A potential climatic index for total Saharan dust: the Sun insolation

Abstract

It is shown that the daily course of the total amount of dust in the atmosphere, based on the TOMS aerosol index (AI), is highly correlated (0.98) with the integrated daily solar insolation over the Sahara region for spatially and temporally averaged data. Moreover, the annual maximum is even reached nearly at the same day, i.e., the 17th (18th) June for the surface (top of the atmosphere) solar insolation, as compared to 21st June for the date of maximum atmospheric dust loading. Other factors like cloudiness, rainfall soil moisture, and wind probably play a more dominant role in smaller areas. The fact that on a very large scale, like the whole of the Sahara, the solar insolation becomes the one single forcing directly correlated with the total amount of dust in the atmosphere is suggesting a simple climatic index for the total dust loading in the atmosphere. This index is shown for the Saharan region only, while more studies are needed for other regions. The 3–4 day time‐lag between the maximum insolation and the maximum dust loading in the atmosphere is compared to about a one‐month delay with the maximum ground or air temperatures and may serve as an estimate to the surface response to the major forcing of the Sun. The maximum ground temperature, delayed to the 17th July, can be explained by the delay in the lagged‐heating of the deeper soil layers. The 3–4 day delay only with the Sun insolation is probably linked to the period in which the dust is maintained in the atmosphere until it is deposited. One application to the present finding may be for climate models, since such a high correlation for spatially and temporarily averaged data may be used as an integrated index for the total amount of Saharan dust and for potentially validating the complex model parameterizations in climate models.