The variability of the Saharan Heat Low intensity and its impact on the West African Climate and precipitation

The Saharan Heat Low (SHL) plays a pivotal role in the West African Monsoon system in spring and summer. The recent trend in SHL activity has been analysed using two sets of numerical weather prediction (NWP) model reanalyses and Atmospheric Models Intercomparison Project (AMIP) simulations from 15 climate models performed in the framework of the 5th Coupled Models Intercomparison Project (CMIP5) exercise. A local increase in the Sahara during the 90's has been found in the two sets of NWP models temperature. This increase is stronger within the SHL region than over the surrounding areas. By using different filters on the signal (under 25 days, 25-100 days and above 300 days), it has been shown that this is accompanied by a slight but large spatial scale increase of temperature, and a change in the filtered signal under 25 days during the transition period of the 90's. The impacts of the different pulsations of the SHL show different scales, from regional (for the high band pass) to a synoptic signature for the low band pass signal. Despite a large variability of the temporal trends between 15 climate models from the CMIP5 project, the trend is observed using the ensemble mean. Nevertheless, large discrepancies are found between the NWP models reanalyses and the climate models simulations regarding the spatial and temporal evolutions of the SHL as well as its impact on West African climate at the different time scales. These comparisons also reveal that climate models represent the west African monsoon interactions with SHL pulsations quite differently. We provide recommendations to use few of them depending on the time scales of the processes at play (synoptic, seasonal, interannual) and based on key SHL metrics (location, mean intensity, global trend, interaction with the west African monsoon dynamic).