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Ocean circulation in the coastal waters around Antarctica 1. Aim The polar continental shelf-slope regions are impacted, particularly in the Antarctic, by energetic atmosphere-ice-ocean interactions. The Antarctic shelves are the sources for dense Antarctic Bottom Water that drives the southern hemisphere branch of the global Meridional Overturning Circulation. The ocean circulation in this region feeds one of the globe's richest ecosystems, impacts carbon sequestration, and influences overlying atmospheric conditions and the adjacent ice cap. The ocean freshwater balance of the Antarctic shelves is presently changing, at least partly in response to surging and melting of the West Antarctic Ice Sheet (WAIS). The most dramatic thinning of the WAIS has been recorded in several glaciers feeding the ice shelves in the Amundsen Sea (Pritchard et al, 2009; Shepherd et al, 2004). Understanding this system is central to our ability to predict behaviour of the WAIS, melting of which in turn can impact global sea-level. The Amundsen Sea remains one of the least sampled and understood of the circum-Antarctic marginal seas. The present project will use in situ measurements, remote sensing and analytical modeling to study warm Circumpolar Deep Water (CDW) circulation on the Amundsen Sea Shelf, and the subsurface melting of icebergs and ice-shelves that it induces. The specific goals are: (1) Use the results of (already funded) shipboard and moored observations, combined with historical data, to quantify time variations in transport and properties of CDW and iceshelf meltwater in the Amundsen Shelf region (2) Use remote sensing to assess the iceberg melt in the Antarctic coastal region, with emphasize on the Amundsen Sea; (3) Use remote sensing to assess the horizontal excursions of the Antarctic Circumpolar Current and trends in its extent; and (4) Develop a new theoretical model for the ventilation of the water masses in the Antarctic coastal regions, with application to the Amundsen Sea. 2. Background Polar shelf regions are important sites for water mass transformation. Shelf sea waters are strongly influenced by fluxes of heat and freshwater between the ocean, the ice and the atmosphere, as well as exchanges with the deep sea. The shelf region is a place where warm and salty ocean water encounters and melts glacier ice, which produces a relatively buoyant water mass (Walker et al, 2007; Martinson et al, 2008; Moffat et al, 2009; Wåhlin et al, 2010b). Large atmospheric heat losses are experienced by coastal boundary currents over the shallow shelf seas (Mauritzen, 1996a, b; Isachsen et al, 2007; Wåhlin and Johnson, 2009), and much of the densest water in the world ocean is formed during ice freezing in wintertime polynyas (e.g. Padman et al, 2009). The Antarctic shelf regions are highly appropriate research sites since they are data poor compared to the Arctic, and they are currently experiencing large changes coincident with seaward surging of the West Antarctic Ice Sheet (WAIS) (Pritchard et al, 2009). The WAIS has declined in volume over the last decade. The mass balance of the WAIS is of global significance, since it has the potential to raise global sea level by 5-6 m. The most dramatic thinning is occurring in the sector feeding into the Amundsen Sea, where it can exceed 9 m per year (Pritchard et al, 2009). The changes are hypothesized to have an oceanic origin (e.g. Pritchard et al, 2009), with warm ocean water circulating below the ice shelves and inducing basal melting and accelerated glacial flow (Rignot et al., 2002). Long term measurements in the Ross Sea show a significant interannual freshening in which meltwater from the WAIS might have played an important role (Jacobs et al., 2002), but this is not the only possibility. Changes in sea ice production/melting and cross shelf exchanges also strongly influence the

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freshwater budget. Despite great potential impact, the Amundsen and Bellingshausen seas remain relatively poorly sampled.

