The Wave-Driven Ocean Circulation (original) (raw)
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An asymptotic theory for the interaction of waves and currents in coastal waters
Oceanic surface gravity waves have a mean Lagrangian motion, the Stokes drift. The dynamics of winddriven, basin-scale oceanic currents in the presence of Stokes drift are modified by the addition of so-called vortex forces and wave-induced material advection, as well by wave-averaged effects in the surface boundary conditions for the dynamic pressure, sea level, and vertical velocity. Some theoretical analyses previously have been made for the gravity wave influences on boundary-layer motions, including the Ekman currents. The present paper extends this theory to the basin-scale, depth-integrated circulation in a bounded domain. It is shown that the Sverdrup circulation relation, with the meridional transport proportional to the curl of the surface wind stress, applies to Lagrangian transport, while the associated Eulerian transport is shown to have a component opposite to the Stokes-drift transport. A wave-induced correction to the relation between sea level and surface dynamic pressure is also derived. Preliminary assessments are made of the relative importance of these influences using a global wind climatology and an empirical relationship between the wind and wave fields. Recommendations are made for further development and testing of this theory and for its inclusion in general circulation models.
Sea Surface Gravity Wave-wind Interaction in the Marine Atmospheric Boundary Layer
Energy Procedia, 2014
In this study, we investigate the turbulence structure over idealized wind-generated surface gravity waves with varying wave age using a wave-modified one-dimensional boundary layer model. To prescribe the shape of the water wave and the associated orbital velocities, we employ an empirical expression for the wave energy spectrum without assigning a prognostic equation for modelling wave evolution under the action of wind. The key element in this model is the the work done by the wave-induced momentum flux on the atmosphere in the presence of waves. This is incorporated into the airflow using an exponential decay function. Finally, we conduct a series of numerical experiments to identify wave effects on the airflow over a wavy moving interface as a function of wave age, and to check the skill of the present model in capturing wave-induced processes in the marine atmospheric boundary layer (MABL). The results obtained confirm again the significant role of wave-induced processes in influencing the MABL, for example, in modifying the wind profile. Meanwhile, it is shown that the modified one-dimensional model is sensitive to wave parameterizations and the wave energy spectrum. However, a number of uncertainties remain for further investigation, such as the choice of wave energy spectrum, wave forcing parametrization, and surface boundary conditions for momentum and energy.
Surface Gravity Waves and Coupled Marine Boundary Layers
2001
The long term objective of our research is to advance the understanding of air-sea interaction and the coupling between the atmospheric and oceanic boundary layers (the ABL and OBL) medi ated by the surface gravity wave field, in order ultimately to develop better parameterizations of the boundary layers and surface fluxes for coupled, large-scale numerical models. Turbulence resolving, large-eddy and direct numerical simulations (LES and DNS) are the main tools to be used to investigate interactions among the ABL, OBL, and the air-sea interface. Using numeri cally generated databases, we intend to investigate: (1) vertical heat and momentum fluxes carried by wave-correlated winds and currents; (2) enhanced small-scale, turbulent energy, mixing, and dissipation due both to enhanced wave-correlated wind and current shears and to wave breaking; and (3) wave-averaged influences due to mean Lagrangian currents (Stokes drift) that give rise to coherent Langmuir circulations in the ocean. These mechanisms will be considered for a variety of surface wave states. Finally, we intend to make an effort to connect our simulation results with the proposed Coupled Boundary Layers Air-Sea Transfer (CBLAST) field campaigns. OBJECTIVES Our recent research objectives have focused on understanding the interaction between an imposed surface gravity wave and stratified turbulence in the ABL and OBL, and generating LES of the CBLAST low-wind site using typical large-scale forcings. APPROACH We are investigating interactions among the ABL, OBL, and the connecting air-sea interface using both LES and DNS. The premise behind this approach is that the fundamental processes that lead to air-sea coupling will manifest themselves in three-dimensional, time-dependent simulations. Hence, the creation of sufficient numerical databases will allow for the interpretation of air-sea interaction. The LES code adopted for our work evolves from the efforts of Moeng (1984), Sulli-Same as Report (SAR) 18. NUMBER OF PAGES 8 19a. NAME OF RESPONSIBLE PERSON
