Wind stress

[2] Stress is the quantity that describes the magnitude of a force that is causing a deformation of an object.

These surface currents are able to transport energy (e.g. heat) and mass (e.g. water or nutrients) around the globe.

The different processes described here are depicted in the sketches shown in figures 1.1 till 1.4.

[2] The Beaufort scale quantifies the correspondence between wind speed and different sea states.

The wind stress is the component of this force that acts parallel to the surface per unit area.

[5] The equation describes how the force exerted on the water surface decreases for a denser atmosphere or, to be more precise, a denser atmospheric boundary layer (this is the layer of a fluid where the influence of friction is felt).

The wind stress can also be described as a downward transfer of momentum and energy from the air to the water.

is the density of the surface air and CD is a dimensionless wind drag coefficient which is a repository function for all remaining dependencies.

Some important assumptions that underlie the Ekman balance are that there are no boundaries, an infinitely deep water layer, constant vertical eddy viscosity, barotropic conditions with no geostrophic flow and a constant Coriolis parameter.

Upwelling due to Ekman transport can also happen at the equator due to the change of sign of the Coriolis parameter in the Northern and Southern Hemisphere and the stable easterly winds that are blowing to the North and South of the equator.

A well known phenomenon that is caused by changes in surface wind stress over the tropical Pacific is the El Niño-Southern Oscillation (ENSO).

Important meridional wind stress patterns are northward (southward) currents on the eastern (western) coasts of continents in the Northern Hemisphere and on the western (eastern) coast in the Southern Hemisphere since these generate coastal upwelling which causes biological activity.

[4][1] Wind stress is one of the drivers of the large-scale ocean circulation with other drivers being the gravitational pull exerted by the Moon and Sun, differences in atmospheric pressure at sea level and convection resulting from atmospheric cooling and evaporation.

However, the contribution of the wind stress to the forcing of the oceanic general circulation is largest.

Important is the Sverdrup balance which describes the relation between the wind stress and the vertically integrated meridional transport of water.

[12][13] Long before these theories were formulated, mariners have been aware of the major surface ocean currents.

Firstly, the Eulerian velocity can be measured using a current meter along a rope in the water column.

[1] Wind-driven upwelling brings nutrients from deep waters to the surface which leads to biological productivity.

Coastal upwelling occurs when the wind stress is directed with the coast on its left (right) in the Northern (Southern) Hemisphere.

All of these currents support major fisheries due to the increased biological activities.

Therefore, the characteristics of wind waves are determined by the coupling processes between the boundary layers of both the atmosphere and ocean.

Wind waves also play an important role themselves in the interaction processes between the ocean and the atmosphere.

A proper description of the physical mechanisms that cause the growth of wind waves and is in accordance with observations has yet to be completed.

[2][16][17][18] The drag coefficient is a dimensionless quantity which quantifies the resistance of the water surface.

A general expression for the drag coefficient does not yet exist and the value is unknown for unsteady and non-ideal conditions.

Still, measurements of the wind stress are important as the value of the drag coefficient is not known for unsteady and non-ideal conditions.

Measurements of the wind stress for such conditions can resolve the issue of the unknown drag coefficient.

[2] The wind can also exert a stress force on land surface which can lead to erosion of the ground.

Figure 1.1 A sketch of an ocean at rest with a zonal wind blowing over the ocean surface.
Figure 1.2 A sketch of an ocean still at rest with wind induced zonal surface stress vector depicted.
Figure 1.3 A sketch of an ocean in the Northern Hemisphere where wind waves and a surface Ekman current have been generated due to shear action of the zonal wind stress. In the Northern Hemisphere, the surface Ekman current is directed 45° to the right of the wind vector.
Figure 1.4 A sketch of the boundary layer of an ocean in the Northern Hemisphere where a zonal wind stress generates a surface Ekman current and other deeper positioned Ekman currents that are turned rightward. At the bottom of the ocean boundary layer the Ekman spiral is depicted. Also, the net Ekman transport which is directed 90° to the right of the wind stress vector is depicted.
Figure 2.1 Climatology over 1990–2020 of annual mean zonal wind stress [N/m ]. Positive values imply that wind stress is directed toward the East. [ 9 ]
Figure 2.2 Climatology over 1990–2020 of annual mean meridional wind stress [N/m ]. Positive values imply that wind stress is directed toward the North [ 9 ]
Figure 2.3 Animation of the climatology over 1990–2020 of monthly mean zonal wind stress [N/m 2 ]. Positive values imply that wind stress is directed toward the East [ 9 ]
Figure 2.4 Animation of the climatology over 1990–2020 of monthly mean meridional wind stress [N/m ]. Positive values imply that wind stress is directed toward the North [ 9 ]