Figure 1. Map of the Amundsen Sea showing the bathymetry, stations occupied during the OSO0809 cruise (green dots), planned sections (red thick lines) and mooring locations (stars). The black square shows the region where icebergs were tracked (Fig. 3). The source of warm oceanic water surrounding Antarctica is Circumpolar Deep Water (CDW), with a temperature of 1-2 oC. CDW intrusions have been observed in the deep troughs and canyons crossing the shelf west of the Antarctic Peninsula (Martinson et al, 2008; Moffat et al, 2009) as well as in the Amundsen Sea (Walker et al, 2008; Wåhlin et al, 2010b). A ship-based survey of the Amundsen Sea region in 2008/2009 (Figure 1; Wåhlin et al, 2010b) revealed a large mass of water produced through cooling and freshening of CDW by subsurface melting of ice shelves and/or icebergs (Figure 2). This mixture between CDW and ice shelf melt water (ISW) was observed in the westernmost of the deep cross-shelf troughs (Fig. 1) and occupied a 100-200 m thick intermediate layer between the CDW and the surface layer. Such a layer has not been previously reported from the area and is not shown in historical data from the World Ocean Database (Boyer et al, 2006). Few stations have however been occupied in the deep troughs, and fewer still in the Western trough. Hence, it is possible that CDW-ISW mixtures have been present but not detected. The heat loss reported by Wåhlin et al (2010b) corresponds to an annual ice melt of 110 - 130 km3. The annual net volume loss (including contributions from e.g. snow accumulation) of the glaciers terminating in the Amundsen Sea was estimated as 40 - 80 km3 (Rignot and Thomas, 2002; Pritchard et al, 2009), indicating that oceanic melt in the Amundsen Sea contributes significantly to the glacier's volume budget. Although the basic mechanism for basal melting of the ice shelves is understood, significant unsolved questions remain. The main pathways of the CDW are not known, and the total inflow rate is not quantified. Furthermore the temporal variations are virtually unknown. It is also not known whether the melt-water originates from ice-shelves or from icebergs farther from land.

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Figure 2. Top panel. T-S diagram showing the freshening and cooling of the CDW (red circle) after contact with the ice shelves in the Western Amundsen Sea. Colours indicate oxygen content, gray dots are historical data. The black line is the 'Gade line', i.e. the T-S relation obtained if the only process affecting the original water is latent cooling from the ice melt and subsequent mixing with the melt water. The red line is the freezing temperature. Bottom panel. Section across the trough with blue colours indicating that the TS properties fall along the Gade line and hence consists of CDW and ice-shelf melt water mixtures. White thick line shows the bound of the CDW. From Wåhlin et al, 2010b. Modeling studies (e.g. Thoma et al, 2008; Dinniman and Klinck, 2004) suggest that the inflow of CDW fluctuates with time scales from days to weeks and peak during wintertime. A similar time dependence has been observed on the West Antarctic Peninsula shelf (Martinson et al, 2008). However, the temporal variation of the CDW inflow has never been measured. One way to quantify the inflow variations, and to relate the summer observations to the yearly mean, is to obtain time series from moored arrays and velocity profilers at strategic locations. Icebergs drift westward or northwestward (e.g. Stuart and Long, 2008), suggesting a clockwise shelf circulation similar to that observed on the west Antarctic Peninsula shelf. The icebergs transport fresh ice into the central Amundsen Sea, and it is possible that a significant part of the subsurface melting takes place there rather than in sub ice-shelf cavities. If this is true the ice-shelves may not be as vulnerable to oceanic melt as previously thought, since melting icebergs do not directly impact the ice-shelves. Further, melting icebergs might contribute to localized diapycnal mixing across the thermocline because of local buoyant convection processes associated with the meltwater, and this might locally impact biology and/or sea ice formation.

