Abstract
Abstract Ocean circulation responses to interhemispheric radiative imbalance can damp north–south migrations of the intertropical convergence zone (ITCZ) by reducing the burden on atmospheric energy transport. The role of the Atlantic meridional overturning circulation (AMOC) in such dynamics has not received much attention. Here, we present coupled climate modeling results that suggest AMOC responses are of first-order importance to muting ITCZ shift magnitudes as a pair of hemispherically asymmetric solar forcing bands is moved from equatorial to polar latitudes. The cross-equatorial energy transport response to the same amount of interhemispheric forcing becomes systematically more ocean-centric when higher latitudes are perturbed in association with strengthening AMOC responses. In contrast, the responses of the Pacific subtropical cell are not monotonic and cannot predict this variance in the ITCZ’s equilibrium position. Overall, these results highlight the importance of the meridional distribution of interhemispheric radiative imbalance and the rich buffering of internal feedbacks that occurs in dynamic versus thermodynamic (slab) ocean modeling experiments. Mostly, the results imply that the problem of developing a theory of ITCZ migration is entangled with that of understanding the AMOC’s response to hemispherically asymmetric radiative forcing—a difficult topic deserving of focused analysis across more climate models.
Highlights
The intertropical convergence zone (ITCZ) is a band of intense rainfall encircling most of the tropics, roughly coinciding with the ascending branch of the Hadley Cell
Recent global climate model (GCM) experiments have shown that oceanic dynamics can damp ITCZ shift responses to high-latitude forcing by imposed cloud brightness (Kay et al 2016), ocean albedo (Hawcroft et al 2016), sea-ice cover (Tomas et al 2016), and stratospheric aerosols (Hawcroft et al 2018)
A southward ITCZ “shift”, a term we will use as shorthand to describe an increase in intensity of the southern zonal-mean, annual-mean rainfall band, is observed in all cases, as expected by the northward cross-equatorial atmospheric heat transport (AHT) response to warming the southern hemisphere
Summary
The intertropical convergence zone (ITCZ) is a band of intense rainfall encircling most of the tropics, roughly coinciding with the ascending branch of the Hadley Cell. Recent global climate model (GCM) experiments have shown that oceanic dynamics can damp ITCZ shift responses to high-latitude forcing by imposed cloud brightness (Kay et al 2016), ocean albedo (Hawcroft et al 2016), sea-ice cover (Tomas et al 2016), and stratospheric aerosols (Hawcroft et al 2018) This is in contrast to studies that do not include ocean dynanmics Chiang and Bitz 2005; Broccoli et al 2006; Kang et al.2008) which exhibit more pronounced ITCZ shifts to extratropical forcings This occurs since perturbations to interhemispheric energy balance need not be restored by the atmosphere alone in a dynamic ocean coupled GCM; they can be mediated by changes in oceanic heat transport
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