On the Role of Open Ocean and Slope-Convection in the Arctic Ocean and Nordic Seas
Jochen Kaempf
Institut fuer Meereskunde, Universitaet Hamburg,
Troplowitzstr. 7, D-22529 Hamburg, Germany
Tel: 49-40-4123-5791
Fax: 49-40-4123-4644
kaempf@akrz.ce
co-authors: Jan Backhaus and Hermann Fohrmann
Slope-convection, or cascading, of dense shelf bottom water and its role in the formation of water masses in the basins of the Arctic Ocean determines the final properties of its deep and intermediate waters. Slope-convection is initiated by convection in the shelf water column and particularly efficient in areas with high ice production (i.e. polynyas). The localised convective formation of dense shelf waters leads to isolated, vertically homogeneous water bodies which will collapse and finally move towards the shelf edge as a bottom-arrested gravity current. Instabilities and mixing during collapse andre-stratification as well as entrainment occuring during the progress to the continental slope may modify the water bodies already on the shelf. On the slope plumes accelerate which accounts for enhanced entrainment. They act as transport vehicles which transfer entrained waters towards intermediate and deep waters of the Arctic Ocean.
The potential role of sediments in water mass formation has as yet not been considered in much detail although substantial amounts of recent sediments of either glacial or fluvial origin, or from melting ice are available in the Arctic Ocean an adjacent seas which are affected by Arctic outflow (e.g. East Greenland Current). Slope convection, enhanced by suspended sediments, therefore, appears as an issue worth further investigation.
All processes mentioned above were simulated with a series of individual proces-oriented models which were specially designed to match the respective physical constraints and processes. A non-hydrostatic convection model, coupled to an ice-model, was applied to a typical shelf polynya situation to simulate convective production of dense shelf waters. It predict the temporal evolution of water mass formation under extreme atmospheric forcing. Reduced-gravity 2.5-dimensional plume models were applied to simulate the export of shelf waters towards the slope and their decent on the slope. Process studies with a sediment-plume model in which typical polar water masses were prescribed show that sediments which are initially suspended in a gravity plume, or later due to erosion on its descent, may account for a larger negative buoyancy in comparison to classical water mass plumes. Their dynamics may become largely ageostrophic allowing for a very rapid descent, a deeper penetration and a different entrainment rate of water masses. Observed localised areas of high accumulation of recent sediments at the base of the continental slope and intermediate and bottom nepheloid (high attenuation) layers in the Norwegian Sea are considered as evidence of these sediment plumes. Nepheloid layers were also observed in the vicinity of continental slopes in the Arctic Ocean.
Once a sediment-plume has reached its equilibrium density horizon reduced turbulence levels allow for a settling of sediments. The plume water masses, void of the sediment-load, may then become lighter than ambient water masses and may hence initiate upward directed convection in the water column which originates from the intrusion level. This was already observed in the tropics and later confirmed by laboratory experiments. Model experiments were conducted to estimate the extent of vertical upward mixing of water masses caused by this interesting facet of convection.
