Robin D. Muench

Particulate matter and nutrient distributions in the ice-edge zone of the Weddell Sea: relationship to hydrography during late summer

Previous estimates of the marginal ice zones quantitative contribution to biogeochemical cycles and annual productivity in the Southern Ocean may be conservative because of assumptions that phytoplankton blooms are associated only with actively retreating ice edges. Observations during March 1986, near an almost stationary ice edge in the northwestern Weddell Sea, revealed very low geostrophic currents, no appreciable horizontal gradients in temperature or salinity and no significant net melting or freezing in the ice-edge region. Vertical stratification within the upper 50 m was evident throughout the study area, and resulted primarily from prior melting of pack ice. In contrast with previous observations in marginal ice zones, the distribution of phytoplankton biomass showed little correlation with the meltwater field; here, significant horizontal biomass gradients occurred in an area where vertical stability was almost uniform laterally and both elevated biomass and diminished nutrient levels extended well below the pycnocline. Absolute levels of chlorophyll were modest (generally 10, 1.5 and 3.5 μmol l−1, respectively) were similar to those found previously during a spring phytoplankton bloom at the Weddell-Scotia Sea ice edge. The mean mole ratio of biogenic silica to organic carbon within the particle assemblage was 0.44, which is very high for surface seawater and about three times higher than that typically found in pure diatom cultures. Therefore, despite relatively low chlorophyll levels, the ice edge remained a localized maximum in biogenic particulate matter at least through March and this material was unusually rich in silica. Combined, these physical and biological data show that elevated phytoplankton biomass in the ice-edge zone can persist well past the time when net melting stops and the ice edge becomes stationary.

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.

Wind-driven transport pathways for Eurasian Arctic river discharge

Distributions of temperature, salinity, and barium in near-surface waters (depth ≤ 50 m) of the Laptev Sea and adjacent areas of the Arctic Ocean are presented for the summers of 1993, 1995, and 1996. The tracer data indicate that while fluvial discharge was largely confined to the shelf region of the Laptev Sea in the summer of 1993, surface waters containing a significant fluvial component extended beyond the shelf break and over the slope and basin areas north of the Laptev Sea in the summers of 1995 and 1996. These distributions of fluvial discharge are consistent with local winds and suggest two principal pathways by which river waters can enter the central Arctic basins from the Laptev Sea. When southerly to southeasterly wind conditions prevail, river waters are transported northward beyond the shelf break and over the slope and adjacent basin areas. These waters can then enter the interior Arctic Ocean via upper layer flow in the vicinity of the Lomonosov Ridge. Under other wind conditions, river waters are steered primarily along the inner Laptev shelf and into the East Siberian Sea as part of the predominantly eastward coastal current system. These waters then appear to cross the shelf and enter the interior Arctic Ocean via upper layer flow aligned roughly along the Mendeleyev Ridge. The extent to which either pathway is favored in a given year is largely determined by local wind patterns during the summer months, when fluvial discharge is greatest and shelf waters are at the lowest salinity of their annual cycle.

Winter oceanographic conditions in the Fram Strait‐Yermak Plateau region

Temperature, salinity and currents were measured in the Yermak Plateau and northern Fram Strait region during March–April 1989. An isothermal upper layer was near the freezing point everywhere except seaward of the ice edge near western Svalbard. Temperatures in the underlying Atlantic Water layer decreased in general toward the northwest from maximum values in the West Spitsbergen Current west of Svalbard. Mesoscale features were superimposed on this trend, and the most apparent of these was a middepth (200–300 m) warm filament overlying the northwestern flank of the plateau. Currents in the warm filament were northward, at 2–4 cm/s, for a 10-day period in April, providing a possible advective explanation for presence of the filament. The mechanism for these currents may involve topographic control over a western extension of the north flowing West Spitsbergen Current or, alternately, rectification of diurnal currents which are enhanced by the bottom topography of the plateau. Our data favor the tidal rectification mechanism. A warm core, anticyclonic eddy associated with the filament contained Atlantic Water which was virtually unmodified from that occurring more than 200 km farther south. Characteristics of this eddy were consistent with an origin in the West Spitsbergen Current. In the upper layer, a generally cyclonic baroclinic circulation was present about the plateau. Measured upper layer current speeds were greatest (∼8 cm/s) in the southward flowing East Greenland Current, where they exceeded the baroclinic currents by approximately 1 cm/s, which may represent a southward barotropic component. Farther to the northeast, both measured and baroclinic upper layer currents were weak and variable.

Internal waves and tides in the western Weddell Sea: Observations from Ice Station Weddell

The upper ocean current and temperature fields in the western Weddell Sea were measured from the drifting pack ice at Ice Station Weddell 1 (ISW) and nearby sites using a vertical profiler and an array of moored sensors in January–June 1992. These data document the structure and variability of the internal gravity wave field and tidal currents in this remote region. The variance of the internal wave continuum (ƒ < frequency < N) at ISW was 0.2–0.6 of the Garrett-Munk (GM) universal level for the first 60 days, increasing to near GM levels during the final 10 days of the deployment. In contrast, the energy density at site C, 50 km west of ISW and farther up the continental slope, was always near GM levels. Variations may be due to a combination of spatial and temporal gradients of the internal wave field. At ISW, coherence between vertically separated sensors was used to estimate vertical wave number bandwidth. Energy and bandwidth estimates are compared with previous studies in both ice-covered and temperate oceans. Using our measurements of the internal wave field and existing parameterizations of mixing, we estimate the vertical heat flux from the Warm Deep Water toward the surface. At ISW the upward heat flux due to mixing associated with the internal waves was about 1 W m−2, much less than the 20 W m−2 average flux required to balance the heat budget for the Weddell Gyre. Tidal currents contributed significantly to the total measured horizontal velocity variance. The tides were primarily barotropic and increased toward the west in both the semidiurnal and diurnal frequency bands. It is suggested that the stronger tidal currents to the west, over the shallower water of the upper continental slope, are indirectly responsible for the higher internal wave energy at site C relative to ISW.

