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

Abstract

The diffusive layering (DL) form of double-diffusive convection cools the Atlantic Water (AW) as it circulates around the Arctic Ocean. Large DL steps, with heights of homogeneous layers often greater than 10 m, have been found above the AW core in the Eurasian Basin (EB) of the eastern Arctic. Within these DL staircases, heat and salt fluxes are determined by the mechanisms for vertical transport through the high-gradient regions (HGRs) between the homogeneous layers. These HGRs can be thick (up to 5 m and more) and are frequently complex, being composed of multiple small steps or continuous stratification. Microstructure data collected in the EB in 2007 and 2008 are used to estimate heat fluxes through large steps in three ways: using the measured dissipation rate in the large homogeneous layers; utilizing empirical flux laws based on the density ratio and temperature step across HGRs after scaling to account for the presence of multiple small DL interfaces within each HGR; and averaging estimates of heat fluxes computed separately for individual small interfaces (as laminar conductive fluxes), small convective layers (via dissipation rates within small DL layers), and turbulent patches (using dissipation rate and buoyancy) within each HGR. Diapycnal heat fluxes through HGRs evaluated by each method agree with each other and range from ~2 to ~8 W m−2, with an average flux of ~3–4 W m−2. These large fluxes confirm a critical role for the DL instability in cooling and thickening the AW layer as it circulates around the eastern Arctic Ocean.

Open access
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.

Open access