Background

Recently there has been an effort to develop simplified dynamical models of the Arctic Ocean and Nordic Seas (Nøst and Isachsen, 2003; Isachsen et al., 2003; Walin et al., 2004; Nilsson et al., 2005). The agreement with these simplified models and the observed flow field is surprisingly good (see Nøst and Isachsen, 2003 and Isachsen et al., 2003), suggesting that they capture the most important physics controlling the large scale circulation. However, these models do not consider the buoyancy budget, but rather uses the observed density to estimate the flow field.

In order to study the mechanisms controlling the transport of different water masses and freshwater we need to consider the main balances in the buoyancy budget. Both modeling and laboratory experiments indicate that buoyancy transport by mesoscale eddies plays a vital role in the buoyancy budget (e.g. Marshall et al., 2002, Spall 2004).  The source/sink of buoyancy is determined by locations of river runoff and atmospheric/sea ice conditions. Due to topographic steering the mean buoyancy advection cannot reach all locations of the buoyancy sources/sinks, and the buoyancy has to be distributed in/out of these areas by mesoscale eddies (Spall, 2004).

WP1, together with previous data collected from the same sites, will especially give valuable information on the production and behaviour of mesoscale eddies in the frontal areas connected to steep topography. We will use this information to improve the above mentioned simplified models, but the new knowledge may also be used to improve parametrisation of subgrid-scale eddy fluxes in coarse resolution climate ocean models. The most common method used now, the Gent and McWilliams (1990) scheme, works good only in regions with flat bottom (see Adcock and Marshall, 2000).

Major outcome of this task

Get a better understanding of the role of mesoscale eddies in the overall buoyancy budget in the Arctic and Subarctic Seas, and use this to get a better understanding of the overall mean circulation.

Proposed activity 3.4.1:  Measure current, temperature and salinity at the West Spitsbergen side of the ASOF section, and especially analyse the data regarding the generation of topographic waves and their role in the cooling of the WSC.

Recent studies have shown that the ASOF mooring section can be used to calculate the heat loss through isopycnal diffusion in the layers below the surface layer (Nilsen et.al., 2006), but that the section is not able to fully capture the water mass exchange between the slope and the West Spitsbergen shelf areas. Hence, In addition to the existing ASOF moorings provided by AWI (Ursula Schauer), a bottom mounted ADCP will be deployed at the shelf break, further inshore of the innermost ASOF mooring. This to better capture the inner border of the topographic waves generated in the area (Nilsen et.al., 2006). The extended mooring section time series will be used to calculate the heat loss of the WSC along West Spitsbergen, and to study other mechanisms for water mass exchanges between the WSC and both the shelf to the east and the deep Greenland Sea to the west.

Proposed activity 3.4.2: Map the distribution of the strength of eddy activity by analysing current meter data.

Current meter data exists from three sites in the NwAC, and one site in the EGC, all of them comprising different density fronts and strength of surface buoyancy fluxes. In this activity we will look for differences in eddy activity at the different sites and mooring positions, and look for consistency with theory.

Proposed activity 3.4.3: Explore processes governing the mixing between Atlantic and Arctic type water masses.

Isopycnal mixing in frontal areas is dominantly governed by mesoscale eddy activity, according to simplified modelling (see e.g. Spall, 2004). A weakness with these models is they don't use salinity as a buoyancy determining variable, only temperature. In this activity we will study instability processes in fronts between warm, saline and cold, fresh watermasses; driving mechanisms and isopycnal/diapycnal mixing.

Proposed activity 3.4.4: Use autonomous glider data from the repeated sections in the Norwegian Sea to estimate eddy overturning.

Eddy induces overturning may be estimated by a method calculating the volume transport within isopycnal layers in a cross section through a geostrophic mean flow  at several snapshots in time, and finding a temporal average of these (McIntosh and McDoughall, 1996). The repeated sections done by the gliders may serve as an excellent opportunity to try to estimate residual mean flow and eddy overturning in the real ocean.

Proposed activity 3.4.5: Development of a simplified model of the mean circulation of the Arctic and Subarctic Seas.

We will use simplified models similar to the ones developed by Nøst and Isachsen (2003) and Isachsen et al., (2003) combined with hydrographic data and data on buoyancy sources/sinks to study the buoyancy budget of the Arctic and Subarctic Seas.