The Role of Cloud Radiative Feedbacks in the Response of the Hadley Circulation to Poleward Ocean Heat Transport

This study examines the influence of poleward ocean heat transport (OHT) on the basic state of the Hadley circulation (HC), with a particular focus on how this influence is mediated by cloud radiative effects. Previous work has demonstrated that the presence of poleward OHT causes the HC to be wider than it would be in the absence of OHT, and that this widening is driven by (1) a weakening of the meridional temperature gradient across the HC, (2) an increase in subtropical static stability, and (3) a decrease in subtropical vertical wind shear induced by a broadening of the annual-mean ITCZ, each of which cause the onset of significant baroclinic instability (and the HC terminus) to shift poleward. The role that cloud radiative changes play in shaping these responses and thus the poleward expansion of the HC, however, has not been investigated. Using a slab-ocean aquaplanet model, this study isolates the effect of cloud radiative changes on the OHT-induced expansion of the HC using a "cloud locking" technique, in which the cloud radiative effect is isolated by locking cloud radiative properties either to their values from a control simulation which includes an idealized representation of poleward OHT, or to their values from a "no OHT" simulation in which poleward OHT is absent. It is found that cloud radiative changes damp the OHT-induced expansion of the HC, seemingly because the cloud radiative changes oppose the OHT-induced relaxation of the across-HC temperature gradient and the OHT-induced poleward expansion of the ITCZ (i.e., they oppose mechanisms (1) and (3) above). Work is ongoing to investigate the separate roles of shortwave and longwave cloud radiative responses in shaping the response of HC extent to OHT. Overall, these results underscore the importance of cloud radiative feedbacks in shaping the response of the large-scale atmospheric circulation to external forcing.