sea ice production in the mertz glacier polynya 2002... · 2009-05-15 · ian allison, nathan...

Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd–6th December 2002 International Association of Hydraulic Engineering and Research

SEA ICE PRODUCTION IN THE MERTZ GLACIER POLYNYA

Ian Allison, Nathan Bindoff, Victoria Lytle, Rob Massom, Matt Paget, Andy Roberts, Mark Rosenberg, Guy Williams, Tony Worby and Xingren Wu1

The Mertz Glacier Polynya (MGP) is a large area of open water and thin sea ice on the coast of the Antarctic continent near 145 °E, in the lee of the floating Mertz Glacier tongue. The MGP is one of the largest and most persistent winter polynyas on the Antarctic coast. Processes within this polynya are implicated in the production of Adelie Land Bottom Water (ALBW), which has been shown to represent a significant fraction of the total volume of Antarctic Bottom Water. In winter 1999 a multi-disciplinary expedition onboard the research ice-breaker RSV Aurora Australis spent 6 weeks within the Mertz Glacier polynya investigating the processes that maintain the open water, the rate of sea ice formation and export, the rate of water-mass conversion in the polynya, and hence the role of the MGP in Adelie Land Bottom Water production. Ice production in Buchanan Bay, in the coastal part of the polynya where ice is rapidly removed by strong katabatic wind funnelling down the Mertz Glacier valley, is very high. Drifting buoys deployed here in newly forming ice measured ice export from this region at an initial-ice drift speed as high as 60 km day–1. Periodic measurements of the ice thickness in the vicinity of the buoys gave an initial growth rate of undeformed ice of about 4 cm day–1. When deformed ice was included, the total growth rate increased to an average of 8 cm day–1. Air temperature and humidity data, measured using a probe extending from a helicopter, provided an estimate of the total turbulent heat loss from this part of the polynya of up to 600 W m–2. Elsewhere, ice removal from the polynya may be restricted by heavy pack to the north and by fast ice to the west. The ice production over the total MGP area is estimated from in-situ and ship based observations, and satellite imagery. These ice production estimates are compared to changes in the ocean structure determined over a 5–week period using CTD data collected at 5 nautical mile spacing during three laps of a study region encompassing the core of the polynya. These confirm that the polynya is primarily a latent heat one with 95 % of the total heat flux due to sea ice formation at a freshwater equivalent growth of 4.9–7.7 cm day–1. The high rate of ice production in the polynya is both necessary and sufficient to form the high salinity water found in the Adelie Depression that is a precursor to Bottom Water.

1 Antarctic CRC, PO Box 252-80, Hobart, Australia


PAST SEA-ICE COVER IN THE SOUTHEAST INDIAN SECTOR OF THE SOUTHERN OCEAN

Leanne Armand1, Lloyd Burckle2, Xavier Crosta3 and Aldo Shemesh4

Sea-ice plays a key role in modulating climate and ecological systems. This includes the regulation of heat flux between the ocean and the atmosphere, the modulation of weather patterns and oceanographic circulation, and the production of a substrate for biological activity. Sea-ice is thus, a significant parameter for the interpretation of past climates and oceanography. Previous estimates have been limited to the Last Glacial Maximum extent of sea-ice in the Southern Ocean. This presentation documents new continuous records of past sea-ice concentration in the Southeast Indian Ocean from siliceous algal remains (diatoms) and biogenic silica content. We provide a comparison of the methods used to determine past sea-ice cover and provide a history of sea-ice variability over the last two glacial cycles.

1 Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart 7001, Tasmania, Australia 2 Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA 3 DGO, UMR-CNRS 5805 EPOC, Universite de Bordeaux I, Avenue des Facultes 33405, Talence

Cedex, France 4 Dept. of Environmental Sciences, Sussman Building, The Weizmann Institute of Science, Rehovot,

76100 Israel


SEA-ICE COVER AND ANNUAL DIATOM FLUX IN THE OFFSHORE PRYDZ BAY

Leanne Armand1, Cindy Pilskain2 and Rob Massom3

In 1998, a collaborative field research program was initiated between the U.S. and China, and later Australia, to quantify the biogeochemical particulate export occurring in the offshore Prydz Bay region of the Southern Indian Ocean, East Antarctica. Peak total mass and biogenic component fluxes are observed at all depths in the austral summer months of Jan.–Feb. following the retreat of the sea ice, with secondary peaks in the winter (July) and early spring (Oct.) recorded by the 1000 m trap. Biogenic opal is the dominant flux component representing over 70 % of the Prydz Bay mass flux, similar to the results from the Ross Sea AESOPS traps deployed in the Antarctic Zone. Samples of the Prydz Bay 1998–1999 sediment trap material from 1000 m are generally dominated by diatom tests and zooplankton fecal pellets with periodic occurrences of silicoflagellates, radiolarians, and planktonic foraminifera. Diatoms are most abundant and display the greatest diversity, representing a mixture of planktonic and sea-ice algal species, in the Jan.–Feb. and Dec. 1998 samples. Little to no sea-ice cover existed at the trap site during these months. This pattern is consistent with the frequent observation of algal blooms associated with sea ice melt-back periods noted in other regions of the Southern Ocean. Diatom dominances and succession will be discussed, as will blooms occurring during maximum sea-ice cover in winter. Additional comments on the occurrence of the common planktonic diatom F. kerguelensis in trap samples collected during maximum ice cover months indicating lateral current input under the ice from adjacent ice-free, open water regions will be covered.

