On The Variability of Mineral Dust and the Intertropical Convergence Zone Over West Africa: A synthesis of observations, climate models and geochemical analysis

Abstract

Mineral dust in the atmosphere is of great importance through its impacts on climate, atmospheric chemistry and biogeochemical processes. The year-to-year and season-to-season variability of mineral dust transport are not well understood. This work focuses on investigating and explaining the inter-annual variability of mineral dust transport by an approach that combines data analysis from in-situ observations, global climate models, multiple reanalysis products and geochemical laboratory analysis. Linking a 38-year record of mineral dust concentration at Barbados to variations in position and intensity of the zone of nearsurface convergence over West Africa, a part of the Intertropical Convergence Zone (ITCZ) a relationship between the latitude of the ITCZ overWest Africa and mineral dust at Barbados is demonstrated during winter (r = 􀀀0:69) and summer (r = 􀀀0:47). This nding represents an improvement over previous studies, which related mineral dust transport to changes in the NAO and ENSO. Southward displacement of the ITCZ leads to favorable winds for dust emission near the surface in both seasons, and in summer also leads to reductions in precipitation over the Sahel, which in turn increases aridity. The newly developed climate index that quanties the variability of the ITCZ is then used to assess the performance of a suite of 11 global climate models (from CMIP3) in both hindcasts of the 20th century and predictions of the 21st century. A northward trend in the position of the ITCZ is observed in the summer of the 20th century, and predicted in both summer and winter of the 21st century. Northward migration of the ITCZ in the 21st century is expected to reduce dust load by 􀀀1:9 gm􀀀3 to 􀀀3:8 gm􀀀3 in summer and by 􀀀1:5 gm􀀀3 to 􀀀2:2 gm􀀀3 in winter. To establish provenance of mineral dust the mineralogy and elemental composition of dust samples collected from four continents are analyzed and via principle component analysis it is demonstrated that continental sources can be separated.