E. P. Jones

Formation and evolution of the surface mixed layer and halocline of the Arctic Ocean

Fresh water from summer ice melt and the total freshwater content of the Arctic Ocean water column above the thermocline are estimated from vertical profiles of temperature and salinity observed on the I/B Oden 1991 cruise. The seasonal ice melt ranges from 0.5 m to slightly above 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater content is calculated relative to the salinity just above the thermocline north of the Barents Sea. The freshwater content increases toward the interior of the Arctic Ocean, showing that fresh water is advected from other regions into the observation area. Regions of different freshwater content are separated by fronts over the Nansen-Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between waters derived from the Eurasian and Canadian Basins. Denser water, homogenized north of the Barents Sea, is recognized by a temperature minimum layer. The absence of the temperature minimum near the Nansen-Gakkel Ridge indicates that heat is transferred from the Atlantic Layer over a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic Layer, i.e., toward the east, and returns above the Nansen-Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water formed north of the Barents Sea becomes covered by low-salinity shelf water. The increased freshwater content limits the winter convection, so it no longer reaches the thermocline and an intermediate halocline is formed. The halocline in the Eurasian Basin consists of water originating from winter convection in the Arctic Ocean north of the Barents Sea, which then circulates around the basin. Such a formation mechanism also explains the observed distribution of low NO water. The strong density increase limits vertical exchange, and the vertical diffusion coefficient in the halocline is small (∼1 × 10−6 m2 s−1). The increased temperature of the halocline shows that the heat lost upward by the Atlantic Layer, mainly by double-diffusive convection, is trapped below the mixed layer.

Waters of the Makarov and Canada basins

Abstract Hydrographic measurements from the 1994 Arctic Ocean Section show how the Makarov and Canada basins of the Arctic Ocean are related, and demonstrate their oceanographic connections to the Eurasian Basin. The inflow into the Makarov Basin consists largely of well-ventilated water within a broad band of densities from a boundary flow over the Siberian end of the Lomonosov Ridge. The boundary flow contains a significant component of dense shelf water likely originating in the Barents, Kara, and Laptev Seas. Earlier ice camp data show that the Canada Basin is relatively more isolated from this ventilation source. In the Canada Basin shelf sources influenced by Bering Sea water appear to add cold waters with high silicate concentrations to the halocline and deeper. In 1994 the halocline silicate maximum over the central Makarov Basin was absent, evidence of the recent displacement of the upper (S∼ 33.1) halocline water from the Chukchi-East Siberian Sea region by water from the Eurasian Basin. Much of the Makarov Basin water in and below the halocline is in fact from the Eurasian Basin, with admixture of waters from the Canada Basin suggested by their higher silicate concentrations. Mid-depth eddies may transport anomalous properties into the central Arctic and create property gradients or fronts in mid-depth and deep waters. The complex topography of the Mendeleyev Ridge-Chukchi Plateau region also may assist spreading of water from the boundary into the interior. Atlantic layer characteristics in 1994 differed from previous general depictions. In particular the core temperatures at the Chukchi-Mendeleyev boundary were at least 0.2°C warmer on average than indicated in earlier work. The recent warming at intermediate depth has resulted from inflow of Atlantic waters that have been cooled relatively little during their transit of the Norwegian Sea.

Water masses and circulation in the Eurasian basin. Results from the Oden-91 expedition

The Oden 91 North Pole expedition obtained Oceanographic measurements on four sections in the Nansen and Amundsen basins of the Eurasian Basin and in the Makarov Basin of the Canadian Basin, thereby proving the feasibility of carrying out a typical Oceanographic program using an icebreaker in the Arctic Ocean. The data show greater spatial variability in water structure and circulation than was apparent from previous data. The results show that a clear front exists between the Eurasian and Canadian basins such that upper halocline water in the Canadian Basin is almost absent from the Eurasian Basin. The lower halocline water produced in the Barents-Kara Sea region permeates much of the Eurasian Basin and flows along the continental slope into the Canadian Basin. The deeper circulation is strongly influenced by topography. Three return flows of the Atlantic layer are identified, one over the Nansen-Gakkel Ridge, one over the Lomonosov Ridge, and a third flowing from the Canadian Basin. The slight differences observed in salinity and temperature characteristics of the deeper waters of the Nansen and Amundsen basins do not lead to an obvious explanation of their origin or flow pattern.

