Ragnheid Skogseth

Internal Waves and Mixing in the Marginal Ice Zone near the Yermak Plateau

Observations were made of oceanic currents, hydrography, and microstructure in the southern Yermak Plateau in summer 2007. The location is in the marginal ice zone at the Arctic Front northwest of Svalbard, where the West Spitsbergen Current (WSC) carries the warm Atlantic Water into the Arctic Ocean. Time series of approximately 1-day duration from five stations (upper 520 m) and of an 8-day duration from a mooring are analyzed to describe the characteristics of internal waves and turbulent mixing. The spectral composition of the internal-wave field over the southern Yermak Plateau is 0.1‐0.3 times the midlatitude levels and compares with the most energetic levels in the central Arctic. Dissipation rate and eddy diffusivity below the pycnocline increase from the noise level on the cold side of the front by one order of magnitude on the warm side, where 100-m-thick layers with average diffusivities of 5 3 10 25 m 2 s 21 lead to heat loss from the Atlantic Water of 2‐4 W m 22 . Dissipation in the upper 150 m is well above the noise level at all stations, but strong stratification at the cold side of the front prohibits mixing across the pycnocline. Close to the shelf, at the core of the Svalbard branch of the WSC, diffusivity increases by another factor of 3‐6. Here, nearbottom mixing removes 15 W m 22 of heat from the Atlantic layer. Internal-wave activity and mixing show variability related to topography and hydrography; thus, the path of the WSC will affect the cooling and freshening of the Atlantic inflow. When generalized to the Arctic Ocean, diapycnal mixing away from abyssal plains can be significant for the heat budget. Around the Yermak Plateau, it is of sufficient magnitude to influence heat anomaly pulses entering the Arctic Ocean; however, diapycnal mixing alone is unlikely to be significant for regional cooling of the WSC.

Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from density overturns

Observations were made of the dense overflow from Storfjorden from a survey conducted at closely spaced stations in August 2002. The field data set consists of conventional CTD (conductivity, temperature, depth) profiles and short-term moored current meters and thermistor strings. Finestructure estimates were made by calculating Thorpe scales over identified overturns using 0.1-dbar verticallyaveraged density profiles. Dissipation rate of turbulent kinetic energy per unit mass, e, is estimated assuming proportionality between Thorpe and Ozmidov length scales. Vertical eddy diffusivity, Kz, is estimated using Osborn’s model assuming a constant mixing efficiency. Survey averaged profiles suggest enhanced mixing near the bottom with values of Kz and e, when averaged within the overflow, equal to 10x10 -4 m 2 s -1 and 3x10 -8 W kg -1 , respectively. Kz is found to decrease with increasing buoyancy frequency as N -1.2 (±0.3) , albeit values of N covered only 0.5–8 cph (1 cph = 2π/3600 s -1 ). Values of heat flux obtained using Kz suggest that the plume gains a considerable amount of heat, 45 ± 25 W m -2 when averaged over the thickness of the plume, from overlying waters of Atlantic origin. This value is lower than, but considering the errors in estimates of Kz comparable with 100 W m -2 , the rate of change of heat in the overflow derived from sections across the sill and 80 km downstream.

Supercooled water in an Arctic polynya: observations and modeling

In situ field measurements of an active polynya in Storfjorden, Svalbard, during April 2006 are presented. A surface heat flux, estimated to be 400 W m −2 , produced frazil ice that was advected away from the fast ice edge during the end of a polynya event driven by cold winds from the northeast. Conductivity, temperature and depth casts from the fast ice edge of the polynya were calibrated by accompanying water samples, and reveal a supercooling event that lasted for 3 days in a 5 m deep water column. Surface salinity reached 35.9 psu from brine release during ice growth. The maximum supercooling measured was 0.037 ± 0.005 ◦ C below the in situ freezing point near the surface and 0.016 ± 0.005 ◦ C at the bottom; the mean supercooling gradient was 0.020 ± 0.005 ◦ C between the surface and the bottom. These measurements are consistent with results from a one-dimensional frazil ice model, confirming that such supercooling levels can be expected. Frazil ice concentrations in the water were modeled to be lower than 0.02 g L −1 , due to advection in the surface layer. Seven frazil/grease ice samples taken from a place where advection was blocked along the fast ice edge showed a mean salinity of 26.2 psu, indicating 25% frazil ice and 75% sea water in the grease ice. The water-column salinity decreased during the measurement period due to less saline water replacing newly formed brine- enriched shelf water flowing down to deeper parts of Storfjorden. The supercooling ceased when the wind direction turned to the east, with higher air temperatures and warmer and less saline water being pushed into Storfjorden by the northward Ekman transport. These are the first in situ observations from an active Arctic polynya with concurrent sampling of hydrography and frazil ice, and the supercooling is the maximum observed in recent years with modern and accurate instrumentation.

A Simple Shelf Circulation Model: Intrusion of Atlantic Water on the West Spitsbergen Shelf

AbstractBarotropic flow along depth contours is found in accordance with standard geostrophic theory. A numerical model is developed that studies the deviation from such a flow. The model gives a good approximation of the dynamical processes on the West Spitsbergen Shelf (WSS) and shows that the West Spitsbergen Current (WSC), the main gateway of Atlantic water (AW) toward the Arctic, connects more easily to the Isfjorden Trough than anywhere else along the shelf. The circulation of AW in the troughs along the WSS is here named the Spitsbergen Trough Current (STC). From hydrographical and ocean current observations it is evident that the STC is primarily barotropic and driven by the sea surface height. A connection between the along-coast wind stress and the STC is established, and it is demonstrated how the increased occurrence of winter cyclones in Fram Strait during January–February accelerates and widens the WSC. Ultimately, this results in a strengthened STC and dominance of AW on the WSS. The STC re...

Circulation and exchange in a broad Arctic fjord using glider-based observations

ABSTRACT In recent years, Svalbard fjords have experienced a substantial reduction in winter sea-ice extent. This has been linked to changes in wind stress patterns over Fram Strait and an increased transport of warm Atlantic Water into the fjords. In November 2014, we deployed two Slocum gliders to Isfjorden and measured the hydrographical properties and depth-averaged currents in the region. The campaign marked the first time gliders have been used inside an Arctic fjord. We observed geostrophically balanced flow patterns both in the mouth, where the heat flux into the fjord was calculated to be 0.13 TW, and in the interior of Isfjorden, where geostrophic flows were up to 20 cm s−1. After a change in the prevailing wind direction on the West Spitsbergen Shelf, we found evidence for a wind-driven geostrophic control mechanism at the fjord mouth, impeding fjord–shelf exchange, and found that the geostrophic circulation inside the fjord had broken down. We conclude that the circulation patterns in Isfjorden are heavily influenced by rotational effects and by wind activity both locally and on the West Spitsbergen Shelf, and that geostrophically balanced exchange flows may deliver Atlantic Water to the fjord interior given the correct conditions at the fjord mouth. The combination of hydrography and high-resolution velocity data from throughout the Isfjorden region provided new insights into the circulation here, suggesting that this approach will be useful for studying high-latitude fjords in the future.