Ekman drift in homogeneous water

Abstract

Measurements made with satellite-tracked buoys drogued in different layers between the sea surface and 30-m depth under homogeneous winter conditions in the North Sea allow analysis of the Ekman currents under a large variety of wind conditions. The experiment lasted from November 20, 1991, until February 29, 1992. The first 4 weeks of this period, during which the buoys stayed close together, are used to determine the Ekman stresses. The total current field is a superposition of barotropic currents due to sea level variations and Ekman currents. The classical Ekman theory is not able to describe properly the observed deflection of the currents to the right of the wind direction and their decay with depth. This deflection is 10° near the sea su1face and increases to approximately 50° in 25-m depth. The relation between wind stress and the stress field in the interior of the water is given by a tensor, which describes the rotation and the variation of the stress with increasing depth. The concept of eddy viscosity is applicable, if a viscosity tensor is used to relate stress and vertical shear. The viscosity tensor is a function of the vertical coordinate only and is independent from the wind stress. It shows maximum values in 15-to 20-m depth and may be due to Langmuir circulation cells. Further studies are needed to determine the physics of this tensor. Its magnitude in the interior of the mixed layer exceeds 1000 cgs units. Consequently, Ekman currents are weak and may not be the dominant currents within the mixed layer.

The Deflection Angle of Surface Ocean Currents From the Wind Direction

“…10.1029/2019JC015454 current from the wind is typically larger during the day than during the night (see Krauss, 1993). These discrepancies are ascribed to Ekman's oversimplified hypotheses, in particular, the assumption of constant eddy viscosity.…”

“…The authors are grateful for helpful comments from the referees. All data for this paper are properly cited and referred to in the reference list; more specifically, the relevant data can be found in the papers (Cronin & Kessler, 2009;Cronin & Tozuka, 2016;Huang, 1979;Krauss, 1993;Polton et al, 2013;Roach et al, 2015;Stacey et al, 1986;Yoshikawa & Masuda, 2009).…”

Ocean color observations of a surface water transport event: Implications for Pseudo-nitzschia on the Washington coast

“…Numerous attempts to verify Ekman theory have been made, but quantitative agreement is difficult to obtain in the field. In general, surface current deflections to the right of the windstress are observed; however, the angle of deflection can vary over a large range (Krauss, 1993). A gradient in wind direction was observed between the two meteorological stations, with winds oriented alongshore at each location.…”

“…The gap in the data record for La Perouse Bank between 21 and 24 May is unfortunate (as is the lack of data from the Cape Elizabeth buoy, just south of the eddy), but given the strength of the upwelling winds observed at Columbia River Bar and the degree to which the two wind records appear correlated thereafter, upwelling-favorable conditions were most likely present at La Perouse Bank during this time period. Theoretical near-surface flow is oriented 458 to the right of the surface windstress under stationary conditions and constant eddy viscosity (Ekman, 1905;Krauss, 1993). Numerous attempts to verify Ekman theory have been made, but quantitative agreement is difficult to obtain in the field.…”

Seasonal changes of water properties and current in the northernmost gulf of aqaba, red sea

“…Several studies showed that winds are effective external forces that govern surface and subsurface currents (Monismith and Genin 2004;Manasrah et al 2004;Berman et al 2000;Brenner et al 1988Brenner et al , 1989. On the other hand, a vertical anticlockwise rotation in current direction from top to bottom was observed specifically during summer and autumn seasons, which may be related to the Ekman wind drift in the coastal region with vertical wall (Price et al 1987;Krauss 1993), which predicts that a steady wind stress acting together with the Coriolis force will produce a transport of water to the right of the wind.…”