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Tuesday, January 14, 2020, 14:00 CET

Heat Wave / Drought
Southern Africa
July 2019 - Dec 2019

Satellite image (visible) from Lake Kariba
(between Simbabwe and Samibia)
November 28, 2019
Source: NASA

Southern Africa is suffering a destructive and dangerous heat wave / drought situation. From October 2019 to December 2019 there have been widely distributed rainfall deficits of more than 35%. This leads to substantial reduction of Lake Kariba's water level and issues in food security.

The southern part of Africa has been experiencing a heat wave with pronounced drought for several months. Contrary to what is often expected, southern Africa's climate is in general not characterised by enormous drought. The metropolis of Johannesburg records an annual average rainfall of 790 mm and an annual average temperature of 16.0°C. Large parts of the total precipitation come together in the local summer (Northern Hemisphere winter). East London, located on the south-east coast of the continent, also has a higher annual precipitation of 822 mm than Karlsruhe, for example (728 mm). The popular tourist destination Capte Town in the southwest of South Africa has an average annual precipitation of 853 mm and an average annual temperature of 16.9°C. Interestingly, dominant amounts of the precipitation are falling in the local winter, i.e. in the Northern hemisphere summer. According to the Köppen climate classification, the climate belongs to the same class as in the Mediterranean, called "Csb". The reason for this are frequent Atlantic low pressure areas, which are more likely to hit Capetown in local winter due to the equatorwards shift of the polar front. In the local summer, there is often a dry foehn wind from the inland Great Karoo, called Berg-Wind.

Precipitation anomaly (reference period 2000-2010) from September 10 until December 08, Source: NASA

According tot he World Food Program, many parts of southern Africa have recieved average rainfall in just on of the past five agricultural seasons. Nevertheless, the recent season is unusually dry and hot. Temperature-wise one can see large positive anomalies in the whole southerly African continent. Looking at the recent 30 days, the temperature anomaly peaks in the area between the Kingdom of Lesotho and northern Simbabwe. Around Lake Kariba the temperature anomaly is approximately +3 to +4°C.

Precipiation anomaly from Jul 12 - Jan12, Oct 01 - Dec 31 and temperature anomaly from December 14 until January 12, NOAA Climate Prediction Center

In a climatological averaged season in southern Africa rainfalls start to increase at the beginning of October. In 2019, rain saison started late and also with very litte and unreliable precipitation. Looking at rainfall percentage referred to average values, the drought is worst in western Namibia (in the Namib-Naukluft national park), where in some parts less than 1% of climatological average rainfall has fallen (180-Day review from January 12). In fact, the total amount of precipitation was 0 mm, where the climatological average in this reference period is 6 mm. Looking at absolute values of the past three months (October 2019 to December 2019), there is a lack of precipitation in the order of 100 to 200 mm in large parts of South Africa, especially in the southeast. You can also see huge precipitation deficits in central Angola with values around 200 mm below average. This corresponds to approximately 35% of rainfall deficit.

SPI time series from Capetown and northern Madagascar, Jan 2018 until Dec 2019 NOAA Climate Prediction Center

In contrast to severe drought in the stated regions, large parts of eastern Africa and northern Madagascar have experienced strongly above average precipitation amounts in the most recent three months. Above average sea surface temperatures led to high amounts of available moisture and a powerful ending of the 2019 North Indian Ocean cyclone Season and an early start of the 2019-20 South-West Indian Ocean cyclone season. For more details head over to: Tropical Cyclone activity in the Indian Ocean in December 2019

A good way to produce time series of precipitation anomalies is the Standardized Precipitation Index (SPI). It's a commonly used index to characterize the meteorogical strength of drought or heavy rainfall events. With the help of the SPI, precipitation perdiods can be compared on different time scales and from different climate zones. On short time scales the SPI is closely related to soil moisture, while on longer time scales the SPI can be related to groundwater and reservoir storage. SPI 1 refers to a one-month reference period, SPI 12 to a one-year period.
As mentioned in the beginning, in southern Africa (here as example Capetown) drought is not only a problem since 2019, but it's a long term problem which became very pronounced in 2019. On the contrary, northern Madagascar seems to experience the opposite effects with above average precipitation in almost every month in the past two years.

Precipitation probability forecast (NMME), Feb until Apr, 2020 and current 7-day SSt anomaly, NOAA Climate Prediction Center

Accorrding to predictions from North American Multi-Model Ensemble (NMME) there is a slight to moderate tilt in the odds to favor above-average rainfall over portions of East Africa through the northern hemisphere spring 2020. However, below-average rainfall over portions of Southern Africa remain likely through the northern hemisphere spring 2020.

The pictures below show the reduction of Lake Kariba's water level between end of 2018 and end of 2019.

Satellite images from Lake Kariba (total see and its coastlines) Source: NASA

Text: FS
January 14, 2019

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