Abstract
Heated from above, the oceans are stably stratified. Therefore, the performance of general ocean circulation models and climate studies through coupled atmosphere–ocean models depends critically on vertical mixing of energy and momentum in the water column. Many of the traditional general circulation models are based on total kinetic energy (TKE), in which the roles of waves are averaged out. Although theoretical calculations suggest that waves could greatly enhance coexisting turbulence, no field measurements on turbulence have ever validated this mechanism directly. To address this problem, a specially designed field experiment has been conducted. The experimental results indicate that the wave–turbulence interaction-induced enhancement of the background turbulence is indeed the predominant mechanism for turbulence generation and enhancement. Based on this understanding, we propose a new parametrization for vertical mixing as an additive part to the traditional TKE approach. This new result reconfirmed the past theoretical model that had been tested and validated in numerical model experiments and field observations. It firmly establishes the critical role of wave–turbulence interaction effects in both general ocean circulation models and atmosphere–ocean coupled models, which could greatly improve the understanding of the sea surface temperature and water column properties distributions, and hence model-based climate forecasting capability.
Highlights
The ocean surface layer is arguably one of the most complicated dynamic systems
The non-breaking waves, modelled successfully as irrotational motions, have far been treated as totally irrelevant to the large-scale ocean dynamical system, yet studied extensively as an isolated and curious phenomenon in itself. This picture has only recently been challenged by the introduction of non-breaking wave-induced mixing in general ocean circulation modelling through wave–turbulence interactions [6]
The rest of this paper is divided into the following sections: §2 summarizes the mathematics of kinematical and dynamical consequences of wave–turbulence interactions in terms of spectral representations; §3 covers a review of the past experimental results to clarify the physics of wave–turbulence interaction; §4 describes the field experiment and the observational results; §5 presents the newly proposed parametrization scheme and §6 is our conclusions
Summary
The ocean surface layer is arguably one of the most complicated dynamic systems. Under the free surface, waves and wave breaking, turbulence and mean shear currents all coexist, acting together and on each other to affect the mixing process downward into the water column below and upward to the air–sea interaction processes above. The non-breaking waves, modelled successfully as irrotational motions, have far been treated as totally irrelevant to the large-scale ocean dynamical system, yet studied extensively as an isolated and curious phenomenon in itself This picture has only recently been challenged by the introduction of non-breaking wave-induced mixing in general ocean circulation modelling through wave–turbulence interactions [6]. We intend to clarify the role of turbulence interactions with non-breaking waves in vertical mixing, and propose a simple parameter to account for their effects for even large-scale geophysical problems as in the general circulation and coupled climate models. The physics of the interactions is exactly as Teixeira & Belcher [17] have pointed out: the net effects of wave-induced straining of the turbulence are through the Stokes drift associated with the wave motions to bend the vorticity lines streamwise and enhance mixing This physical picture is directly confirmed in our field experiments. The rest of this paper is divided into the following sections: §2 summarizes the mathematics of kinematical and dynamical consequences of wave–turbulence interactions in terms of spectral representations; §3 covers a review of the past experimental results to clarify the physics of wave–turbulence interaction; §4 describes the field experiment and the observational results; §5 presents the newly proposed parametrization scheme and §6 is our conclusions
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