Field measurements of the vertical shears for a drift current

1987

Journal of the Oceanographical Society of Japan, 1978

An examination of behaviors of a subsurface drogue and a subsequent examination of current measurement with a drifter comprising the drogue as its important part are made in the channel between Oshima and Izu Peninsula. The drogue submerged to the anticipated depth of 300 m within 10 minutes after launching. Since then the drogue kept its depth and operated normally. From the comparison with the velocity measurement by the use of a currentmeter moored at a station in the vicinity of the drifter's track, it is verified that the drifter's motion well reflects the motion of a water parcel around the drogue.

Velocity measurements with vertical resolution 0.02 m were conducted in the lowest 0.5 m of the water column using acoustic Doppler current profiler (ADCP) at a test site in the western part of the East China Sea. The friction velocity u * and the turbulent kinetic energy dissipation rate ε wl (ζ) profiles were calculated using loglayer fits; ζ is the height above the bottom. During a semidiurnal tidal cycle, u * was found to vary in the range (1-7)Â10 −3 m/s. The law-of-the-wall dissipation profiles ε wl (ζ) were consistent with the dissipation profiles ε mc (ζ) evaluated using independent microstructure measurements of small-scale shear, except in the presence of westward currents. It was hypothesized that an isolated bathymetric rise (25 m height at a 50-m seafloor) located to the east of the measurement site is responsible for the latter. Calculation of the depth integrated internal tide generating body force in the region showed that the flanks of the rise are hotspots of internal wave energy that may locally produce a significant turbulent zone while emitting tidal and shorter nonlinear internal waves. This distant topographic source of turbulence may enhance the microstructure-based dissipation levels ε mc (ζ) in the bottom boundary layer (BBL) beyond the dissipation ε wl (ζ) associated with purely locally generated turbulence by skin currents. Semi-empirical estimates for dissipation at a distance from the bathymetric rise agree well with the BBL values of ε mc measured 15 km upslope.

Journal of Geophysical Research: Oceans, 2012

From the Surface Velocity Program (SVP) drifter current and QuikSCAT wind data, the relationship between the observed near-surface current vectors and surface wind vectors for the northwestern Pacific Ocean under high winds (20-50 m s À1) are obtained with quantitative estimations of near-surface drift ratio (current speed versus wind speed) r ($2%) and near-surface drift angle a ($0-10 to the right of the winds). These estimations keep unchanged after removing the surface geostrophic component. From the SVP drifter current and daily WindSat wind data, the estimated r is still approximately 2%. Three linear regression equations are obtained between the observed near-surface current speeds and the surface wind stress for the high wind range.

Environmental Fluid Mechanics, 2011

A modification of the classical Ekman model of oceanic wind-driven currents including the Stokes drift and stratification effects is discussed. The modification is formulated as an application of turbulence mechanics accounting for the curvature effect of velocity fluctuation streamlines. It is shown that similar to the Stokes drift effect, the presence of a density jump layer (pycnocline) decreases the veering of the flow velocity vector at the surface from the direction of the wind stress. It is shown also that in the pycnocline the decrease of the norm of the velocity vector as well as its rotation with depth is smaller than in the regions adjacent to the pycnocline. If the Stokes drift and stratification effects are neglected, the model reduces to the classical Ekman solution with the coefficient of the turbulent shear viscosity replaced by an effective viscosity coefficient. The vertical distributions of velocity predicted by the modified model are compared with the velocity data measured in the Drake Passage and within the Long-Term Upper Ocean Study (LOTUS) in the North Atlantic.

Journal of Geophysical Research: Oceans

Many years of simultaneous hourly buoy wind and directional wave spectra data in the Gulf of Mexico and the Pacific were used to estimate Stokes drift and u * e w where u * = (magnitude of the local windstress/water density) 1/2 and e w is a unit vector in the direction of the local wind. Stokes drift and u * e w were strongly vectorally correlated, the two vectors on average being within a few degrees of one another. This result remained valid even when there was evidence of remotely forced swell. Extension of the observed wave spectra above 0.35 Hz to the u *-dependent wave breaking frequency shows that typically the e-folding scale of the Stokes drift with depth is less than 1.8 m, much smaller than the Ekman layer e-folding scale. Therefore, there is negligible induced Eulerian cancellation of the Stokes drift, and the surface particle movement is governed by the Eulerian velocity + |u Stokes |e w. Taking into account wave spreading, |u Stokes | typically ranges from about 3 to 13 cm/s. Thus, the Stokes drift, which can be estimated directly from the wind stress, is an order one contributor to the surface transport of particles. Plain Language Summary Although crucial for the movement of oil spills, red tide, fish eggs and larvae, and floating garbage, much still has to be learned about net particle movement in the top 1 or 2 m of the ocean. George Gabriel Stokes showed mathematically in 1847 that ocean surface waves may affect the net movement of particles at the ocean surface, but later it was shown that because we live on a rotating Earth, the net particle movement in the direction of the waves (the "Stokes drift") might be canceled by another opposite flow. In this paper we demonstrate that because of the ocean turbulence generated by the wind, the main part of the Stokes drift in the top 2 m of the ocean is not canceled by an opposing flow. Furthermore, analysis of simultaneous hourly wind and wave measurements for many years at Christmas Island in the equatorial Pacific, ocean station Papa in the north Pacific, and 10 stations in the Gulf of Mexico shows that Stokes drift is strongly related to the local wind and is in the direction of the wind. Stokes drift is therefore not primarily due to remotely driven swell; rather, it is mainly due to the much shorter waves that the local wind generates. By taking into account when the short waves break, it is shown how Stokes drift can be approximately estimated directly from the local wind.

