Energetic Controls on the ITCZ Position

We use energy flux constraints for the atmospheric general circulation to approximate the mean Iatitude of the Intertropical Convergence Zone (ITCZ). We argue that the ITCZ position is determined by interhemispheric asymmetries in subtropical eddy moist static energy fluxes and the equatorial excess shortwave radiation. We use closures for the dry and moist component of the eddy moist static energy fluxes in the subtropics and an approximation for the outgoing longwave flux to relate the ITCZ position to subtropical and extratropical surface and near-surface temperatures. We illustrate the workings of the energetic constraint with simulations from an idealized moist aquaplanet GCM for global warming and tropical warming scenarios, representing changes in greenhouse gas concentration and equatorial ocean upwelling respectively. We show that in our simulations, the ITCZ shifts away from the equator under global warming, but towards the equator under tropical warming. We find that under global warming, subtropical moisture fluxes are responsible for the ITCZ shift, while under tropical warming, it is the changing sensitivity of the ITCZ location to hemispherically asymmetric energy fluxes that drives the shift. This has important implications for the interpretation of the paleoclimatologcial record in the light of atmospheric carbon dioxide and ENSO variability, as well as for possible future climate changes under elevated greenhouse gas and aerosol concentrations.