Anti-correlated Hadley and Walker circulations through cloud radiative feedback

The Pacific Ocean plays a pivotal role in driving Earth’s climate, as it is the primary region for strong deep convection that forms the ascending branch of the zonally symmetric Hadley circulation (HC) and the zonally asymmetric Walker circulation (WC). The HC is characterized by ascending motion in the equatorial region and descending motion in the subtropics, governed by energy and angular momentum conservation. In contrast, the WC features strong ascent over the western Pacific and descent over the central to eastern equatorial Pacific, driven by the zonal sea surface temperature (SST) gradient. Although the HC and WC share an ascending branch, their coupling remains poorly understood from a theoretical perspective. To investigate their coupling, we use an idealized aquaplanet slab ocean model. By prescribing zonally asymmetric warm and cold surface fluxes in the tropics, we deliberately alter the strength of the WC. The results show an anticorrelated relationship between the two circulations, with the HC weakening in proportion to the WC strengthening. This behaviour is attributed to the dominant cloud radiative effect in the cold patch region compared to the warm patch. These findings are further supported by cloud-locked experiments that isolate the contributions of cloud radiative feedback. While the shared ascending branch in the western Pacific is often considered the determining factor governing the dynamics, our idealized experiments suggest that the interplay between the two circulations may instead be driven by cloud radiative feedback in the eastern Pacific. The results therefore highlight the critical role of the cold tongue region in shaping the pattern effect in the tropical Pacific.