CGS Theme: Climate Variability

« Decadal modulation of Sahel extreme precipitation events »

My name is Pablo Fernández and I studied Physics at the Universidad Complutense de Madrid. During the last year of my bachelor’s degree, I became very interested in climate science and I decided to study the master in Meteorology and Geophysics in the same university. After an internship in LOCEAN, I did my thesis in Physical Oceanography in the Laboratoire de Météorologie Dynamique / École Normale Supérieure. There I focused on fine-scale air-sea interactions. Now, I am a postdoc at LOCEAN analysing Sahel extreme precipitation events.

Sahelian rainfall has experienced marked decadal modulations in the twentieth century. Humid conditions dominated between 1950 to 1970 followed by a drought in 1970-1990 and a partial recovery in 1994-2014. The majority of modelling studies assessing these decadal fluctuations focus on total Sahel rainfall. Thus, the decadal modulations of Sahel intraseasonal rainfall features (like the number of extreme, heavy, moderate rainfall days, their intensities and the timing of the wet season) remains poorly known. Furthermore, the studies assessing these features remain observational and constrained to certain countries in the Sahel.

In the first part of the project, we use three large ensemble historical simulations (IPSL-CM6A-LR, CESM2 and MIROC6) and the gauge-based observational dataset REGEN. Here, we show that the 1950-1970 and 1994-2014 humid periods are characterized by more extreme and heavy rainfall events. Their intensity of extreme rainfall events experiences less variations. We also find larger wet seasons due to both an anticipation and a delay of the onset and demise of the rainy season respectively. The larger wet seasons of 1994-2014 contain a higher concentration of extreme rainfall days: we do not find a larger amount of extreme rainfall days in longer wet seasons just because the seasons are larger, but also because the frequency of extreme rainfall days increases.

Furthermore, we show that in the 1950-1970 – 1970-1990 transition, the decrease in the extreme rainfall days (and other intraseasonal rainfall indices) contains an important effect of the internal variability whereas the 1970-1990 – 1994-2014 recovery is mainly forced. By analysing the different external forcings, we find that atmospheric aerosol concentrations and GHGs are the main drivers of the changes in the forced signal. In addition, they induce sea-surface temperature (SST) changes which could be impacting Sahel precipitation as well via an ocean-mediated indirect mechanism.

In the next steps of the postdoc project, we will be running SST-forced atmospheric simulations in which we will change the SST and sea-ice concentration as well as the GHG and AA concentrations between the three periods in order to isolate this ocean-mediated from the direct atmospheric response.