1 Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart, Tas 7001, Australia 2 Bigelow Lab for Ocean Sciences, W. Boothbay Harbor, ME 04575, USA 3 Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart, Tas 7001, Australia


SL-MIP (SURFACE LAYER MODEL INTERCOMPARISON PROJECT): RESULTS USING SHEBA OBSERVATIONS

John Cassano, Amanda Lynch, Judy Curry and Ola Persson1

Surface layer parameterizations are used in numerical models to diagnose surface turbulent fluxes of momentum, heat, moisture, and other scalar variables. As such these parameterizations are critical in linking the atmosphere to the underlying surface in regional climate models. The goal of SL-MIP is to evaluate current state-of-the-art surface layer parameterizations, that are being used in regional and global climate models, using surface layer observations over the Arctic sea ice made during the SHEBA project. Key issues being addressed include the description of common biases, the characteristics of successful parameterizations, the influence of small-scale waves, and the role of the lowest model level.

1 CIRES, University of Colorado, 216 UCB, Boulder, CO 80309


ICE THICKNESS PROFILE EVOLUTION IN A WAVE FIELD

Mingrui Dai1, Hayley Shen2 and Mark Hopkins3

In this study, a discrete element computer model is utilized to simulate the behaviors of ice floes in wave field. The ice floes are circular disk in shape and are subject to water drag, added mass, gravity and buoyant forces. Second order nonlinear effects are incorporated in the model. The drift velocity imposed by the wave drives the ice floes toward the land fast ice. The equilibrium thickness of the ice accumulation in front of the land fast ice is determined by wave parameters, ice floe properties, water drag and added mass coefficients.

1 Department of Civil & Environmental Engineering, Clarkson University, Potsdam, NY 13699-

5710, USA. Email [email protected] 2 ibid, Email [email protected] 3 Ice Engineering, USACRREL, Hanover, NH 03755, USA. Email [email protected].

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THE FRACTURE BEHAVIOR OF SEA ICE IN McMURDO SOUND: OBSERVATIONS FROM IN-SITU EXPERIMENTS

John P. Dempsey1, David M. Cole2, Saul Shapiro1, Guro Kjestveit1 and Lewis H. Shapiro3

The breakup process plays an important role in the ice dynamics and thermodynamics of the Antarctic region. A current research program is studying the relevant mechanical properties of the annual ice in Antarctica in an effort to develop improved, physically based models of the breakup process. Because of the influence of natural growth conditions and scale on mechanical properties, it is particularly important to investigate the in-situ mechanical properties of sea ice in order to fully understand the breakup process and verify the analytical models. To accomplish this, field trips were conducted on the ice in McMurdo Sound in the Austral springs of 2000 and 2001, and the present paper describes the experimental approach and presents selected results from the 2000 season. The test site was several kilometers offshore near Cape Barne, Ross Island, Antarctica. The in-situ experiments involved cyclic loading and fracture tests on floating, edge-notched, square plate specimens of first-year ice. The experiments investigated the con-stitutive and fracture behavior of first-year sea ice specimens of varying size, and involved a detailed characterization of the physical properties of the ice (salinity, bulk density, grain size and orientation) as well as its thermal state. The specimens were subjected either to sets of cyclic loads with varying frequency and amplitude, or to a monotonically increasing strain-controlled ramp to failure. Surface deformations were measured at several locations along the starter crack, at the initial crack tip, and in the bulk specimen ahead of the crack tip. An acoustic emissions monitoring system prov-ided a measure of the microcracking activity at two locations ahead of the initial crack tip. This paper discusses the fracture behavior of edge cracked square plates subjected to selected cyclic and ramp loadings.

1 Clarkson University, Department of Civil and Environmental Engineering, Potsdam NY 13699-

5710 2 U.S. Army Cold Regions Research and Engineering Laboratory, Hanover NH 03755 3 Geophysical Institute, University of Alaska – Fairbanks, Fairbanks, AK.


HISTORICAL TRENDS IN LAKE ICE COVER IN NORTHERN CANADA

Claude R. Duguay, Frédéric Lenormand, Ross D. Brown and Terry D. Prowse1

Long series of lake ice observations can serve as a proxy climate record, and the monitoring of freeze-up and break-up trends provide a convenient integrated and seasonally specific index of climatic perturbations. The most recent study on historical freeze-up and break-up records of ice on lakes (Magnuson et al., Science, 2000) indicates that later freezing and earlier break-up dates are observable around the Northern Hemisphere from 1846 to 1995. However, some lake sites may deviate from this general trend depending on the hydroclimatic region in which they are located. In a country as large as Canada, lakes are likely to respond differently whether they are found below or above treeline, for example. Given this and the fact that global climate models predict that high latitude regions will be the most strongly affected by climate warming, we undertook a study to examine historical trends in ice cover for lakes located in the sub-arctic and arctic regions of Canada. In this presentation, we will report results from the analysis of a set of 28 lake ice sites over a 23-year period (1963–64 to 1987–88).

