John T. Gunn

The Arctic Ocean Boundary Current along the Eurasian slope and the adjacent Lomonosov Ridge: Water mass properties, transports and transformations from moored instruments

Year-long (summer 1995 to 1996) time series of temperature, salinity and current velocity from three slope sites spanning the junction of the Lomonosov Ridge with the Eurasian continent are used to quantify the water properties, transformations and transport of the boundary current of the Arctic Ocean. The mean flow is cyclonic, weak (1 to 5 cm s−1), predominantly aligned along isobaths and has an equivalent barotropic structure in the vertical. We estimate the transport of the boundary current in the Eurasian Basin to be . About half of this flow is diverted north along the Eurasian Basin side of the Lomonosov Ridge. The warm waters (>1.4°C) of the Atlantic layer are also found on the Canadian Basin side of the ridge south of 86.5°N, but not north of this latitude. This suggests that the Atlantic layer crosses the ridge at various latitudes south of 86.5°N and flows southward along the Canadian Basin side of the ridge. Temperature and salinity records indicate a small (0.02 Sv), episodic flow of Canadian Basin deep water into the Eurasian Basin at , providing a possible source for an anomalous eddy observed in the Amundsen Basin in 1996. There is also a similar flow of Eurasian Basin deep water into the Canadian Basin. Both flows probably pass through a gap in the Lomonosov Ridge at 80.4°N. A cooling and freshening of the Atlantic layer, observed at all three moorings, is attributed to changes (in temperature and salinity and/or volume) in the outflow from the Barents Sea the previous winter, possibly caused by an observed increased flow of ice from the Arctic Ocean into the Barents Sea. The change in water properties, which advects at along the southern edge of the Eurasian Basin, also strengthens the cold halocline layer and increases the stability of the upper ocean. This suggests a feedback in which ice exported from the Arctic Ocean into the Barents Sea promotes ice growth elsewhere in the Arctic Ocean. The strongest currents recorded at the moorings (up to ) are related to eddy features which are predominantly anticyclonic and, with a few exceptions, are of two main types: cold core eddies, confined to the upper 100–300 m, probably formed on the shelf, and warm core eddies of greater vertical extent, probably related to instabilities of an upstream front.

Validation and Error Analysis of OSCAR Sea Surface Currents

Abstract Comparisons of OSCAR satellite-derived sea surface currents with in situ data from moored current meters, drifters, and shipboard current profilers indicate that OSCAR presently provides accurate time means of zonal and meridional currents, and in the near-equatorial region reasonably accurate time variability (correlation = 0.5–0.8) of zonal currents at periods as short as 40 days and meridional wavelengths as short as 8°. At latitudes higher than 10° the zonal current correlation remains respectable, but OSCAR amplitudes diminish unrealistically. Variability of meridional currents is poorly reproduced, with severely diminished amplitudes and reduced correlations relative to those for zonal velocity on the equator. OSCAR’s RMS differences from drifter velocities are very similar to those experienced by the ECCO (Estimating the Circulation and Climate of the Ocean) data-assimilating models, but OSCAR generally provides a larger ocean-correlated signal, which enhances its ratio of estimated signal...

Evolution of the Arctic Ocean boundary current north of the Siberian shelves

Abstract The Arctic Mediterranean Sea is the most important source for the North Atlantic Deep Water, and the Arctic Ocean, often neglected in this respect, may provide a significant amount of the overflow waters crossing the Greenland–Scotland Ridge. Warm water from the south enters the Arctic Ocean through two main passages, Fram Strait and the Barents Sea, and the inward flowing boundary current that overlies the Eurasian continental slope of the Arctic Ocean supplies heat to the Arctic Ocean and exerts a dominant influence over its internal temperature and salinity characteristics. Major transformations of the inflow occur in the Barents Sea and as the two inflow branches meet in the boundary current north of the Kara Sea their characteristics are different. Lateral mixing between the two branches dominates the further transformations of the Atlantic and intermediate layers occurring in the Eurasian Basin. Ice formation, brine rejection and dense water formation on the shelves and subsequent convection down the slope lead to transformation of the boundary current that crosses the Lomonosov Ridge, and determine the properties of the Canadian Basin water column. Changes in the inflow characteristics of the boundary current will gradually, but slowly, affect also the intermediate and deep-water characteristics of the water column in the interior of the Canadian Basin. In the Eurasian Basin the influences of the shelf processes and pure slope convection are smaller and the water mass characteristics are mostly determined by advection and mixing of the two inflows. Only in the deepest part of the water column does slope convection appear to dominate the water mass transformations.

