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Sixth Meeting of the UK Sea Ice Group

Sea Ice-Ocean Modelling at the National Oceanography Centre, Southampton.  Yevgeny Aksenov, Steven Alderson, Andrew Coward, Beverly de Cuevas, Gurvan Madec, Adrian New and Katya Popova

A rapid Transition from ice covered CO2 rich Waters to a biologically mediated CO2 Sink in the eastern Weddell Gyre.  Dorothee Bakker.

Radar Altimetry of Sea Ice.  John Fletcher

Basin-wide thinning of Arctic sea ice following the 2007 record ice extent minimum.  Katharine Giles, Seymour W. Laxon and Andy L. Ridout.

Comparison of coincident profiles of sea ice surface elevation and ice thickness derived from helicopter-borne laser altimetry and electromagnetic induction in the Arctic and Antarctic. Sibylle Goebell.

Iceberg - climate interactions Kathryn Jones and Miguel Morales Maqueda.

Sea Ice Modelling at the Met Office. Ann Keen.

The role of sea ice dynamics in carbonate mineral production in polar waters. Hilary Kennedy, S. Papadimitriou, G. Nehrke, G., Dieckmann and D.N. Thomas.

Ice tank experiments with application to sea ice mechanics. Ben Lishman, Daniel Feltham, Peter Sammonds, Alexander Wilchinsky, Steve Boon and Adrian Turner.

Investigation of wind-driven polynya dynamics with a mass and momentum conserving, one-dimensional model. Miguel Morales Maqueda, Ian A. Walkington and Andrew J. Willmott.

The effect of tides on dense water formation in Arctic Shelf Seas. Clare Postlethwaite, Miguel Morales Maqueda, Graham Tattersall, Jason Holt and Andrew Willmott.

Attribution of Arctic change. Jeff Ridley.

Experimental investigation of the flooding of snow-laden sea ice. Lucas Stone-Drake.

Arctic Sea Ice Thickness from Royal Navy Submarine Cruises in 2004 and 2007. Nicholas Toburg.

The CPOM coupled sea ice-ocean model. Adrian Turner.

On the role of salinity variations on the Arctic and Antarctic sea ice mass balance Martin Vancoppenolle.

Recent use of through-the-ice AUV to map the underside of sea ice. Peter Wadhams.

Generation of a dense coastal current by an Antarctic polynya. Alexander Wilchinsky and Daniel Feltham

Field investigations of Ku-band radar penetration into snow cover on Antarctic sea ice. Rosemary Willatt

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Sea Ice-Ocean Modelling at the National Oceanography Centre, Southampton.
Yevgeny Aksenov, Steven Alderson, Andrew Coward, Beverly de Cuevas, Gurvan Madec, Adrian New and Katya Popova. National Oceanography Centre, Southampton

We present the results from various coupled sea ice-ocean modelling projects being carried out at the National Oceanography Centre, Southampton (NOCS). The foci are on the model development and testing of the new version 3 of NEMO, and also on high resolution global simulations. We have performed inter-annual simulations with a ¼ degree global NEMO forced with DFS3 and with a double-pass through the period 1958-2001. The results have been compared to the DRAKKAR simulations. The NEMO model includes the Adaptive Grid In Fortran nesting technology (AGRIF), however the current implementation does not work with the sea ice model. The nesting scheme in the model has been modified at NOCS to solve this problem, and now allows us to set up a model nest in ice regions; the scheme is currently being tested. A new sea ice-ocean coupling scheme accounting for the correct mass exchange between ice and ocean has been developed and implemented in version 3 NEMO. We have carried out a pilot study to investigate biological and physical interactions through the solar radiation attenuation mechanism with the emphasis on the Arctic Ocean and Nordic Seas. In collaboration with the UK Met Office we have also started an inter-comparison between NEMO-LIM3 and NEMO-CICE coupled models.

