Ole Anders Nøst

The large-scale time-mean ocean circulation in the Nordic Seas and Arctic Ocean estimated from simplified dynamics

A simplified diagnostic model of the time-mean, large-scale ocean circulation in the Nordic Seas and Arctic Ocean is presented. Divergences in the surface Ekman layer are extracted from observed climatological wind stress fields. Similarly, divergences caused by the meridional thermal wind transport (relative to the bottom) are calculated from an observed climatological density field. These known quantities are then used to force the models bottom geostrophic velocities. Both scaling arguments and direct observations show that for long time scales the bottom currents are closely aligned with contours of flH, (where f is the Coriolis parameter and H is the depth of the seabed). Due to the weak planetary vorticity gradient at high latitudes, the f/H field is dominated by topography and is characterized by multiple regions of closed isolines. The only frictional effect included in the model is bottom stress. By then integrating the depth-integrated vorticity equation over the area spanned by a closed f/H contour, and assuming that the same contour is a streamline of the bottom geostrophic flow, we derive an analytical expression for the bottom geostrophic velocity on this f/H contour. For the few contours that are not closed, current measurements are used as boundary conditions. Model results are compared with near-bottom current measurements in both the Nordic Seas and the Arctic Ocean. In addition comparison is made with observations from surface drifters in the Nordic Seas by adding the observed thermal wind shear to the modeled bottom flow. The agreement is surprisingly good, suggesting that the simple model is capturing some of the most important processes responsible for the large-scale circulation field. Features like the subgyre recirculations in the Nordic Seas, the gyres in the Canadian and Eurasian Basins, the East Greenland Current, the Norwegian Atlantic Current and the Arctic Circumpolar Boundary Current are all well reproduced by the model. The simplicity of the model makes it well suited as a dynamical framework for interpreting the large-scale circulation pattern in the Nordic Seas and Arctic Ocean.

Salinity and temperature structure of a freezing Arctic fjord: monitored by white whales (Delphinapterus leucas)

Received 10 May 2002; accepted 29 August 2002; published 11 December 2002. [1] In this study we report results from satellite-linked conductivity-temperature-depth (CTD) loggers that were deployed on wild, free-ranging white whales to study the oceanographic structure of an Arctic fjord, Storfjorden, Svalbard. The whales dove to the bottom of the fjord routinely during the study and occupied areas with up to 90% ice-cover, where performance of conventional ship-based CTD-castswouldhavebeendifficult.Duringtheinitialperiod of freezing in the fjord, over a period of approximately 2 weeks, 540 CTD profiles were successfully transmitted. The dataindicatethatStorfjordenhasasubstantialinflowofwarm NorthAtlanticWater;thisiscontrarytoconventionalwisdom thathassuggestedthatitcontainsonlycoldArcticwater.This study confirms that marine-mammal-based CTDs have enormous potential for cost-effective, future oceanographic studies; many different marine mammal species target oceanographic discontinuities for foraging and thus may be good ‘adaptive samplers’ that naturally seek areas of high oceanographic interest. INDEX TERMS: 4294 Oceanography: General:Instrumentsandtechniques;4536Oceanography:Physical: Hydrography; 4219 Oceanography: General: Continental shelf processes; 1635 Global Change: Oceans (4203); KEYWORDS: CTD-measurements, Arctic oceanography, marine mammals, satellite telemetry. Citation: Lydersen, C., O. A. Nost, P. Lovell, B. J. McConnell, T. Gammelsrod, C. Hunter, M. A. Fedak, and K. M. Kovacs, Salinity and temperature structure of a freezing Arctic fjord—monitored by white whales (Delphinapterus leucas), Geophys. Res. Lett. , 29(23), 2119, doi:10.1029/2002GL015462, 2002.

Estimates of the Southern Ocean general circulation improved by animal‐borne instruments

Over the last decade, several hundred seals have been equipped with conductivity-temperature-depth sensors in the Southern Ocean for both biological and physical oceanographic studies. A calibrated collection of seal-derived hydrographic data is now available, consisting of more than 165,000 profiles. The value of these hydrographic data within the existing Southern Ocean observing system is demonstrated herein by conducting two state estimation experiments, differing only in the use or not of seal data to constrain the system. Including seal-derived data substantially modifies the estimated surface mixed-layer properties and circulation patterns within and south of the Antarctic Circumpolar Current. Agreement with independent satellite observations of sea ice concentration is improved, especially along the East Antarctic shelf. Instrumented animals efficiently reduce a critical observational gap, and their contribution to monitoring polar climate variability will continue to grow as data accuracy and spatial coverage increase.