Journal of Fluid Mechanics

The work examines upper-ocean response to time-varying winds within the Ekman paradigm. Here, in contrast to the earlier works we assume the eddy viscosity to be both time and depth dependent. For self-similar depth and time dependence of eddy viscosity and arbitrary time dependence of wind we find an exact general solution to the Navier-Stokes equations which describes the dynamics of the Ekman boundary layer in terms of the Green's function. Two basic scenarios (a periodic wind and an increase of wind ending up with a plateau) are examined in detail. We show that accounting for the time dependence of eddy viscosity is straightforward and that it substantially changes the ocean response, compared to the predictions of the models with constant-in-time viscosity. We also examine the Stokes-Ekman equations taking into account the Stokes drift created by surface waves with an arbitrary spectrum and derive the general solution for the case of a linearly varying with depth eddy viscosity. Stability of transient Ekman currents to small-scale perturbations has never been examined. We find that the Ekman currents evolving from rest quickly become unstable, which breaks down the assumed horizontal uniformity. These instabilities proved to be sensitive to the model of eddy viscosity, they have small (∼ 10 2 m) spatial scales and can be very fast compared to the inertial period, which suggests spikes of dramatically enhanced mixing localized in the vicinity of the water surface. This picture is incompatible with the Ekman paradigm and thus prompts radical revision of the Ekman-type models.

Proceedings of the IEEE Fourth Working Conference on Current Measurement, 1990

Point measurement of the motional horizontal electric field (HEF) is an important and underutilized tool for the study of the barotropic component of the velocity field at subinertial frequencies. This paper reviews the theoretical relationship between the HEF and the verticallyintegrated, conductivity-weighted horizontal water velocity including the effect of finite electrical conductivity beneath the d o o r. This is validated by comparing data from a six element taut current meter mooring to nearby HEF data collected in 1986-1987 in the central North Pacific. The similarity of the two data sets is limited only by a high noise level in the mooring data due to rotor stalling and by large vertical wavenumber components of the velocity field that cannot be resolved with a small number of discrete velocity samples. The results of this study constitute further verification of the utility of electric field methods in oceanography.

Journal of Physical Oceanography, 2009

The datasets of the Eddies and Gyre Path Tracking (EGYPT)/EGITTO program in the eastern Mediterranean Sea reveal a large mesoscale anticyclone traveling along the Libyan shelf. Surface drifter trajectories combined with a CTD transect accurately quantify the horizontal velocity and the vertical structure of this surface-intensified anticyclone. The observed westward drift speed is significantly higher than expected from the beta effect alone. To study the impact of a steep shelf topography on the propagation of compact surfaceintensified vortices, the authors used a two-layer beta-plane model with steep continental slope and nearly zonal boundary. A perturbation theory derived by G. Sutyrin for a circular vortex in the upper layer with the lower layer at rest as a basic state is generalized for nonuniform slope in the presence of the image effect. An integral momentum balance is used to derive the drifting velocity of an upper-layer vortex with the main assumption that a stable and steady drifting solution of the two-layer system exists. The interface is described by a steady drifting circular dome at the leading order. This approach allows the problem to be reduced to the calculation of the deep-flow pattern, depending on the interface shape and topography. When the topographic slope beneath the eddy changes rapidly from a steep continental slope to a gentle continental rise, most of the deep-flow pattern is shifted offshore. The corresponding anticyclonic deep-flow feedback provides an additional along-slope propagation, which is proportional to the basic drift speed and the steepness parameter.

Physics of Fluids

The angle between the wind stress that overlies the ocean and the resulting current at the ocean surface is calculated for a two-layer ocean with uniform eddy viscosity in the lower layer and for several assumed eddy viscosity profiles in the upper layer. The calculation of the deflection angle is greatly simplified by transforming the linear, second-order, vertical structure equation to its associated nonlinear, first-order, Riccati equation. The transformation to a Riccati equation can be used as an alternate numerical scheme, but its main advantage is that it yields analytic expressions for several eddy viscosity profiles.