Ajoku, O., J. R. Norris and A. J. Miller, 2019:
Observed monsoon precipitation suppression caused by anomalous
Climate Dynamics, in press.
This study uses observations and atmospheric reanalysis products in order to understand the impacts
of smoke aerosols advected from the Southern Hemisphere on the dynamics of the West African Monsoon.
Seasonal biomass burning and resulting aerosol emissions have been well documented to affect regional
weather patterns, especially low-level convection. Out of all monsoon months, precipitation shows the most
variability over land during August, in which anomalous smoke aerosol values can increase (decrease) by 33%
(29%) in the Northern Gulf of Guinea and precipitation can decrease (increase) by up to ~2.5 mm/day (~3
mm/day) along the West African monsoon region accounting for a 17% (18%) change in precipitation.
Smoke aerosols produced by biomass burning occurring near Central Africa are advected towards the Gulf of
Guinea at elevations around the 850 hPa level. Satellite observations show an increase (decrease) in cloud
fraction and optical depth below (above) the 300-hPa level in the Gulf of Guinea and along the West African
coastline along with concurrent decreases (increases) in cloud droplet radius during dirty (clean) aerosol
episodes. Additional observations of shortwave radiation quantify changes in cloud coverage and monsoon
dynamics. On average, reductions in surface shortwave radiation of ~10-15 W m-2 occur over the Gulf of
Guinea during increased aerosol transport, with aerosols accounting for ~ 33%-50% of that reduction.
Reductions in shortwave radiation are associated with decreased convective available potential energy
(CAPE). This demonstrates that increased transport of aerosols perturbs surface radiation, convection in the
lower troposphere and eventually cloud coverage, potentially leading to the observed monsoon precipitation
suppression. In a broader social context, this region houses 200 million people and thus understanding these
climate patterns may carry great importance.