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Eddy C. Carmack

Abstract

Kamloops Lake is a long (25 km), deep (maximum depth, 145 m) intermontane lake in central British Columbia fed at its eastern end by the Thompson River (mean annual flow, 720 m2 s−1). Here I describe spring overturn and the onset of stratification on the basis of three conceptual models distinguishing among river-induced, surface-induced and edge-induced circulations. The lake during winter is characterized by weak reverse stratification; the incoming river waters are less dense than ambient lake water and thus tend to remain at the lake surface. During spring,, the shallow river water warms more rapidly than the deep water of the lake; as inflow water warms toward the temperature of maximum density (4°C), it becomes denser than lake water and thus tends to sink on entry into the lake. Further warning of the inflow water above 4°C decreases its density causing it to again enter the lake as a surface overflow. Although the inflow itself is less dense than lake water, some mixtures of the two will necessarily have temperatures near 4°C, and thus be denser than either parent water mass; this process is called cabbeling (cf. Foster, 1972). The dense mixtures then sink along a narrow frontal zone, filling the lake basin with 4°C water from the bottom upward, while new (unmixed) inflow water is held as an arrested wedge near the point of entry. When the whole lake is warmed above 4°C, the cabbeling instability disappears and the wedge of warm water is released to spread down the lake. Transport of warm water across the lake subsequently forms the spring thermocline. Budget considerations show that although surface heating of the open lake contributes the major portion of the spring heat income, the riverine flow dominates lakewide circulation patterns, and thus determines the distribution of material properties during and subsequent to spring overturn.

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Andreas Münchow and Eddy C. Carmack

Abstract

Analyses of data from three shipborne surveys describe the quasi-synoptic density and velocity fields near Barrow Canyon, Alaska. The canyon parallels the northwestern coast of Alaska and contains three different water masses. These are 1) warm and fresh Alaskan coastal waters that originate from the Bering Strait; 2) cold and moderately salty waters that originate from the Chukchi shelf; and 3) warm and salty waters that originate from the Atlantic layer of the Arctic Ocean. A halocline separates the Chukchi shelf and Atlantic layer waters. The halocline slopes upward into the canyon where it is then twisted to slope across the wide canyon. An intensification of the Beaufort gyre near the shelf break just seaward of Barrow Canyon raises the halocline more than 100 m toward the surface. Locally upwelling favorable winds raise the Arctic halocline, which thus is ventilated within Barrow Canyon adjacent to the coast. In the absence of winds the halocline slopes across-canyon in the thermal wind sense due to a northward flowing coastal current.

Velocity measurements from a towed acoustic Doppler current profiler reveal a northward flowing jet that transports about 0.3 Sv (Sv ≡ 106 kg m−3) of Bering Sea summer water into the Arctic Ocean at speeds that exceed 0.7 m s−1. Total northward transports through the canyon exceed 1.0 Sv. The warm waters of this coastal current supply more than 100 W m−2 of heat to the atmosphere. The jet separates both from the bottom and from the coast. Hence, a laterally and vertically sheared jet forms, which breaks into three branches at about 71.8°N latitude.

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Theodore D. Foster and Eddy C. Carmack

Abstract

The general circulation of water in the Weddell Sea is part of a large cyclonic gyre. A section taken across this gyre from the Scotia Ridge to Cape Norvegia shows that the Warm Deep Water forms an asymmetric lens-like structure with the thickest portion south of the center of the sea. This large-scale feature of the Weddell Sea is evidently due to a divergent Ekman flux driven by the general atmospheric circulation and upwelling in the center of the gyre. Vertical profiles of temperature and salinity in the center of the gyre show small step-like structures in the upper part of the transition from colder, less salty Winter Water to the warmer, saltier Warm Deep Water below and large step-like structures in the tower part of the transition region. Double-diffusive convection can take place in both regions. Circumstantial evidence leads one to believe that the cabbeling instability is effective in the large-step region. Internal waves and shear instabilities may also he mechanisms that contribute to the formation of the step-like structures.

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Igor V. Polyakov, Andrey V. Pnyushkov, Robert Rember, Laurie Padman, Eddy C. Carmack, and Jennifer M. Jackson

Abstract

A 1-yr (2009/10) record of temperature and salinity profiles from Ice-Tethered Profiler (ITP) buoys in the Eurasian Basin (EB) of the Arctic Ocean is used to quantify the flux of heat from the upper pycnocline to the surface mixed layer. The upper pycnocline in the central EB is fed by the upward flux of heat from the intermediate-depth (~150–900 m) Atlantic Water (AW) layer; this flux is estimated to be ~1 W m−2 averaged over one year. Release of heat from the upper pycnocline, through the cold halocline layer to the surface mixed layer is, however, seasonally intensified, occurring more strongly in winter. This seasonal heat loss averages ~3–4 W m−2 between January and April, reducing the rate of winter sea ice formation. This study hypothesizes that the winter heat loss is driven by mixing caused by a combination of brine-driven convection associated with sea ice formation and larger vertical velocity shear below the base of the surface mixed layer (SML), enhanced by atmospheric storms and the seasonal reduction in density difference between the SML and underlying pycnocline.