An Arctic Ocean cold core eddy

Detailed physical and chemical observations were obtained during September 1997 of a cold core eddy situated in the southern central Canada Basin of the Arctic Ocean. The eddy was about 20 km in diameter, had maximum current speeds exceeding 20 cm s 21 , and extended from the base of the upper mixed layer, near 40 m, down to ;400 m. Excess salt in the eddy core was consistent with addition of brine from ; 1mo f ice formation. Core tracer distributions indicated a lifetime exceeding 1 year and were consistent with an origin as near-surface Pacific water with some admixed terrestrial runoff. The eddy was probably formed in association with a polynya along the Alaskan Chukchi Sea coast through local water densification from surface ice formation followed by development of frontal instabilities. Formation and subsequent migration of such eddies, which may have lifetimes of several years, provide a mechanism for transporting water from the Beaufort and Chukchi Sea coastal and shelf regions into the interior Canada Basin and for contributing to maintenance of the permanent halocline. Present light ice conditions throughout the Arctic Ocean favor the formation of such eddies and may contribute to enhanced ventilation of the halocline waters.

Observed changes in Arctic Ocean temperature structure over the past half decade

Ocean temperature data obtained from the central Arctic Ocean during 1995-1999 show interannual changes in the temperature of the warm Atlantic Water core. Widespread warming continued from 1995 until 1998 but had ceased during 1998-1999 and was replaced by a slight cooling. The timing of these changes differed regionally, as the warming had occurred prior to 1995 in the Nansen and Amundsen basins, and between 1995 to 1998 in the Makarov Basin. The regional phase differences are consistent with advection along mid-ocean ridges from the Eurasian margin slope current, which provides the source for warm Atlantic Water. Anomalous behaviors were associated with the Arctic Mid-Ocean and Lomonosov ridges and probably reflected the presence of overlying topographically trapped circulations.

The Weddell-Scotia marginal ice zone: Physical oceanographic conditions, geographical and seasonal variability

Abstract Physical oceanographic conditions were measured in the Weddell and Scotia Sea marginal ice zones (MIZs) during 1983, 1986 and 1988. The field work encompassed spring, autumn and mid-winter periods and included retreating, advancing and steady-state ice edges. Observed upper ocean structures, which typify MIZs and reflect input of low salinity water from melting ice, included low salinity upper layers, lenses and fronts. An upper mixed layer was always present and was generally more fully developed in autumn and winter than at other times of year. Conditions in the deeper waters reflected regional oceanographic processes and significant differences were present between the Weddell and Scotia seas. Weddell Sea Water is a major source of water for the southern Scotia Sea, however, the upper Scotia Sea was dominated by warmer, less saline waters from Drake Passage. The colder, denser Weddell water appeared to have mixed isopycnally with deeper water in the Scotia Sea, present there at depths exceeding 500 m. The Scotia Sea was dominated by strong gradients and energetic mesoscale features, with currents exceeding 50 cm/s. The northwestern Weddell Sea had, in contrast, current speeds well below about 5 cm/s and small to negligible lateral water property gradients. Our observations suggest that the Weddell western boundary current was weaker than has been estimated in the past. In addition, we found scant evidence of deep winter convection in the Scotia Sea, a process which has been hypothesized in the past to contribute to deep water formation. No evidence was found during winter 1988 in the Scotia Sea of the modified water known as Weddell-Scotia Confluence water.