Wave-Modified Ekman Current Solutions for the Time-Dependent Vertical Eddy Viscosity

This study aimed to highlight a general lack of clarity regarding the scale of the temporal averaging implicit in Ekman-type models. Under the assumption of time and depth-dependent eddy viscosity, we present an analytical Fourier series solution for a wave-modified Ekman model. The depth dependence of eddy viscosity is based on the K-Profile Parameterization (KPP) scheme. The solution reproduces major characteristics of diurnal variation in ocean velocity and shear. Results show that the time variability in eddy viscosity leads to an enhanced mean current near-surface and a decrease in the effective eddy viscosity, which finally results in an intensified near-surface shear and wakes a low-level jet flow. Rectification values are dominated by the strength of diurnal mixing, and partly due to the nonlinear depth dependence of the eddy viscosity.

What happens with the Ekman current under constant wind?

<p>The work examines the Ekman current  response to a steady<br>wind within the Stokes-Ekman paradigm. Under constant wind<br>in the classical Ekman model there is a single attractor<br>corresponding to the Ekman (1905)steady solution. It is<br>known that the account of wind waves  strongly affects the<br>Ekman current dynamics via the Stokes drift, which is<br>described by the Stokes-Ekman  model. Waves continue to<br>evolve even under constant wind, which makes  steady<br>solutions of the Stokes-Ekman equation impossible. Since<br>the dynamics of the Ekman response in the presence of<br>evolving wave field have not been considered,  the basic<br>questions on how  the Ekman current evolves and,<br>especially, whether it grows or decays at large times,<br>remain open.</p><p>Here by employing the known self-similar laws of wave<br>field evolution and  solving analytically the<br>the Stokes-Ekman equation we  find and analyse<br>evolution of the Ekman current. We show that the system has<br>a single time dependent attractor which can be described<br>asymptotically. The large time asymptotics of the Ekman<br>current is found to be determined by the regime of wave<br>field evolution:  for the regimes typical of young waves<br> the Ekman current grows with time to infinity, in contrast, for<br>`old waves'  the Ekman current asymptotically decays.</p><p> </p>

Wind drag in oil spilled ocean surface and its impact on wind-driven circulation

The drag coefficient is a key parameter to quantify the wind stress over the ocean surface, which depends on the ocean surface roughness. During oil spill events, oil slicks cover the ocean surface and thus change the surface roughness by suppressing multi-scale ocean surface waves, and the drag coefficient is changed. This change has not been included in the current ocean circulation models. In this study, such change in sea surface roughness is studied by satellite remote sensing via synthetic aperture radar (SAR) data to quantify the changes in the wind effect over the oil-covered ocean surface. The concept of effective wind speed is introduced to quantify the wind work on the ocean. We investigate its influence on the wind-driven Ekman current at the ocean surface. Using observations from the Deepwater Horizon oil spill (2010) as an example, we find that the presence of oil can result in an effective wind speed of 50%∼100% less than the conventional wind speed, causing overestimates by 75%∼100% in the wind driven Ekman current. The effect of such bias on oil trajectory predictions is also discussed. Our results suggest that it is important to consider the effect of changes in the drag coefficient over oil-contaminated areas, especially for large-scale oil spill situations.

Can Drake Passage Observations Match Ekman's Classic Theory?

Ekman's theory of the wind-driven ocean surface boundary layer assumes a constant eddy viscosity and predicts that the current rotates with depth at the same rate as it decays in amplitude. Despite its wide acceptance, Ekman current spirals are difficult to observe. This is primarily because the spirals are small signals that are easily masked by ocean variability and cannot readily be separated from the geostrophic component. This study presents a method for estimating ageostrophic currents from shipboard acoustic Doppler current profiler data in Drake Passage and finds that observations are consistent with Ekman's theory. By taking into account the sampling distributions of wind stress and ageostrophic velocity, the authors find eddy viscosity values in the range of 0.08–0.12 m2 s−1 that reconcile observations with the classic theory in Drake Passage. The eddy viscosity value that most frequently reconciles observations with the classic theory is 0.094 m2 s−1, corresponding to an Ekman depth scale of 39 m.

The coupling of wind and internal waves: modulation and friction mechanisms

The interaction between internal waves (IW) and wind waves (WW) is studied. Three types of interaction are considered: spontaneous IW generation by a random field of WWs, and two feedback mechanisms - modulation and friction.The latter mechanism has not been studied before. Its influence on the IW-WW coupling is of primary importance. The modulation and friction mechanisms result in exponential attenuation of the IWs. Attenuation of IWs propagating against wind is the strongest. The IW attenuation has a dimensionless decrement of order 10-3, whereas for storm winds it attains the value of 10-2. Joint action of the spontaneous generation of IWs and their attenuation due to feedback mechanisms permits a stationary ‘wind-IW’ spectrum to exist. For strong winds the ‘wind-IW’ energy is of order 105 erg cm-2. The effect of IWs on currents in the ocean's upper layer is considered. Momentum and energy lost by IWs due to their interaction with WWs generates inertial oscillations. Under the attenuation of intensive IWs, the amplitude of inertial oscillations may be compared with the background Ekman current.