The Green Sahara Re-desertification transition and its Climate Impacts over Northern Africa, Mediterranean Basin and the Levant

Abstract. This study presents an ensemble of atmosphere-ocean coupled Regional Climate Model (RCM) simulations for the Mid-Holocene (MH) climate across the Middle East, Mediterranean and North Africa (MENA) regions. This ensemble is generated using the Weather Research and Forecast (WRF) model with online coupling to the Regional Ocean Modelling System (ROMS) ocean model, which simulates the dynamics of the entire Mediterranean Sea, utilizing forcing derived from the Mid-Holocene (MH) climate generated from the UofT-CCSM4 General Circulation Model (GCM) with a prescribed Green Sahara (GS), and having a prescribed GS in the land surface in WRF, but allowing the land surface scheme in WRF to compute influence from GS. The results of this ensemble are characterized by an increased precipitation field, similar to previous results, and a 2 m surface temperature that is lower than those in results that have a fully prescribed GS in WRF. Further analysis determined that under the same GS prescription as in a previous study, the land surface scheme used in this study produces higher evaporation and a smaller change in albedo, jointly producing a lower 2 m surface temperature. This conclusion is supported by a new set of sensitivity experiments that modifies the prescribed land surface field, which also showed that the climate over the Middle East is sensitive to land surface states over northern Africa.