Observations with Moored Acoustic Doppler Current Profilers in the Convection Regime in the Golfe du Lion

Abstract

In the Golfe du Lion, south of France, favorable conditions for deep winter convection exist and were documented by the MEDOC experiments during 1969–75. A renewed investigation of that regime with upward-looking moored acoustic Doppler current profilers (ADCPs) was carried out during 24 January–5 March 1987, to record the three-dimensional currents associated with the deep mixing. While in the earlier studies initial deep convection did not begin until fairly late in the winter season, a very strong Mistral around 10 January 1987 had already generated a 1arge deep-mixed patch, homogeneous down to around 2000 m at deployment time. Three ADCPs, two working at 150 kHz and one at 75 kHz, were moored in a triangle of 15 km sidelength at 550–780 m depth. Full records at 1-hour ensemble time intervals, 400 pings per ensemble, 8 m bin lengths were obtained by the 75 kHz and one of the 150 kHz ADCPs. In mid-February, a second Mistral hit the region. With the onset of strong winds and surface cooling the occurrence of short-period current fluctuations, in the period range of hours, was observed which lasted for the duration of the negative heat flux. The vertical currents recorded by the ADCPs during this period included downward events of 5–10 cm s−1 velocity with weaker upward motion in between. These events appeared to occur simultaneously over the depth range of several 100 m covered by the ADCPs. An interpretation of these events as frozen structures, advected by with the mean current, yielded a horizontal scale estimate of only order 1 km. The mean vertical velocity during the Mistral week due to the integrated effect of these events was of order 1 cm s−1 downward.