Observations from the High Wind Gas Exchange Study (HiWinGS): Gas transfer in the Labrador Sea at wind speeds above 15 m/s

The nature of kinematic forcing driving the air-sea exchange of momentum, heat, and dissolved gases evolves with increasing wind speed. At wind speeds of 7-10 m/s whitecaps appear, increasing turbulent kinetic energy dissipation in the ocean surface and injecting air bubbles into the liquid phase. Significant quantities of sea spray are produced at wind speeds of 15 m/s and above. Transfer coefficients up to wind speeds of ~15 m/s have been obtained during many research programs over the past two decades. However, few gas transfer observations at wind speeds of 15-30 m/s exist, and almost none with coincident wave physics observations. In physical models, the effects of whitecaps, sea spray, and sea state on gas transfer remain crudely parameterized. The HiWinGS study was conceived to address the paucity of field observations under these conditions and develop improved representations for the effects bubbles and waves in the NOAA COARE bulk flux model. Observations in high winds were obtained in Oct-Nov 2013 from a cruise in the Labrador Sea on the R/V Knorr. The experimental strategy focused on placing the ship in the predicted path of weather systems, hove-to (bow into wind) and drifting with the mean currents for the duration of each event. Seven stations of intensive observations were conducted over 36 days. Hourly average wind speeds reaching 25 m/s with well-developed seas (Hs ~9m) were encountered. Wind speeds exceeded 15 m/s 25% of the time. UH and NOAA investigators obtained transfer coefficients for two trace gases, dimethylsulfide (DMS) and CO2, from continuous eddy correlation flux and seawater concentration measurements, in addition to momentum flux and bulk meteorological variables. Wave height and period were monitored from the ship's bow tower with a laser rangefinder. Collaborating groups made wave measurements from free-drifting Waverider and spar buoys at each station and collected continuous daytime whitecap imagery. Other groups focused on flux measurements of soluble organic trace gases. Transfer coefficients for winds above 15 m/s for the more soluble gas DMS are broadly consistent with prior observations and generally consistent with the trend predicted by the COARE gas transfer scheme. For the less soluble gas CO2, the observed transfer coefficients are greater than for DMS but significantly less than predicted by either empirical power-law relationships or the COARE model at high wind speeds. These results imply the physical understanding of air-side resistance and interfacial transfer, as represented in COARE, are largely correct, but the effects of bubble-mediated transfer and waves (important for low solubility gases) require further development.