1 Department of Geography, Laval University, Quebec, G1K 7P4, Canada


ICE PRESSURE VARIATIONS DURING INDENTATION

R. Frederking1

During the JOIA project detailed measurements of load and pressure distributions were made. One data file, from a test conducted February 4, 1999 was made available to IAHR Ice Crushing Working Group. Indenter width was 1.5 m, ice thickness 168 mm and indentation velocity 3 mm/s. The entire indenter face was covered with “tactile” sensor elements, each 10 mm by 10 mm, for a total of 8448 sensor elements. The ice was not thick enough to ensure contact with all the sensor elements, but usually 2000 to 3000 elements experience loads. Spatial and temporal distributions of pressure will be examined, both on the basis of average distributions, shape of contours, simultaneity and variations during load cycles. Implications for describing ice crushing processes will be discussed.

1 Canadian Hydraulics Centre, National Research Council, Ottawa, ON, K1A 0R6 Canada


HELICOPTERBORNE EM THICKNESS MEASUREMENTS IN THE TRANSPOLAR DRIFT

Christian Haas1

During a Polarstern voyage in August and September 2001 to the Transpolar Drift and North Pole, a newly developed helicopter-borne EM thickness sensor was evaluated and operated. System characteristics as well as some results of the thickness measurements will be presented. The sensor is a towed bird suspended below the helicopter on a 20 m long power cable at altitudes of 10 to 20 m above the ice surface. It is 3.4 m long and weighs 100 kg, small enough to be operated from helicopter decks of ice breakers. The bird operates in the frequency domain, using signal frequencies of 3.6 and 112 kHz. Signal generation and reception are fully digital. The bird’s thickness retrievals agree very well within 0.1 m with extensive ground-based thickness measurements, both statistically by comparing the derived thickness distributions as well as by comparing collocated profiles across single floes. Thus, with the bird high resolution thickness surveys along extended transects are possible. The bird measurements revealed the existence of two main ice types in the Transpolar Drift, first-year ice with a typical thickness of 1.2 to 1.4 m, and second- or multiyear floes with typical thicknesses of 2 m. The latter is considerably thinner than our previous observations performed in 1991, 1996, and 1998, which show a steady decrease from a maximum typical thickness of 2.5 m in 1991. The typical thickness is representative for level ice, whose thickness is mainly determined by thermodynamic growth processes. In contrast, recent submarine observations of ice draft decreases refer to the mean thickness, which is comprises both level and deformed ice. Reductions of the mean ice thickness have therefore mainly been attributed to changes in ice dynamics. Our measurements may thus provide some complementary and so far hidden information about changes in the thermodynamic boundary conditions.

1 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany


AN EAST ANTARCTIC POLYNYA CLIMATOLOGY USING MULTI-SENSOR SATELLITE DATA ANALYSIS

K.L. Hill, K. Michael and R.A. Massom1

Polynyas, defined as non-linear areas of open water or thin ice up to 0.3m thick and enclosed by thick ice and coastline, play a key role in the Antarctic air-sea-ice inter-action system. In the case of latent heat polynyas, sea ice forms and is quickly removed by ocean currents and/or winds (synoptic and/or katabatic), with heat loss from the ocean surface being balanced by the latent heat of new ice formation. Importantly, polynyas recur and persist in approximately the same locations each year. They act as "ice factories" of the Antarctic sea-ice zone, and are sites of significant water-mass modification and very intense ocean-atmosphere fluxes of heat and moisture compared to the surrounding insulative pack. Indeed, these regions can dominate the total heat exchange, and gaining improved estimates of the spatial and temporal variability in their areal extent is a key to better understanding their overall role. In this paper, we utilise the long-term satellite data record from the NOAA Advanced Very High Resolution Radiometer (AVHRR) to develop an improved climatology of polynyas along the East Antarctica coast between 40º E and 170º E and over the period 1992–1999. Compared to the coarse (25 km) resolution of passive microwave sensor data used in previous studies, the AVHRR offers coverage at a finer resolution of 1 km, and potentially a more accurate means of determining polynya size, although the AVHRR is cloud-limited. Thirty two polynya areas were identified and characterised. A short case study is presented of the Mertz Glacier Polynya, with satellite data being validated by in situ measurements collected during a cruise of the RSV Aurora Australis during the austral winter of 1999. High-resolution radar backscatter data from the Radarsat ScanSAR are combined with AVHRR thermal infrared data, creating a false color composite image (hue-lightness-saturation), in an effort to utilise the unique attributes of both datasets.