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Igor V. Polyakov, Andrey V. Pnyushkov, Robert Rember, Vladimir V. Ivanov, Y.-D. Lenn, Laurie Padman, and Eddy C. Carmack

Abstract

A yearlong time series from mooring-based high-resolution profiles of water temperature and salinity from the Laptev Sea slope (2003–04; 2686-m depth; 78°26′N, 125°37′E) shows six remarkably persistent staircase layers in the depth range of ~140–350 m encompassing the upper Atlantic Water (AW) and lower halocline. Despite frequent displacement of isopycnal surfaces by internal waves and eddies and two strong AW warming pulses that passed through the mooring location in February and late August 2004, the layers preserved their properties. Using laboratory-derived flux laws for diffusive convection, the authors estimate the time-averaged diapycnal heat fluxes across the four shallower layers overlying the AW core to be ~8 W m−2. Temporal variability of these fluxes is strong, with standard deviations of ~3–7 W m−2. These fluxes provide a means for effective transfer of AW heat upward over more than a 100-m depth range toward the upper halocline. These findings suggest that double diffusion is an important mechanism influencing the oceanic heat fluxes that help determine the state of Arctic sea ice.

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Till M. Baumann, Igor V. Polyakov, Andrey V. Pnyushkov, Robert Rember, Vladimir V. Ivanov, Matthew B. Alkire, Ilona Goszczko, and Eddy C. Carmack

Abstract

The Eurasian Basin (EB) of the Arctic Ocean is subject to substantial seasonality. We here use data collected between 2013 and 2015 from six moorings across the continental slope in the eastern EB and identify three domains, each with its own unique seasonal cycle: 1) The upper ocean (<100 m), with seasonal temperature and salinity differences of Δθ = 0.16°C and ΔS = 0.17, is chiefly driven by the seasonal sea ice cycle. 2) The upper-slope domain is characterized by the influence of a hydrographic front that spans the water column around the ~750-m isobath. The domain features a strong temperature and moderate salinity seasonality (Δθ = 1.4°C; ΔS = 0.06), which is traceable down to ~600-m depth. Probable cause of this signal is a combination of along-slope advection of signals by the Arctic Circumpolar Boundary Current, local wind-driven upwelling, and a cross-slope shift of the front. 3) The lower-slope domain, located offshore of the front, with seasonality in temperature and salinity mainly confined to the halocline (Δθ = 0.83°C; ΔS = 0.11; ~100–200 m). This seasonal cycle can be explained by a vertical isopycnal displacement (ΔZ ~ 36 m), arguably as a baroclinic response to sea level changes. Available long-term oceanographic records indicate a recent amplification of the seasonal cycle within the halocline layer, possibly associated with the erosion of the halocline. This reduces the halocline’s ability to isolate the ocean surface layer and sea ice from the underlying Atlantic Water heat with direct implications for the evolution of Arctic sea ice cover and climate.

Open access
Igor V. Polyakov, Tom P. Rippeth, Ilker Fer, Matthew B. Alkire, Till M. Baumann, Eddy C. Carmack, Randi Ingvaldsen, Vladimir V. Ivanov, Markus Janout, Sigrid Lind, Laurie Padman, Andrey V. Pnyushkov, and Robert Rember

Abstract

A 15-yr duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (~150–900 m) warm Atlantic Water (AW) to the surface mixed layer and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017–18 showing AW at only 80 m depth, just below the wintertime surface mixed layer, the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3–4 W m−2 in 2007–08 to >10 W m−2 in 2016–18. This seasonal AW heat loss in the eastern EB is equivalent to a more than a twofold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback.

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Igor V. Polyakov, Vladimir A. Alexeev, Igor M. Ashik, Sheldon Bacon, Agnieszka Beszczynska-Möller, Eddy C. Carmack, Igor A. Dmitrenko, Louis Fortier, Jean-Claude Gascard, Edmond Hansen, Jens Hölemann, Vladimir V. Ivanov, Takashi Kikuchi, Sergey Kirillov, Yueng-Djern Lenn, Fiona A. McLaughlin, Jan Piechura, Irina Repina, Leonid A. Timokhov, Waldemar Walczowski, and Rebecca Woodgate

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