Deep mixing and sinking

Work package 1

The colour scale is nmol/m² of SF₆. This tracer was released in the Gyre area marked by the rectangle in August 1996. The initial concentration was 120. If one assumes that the rectangle represents an area initially ventilated (100%) by open ocean convection, then the colour scale roughly corresponds to the percentage of ventilated water in a column.

Spreading of Greenland Sea Deep Water (animation)

Key personnel and responsibilities:
Tor Eldevik (NERSC):
Principal investigator, convection and SCV modelling.

Ole Anders Nøst (NP): Theoretical interpretation and supervision of Ph.D. student.
Truls Johannessen and Yoshie Kasajima (Bjerknes): Field work and analysis of hydrography and SF6.
Doroteaciro Iovino (PhD student NERSC) : Coarse resolution GCM study with idealized domains
Signe Aaboe (Ph.D. student NP): Eddy resolving GCM study.

Activities

Modelling:
(i) Following Straneo and Pickart (2001), a model for the preconditioning and rate of convection associated with the Greenland Sea Gyre will be set up.

(ii) High resolution numerical simulations of Submesoscale Coherent Vortices (SCV) dissolution will be performed using the MITgcm (Marshall et al. 1997) in its non-hydrostatic mode.

(iii) A coarser version, although still eddy resolving in the gyre area, will be set up for the Greenland Sea. It will be forced at its open boundaries by fluxes estimated from regional or GCM model runs available at NERSC, and from the observational data provided through ASOF and other projects. This will constitute a Ph.D. project. The student will be supervised at NP, but the numerical modelling will take place in close collaboration with NERSC.

(iv) Studies of the Meridional Overturning Circulation (MOC) using full GCMs on idealized computational domains mimicking the North Atlantic (Marotzke and Scott, 1999, Spall and Pickart, 2001), will be extended with an idealized Greenland - Scotland ridge and a 30º × 20º Nordic Seas box using MICOM (Bleck et al. 1992). A further extension will be to add a shelf sea box representing the Barents Sea and thereby include the important exchange between a shelf sea and the deep ocean. The convective rate/mode-model developed in (i) should be used for parameterization when available. This activity will be a Ph.D. study at NERSC.

Field work: There will be continued sampling of SF6 and hydrography in the Greenland Sea in 2003 in collaboration with the EU TRACTOR project (coordinated by Truls Johannessen) with a focus on the interface between the intermediate and bottom layers, i.e. at the maximum convection depth. The data set will be analysed with respect to mixing processes and larger scale flow (sinking).

Discussion, description and motivation:
The understanding of Nordic Seas mixing, sinking, and circulation is important in its own right, but to touch upon the region's climatic influence one has to couple it to (at least) the North Atlantic. Broecker's visualization called the great conveyor belt has enjoyed popular support. Some GCM simulations support the concept, while those of Mauritzen and Häkkinen (1999), and Bentsen et al. (2002) find little influence of the variability of convective mixing in the Nordic Seas on the MOC. The small scale process studies will address the role of the convective eddies in the larger system. The larger scale but eddy-resolving modelling will allow an estimate of the ageostrophic Ekman and eddy transfer of properties in and out of the topographically controlled gyre, and estimation of the individual contributions to the circulation from resolved convective mixing and frontal subduction. The idealized coarse resolution GCM modelling will address fundamental aspects of the control and leads/lags in MOC variability. (See also recent work by Eldevik and Drange / Nøst / Kasajima and Haugan at www.noclim.org).

References
Bentsen, M. Drange, H., T. Furevik, and T. Zhou, 2002:Variability of the Atlantic meridional overturning circulation in an isopycnic coordinate OGCM. Submitted to Clim. Dyn.

Bleck, R., C. Rooth, D. Hu, and L. T. Smith, 1992: Salinity-driven thermocline transients in a wind- and thermocline-forced isopycnic coordinate model of the North Atlantic. J. Phys. Oceanogr., 22, 1486-1505.

Marotzke, J., and J. R. Scott, 1999: Convective mixing and the thermohaline circulation. J. Phys. Oceanogr., 29, 2962-2970.

Marshall, J., C. Hill, L. Perelman, and A. Adcroft, 1997: Hydrostatic, quasi-hydrostatic, and nonhydrostatic ocean modelling. J. Geophys. Res., 102, C3, 5733-5752.

Mauritzen, C., and S. Häkkinen, 1999: On the relationship between dense water formation and the "Meridional Overturning Cell" in the North Atlantic Ocean. Deep Sea Res. I, 46, 877-894.

Spall, M. A., and R. S. Pickart, 2001: Where does dense water sink? A subpolar gyre example. J.Phys. Oceanogr., 31, 810-826.

Straneo, F., and R. S. Pickart, 2001: Interannual variability in Labrador Sea water formation and export: how does it correlate to the atmospheric forcing? Extended abstract for the US CLIVAR Meeting 2001 in Boulder, Colorado, USA.