The African Sahel is one of the most vulnerable regions to climate variability. It is an arid to semi arid zone that has very limited water resources, see Figure 1. Annual rainfall is the major one, but is characterized by a very strong inter-annual variability. El Niño–Southern Oscillation (ENSO) is believed to be one of the driving mechanisms of this variability. ENSO refers to a climate variability caused by the interaction between the ocean and the atmosphere of the eastern tropical Pacific. More in detail, El Niño refers to a warming of the ocean’s surface, while the Southern Oscillation is the change in atmospheric surface pressure between Darwin in northern Australia and Tahiti in the central pacific. Therefore, the Southern Oscillation also affects the speed of the surface wind across the Pacific. ENSO is a semi-cyclic phenomenon that impacts the climate on a global scale every 2 to 7 years. Before we discuss the variability further, let us examine the prevailing state of the ocean and the atmosphere in the tropical Pacific.
The Pacific basin extends from the coast of South America in the east, to Indonesia in the west. Due to the westward surface wind, the sea surface temperature (SST) and the sea level are higher in the western Pacific than in the eastern part. The relatively hot ocean surface causes vertical motion in the atmosphere, which produces low surface pressure and heavy rainfall. The rising air splits, moves both to the east and west and then descends, resulting in higher atmospheric pressures to the east and west of the zone of ascent. Therefore, the eastern Pacific attains a higher surface pressure, which sustains the westward surface wind. This circulation is named Walker circulation, after the British meteorologist Gilbert Walker. Figure 2 shows the global vertical motion across the tropics; the pacific Walker circulation is emphasized.
During El Niño episodes the dominant state in the ocean and the atmosphere breaks down. The chain of actions starts when the westward surface winds weaken, leading to two effects. Firstly, this weakens the upwelling of cold water in the eastern pacific. Secondly, more warm water from the western Pacific will be transported eastward. In this way, the east-west difference in SSTs and atmospheric pressure decreases. Consequently, the westward wind will be weakened, which further reduces the upwelling of cold water in the eastern Pacific. We can summarize the whole series of actions by saying that El Niño occurs when the eastern Pacific fails to stay colder than the western part. ENSO also has another phase, called La Niña, in which the eastern Pacific becomes colder than its normal state. It is typically the opposite of El Niño.
As mentioned earlier, ENSO affects the climate in many places around the globe. El Niño affects the weather in countries like Peru, Chile and western part of the United States, causing heavy rainfall and floods. On the contrary, droughts and heat waves strike Indonesia and Australia. The situation is reversed during La Niña. The signal of ENSO reaches even further, influencing the dynamics of monsoons in Asia and Africa and the SSTs of the Indian Ocean.
Here, we consider the eastern part of the Sahel, covering the area between 22° to 36°E and 11° to 15°N. This corresponds to the central part of Sudan. To examine the link between rainfall and ENSO, we have obtained the area averaged annual rainfall of Sahelian Sudan from the Global Climatology Precipitation Project (GPCP) and the SSTs of the central Pacific from the National Oceanic and Atmospheric Administration (NOAA). The two datasets cover 35 years, from 1979 to 2013. We compute the annual mean rainfall and SST, and then calculate the differences from the mean in each dataset. This is supposed to reveal the years that have values of rainfall and SSTs that are higher or lower than normal.
Figure 3 presents the year-to-year variation in rainfall and SSTs. The general picture shows that difference in SSTs and rainfall are opposite to one another. Several years have low rainfall amounts are accompanied by an increase in SSTs. Note that these changes in rainfall amount seem to follow the ones in the SSTs. Along the same line, some years show higher rainfall than normal co-existing with SSTs which are lower than normal. A study by Joly and Voldoire (2008) showed that ENSO’s influence on the rainfall occurs either during the development period of El Niño or during the decaying phase of a relatively long La Niña episode.
The link between eastern Sahel rainfall and ENSO has been widely highlighted in the literature. For example, Osman and Shamseldin (2002) analyzed a 49-year record of rainfall from 12 weather stations in central (Sahelian) and South (Equatorial) Sudan. Their results showed that the driest years coincided with a warm ENSO. The study also indicated that the ENSO signal is stronger on the Sahelian rainfall compared to the equatorial one. The same signal is also found in the annual flow of the river Nile. Eltahir (1996) showed that 25% of the variability in the amount of water in the Nile River could be explained by ENSO. Because the Blue Nile mainly originates from Ethiopia, Eltahir’s result indicates that the rainfall over the Ethiopian highlands is also sensitive to ENSO.
But how does ENSO affect rainfall in Sahelian Sudan? What is the physical mechanism that explains this link? A possible explanation combines the moisture transport and atmospheric circulation. The Indian Ocean is one of the moisture sources of Sahelian Sudan. If the SSTs change for any reason, this will affect the total moisture transported into the region. Also, the variation in temperature between the Indian Ocean and the land body of the Indian continent is responsible for very important wind patterns. These patterns are assumed to transport moisture to the Sahelian zone. However, no conclusive explanation has been developed yet. Thus, there is still the need to fully understand the physical connection between ENSO and Sahelian rainfall.
Eltahir EAB. 1996. El Niño and the natural variability in the flow of the Nile River. Water Resources Research Vol. 32. No 1: 131 – 137
Joly M, Voldoire A. 2009. Influence of ENSO on the West African Monsoon: Temporal Aspects and Atmospheric Processes. Journal of Climate, 22, 3193–3210. doi: http://dx.doi.org/10.1175/2008JCLI2450.1
Osman YZ, Shamseldin AY. 2001. Qualitative rainfall prediction models for central and southern Sudan using El Niño southern oscillation and Indian Ocean Sea surface temperature indices. International Journal of Climatology 22: 1861–1878