Asymmetric response of cross-equatorial ocean heat transport to latitudinal thermal forcing in CESM

Abstract

The response of cross-equatorial ocean heat transport (OHT) to interhemispheric radiative imbalances is investigated using a set of fully coupled experiments with external radiative forcing imposed at different latitudinal bands at the top of the atmosphere. The modeling results reveal a first-order collaborative relationship between the ocean and the atmosphere, both working in tandem to facilitate the cross-equatorial heat transport to offset the imposed energy imbalance. Considerable asymmetry is also found between the responses to the radiative forcing placed at the same latitude, but in opposite hemispheres. This asymmetry is mainly attributed to the different changes in ocean circulation. When the radiative forcing is placed over the Southern Ocean, a buoyancy-driven clockwise cell is created that extends from the Southern Ocean into the tropics in both the Indo-Pacific and Atlantic basins, effectively transporting energy into the northern hemisphere. Conversely, when the radiative forcing is placed over the northern high latitudes, the Atlantic Meridional Overturning Circulation (AMOC) is weakened, leading to anomalous southward energy transport. In addition, our study reveals the reduced effectiveness of tropical forcing relative to higher latitude forcing in generating anomalous cross-equatorial OHT. This reduced effectiveness is due to the cancellation of the overturning cell-induced cross-equatorial OHT anomaly by the contribution of the horizontal gyre.