Effect of SST Distribution and Radiative Feedbacks on the Simulation of Intraseasonal Variability in an Aquaplanet GCM

Abstract

An aquaplanet atmospheric general circulation model with prescribed sea surface temperatures (SST) is used to provide insight on tropical intraseasonal variability. Previous work showed that a zonally asymmetric SST distribution that resembles observations but with the meridional SST gradient reduced to one-quarter of its observed value poleward of 10° could produce a robust Madden-Julian oscillation (MJO) with 50-day periodicity and amplitude greater than observations in this model. Wind-evaporation feedback was shown to be necessary to destabilize the model MJO. This present work extends these aquaplanet sensitivity experiments to examine the impact of other SST basic states on intraseasonal variability, and also determines whether cloud-radiative and moisture-radiative feedbacks are important for destabilizing the model MJO. A zonally symmetric model with reduced meridional SST gradient produces dominant intraseasonal power at too short of timescale (~30 days) and reduced amplitude relative to the zonally asymmetric simulation, suggesting that basic state zonal asymmetries and surface mean westerlies are important for producing a realistic MJO. The intraseasonal mode in the zonally symmetric model resembles the WISHE mode of linear theory, with wind speed and latent heat flux anomaly maxima occurring in surface easterly anomalies to the east of maximum convection, unlike the zonally asymmetric model in which latent heat fluxes maximize near and to the west of convection like in observations. An experiment with in which longwave radiative heating is prescribed to be near climatology suggests that both radiative and wind-evaporation feedbacks may help to destabilize the model MJO.