Are vegetation-related roughness changes the cause of the recent decrease in dust emission from the Sahel?

Abstract

[1] Since the 1980s, a dramatic downward trend in North African dustiness and transport to the tropical Atlantic Ocean has been observed by different data sets and methods. The precise causes of this trend have previously been difficult to understand, partly due to the sparse observational record. Here we show that a decrease in surface wind speeds associated with increased roughness due to more vegetation in the Sahel is the most likely cause of the observed drop in dust emission. Associated changes in turbulence and evapotranspiration, and changes in large-scale circulation, are secondary contributors. Past work has tried to explain negative correlations between North African dust and precipitation through impacts on emission thresholds due to changes in soil moisture and vegetation cover. The use of novel diagnostic tools applied here to long-term surface observations suggests that this is not the dominating effect. Our results are consistent with a recently observed global decrease in surface wind speed, known as "stilling", and demonstrate the importance of representing vegetation-related roughness changes in models. They also offer a new mechanism of how land-use change and agriculture can impact the Sahelian climate. Citation: Cowie, S. M., P. Knippertz, and J. H. Marsham (2013), Are vegetation-related roughness changes the cause of the recent decrease in dust emission from the Sahel?, Geophys. Res. Lett., 40, 1868-1872, doi:10.1002/grl.50273.

Keywords: Sahel; decadal trends; dust emission; roughness length; vegetation; wind-speed.

Figure 1

Map showing the location of the seven Sahelian stations used in this study (red dots with labels), the orography (shaded in m above mean sea level according to the legend), and the domain used for averaging ERA-Interim reanalysis data in blue. Winter mean (December–February) 10 m wind vectors from ERA-Interim are also included (scale in bottom right corner).

Figure 2

Trends in mean annual 10 m wind speed (V, black lines), dust uplift potential (DUP, red lines), and frequency of dust events (FDE, blue line) from observations averaged over seven surface stations in the Sahel (see Figure 1 for locations; solid lines) and ERA-Interim reanalysis averaged over the blue box shown in Figure 1 (dashed lines) for the time period 1984–2010. Numbers in brackets in the legend indicate the relative change over the time period estimated from the linear trend line as in Table 1. Definitions of DUP and FDE are given in section 2. Note that there is no FDE from reanalysis data. A fixed threshold of 7 ms^–1 was used for the DUP computations.

Figure 3

(a) Schematic illustrating the estimation of emission threshold wind velocities using probability density distributions for all wind observations and those during dust emission events. Probabilities of 25% (v₂₅) and 75% (v₇₅) were arbitrarily selected to characterize the range of wind speeds typical for the beginning of dust emission. (b) Time evolution of v₂₅ and v₇₅ threshold velocities computed for each station, for 5 year periods from 1985–2010, then averaged over all stations (dashed lines, left axis). Standard deviations of v₂₅ and v₇₅ wind speeds are given by the error bars. Corresponding DUP calculations are also shown (right axis) using (1) a mean wind distribution over the whole time period and the v₂₅ threshold velocity (green), (2) a mean wind distribution and a probability weighting (purple), and (3) a varying wind distribution representative of each 5 year period and a probability weighting (black).