The presence of a seasonal thermocline likely plays a key role in restraining methane released from aseabed source in the deeper water column, thereby inhibiting exchange to the atmosphere. The bubbleplume itself, however, generates an upward motion of fluid, e.g. upwelling and may thereby be partiallyresponsible for an early breakdown of the seasonal thermocline. Measurements at site 22/4b, located at(57?550N, 1?380E) in the UK Central North Sea, 200 km east of the Scottish mainland, where gas is stillbeing released since a blow out in 1990, have been used to identify the generation of the seasonalthermocline, and thus, the depth of the upper mixed layer and its breakdown in autumn. Data derivedfrom two landers, containing an Acoustic Doppler Current Profiler and a Conductivity Temperature Depthrecorder, were used to determine the mixed layer depth and the breakdown of the thermocline. Mixingof upper layer fluid into the lower layer has been inferred from large amplitude variations in the nearbottomtemperature.The ADCPs estimate velocity profiles in four beam directions using Doppler shifted frequency fromacoustic pings sent out and received by four different transducers in a specific configuration. Besides that,the intensity of the backscattered sound per transducer is also recorded. Bubbles from the nearby plumecontaminate the signal during part of the tidal cycle, but in bubble free periods, the mixed layer depthcan be estimated using the acoustic backscatter signal as local maxima. Results show that the thermoclinebroke down between mid-October and early November, several weeks earlier than the breakdownof the thermocline in nearby/comparable areas, likely caused by bubble-induced downwelling at the site.The early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet-likestructure, associated with the seasonal tidal mixing front.
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