AUV observations of surface mixing and bubble entrainment in the Clyde estuary, Scotland

Most of the mixing throughout the world ocean is driven by wind forcing of the ocean surface, which also exerts a controlling influence of the rate of exchange of gases between the atmosphere and the ocean. As part of the UK NERC-funded surface boundary layer consortium 'OSMOSIS' we conducted AUV-based observations of near-surface turbulent mixing and bubble entrainment in the Clyde estuary, Scotland, in conjunction with a program of ship-, mooring, and glider-based measurements. A Hydroid REMUS 600m AUV equipped with a forward-mounted microstructure sensing package designed by Rockland Scientific was instrumented for this experiment with an upward-looking 1.1 MHz echo sounder, in order to measure bubbles that are injected into the near surface by breaking waves and often organized into quasi-linear vertical curtains by Langmuir circulation cells. The AUV-based observations reported here were conducted over a seven day period in September, 2011, in a two-day window leading up to, and subsequent three-day window recovering from, a period of force 12 winds. As the wind speed reduced a thin layer of fresher water, previously pinned to the coast by the gale-force winds, rapidly slumped over the survey area. Measurements revealed that wind-generated turbulence was largely confined to this brackish surface layer, a layer too thin to be sampled by conventional means. In this paper we describe the vehicle configuration during this pilot study, the resulting data, and further vehicle modifications that will enable both more robust measurements and more flexible use of the AUV as a test-bed sampling platform.