Observations using ocean gliders and Argo profiling floats in the Lofoten Vortex in the Nordic Seas
The Lofoten Basin is the largest heat and salt reservoir of the Nordic Seas and plays a particularly important role in water mass transformations and fisheries. The region is dynamic with ocean eddies, as well as a permanent vortex. The Lofoten vortex is centered at a location where the SWOT tracks cross over. We will deploy two ocean gliders and several profiling Argo floats to support SWOT in the fast-sampling phase. Our observations will contribute to the validation of the SWOT surface topography retrievals. The subsurface observations supplemented by the sea surface topography offered by wide-swath measurements of SWOT will help capture the dynamical processes central to water mass transformation and subduction in the region, and the coupled physical and biological processes.
The Nordic Seas (Greenland, Iceland and Norwegian Seas) are a key transit region for the Atlantic Meridional Overturning Circulation. Warm, saline Atlantic Water (AW) flows over the Greenland-Scotland Ridge into the Nordic Seas, while cold, dense water returns to the south through gaps in the ridge. The inflowing AW is progressively cooled and densified as it moves poleward. The Lofoten Basin is the largest heat and salt reservoir of the Nordic Seas and plays a particularly important role in the transformation of AW. Mesoscale eddy activity is intense and a permanent vortex in the Lofoten Basin is a key component of the dynamics. While the mesoscale contributes considerably to the heat and vorticity budgets, the submesoscale plays a major role in closing the budgets.
Lofoten Vortex is centered at a location where Surface Water and Ocean Topography (SWOT) tracks cross over. Motivated by this opportunity provided through the fast-sampling phase and the impeccable horizontal and temporal resolution of the sea surface height variability, we will deploy two ocean gliders and several profiling Argo floats during the fast-sampling phase of the SWOT.
Data from SWOT will provide unprecedented wide-swath (140 km) and high-resolution (2 km) observation of the sea surface elevation, especially during the 3 months daily fast-sampling phase. Orbits cover the Lofoten Basin, Mohn Ridge and Greenland Sea (daily) and cross-overs (doubles sampling) exist over the LBE and the Greenland Sea.
We will take advantage of the existing infrastructures and ongoing programmes in Bergen to supplement SWOT in the fast-sampling phase. The ocean glider programme of NorEMSO operates sections in the Nordic Seas (Fig 1). The NorEMSO infrastructure led by Fer is funded for instruments, operations and collecting observations. Gliders are suitable platforms for high-quality observations of submesoscale processes. NorArgo2 operates today more than 40 Argo floats in the Nordic Seas and 15 of these are located in the Lofoten Basin. The NorArgo2 infrastructure led by Mork is funded for instruments, operations and collecting observations.
We will coordinate and deploy already funded and planned instruments in the LBE, to sample for an extended period. Ocean glider missions and the NorArgo2 programme in the Nordic Seas will also continue. One glider will be deployed in mid-January 2023 in the LBE and will sample there at least for two months before moving toward the Mohn Ridge. Another glider (rechargeable, 100-day endurance) will be deployed in the LBE in late February 2023 and will be maintained in the region until the end of 2023. Presently there are two profiling Argo floats in the LBE (one standard float at the rim of the eddy and another with biogeochemical sensors in the core). Particularly the float in the core is trapped in the vortex and will collect data through the fast- sampling phase of the SWOT. New deployments are scheduled for May 2023. The subsurface observations supplemented by the SWOT surface observations will help capture the dynamical processes central to water mass transformation and subduction in the region. Our observations will contribute to the validation of the SWOT surface topography retrievals.
The significant increase in regular access to multi-sensor satellite and in-situ observations in combination allow for dedicated studies of the three-dimensional thermodynamic and biological properties of the mesoscale eddies in the Lofoten Basin. NERSC is leading an initiative focused on the role of eddies in the phytoplankton bloom in the Lofoten Basin, the S23Eddy ESA-PRODEX funded project. The study builds on the synergy between satellite-based multi-sensor high-resolution microwave, infrared and optical data collocated with Argo profiling floats to characterize the 3D eddy structure. Automated eddy-detection methods are used to obtain a 2D representation of the mesoscale field from conventional altimetry. The eddy population is then collocated at the surface with high-resolution remote-sensing retrievals (from Sentinels, MODIS and Radarsat-2) missions, and with Argo profiling floats measuring the physical and biological parameters of the water column. Vertical structures of ocean biological parameters from remote-sensing maps are reconstructed, and allow for disclosing the covariance of eddy-driven anomalies from the ocean surface to eddy-trapping depth. We aim to extend this method with new SWOT data to link the sea surface topography offered by wide-swath measurements with the 3D structure of meso- to-submesoscale features of the ocean dynamics affecting the coupled physical and biological processes.
The Norwegian Meteorological Institute delivers daily operational ocean forecasts for this region using the ocean model ROMS coupled to the CICE sea ice model. It has a lateral resolution of 2.5 km, and high resolution (2.5 km) atmospheric forcing is applied hourly. The model is run with 24 ensemble members, and sea ice concentration, sea surface temperature, and in situ temperature and salinity are assimilated every 24 hours. Ongoing efforts on including assimilation of satellite altimetry in this system will greatly benefit from the collocated surface and subsurface observations and allow for exploration of potential synergy between in situ hydrography profiles and sea surface height on improving the description of the Lofoten Vortex in the model.
Location of the Norwegian Sea, Lofoten Basin campaign. a) Bathymetry of the Nordic Seas, warm AW circulation (red) and the cold East Greenland Current outflow (blue). Glider missions coordinated under the NorEMSO infrastructure (black transects, 1 to 3), and the location of the Lofoten Vortex (Lofoten Basin Eddy, LBE) are marked b) Available glider data (mission tracks) from ProVoLo (color), and recent NorEMSO missions (black).
Principal investigators: A.D. Brakstad, F. Elliott, I. Fer (University of Bergen), K.A. Mork, H. Søiland (Institute of Marine Research), A.K. Sperrevik (Norwegian Meteorological Institute), A. Bonaduce, R.P. Raj, J.A. Johannessen (Nansen Environmental and Remote Sensing Center, NERSC)
Contact point for the study site: Ilker Fer (ilker.fer@uib.no)