Mixing and vertical heat flux estimates in the Arctic Eurasian Basin

Abstract This paper presents results of an analysis into the magnitude and distribution of vertical heat flux from the warm Atlantic Water core into the upper ocean over the southern Eurasian Basin of the Arctic Ocean as determined by Acoustic Doppler Current Profiles (ADCP) and Conductivity, Temperature, and Depth (CTD) casts made in 1993 and 1995. Vertical diffusivity parameterizations based on shear and buoyancy frequency squared are utilized, including one that assumes that turbulent dissipation rates associated with internal wave variance dissipation can be estimated from vertical shear and buoyancy frequency profiles [Gregg, M.C., 1989. Scaling turbulent diffusion in the thermocline. J. Geophys. Res., 94, pp. 9686–9698; DAsaro, E.A., Morison, J.H., 1992. Internal waves and mixing in the Arctic Ocean. Deep-Sea Res., 39 (Suppl. 2), pp. S459–S484], and another based on an inverse Richardson number parameterization [Pacanowsky, R.C., Philander, S.G.H., 1981. Parameterization of vertical mixing in numerical models of tropical oceans. J. Phys. Oceanogr., 11, pp. 1443–1451]. Due to the scarcity of measurements in the Arctic, these data provide a preliminary indication as to the magnitude and spatial distribution of vertical heat flux from the warm core, below the halocline, into the upper ocean. The data suggest that vertical heat flux peaks over the continental slope region of the Eurasian Basin. It is possible that this diffusion is associated with elevated tidal forcing over the shelf break and slope regions, although our ship board ADCP records are too short to resolve tidal currents directly.

On some possible interactions between internal waves and sea ice in the marginal ice zone

The ice edges of the world ocean are generally the site, at times when winds are blowing off the ice, of regularly spaced surface bands of ice floes. These bands have size scales of the order of 1–10 km, and their long axes are oriented approximately normal to the wind direction. Available oceanic temperature and salinity data from the Bering and Greenland Sea ice edge regions suggest that these ice bands are commonly underlain by a two-layered density structure which is maintained by net melting along the ice edges. Linear internal wave theory is applied to these data to compute first-mode interfacial wave phase speeds. A simple analytical model is developed that demonstrates the feasibility of generation of such interfacial internal waves by the stress discontinuity due to off-ice winds blowing over either a stationary or a moving ice edge. It is qualitatively shown that the computed internal wave phase speeds and wavelengths are, under many conditions, compatible with the speeds and spacings of the surface ice bands. This compatibility suggests, in turn, that coupling between internal waves and ice bands may commonly occur. Some possible implications of the generation and presence of these internal waves upon other ice edge processes, such as air-sea heat and momentum transfer, are qualitatively discussed.

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.

Convection beneath freezing leads: New observations compared with numerical model results

Vertical distributions of temperature, salinity, and horizontal and vertical current speed were measured along the edges of a number of rapidly freezing leads in the southern central Beaufort Sea during March-April 1992. These observations were restricted to cases having sufficiently small ice water relative speeds that brine-driven convection was expected, based on scaling arguments, to dominate shear turbulence generated by the relative ice water motion. The observed salinity and current features were consistent with an existing conceptual model for sublead convection consequent to brine rejection from ice formation, and with the results of a numerical model which assumes two-dimensionality in the cross-lead direction, and uses a brine input consistent with the field observations. These models predict convection of brine-enriched plumes beneath the lead downward to the pycnocline, where they then spread outward away from the lead. Volume continuity is satisfied by horizontal inflow of water to the lead at the surface. Observed mean downward convection speeds varied from 0.2 to 0.7 cm s−1 and had maxima up to about 2 cm s−1, consistent with the numerical model. Salinities associated with the convecting parcels were about 0.001 practical salinity units (psu) above ambient, which was an order of magnitude less than predicted by the model, while plumes of water flowing away from the lead on the pycnocline had salinities as high as 0.004 psu above ambient. These similarities and discrepancies are discussed within the context of a steady state model versus a nonsteady field situation, and it is concluded that the steady “convection cell” concept is probably valid if viewed as a time-averaged sequence of discrete convective plumes or “thermals.”

Introduction: Third Marginal Ice Zone research collection

The decade of the 1980s might well have been designated “The Decade for Marginal Ice Zone (MIZ) Research.” These highly energetic regions where air, ice, and water intermingle and interact thermodynamically and dynamically have undergone an unprecedented amount of study during this past decade. Relevant major programs have included the Office of Naval Research-sponsored Marginal Ice Zone Experiment (MIZEX) West and East experiments, the Coordinated Eastern Arctic Experiment (CEAREX) in the Arctic, and the National Science Foundation-sponsored Antarctic Marine Ecosystem Research in the Ice Edge Zone (AMERIEZ) program in the Antarctic. There have been a host of smaller experiments as well.