Intraseasonal Variability in a Cloud-Permitting Equatorial Aqua-Planet Model

We perform simulations with the System for Atmospheric modeling (SAM) run at a horizontal grid spacing of 4 km on an equatorial aqua planet, extending from 46^o S to 46^o N latitude with a fixed, constant sea surface temperature with perpetual equinox solar radiation. The model is run for 280 days.

Despite the constant sea surface temperature, easterlies develop in the tropics with westerlies at higher latitudes. A spectrum of disturbances develop that include a strong, eastward-propagating wavenumber 1 mode that strongly resembles the Madden Julian Oscillation (MJO), with a Gill-like pattern of outgoing longwave radiation, winds, and temperature. Horizontally homogenizing the radiative heating eliminates this mode, demonstrating that it owes its existence to feedbacks among clouds, water vapor and radiation. In this and in other respects, the mode behaves like self-aggregation on a sphere, as found by Arnold and Randall (2015) using a super-parameterized general circulation model. In an experiment in which the wind speed is fixed in the surface flux formulation, the MJO mode still develops but does not propagate, showing that its eastward propagation in the model is owing to wind-dependent surface fluxes (WISHE).

In our talk, we will discuss this mode in greater detail and also diagnose the physics of higher frequency equatorially trapped modes, such as Kelvin and mixed Rossby-gravity waves, produced in these simulations.