Seeing under the ice: a strategy for observing the Southern Ocean beneath sea ice and ice shelves

Transcription

1 Seeing under the ice: a strategy for observing the Southern Ocean beneath sea ice and ice shelves Steve Rintoul CSIRO Marine and Atmospheric Research Wealth from Oceans National Research Flagship Antarctic Climate and Ecosystems CRC Hobart, Tasmania, Australia

2 Antarctic sea ice: 19 million km 2 in winter

3 Southern Ocean overturning connects the upper and lower limb of global overturning Rintoul, 2001

4 The Southern Ocean is warming 200 m Temp trend ( C/decade) 1800 m 60 S 35 S Böning et al., Nature Geoscience, 2008

5 and freshening 200 m 1800 m salinity trend (psu/decade) 60 S 35 S Böning et al., Nature Geoscience, 2008

6 Warming of Antarctic Bottom Water Purkey and Johnson, 2010

7 Large regional changes in Antarctic sea ice Changes in sea ice duration: ± 23 days 57 ± 13 days Stammerjohn et al. (2008)

8 Antarctic ice-sheet loss driven by basal melting of ice shelves the most profound contemporary changes to the ice sheet and its contribution to sea level can be attributed to ocean thermal forcing Pritchard et al. 2012

9 Sea ice zone remains almost unobserved Southern Ocean Data Base: 1400 CTD stations south of 60S in Southern Ocean database in winter (May Oct). Only 330 stations outside of western Antarctic peninsula and 0 E.

10 A strategy for observing under Antarctic sea ice and ice shelves Structure of report: Background and motivation Circulation and inventory of heat, FW and carbon Ocean sea ice interaction Ocean ice shelf interaction Objectives and Key questions Integrated strategy for under-ice observing Summary of recommendations

11 Circulation and inventory of heat, FW and carbon Objectives: 1. To quantify how much heat, freshwater and carbon are stored by the ocean between the winter sea ice edge and the Antarctic continent. 2. To understand the processes responsible for ocean storage of heat, freshwater and carbon and their sensitivity to changes in forcing.

12 Circulation and inventory of heat, FW and carbon Key science questions: 1. What is the time-evolving inventory of ocean heat and freshwater content between the winter ice edge and the Antarctic continent? 2. How do Antarctic and Southern Ocean processes influence the distribution of sea level rise? 3. How much heat, freshwater and momentum is exchanged between the ocean and atmosphere in the sea ice zone and how do air-sea fluxes vary in space and time? 4. What are the key physical processes regulating exchange between the open ocean and the continental shelf? 5. What processes set the stratification of the upper ocean and its response to changes in forcing? 6. What are the relative contributions of air-sea fluxes, sea ice formation and melt, and mixing in driving water mass transformations in the sea ice zone? 7. What is the strength of the overturning circulation in the sea ice zone and how and why does it vary in time? 8. Where and how is Antarctic Bottom Water formed? 9.

13 Ocean sea ice interaction Objectives: 1. To determine the processes controlling the circumpolar and regional distribution of sea ice concentration and thickness. 2. To determine how and why the concentration and thickness of Antarctic sea ice varies over time-scales from days to millennia. 3. To understand and quantify coupled interactions between Antarctic sea ice, the ocean, the atmosphere, and ice shelves.

14 Ocean ice shelf interaction Objectives: 1. To determine the sensitivity of Antarctic ice shelves to changes in ocean circulation and temperature. 2. To assess the affect of basal melt of floating ice shelves on the mass balance of the Antarctic ice sheet and its contribution to sea level rise. 3. To determine the response of the ocean to changes in the freshwater input by the Antarctic ice sheet.

15 A strawman strategy for an integrated underice observing system 5 Diagram under development. 4 shelf Argo Five domains in the sea ice zone, each with own sampling needs/opportunities: 1. Open ocean above 2000 m 2. Deep ocean 3. Continental shelf and slope 4. Ice shelf cavity 5. Sea ice and atmosphere 3 glider moorings 1 ice Argo Tracked floats ITP 2 vanilla Argo deep Argo hydrography moorings 2000 m

16 Broad-scale sampling in the upper 2000 m

17 Broad-scale sampling in the upper 2000 m Pla$orm Ice- capable Argo in water depths greater than 2000 m AcousDcally- tracked Argo in Weddell and Ross gyres Seal tags Hydrographic secdons Satellite aldmetry Sampling requirements Minimum requirement is consistency with global Argo design of 1 profile per 3 x 3 square every 10 days. Array of ~8 sound sources and maintain 50 floats in each gyre Maintain or enhance MEOP sampling Occupy GO- SHIP full- depth repeat hydrography lines. Add addidonal short meridional transects crossing the AntarcDc slope and shelf where feasible (e.g. near AntarcDc bases) Maintain JASON sampling; validate use of aldmeter in ice- covered seas in AntarcDca

18 Deep ocean Repeat hydrographic sections will be the backbone of the deep ocean observing system. Full-depth repeats, with full tracers and ADCP, are needed.

