Investigation of observed dust trends over the Middle East region in NASA GEOS Earth system model simulations

Abstract. Satellite observations and ground-based measurements have indicated a high variability in the Aerosol Optical Depth (AOD) in the Middle East region in recent decades. In the period that extends from 2003 to 2012, observations show a positive trend of 0.01–0.04 AOD per year or a total increase of 0.1–0.4 per decade. This study aimed to investigate if the observed trend was also captured by the NASA Goddard Earth Observing System (GEOS) Earth system model. To this end, we examined changes in the simulated dust emissions and dust AOD during this period. To understand the factors driving the increase of AOD in this region we also examined meteorological and surface parameters important for dust emissions, such as wind fields and soil moisture. Two GEOS model simulations were used in this study: the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) Reanalysis (with meteorological and aerosol AOD data assimilated) and MERRA-2 GMI Replay (with meteorology constrained by MERRA-2 Reanalysis, but without aerosol assimilation). We did not find notable changes in the modeled 10-meter wind speed and soil moisture. However, analysis of MODIS Normalized Difference Vegetation Index (NDVI) data, did show an average decrease of 8 % per year in the region encompassing Syria and Iraq, which prompted us to quantify the effects of vegetation on dust emissions and AOD in the Middle East region. This was done by performing a sensitivity experiment in which we enhanced dust emissions in grid cells where NDVI decreased. The simulation results supported our hypothesis that the loss of vegetation cover and the associated increase of dust emissions over Syria and Iraq can partially explain the increase of AOD downwind. The model simulations indicated dust emissions need to be tenfold larger in those grid cells in order to reproduce the observed AOD and trend in the model.