Recorded presentation11B.4
Wind stress under the presence of swell in moderate to strong wind conditions
Hector Garcia-Nava, CICESE, Ensenada, Baja California, Mexico; and F. J. Ocampo-Torres, B. K. Haus, and I. Savelyev
Wind stress is a key parameter to a number of oceanic and atmospheric processes at different scales, including the global climate, large scale atmospheric and oceanic circulation, storm development, and wave generation. Thus, estimation of wind stress represents an issue of increasing importance for oceanic and atmospheric modelling, coupling and dynamic studies.
It is well known that the transference of momentum between the atmosphere and the ocean depends on the sea state. In particular, it has been shown that the presence of swell can modify both the magnitude and direction of wind stress. The presence of swell enhances the wind stress mangnitude when swell propagates opposite to the wind direction and reduces it when it travels in an along wind direction; deviations of the stress direction from the mean wind direction can be as much as 180o. However, those conclusions are mainly based on data acquired in low wind speed conditions and it is not clear to what extent the effect of swell persists at higher winds.
Aiming for a better understanding of the effect of swell on momentum flux, simultaneous measurements of wind stress and wave field were carried out in the Gulf of Tehuantepec Mexico from an Air-Sea Interaction Spar (ASIS) buoy. ASIS was moored in the central part of the Gulf of Tehuantepec roughly 20 km from coast in a 60m depth area from February to April 2005.
The Gulf of Tehuantepec is a region of the Mexican South Pacific characterized by the occurrence of strong gap wind events, know as Tehuanos, that blows offshore against long period Pacific swell. The Tehuanos are stronger and more common during winter. A single event can last from a few hours to several days, with wind speeds reaching 30 ms-1.
The observations made at high winds conditions point out that the swell causes significant reduction of wind stress at winds as high as 20 ms-1. The current hypothesis is that swell causes a reduction of drag by modifying the wind-sea roughness. Discussions of this hypothesis in the light of field data along with measurements of wind stress, wave field, and form drag made in the Air-Sea Interaction Salt water Tank (ASIST), of the University of Miami, in similar conditions will be presented here.
Session 11B, In situ turbulent air-sea flux measurements, including gas exchange
Wednesday, 14 January 2009, 4:00 PM-5:30 PM, Room 128B
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