HiWinGS Observations: Whitecap Fraction and Sea State

From October 9th to November 13th 2013, the High Wind Gas Exchange Study (HiWinGS) research cruise took place on board the R/V Knorr in the Labrador Sea, with the aim of gaining a greater understanding of the dependence of air-sea fluxes on sea state under high wind conditions. Fluxes of momentum, heat, moisture, sea-spray aerosol and several trace gases including CO2, DMS, and methanol were determined using the eddy covariance technique. Spar buoys, designed and build at the UK National Oceanography Centre, and a Datawell DWR-G4 Directional Waverider buoy were deployed to measure wave spectra and wave breaking.

Breaking waves entrain air into the upper ocean, forming plumes of bubbles that can enhance gas exchange and generate sea-spray aerosol when they burst at the surface. Whitecaps are the surface manifestation of these bubble plumes and often used as a proxy measure for the associated processes. Whitecap fraction data was calculated using images from multiple cameras situated on the ship that operated autonomously throughout the cruise.

A wide variety of wind speeds and sea states were observed; 10-minute averaged Hs and U10 values during HiWinGS were between 1.38 and 11.4 m, and 0.1 m/s and 27.7 m/s respectively. Here we discuss whitecap fractions for these different observed conditions, comparing observations with wave statistics and spectral energies.