Water vapor buoyancy and the African easterly jet

TL;DR

This study reveals that water vapor buoyancy significantly influences the African easterly jet's strength and its response to global warming, emphasizing the importance of moisture distribution in climate modeling.

Contribution

It demonstrates the substantial impact of water vapor buoyancy on the AEJ and highlights discrepancies in climate models regarding vapor buoyancy inclusion.

Findings

Water vapor buoyancy reduces AEJ magnitude by 30%.

The effect of vapor buoyancy on AEJ strengthens with global warming.

Some CMIP6 models omit vapor buoyancy, affecting climate projections.

Abstract

The African easterly jet (AEJ) is a prominent circulation feature in the tropical atmosphere. It transports mineral dust and generates easterly waves that serve as seeds for hurricanes. Conventional wisdom holds that the AEJ is in thermal wind balance with the positive meridional temperature gradient over North Africa. Here, using reanalysis data, we show that the negative meridional moisture gradient substantially counteracts the effect of the temperature gradient on density in that balance, diagnostically accounting for a 30\% reduction of the AEJ magnitude. Using CMIP6 data, we further show that this effect of vapor buoyancy on the AEJ strengthens under global warming, highlighting the critical role of the spatial distribution of moisture on large-scale circulation. Analysis of the AEJ in CMIP6 models confirms that some models do not include vapor buoyancy in their governing equations, raising questions about the relevance of their projections of climate change in that region.