1 IASOS, Po Box 252-77, Hobart, Australia, 7001


AIRCRAFT MEASUREMENTS OVER BROKEN SEA ICE

Amélie Kirchgäßner1, David Schröder1, Timo Vihma2 and Burghard Brümmer1

A total of 27 flight missions were flown over the ice edge zone of the northern Baltic Sea (BASIS 1998 and BASIS 2001) and the Fram Strait (ACSYS 1998 and FRAMZY 1999) by the German research aircrafts Falcon and DO-128. During all flights horizontal legs were flown at low levels between 10 and 35 m. The flights took place under a wide range of synoptic situations and thus stratification conditions. For the three experiments in 1998 and 1999 the effective roughness lengths for moment-um 0 ,z sensible heat Tz and latent heat qz under unstable conditions were calculated applying bulk formulas and the universal stability functions. Over broken sea ice the resulting mean values were 4 8

0 4 10 m, 1 10 mTz z− −= ⋅ = ⋅ and 71 10 m.qz −= ⋅ Observed

over a wide range of ( )5 20 010 m 10 mz z− −< < the ratio 4

0 / 10Tz z ≈ is in contradiction

to previously published results over compact sea ice ( )1 3010 / 10 .Tz z− < < For less

rough surfaces ( )30 10 mz −< the discrepancy is largest and is found to be significant.

The ratio is approximately in agreement with several observational results obtained for heterogeneous land surfaces. The corresponding neutral transfer coefficients reduced to 10 m height amount to ( ) ( )3 3

10 101.8 0.7 10 , 0.9 0.2 10DN HNC C− −= ± ⋅ = ± ⋅ and 10ENC =

( ) 31.0 0.3 10 .−± ⋅ As a consequence, parameterizations with commonly used heat trans-fer coefficients overestimate surface heat fluxes over broken sea ice by more than 100 %. The aircraft measurements carried out during BASIS 2001 provide a set of low level flights (at heights between 5 and 15 m) that allow us to study the validity of the results described above and extend the investigations to the stably-stratified boundary layer. Schröder, D., Vihma, T., Kerber, A. and Brümmer, B. On the parameterization of turbulent surface

fluxes over heterogeneous sea ice surfaces. J Geophys. Res. 108(C6): 3195, doi:10.1029/2002JC001 385 (2003).

1 Meteorological Institute, Center for Marine and Atmospheric Research, University of Hamburg,

Hamburg, Germany 2 Finnish Institute of Marine Research, Helsinki, Finland

Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd–6th December 2002 International Association of Hydraulic Engineering and Research PROPOSED OBSERVATIONS OF THE GROWTH OF LAND-FAST

SEA ICE IN MCMURDO SOUND, ANTARCTICA DURING THE WINTER OF 2003

Greg Leonard1, Craig Purdie1, Mike Williams2, Tim Haskell3 and Pat Langhorne1

We present some preliminary oceanographic and ice structure measurements taken in McMurdo Sound during September 2002, and describe proposed measurements to be undertaken during the Antarctic winter of 2003. The purpose of these measurements is to quantitatively describe the formation of the land-fast sea ice in McMurdo Sound and to determine the link between this formation and the underlying oceanographic conditions. Specifically we are concerned with determining the relationship between the underlying oceanographic conditions and the formation of a type of ice termed platelet ice, as well as investigating the cause of horizontal layering which has been observed in McMurdo Sound. Almost all that is known about the sea ice growing attached to the Antarctic landmass has been measured in the spring when the ice has grown to over 1 meter thick. Here we intend to measure the physical conditions of the thin sea ice that prevails in Antarctic winter conditions.

1 Department of Physics, University of Otago, P,O. Box 56, Dunedin New Zealand 2 National Institute of Water and Atmospheric Research Ltd, Wellington New Zealand 3 Industrial Research Ltd, Lower Hutt, New Zealand


ANTARCTIC SEA ICE CORE DATA BASE

Victoria Lytle1

Sea ice cores around Antarctica have been collected by numerous people and organisations over the past 20 years or so. The core properties and characteristics are usually analysed and reported from isolated voyages or from individual regions around Antarctica. There have been no comprehensive studies which attempt to compile these cores from different regions, seasons and years into a consistent data base. A recent initiative by ASPeCt (Antarctic Sea Ice Processes and Climate) has started to compile these sea ice core results so that analysis on the entire available data is possible. This includes compiling data on salinity, crystal structure, temperature and biological properties. Ancillary data on snow thickness, and meteorological conditions when the cores were collected is also included. Although all data are not collected on all cores, basic data such as crystal structure and bulk salinity are usually measured. This paper presents initial results from this data base which now contains hundreds of cores.

1 GPO Box 252-80 Hobart, Tas 7001, Australia


FAST ICE EXTENT AROUND EAST ANTARCTICA USING SYNTHETIC APERTURE RADAR IMAGES

V.I. Lytle and A.B. Giles1

Here we present results from a study of fast ice extent, along the coast of East Antarctica. We present 2 snapshots of the fast ice extent during November, the time of maximum extent for two year; 1997 and 1999. We use pairs of synthetic aperture radar (SAR) images from Radarsat-1 to determine the location of the fast ice. By using a standard cross-correlation technique to determine sea ice motion vectors we define fast ice as sea ice which has near-zero velocity. Although the availability of these images is limited both in time and space, the available dataset provides a high resolution mapping of fast ice along most of the coastline between longitudes 80 E and 170 E, for at least one of the two years. Massom et. al. (these proceedings) use a series of visible and infrared satellite images to extend these results to other years and seasons. From the calibrated backscatter intensity and in-situ data, we identify 3 different types of fast ice in this region; a) smooth first year ice with low backscatter and few textural features in the SAR images, b) highly deformed first year fast ice which accumulates around coastal protrusions and grounded icebergs, and c) very thick multi-year fast ice. Using these classifications as a proxy indicator of ice thickness, we estimate the total volume of fast ice in the region.

