Influence of the ocean surface velocity dependence of the wind stress on the dynamics of the Southern Ocean
Final Report Abstract
Most of the energy that is input to the ocean comes for the wind which exerts a stress at the surface of the ocean. This energy is used to drive ocean currents but, most importantly, is used for mixing in the ocean. Without mixing, the ocean would fill up with cold, dense water apart from a very thin layer at the surface where there is direct contact with the atmosphere. Mixing is, therefore, the essential ingredient for setting the ocean stratification and the structure of the ocean circulation that we observe. Since the level of mixing depends on the wind power input (WPI) it is essential that we have a good estimate of the WPI. Conventional ways of representing the wind stress do not take account of the ocean surface velocity. The argument for doing this is that the ocean surface velocity is almost always small compared to the overlying wind strength. This, however, can be quite misleading. As shown here, the effect of including the ocean surface velocity is to decrease the WPI to the ocean by roughly 20% on average over a broad range of frequency bands. The wind is particularly efficient at inputting energy in what is called the near-inertial frequency band, that is at frequencies close to one day. We find that taking account of the ocean surface velocity leads to a reduction of 40% on average, and locally by as much as 60%, in the energy in this frequency band in surface layer of the ocean. It is also shown that the WPI in the near-inertial band is proportional to the variance of the traditional wind stress in the near-inertial frequency band (that is, the wind stress without taking account of the ocean surface velocity). This enables the near-inertial WPI to be estimated from knowledge of the wind alone. This is important because it allows us to estimate the changes in near-inertial WPI to the ocean during the past 30 years when the westerly winds over the Southern Ocean have shown a marked upward trend. In summary, these results have important consequences for estimating the wind power input to the ocean and the associated energy that is available for mixing.
Publications
- 2013, On the spatial and temporal distribution of near-inertial energy in the Southern Ocean, Journal of Geophysical Research - Oceans
Rath, W., Greatbatch, R. J. and Zhai, X.
(See online at https://doi.org/10.1002/2013JC009246) - 2013, Reduction of near-inertial energy through the dependence of wind stress on the ocean-surface velocity, Journal of Geophysical Research - Oceans, 118, 2761-2773
Rath, W., Greatbatch, R. J. and Zhai, X.
(See online at https://doi.org/10.1002/jgrc.20198) - 2013, The influence of ocean-surface-velocity-dependent wind stress on the dynamics of the Southern Ocean: The near-inertial and the sub-inertial response. Ph.D. dissertation, Christian-Albrechts-Universität zu Kiel
Rath, W.
