• What is the mean circulation of the Mediterranean Water and the Labrador Sea Water in the interqyre region of the eastern North Atlantic Ocean?

• Is there a lagrangian drift or eddy mixing of the Mediterranean Water and the Labrador Sea Water?

Modeling studies are underway to gain a better understanding of the mechanisims for generating flow and mixing of water masses in the presence of waves and instabilities.

Descriptive:

According to Saunders' (1982) description of the circulation in the eastern North Atlantic, there is, roughly, southward flow above 500m, northward flow in the depth range 500-1000m north of 40°N, and southward flow below 1000m. His study was based on the analysis of hydrographic data, with flow referenced such that the vertically integrated transport agreed with Ekman pumping, generally weak in this area. Recent analyses of historical data in the region by Paillet (1996) refine and support the earlier work, and provide quantitative estimates of geostrophic transport in various density classes.

The Med Water itself has been suggested to have a dynamical impact on the eastern Atlantic through the flow generated by mixing or double diffusion. Mixing tends to stretch water columns locally by permitting a diapycnal velocity to arise, generating vorticity and meridional flow. The Med outflow from the Strait of Gibralter tends to flow north along the topography, feeding the interior with salt and mass via instabilities. Modeling and laboratory studies are underway by a number of groups to understand the diverse dynamics involved in the overflow, eastern boundary current, and interior circulation. Ekman suction in the subpolar gyre is one mechanism for creating meridional flow at mid-depths since isopycnal surfaces within the Med Water rise to the surface in the subpolar gyre (Schopp). Other mechanisms may be relevant near the eastern boundary where topographic waves occur. Also, the basic instability of the large-scale flow in the eastern Atlantic could lead to the mixing of Med Water with surrounding water and generate salt fluxes.

Statistical:

Very little lagrangian data exists for the intergyre region of the eastern North Atlantic. The TOURBILLON experiment at 47 N, 15 W provided a few month-long trajectories which suggest a suprisingly short 2 day lagrangian time scale, and 15-30km spatial correlation scale (Mercier and Colin de Verdiere, 1985). For comparison, Rossby, Price, and Webb (1986) estimated decorrelation time scales of 7-8 days in POLYMODE data (western North Atlantic). Freeland et al. (1975) estimated a spatial correlation scale of 55 km from SOFAR data obtained during MODE, drifting at 1500m depth in the Sargasso Sea.

Float data from the middle of the Med salt tongue, roughly 20 N - 35 N, show two types of behavior: 1) zonal jet-like flow, with periods of 200 days or more, and 2) wave-like oscillations with periods of 50-200 days (Spall, Richardson, and Price, 1993). The spatial scale of the jets is thought to be about 75-150 km, based on instability studies (Spall, 1994). Integral time scales for the floats in the zonal jets are (23 d, 13 d) [for (u,v)], while the wave-like floats had time scales much shorter, at (8 d, 5 d).

Current meter mean flows are somewhat supportive of Saunder's circulation. The long term IFM Kiel mooring is to the south of the Lab Sea Water deployment, closer to the southern, Med Water deployment region, well within the subtropical gyre.

Modeling:

Modeling studies are being carried out to develop an understanding of the relative roles of eddy mixing and advection in idealized circumstances. The methods developed will be applied to more realistic water mass distributions in order to find the conditions under which eddies transport significant amounts of Med Water and Lab Sea Water properties. These two water masses are distinguished by temperature, salinity, and potential vorticity, and provide a canonical example of the importance of eddy-driven transformation to water mass evolution.

Assimilation studies are to be carried out in collaboration with ARCANE.

Efforts will be made to use the mesoscale information present in altimetric data to understand more about the flow field in which the floats move.