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3. Project description. The ship time and equipment necessary for the field work proposed here has already been granted and one of the moorings is already deployed. In this project funding for analyzing the data (historical as well as new acquisitions during 2010 - 2012), working with the satellite data, developing the theoretical model, and publishing the results is requested. The historical data sets that will be used include the SASSI network database, hydrographic and ADCP transects in the Amundsen Sea during the OSO 2008/09 cruise, hydrographic data sampled in the Amundsen Sea during the OSO 2010 cruise (through collaboration with Prof. A. Orsi); a number of hydrographic/ADCP transects from the Ross Sea collected during the AnSlope cruises (through collaboration with Dr. R. Muench), and publicly available oceanographic databases (e.g. NODC). The new field program will be to sample hydrographic/LADCP transects across the deep troughs in the central Amundsen Sea and deploy two moored instrument arrays (see Figure 2). One of the moorings was deployed on Feb. 16th, 2010 and will be recovered and redeployed in December 2010, with final recovery in 2011/2012 to provide a 2-year data time series. The hydrographic sections will be sampled using CTD, oxygen sensors and LADCP; the moorings will have microcats (CTD) at different depths and an upward-looking ADCP. The field work will take place during 2010/2011 and 2011/2012 from IB Oden. The first of these cruises will be a joint expedition with the American icebreaker N.B. Palmer. Palmer will operate primarily in the polynya, and data will be shared between the groups. Water mass analyses will be used, along with submarine trough bathymetry, to define the major pathway(s) of CDW from the outer shelf. Geostrophic calculations in combination with detided ADCP and LADCP data will allow transport estimates for the different troughs. Freshwater and heat budgets will be constructed from the hydrographic sections, meteorological forcing and satellite data. Mooring data will be analyzed in terms of the depth distribution of CDW, flux of CDW and heat towards the ice shelf, and their temporal variations. This is the first mooring deployment in one of the deep troughs, and there is no prior information about temporal variations. In addition to ship-borne measurements, remote sensing will be used to quantify the iceberg melt rate. Satellite radar images (ASAR) images will be used to estimate the surface area occupied by icebergs, which return a strong radar signal and can normally be clearly distinguished from sea ice (Fig. 3). The recently launched SMOS satellite is equipped with a two-dimensional interferometric radiometer instrument that can measure surface salinity, which will greatly aid in the tracking of the surface water. An application for SMOS data access has been filed to ESA and is awaiting approval. The work with the SMOS satellite will be done in collaboration with L. Eriksson, ice remote sensing expert at Chalmers University of Technology. The remotely sensed surface layer drift will be compared to and complemented with the CATS2008b tide model (Padman et al, 2002), which has shown good agreement with LADCP data on the shelf (Wåhlin et al, 2010b).

Figure 3. ASAR images from the black square in Fig. 2 for 3 different days in October. Yellow-red colours indicate icebergs. The white arrows in (b) and (c) show the velocity field obtained by tracking the icebergs using PIV software (Sveen, 2004). (a) Position of icebergs on on Oct 2nd (b) Position of icebergs on Oct 5th, together

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with the drift velocity calculated between Oct 2nd and Oct 5th. (c) Position of icebergs on Oct 8th, together with the drift velocity calculated between Oct 5th and Oct 8th. 4. Preliminaries. A first estimate from the ASAR images indicates that the area occupied by large icebergs on the Amundsen Sea shelf was ~ 2500 km2 in January 2009, i.e. ~ 10% of the area occupied by the floating ice shelves. Since the icebergs have a much larger ice-ocean contact surface than the ice shelves and are closer to the warm source of the CDW, the iceberg melt can be a significant portion of the subsurface melting process despite the comparatively small horizontal surface area. If the CDW-ISW mixture is produced by iceberg melt on the shelf rather than ice-shelf melt close to the coastline it might also explain why the water mass has not been previously observed; much of the melting would have occurred on the central shelf and in the deep troughs. The ASAR images can also be used to track iceberg motions. Fig. 3 shows a time series of the icebergs, together with the velocity field derived from using PIV (Particle Image Velocimetry, e.g. Sveen 2004) software on the images. The iceberg drift is highly intermittent, with fast movement between the 2nd and the 5th and then no movement between the 5th and the 8th. Initial scanning of the available satellite data indicates that at least one time series of at least 3 images within a week can be found every month in most areas in the Amundsen Sea. Coverage is considerably better in the southern part of the shelf than in the northern. By combining drift velocities with iceberg surface area, local melt rates can be estimated. A (lower) estimate of the melt rate in a region is given by the area of the icebergs that flow into the region minus the area of the icebergs that flow out of the region, i.e. the divergence of the velocity weighted by the surface area. From the position of the stranded icebergs and the bathymetry it will also be possible to estimate their thickness, which will aid in the iceberg melt calculations. The new ESA SMOS satellite also has capabilities to measure the iceberg thickness and this will be used.

