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2.3.2 Wind Stress

Winds blowing at the surface constitute a very important driving force for ocean currents. Wind stress at the surface is therefore a necessary forcing parameter for an ocean model. Winds are routinely observed at many coastal locations around the world, but these are little help in deducing the winds over the open ocean. Historically, observations from ships constitute a major portion of our knowledge of winds over the oceans, although remote sensing using microwave scatterometers and radar altimeters (instruments that essentially measure the wind-induced roughness of the sea surface and thereby infer the wind stress acting at the sea surface) might become an important future source. Since winds vary over a variety of time scales including hourly to annual, it is difficult to obtain complete wind information at all temporal and spatial scales over the oceans. Numerical weather prediction models are one source of ``synoptic'' winds, however imperfect.

For longer time scales, marine surface observations from ships are an important source. These observations have been collected and archived, and long-term averages over many decades incorporated into a historical data base. Hellerman and Rosenstein [29] derived monthly average wind stresses from such a data base over global oceans at a resolution of 2 degrees (roughly 200 km). This data base is often adequate for simulating seasonal evolution of currents in an ocean model. Most ocean model results to date have been obtained with such climatological wind fields. Provided is a program to access the Hellerman--Rosenstein data base and extract monthly wind stresses for any region with prescribed latitudinal--longitudinal extent. These fields can be displayed using GMT and can also be animated to look at the seasonal evolution of wind stress/wind over the region. Below are figures depicting the monthly averaged winds for the Sea of Japan during the four seasons.

Figure 10: Spring Monthly Wind Averages in the Sea of Japan.

Figure 11: Summer Monthly Wind Averages in the Sea of Japan.

Figure 12: Autumn Monthly Wind Averages in the Sea of Japan.

Figure 13: Winter Monthly Wind Averages in the Sea of Japan.