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Abstract A recent theory proposes that tropical depression (TD)‐type waves grow by flattening the mean meridional moisture gradient, consequently weakening the Hadley Cell through a poleward moisture flux. To evaluate this theory, we investigate the seasonality of TD‐type waves and their relation to the Hadley Cell in ERA5 and Coupled Model Intercomparison Project Phase 6 (CMIP6) models. On the basis of the theory, a Hadley Cell instability metric is defined whose variability is largely determined by the background meridional moisture gradient and the sensitivity of rainfall to moisture fluctuations. Results show that both TD‐type wave column moisture variance and eddy moisture fluxes peak when the Hadley Cell instability metric is a maximum. These conditions typically occur when the mean meridional precipitation gradient is strongest and the Hadley Cell is weak and narrow. CMIP6 models that exhibit higher Hadley Cell instability metric simulate stronger TD‐type wave activity in the Northern Hemisphere.
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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 anf-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 quasi-geostrophic 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.
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- Award ID(s):
- 2236433
- PAR ID:
- 10487340
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- ISSN:
- 0894-8755
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JCLI-D-23-0147.1
