Subtleties of Wind Bulk Formulae and the Southern Ocean Circulation
There are some interesting subtleties buried in the bulk formulae typically used to translate atmospheric wind into the wind stress felt by the ocean. In particular, the mesoscale eddy field can be directly damped by the wind, due to difference in stress across the eddy, and variability of the wind influences the mean wind stress in an unobvious way. Both of these could potentially lead to differences in the way that the Southern Ocean circulation, its overturning and circumpolar transport, reacts to change in wind stress.
Previous work has highlighted that when the wind stress depends on the difference between the ocean and wind velocities, so-called relative wind stress, the circumpolar transport is increased. Our simulations show the same, to a lesser degree. However, the impact on overturning is limited, due to the adiabatic nature of the damping effect at the surface, which can be explained simply using residual mean theory ideas.
Strongly varying atmospheric wind is found to increase the (parameterised) near-surface viscous and diffusive mixing and lead to a change in the primary dissipation mechanism for the power budget. This leads to thicker mixed layers and higher sensitivity of the residual circulation to increasing wind stress, when compared to equivalent experiments with the same wind stress held constant in time.