Figure 4. Temperature section across the eastern trough (for location, see Figure 1). Left panel: Temperature recorded from Oden during the OSO 2008/09 cruise. Right panel: Historical data from 2002 (Walker et al, 2006). Thick white line is the boundary of the CDW, and the temperature scale is the same in the left and right panels. Vertical gray lines in the left panel show the horizontal boundaries of the section in the right panel.

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Figure 5. Historical data (plus signs) together with data from the OSO 2008/09 cruise (circles) for the western as well as the eastern troughs (a) Dissolved oxygen versus temperature (b) Dissolved oxygen versus salinity. The black dashed line shows the CDW-WW mixing line. The thick layer of CDW-meltwater mixture that was recorded in the western trough during the OSO 2008/09 cruise (Wåhlin et al, 2010b) has not previously been reported in the region. In addition to the western transect, a cross-section over the eastern trough (Fig. 1) was also performed during the 2008/09 cruise. The eastern trough has previously been transected in 2002 (Walker et al, 2006), and a comparison of the eastern section (2008/2009) with Walker et al data (recorded 2002) (Figure 4) indicates that the inflow there is a persistent feature. The oxygen data (Fig. 5) show that the intermediate water was produced by subsurface melting also in the eastern trough. The black dashed line in Figure 5 shows the mixing line between CDW and Winter Water. Unlike the historical data, a substantial part of the OSO data deviates from the mixing line in that it is become fresher and colder than CDW but lower oxygen content than mixing with WW would have entailed. This is an indication of subsurface ice melt (Wåhlin et al, 2010b). Preliminary estimates of the CDW inflow through the eastern section shows an inflow of 0.45 Sv, larger than that through the western trough and nearly twice as large as the only existing previous estimate of 0.25 Sv (Walker et al, 2006). 5. Developing heat- and freshwater budgets for the Antarctic Shelf Sea region. Because of the historical paucity of observations, basic quantities such as the freshwater balance, surface circulation and adjustment time scales are largely unknown for substantial parts of the Antarctic Shelf Seas, and in particular for the Amundsen Sea. There exists no unified theory to describe the process balances that impact a specific Antarctic shelf region. For example, why is the Amundsen Sea shelf dominated by subsurface ice shelf and iceberg melting rather than buoyancy loss at the coastal polynya which forms regularly every winter, as for the Ross Sea (Gordon et al, 2009)? Why is the west Antarctic Peninsula dominated by

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lateral exchange with the open ocean rather than coastal air-ice-ocean exchange processes (Martinson et al, 2008)? An important part of the present project will be to analyze the hydrographic and remotely sensed data and from the results develop further the physical model which has been outlined in Figure 6. A brief explanation is provided in the caption. Understanding the ocean physics and the interaction between ocean, ice and atmosphere is necessary in order to predict how the fluxes of heat and freshwater evolve in a changing climate and to assess potential future threats to the WAIS. A first understanding of the system in Figure 6 can be obtained by assuming that the exchange of water between the shelf and the deep sea is constant, set e.g. by topographic constraints. This assumption yields a family of analytical solutions describing exponential adjustments from initial to equilibrium states (see a similar treatment in, e.g., Wåhlin and Johnson, 2009), from which some physical understanding can be gained. In the equilibrium state, subsurface heat loss to the glacier ice (and icebergs) and the atmospheric heat loss is precisely balanced by the heat content in the inflowing warm ocean water.

