Seasonality of the Observed Amplified Sahara Warming Trend and Implications for Sahel Rainfall

Abstract

Prior results indicate an amplified annual mean warming trend over the Sahara, with temperature trends that are 2–4 times that of the tropical mean rate. Trend analysis is conducted using five atmospheric reanalyses and three observational datasets to better understand the seasonality and physical processes of this amplified warming and the implications for Sahel precipitation. The seasonality of the amplified warming is maximum during July–October with a minimum during June. Two processes related to the amplified warming are identified. A “dry process” supports amplified warming over the Sahara when there is limited latent heating and/or evaporation to cool the surface and distribute heat to the atmosphere. In this mechanism, the warming results from changes in the upward longwave and downward longwave fluxes that are tightly coupled to each other. The second, termed a “wet process,” occurs during the summer West African monsoon season. In this mechanism there are increases in the low- and midlevel atmospheric moisture over the Sahara that add to the surface warming by increasing the longwave downward radiation. This additional atmospheric moisture is transported over the Sahara because of a strengthening of the thermal low/Saharan high circulation system. A positive feedback mechanism is discussed in which enhanced moisture transport due to the stronger Saharan warming leads to increased Sahel rainfall that further strengthens the meridional temperature and height gradients by cooling the Sahel surface, further enhancing moisture transport into the region. Both processes contribute to the amplified warming, with the amplification being greater during the summer.