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New insights into century-scale rainfall variability over Africa

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Figure: Annual time series over the length of record for multi-regional sectors shown in the inset. Rainfall is expressed as a spatially-averaged standardized departures (Nicholson et al. 2018, DOI: 10.1016/j.gloplacha.2017.12.014).

Due to rain-fed smallholder farming practices across much of sub-Saharan Africa, hydro-meteorological extreme events such as droughts and floods often mean famine and sometimes even political unrest. A salient example are the recent droughts in the Greater Horn of Africa region that also stirred a vivid discussion among scientists whether these droughts can be attributed to ongoing climate change or are reflecting longer-term, but natural climate variations. A substantial contribution towards answering this question is the investigation of past rainfall fluctuations – yet Africa is notorious for its data sparseness.

Over the last two decades, researchers of the Institute of Meteorology and Climate Research (IMK) led by Prof. Dr. Andreas H. Fink have compiled observations from many different sources into the flexible and easy-to-use “Karlsruhe African Surface Station Database” (KASS-D). This was only possible through close collaboration with many African researchers and institutions. Containing several millions of rainfall and other weather observations already and growing steadily, KASS-D is an invaluable and rich resource for climate and weather research in Africa at KIT and beyond. For two recent publications, KASS-D was combined with rain gauge archives from two renowned climatologists – Prof. Dr. Sharon Nicholson (Florida State University) and Dr. Chris Funk (University of California at Santa Barbara) – to create the so far longest and most comprehensive gauge series for Africa encompassing most of the continent for the last 125–175 years.

In the first publication (Nicholson et al. 2018, DOI: 10.1002/joc.5530), the recent recovery of rainfall in the West African Sahel is investigated. While the Sahelian drought in the 1970s/1980s is unprecedented in the instrumental period since 1854, a full recovery is not observed, the recovery is stronger in the eastern Sahel than in the western part, and characterized by stronger year-to-year fluctuations. A climate regime shift occurred in 1968 that also changed teleconnections to the ocean temperature fluctuations known as Atlantic and Pacific El Niños. Also at the populous Guinea Coast, annual rainfall since about the late 1960s has been lower than ever since 1886. Taking the explosive population growth into account, this causes increasing water scarcity across West Africa.

A continent-wide view on centennial rainfall variability is given in the second invited publication (Nicholson et al. 2018, DOI: 10.1016/j.gloplacha.2017.12.014). The change to more arid conditions around 1968 occurred in parallel south and north of the Sahara (see figure). For the continent as a whole, more arid conditions began in the 1980s. In equatorial regions, the first rainy season in boreal spring has become drier, while the second rainy season in boreal autumn has gotten wetter. The study also shows that due to a recent decline in meteorological stations, ground truth on rainfall trends is not available for some important regions such as the large Congo rainforest.  Both studies corroborate the value of surface meteorological observations in Africa to quantify rainfall variability and their potential causes.

(Working group: Atmospheric Dynamics)