Turbulence and equatorial waves in moist and dry shallow water systems

Abstract: Turbulence and large-scale waves in the tropical region are studied using the spherical shallow water equations. With mesoscale vorticity forcing, both moist and dry systems show kinetic energy scaling that is dominated by rotational modes, has a -5/3 exponent. At larger planetary scales, the divergent component of the energy increases and we see a footprint of tropical waves. The dry system shows a signature of the entire family of equatorial waves, while the moist simulations only show low frequency Rossby, Kelvin and mixed Rossby gravity waves with an equivalent depth that matches rapid condensation estimates. Further, runs with interactive moisture exhibit spontaneous aggregation with the equilibrium moist energy spectrum obeying a -2 power-law. Synoptic scale moisture anomalies form in heterogeneous background saturation, and are sustained by advection and convergence, within rotational gyres that dominate the tropical region. In that case, along with equatorial waves and turbulence, a large-scale eastward moving moisture wave appears in the midlatitudes. In all simulations with upscale energy transfer, a systematic equatorial zonal mean zonal flow develops which is easterly and westerly for the dry and moist ensembles, respectively.The interaction of this zonal mean flow with a spatially heterogeneous saturation field results in the formation of a moist stationary tropical wave. The super-rotating flow is driven by rotational eddy momentum fluxes due to enhanced equatorial Rossby wave activity in the moist runs. The nature of eddies is such that the tropical circulation in the dry and moist cases tends to homogenize and exaggerate potential vorticity gradients, respectively. These experiments demonstrate the co-existence of tropical waves and turbulence, and highlight the fact that the vortical and divergent wind are inextricably linked with the evolving moisture field.