1 GPO Box 252-80 Hobart, TAS 7001 Australia


COMPOSITION AND ORIGIN OF BASAL ICE IN COLD AND POLYTHERMAL GLACIERS

Sarah Mager and Sean Fitzsimons1

The study of ice composition sheds light on the various processes operating at the ice-bedrock interface and on glacier dynamics. By analyzing the ice composition inferences can be made about ice formation and its origin. This paper compares the isotopic and solute composition of basal ice from the Rhone and Taylor glaciers which are located in the Taylor Valley, South Victoria Land, Antarctica. Rhone Glacier is a small dry-based glacier with a basal ice temperature of –17 ºC. Its basal zone is characterised by 3.5 m thick basal sequences of stratified and amber ice that rests below 18 m of clean englacial ice. Taylor Glacier is a large thick outlet glacier that flows from Taylor Dome, an independent dome within the east Antarctic ice sheet. At the terminus Taylor Glacier has a basal temperature of –18 ºC but 8 km upstream it is believed to be at pressure melting point (Robinson 1984). The Taylor basal zone consists of over 2.5 m of debris-bearing ice that is overlain and underlain by clean englacial ice. Tunnels excavated in both glaciers provided access to the basal ice and facilitated the removal of frozen samples. The basal ice facies were mapped detailing sedimentary structures, ice facies, and physical characteristics, and blocks of ice were extracted for laboratory analysis. In the laboratory these blocks of ice were sub-sampled and the stable isotope signatures (δ18O and δD) and solutes (Sodium, Calcium, Magnesium, Potassium, and Chloride). This analysis revealed a strong contrast between the different ice facies. The ice facies strongly contrast in physical appearance, as there is a clear distinction between englacial ice, amber ice, solid debris, and stratified ice facies. The englacial ice is characterised by very low solute concentrations and isotopic values plot on the local meteoric water line. The amber ice has a distinct yellow-greenish discoloration, low debris concentration and relatively high solute concentrations. The stratified ice faces contains layers of debris and clean ice and is characterised by high debris concentration and very high solute concentrations. The isotopic values of debris-bearing ice from Rhone Glacier show a linear relationship that is very similar to the local meteoric water line. In contrast isotopic values of debris-bearing ice from Taylor Glacier show a linear relationship with a slope that is considerably lower than that of the local meteoric water line. The differences in the physical and chemical signatures of the ice facies reflect different origins of the basal ice. The high solute concentrations and low slope of the isotopic values from Taylor Glacier show that meltwater plays a crucial

1 Department of Geography, University of Otago, P.O. Box 56, Dunedin, New Zealand

role in the formation of the basal ice. Differences in the basal ice stratigraphy between the Rhone and Taylor glaciers reflect intrinsic differences in their thermal regimes. Robinson, P.H. (1984) Ice dynamics and thermal regime of Taylor Glacier, South

Victoria Land, Antarctica. Journal of Glaciology 30(105): 153–160.


THE SIGNIFICANT EFFECT OF EXTRA-POLAR BLOCKING ANTICYCLONES ON SEA ICE IN THE PALMER LTER REGION,

LATE WINTER-EARLY SPRING 2001

Robert Massom, Sharon Stammerjohn, Ray Smith, Mike Pook, Rich Iannuzzi and Yuko Massom1

This paper describes sea-ice conditions encountered during cruise NBP0105 of the R/V Nathaniel B. Palmer to Marguerite Bay (~ 68 ºS, 70 ºW) on the central western Antarctic Peninsula continental shelf in September–October 2001 as part of the US NSF's Palmer Long-Term Ecological Research programme. Of overriding importance in terms of its effect on regional sea-ice conditions was the establishment of a series of blocking anticyclones in the South Atlantic Ocean (examined here by analysis of NCEP data). This steered approaching cyclones to the southeast, and led to persistent and strong northerly winds in the study region and the transport of significant moisture (and significant snowfall) and relatively high surface air temperatures across the entire sea-ice zone over a prolonged period (i.e. a number of weeks). The overall effect on regional sea-ice conditions was complex. On the one hand, persistent high air temper-atures caused significant sea ice and snowcover melt, and widespread ice-surface flooding. Flooding was compounded by i) significant aeolian redistribution and accumulation of snow, and ii) dynamic depression of the sea-ice surface in the vicinity of pressure ridges. Intervening and intermittent cooling episodes led to significant snow-ice formation, which to some extent counterbalanced sea-ice basal melt (observed both directly in sea-ice core analysis and deduced from CTD time series measurements). On the other hand, the strong northerly winds also caused significant deformation and dynamic thickening of the pack ice by forcing it against the Antarctic Peninsula coast and islands in Marguerite Bay. This led to a significant narrowing and compaction of the sea-ice zone in this sector (as determined by satellite data analysis). Resultant sea ice thicknesses of >10 m were encountered in consolidated pack, with a snow cover of 0.5–1.5 m.