Eddy overturning of the Antarctic Slope Front controls glacial melting in the Eastern Weddell Sea

[1] The Eastern Weddell Sea is characterized by narrow continental shelves and Warm Deep Water (WDW) is located in close proximity to the ice shelves in this region. The exchange of WDW across the Antarctic Slope Front (ASF) determines the rate of basal ice shelf melting. Here, we present a unique data set consisting of 2351 vertical profiles of temperature and salinity collected by southern elephant seals (Mirounga leonina) and a profile beneath the Fimbul Ice Shelf (FIS), obtained via drilling through 395 m of ice. This data set reveals variations in salinity and temperature through winter, and using a conceptual model of the coastal salt budget we quantify the main exchange processes. Our data show that modified WDW, with temperatures below −1.5°C, is advected onto the shelf and into the ice shelf cavities by an eddy overturning of the ASF. The onshore Ekman flux of surface waters during summer is the main source of freshwater that leads to the formation of low salinity shelf waters in the region. The modified WDW that reaches beneath the ice shelves is too cold for basal ice shelf melting to create such low salinity water. A high‐resolution model of an idealized ASF–continental shelf–ice shelf system supports the conclusions from the data analysis. The inflow of WDW onto the continental shelf and into the ice shelf cavity occurs within a bottom boundary layer where the eddy advection in the model is particularly strong, in close agreement with the observed vertical profile of temperature beneath the FIS. Citation: Nost, O. A., M. Biuw, V. Tverberg, C. Lydersen, T. Hattermann, Q. Zhou, L. H. Smedsrud, and K. M. Kovacs (2011), Eddy overturning of the Antarctic Slope Front controls glacial melting in the Eastern Weddell Sea, J. Geophys. Res., 116, C11014, doi:10.1029/2011JC006965.

A model of ice shelf-ocean interaction with application to the Filcher-Ronne and Ross Ice Shelves

A simple analytical model has been developed to study the formation of Ice Shelf Water (ISW). ISW is assumed to flow as a buoyancy-driven layer underneath the ice shelf. A relation between potential temperature and salinity in the ISW layer is calculated from the mass and energy balance. This temperature-salinity relation is shown to depend only on the temperature and the salinity of the source water mass and to be practically independent of entrainment and melt rates. The model results are obtained without making any assumptions about entrainment and melt rates. The model is in good agreement with observations under the Ronne Ice Shelf, and it indicates that ISW in the Filchner Depression is formed from Western Shelf Water (WSW) with salinity higher than 34.75 practical salinity units. Such high-salinity water is only observed in the Ronne Depression in the western part of the continental shelf. This implies a circulation of WSW, under the Filchner-Ronne Ice Shelf, from the Ronne Depression into the Filchner Depression. Similarly, the model shows that the ISW observed under J9 at the Ross Ice Shelf has been formed from Low Salinity Shelf Water (LSSW) from the eastern parts of the Ross Sea continental shelf. LSSW must therefore circulate under the eastern parts of the Ross Ice Shelf.

Effects of Hydrographic Variability on the Spatial, Seasonal and Diel Diving Patterns of Southern Elephant Seals in the Eastern Weddell Sea

Weddell Sea hydrography and circulation is driven by influx of Circumpolar Deep Water (CDW) from the Antarctic Circumpolar Current (ACC) at its eastern margin. Entrainment and upwelling of this high-nutrient, oxygen-depleted water mass within the Weddell Gyre also supports the mesopelagic ecosystem within the gyre and the rich benthic community along the Antarctic shelf. We used Conductivity-Temperature-Depth Satellite Relay Data Loggers (CTD-SRDLs) to examine the importance of hydrographic variability, ice cover and season on the movements and diving behavior of southern elephant seals in the eastern Weddell Sea region during their overwinter feeding trips from Bouvetøya. We developed a model describing diving depth as a function of local time of day to account for diel variation in diving behavior. Seals feeding in pelagic ice-free waters during the summer months displayed clear diel variation, with daytime dives reaching 500-1500 m and night-time targeting of the subsurface temperature and salinity maxima characteristic of CDW around 150–300 meters. This pattern was especially clear in the Weddell Cold and Warm Regimes within the gyre, occurred in the ACC, but was absent at the Dronning Maud Land shelf region where seals fed benthically. Diel variation was almost absent in pelagic feeding areas covered by winter sea ice, where seals targeted deep layers around 500–700 meters. Thus, elephant seals appear to switch between feeding strategies when moving between oceanic regimes or in response to seasonal environmental conditions. While they are on the shelf, they exploit the locally-rich benthic ecosystem, while diel patterns in pelagic waters in summer are probably a response to strong vertical migration patterns within the copepod-based pelagic food web. Behavioral flexibility that permits such switching between different feeding strategies may have important consequences regarding the potential for southern elephant seals to adapt to variability or systematic changes in their environment resulting from climate change.

