Chapter 9 - Response of the
Upper Ocean to Winds

9.5 Important Concepts

1. Changes in wind stress produce transient oscillations in the ocean called inertial currents
   (a) Inertial currents are very common in the ocean.
   (b) The period of the current is (2π)/f.



2. Steady winds produce a thin boundary layer, the Ekman layer, at the top of the ocean. Ekman boundary layers also exist at the bottom of the ocean and the atmosphere. The Ekman layer in the atmosphere above the sea surface is called the planetary boundary layer.



3. The Ekman layer at the sea surface has the following characteristics:
   
   (a) Direction: 45° to the right of the wind looking downwind in the Northern Hemisphere.
   (b) Surface Speed: 1-2.5% of wind speed depending on latitude.
   (c) Depth: approximately 40-300 m depending on latitude and wind velocity.



4. Careful measurements of currents near the sea surface show that:
   (a) Inertial oscillations are the largest component of the current in the mixed layer.
   (b) The flow is nearly independent of depth within the mixed layer for periods near the inertial period. Thus the mixed layer moves like a slab at the inertial period.
   (c) An Ekman layer exists in the atmosphere just above the sea (and land) surface.
   (d) Surface drifters tend to drift parallel to lines of constant atmospheric pressure at the sea surface. This is consistent with Ekman's theory.
   (e) The flow averaged over many inertial periods is almost exactly that calculated from Ekman's theory.



5. Transport is 90° to the right of the wind in the northern hemisphere.



6. Spatial variability of Ekman transport, due to spatial variability of winds over distances of hundreds of kilometers and days, leads to convergence and divergence of the transport.
   (a) Winds blowing toward the equator along west coasts of continents produces upwelling along the coast. This leads to cold, productive waters within about 100km of the shore.
   (b) Upwelled water along west coasts of continents modifies the weather along the west coasts.



7. Ekman pumping, which is driven by spatial variability of winds, drives a vertical current, which drives the interior geostrophic circulation of the ocean.

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