Topographic effects on wind driven oceanic circulation
2005
The subject of localized topographic circulation has received much attention in the past, in particular Taylor columns have been extensively studied and several observations indicate their existence in the real ocean. In contrast, we study here the closed circulation created above a large scale seamount, in the absence of a mean flow, by the interaction of the eddy field with
Global surface wave drift climate from ERA-40: the contributions from wind-sea and swell
Ocean Dynamics, 2014
By using 45 years of reanalysis data from the European Centre for Medium-Range Weather Forecasts, ERA-40, we present and discuss the global climatological wave-induced velocity, surface Stokes drift (SD), and its vertically integrated transport for deep water waves. We find that in most of the oceanic basins, the global surface SD is mainly wind-sea-driven while its vertically integrated transport is mainly swell-driven. The total surface SD does not always coincide in orientation with its vertically integrated transport. We suggest that such regions of misalignment are linked to "wave-driven wind" regimes. Coastal wave-induced transport divergences are mostly linked to wind-sea while divergences in the interior of the oceans basins are linked to swell. Analysis of trends indicates that the vertically integrated transport has generally increased during the 45 years analyzed. The largest increases are due to wind-sea in limited high-latitude areas while the swell has a minor increase, but over larger areas. Keywords Global Stokes drift. ERA-40. Surface gravity waves. Wave-induced currents. Global wind-sea and swell currents. Global depth-integrated Stokes transport. Divergence of depth-integrated Stokes transport
2008
The surface current response to winds is analyzed in a two-year time series of a 12 MHz (HF) Wellen Radar (WERA) off the West coast of France. Consistent with previous observations, the measured currents, after filtering tides, are of the order of 1.0 to 1.8% of the wind speed, in a direction 10 to 40 degrees to the right of the wind, with systematic trends as a function of wind speed. This Lagrangian current can be decomposed as the vector sum of a quasi-Eulerian current U E , representative of the top 1 m of the water column, and part of the wave-induced Stokes drift U ss at the sea surface. Here U ss is estimated with an accurate numerical wave model, thanks to a novel parameterization of wave dissipation processes. Using both observed and modelled wave spectra, Uss is found to be very well approximated by a simple function of the wind speed and significant wave height, generally increasing quadratically with the wind speed. Focusing on a site located 100 km from the mainland, the wave induced contribution of Uss to the radar measurement has an estimated magnitude of 0.6 to 1.3% of the wind speed, in the wind direction, a fraction that increases with wind speed. The difference U E of Lagrangian and Stokes contributions is found to be of the order of 0.4 to 0.8% of the wind speed, and 45 to 70 degrees to the right of the wind. This relatively weak quasi-Eulerian current with a large deflection angle is interpreted as evidence of strong near-surface mixing, likely related to breaking waves and/or Langmuir circulations. Summer stratification tends to increase the UE response by up to a factor 2, and further increases the deflection angle of U E by 5 to 10 degrees. At locations closer to coast, Uss is smaller, and UE is larger with a smaller deflection angle. These results would be transposable to the world ocean if the relative part of geostrophic currents in U E were weak, which is expected. This decomposition into Stokes drift and quasi-Eulerian current is most important for the estimation of energy fluxes to the Ekman layer.
On the Wind Power Input to the Ocean General Circulation
Journal of Physical Oceanography, 2012
The wind power input to the ocean general circulation is usually calculated from the time-averaged wind products. Here, this wind power input is reexamined using available observations, focusing on the role of the synoptically varying wind. Power input to the ocean general circulation is found to increase by over 70% when 6-hourly winds are used instead of monthly winds. Much of the increase occurs in the storm-track regions of the Southern Ocean, Gulf Stream, and Kuroshio Extension. This result holds irrespective of whether the ocean surface velocity is accounted for in the wind stress calculation. Depending on the fate of the high-frequency wind power input, the power input to the ocean general circulation relevant to deep-ocean mixing may be less than previously thought. This study emphasizes the difficulty of choosing appropriate forcing for ocean-only models.