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

Antarctic sea ice: 19 million km 2 in winter

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

The Southern Ocean is warming … Böning et al., Nature Geoscience, m 1800 m 35  S60  S Temp trend (  C/decade)

… and freshening Böning et al., Nature Geoscience, 2008 salinity trend (psu/decade) 200 m 1800 m 35  S60  S

Warming of Antarctic Bottom Water Purkey and Johnson, 2010

Large regional changes in Antarctic sea ice Stammerjohn et al. (2008) Changes in sea ice duration: 1979 –  23 days 57  13 days

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

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.

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

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.

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.………

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.

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.

A strawman strategy for an integrated under- ice observing system ‘vanilla’ Argo ice Argo Tracked floats ITP 2000 m deep Argo hydrography moorings glider moorings 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 Diagram under development …

Broad-scale sampling in the upper 2000 m

PlatformSampling requirements Ice-capable Argo in water depths greater than 2000 m Minimum requirement is consistency with global Argo design of 1 profile per 3 x 3° square every 10 days. Acoustically-tracked Argo in Weddell and Ross gyres Array of ~8 sound sources and maintain 50 floats in each gyre Seal tagsMaintain or enhance MEOP sampling Hydrographic sections Occupy GO-SHIP full-depth repeat hydrography lines. Add additional short meridional transects crossing the Antarctic slope and shelf where feasible (e.g. near Antarctic bases) Satellite altimetryMaintain JASON sampling; validate use of altimeter in ice-covered seas in Antarctica

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.

Deep ocean PlatformSampling requirements Hydrographic sections Occupy GO-SHIP full-depth repeat hydrography lines, with tracers. Add additional short meridional transects crossing the Antarctic slope and shelf where feasible (e.g. near Antarctic bases). Deep Argo Pilot deployments underway now. When proven, need broad- scale deployments to sample deep ocean. Sampling requirements not yet quantified. MooringsDeployed in key locations, including dense overflows and boundary currents to measure temperature, salinity, velocity and bottom pressure. Development of long endurance moorings with data telemetry is needed to allow broader deployment.

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.

Continental shelf and slope PlatformSampling requirements Ice-capable profiling floats, adapted for use on shelf Floats may ground between profiles, include active bottom-avoidance, or be tethered. Ice-tethered profilers Most cost-effective in multi-year or fast ice given short lifetime of most Antarctic sea ice. Seal tags Maintain or enhance MEOP sampling. Coverage of the shelf optimised by deployments in Antarctica, including shelf-resident species (Weddell seals). Hydrographic sections Only platform capable of collecting full suite of physical, biogeochemical and biological variables. Gliders Only platform capable of frequent, high resolution transects on the shelf and slope. MooringsDeployed in key locations (e.g. polynyas, dense overflows).

Ice shelf cavities

PlatformSampling requirements Unmanned submarinesOnly proven technology for transects in ice shelf cavity Sensors deployed through boreholes Provide time series of sub-ice shelf properties and circulation. Both traditional oceanographic sensors and DTS from fibre optic cables Exploit “boreholes of opportunity”. Moorings deployed by submarine Not yet a proven technology. Ship and glider transects & moorings across the ice front Needed to measure ocean heat flux to ice shelf cavity. Year-round sampling needed. May require acoustic navigation under sea ice (and under ice shelf?) Phase sensitive radar on ice shelves and glacier tongues Provide direct measurements of basal melt. Acoustic tomographyPotential to resolve time series of circulation and temperature within the full ice shelf cavity. Use acoustics for multiple purposes (navigation, tomography)?

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

Sea ice and atmosphere PlatformSampling requirements Sea ice mass balance buoys Most to be gained by combining these top 4 platforms into an integrated ice-ocean-atmosphere observing platform. Ice-tethered profilers Air-sea flux stations Turbulence sensors at ice- ocean interface Ice thickness sonars on floats, moorings and gliders/submarines Ice stations Process studies with simultaneous measurements of ocean, ice and atmosphere. Ship-based observations Visual observations of sea ice characteristics while underway, including automated approaches (e.g. Ice-cam). Air-borne observations Measurements of ice and snow thickness (e.g. EM, lidar), sea ice concentration. Airplanes, helicopters, UAVs. Remote sensingIn situ observations essential for validation and calibration.

Air-sea fluxes PlatformSampling requirements Meteorological sensors on ships As per SAMOS Direct flux measurements Needed to improve parameterisations of air-sea fluxes from met measurements. Direct flux measurements can be made from ships, aircraft and UAVs. Automatic weather stations Expand array of AWS on coastline and islands Remote sensingDedicated air-sea flux missions Antarctic reanalysisAssimilation of in situ and remotely sensed observations in a regional, high resolution Antarctic reanalysis is needed.

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).