Double‐ITCZ Bias Reduces Southern Hemisphere Influence on Tropics via Oceanic Pathways

Abstract

Many coupled climate models exhibit an Intertropical Convergence Zone (ITCZ) south of the Equator in the annual‐mean tropical Pacific that is more pronounced than observed. This bias impacts winds, wind‐driven ocean circulation, including the meridional subtropical cells, and the zonal distribution of waters supplying the equatorial thermocline. To explore the impact of these biases, we compare particle pathways from a double‐ITCZ‐biased coupled climate simulation with those from an ocean simulation forced by atmospheric reanalysis. In the forced ocean simulation, Southern Hemisphere (SH) subducted waters travel directly to the equator in the Central and Eastern Pacific. In the coupled simulation, surface winds associated with the South Pacific Convergence Zone intensify the SH subsurface meridional potential vorticity gradient, blocking and diverting interior flow to western boundary currents. These circulation changes are accompanied by modifications of subducted water potential densities, altering equatorial water mass properties and potentially the simulation of decadal climate variability.