Wind stress

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Wind stress is the shear force that the wind applies to the surface of large bodies of water. It is the part of the wind’s force that acts parallel to the surface and carries momentum from air to water. This force helps generate wind waves and drives the surface currents that move heat, nutrients, and water around the globe. The strength of wind stress depends on how fast the wind is blowing, the state of the waves, and how the air and water mix.

We describe wind stress with a simple idea: it grows with wind speed and air density and depends on a drag coefficient that captures how the water surface resists the wind. A common expression is tau = rho_air * CD * U^2, where U is the wind speed about 10 meters above the surface. The drag coefficient changes with wind history and water depth, so there is no single universal formula for all conditions.

The wind mainly stirs the ocean’s top layer, about 10 meters deep. Because the Earth rotates, the surface current is deflected to the right of the wind in the Northern Hemisphere (and to the left in the Southern Hemisphere). Deeper currents bend even more in what is called the Ekman spiral, and the overall transport is perpendicular to the wind. This Ekman transport can push water away from or toward the coast, helping cause coastal upwelling or downwelling.

Global wind patterns shape ocean circulation. Trade winds near the equator and westerlies at midlatitudes drive large circular currents (gyres), while strong westerlies around Antarctica fuel the Antarctic Circumpolar Current. Winds are variable, so the ocean’s currents and waves also change over time, and these changes influence climate phenomena like ENSO.

Wind stress is measured indirectly because the drag coefficient isn’t fixed for all conditions. Scientists estimate it from wind speed using methods such as Reynolds stress, energy dissipation, profile measurements, or radar. Typical wind-stress values are around 0.1 pascals on average, with larger values in the Southern Ocean.

Wind stress and the resulting waves interact with the wind field, creating a dynamic link between the air and sea. The same idea also applies to wind loading on land, which can cause erosion.