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Figure 6. Sketch of the fluxes of heat and freshwater between the polar shelf sea and the atmosphere, the ice, and the open ocean. The air-sea heat flux AIRQ is proportional to the temperature difference between surface water

and air, with a proportionality constant AIRγ that varies depending on wind speed, relative humidity and sea ice (Haney, 1971; Wåhlin & Johnson, 2009; Wåhlin et al, 2010a). Sea ice melting creates cold, low salinity surface layers that block ocean to atmosphere heat transfer. In the shelf seas mixing is enhanced e.g. by tides and irregular topography (e.g. Padman, 1995; Holloway and Proshutinsky, 2007) compared to the open ocean (e.g. Padman and Dillon, 1989; Timmermans et al., 2008; Fer, 2009). This leads to an upward heat transport from deep water to the fresh surface layer and the atmosphere (e.g. Martinson, 1990; Martinson et al, 2008). Sea-air heat transfer in the shelf regions consequently increases with bulk ocean temperature according to

()AIRAIRAIRQTT γ=− . It is expected that AIRγ increases in regions of enhanced mixing (due e.g. to strong tides) and decreases as a function of the salinity stratification. A similar relation is obtained for the ice-ocean heat transfer ICEQ . Unlike for temperature, there is no strong coupling between surface salinity and the freshwater flux F. A positive freshwater flux is caused e.g. by melting of ice or net precipitation. A negative F is induced primarily by freezing processes. The water exchange with the deep ocean is given by M (m2s-1). Part of this exchange is independent of shelf processes and is driven e.g. by the wind (Dinniman and Klinck, 2004) and eddy-exchanges (Kanzow et al, 2009). Other possible contributions depend on the net buoyancy flux on the shelf. In freezing coastal polynyas (as in the western Ross Sea) a net buoyancy loss from the shelf sea to the atmosphere drives a dense bottom outflow with magnitude proportional to the density of the polynya water (Wåhlin, 2004; Muench et al, 2009). Subsurface melting of ice-shelves (as in the Amundsen Sea) produces a mid-depth outflow of relatively buoyant water that generates an inflow of warm salty water at the bottom. 6. International and national collaboration. IB Oden field work will be done in collaboration with Prof. G. Björk and Dr. L. Arneborg. Historical data, and 2010 hydrographic and LADCP data being collected from IB Oden, will be obtained from Prof. A. Orsi, Texas A&M Univ.. Ross Sea data will be obtained from Dr. R. Muench. Processing of SMOS data will be done in collaboration with Dr. L. Eriksson, Chalmers Univ. of Tech. Analyses of tidally driven ocean mixing and comparisons with numerical modeling will be done in collaboration with Dr. L. Padman. References Boyer, T. P., J.I. Antonov, H.E. Garcia, D.R. Johnson, R.A. Locarnini, A.V. Mishonov, M.T. Pitcher, O.K. Baranova, I.V. Smolyar, 2006.World Ocean Database 2005. S. Levitus, Ed., NOAA Atlas NESDIS 60, U.S. Government Printing Office, Washington, D.C., 190 pp., DVD Dinniman and Klinck, 2004. A model study of circulation and cross-shelf exchange on the west Antarctic Peninsula continental shelf. Deep Sea Research part II, 51, 2003 - 2022. Fer, I. 2009. Weak Vertical Diffusion Allows Maintenance of Cold Halocline in the Central Arctic. Geophysical Research Letters, 2 (3), 148 - 152. Gade, H. 1979. Melting of Ice in Sea Water: A Primitive Model with Application to the Antarctic Ice Shelf and Icebergs. Journal of Physical Oceanography, 9 (1), 189 - 198. Goosse, H. and Fichefet, T., 2001. Open ocean convection and polynya formation in a large-scale ice-ocean model. Tellus A, 53, 94 - 111. Haney, R. L., 1971. Surface thermal boundary condition for ocean circulation models. J. of Ph. Oc., 1, 241–248.

