The sensitivity of multiple equilibria to sea surface temperature changes in WTG simulations

A cloud resolving model (CRM) implementing the weak temperature gradient (WTG) approximation is used to explore multiple equilibria, here defined as the precipitating or non-precipitating steady state in a limited model domain. The WTG approximation is based on the premise of horizontal homogeneity of virtual temperature in the tropics. Gravity waves are the main redistribution mechanism which enforce horizontal homogeneity. In the model, this is achieved by artificially maintaining vertical advection of potential temperature sufficient to to counteract the effects of heating.

Radiative Convective Equilibrium (RCE) simulations are used to generate reference profiles which represent the large scale environment. A series of reference profiles with different SSTs are used to simulate the effect of large scale warming or cooling. For each reference profile, we perform a series of simulations in which we vary the wind speed in initially moist or dry environments to determine the sensitivity of multiple equilibria to different SSTs. We find that there is a range of wind speeds at which both equilibria exist, and this range depends non-linearily on the SST of the large scale environment.

Multiple equilibria in limited domain WTG simulations is analogous to the tendency for convection in large domains to self-aggregate under certain conditions: The dry and precipitating equilibria in the smaller domains represent the corresponding sub-regions in the large domain self-aggregated state. Understanding the sensitivity of multiple equilibria to changes in SST in WTG reference profiles may help to elucidate the non-linear dependence of self-aggregation on SSTs.