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.
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.
(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
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,
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.