Influence of dry season vegetation (and wind) variability on Sahelian dust 2002-2015.

Abstract:

The role of the land surface on dust variability over West Africa is still debated. Early studies have identified a correlation between dust optical depth over the Atlantic and previous year Sahelian precipitation and NDVI. Recent studies, however, point to a modest role of vegetation on dust emission variability, either with model attribution studies or analyses of SYNOP data. Part of the debate comes from the fact that 'previous year precipitation or NDVI' is only a vague cause of reduced dust emission during the dry season. It is implicitly assumed that wet or vegetated soils persist during the dry season, without any solid evidence.

Here we present a method to derive dry-season vegetation in the Sahel, applied to the aqua-MODIS data (2002-2015). The ratio of the SWIR bands at 1.6 and 2.1 microm, referred to as STI, is shown to depend on dry vegetation cover.

We show that the anomalies of vegetation, emerging in the rain season, persist throughout the dry season in the Sahel. Monthly STI anomalies for each month from October to June are strongly correlated to the July-September NDVI anomaly for every pixel in the Sahel.

Then we correlate dry-season averaged STI and Dust Uplift Potential (DUP from ERA-Interim) to MODIS DeepBlue AOD and to AOD from Aeronet stations in the Sahel. STI and DUP explain 40% to 50% of the variance of dust AOD. Areas where STI is best correlated with AOD variability are mostly found in Niger but extend from Chad to Mauritania, whereas DUP shows the strongest correlations near Bodele and similar places in Niger. The role of STI is best seen when averaging over a few months whereas DUP operates also a shorter time-scales.

We conclude that the dry-season vegetation plays a role in controlling dust AOD over the Sahel, in addition to wind.

STI estimated from Landsat data for the large drought of 1984 was much smaller than during 2002-2015, which suggests that vegetation was also an important cause of the high dust AOD of the 80s.