The seesaw response of the Intertropical and South Pacific convergence zones to hemispherically asymmetric thermal forcing

Considerations based on atmospheric energetics and aqua-planet model simulations link the latitudinal position of the global intertropical convergence zone (ITCZ) to atmospheric cross-equatorial energy transport—a greater southward transport corresponds to a more northerly position of the ITCZ. This study, rather than concentrating of the zonally-averaged ITCZ, focuses on the tropical Pacific and looks separately at precipitation in the northern and southern hemispheres. Using numerical experiments, we show that in the tropical Pacific the response of the fully coupled ocean-atmosphere system to a hemispherically asymmetric thermal forcing, modulating atmospheric cross-equatorial energy transport, involves an interplay between the ITCZ and its counterpart in the South Pacific—the Southern Pacific convergence zone (SPCZ). This interplay leads to interhemispheric seesaw changes in tropical precipitation, such that the latitudinal position of each rain band remains largely fixed, but their intensities follow a robust inverse relationship. The seesaw behavior is also evident in the past and future coupled climate simulations of the Climate Model Intercomparison Project Phase 5 (CMIP5). We further show that the tropical Pacific precipitation response to thermal forcing is qualitatively different between the aquaplanet (without ocean heat transport), slab-ocean (with climatological ocean heat transport represented by a “Q-flux”) and fully-coupled model configurations. Specifically, the induced changes in the ITCZ latitudinal position successively decrease, while the seesaw precipitation intensity response becomes more prominent, from the aqua-planet to the slab-ocean to the fully-coupled configuration. The ITCZ/SPCZ seesaw can explain a precipitation dipole pattern observed in paleoclimate without invoking a too strong climate forcing and is relevant to future projections of tropical precipitation.