1 Antarctic CRC, GPO Box 252-80, c/o University of Tasmania, Hobart, Tasmania 7001, Australia


FAST ICE DISTRIBUTION AND ITS INTERANNUAL VARIABILITY IN EAST ANTARCTICA, DETERMINED FROM

SATELLITE DATA ANALYSIS

Robert Massom, Ted Scambos, Vicky Lytle, Katrina Hill, Dan Lubin and Barry Giles1

The distribution of Antarctic fast ice is of key climatic and ecological importance. For example, fast-ice formation and breakout behaviour have an important impact on the Mertz Glacier Polynya, which is a significant site of Antarctic Bottom Water formation, the characteristics of the regional sea-ice zone over a large area, and penguin populations. In spite of this importance, fast-ice areal extent and seasonal and inter-annual variability are poorly understood. This paper presents preliminary results from an analysis of the distribution of fast ice, and its variability, in the Dibble Iceberg Tongue-Cook Ice Shelf region of East Antarctica (~ 135–155 ºE) over the period 1985 to 2001. It uses cloud-free satellite data from wide-swath, medium-resolution sensors, namely the NOAA Advanced Very High Resolution Radiometer (AVHRR, spatial resolution ~1km) and the DMSP Operational Linescan Sensor (OLS, resolution ~ 0.56 km). This work complements, and extends in time, the study by Lytle et al. (this proceedings), which uses higher resolution Radarsat ScanSAR imagery to produce snapshots of fast-ice distribution. It examines the use of a standard cross-correlation technique to distinguish fast ice from moving pack ice by the application of a threshold velocity. Fast-ice extent is also determined by visual inspection. Comparisons are made between the SAR and AVHRR/OLS fast-ice retrievals where the two datasets are coincident in space and time. Particularly complex fast-ice conditions occur around the floating Mertz Glacier tongue and regions of grounded icebergs to the east and west. Interactions between fast-ice distribution and icebergs (both drifting and grounded) are highlighted, with fast ice often "locking in" icebergs. Distinctions are made between regions of perennial and annual fast ice. Large variability is noted in the distribution of annual fast ice in the vicinity of Dumont d'Urville base, on synoptic to inter-annual time-scales.

1 Antarctic CRC, GPO Box 252-80, c/o University of Tasmania, Hobart, Tasmania 7001, Australia


GEOMETRIC REPRESENTATION OF RUBBLE PILE-UP ON A CONICAL STRUCTURE

D.C. Mayne and T.G. Brown1

The load contribution of rubble pile-up on conical structures is seen as a significant portion of the overall load. Using observations from the Confederation Bridge Monitoring Programme and a field programme conducted in March 2001 a geometric representation of a general rubble pile has been derived. These observations show that the rubble formations are consistently non-linear in slope and non-circular in footprint. The new representation will examined with existing algorithms for volumetric and overall load comparisons.

1 Department of Civil Engineering, University of Calgary, Calgary, Canada


HIGH-RESOLUTION STUDY OF THE THERMAL CONDUCTIVITY OF FIRST YEAR SEA ICE

Daniel Pringle, Joe Trodahl and Mark McGuinness1

Historically the thermal conductivity, κ, of sea ice has been represented very simplist-ically in sea-ice and climate models. This treatment has been based on simple theoretical heat conduction models which don’t address ice structure or non-conductive heat transfer processes. We have established a program of high-resolution in-situ measurements of the temperature field in first year, land fast sea ice in McMurdo Sound, Antarctica. This is aimed at parameterising κ by temperature, temperature gradient, and ice structure Of particular interest is the conductivity of the highly disordered, near-surface ice. Into growing first year sea ice we have frozen in 2 m long, 6mm diameter thin-walled stainless steel tubes housing 20 thermistors at 10 cm spacings. Every 30 minutes the temperature at each depth T z t,b g is recorded with a resolution better than 0.01 °C. We will summarise the operating principles and performance of this equipment which has also been successfully deployed in dry Antarctic Permafrost and in Elson Lagoon, Barrow, AK. Five data sets from 1991 – 2002 have been analysed with several different techniques. We will discuss the implementation and results of a finite difference technique to calculate the thermal conductivity as a function of temperature, depth and temperature gradient, and of an examination of the near-surface region by propagating the surface temperature T t0,b g down. Possible signatures of non-conductive heat transfer will be discussed. Our experimental program is continuing this Austral winter. We will deploy two thermal arrays including new custom-built electronics with improved temperature resolution at adjacent sites in McMurdo Sound off Arrival Heights. We will also make direct measurements of the thermal conductivity of sea ice cores at this site in November 2002. We expect to be able to present these results at this meeting.

