Scientists have long known that winds can generate internal gravity waves in the surface layer of Earth’s oceans. Because the kinetic energy of these waves peaks near the inertial frequency, these motions are called near-inertial oscillations (NIOs), and they provide part of the estimated 2 terawatts of energy required to sustain the global system of thermohaline circulation. But the exact contribution of wind power to these near-inertial motions and wind’s relative importance compared to tidal forces remain topics of vigorous debate.
Although a number of previous studies have estimated this wind power input using numerical models, the results have varied widely from 0.3 to 1.5 terawatts. Now, for the first time, Liu et al. have estimated the near-inertial input of wind power solely on the basis of observations. Using hourly ocean current measurements from surface drifters combined with satellite-derived surface wind measurements, the team calculated that from 1993 to 2016 the worldwide near-inertial wind power contribution was 0.3 to 0.6 terawatt. The researchers also found that the strongest flux of energy occurs between 30° and 60° latitudes during the winter season, when storms are the most prevalent.
The team’s revised estimates are considerably lower than those derived from numerical models, a fact the authors attribute to the models’ disregard of the effects of ocean currents on wind stress. Their sensitivity testing suggests this omission could overestimate local, near-inertial wind power by up to 120%.
By narrowing the estimated contribution of near-inertial wind power to the energy driving global ocean circulation, this study has the potential to reduce the uncertainties of parameterizing the effects of NIOs on thermohaline circulation and climate in numerical models. As such, this contribution is likely to provide a new benchmark for comparing future calculations. (Geophysical Research Letters, https://doi.org/10.1029/2018GL081712, 2019)
—Terri Cook, Freelance Writer