trump I suspect that depends on which part of the US you live in. (With Agence France-Presse)
For my own part of Gauteng province in South Africa the temperature difference relative to 2015’s November was marked, as illustrated by the table below. The perceived temperature reduction was dependent on afternoon maxima while the night minima differences were less noticeable. The considerably reduced daily range of temperatures can be attributed to a generally moister circulation reducing radiation in and out.
|Number of days||2015||2016|
|4 to 6||3|
|7 to 9||3|
|10 to 12||11||9|
|13 to 15||7||15|
|16 to 18||6||6||1|
|19 to 21||5||5|
|22 to 24||9||9|
|25 to 27||6||13|
|28 to 30||5||2|
|31 to 33||5|
Rainfall has been dramatically more in the 2016/17 rainy season to date, correlating with the more humid conditions noted above.
Our home area has received the good rains above but in the Kruger National Park last month the grazing was extremely sparse. Then driving down from Johannesburg to near Cape Town in the last few days brings home the patchy nature of the rains. While the maize areas of the northern Free State were looking quite good the areas further south were very dry. As we drove into Colesberg (Northern Cape) in the late afternoon the car was indicating an outside temperature of 41°C (normally inflated by two or three degrees on the tar roads) and even the owners of the overnight accommodation were feeling that, together with last year being similar, they may need to move. One thing we had never seen before was the turksvye (prickly pear cactus, Opuntia ficus-indica, famed for resistance to heat and drought) in Beaufort West being killed by the heat and drought.
Dam Levels. Despite the good rains in some areas the drought is still critical as reflected in dam levels. In the last 2½ months the region’s levels have dropped from 51 to 49% of full storage capacity (FSC) and over one year from 59 to 49%. The most alarming drop contributing to the latter change is for the Western Cape, from where I am writing, dropping from 63 to 49%. We look out onto the effects on gardens as water restrictions start to impact. An interesting feature of the figures for the Free State, wherein lies nearly 50% of the region’s capacity, is the increase in the Vaal catchment area numbers but decline in those for the Orange, reflecting the good rains in the northern part but continuing drought further south as noted above. In particular Vaaldam has increased 12% points over the last year while releasing water to it from Sterkfontein has only reduced that dam by 4%. We continue to be amazed at how kwaZulu Natal dams continue below 50% capacity when it always seems to be raining there! The region’s figures (for SA and Lesotho) are as follows:
|Percentage of capacity||19.12.16||19.12.15||3.10.16|
|Free State total||15,971||50||59||53|
|W Cape total||1,853||49||57||63|
|Albert Falls Dam||288||28||41|
|E Cape total||1,826||59||74||64|
|Katse / Mohale (Lesotho)||2,376||38||51||37|
|Total RSA and Lesotho||31,913||49||59||51|