Negative sensitivity of Southern Ocean Circumpolar Transport to increased wind stress controlled by residual overturning

The transport of the Southern Ocean’s Antarctic Circumpolar Current, closely linked to the global stratification to the north and in turn the inter-hemispheric overturning circulation, is a key metric for quantifying ocean circulation. Understanding the sensitivity of transport to changes in forcing is important in understanding the role of the Southern Ocean in past, present and future climates. Here, we report on an investigation of a negative sensitivity regime, whereby the circumpolar transport decreases with increasing wind forcing, a phenomenon previously reported in ocean modelling investigations where the residual overturning circulation is oriented opposite to the present-day configuration. The present study finds that this negative sensitivity is a subtle effect resulting from both eddy saturation and a negative residual overturning circulation, the latter referring to a poleward mass flux in the warm surface layers. The work provides an examination and rationalisation of the sensitivities relating to the Southern Ocean circumpolar transport, and additionally touches on a numerical methodology that is particularly adept for the study of equilibrium sensitivities, with implications for analogous explorations in the paleoclimate context.