Balanced Tropical Dynamics and Convection

The fundamental dynamics of the atmosphere in middle latitudes is balanced. I argue that the same is mostly true of tropical atmospheric dynamics. With the exception of equatorial Kelvin waves and related gravity wave modes as well as certain aspects of zonally symmetric equatorial flows, tropical disturbances, including westward-moving mixed Rossby-gravity modes, equatorial Rossby waves, tropical easterly waves, tropical cyclones, and the Madden-Julian oscillation, as well as the Hadley and Walker circulations are essentially balanced.

Mid-latitude and tropical dynamics differ in the following way: Baroclinic instability dominates vertical motion in middle latitudes. However, horizontal temperature gradients are generally insufficient to support baroclinic instability in the tropics. Deep convection takes its place there, with a virtual monopoly on ascending motion and rainfall. The key problem in tropical atmospheric dynamics is therefore understanding how convection interacts with balanced tropical flows.

Observations from field programs over the past 25 years have yielded three insights as to how this interaction works: (1) Surface latent heat fluxes drive convection and precipitation to a large degree; (2) Small values of gross moist stability amplify the importance of these surface fluxes; (3) Small gross moist stability occurs in the presence of strong deep-tropospheric vorticity by virtue of associated temperature perturbations that affect moist convective instability. Convection in turn alters the vorticity via the pattern of convergence and divergence it imposes on the environment. This interaction leads to rich behavior that has not been extensively explored except in the formation of tropical cyclones.

Tropospheric moisture, or the lack thereof, turns out not to control convection except in the case of very dry atmospheres. In moister atmospheres in which significant deep convection exists, the column relative humidity tends to vary inversely with moist convective instability as a result of a moisture quasi-equilibrium process brought about by the convection itself. Tropospheric moisture thus co-varies with convection and is therefore not a cause of convection.