Deep waters of the Arctic Ocean: origins and circulation

The Oden 91 Expedition has provided a data set that makes it possible to deduce more detailed ideas regarding the origin and circulation of waters in the Arctic Ocean. The three possible sources for the deep water of the Arctic Ocean are: (1) density flows down the continental slope triggered by brine enhanced waters formed on the continental shelves but consisting primarily of waters entrained from the Atlantic and intermediate layers; (2) inflow of Atlantic Water over the Barents Sea shelf that has experienced a density increase by cooling and freezing in that sea and then sinks with little entrainment down the St Anna Trough into deep layers of the Arctic Ocean; and (3) the inflow of Norwegian Sea Deep Water through Fram Strait. Of these three sources, the first appears to contribute the most to the Arctic Ocean deep water and the third the least. The Eurasian Basin communicates with the Canadian Basin through a boundary current that enters the Canadian Basin north of Siberia and leaves it north of Greenland. The fact that both the temperature and salinity are higher in the Canadian Basin than in the Eurasian Basin at levels above as well as below the sill depth of the Lomonosov Ridge indicates that slope convection is active in the Canadian Basin. The deepest layers have constant salinity, but show a weak temperature decrease towards the bottom. This suggests that these layers of the Canadian Basin are not primarily renewed by convection down the continental slope but by a spill over of Eurasian Basin Deep Water across the central part of the Lomonosov Ridge. A model which incorporates density flows triggered by high salinity shelf water and water overflowing the Lomonosov Ridge flom the Eurasian Basin is applied to reproduce the observed profiles of the Canadian Basin and to establish the relative importance of these two sources. By incorporation of I4C profiles into the model, estimates of the exchange rates of water in these layers can be made.

The first oceanographic section across the Nansen Basin in the Arctic Ocean

Abstract The first quasi-synoptic oceanographic section across a major deep basin of the Arctic Ocean reveals three different regimes: a narrow boundary current system along the northern Barents Shelf slope, a wide interior basin regime and a northern boundary curret regime with several distinct cores along the Nansen-Gakkel Ridge at 86°N. The southern boundary current cores are marked by high oxygen concentrations, high salinities and low temperatures that indicate sources on the shelf and in Fram Strait. The northern boundary current regime contains water mass signatures that are thought to come from the Amundsen Basin as well as from Fram Strait. The Nansen Basin interior is only slowly ventilated from the boundary currents and shelves, the deep water having an age of several decades. At 83°N characteristics change abruptly from those representative of the southern half of the section to those typical for the northern half and deep Arctic basins. Waters of the southern half of the basin, which have a strong melt-water component in their surface layers, largely originate directly in Fram Strait River runoff from the Siberian rivers and sea ice from the Laptev Sea contributes to the northern part of our section.

Ventilation of the Arctic Ocean estimated by a plume entrainment model constrained by CFCs

Intermediate and deep water formation rates in the Arctic Ocean are estimated using a plume entrainment model based on shelf-slope processes and constrained by tracer distributions within the deep basin. Each plume is initiated by a fraction, rj, leaving the shelf break at 200 m, followed by an entrainment of rj for every 150 m depth the plume descends. The model is tuned by varying rj to achieve the transient tracer (CFC-12 and carbon tetrachloride) distribution as measured in the Nansen, Amundsen and Makarov Basins during the Oden 1991 expedition, and the concentrations in the source waters are calculated assuming a water in 100% equilibrium with the atmosphere. The formation of water entering below 500 m is computed to be 1.5 and 1.9 Sv when constrained by CFC-12 and CCl4, respectively, with a total uncertainty of ±0.45 Sv. Sensitivities of the model settings to the entrainment rate, degree of saturation of the transient tracer in the source waters, and age of the Atlantic Layer water are investigated. Processes in the Arctic Ocean contribute around 1/3 of the approximately 5.6 Sv that flows over the Scotland-Greenland Ridge, with the rest likely attributed processes in the Greenland and Iceland Seas. We thus conclude that the Arctic Ocean has to be included in the discussion of the sensitivity of the Greenland-Scotland overflow to a climate change.

U.S., Canadian researchers explore Arctic Ocean

During July–September 1994, two Canadian and U.S. ice breakers crossed the Arctic Ocean (Figure 1) to investigate the biological, chemical, and physical systems that define the role of the Arctic in global change. The results are changing our perceptions of the Arctic Ocean as a static environment with low biological productivity to a dynamic and productive system. The experiment was called the Arctic Ocean Section (AOS) and the ships were the Canadian Coast Guard ship Louis S. St.-Laurent and the U.S. Coast Guard cutter Polar Sea.

Oxygen utilization rates in the Nansen Basin, Arctic Ocean: implications for new production

Abstract In situ consumption of oxygen is balanced by ventilation if the observed distribution of dissolved oxygen below the euphotic zone is in steady state. Apparent oxygen utilization rates (AOURs) can be estimated from the observed oxygen distribution if the waters of the upper layers can be dated. It has been shown previously that tritium/ 3 He ages can be used, together with observed oxygen concentrations, to estimate AOURs for waters with ages of several months to several decades. This method is applied to data obtained from the Nansen Basin, Arctic Ocean, during the 1987 cruise of F.S. Polarstern . New production is estimated by depth integration of AOURs calculated for several isopycnals to be 19±5 g C m −2 year −1 for the southern part and 3±2 g C m −2 year −1 for the northern part of the Nansen Basin section. The results are discussed and compared with previous estimates based on different methods.