1 Victoria University of Wellington, Wellington, New Zealand


INVESTIGATIONS OF CHEMICAL COMPOSITION OF SNOW AND ICE OF THE ARAL SEA

G.A. Tolkacheva1

The Aral Sea basin is internal-drainage one and occupies about 690000 km2 within the Central Asian region. The Aral Sea is an end point of surface waters runoff and discharges of this basin. Resulting from an intensive irrigation development, population growth, agro-industrial complex capacity buildup consumption of water resources has been increased in the region. As a result water entering in the Aral Sea has been sharply reduced what has led to shrinking the area sea, reduction of water volume, sharp deterioration of water quality both in the sea and adjacent territories. Water salinity in the Aral Sea has increased by 3–4 times. Due to these processes dramatic ecological changes have occurred in the Aral Sea basin. The goal of our activity was investigation of the chemical composition of show and ice sampled in the Aral Sea during two seasons. The object of investigation was the frozen sea surface covered with snow. Electro-conductivity, pH, sum of ions, chlorides, sulphates, nitrates, ammonia, sodium, potassium, calcium, magnesium were defined in these samples. The main methods of analyzing are ion chromatography, atom adsorption and photo-calorimetry. Due to conducted investig-ations the following has been revealed. Chemical composition of ice and snow differs from water composition. On the whole, the content of main ions of chlorines, sulphates, sodium, potassium in snow and ice samples is lower than in water samples. The most high chlorine ions content is observed in upper ice layers. Similar regularity was detected also for hydrocarbonates. On the whole, in snow and ice samples of the Aral Sea total salts content is by 1000 times higher than in snow and ice samples taken in the runoff formation area. It should mention that calculated values of geo-chemical coefficients have shown prevalence of chlorine ions in all cases. Decrease in total mineralization of snow and ice samples testifies to the possibility to desalinate surface water by freezing.

1 Central Asian Research Hydrometeorological Institute (SANIGMI), 72 K. Makhsumov str.,

Tashkent, Uzbekistan, Tel: 998 712 35 84 68, Fax: 998 71 133 11 50, e-mail: [email protected]


MIGRATION OF MILLIMETER-SIZE BRINE POCKETS IN A TEMPERATURE GRADIENT

B.J. Tuckey1 and P.J. Langhorne2 Brine movement has a significant influence on the decay of sea ice. It occurs by various mechanisms, including gravity-driven convection in brine drainage channels, and the migration of enclosed brine pockets in response to an imposed temperature gradient. Using brine pocket dimensions that are characteristic of those during the decay of sea ice, this study examines the migration of an enclosed cylindrical brine pocket in an imposed temperature gradient, and makes some deductions about the processes that directly affect this movement. In previous experiments, the small size of brine pockets has led researchers to conclude that the diffusion of salt in the pocket and the crystal orientation of the host dominate the migration and geometry of brine pockets. In the present research on pockets of larger dimensions, we find evidence of the presence of buoyancy-driven convection. Our cylindrical brine pocket is tilted with the long axis at an angle to the vertical, a geometry that, with the exception of McGuinness et al. (1998), has been largely overlooked. A study of fluid convection in a slot tilted at an angle to the vertical (Woods and Linz, 1992) explains that this geometry ensures free convection always occurs, independent of the nature of the imposed temperature gradient. Here a single brine pocket in ice is simulated by drilling a hole (2−5 mm diameter) in a block of bubble-free ice. The pocket is filled with NaCl solution. The block is placed in a temperature gradient (10 or 50 °C m–1) and the migration of the pocket is logged by video. The brine pocket not only moves in the direction of the warmer material, but the cylindrical geometry becomes more bulbous. The velocity of migration, which appears to have a weak dependence on brine pocket diameter and temperature gradient, is an order of magnitude larger than would be expected if the process was diffusion limited. McGuinness, M.J., Trodhal, H.J., Collins, K., and Haskell, T.G. (1998) Non-linear thermal

transport and brine convection in first-year sea ice. Annals of Glaciol. 27: 471–476. Woods, A.W. and Linz S.J. (1992) Natural convection and dispersion in a tilted fracture. J.

Fluid Mech. 241: 59–74.

1 Cawthron Institute, Nelson, New Zealand 2 Department of Physics, University of Otago, P.O. Box 56, Dunedin, New Zealand


ON THE FORMATION AND CIRCULATION OF ADÉLIE LAND BOTTOM WATER IN EAST ANTARCTICA: SEA-ICE

PRODUCTION AND OCEAN/ICE INTERACTION IN THE MERTZ GLACIER POLYNYA

G.D. Williams, N. L. Bindoff, S.S. Jacobs, S. Marsland and S.R. Rintoul1

Recent studies suggest that the Adélie Land coast is the second largest source of Antarctic Bottom Water. Oceanographic work carried out aboard the RV Aurora Australis and RSV Nathaniel B. Palmer between July 1998 and February 2001 investig-ated a specific source previously identified next to the Mertz Glacier in the Adélie Depression off the Antarctic coast between 145 °E and 150 °E. Analysis of winter and summer CTD profiles, ship-mounted ADCP profiles and time series measurements from 7 moorings show the seasonal evolution of water masses. Intrusions of warm (up to –0.8 °C), saline and oxygen depleted Modified Circumpolar Deep Water over the continental shelf break dominate the Adélie Depression in summer. During winter, brine-enhancement from intense sea-ice production (5–10 cm day–1) within the Mertz Polynya converts these summer water masses into dense shelf waters (> 34.63 psu). Ice-shelf exchanges with the Mertz Glacier tongue occur throughout the year producing Ice Shelf Water (< –1.91 °C) with a cooling and freshening feedback to the overall system that is a strongest in summer. Time series data from Seabird micro-cats and OEI current meters deployed on the continental shelf, across the shelf break and within the Adélie Depression show the volume of dense shelf waters increasing until August. In August the dense shelf water reaches the depth of the sill and escapes over the shelf break. A process modeling experiment applying the C-HOPE global ocean model to the region provides interesting comparisons with the observed results and greater scope for understanding the sensitivities of the overall system. Using local bathymetry to define the water mass volumes, we examine freshwater and heat exchanges to define the influence of the sea-ice production in the Mertz Polynya and ice-shelf melting beneath the Mertz Glacier on the production of Antarctic Bottom Water from this source.

