Ocean motion on the Yermak Plateau : - tidal and air-ocean interactions

This study focuses on the tidal and atmospheric dynamics controlling the overflow of warm Atlantic Water, crossing over the Yermak Plateau, which can be seen as a doorstep to the Arctic Ocean. The Arctic conditions are changing due to the general global warming, and in order to make good predictions of the future climate north of Svalbard and further into the Arctic Ocean, a good understating of the dynamics controlling the overflow is essential. The Yermak Plateau is known for enhanced diurnal tides caused by topographically trapped waves (TTW). A numerical shelf model has been set up for the southwestern side of the plateau to investigate when the TTW near the diurnal frequency become resonant with zero group velocity, meaning that the diurnal energy does not radiate out of the region, but will accumulate along the slope. The model inputs are slope steepness, background current and stratification, and the result indicates that the group velocity of the TTW near the diurnal frequency becomes zero when the background current is strong, i.e. during winter. Four moorings have measured ocean currents and ocean bottom pressure (OBP) on top of the plateau and the data revealed significant monthly and fortnight tidal periods during winter. The low-frequency Lunar Monthly, Mm, and the fortnightly, MSf, are astronomically forced, but their potentials are weak, especially the potential of MSf. Therefore, we suggest that the observed enhancement of Mm and MSf on top of the plateau during winter, is caused by an energy contribution from the diurnal tides. The superposition of Mm and MSf have been termed the Nonlinear Yermak Tidal Overflow (NYTO), and reached a maximum speed of 15 cm s$^{-1}$ in February 2016. From December to May, the mean volume transport was 1.1 Sv by the NYTO alone. The four moorings located on top of the plateau have been set up to target the Svalbard Branch (SB) and the Spitsbergen Polar Current (SPC). After evaluating the tidal effect on the Atlantic Water flow across the plateau, these ocean data were coupled with high resolution atmospheric hindcast data to get a deeper understanding of the air-ocean dynamics. The volume transports in the SB and the SPC calculated from the OBP data were correlated with the wind stress curl over Svalbard. As a negative wind stress curl is linked to surface water convergence, southerly alongshore winds stress in the Fram Strait will generate onshore Ekman transport resulting in a convergence zone with a negative wind stress curl on the shelf. This effect steepens the sea surface tilt over the slope, accelerating the ocean current along the slope. In the opposite case, a northerly wind stress generates a westward Ekman transport, weakening the sea surface tilt and decreasing the current speed. Satellite altimetry measurements from 1994 to 2018 were included to investigate the interannual and decadal variations of the oceanic flow passing over the plateau. To validate the performance of satellite altimetry in the SB north of Svalbard, the calculated current fluctuation from satellite altimetry were compared with the current fluctuation derived from the OBP data. Satellite altimetry was found to be useful in calculating the volume transports in winter when the water column has constant density. The winter volume transports from 1994 to 2018 were calculated in three sections, the barotropic West Spitsbergen Current (WSC core), the SB, and the Yermak Pass Branch (YPB) with mean values of 1.4 Sv, 1.1 Sv, and 1.4 Sv, respectively. The flow in the WSC core and the SB correlated with each other throughout the winter season and with the southerly wind stress on the West Spitsbergen Shelf. The flow in the YPB correlated with the northerly wind stress on the West Spitsbergen Shelf and were anti-correlated with the WSC core and the SB from March to May.Doktorgradsavhandlin

Wind-Driven Variability in the Spitsbergen Polar Current and the Svalbard Branch Across the Yermak Plateau

The Yermak Plateau (YP) acts as a guidance or barrier for the West Spitsbergen Current (WSC), which either crosses the plateau or flows around it to enter the Arctic Ocean. Closer to the West Spitsbergen coast, the Spitsbergen Polar Current (SPC) also flows over the YP in a narrow passage between the Svalbard Branch (SB) and the coast. A 2-year ocean observing program combined with altimetry and re-analysis wind data has given new knowledge on the variability and dynamics of the SPC and SB. The variability in the SPC and SB is controlled by the sea surface pressure gradient driven by the wind stress along the West Spitsbergen coast and locally on the YP. A peak-to-peak volume transport variability of 0.8 Sv and a positive heat transport anomaly of 3 TW were found in the SPC. The variability in the SB is mainly controlled by the upstream wind stress curl field along the West Spitsbergen Shelf where the negative wind stress curl field force the barotropic WSC branch directly into the SB. The peak-to-peak variability in the SB can exceed 4 Sv and in January 2016, an episodic heat flux was estimated to be 10 TW. Hence, an increasing number of winter cyclones affecting Svalbard will increase the volume transport variability and pulses of warm water to the shelf areas north of Svalbard.publishedVersio