Complex network of channels beneath an Antarctic ice shelf

Ice shelves play an important role in stabilizing the interior grounded ice of the large ice sheets. The thinning of major ice shelves observed in recent years, possibly in connection to warmer ocean waters coming into contact with the ice-shelf base, has focused attention on the ice-ocean interface. Here we reveal a complex network of sub ice-shelf channels under the Fimbul Ice Shelf, Antarctica, mapped using ground-penetrating radar over a 100 km2 grid. The channels are 300–500 m wide and 50 m high, among the narrowest of any reported. Observing narrow channels beneath an ice shelf that is mainly surrounded by cold ocean waters, with temperatures close to the surface freezing point, shows that channelized basal melting is not restricted to rapidly melting ice shelves, indicating that spatial melt patterns around Antarctica are likely to vary on scales that are not yet incorporated in ice-ocean models.

North Atlantic Water in the Barents Sea Opening, 1997 to 1999

North Atlantic Water (NAW) is an important source of heat and salt to the Nordic seas and the Arctic Ocean. To measure the transport and variability of one branch of NAW entering the Arctic, a transect across the entrance to the Barents Sea was occupied 13 times between July 1997 and November 1999, and hydrography and currents were measured. There is large variability between the cruises, but the mean currents and the hydrography show that the main inflow takes place in Bjørnøyrenna, with a transport of 1.6 Sv of NAW into the Barents Sea. Combining the flow field with measurements of temperature and salinity, this results in mean heat and salt transports by NAW into the Barents Sea of 3.9 × 1013 W and 5.7 × 107 kg s-1, respectively. The NAW core increased in temperature and salinity by 0.7 °C yr-1 and 0.04 yr-1, respectively, over the observation period. Variations in the transports of heat and salt are, however, dominated by the flow field, which did not exhibit a significant change.

Wind‐driven spreading of fresh surface water beneath ice shelves in the Eastern Weddell Sea

Solar heated, fresh Antarctic Surface Water (ASW) is a permanent feature along the Eastern Weddell Sea (EWS) coast in summer down to a depth of roughly 200 m. Recently, ASW has been observed beneath the Fimbul Ice Shelf, suggesting that it might play an important role in basal melting. We propose that wind-driven coastal downwelling is the main mechanism that spreads ASW beneath the ice shelf in this sector of Antarctica. We validate this hypothesis with observations, scaling analyses, and numerical modeling, along three principle lines: (i) data analyses of about 1500 salinity profiles collected by instrumented seals indicate that the observed freshening of the coastal water column is likely explained by the on-shore Ekman transport and subsequent downwelling of ASW; (ii) an analytical model of the coastal momentum balance indicates that wind-driven downwelling is capable of depressing the buoyant surface water to a depth similar to the ice shelf draft; and (iii) simulations from both idealized and regional eddy-resolving numerical ice shelf/ocean models support our proposition. Our main conclusion is that wind-driven spreading of ASW beneath the ice shelf occurs when downwelling exceeds the depth of the ice shelf base. Furthermore, our study adds to the understanding of the oceanic processes at the Antarctic Slope Front in the EWS, with possible implications for other sectors of Antarctica.

Winter sea ice melting in the Atlantic Water subduction area, Svalbard Norway

Herein, we study a small area along the shelf west of Spitsbergen, near Prins Karls Forland, where warm, saline Atlantic Water of the West Spitsbergen Current currently first encounters sea ice. This sea ice is drifting in a coastal current that carries Arctic Water originating from the Barents Sea northward over the shelf. Our aim was to investigate whether melting of sea ice by Atlantic Water in this area might be a significant factor that could contribute to the formation of a cold halocline layer that isolates the sea ice from further melting from below. Observations of temperature and salinity profiles were collected during two winters, via CTD-SRDL instruments deployed on harbor seals (Phoca vitulina), and fed into a heat and freshwater budget box model in order to quantify the importance of melting relative to other processes that could transform the shelf water mass during winter. Cross-frontal exchange of Atlantic Water from the West Spitsbergen Current, driven by buoyancy forcing rather than Ekman upwelling, was determined to be the source of the heat that melted drift ice on the shelf. Some local sea ice formation did take place, but its importance in the total heat and freshwater budgets appeared to be minor. The data suggest that the production of a cold halocline layer was preceded by southerly winds and rapid drift ice melting.