African Easterly Jet

African Easterly Jet
Name	African Easterly Jet
Type	atmospheric jet
Location	West Africa, Sahara, Sahel
Latitude	~10°–20°N
Season	summer (June–September)
Typical height	lower troposphere (~600–700 hPa)
Typical speed	~10–20 m/s

African Easterly Jet The African Easterly Jet is a mid-tropospheric wind maximum across West Africa that influences weather over the Sahara, Sahel, Guinea Coast, Mali, Niger, and Chad. It forms during the boreal summer and modulates mesoscale convective systems, African easterly waves, and Atlantic tropical cyclone precursors over the Gulf of Guinea and eastern Atlantic Ocean. Researchers from institutions such as the National Oceanic and Atmospheric Administration, European Centre for Medium-Range Weather Forecasts, National Aeronautics and Space Administration, and universities in Nigeria, Senegal, Ghana, and Morocco have studied its role in interannual climate variability and regional socioeconomic impacts.

Overview

The jet appears as an easterly wind maximum in the lower to mid-troposphere typically centered near 600–700 hPa over the West African Monsoon zone, stretching from the western Senegal coastline eastward through Mauritania, Mali, Burkina Faso, Niger, and into Sudan and Chad. Its existence is tied to the meridional thermal contrast between the hot Sahara Desert and the cooler Gulf of Guinea sea surface temperatures influenced by phenomena like the Atlantic Multidecadal Oscillation and the El Niño–Southern Oscillation. Observational campaigns such as NAME and AMMA combined with reanalyses from ERA-Interim and MERRA have mapped its climatology and variability.

Dynamics and formation

Formation of the jet arises from baroclinic and barotropic processes associated with the north–south temperature gradient between the Sahara and the Guinea Highlands, the seasonal migration of the Intertropical Convergence Zone, and the thermal low over the Sahel. Baroclinic instability of the jet fosters the growth of synoptic-scale disturbances like African easterly waves, whose dynamics are described using frameworks developed in studies at MIT, Princeton University, University of Oxford, and Columbia University. The juxtaposition of the jet with the low-level southwesterly monsoon flow produces wind shear and potential vorticity anomalies verified in simulations with the Weather Research and Forecasting Model and the ECMWF Integrated Forecasting System.

Seasonal variability and climatology

The jet is strongest during boreal summer months (June–September) when the West African Monsoon reaches its northernmost position over the Sahel and recedes in autumn. Interannual modulation links to remote forcings from the North Atlantic Oscillation, Mediterranean Sea surface anomalies, and variability in the Indian Ocean Dipole. Paleoclimate proxies from lake sediments in Lake Chad and tree-ring reconstructions in Mali and Senegal indicate multidecadal changes in jet behavior correlated with droughts and pluvials that affected historical societies such as the Songhai Empire and colonial-era administrations in French West Africa.

Interaction with African easterly waves and tropical cyclogenesis

Baroclinic instability of the jet generates and amplifies African easterly waves, which can organize convective activity and, under favorable conditions, seed tropical cyclones over the eastern Atlantic Ocean. Major studies linking easterly waves to Atlantic hurricane activity involve collaborations among researchers at the National Hurricane Center, University of Miami, Scripps Institution of Oceanography, and the Plymouth Marine Laboratory. The role of the jet in tropical cyclogenesis has been assessed in cases such as storms tracked by Hurricane Hunter (US Air Force), analyses of Hurricane Katrina precursors, and composite studies of Cape Verde hurricane seasons. Modulation by the jet influences moisture flux from the Gulf of Guinea and interactions with the Saharan Air Layer.

Impacts on West African climate and socioeconomic effects

Variability of the jet affects seasonal rainfall distribution, the frequency of harmful dust outbreaks from the Bodélé Depression, and heat extremes that have consequences for agriculture in Nigeria, Senegal, Burkina Faso, and Ghana. Changes in monsoon onset and intraseasonal breaks linked to jet dynamics alter crop yields for staples like millet and sorghum, with socioeconomic consequences documented by organizations such as the Food and Agriculture Organization, United Nations Development Programme, International Federation of Red Cross and Red Crescent Societies, and regional bodies like the African Union and the Economic Community of West African States. Historical droughts in the 1970s and 1980s examined by the World Bank and climate research centers led to migration, conflict, and shifts in pastoralist practices studied by social scientists at Oxford University and University of Cape Town.

Observations and modeling studies

Observation platforms include radiosonde networks maintained by national meteorological services of Senegal, Mali, Niger, and Nigeria; satellite sensors aboard platforms such as Meteosat, TRMM, Aqua, and Jason-3; and airborne campaigns by NCAR and UK Met Office research aircraft. Numerical experiments utilize general circulation models developed at the UK Met Office Hadley Centre, NOAA Geophysical Fluid Dynamics Laboratory, NASA Goddard Institute for Space Studies, and regional models applied in projects funded by the European Union and the National Science Foundation. Recent advances include high-resolution convection-permitting simulations, data assimilation studies linking GPS radio occultation observations, and coupled atmosphere–ocean modeling that incorporate interactions with the Atlantic Meridional Mode.

Category:Atmospheric jets Category:Climate of Africa Category:Weather of West Africa