Impact of explicit atmosphere-ocean coupling on MJO-like coherent structures in idealized aquaplanet simulations

This paper discusses the impact of the atmosphere-ocean coupling on the large-scale organization of tropical convection simulated by an idealized global model applying the Cloud-Resolving Convection Parameterization (CRCP; superparameterization). Because the organization resembles the Madden-Julian Oscillation (MJO), the results contribute to the debate concerning the role of atmosphere-ocean coupling in tropical intraseasonal oscillations. The modeling setup is an aquaplanet with globally uniform mean sea surface temperature (SST) of 30°C (tropics everywhere) in radiative-convective quasi equilibrium. The simulations apply an interactive radiation transfer model and a slab ocean model with a fixed oceanic mixed layer depth. Results from several 80-and 100-day-long simulations are discussed, where the only difference between the simulations is the prescribed oceanic mixed layer depth, which varied from 5 to 45 m. A simulation with a very deep oceanic mixed layer is also performed to represent constant-SST conditions. The simulations demonstrate that the interactive SST impedes the development of large-scale organization and has insignificant impact on the dynamics of mature MJO-fike systems. The impediment is the result of a negative feedback between the large-scale organization of convection and SST, the convection-SST feedback. In this feedback, SST increases in regions of already suppressed convection and decreases in regions with enhanced convection, thus hindering the large-scale organization. Once developed, however, the MJO-like systems are equally strong in interactive and constant-SST simulations, and compare favorably with the observed MJO. The above impacts of the atmosphere-ocean coupling contradict the majority of previous studies using traditional general circulation models, where, typically, an enhancement of the intraseasonal signal occurs compared to prescribed-SST simulations. An explanation of this discrepancy is suggested.