83rd Annual

Wednesday, 12 February 2003
Ocean Currents Inferred from Differences Between QuikSCAT and TAO Winds
Kathryn A. Kelly, University of Washington, Seattle, WA; and S. Dickinson and G. C. Johnson
Differences between the winds from the Tropical Atmosphere-Ocean (TAO) buoys and from the QuikScat scatterometer are used to estimate time-varying ocean surface currents. The scatterometer measures backscatter from centimeter waves caused by the wind blowing over the ocean; when ocean and atmosphere move together, no waves are generated. Thus, the scatterometer measures the motion of the air relative to the moving ocean surface, the quantity needed to estimate surface stress. On the other hand, an anemometer measures the motion of the air relative to a fixed platform. Therefore, the difference between an anemometer wind and a scatterometer ``wind" contains the signature of the ocean surface current.

Previous estimates of currents from TAO anemometer winds minus NSCAT or QuikSCAT scatterometer winds showed good agreement with carefully collocated current measurements from moored current meters on the equator and from acoustic Doppler current profilers (ADCP) mounted on ships servicing the TAO moored array. In the comparison with QuikSCAT winds, currents in the South Equatorial Current (SEC) were about 1.2 m/s westward and currents in the North Equatorial Countercurrent (NECC) were about 0.6 m/s eastward. Neglect of these strong tropical currents in this region of relatively weak winds (5-7 m/s) leads to errors in stress of 25-50\% and to even larger errors in wind stress curl.

Here we estimate time-varying surface currents over 2.5 years and compare with the current estimates derived from the ADCP. An expected discrepancy in the current comparisons arises because the ADCP measures currents at depths below 25m; surface currents differ from a 25-m current both by the vertical shear in the geostrophic current and by the Ekman current, which is much stronger at the surface. Preliminary estimates of the surface Ekman component from the scatterometer suggest that it is larger than expected. If currents estimated from anemometer/scatterometer differences can be shown to be sufficiently accurate, routine estimates of surface currents can be made to enhance the current sparse measurements and the implications for atmosphere-ocean momentum coupling can be quantified.

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