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 (Deep-Sea 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.
Highlights
The excitation of near-inertial internal gravity waves by wind stress and the corresponding energy transfer mechanisms have been studied for decades
This paper presents internal wave energy fluxes derived from simulations of the hybrid slab model tuned to represent the North Atlantic during the years 1989 and 1996
To interpret the results described in the previous section, we first consider the differences between the 2 years as an indicator to the sensitivity of the energy fluxes to the wind stress magnitude
Summary
The excitation of near-inertial internal gravity waves by wind stress and the corresponding energy transfer mechanisms have been studied for decades. D’Asaro (1985) extended the model by subtracting a time-dependent Ekman component to isolate the inertial response. Its use ranges from comparison with and interpretation of local measurements (e.g., Alford 2001; Plueddemann and Farrar 2006; Chaigneau et al 2008) to global estimates of near-inertial wind power input (e.g., Alford 2001; Watanabe and Hibiya 2002; Furuichi et al 2008). Whitt and Thomas (2015) and Jing et al (2017) extended the slab model of Pollard and Millard (1970) by an underlying geostrophic motion and showed that the geostrophic flow may affect the near-inertial wind power The formulation of a frequency-dependent attenuation of the induced surface motion has received little attention so far (Alford 2003; Alford et al 2012). Whitt and Thomas (2015) and Jing et al (2017) extended the slab model of Pollard and Millard (1970) by an underlying geostrophic motion and showed that the geostrophic flow may affect the near-inertial wind power
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