19 Deep ocean Pla$orm Hydrographic secdons Deep Argo Moorings Sampling requirements Occupy GO- SHIP full- depth repeat hydrography lines, with tracers. Add addidonal short meridional transects crossing the AntarcDc slope and shelf where feasible (e.g. near AntarcDc bases). Pilot deployments underway now. When proven, need broad- scale deployments to sample deep ocean. Sampling requirements not yet quandfied. Deployed in key locadons, including dense overflows and boundary currents to measure temperature, salinity, velocity and boyom pressure. Development of long endurance moorings with data telemetry is needed to allow broader deployment.

20 Continental shelf and slope

21 Continental shelf and slope Sections (Iines) and moorings (circles) completed during the SASSI IPY program. Sustained occupations of these sections and arrays would make a substantial contribution to an under-ice observing system.

22 Continental shelf and slope Pla$orm Sampling requirements Ice- capable profiling Floats may ground between profiles, include acdve floats, adapted for boyom- avoidance, or be tethered. use on shelf Ice- tethered profilers Most cost- effecdve in muld- year or fast ice given short lifedme of most AntarcDc sea ice. Seal tags Maintain or enhance MEOP sampling. Coverage of the shelf opdmised by deployments in AntarcDca, including shelf- resident species (Weddell seals). Hydrographic Only pla]orm capable of collecdng full suite of physical, secdons biogeochemical and biological variables. Gliders Only pla]orm capable of frequent, high resoludon transects on the shelf and slope. Moorings Deployed in key locadons (e.g. polynyas, dense overflows).

23 Ice shelf cavities

24 Ice shelf cavities

25 Ice shelf cavities Pla$orm Unmanned submarines Sensors deployed through boreholes Moorings deployed by submarine Ship and glider transects & moorings across the ice front Phase sensidve radar on ice shelves and glacier tongues AcousDc tomography Sampling requirements Only proven technology for transects in ice shelf cavity Provide Dme series of sub- ice shelf properdes and circuladon. Both tradidonal oceanographic sensors and DTS from fibre opdc cables Exploit boreholes of opportunity. Not yet a proven technology. Needed to measure ocean heat flux to ice shelf cavity. Year- round sampling needed. May require acousdc navigadon under sea ice (and under ice shelf?) Provide direct measurements of basal melt. PotenDal to resolve Dme series of circuladon and temperature within the full ice shelf cavity. Use acousdcs for muldple purposes (navigadon, tomography)?

26 Sea ice and atmosphere Arctic example, J. C. Gascard

27 Sea ice and atmosphere Pla$orm Sea ice mass balance buoys Ice- tethered profilers Air- sea flux stadons Turbulence sensors at ice- ocean interface Ice thickness sonars on floats, moorings and gliders/submarines Ice stadons Ship- based observadons Air- borne observadons Remote sensing Sampling requirements Most to be gained by combining these top 4 pla]orms into an integrated ice- ocean- atmosphere observing pla]orm. Process studies with simultaneous measurements of ocean, ice and atmosphere. Visual observadons of sea ice characterisdcs while underway, including automated approaches (e.g. Ice- cam). Measurements of ice and snow thickness (e.g. EM, lidar), sea ice concentradon. Airplanes, helicopters, UAVs. In situ observadons essendal for validadon and calibradon.

28 Air-sea fluxes Pla$orm Meteorological sensors on ships Direct flux measurements AutomaDc weather stadons Remote sensing AntarcDc reanalysis Sampling requirements As per SAMOS Needed to improve parameterisadons of air- sea fluxes from met measurements. Direct flux measurements can be made from ships, aircrai and UAVs. Expand array of AWS on coastline and islands Dedicated air- sea flux missions AssimilaDon of in situ and remotely sensed observadons in a regional, high resoludon AntarcDc reanalysis is needed.

29 Next steps Steve apologises for taking so long to get a draft of the report out. Feedback welcome on the approach taken. How can we most effectively catalyse an enhanced observing system in the Antarctic sea ice zone? It might be useful for the SOOS committee to compile a list/map of recent and planned advances in under-ice observing (to provide evidence of progress, feasibility and strong community interest).