Role of the convergence zone over West Africa in controlling Saharan mineral dust load and transport in the boreal summer

Abstract

During summer, large amounts of mineral dust are emitted and transported from North Africa over the tropical North Atlantic towards the Caribbean with the exact quantity varying greatly from year to year. Much effort has been made to explain the variability of summer season mineral dust load, for example, by relating dust variability to teleconnection indices such as ENSO and the NAO. However, only weak relationships between such climate indices and the abundance of mineral dust have been found. In this work, we demonstrate the role of the near-surface convergence zone over West Africa in controlling dust load and transport of mineral dust. We apply the ‘Center of Action’ approach to obtain indices that quantify the movement and strength of the convergence zone using NCEP/NCAR Reanalysis data. The latitudinal position of the convergence zone is significantly correlated with the quantity of mineral dust at Barbados over the period 1965–2003 (r=−0.47). A southward displacement of the convergence zone is associated with both increased near-surface flow and decreased precipitation over the dust source regions of the southern Saharan desert, Sahel and Lake Chad. This in turn reduces soil moisture and vegetation, furthering the potential for dust emission. In contrast, the intensity of the convergence zone is not correlated with dust concentration at Barbados. We conclude that the coupling of changes in near-surface winds with changes in precipitation in source regions driven by a southward movement of the convergence zone most directly influence dust load at Barbados and over the tropical North Atlantic during summer.

• Saharan mineral dust
• ITCZ
• West Africa
• interannual variability
• climate
• Centres of Action

6. Acknowledgements

We thank the two anonymous reviewers for their constructive comments. We acknowledge and thank Dr. Joe Prospero and his staff for producing and sharing the record of mineral dust concentrations at Barbados. We thank Dr. Amato Evan for providing AOT data and for comments that greatly improved the manuscript.

NCEP Reanalysis and GPCP Precipitation data provided by the NOAA/OAR/ESRL PSD, Boulder, CO, USA, from their Web site at http://www.esrl.noaa.gov/psd.

GIMMS NDVI produced by the Global Land Cover Facility (www.landcover.org) and the Department of Geography, University of Maryland.