A rapid Transition from ice covered CO2 rich Waters to a biologically mediated CO2 Sink in the eastern Weddell Gyre
Dorothee Bakker School of Environmental Sciences, University of East Anglia

Circumpolar Deep Water (CDW), locally called Warm Deep Water, (WDW), enters the Weddell Gyre in the southeast, roughly at 25-30 °E. In December 2002-January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO2) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO2 difference across the sea surface drives CO2 air-sea fluxes. Deep CTD sections and surface transects of fCO2 were made along the Prime Meridian, a northwest-southeast section, and along 17-23°E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO2 below the sea ice along 0°W and 17 to 23°E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO2 to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO2 by up to 100 µatm, thus creating a sink for atmospheric CO2. Despite the tendency of the surfacing of WDW to cause CO2 supersaturation, the Weddell Gyre may well be a CO2 sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO2 reduction. The CO2 source tendency deriving from the upward movement of "pre-industrial" CDW is declining, as atmospheric CO2 levels continue to increase and thus the CO2 sink of the Weddell Gyre will continue to increase as well (provided the upward movement of WDW does not change much).

Radar Altimetry of Sea Ice.
John Fletcher. University of Cambridge

If sea ice freeboard can be determined through radar altimetry, then ice thickness can be inferred from the mean density of the ice and the sea water. However, since most of the ice lies below the level of the water, any error in the freeboard will be multiplied by a large factor. A major source of such error is due to partial scattering of the radar pulse by a layer of snow which often covers the ice. As I will explain, the roughness of the snow surface is of particular interest. I present a strategy for estimating the snow surface roughness from variations in the return radar pulse.

Basin-wide thinning of Arctic sea ice following the 2007 record ice extent minimum
Katharine A. Giles, Seymour W. Laxon and Andy L. Ridout Centre for Polar Observation & Modelling

September 2007 marked a record minimum in Arctic sea ice extent, 24% lower than the previous record low in September 2005, and 37% below the climatological mean. Model studies have suggested that ice thickness and ice extent are intrinsically linked, and while there have been many studies published recently describing the minimum and its causes, little is known about how the ice thickness has changed in the run up to, and following, the summer of 2007. Using satellite radar altimetry data, covering the Arctic Ocean up to 81.5° North, we investigate changes in sea ice thickness in the run up to, and following, the 2007 minimum. These results show no evidence of preconditioning through ice thinning between 2002 and 2007 but show that, after the record minimum ice extent in 2007, the average ice thickness was reduced, particularly in the Western Arctic.

Comparison of coincident profiles of sea ice surface elevation and ice thickness derived from helicopter-borne laser altimetry and electromagnetic induction in the Arctic and Antarctic
Sibylle Goebell Newcastle University

We have performed surface elevation and sea ice thickness measurements with a combined airborne single-beam laser altimeter and differential GPS (DGPS) using a helicopter suspended sensor. Surface elevation is derived from the difference between the laser range measurement above the snow surface and the instrument's height above the geoid determined by DGPS (GPS height) yielding the geolocated elevation above the geoid (ground elevation). Ground elevation is different from surface elevation because the local sea level deviates from the geoid which is used as reference for the GPS height due to geoid errors and the unknown dynamic sea surface topography. Therefore, after the first processing, the ground elevation of open water sites between the ice floes is not zero. This bias can be removed by implementing a specific filtering operation. On average, the accuracy of the surface elevation is estimated as +- 0.1m. However, it can increase considerably depending on the roll angle of the sensor.

Results of surface elevation are compared with coincident profiles of sea ice thickness measured in the Lincoln Sea in the Arctic and in the Weddell Sea in the Antarctic. The coincident thickness profiles have been obtained using an electromagnetic thickness sounding instrument that was also included in the sensor suspended beneath the helicopter. Results show that thickness/surface elevation ratios are smaller over sea ice in the Weddell Sea than in the Lincoln Sea according to a thicker snow cover in the Antarctic. This has fundamental consequences for the ice thickness retrieval from spaceborne altimeter missions.

The high variability of R-values which result from the coincident measurements of surface elevation and ice thickness is addressed and R-values are presented for the Lincoln Sea and for the Weddell Sea. Transforming surface elevation to total thickness by solving the hydrostatic equilibrium equation and assuming specific densities for sea ice, sea water, and snow as well as snow depth, resulted in unacceptably large uncertainties. However, snow depth can be computed from the coincident surface elevation/thickness measurements which in turn is suitable in supporting airborne and spaceborne missions with respect to transforming surface elevation, and freeboard.

