The Relationship Between the ITCZ and the Southern Hemispheric Eddy-Driven Jet

We study the effect of a thermal forcing confined to the midlatitudes of one hemisphere on the eddy-driven jet in the opposite hemisphere, using an idealized aquaplanet general circulation model. We demonstrate the existence of an ”interhemispheric teleconnection,” whereby warming (cooling) the Northern Hemisphere causes both the intertropical convergence zone (ITCZ) and the Southern Hemispheric midlatitude jet to shift northward (southward). The interhemispheric teleconnection is effected by a change in the asymmetry of the Hadley cells: as the ITCZ shifts away from the Equator toward the warmed hemisphere, the cross-equatorial Hadley cell intensifies, fluxing more momentum toward the subtropics and sustaining a stronger subtropical jet. Changes in subtropical jet strength, in turn, alter the propagation of extratropical waves into the tropics, affecting eddy momentum fluxes and the eddy-driven westerlies.

The effect of the subtropical jet on meridional wave propagation is first demonstrated in simple barotropic model experiments with fixed eddy stirring in midlatitudes and a prescribed subtropical jet of varying amplitude. Then the relevance of this mechanism is shown in the context of future climate change simulations, where shifts of the ITCZ are significantly related to shifts of the Southern Hemispheric eddy-driven jet in austral winter. Finally, we also discuss the possible relevance of the proposed mechanism to paleoclimates, particularly with regard to theories of ice age terminations.