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Holloway, G., and A. Proshutinsky, 2007: Role of tides in Arctic ocean/ice climate, J. Geophys. Res., 112, C04S06, doi:10.1029/2006JC003643. Isachsen, P. E., Mauritzen, C. and H. Svendsen, 2007. Dense water formation in the Nordic Seas diagnosed from sea surface buoyancy fluxes. Deep Sea Research I, 54, 22 - 41. Jacobs, S., C. Giulivi and P. A. Mele, 2002. Freshening of the Ross Sea during the late 20th century, Science, 298. Jenkins, A. 1999. The Impact of Melting Ice on Ocean Waters. J. of Physical Oceanography, 29, 2370 - 2381. Kanzow, T., H. L. Johnson, D. P. Marshall, S. A. Cunningham, J. J-M. Hirschi, A. Mujahid, H. L. Bryden and W. E. Johns, 2009. Basin-wide integrated volume transports in an eddy-filled ocean. J. of Phys. Oc., in press. Martinson, D. G. (1990), Evolution of the Southern Ocean Winter Mixed Layer and Sea Ice: Open Ocean Deepwater Formation and Ventilation, J. Geophys. Res., 95(C7), 11,641–11,654. Martinson, D. G., S. E. Stammerjohn, R. A. Iannuzzi, R. C. Smith, M. Vernet, 2008. Western Antarctic Peninsula physical oceanography and spatio–temporal variability. Deep Sea Research II, 55, 1964 - 1987. Martinson, D, Killworth, P, Gordon, A, 1981. A convective model for the Weddell polynya. J. Ph. Oc., ll, 466-488 Mauritzen, C. 1996a. Production of the dense overflow waters feeding the North Atlantic across the Greenland-Scotland Ridge. Part 1. Evidence for a revised circulation scheme. Deep Sea Research I, 43 (6), 769 - 806. Mauritzen, C. 1996b. Production of the dense overflow waters feeding the North Atlantic across the Greenland-Scotland Ridge. Part 2. An inverse model. Deep Sea Research I, 43 (6), 807 - 835. Moffat, Owen and Beardsley, 2009. On the characteristics of Circumpolar Deep Water intrusions to the west Antarctic Peninsula Continental Shelf. Journal of Geophysical Research C: Oceans. 114 (5), C05017. Moffat, C., Beardsley, R.C., Owens, B., van Lipzig, N., 2008. A first description of the Antarctic Peninsula Coastal Current. 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Eprint no. 2, ISSN 0809-4403, Dept. of Mathematics, University of Oslo. http://www.math.uio.no/˜jks/matpiv Thoma, M., A. Jenkins, D. Holland and S. Jacobs, 2008. Modelling Circumpolar Deep Water intrusions on the Amundsen Sea continental shelf, Antarctica. Geophysical Research Letters, 35, L18602. Timmermans, M.-L., J. Toole, R. Krishfield, et al., 2008: Icetethered profiler observations of the double-diffusive staricase in the Canada Basin thermocline, J. Geophys. Res., 113, C00A02, doi:10.1029/2008JC004829. Walker, D., M. A. Brandon, A. Jenkins, J. T. Allen, J. A. Dowdeswell and J. Evans, 2007. Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough. Geophysical Research Letters, 34, L02602. Wåhlin, A. and G. Walin, 2001: Downward migration of dense bottom currents. Env. Fluid Mech 1 (2), 257 - 279. Wåhlin, A., 2002: Topographic steering of dense bottom currents with application to submarine canyons, Deep-Sea Research part I, 49 (2), 305 - 320. Wåhlin, A., 2004: Topographic advection of dense bottom water, Journal of Fluid Mechanics, 210, 95 - 104. Wåhlin, A. and C. Cenedese, 2006: How entraining density currents influence the stratification in a one-dimensional ocean basin, Deep-Sea Research II, 53, 172 - 193. Wåhlin, A. and H. Johnson (2009). The Salinity, Heat, and Buoyancy Budgets of a Coastal Current in a Marginal Sea. Journal of Physical Oceanography, 39, 2562 - 2580 . Wåhlin, A., Ericsson, M., Aas, E., Brostrøm, G., J. Weber and J. Grue, 2010a. Horizontal convection in water heated by infra-red radiation and cooled by evaporation. Tellus A, 62, 154 - 169. Wåhlin, A., Yuan, X., Nohr, C. and Björk, G., 2010b. Inflow of warm Circumpolar Deep Water in the Western Amundsen Sea, Journal of Physical Oceanography (accepted). Voropayev, S. I. and Fernando, H. J. S, 1999. Evolution of two-layer thermohaline systems under surface cooling. Journal of Fluid Mechanics, 380, 117-140