Congo forest weather and tropical storms may delay rainfall in southern Africa

Researchers expect less spring and summer rainfall in southern Africa. This is according to a study that compared results from several computer models. 

Scientists use computers to predict future weather. Rainfall predictions are especially important for farmers - if rainfalls are delayed it means they will produce less food, which can lead to hunger and poverty. 

In this paper, the researchers wanted to see if the changes of rainfall experienced in southern Africa could possibly be due to the influence of other nearby weather systems. 

They checked that models catered for the Congo Air Boundary (CAB), a region in the Congo forest that experiences rapid changes from very humid to very dry, and ‘tropical lows’ that cause mild stormy weather (tropical storms). 

Then, researchers studied humidity and temperature data between August 1970 and December 1990 for the CAB. For tropical lows, they studied wind data between November 1969 and March 1999. They then compared their results with data for 1980-2010.

They looked for relationships between spring drying and the Congo forest weather, and also between summer rainfalls and tropical storms and the Congo forest weather.

Most models that the researchers used gave similar predictions. They reported that rainfalls in southern Africa would decline, specifically for October, November and December, and cause droughts. They said if rainfalls were delayed, the spring season was most likely to be dry.  Delays in rainfall shorten the growing season, and affect agriculture.

Their summer predictions showed a lot of variations but generally pointed to little rains. The researchers also said that spring would be drier than summer, but that the effects of the two seasons on agriculture would be the same. The researchers said spring drying is more likely, especially around the Congo forest, and between October and December. 

The researchers observed that model predictions on where and how often tropical storms would occur were not consistent, but showed a general 15% decrease. The storms were expected to occur mostly towards the north. They said the changes in tropical storm occurrence were slightly related to summer rains in southern Africa.

Scientists previously predicted droughts in southern Africa because of delayed rains. In this study researchers contributed a potential answer as to what causes the delay, namely the Congo forest weather changes and storms in tropical regions described above.

The researchers recommended more research to understand the models that are used to predict rainfall patterns. 

Abstract

In southern Africa, models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) predict robust future drying associated with a delayed rainy-season onset in the austral spring and a range of wetting and drying patterns in the austral summer. This paper relates these rainfall changes to dynamical shifts in two classes of weather systems: the Congo Air Boundary (CAB) and tropical lows. Objective algorithms are used to track these features in CMIP5 model output. It is then established that the climatological locations and frequencies of these systems are reasonably well represented in the CMIP5 models. RCP8.5 end-of-twenty-first-century projections are compared with historical end-of-twentieth-century simulations. Future projections in tropical-low locations and frequencies diverge, but indicate an overall average decrease of 15% and in some cases a northward shift. The projected spatial change in the tropical-low frequency distribution is weakly positively correlated to the projected spatial change in the austral summer rainfall distribution. Meanwhile, future projections indicate a 13% increase in CAB frequency from October to December. This is associated with the gradual climatological CAB breakdown occurring half a month later on average in end-of-twenty-first-century RCP8.5 projections. A delay in the gradual seasonal decline of the CAB prevents rainfall to the south of the CAB’s mean position, most of which is shown to occur on CAB breakdown days, hence creating the austral spring drying signal and delayed wet-season onset. Intermodel variability in the magnitude of CAB frequency increase is able to explain intermodel variability in the projected drying.

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