Iceberg - climate interactions
Kathryn Jones and Miguel Morales Maqueda Proudman Oceanographic Laboratory

This investigation is into the relationship between the iceberg count in the Northwest Atlantic and climatic variables. This study was carried out to see how the changes in iceberg numbers depend on climate variability and to gain an understanding about how icebergs operate in one of the busiest shipping districts in the world.

Sea Ice Modelling at the Met Office
Ann Keen Met Office

Recent work has focused on the development of our new coupled climate model HadGEM3. This consists of the Met Office UM atmospheric model, coupled using the OASIS coupler to the NEMO ocean model, which has been modified to call the CICE sea ice model. After much technical work, a first 100 year integration of the prototype model has been completed, and a second development integration is now underway. First results will be presented.

The role of sea ice dynamics in carbonate mineral production in polar waters
Hilary Kennedy (1), S. Papadimitriou1, Nehrke G.(2,3) Dieckmann G.(2) and Thomas D.N (1)
1. School of Ocean Sciences, Bangor University
2. Alfred-Wegener-Institute for Polar & Marine Research, Bremerhaven, Germany
3. Also at Geochemistry, Department of Earth Sciences, Faculty of Geosciences, Utrecht, Netherlands.

Although the interplay between biology and climate change is a major focus of current studies in polar waters, less attention has been paid to abiotic drivers that may influence carbon cycling and sequestration. Thermodynamic principles predict that, during the freezing of seawater, minerals should precipitate in sea ice, including calcium carbonate (CaCO3). The recent discovery of ikaite, a metastable phase of hydrated CaCO3 (CaCO3.6H2O), has confirmed the presence and mineralogy of CaCO3 in sea ice but has left us unable to assess the significance of its role in carbon cycling in polar oceans. Our initial observations suggest that the sea ice in the Weddell Sea would contain on average, 7.4 x 1012 ± 4.9 x 1012 g ikaite, equivalent to 0.4 x 1012 ± 0.3 x 1012 g carbon. By comparison, primary production in sea ice at the same time in the Weddell Sea would sequester 1.19x1012 g carbon, indicating that ikaite formation represents an additional carbon sink equivalent to 36% of that by primary production. However, key aspects of the dynamics of this mineral need to be studied before a true appreciation of its role in the polar carbon cycle can be assessed.

Ice tank experiments with application to sea ice mechanics
Ben Lishman, Daniel Feltham, Peter Sammonds, Alexander Wilchinsky, Steve Boon and Adrian Turner. Centre for Polar Observation & Modelling

This talk will briefly survey the type of experiments our group recently performed in the HSVA environmental ice tank facility. These experiments focussed on the deformation of granular assemblies of sea ice "floes". Only an outline is given here of the experiments as we have not had time to analyse the data yet. An indication is given of how these experiments relate to the geophysical scale deformation of the ice cover and how they may be used to improve the representation of sea ice mechanics in global climate models.

Investigation of wind-driven polynya dynamics with a mass and momentum conserving, one-dimensional model.
Miguel A. Morales Maqueda1 (1), Ian A. Walkington (2) and Andrew J. Willmott (1).
1. Proudman Oceanographic Laboratory
2. Department of Engineering, University of Liverpool

The dynamics of polynyas have been often investigated with relatively simple flux models that use the ice continuity, or mass balance, equation to calculate polynya evolution. In these models, steady wind and surface heat flux forcing lead to polynyas opening to a steady-state width in timescales of hours to a few days. Here we discuss the strengths and weaknesses of the flux model approach and, in particular, examine critically the concept of a steady-state polynya.

