My 23 June blog on this topic emphasised that despite the decline in the El Niño there was little indication of a La Niña developing (except for the forecast of the Scripps Institution of Oceanography of the University of California, San Diego). In its July update the International Research Institute for Climate and Society of Columbia University’s Earth Institute (IRI) continues to ascribe a low probability to a La Niña, with the likelihood that sea surface temperatures will maintain an El Niño Southern Oscillation (ENSO) neutral position for the coming months. Chances of an El Niño for the next nine months or so are put at 35 to 40%. The following chart shows the official forecast based on models which have been adjusted for the input of IRI and other forecasters. It is interesting to note that the chances of a La Niña based on unadjusted model forecasts are very much lower in the later months, declining to almost negligible figures towards Q2 2018.
Western Cape rainfall continues to disappoint and dismay. The graph below indicates how rainfall has been far below normal up to early July and compared to virtually all the years since 1977. Interestingly it is currently tracking very closely the 2016 line which itself was at the lowest end of the 40 year data. The University of Cape Town’s Climate System Analysis Group website shows the near match continuing through July but then failing to show the same increase in early August compared to 2016. Both 2016 and 2017 to date feature negligible summer rainfall and at best only a short period of winter rainfall where the graph is at a similar gradient to normal. 2016’s rainfall was only 220mm compared to the 40 year median of around 515mm. The cumulative figure for the year to 1 August 2017 is 101mm compared to 114mm for the same period last year and to the 40 year median of 303mm. The one prior year line which can be seen on the graph tracking a similar path to 2016 and 2017 was 1994.
Apologies for the poor definition of the graph. The main point is that the red 2017 line is closely following the mauve line for 2016 and well below nearly all the lines for some 38 earlier years.
The graph in fact surprises me in that the normal winter rainfall is not as pronounced relative to the summer months as I would have expected or compared to the similar summer rainfall on the Highveld (see below). In contrast the 2016 data depicts a year in which the great majority of the rain fell in just three winter months and 2017 is exhibiting the same trend of much reduced summer rainfall and shorter winter rains. Of note over the last week or so has been that cold fronts (which bring the winter rain) have been merely brushing the south coast and bringing little rain to the south west corner, even while areas further east have been getting better rains.
The heavy rain of early June gave a welcome boost to the cumulative rainfall figure and to the Theewaterskloof Dam, the main source of Cape Town’s water, but the level has not increased greatly since then, leaving it at 21.3% on 31 July when Western Cape dams in total were at 27.4% of capacity. (The 4 August figure for Theewaterskloof was 21.7%). The comparable figures for a year ago were 46.9 and 54.8% respectively. The outlook appears disastrous given the 12 month drop in levels, when rainfall has shown no improvement and restrictions of varying severity have been in place. A rainfall pattern for the remainder of 2017 similar to 2016 would likely leave dams with effectively nil extractable reserves well before midsummer. Residential usage has been restricted to extremely low levels but industry and agriculture cannot make similar savings without major reductions in output.
The following graph (again with poor definition but covering the period 23 July to 4 August and with a vertical scale 20.2 to 21.8% of capacity) shows the small increase (1.4% points) in the level of Theewaterskloof Dam from 23 July to 4 August. The shape of the graph leaves the distinct impression that if there is reasonable rain the dam level increases but with no lasting effect beyond a few days, after which inflow does no more than match outflow/usage for a time. This is possibly understandable in the light of the very small catchment area of the dam (via the Riviersonderend, somewhat ironically “the river without end” but flowing eastwards away from Cape Town) and a drought related low water table.
The abnormal Cape weather is to some extent mirrored in the Highveld area of northern South Africa. Strong cold fronts affecting the Western Cape bring colder weather to the Highveld a couple of days later. This winter it has been more a case of catching the tail end of the cold front. The only sustained approximation to a normal Highveld winter was during the period 17 to 22 July when average daily minimum temperatures were 0.2°C and maximum 20.7°C; not exactly bone chilling weather even then. The number of readers who used to live on the Highveld would be surprised at the condition of the local suburban lawns which for the first time in my experience were not reduced to brown, freeze dried shreds which broke up when trampled, under the impact of severe ground frosts, clear skies, drought and very low humidity air. (Reminding me of a comment the other day by a visitor to the country that the best summer he ever had was a Highveld winter).
As has been mentioned before the Highveld summer produced excellent rains, as the Excel screenprint below indicates. Even April (85mm) and May (58mm) chipped in with relatively good rains. The 1mm since 14 May is typical of winter. The total for the year to 30 June was 1069mm
Sources: http://www.dwa.gov.za http://www.2oceansvibe.com http://iri.columbia.edu http://www.csag.uct.ac.za/