Reduced surface fine dust under droughts over the southeastern United States during summertime: observations and CMIP6 model simulations

Abstract

Drought is an extreme hydroclimate event that has been shown to cause an increase in surface fine dust near source regions, yet the drought–dust relationship in regions predominantly influenced by long-range-transported dust such as the southeastern USA (SEUS) has received less attention. Using long-term surface fine-dust observations, the weekly US Drought Monitor (USDM), and the monthly standardized precipitation–evapotranspiration index (SPEI), this study unmasks spatial disparity in drought–dust relationships in the contiguous USA (CONUS) where the SEUS shows a decrease in surface dust concentrations during drought in contrast to the expected increase in dust found in other CONUS regions. Surface fine dust was found to decrease by ∼ 0.23 µg m−3 with a unit decrease in SPEI in the SEUS, as opposed to an increase of ∼ 0.12 µg m−3 in the west. The anomalies of dust elemental ratios, satellite aerosol optical depth (AOD), and dust extinction coefficients suggest that both the emissions and trans-Atlantic transport of African dust are weakened when the SEUS is under droughts. Through the teleconnection patterns of the negative North Atlantic Oscillation (NAO), a lower-than-normal and more northeastward displacement of the Bermuda High (BH) is present during SEUS droughts, which results in less dust being transported into the SEUS. At the same time, enhanced precipitation in the Sahel associated with the northward shift of the Intertropical Convergence Zone (ITCZ) leads to lower dust emissions therein. Of the 10 selected models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6), GISS-E2-1-G was found to perform the best in capturing the drought–dust sensitivity in the SEUS. This study reveals the mechanism of how droughts influence aerosol abundance through changing long-range transport of dust.