The effect of tides on dense water formation in Arctic Shelf Seas
Clare Postlethwaite, Miguel Morales Maqueda, Graham Tattersall, Jason Holt and Andrew Willmott. Proudman Oceanographic Laboratory

Dense water formation occurs in ice covered seas when brine is rejected from newly forming ice. Dense water formation can be modulated by tides in several ways. Tidal mixing can hinder ice growth or even cause melting, as oceanic heat is transported upwards. A layer of fresh melt water can inhibit convection. Additionally the ebb and flow of the tide can cause sea-ice to pile up (in areas of convergence) or separate (in areas of divergence). Thicker, piled up ice thermally insulates the ocean from the atmosphere and thus further dense water formation becomes less likely. Conversely, areas of open water exposed as the tides pull the ice cover apart, start to produce dense water as brine is rejected from newly formed sea-ice. We present results from a dynamic/thermodynamic sea-ice model (CICE) coupled to a baroclinic coastal ocean model (POLCOMS) of the Barents and Kara Seas. Although introducing tides into the model does not alter the total annual salt flux to the ocean significantly, some regions show significant changes to ice volume and ocean salinity. In particular, the seasonally ice covered seas in the south of the domain have up to 25‰ less ice volume during freeze-up and melting when tides are included in the model. Conversely, the shallow area around Svalbard has increased ice volume throughout the year. The distribution of increased brine rejection due to tides is similarly inhomogeneous and, although some coastal regions show significantly increased salinity throughout the water column, this appears to be dominated by advection by residual tidal currents. Further work will determine the significance of these results and indicate whether future Global Climate Models should include tide/sea ice interactions to make more accurate predictions.

Attribution of Arctic change
Jeff Ridley Met office

The Met Office climate model HadGEM1 is one of the IPCC models which accurately depicts recent sea ice change in the Arctic. A formal method of attribution of ice seasonality is undertaken to determine that observed changes may be attributed to global warming. A four member ensemble of climate simulations since 1979 is examined and compared with observations. We show that even the largest single annual change in the observations, that of summer 2007, is consistent with the model. Thus, there is no case to reject model projections of an ice free Arctic between 2050 and 2080.

Experimental investigation of the flooding of snow-laden sea ice
Lucas Stone-Drake Centre for Polar Observation & Modelling

The surfaces of sea ice floes can become flooded with seawater. This is usually a result of the floe sinking under the weight of snow on its surface, and the subsequent infiltration of seawater into the snow layer by overflowing the sides of a floe, or by percolating upwards through the permeable sea ice. This process, and the subsequent formation of snow ice, impacts on ice and snow thermodynamics, influences the physical and compositional properties of sea ice, and plays a role in sea ice ecosystems. Tank experiments have been designed to investigate the dynamics and thermodynamics of these processes. The experiments take place in a cold room and involve the use of thermistors, hypodermic needles, thin- sectioning techniques, and a digital refractometer. Measurements of temperature and salinity, and inferred local solid fractions, reveal the changing distribution of salt during flooding and refreezing.

Arctic Sea Ice Thickness from Royal Navy Submarine Cruises in 2004 and 2007.
Nicholas Toberg University of Cambridge

Statistics obtained from sea ice measurements done by Royal Navy submarine HMS Tireless during the Spring of 2004 and 2007 are analyzed. Areas of coincident tracks, off the northern coast of Greenland, show few differences in mean draft and suggest a trend toward ice build up in this region. Methods of data extraction are discussed, including future work to be carried out with three dimensional multibeam sonar equipment.

The CPOM coupled sea ice-ocean model
Adrian Turner Centre for Polar Observation & Modelling

We present preliminary simulations of the Arctic Ocean and sea ice cover from a newly coupled, sea-ice/ocean model. The sea ice component is the Los Alamos CICE model version 4, and the ocean component is OPA version 9. The simulation results from the coupled model are compared to estimates of basin-wide sea ice thickness obtained from satellite altimetery and sea ice extent, and the ocean circulation pattern is described and discussed in light of previous hydrographic surveys.

On the role of salinity variations on the Arctic and Antarctic sea ice mass balance
Martin Vancoppenolle Institut d'astronomie et de geophysique Georges Lemaitre

