Coupling between Hadley Circulation Strength Variability and Wind-stress-driven Ocean Circulation is Hemisphere Dependent

The Hadley circulation plays an essential role in shaping tropical and subtropical climate variability by regulating the horizontal and vertical heat and moisture distributions. Sea surface temperature (SST), acting as a lower boundary for the Hadley circulation (HC), exerts influence on the HC strength through thermal forcing. A stronger meridional SST gradient in the tropics drives a stronger Hadley circulation following HC’s angular momentum conserving solution. Thus, the variation of the tropical SST directly influences the variation of the HC strength. On a large scale, the wind-driven ocean circulation plays an important role in the tropical SST variability along with the air-sea heat fluxes. However, our understanding of the dynamic ocean processes that modulate the HC strength variability via SST variations remains limited. Previous studies attempt to characterize the role of ocean circulation variability on HC variability by comparing coupled models with a dynamic ocean and a mixed-layer slab ocean. Nonetheless, for many reasons, the slab ocean model is substantially different from a dynamic ocean model and the differences between the two coupled model types remain difficult to interpret. As a result, how much wind-stress driven ocean dynamics, including El-Niño Southern Oscillation (ENSO), impact HC strength through SST variations remains an open question. To address these concerns, we investigate the coupling between interannual variability of the Hadley circulation strength (HCS) and anomalous wind-stress-driven ocean circulation variability using Community Earth System Model (CESM) experiments. We use three distinct model experiments: (i) fully interactive coupled ocean and atmosphere (ii) coupled ocean and atmosphere without anomalous wind-stress driven ocean circulation variability and (iii) coupled ocean and atmosphere without anomalous wind-stress driven ocean circulation only in the equatorial Pacific. The latter essentially damps ENSO variability while maintaining other tropical dynamic ocean models in other basins and in the extratropics. We find that the anomalous wind-stress driven ocean circulation variability significantly amplifies HCS variability in the Southern Hemisphere (SH), primarily driven by the influence of ENSO. The tropical diabatic thermal forcing associated with ENSO triggers the anomalous circulation variability in the SH through convection. Conversely, the variability of the Northern Hemisphere (NH) HCS is predominantly influenced by the eddy-driven internal atmospheric variability with little role of ocean dynamics, attributed to the HC’s deviation from the angular momentum conserving solution at the upper level. The study emphasizes the hemispherically asymmetrical impact of the eddy-driven atmospheric circulation and wind-stress-driven ocean circulation on the interannual HCS variability, thereby providing crucial insights into the internal HC variability. Furthermore, a substantial modulation of the HCS variability by ENSO in the SH has the potential implication to enhance the predictability of hydrological consequences related to the HC.