Upward momentum transfer in the marine boundary layer

This paper focuses on the study of momentum flux between ocean and atmosphe re in light winds and is based on the data collected during several field c ampaigns, the Atlantic Stratocumulus Transition Experiment, the Tropical Oc ean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment, and the San Clemente Ocean Probing Experiment. Weak wind at sea is frequently acco mpanied by the presence of fast-traveling ocean swell, which dramatically a ffects momentum transfer. It is found that the mean momentum flux (uw covar iance) decreases monotonically with decreasing wind speed, and reaches zero around a wind speed U approximate to 1.5-2 m s(-1), which corresponds to w ave age c(p)/U approximate to 10 for wave/swell conditions of the experimen ts in this study. Further decrease of the wind speed (i.e., increase of the wave age) leads to a sign reversal of the momentum flux, implying negative drag coefficient. Upward momentum transfer is associated with fast-traveli ng swell running in the same direction as the wind, and this regime can be treated as swell regime or mature sea state. In the swell regime the surfac e stress vector is nearly opposite to wind and swell directions, and the wi nd is roughly aligned in the swell direction. Thus, a weak wind over ocean swell can be frequently associated with upward momentum transfer (i.e., fro m ocean to atmosphere).