1 University of Tasmania, Hobart, Australia


HETEROGENEOUS NUCLEATION OF WATER - WITH AND WITHOUT AN ADDED CATALYST

P. Wilson, A.D.J. Haymet and A. Henaghan1

Heterogeneous nucleation of water is examined by an automated lag time apparatus. We freeze a single supercooled sample many times and analyse the data generated to determine the nucleation curve. We also determine the functional form relating average lag time to level of supercooling. This is done for water with and without silver iodide added. A new definition for the kinetic freezing point or supercooling point will be put forward and data from a variety of fields of science looked at in this new way.

1 Physiology Dept., Otago Medical School, University of Otago, New Zealand


CIRCUMPOLAR SEA ICE THICKNESS MAPS FOR THE ANTARCTIC

Anthony P. Worby1 and Cathleen Geiger2

A compilation of sea ice data from 77 voyages to the Antarctic is presented. These data have been collected between 1980 and 2001 and total almost 20,000 individual ice observations. The data have been standardized and quality controlled using the ASPeCt protocol (Worby, 1999), which provides a unique format for observing and recording sea ice observations. Much of the early data, in particular from the Russian and German programs, was not recorded using this standard procedure, making it necessary to translate the original log books. The data from each voyage include total ice concentr-ation and open water fraction as well as the concentration, thickness, floe size, topography, and snow cover characteristics of each of the three dominant ice types within the pack. In total, data have been contributed from 28 Russian voyages, 28 Australian voyages, 14 US voyages, and 7 German voyages. The data have been gridded around the continent by season, to produce horizontal, circumpolar maps of sea ice thickness and concentration for the period 1980–2000. Some regions in the vicinity of coastal stations have a high density of data, which in a few in cases may be sufficient to examine inter-annual variability in ice conditions. In other areas of heavily ridged and multi-year ice, such as the western Weddell Sea, data coverage is minimal. The thickness of level ice floes is estimated from a ship as they break and turn on their sides. The mean thickness of ridged floes is calculated using a model formula that takes into account the areal coverage and sail height of ridges, based on drill-hole profiles across ridged floes. We estimate an error of ±10 % for level ice thickness and ±25 % for mean ridged ice thickness. The results show the first seasonally variable maps of sea ice thickness and concentration around Antarctica. Despite some gaps, these provide a useful and necessary baseline for many scientific studies that aim to understand the important

1 Antarctic CRC, University of Tasmania, PO Box 252-80, Hobart, Tasmania, 7001, Australia 2 Center for Climatic Research, Geography Department, University of Delaware, 216 Pearson Hall,

Newark, DE 19716, USA

physical and biological processes within the Antarctic sea ice zone. In particular the data will be useful for initializing and validating numerical models.

Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd–6th December 2002 International Association of Hydraulic Engineering and Research ANTARCTIC SEA ICE EXTENT: A LOOK AT HISTORICAL AND

CONTEMPORARY RECORDS

Anthony P. Worby1 and Josefino Comiso2

A comparison of in situ ice edge locations from 30 voyages to the Antarctic with SSM/I data shows remarkably good agreement in the growth season. This is during the months of March and October when the ice edge is typically quite consolidated and the transition from open ocean to consolidated pack occurs over a short distance (typically 1–2 pixels of SSM/I data). In the summer months however, the agreement is much poorer and substantial amounts of ice are routinely observed 1–3 ° latitude north of the SSM/I ice edge. During the summer months bands of ice are common at the ice edge, and the ice in these bands is usually saturated with a passive microwave signature closer to that of open water than sea ice. A key question to be addressed is how consistently the ice edge is defined across different data sets, and therefore whether or not different data sets can be compared. Records suggesting a substantial decrease has occurred in Antarctic sea ice extent include palaeo reconstructions dating back 21,000 years as well as much more recent whaling data from the 20th Century. Contradictory evidence from the records of explorers of the 18th and 19th Centuries suggests that the ice conditions 150–200 years ago were very similar to present conditions. To add to the confusion, different passive microwave algorithms show different trends in Antarctic sea ice extent since the early 1970s. This paper will take a close look at how the ice edge is defined across different data sets and how readily they can be compared.

1 Antarctic CRC, University of Tasmania, PO Box 252-80, Hobart, Tasmania, 7001, Australia 2 Goddard Space Flight Center, NASA,

sea ice production in the mertz glacier polynya 2002... · 2009-05-15 · ian allison, nathan...

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