The Stirring Tropics: Theory of Moisture Mode–Hadley Cell Interactions

Abstract Interactions between large-scale waves and the Hadley cell are examined using a linear two-layer model on an f plane. A linear meridional moisture gradient determines the strength of the idealized Hadley cell. The trade winds are in thermal wind balance with a weak temperature gradient (WTG). The mean meridional moisture gradient is unstable to synoptic-scale (horizontal scale of ∼1000 km) moisture modes that are advected westward by the trade winds, reminiscent of oceanic tropical depression–like waves. Meridional moisture advection causes the moisture modes to grow from “moisture-vortex instability” (MVI), resulting in a poleward eddy moisture flux that flattens the zonal-mean meridional moisture gradient, thereby weakening the Hadley cell. The amplification of waves at the expense of the zonal-mean meridional moisture gradient implies a downscale latent energy cascade. The eddy moisture flux is opposed by a regeneration of the meridional moisture gradient by the Hadley cell. These Hadley cell–moisture mode interactions are reminiscent of quasigeostrophic interactions, except that wave activity is due to column moisture variance rather than potential vorticity variance. The interactions can result in predator–prey cycles in moisture mode activity and Hadley cell strength that are akin to ITCZ breakdown. It is proposed that moisture modes are the tropical analog to midlatitude baroclinic waves. MVI is analogous to baroclinic instability, stirring latent energy in the same way that dry baroclinic eddies stir sensible heat. These results indicate that moisture modes stabilize the Hadley cell and may be as important as the latter in global energy transport. Significance Statement The tropics are characterized by steady circulations such as the Hadley cell as well as a menagerie of tropical weather systems. Despite progress in our understanding of both, little is known about how the mean circulations and the weather systems interact with one another. Here we show that tropical waves can grow by extracting moisture from the Hadley cell, thereby weakening it. They also transport moisture to higher latitudes. Our results challenge the notion that the Hadley cell is the sole transporter of energy out of the tropics and instead favor a view where tropical waves are also essential for the global energy balance. They dry the humid regions and moisten the drier regions via stirring.