ITCZ Shift Dynamics May be Muted By a Hemispheric Rearrangement of Surface Fluxes That Thermodynamically Promote the Ocean's Efficiency to Mediate Asymmetric Top-of-Atmosphere Forcing

While theoretical frameworks exist for the cross-equatorial energy transport partitioning (e.g. Held, 2001, J. Atmos. Sci. / Czaja and Marshall, 2006, J. Atmos. Sci.), it is still unclear how the cross-equatorial partitioning between atmosphere and ocean will respond to climate forcings. Recent work in an idealized setting has revealed a striking role for interactive ocean dynamics to mute ITCZ shifts, through wind stress interactions that modulate ocean current magnitudes (Greene and Marshall, 2017). To further understand this interesting problem we performed fully-coupled general circulation model experiments, perturbed by hemispherically asymmetric, top-of-atmosphere, non-UV band solar forcing at various latitudinal bands (to identify feedbacks specific to different forcing latitudes). The fully coupled Community Earth System Model 1.2 was used to include as rich a set of feedbacks as possible in this study. The results emphasize that hemispherically asymmetric changes in the net column diabatic heating of the ocean, especially involving a canonical surface flux anomaly linked to ITCZ shifts, is a better predictor of the changes in partitioning than the equatorial mass transport responses alone do. These results suggest that it may be important for a complete ITCZ migration theory to include thermodynamic consequences (e.g. flux-induced asymmetric modulations of ocean surface diabatic heating) in addition to the dynamic coupling between atmosphere and ocean via Ekman transport. The results also reveal a more ocean-centric cross-equatorial energy transport response with more poleward forcing cases.