POMME : Pluridisciplinary Mesoscale Process Experiment
ABSTRACT
POMME is a research project aimed at understanding the subduction mechanisms of 11-12℃ mode water in the northeast Atlantic, how this affects the biological production and the carbon budget of the northeast Atlantic, and at describing the fate of organic matter after subduction. The study area is known to be a sink of carbon (Takahashi et al., 1995) and to present a high export of primary production (Jickells et al., 1996). The coupling of mesoscale dynamical and biological processes is therefore one of the major objectives of the project.
A key stone of the project is a network of five tomography instruments, and the associated analysis scheme, which will assimilate all available data with a 4-D Kalman filter model.
Introduction
The POMME project includes both a field experiment and a data assimilation/analysis phase. One of the corner stones of the experimental set up is the deployment of 5 moorings including tomographic sources and receivers. In this presentation, the accent is on the physical oceanography, but it should be kept in mind that a very significant biogeochemical component is an essential part of the experiment.
The domain we focus on is a 500km longitude by 750km latitude area centered on 41.5｡?/19｡? ( 16｡?-22｡?/38｡?-45｡?), and the field experiment is scheduled for a one-year period starting in the autumn 2000 and lasting until the autumn 2001. Efforts will be made to transmit a large amount of physical oceanographic data in near-real time in order to feed assimilation schemes and provide preliminary analyses and forecasts. The experiment is therefore also envisioned as a test of real-time oceanography. It will be linked with high resolution modeling efforts.
The area investigated is a transition zone between quite deep late winter mixed layers in the North (reaching at least 500m) and relatively shallow mixed layers (100-150m) in the south. The water to the North is advected into the area by the southern branches of the North Atlantic current, where it experiences a net cooling and buoyancy loss, and has very high nutrient levels in winter. Part of this water then flows southward, at least intermittently. This has been diagnosed from hydrography (Paillet and Arhan, 1996b), as well as by inverse modeling (Paillet and Mercier, 1997). During the southward drift, the late winter mode water subducts into the thermocline, where it influences the vertical stratification (low potential vorticity) equatorward to 28｡? (Figure 1).
On individual meridional sections, the change in the stratification is often quite sharp with the presence of mesoscale fronts and eddies (Pollard et al., 1996). From the eddy energy statistics estimated from altimetry and from drifters, this region is a boundary between a region of high energy levels to the North and one with much lower energy levels to the south. This transition probably has a dynamic origin, but this has not been thoroughly investigated.
