Dynamics of post-convection in the Gulf of Lion
1 - Introduction
We present here a dynamical analysis of the full three-dimensional fields of temperature and advection based on a dataset collected during the european MAST-I program Thetis 1 and french experiment ConvHiv. We have extended the analysis of the thermal structure presented in Gaillard et al. (1997), by including velocity measurements from moored current meters, deep floats and reciprocal tomography. The largest velocity dataset has been collected during the post convection period, i.e. right after the violent mixing. So our study focuses on this period. The main objective is to present a methodology, its feasibility and to demonstrate the efficiency and added value of such a method seen here as an extension of the previous analysis which have been performed on individual datasets. A second objective is to extract quantitative informations on the space and time scale variability, on the heat transport and on the most salient dynamical features for the time period following the convection phase in the Gulf of Lion.The Thetis 1 experiment was designed to observe the development of deep convection in the ulf of Lion. It was conducted by the Thetis group, formed in 1990 within the european MAST-I program. Thetis 1 relied on the combination of CTD casts and a moored array equiped with current meters, temperature/salinity sensors, Doppler profilers and acoustic tomography. Such combination of in-situ measurement techniques was deployed to cover the wide range of scales involved. This implied devising new methods for the simultaneous analysis of heterogeneogeneous data.
2 - The dataset
The measurements performed during the winter 1991-92 were organized as follows:
･ A small scale array of 3 moorings was set at the central point of the convection area (42N-5E). It was designed to study the plumes and equiped with ADCPs and thermistors chains.
･ Acoustic tomography was the core of a monitoring array set to observe the mixed patch equipped with current meters, temperature sensors and tomographic instruments were deployed in November at the positions shown on Fig. 1.The moorings were retrieved in February in order to repair a malfunction of the new tomography controller, they were immediately redeployed.
･ Three VCM floats were operating from February to April (ConvHiv).
･ Five hydrographic cruises performed during the course of the experiment provided a fine description of the veertical structure of temperature and salinity and of its time evolution from fall to the end of winter.
When the instruments combine the transmitting and receiving functions, tomography data give access to temperature and current. The simplest relation is obtained by linearizing the exact relation around a mean ocean state defined by the mean vertical sound speed profile. A first equation relates the mean travel time perturbation to the sound speed anomaly, a second relates the differential travel time to the current component along the section. While the impact of the temperature signal on the travel time anomaly is of the order of a 20 to 100 milliseconds, the impact of the velocity signal is at least one order of magnitude smaller.
