Extracting Tidal Variability of Sea Ice Concentration from AMSR-E Passive Microwave Single-Swath Data: A Case Study of the Ross Sea

The periodic divergence of stress applied by ocean tidal currents to sea ice affects the time-averaged ice concentration (Cice) and heat and freshwater fluxes at the ocean surface. We demonstrate that, at sufficiently high latitudes, tidal variability in Cice can be extracted from single-swath data from the Advanced Microwave Scanning Radiometer–EOS (AMSR-E) satellite passive microwave sensor, although time intervals between swaths are irregular. For the northwest Ross Sea where tidal currents are large, tidal divergence is the dominant cause of Cice variability in winter, with a range of ±0.2 about a mean of ~0.8. Daily-averaged Cice values vary from \u3e0.9 at neap tides to ~0.7 at spring tides. Variability at the fundamental tidal periods is about half that expected from an inverse barotropic tide model for the Ross Sea, suggesting that the measured tidal signal in Cice may be used to diagnose sea ice mechanical properties and ice/ocean coupling

Analysis of Ice Plains of Filchner/Ronne Ice Shelf Using ICESat Data

We use repeat-track laser altimeter data from the Ice, Cloud, and land Elevation Satellite (ICESat) to map the grounding zone of Filchner/Ronne Ice Shelf (FRIS), Antarctica. Repeated passes of ICESat reveal ice flexure in the grounding zone occurs as the ice shelf responds to ocean height changes due primarily to tides. In the course of our mapping, we have confirmed or identified three major "ice plains", regions of low surface slope near the GZ where the ice is close to hydrostatic equilibrium: one on Institute Ice Stream, another to its east, and another west of Foundation Ice Stream. The vertical information from repeated ICESat tracks enables us to study the topography and flexure characteristics across these three ice plains, and we use this to develop a classification scheme for ice plains based on their surface topography and their state of flotation. We show that one of these ice plains indicates changes in lateral extent on short time-scales, depending on the state of the ocean tide. Understanding the location and nature of ice plains is important for ice sheet modeling, since they add uncertainty to the absolute boundary between floating and grounded ice

Tidal modulation of buoyant flow and basal melt beneath Petermann Gletscher Ice Shelf, Greenland.

A set of collocated, in situ oceanographic and glaciological measurements from Petermann Gletscher Ice Shelf, Greenland, provides insights into the dynamics of under‐ice flow driving basal melting. At a site 16 km seaward of the grounding line within a longitudinal basal channel, two conductivity‐temperature (CT) sensors beneath the ice base and a phase‐sensitive radar on the ice surface were used to monitor the coupled ice shelf‐ocean system. A six month time series spanning August 23, 2015 to February 12, 2016 exhibited two distinct periods of ice‐ocean interactions. Between August and December, radar‐derived basal melt rates featured fortnightly peaks of ~15 m yr‐1 which preceded the arrival of cold and fresh pulses in the ocean that had high concentrations of subglacial runoff and glacial meltwater. Estimated current speeds reached 0.20‐0.40 m s‐1 during these pulses, consistent with a strengthened meltwater plume from freshwater enrichment. Such signals did not occur between December and February, when ice‐ocean interactions instead varied at principal diurnal and semidiurnal tidal frequencies, and lower melt rates and current speeds prevailed. A combination of estimated current speeds and meltwater concentrations from the two CT sensors yields estimates of subglacial runoff and glacial meltwater volume fluxes that vary between 10 and 80 m3 s‐1 during the ocean pulses. Area‐average upstream ice shelf melt rates from these fluxes are up to 170 m yr‐1, revealing that these strengthened plumes had already driven their most intense melting before arriving at the study site

