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Atmosphere-Ice-Ocean Interactions in the Eastern Ross Sea

Eastern Ross SeaEastern Ross Sea

This multidisciplinary project is currently being carried out in collaboration with Stan Jacobs of the Lamont-Doherty Earth Observatory (LDEO), Richard Cullather of the University of Maryland, and David Holland of New York University. It takes advantage of annual transits of the research icebreaker Oden through the eastern Ross Sea (ERS) to investigate a key problem in the physics of air-sea-ice interactions: What processes control the flow of warm Circumpolar Deep Water (CDW) onto the Antarctic continental shelf in the ERS, and how might such incursions change in an evolving polar climate?

It is now widely perceived that in some locations the warmth of CDW drives rapid ice shelf basal melting, which in turn allows grounded ice streams to move more rapidly into the sea. In summer, a sharp transition occurs around the Edward VII Peninsula between a warm region that experiences CDW intrusions and a cold shelf water region that may not. We postulate that a minor shift in the ocean circulation could expose the base of the eastern Ross Ice Shelf to seawater several degrees warmer than today. The ERS lies near the winter residence of the dynamic Amundsen Sea Low on the southern limb of the Ross Gyre, has a nearly perennial sea ice cover, and is devoid of winter measurements.

Goal

To identify key present-day dynamical and thermodynamical mechanisms controlling the net onshore transport of warm CDW - or lack thereof - around the Edward VII Peninsula in the Eastern Ross Sea.

Science Questions

  1. What are the spatial characteristics of the slope front, CDW intrusions, meltwater outflows and surface waters in the ERS in reference to those of neighboring regions in the Ross and Amundsen Seas?

  2. What are the influences of the atmosphere, sea ice, bathymetry and ice shelves on the ocean circulation in this region?

  3. What are the roles of model parameterizations and spatial resolution in providing realistic estimates of poleward mass, heat and freshwater in the ERS?

  4. What is the annual cycle of water properties and circulation on the continental shelf of this region?

  5. What climate scenarios would enhance basal melting and the risk of Ross Ice Shelf retreat associated with cross-shelf processes in the ERS?

Field Program

Field work included a cruise on the Swedish icebreaker Oden sailing from McMurdo, Antarctica in the austral summer of 2010 to carry out all planned CTD/LADCP work and all mooring deployments.

Broad summary of the two funded components of this project:

  • Ocean Station Work We made high-resolution conductivity/temperature/depth (CTD) measurements to characterize the summer regional water mass stratification and circulation, the meridional exchange of waters between the oceanic and shelf regimes, and ocean interactions with the local ice shelves, sea ice and atmosphere. In combination with existing data, these observations were adequate to resolve the local scales of spatial variability, characterize the Antarctic Slope Front and its freshwater transport, and estimate the main cross-frontal transports of CDW mass and heat into the Sulzberger and Little America troughs. CTD data provide the information needed to describe the boundaries and spreading of water masses, infer their mixing histories and interactions with sea-ice and continental ice. The CTD/LADCP/XBT sections provided the spatial framework needed to interpret time series information from the mooring locations, along with model validation data. CTD data also provide information about the internal density field, and thus indirectly the relative geostrophic currents. Referenced to the concurrent LADCP measurements, absolute geostrophic volume and property transports can be calculated across the sections. These fluxes in turn provide the information to detect changes in water mass characteristics and estimate melting rates. On each Oden cruise we profile the full water column for temperature, salinity, dissolved oxygen (T/S/DO) and currents, using a SeaBird CTD system and LADCP provided by TAMU. Water samples are also collected for S, DO to monitor CTD/rosette performance, and for oxygen isotopes (delO18) to assess meltwater content. The proposed sampling along and near the planned route onboard the Oden each year. Observations of the thermal and density structure along other portions of the Oden track will be made using XBTs and XCTDs. The number and spacing of stations will be adjusted according to the sea ice conditions and ocean features encountered.

  • Moored Times Series We acquire the first long-term (approximately two-year) records of current, temperature, salinity (conductivity) and pressure in the interior of the Sulzberger and Little America Troughs using three short bottom moorings. That record length should be adequate to resolve most temporal variability in ocean heat transport at scales from tidal to seasonal. One mooring is deployed off the Ross Ice Shelf, but actual deployment locations are influenced by the incoming CTD data stream. The moorings are furnished with two current meters paired with C/T/P recorders and two T/P recorders. Sensors are paired near the bottom and instruments are kept below 300 m to minimize the likelihood of mooring damage from icebergs. Time series are analyzed in combination with data from CTD/LADCP hydrographic stations to compute the baroclinic and total currents at the time of the cruises. Direct current measurements at the bottom are used to estimate barotropic velocity, the unknown constant of integration for the thermal wind component obtained from CTD measurements. The time series of currents and water properties provide the temporal framework needed to interpret information from the summer snapshots provided by the hydrographic observations.

Acknowledgements

This project is funded by the National Science Foundation, Office of Polar Programs.

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Deployment Cruise aboard Oden: February-March 2010

TAMU component report

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