Role of entrainment in shaping the pattern effect

Recent work has established how the sensitivity of tropical low clouds to patterns of SST warming influences  radiative feedbacks and estimates of equilibrium climate sensitivity. This is known as the pattern effect. Central to the pattern effect is the response of low clouds in subsidence regions, which has been linked to the efficiency through which surface warming influences free-tropospheric temperature and thus changes in lower-tropospheric inversion strength.  

Mechanistic understanding of this effect is underpinned by two conceptual models of the tropical atmosphere: (i) convective quasi-equilibrium (CQE) and (ii) weak free-tropospheric temperature gradients (WTG). Together, CQE and WTG imply that a quasi-uniform change in free-tropospheric temperature is set by warming in regions which are convectively coupled. The extent to which these convectively coupled regions can influence the free troposphere is partially controlled by the rate at which dry air is entrained into convective plumes, a process which is parameterized in global climate models and highly uncertain. 

Here, we explore how dry-air entrainment impacts the pattern effect through idealised simulations with CESM2. Using a control entrainment parameter, we perturb an atmosphere-only model with prescribed  warming and cooling SST patches at 4 locations between 100E and 220E along the equator. The simulations are then repeated for a range of entrainment parameter rates. We observe a nonlinear sensitivity of pattern effect to entrainment rate. Specifically, we find evidence that modifying the entrainment parameter influences which regions are most influential in setting free-tropospheric temperatures and affects the sensitivity of top-of-atmosphere fluxes to SST perturbations. Finally, we note contrasting responses over land and ocean when modifying the entrainment parameter, for which we describe a hypothesised mechanism.