Water mass exchange, pathways and the mesoscale eddy field in the Lofoten Basin of the Norwegian Sea

Abstract

The Lofoten Basin situated in the Norwegian Sea, plays a central role in redistributing and modifying the warm Atlantic Water carried poleward with the Norwegian Atlantic Current. Increased residence time of the warm Atlantic Water in this region, leads to a large cooling and the largest surface heat losses in the Nordic Seas.

This thesis studies the exchange of Atlantic Water with the Lofoten Basin using observations and numerical models, and Lagrangian and Eulerian approaches. A key focus is the study of the mass and heat exchange with the basin outlined by the 3000-m isobath. Surface drifters are analyzed to study the surface circulation in the Nordic Seas and to estimate the water mass exchange with the Lofoten Basin. Fields from Eulerian models and trajectories from Lagrangian simulations at multiple levels are further used to study the processes leading to the exchange, by delineating the mean and eddy component of the flows. Analyses aimed to quantify the mesoscale eddy properties, their interaction with the ambient, heat and vorticity budgets, and to assess the importance of eddies relative to the ambient flow and other submesoscale processes in the mass and heat exchange with the Lofoten Basin. The geographical origins of the water masses having largest interaction with the basin are identified, and these sites are studied in detail to investigate the processes behind the exchange. The thesis also investigates the fate of water masses in the basin to study how their properties evolve with time, and compare this with other regions.

The first main finding, obtained from surface drifter observations, indicates an increased exchange of Atlantic Water across the southern sector of the Lofoten Basin. The drifters show a meandering motion between the eastern and western branches of the Norwegian Atlantic Current towards the basin, and Eulerian simulations suggest that the inflow is primarily related to a mean component of the flow. The warm waters experience long residence times and large temperature losses in the basin. In contrast with earlier literature, there is less evidence of near-surface exchange with the waters carried by the slope current along the continental slope off Norway. However, the net heat transport into the basin is dominated by eddy fluxes. Furthermore, the divergence of eddy heat fluxes obtained from Eulerian calculations on the continental slope is large, and particularly enhanced at depths of about 400 m. It is therefore suggested that the flow from the south dominates the near-surface exchange of Atlantic Water with the basin, but eddy fluxes from the slope region are important at deeper levels.

Lagrangian simulations of particles deployed at several depths reveal variations in the vertical structure of the inflows to the Lofoten Basin. Of the water parcels that are cooled most (more than 1^oC) while in the basin, those at the surface mainly enter from the south, and those at deeper levels (about 500 m) come from the slope. The inflows also have a seasonal variability. In winter, cooling and vertical mixing result in weak stratification and distribute the particles vertically, while strong stratification in summer limits their vertical excursions from their deployment depths. During winter, water masses close to the surface therefore tend to sink and give weaker inflows (less particles) close to the surface and stronger inflows (more particles) at deeper levels (100-300 m).

The eddy activity in the basin and on the continental slope is quantified. Eddy signals extracted from Lagrangian trajectories, using multivariate wavelet ridge analysis, show that water masses in coherent vortices experience larger changes in their water properties (such as temperature and density) than water masses in the ambient flow, with enhanced warming in cyclones and enhanced cooling in anticyclones. There is also evidence of upwelling in the cyclones and downwelling in the anticyclones. The change of water properties and net vertical displacement is most pronounced in the Lofoten Basin. The anticyclones have a longer lifetime, more circular shape and larger radius than the cyclones. However, the eddies only cover a small portion of the Nordic Seas (about 6%) and the ambient flow and filaments around eddies therefore play an important role in balancing the Lofoten Basin heat and vorticity budgets. Ridge analysis confirms the role of eddy activity at deeper levels on the slope, and further reveals that the anticyclonic eddies generated on the slope bring warm water into the basin. Energetics and energy-conversion rates calculated from mooring observations from the upper slope, supported by volume-averaged calculations from an Eulerian model, are consistent with the Lagrangian and Eulerian analyses. Estimated baroclinic conversion rates imply that potential energy is extracted from the mean flow to eddies. The role of filaments in the upper layers, the link between the generation of eddies on the slope and their exchange with the LB at deeper layers, and contribution of these eddies and filaments to the Lofoten Basin heat and vorticity budgets merit further studies.