Absolute vs. Relative Wind Stress in an Eddy-Resolving Model of the Southern Ocean

The calculation of wind stress, the turbulent transfer of momentum between the atmosphere and ocean, is based upon the velocity difference between the atmospheric wind and the surface ocean current. This is usually known as relative wind stress, ocean current interaction or ocean current feedback. When the atmospheric wind velocity is much greater than that of the surface ocean, the resting ocean approximation may be made. This neglects the ocean surface velocity in the wind stress calculation and is known as absolute wind stress. Typically, relative wind stress gives a wind stress a few percent weaker than absolute wind stress for the same atmospheric wind. Specifying a wind stress, instead of an atmospheric wind, when running an ocean model is tacitly using the resting ocean approximation.

It is well established that relative wind stress results in a 20-35% reduction in the power input to the ocean and introduces an additional source of friction at the surface. This friction acts to damp the mesoscale eddy field by locally opposing the circulation of individual eddies. As a result, under relative wind stress, Eddy Kinetic Energy (EKE) may be up to 50% lower than for an equivalent absolute wind stress experiment. This reduction in EKE has the potential to be reflected in the ocean transport of tracers, such as heat or carbon, as well as impacting the overall circulation and energetics of the ocean.

Using an eddy-resolving NEMO configuration of the Southern Ocean forced by JRA55, we compare twin experiments run under relative and absolute wind stress. Changes to the wind power input and EKE, under relative wind stress, leads to modification of the ocean heat transport. This is matched by changes in net surface heat flux and ice cover, which could feedback on other metrics of global climate. This suggests the potential for biases in model simulations to develop based purely upon the formulation of their bulk formulae.