Trend changes of African airmass intrusions in the marine boundary layer over the subtropical Eastern North Atlantic region in winter

Abstract

African dust intrusions in the marine mixing layer of the Eastern North Atlantic subtropical region (23.5◦N to 35◦N) are favoured in winter when the eastern edge of the Azores High covers Southwestern Europe and North Africa. In situ ground pressure observations and reanalysis from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and European Centre forMedium-RangeWeather Forecasts (ECMWF) indicate that the Azores High has strengthened and shifted eastward in winter over the last three decades. This is evidenced by the increase over time of the Madrid–Tenerife Index which is defined as the geopotential height anomaly difference at 1000 mb between Tenerife (28.5◦N; 16.3◦W) and Madrid (40.5◦N; 3.5◦W) in winter and of the African Index which is defined as the residence time over Africa of air mass trajectories entering the subtropical Eastern North Atlantic Ocean. Barcelona Supercomputing Center/Dust Regional AtmosphericModel (BSC-DREAM) dust regional model simulations from 1958 to 2006 were performed, assuming that the soil characteristics of dust sources remained unchanged over time. Simulated winter dust concentration levels are well correlated (0.67) with the available background observations for the 1998–2004 period. The model results show a two-fold increase in winter dust concentrations over the 1980–2006 period with respect to the 1958–1979 period, corresponding to the strengthening and eastward shift of the Azores High.