The Effect of Atmosphere-Ocean Coupling on the Sensitivity of the ITCZ to Convective Mixing

Abstract:

The Intertropical Convergence Zone (ITCZ) is a discontinuous, zonal precipitation band that plays a crucial role in the global hydrological cycle. Previous studies using prescribed-SST aquaplanet models show the simulated ITCZ is sensitive to convective mixing, but such a framework is energetically inconsistent. As studies have already shown that atmosphere-ocean interactions reduce the sensitivity of the ITCZ to hemispherically asymmetric forcing, we investigate the effect of atmosphere-ocean coupling on the sensitivity of the ITCZ to convective mixing.

Using an idealised modelling framework where the atmosphere is coupled to a simplified ocean with an Ekman-driven ocean energy transport (OET), we show that coupling induces SST changes that reduce the sensitivity of the ITCZ location to convective mixing. In prescribed-SST simulations reducing convective mixing promotes a double ITCZ, whilst in a coupled configuration, decreasing convective mixing increases the meridional SST gradient and promotes a single ITCZ. Prescribing OET does not affect the ITCZ location, but it does decrease the sensitivity of tropical precipitation rates by constraining the net-downward surface energy flux. Decreasing convective mixing increases net-downward shortwave cloudy-sky radiation associated with increased latent heat fluxes and tropical precipitation. We also show that for all coupled simulations the atmospheric energy input framework is inadequate to study ITCZ dynamics due to the contribution of strong transient eddies to the total atmospheric energy transport. Our idealised atmosphere-ocean coupled simulations highlight that prescribing either SST or OET may incorrectly strengthen the sensitivity of the ITCZ to a change in a physics parameterisation or atmospheric forcing. Future modelling studies investigating the sensitivity of tropical precipitation to such changes should use an atmosphere-ocean coupled model configuration.