National Oceanography Centre, Southampton, UK
Pierre Testor, LOCEAN-IPSL, Paris, FRANCE
Sonya Legg, GFDL, Princeton, USA
Isabelle Taupier-Letage, LOB, La Seyne sur Mer, FRANCE.
For more information about the project, and real-time data,
see the NOC Glider pages here
In this project we will investigate deep convection in the ocean using undersea gliders; develop a control system to enable the gliders to search, identify and sample convective events; and carry out novel field observations in late-winter in the northwest Mediterranean Sea. Our objectives are to survey a convective patch, to measure for the first time the amplitudes and scales of convective plumes, and to examine the processes at the edge of the patch that exchange water between the patch and the surrounding ocean.
Convection
Deep water formation is a key process for understanding thermohaline circulation in the ocean. It occurs as deep convection events over relatively short periods in late winter at just a few locations, notably the Norwegian-Greenland Sea, the Labrador Sea and the northwest Mediterranean Sea. Because of the inhospitable conditions it is difficult to make observations from ships at times of active convection.
Figure 1.Temperature at the surface of a convecting patch. Numerical simulation courtesy of Sonya Legg.
There are two key issues with deep convection that we propose to address with glider measurements in the Mediterranean Sea. The first is the size of the active convective elements or plumes: what are the length scales, how big is their density anomaly, how fast is their vertical velocity, and how many are there within the patch? Theoretical studies do predict the scale of the plumes based on large-scale parameters for the convection but these scale estimates need to be tested. The second issue is how the newly formed dense water is exchanged across the edges of the convective patch.
Gliders
Figure 2. Webb Research Corporation glider "Slocum".
Gliders represent a recent technological development that is just starting to be in for scientific experiments. Until now they have only been used by the handful of groups who are developing these vehicles. However, we believe that in the near future their use will become widespread.Gliders are typically able to 'fly' from the surface to 1000m depth and back to the surface, whilst travelling about 5km horizontally, in around 3 hours. Along their trajectory they measure pressure, temperature and conductivity, from which salinity can also be derived. Depth averaged and surface currents can be estimated from GPS fixes at the surface. Two-way communication via satellite provides an exciting opportunity to adapt the sampling strategy in response to the data acquired in real-time.
The aim of the project is to develop innovative techniques for the use of gliders and apply them in a study of deep convection in the Mediterranean Sea. Our overall objectives are:
- To develop a system for observing ocean processes and features using a network of ocean gliders directed by a real-time control system incorporating a numerical forecast model.
- To make novel observations of open-ocean deep convection that will enable us to determine the characteristics of plumes within convective patches and the mechanisms and rates of lateral spreading of the convective patches.
Field program
We propose a field experiments in February 2007 in the Gulf of Lions, Mediterranean Sea. The specific objectives of this experiment will be:To map out convective patches, designing criteria that define the edge of active convection and then using gliders to identify the edges of the patch.
To characterise the amplitude and horizontal and vertical scales of the plumes, again using gliders to find the core of the active convection region and then to repeatedly sample the plumes.
To examine the exchange processes at the edge of the convection patch using gliders to see if baroclinic instability is the principal mechanism in the exchange and to identify any smaller scale interleaving, slumping or spreading outwards of the newly formed dense waters from the patch.
Figure 3.The deep convection site in the Gulf of Lions.
Methodology
Unlike profiling floats, or moorings, it is possible to direct gliders to move and make measurements in required locations. To realise their full potential the guidance communicated to the gliders must make use of all of the available information. This requires a sophisticated adaptive guidance system, the development of which is at the core of the study. The proposed operation of the system is illustrated in Figure 4.
Figure 4. Schematic diagram of the adaptive glider network for observing deep convection.