In the mid-latittudes, the large-scale dynamics of the atmosphere can be explained using the quasi-geostrohpic (QG) theory; the flow is characterized by a geostrophic balance between the winds and pressure, as well as the conservation of potential vorticity (PV). The QG model is an example of a balance model, where fast inertial gravity waves are filtered while the slow advective motions are retained. Although the QG model is immensely useful for the midlatitudes, it is not applicable to the tropics as a singularity develops in the geostrophic balance. Furthermore as Kelvin waves, an importantly component in tropical variability, are invisible the under a PV-based dynamics, the QG theory will not adequately describe equatorial dynamics.
In this talk I will focus on the tropical troposphere, where the dynamics are driven by diabatic heating. There are currently two prevailing school of thoughts regarding tropical dynamics: the tradition equatorial wave theory, where diabatic heating excite waves that are highly divergent, and the weak temperature gradient (WTG) model, where the divergent flow is weak compared to the rotational component. Although the latter is formally valid for mesoscale motions, observations suggests that the WTG theory may be valid even at the synoptic/planetary scale; however the WTG model cannot be a complete description of large scale equatorial dynamics as it filters out Kelvin waves. In this talk I will show that a modified asymptotic approach can be used to systematically derive equatorial balance models in the planetary scale regime, and compare it to the WTG model.