Measurements with an Automatically Recording Inverted Echo Sounder; ARIES and the Bubble Clouds

Abstract

We report here analysis of the first observations with a new instrument, ARIES, designed to record the acoustic backscatter from bubble clouds at several levels below the surface of the ocean. The instrument is deployed on a subsurface mooring and records internally. Data has been analysed from deployments of ARIES for one week in the Irish Sea and for periods of 4 and 9 weeks near the edge of the UK continental shelf, during periods in which meteorological data, waves, and near-surface temperatures and, on occasion, currents, were being recorded by other instruments. The results, while being generally similar to earlier measurements in coastal waters, show that the bubble clouds are more intense, having greater acoustic scattering cross per unit volume, Mv, and extend, on average, deeper into the water column. A simple model accounting for the shape of the clouds, their advection through the sonar beam, and their decay (through bubbles returning to the surface or through their gas dissolving into the surrounding sea water) is developed and used to analyse and interpret the recorded bubble cloud duration at the sonar and the intervals of time between successive clouds. This model allows the observations of clouds at a fixed position to be related to the breaking waves which generate them and to the whitecap coverage, and provides a unified model of these phenomena and their spatial and temporal features. The area of bubble clouds at a fixed depth appears to be related to the wind speed in a similar way to whitecap coverage. Generally, the cloud area just below the sea surface is much in excess of the whitecap area; the area of bubble clouds reaching 10.3 m while being actively generated by breaking waves is much less. Following a theory developed in an earlier paper by Thorpe, the mean vertical distribution of a bubble scattering cross section is analyzed to infer the vertical turbulent eddy diffsion coefficient, Ko. The shape of the distribution favors the form Kv = ku*z where u* is the friction velocity in the water, z is depth and k is Von Kármán's constant. The results suggest, however, higher values of u* than can be explained by the wind stress alone. The increased levels of Mv imply that the gas flux from the air to the water via bubbles is greater than previously estimated. An equilibrium supersaturation level of oxygen of about 3.6% is predicted in wind speeds of about 8 m s?1.