The theory of sea ice thickness distribution is generalized to include sea ice salinity distribution. The formulation of the thermodynamic changes in ice salinity is done through a parameterization of brine entrapment and drainage, which is validated against a more complex sea-ice halodynamic module in a 1-D test case. The salinity distribution is included in LIM3, a 3D dynamic-thermodynamic model including ice thickness, enthalpy and age distributions as well as a description of the impact of ice salinity on ice growth and decay. LIM3 is coupled to the ocean model OPA9 and run over 1970-2006, forced by a combination of daily NCEP/NCAR reanalysis data and various climatologies. The seasonal cycle of the simulated sea ice salinity averaged over thickness categories agrees well with historical ice core salinity data. The simulated Arctic salinity distribution follows the salinity-thickness relations of Cox and Weeks (1974). Due to hemispherical differences in the forcings, the model simulates Arctic and Antarctic salinity fields that differ significantly. The simulated large-scale sea ice mass balance is found quite sensitive to the model representation of ice salinity. In the Arctic, including an interactive salinity distribution enhances ice growth / melt rates through a direct impact on ice thermodynamics. Around Antarctica, the effect of an interactive salinity distribution is even larger. But, in contrast to the Arctic, the role of ice-ocean interaction is dominant: using a variable salinity enables to maintain significant ice growth with relatively small salt fluxes to the ocean, which in turn further reduces oceanic heat fluxes and enhances ice growth. Given the importance of salinity on the simulated sea ice characteristics, the salinity distribution should be included in assessments of the response of the high-latitude oceans to ongoing and future climate change.

Recent use of through-the-ice AUV to map the underside of sea ice
Peter Wadhams University of Cambridge

High resolution three-dimensional mapping of the underside of sea ice has been achieved by use of a small Gavia AUV launched through the ice from ice stations. The first experiment was in April 2007 from the SEDNA ice station in the Beaufort Sea, and the second in May 2008 from a small ice station established by the author and his colleague Martin Doble on fast ice north of Alert on Ellesmere Island. First-year and multi-year ridges were studied, with accompanying surface drilling and validation by HEM electromagnetic and airborne scanning laser techniques. The characteristic block size and angularity of the two types of ridge can be clearly discriminated.

Generation of a dense coastal current by an Antarctic polynya.
Alexander Wilchinsky and Daniel Feltham Centre for Polar Observation & Modelling

Descent and spreading of high salinity water generated by salt rejection during sea ice formation in an Antarctic coastal polynya is studied using a hydrostatic, primitive equation three-dimensional ocean model called POLCOMS. The shape of the polynya is assumed to be a rectangle 100 km long and 30 km wide, and the salinity flux into the polynya at its surface is constant. The model has been run at high horizontal spatial resolution (500 m) and numerical simulations reveal a buoyancy-driven coastal current. The coastal current is a robust feature and appears in a range of simulations designed to investigate the influence of a sloping bottom, variable bottom drag, variable vertical turbulent diffusivities, higher salinity flux, and an off-shore position of the polynya. It is shown that bottom drag is the main factor determining the current width. This coastal current has not been produced with other numerical models of polynyas, which may be because these models were run at coarser resolutions. The coastal current becomes unstable upstream of its front when the polynya is adjacent to the coast. When the polynya is situated off-shore an unstable current is produced from its outset due to the capture of cyclonic eddies. The effect of a coastal protrusion and a canyon on the current motion is investigated. In particular, due to the convex shape of the coastal protrusion the current sheds a dipolar eddy.

Field investigations of Ku-band radar penetration into snow cover on Antarctic sea ice.
Rosemary Willatt Centre for Polar Observation and Modelling

We investigated the penetration of Ku-band radar into snow cover over Antarctic sea ice. Satellite radar altimetry is used to calculate sea ice thickness in the Arctic where it is assumed that the radar reflection originates at the snow/ice interface, due to the cold, dry snow conditions. However, the more complicated snow stratigraphy and frequent flooding of Antarctic snow may mean that this assumption is not valid. The data for this investigation were obtained in sea ice off East Antarctica in September and October 2007 with a radar deployed from an icebreaker. Radar data were collected with field measurements of snow depth, density, wetness and stratigraphy in order to examine the effect of the physical conditions of the snow on the radar return echo. Data were taken over a range of locations and the snow conditions included icy layers, flooding and hard crusts. Analysis of data from detailed snow pit studies showed that the snow/ice interface was only the dominant scattering surface of the radar under conditions of very dry and low density snow. Data taken along a transect showed that the mean depth of the dominant scattering surface observed in radar data was less than the mean measured snow depth. In the literature Antarctic sea ice elevations calculated with ERS-2 satellite altimeter data were higher than in-situ freeboard measurements, in accordance with our results.