Abstract:

Convection in the tropics is observed to form coherent large-scale structures, as part of moist tropical systems. These large-scale moist systems, by diverse means, play a major role in shaping the kind of weather occurring during a given season, and are responsible for much of the extreme weather in the tropics. Spectral analysis of brightness temperatures in the tropics - a proxy for deep convection- reveals peaks along linear dispersion relation curves for planetary scale waves, alongside a red noise spectrum. Another main characterizing feature of the tropical flow is the direction of the zonal mean zonal winds at the equator, which can be super-rotating (westerly) or sub-rotating (Easterly). 

In this talk we examine the possibility that mesoscale stochastic forcing can excite both the tropical modes and the red noise background spectrum via a turbulent upscale cascade, and in the process also shape the zonal mean flow. We will examine this process for different kinds of forcing, in moist and dry shallow water models, run in a tropical-planet configuration, in which there are no large-scale gradients in heating or sea surface temperatures. This allows us to examine these interactions without the influence of midlatitude weather. We find that mesoscale structures aggregate to larger scales only if vorticity anomalies are excited strongly enough. We will further examine the formation of equatorial jets in the vorticity-forced runs. Super-rotation is a striking feature in planetary circulations, also found in atmospheric circulation models. While it is well established that a tropical wave source is needed for super-rotation, the mechanism that provides this wave source, and what conditions allow it to be maintained are still not understood and vary between different models. Specifically, in shallow water models with Earth like parameters, super-rotation has only been found when relatively strong thermal damping was added. In our model, the dry runs exhibit both super-rotation and sub-rotation, with spontaneous transitions between the two states, while the majority of moist runs exhibit a stable super-rotating state. We will discuss some potential processes which determine the direction of the equatorial jets, as well as the persistence of super-rotation in the moist runs.