Ocean motion on the Yermak Plateau : - tidal and air-ocean interactions

This study focuses on the tidal and atmospheric dynamics controlling the overflow of warm Atlantic Water, crossing over the Yermak Plateau, which can be seen as a doorstep to the Arctic Ocean. The Arctic conditions are changing due to the general global warming, and in order to make good predictions of the future climate north of Svalbard and further into the Arctic Ocean, a good understating of the dynamics controlling the overflow is essential. The Yermak Plateau is known for enhanced diurnal tides caused by topographically trapped waves (TTW). A numerical shelf model has been set up for the southwestern side of the plateau to investigate when the TTW near the diurnal frequency become resonant with zero group velocity, meaning that the diurnal energy does not radiate out of the region, but will accumulate along the slope. The model inputs are slope steepness, background current and stratification, and the result indicates that the group velocity of the TTW near the diurnal frequency becomes zero when the background current is strong, i.e. during winter. Four moorings have measured ocean currents and ocean bottom pressure (OBP) on top of the plateau and the data revealed significant monthly and fortnight tidal periods during winter. The low-frequency Lunar Monthly, Mm, and the fortnightly, MSf, are astronomically forced, but their potentials are weak, especially the potential of MSf. Therefore, we suggest that the observed enhancement of Mm and MSf on top of the plateau during winter, is caused by an energy contribution from the diurnal tides. The superposition of Mm and MSf have been termed the Nonlinear Yermak Tidal Overflow (NYTO), and reached a maximum speed of 15 cm s$^{-1}$ in February 2016. From December to May, the mean volume transport was 1.1 Sv by the NYTO alone. The four moorings located on top of the plateau have been set up to target the Svalbard Branch (SB) and the Spitsbergen Polar Current (SPC). After evaluating the tidal effect on the Atlantic Water flow across the plateau, these ocean data were coupled with high resolution atmospheric hindcast data to get a deeper understanding of the air-ocean dynamics. The volume transports in the SB and the SPC calculated from the OBP data were correlated with the wind stress curl over Svalbard. As a negative wind stress curl is linked to surface water convergence, southerly alongshore winds stress in the Fram Strait will generate onshore Ekman transport resulting in a convergence zone with a negative wind stress curl on the shelf. This effect steepens the sea surface tilt over the slope, accelerating the ocean current along the slope. In the opposite case, a northerly wind stress generates a westward Ekman transport, weakening the sea surface tilt and decreasing the current speed. Satellite altimetry measurements from 1994 to 2018 were included to investigate the interannual and decadal variations of the oceanic flow passing over the plateau. To validate the performance of satellite altimetry in the SB north of Svalbard, the calculated current fluctuation from satellite altimetry were compared with the current fluctuation derived from the OBP data. Satellite altimetry was found to be useful in calculating the volume transports in winter when the water column has constant density. The winter volume transports from 1994 to 2018 were calculated in three sections, the barotropic West Spitsbergen Current (WSC core), the SB, and the Yermak Pass Branch (YPB) with mean values of 1.4 Sv, 1.1 Sv, and 1.4 Sv, respectively. The flow in the WSC core and the SB correlated with each other throughout the winter season and with the southerly wind stress on the West Spitsbergen Shelf. The flow in the YPB correlated with the northerly wind stress on the West Spitsbergen Shelf and were anti-correlated with the WSC core and the SB from March to May

Wind-Driven Variability in the Spitsbergen Polar Current and the Svalbard Branch Across the Yermak Plateau

The Yermak Plateau (YP) acts as a guidance or barrier for the West Spitsbergen Current (WSC), which either crosses the plateau or flows around it to enter the Arctic Ocean. Closer to the West Spitsbergen coast, the Spitsbergen Polar Current (SPC) also flows over the YP in a narrow passage between the Svalbard Branch (SB) and the coast. A 2-year ocean observing program combined with altimetry and re-analysis wind data has given new knowledge on the variability and dynamics of the SPC and SB. The variability in the SPC and SB is controlled by the sea surface pressure gradient driven by the wind stress along the West Spitsbergen coast and locally on the YP. A peak-to-peak volume transport variability of 0.8 Sv and a positive heat transport anomaly of 3 TW were found in the SPC. The variability in the SB is mainly controlled by the upstream wind stress curl field along the West Spitsbergen Shelf where the negative wind stress curl field force the barotropic WSC branch directly into the SB. The peak-to-peak variability in the SB can exceed 4 Sv and in January 2016, an episodic heat flux was estimated to be 10 TW. Hence, an increasing number of winter cyclones affecting Svalbard will increase the volume transport variability and pulses of warm water to the shelf areas north of Svalbard

Present and past flow regime on contourite drifts west of Spitsbergen: preliminary results from Eurofleet 2 PREPARED cruise (June 2014).

Eurofleets-2 PREPARED cruise was conducted during June 5–15, 2014 on board the Norwegian R/V G.O. Sars to investigate the present and past oceanographic flow regime and patterns around two contourite drifts located in the eastern side of the Fram Strait (south-western margin of Spitsbergen). To achieve the main objective of the project, a full range of time scaled measurements was planned, from instantaneous (CTD) and seasonal (moorings) oceanographic measurements, to the recent (Box corer) and geologic (Calypso core) past record. The successful cruise recovered about 2780 km of underway measurements (hull-mounted ADCP and thermosalinograph); 60 CTD sites along 5 main transects; 22 sites for water sampling at different depths for biogeochemical characterization of water masses; 13 meso-zooplankton samplings carried out by vertical hauls (WP2 net) and 20 by horizontal hauls (Manta net) for the study of the present biological productivity of the area; about 120 km of site survey including high-resolution multibeam map and sub-bottom profiles for the identification of current related structures; 5 Box cores; and 2 Calypso piston cores 19.67 and 17.37 m long with an excellent sediment recovery up to 92%. In addition, 3 moorings were deployed for seasonal measurements of water currents direction and velocity, water mass temperature and salinity and to determine the annual amount of local sediment input. Preliminary onboard analyses outlined the presence of a cold-oxygenated and low salinity water mass moving in the deep northern part of the Storfjorden Trough under the effect of the Corilis force and tide configuration considerably affecting the velocity and bottom distribution of the cold water mass. The long Calypso cores contain the record of the past 20 ka with an exceptionally expanded Holocene sequence (over 5 m-thick) that will allow us to obtain very-high resolution palaeoceanographic and palaeoenvironmental reconstructions in the area