Microstructure profiles during CEAREX

The Coordinated Eastern Arctic Experiment (CEAREX) was a multi-platform geo­physical study covering the period from winter 1988 to spring 1989 in the vicinity of the Yermak Plateau and Fram Strait. The Oceanography ("0") Camp component of CEAREX was designed to study the physical oceanographic conditions from the deep Nansen Basin north of the Yermak Plateau, then into the shallower water over the Plateau. This area , was known from previous experiments (Fram III, Fram IV, MIZEX '83) to be a very en­ergetic region, in which much of the decay of the Atlantic Water flowing into the Arctic Basin occurs. The various programs at "0" Camp obtained data which will improve our understanding of the decay mechanisms, including the role of topographically enhanced diurnal tides, internal waves, and mixing by ice stress at the surface. Our component of this project was the direct measurement of microscale velocity shear, temperature and salinity, using the Rapid Sampling Vertical Profiler. These data are used to estimate (a) the rate at which turbulent kinetic energy is dissipated, (b) vertical eddy diffusivities, and ( c) vertical fluxes of heat, salt, and momentum. More than 1500 profiles were obtained over a depth range of 0-350 m for the period March 30 to April 24, 1989, including 20 days of continuous operation (more than 1 profile per hour), from April 4 to April 24. The data show both the geographic variability associated with the topography, as well as rapid variations due to high frequency internal waves. Good data was obtained both in the surface layer (mixed by surface stress), and in the pycnocline (mixed by shear instabilities). The results confirm the importance of topographic effects on the decay of the Atlantic Water in this region

Summer surface melt thins Petermann Gletscher Ice Shelf by enhancing channelized basal melt

Increasing ocean and air temperatures have contributed to the removal of floating ice shelves from several Greenland outlet glaciers; however, the specific contribution of these external forcings remains poorly understood. Here we use atmospheric, oceanographic and glaciological time series data from the ice shelf of Petermann Gletscher, NW Greenland to quantify the forcing of the ocean and atmosphere on the ice shelf at a site ~16 km from the grounding line within a large sub-ice-shelf channel. Basal melt rates here indicate a strong seasonality, rising from a winter mean of 2 m a−1 to a maximum of 80 m a−1 during the summer melt season. This increase in basal melt rates confirms the direct link between summer atmospheric warming around Greenland and enhanced ocean-forced melting of its remaining ice shelves. We attribute this enhanced melting to increased discharge of subglacial runoff into the ocean at the grounding line, which strengthens under-ice currents and drives a greater ocean heat flux toward the ice base

Eastern Arctic Ocean Diapycnal Heat Fluxes through Large Double-Diffusive Steps

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

Impact of Tide-Topography Interactions on Basal Melting of Larsen C Ice Shelf, Antarctica

Basal melting of ice shelves around Antarctica contributes to formation of Antarctic Bottom Water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers. Tides influence ice shelf basal melt rate (w(b)) by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf. We use a three-dimensional ocean model, thermodynamically coupled to a nonevolving ice shelf, to investigate the relationship between topography, tides, and w(b) for Larsen C Ice Shelf (LCIS) in the northwestern Weddell Sea, Antarctica. Using our best estimates of ice shelf thickness and seabed topography, we find that the largest modeled LCIS melt rates occur in the northeast, where our model predicts strong diurnal tidal currents (similar to 0.4 m s(-1)). This distribution is significantly different from models with no tidal forcing, which predict largest melt rates along the deep grounding lines. We compare several model runs to explore melt rate sensitivity to geometry, initial ocean potential temperature (theta(0)), thermodynamic parameterizations of heat and freshwater ice-ocean exchange, and tidal forcing. The resulting range of LCIS-averaged w(b) is similar to 0.11-0.44 m a(-1). The spatial distribution of w(b) is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of w(b) is influenced by theta(0). These sensitivities in w(b) predictions reinforce a need for high-resolution maps of ice draft and sub-ice-shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

Intensification of Near-Surface Currents and Shear in the Eastern Arctic Ocean:A More Dynamic Eastern Arctic Ocean

A 15-year (2004–2018) record of mooring observations from the upper 50 m of the ocean in the eastern Eurasian Basin reveals increased current speeds and vertical shear, associated with an increasing coupling between wind, ice, and the upper ocean over 2004–2018, particularly in summer. Substantial increases in current speeds and shears in the upper 50 m are dominated by a two times amplification of currents in the semidiurnal band, which includes tides and wind-forced near-inertial oscillations. For the first time the strengthened upper ocean currents and shear are observed to coincide with weakening stratification. This coupling links the Atlantic Water heat to the sea ice, a consequence of which would be reducing regional sea ice volume. These results point to a new positive feedback mechanism in which reduced sea ice extent facilitates more energetic inertial oscillations and associated upper-ocean shear, thus leading to enhanced ventilation of the Atlantic Water