Estimates of wind power and radiative near-inertial internal wave flux
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Estimates of wind power and radiative near-inertial internal wave flux The hybrid slab model and its application to the North Atlantic Georg S. Voelker1
· Dirk Olbers1,2 · Maren Walter1,3 · Christian Mertens3 · Paul G. Myers4
Received: 11 November 2019 / Accepted: 29 June 2020 © The Author(s) 2020
Abstract Energy transfer mechanisms between the atmosphere and the deep ocean have been studied for many years. Their importance to the ocean’s energy balance and possible implications on mixing are widely accepted. The slab model by Pollard (DeepSea Res Oceanogr Abstr 17(4):795–812, 1970) is a well-established simulation of near-inertial motion and energy inferred through wind-ocean interaction. Such a model is set up with hourly wind forcing from the NCEP-CFSR reanalysis that allows computations up to high latitudes without loss of resonance. Augmenting the one-dimensional model with the horizontal divergence of the near-inertial current field leads to direct estimates of energy transfer spectra of internal wave radiation from the mixed layer base into the ocean interior. Calculations using this hybrid model are carried out for the North Atlantic during the years 1989 and 1996, which are associated with positive and negative North Atlantic Oscillation index, respectively. Results indicate a range of meridional regimes with distinct energy transfer ratios. These are interpreted in terms of the mixed layer depth, the buoyancy frequency at the mixed layer base, and the wind field structure. The average ratio of radiated energy fluxes from the mixed layer to near-inertial wind power for both years is approximately 12%. The dependence on the wind structure is supported by simulations of idealized wind stress fronts with variable width and translation speeds. Keywords Near inertial waves · Wind ocean coupling · Internal gravity waves
1 Introduction The excitation of near-inertial internal gravity waves by wind stress and the corresponding energy transfer mechanisms have been studied for decades. Many experiments were conducted in the 1970s and 1980s (e.g., Kundu 1976; Pollard 1980; Fu 1981; Price 1983; D’Asaro 1985) followed Responsible Editor: Emil Vassilev Stanev Georg S. Voelker
[email protected] 1
MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany
2
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
3
Institute of Environmental Physics, University of Bremen, Bremen, Germany
4
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
by the work of D’Asaro et al. (1995). These studies linked the velocity variability below the mixed layer to the wind field and found near-inertial components to be remarkably dominant. In particular, most observations showed upward propagation of phase and thus downward propagation of energy (Leaman and Sanford 1975; Leaman 1976). At the same time, a range of models were developed to understand the corresponding excitation mechanisms (e.g., Pollard and Millard 1970; K¨ase and
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