The Contrasting Dynamics of the Buoyancy-Forced Lofoten and Greenland Basins

Abstract

The Nordic seas are commonly described as a single basin to investigate their dynamics and sensitivity to environmental changes when using a theoretical framework. Here, we introduce a conceptual model for a two-basin marginal sea that better represents the Nordic seas geometry. In our conceptual model, the marginal sea is characterized by both a cyclonic boundary current and a front current as a result of different hydrographic properties east and west of the midocean ridge. The theory is compared to idealized model simulations and shows good agreement over a wide range of parameter settings, indicating that the physics in the two-basin marginal sea is well captured by the conceptual model. The balances between the atmospheric buoyancy forcing and the lateral eddy heat fluxes from the boundary current and the front current differ between the Lofoten and the Greenland Basins, since the Lofoten Basin is more strongly eddy dominated. Results show that this asymmetric sensitivity leads to opposing responses depending on the strength of the atmospheric buoyancy forcing. Additionally, the front current plays an essential role for the heat and volume budget of the two basins, by providing an additional pathway for heat toward the interior of both basins via lateral eddy heat fluxes. The variability of the temperature difference between east and west influences the strength of the different flow branches through the marginal sea and provides a dynamical explanation for the observed correlation between the front current and the slope current of the Norwegian Atlantic Current in the Nordic seas.

Pathways and Water Mass Transformation Along and Across the Mohn‐Knipovich Ridge in the Nordic Seas

“…On longer time scales, it is known that the strength of the front current depends on the phase of the North Atlantic Oscillation (NAO); the front is weaker during the negative phase of the NAO (Raj et al., 2019). Also, under future scenarios, the front is expected to weaken, due to a reduction in the density gradient between the Lofoten Basin and the Greenland Basin (Ypma et al., 2020). If, due to the decrease in baroclinic instability, the exchange between the east and west reduces, the AW heat loss in the eastern basins could decrease and convection in the Greenland Basin could weaken due to a smaller supply of salty waters from the east.…”

“…Details on the model configuration, as well as on the hydrodynamic properties of the circulation, can be found in Spall (2011) and Ypma et al. (2020).…”

“…Furthermore, the importance of the frontal instability for AW transformation is investigated using this model. Although idealized, the model captures the dynamics of the Nordic Seas pertinent for this study (Ypma et al., 2020). The model has a 5 km horizontal resolution and 33 vertical levels ranging from 20 m thickness at the surface to 200 m thickness near the bottom.…”

“…The area north of the island represents the Lofoten and Greenland Basins. There, the ocean surface temperature is restored to an atmospheric temperature of 4°C with a timescale of 2 months to resemble a realistic surface heat flux (Ypma et al., 2020). Furthermore, temperature and velocity are restored over the full water column south of the island (see black box Figure 4a); no other surface forcing is applied.…”