Asymmetric responses of the meridional ocean heat transport to climate warming and cooling in CESM

This study investigates the responses of the meridional ocean heat transport (OHT) to heat fluxes of equal amplitude but opposite sign into the global ocean surface using the Community Earth System Model (CESM). Results show that the poleward OHT in both hemispheres are weakened in response to the positive forcing (i.e., warming) and strengthened in response to the negative forcing (i.e., cooling), with the latter change exceeding the former, manifesting an overall asymmetric response. The OHT responses in the Indo-Pacific to both the warming and the cooling and thus their asymmetry are dominated by its Eulerian-mean component, due primarily to changes in the Indo-Pacific Subtropical Cells. Similarly, the OHT responses in the Atlantic are determined by changes in the Atlantic Meridional Overturning Circulation, yet their asymmetry is small due to the mediation from the nonlinear effect of temperature and velocity changes. For the Southern Ocean, the Eulerian-mean component still controls the total OHT responses, yet the largest contribution is from changes in vertical temperature structure. In addition, the asymmetric response in the Southern Ocean is a joint effort among the overturning circulation cells, horizontal gyres, eddies, as well as temperature changes.