Abstract/Summary

The Intertropical Convergence Zone (ITCZ) is a discontinuous, zonal precipitation band that plays a crucial role in the global hydrological cycle. Across a hierarchy of models, including prescribed sea surface temperature (SST) aquaplanet simulations, a substantial inter-model variability in ITCZ characteristics exists and the ITCZ is sensitive to the representation of convection. To understand the mechanisms driving ITCZ characteristics the sensitivity of the ITCZ to convective mixing is investigated. In prescribed-SST aquaplanet simulations stronger convective mixing favours a single ITCZ, associated with increased boundary-layer moist static energy (MSE) required for deep convection. However,varyinginsolationandprescribedSSTsdemonstratesthatboundaryconditions affect this sensitivity. Further analysis focuses on two key processes in the sensitivity of the ITCZ to convective mixing: cloud-radiation effects (CRE; atmospheric heating due to cloud-radiation interactions) and atmosphere-ocean coupling. The role of CRE is explored using simulations with CRE removed or prescribed as a diurnally-varying, zonal-mean climatology. Removing CRE promotes a double ITCZ, associated with upper-tropospheric cooling, whilst prescribing CRE reduces the sensitivity by approximately 50%. The remaining sensitivity when prescribing CRE is associated with surface latent heat fluxes. The effect of coupling is investigated using an idealised modelling framework in which the atmosphere is coupled to a two-layer ocean with an Ekman-driven ocean energy transport (OET).CouplingsubstantiallyreducesthesensitivityoftheITCZlocationassociatedwithmeridional SST gradient changes. Decreasing convective mixing increases the meridional SST gradient which promotes a single ITCZ and offsets the effect of a reduced boundary-layer MSE required for deep convection. Prescribed OET simulations reveal that OET plays a minimal role in the reduced sensitivity. The important role of interactive SSTs indicates that prescribing SST may overemphasise the sensitivity of the ITCZ to a parameterisation or atmospheric forcing. Future modelling development efforts need to consider surface-atmosphere interactions.