Coupling between Walker and Hadley circulations mediated by low cloud feedback

Rolf Schimmer , Sarah M Kang

The tropical Pacific hosts the primary region of deep convection and forms the shared ascending branch of the Walker and Hadley circulations, which regulate the distribution of energy, angular momentum, and moisture across the climate system. Despite their importance and spatial entanglement, the dynamical coupling between these two circulations remains poorly understood. Here we show that the Walker and Hadley circulations are intrinsically linked by cloud radiative feedbacks. Using an idealized aquaplanet slab-ocean configuration, we introduce a Walker circulation of varying strength by altering the amplitude of prescribed zonally antisymmetric surface heat flux. The Hadley circulation remains largely insensitive to weak Walker circulation forcing, but weakens strongly once the forcing exceeds a critical threshold. This regime transition occurs when cold-patch cooling becomes sufficiently strong to trigger a positive low-cloud feedback. Once triggered, this feedback induces substantial equatorial cooling and lowers the threshold for deep convection in the off-equatorial tropics, thereby reorganizing tropical convection meridionally. Cloud-locked simulations confirm the key role of cloud radiative feedbacks in controlling this transition. We further show that the low-cloud feedback is driven primarily by local cooling over the cold patch, rather than by remote warming over the warm patch. These results suggest that eastern Pacific low-cloud feedbacks are a key control on tropical convective organization – offering an alternative conceptual view of tropical Pacific atmospheric dynamics in which the eastern Pacific cold tongue plays an active role in organizing tropical convection and mediates the interactions between the Walker and Hadley circulations.