Observing dynamical and thermodynamical Processes involved in The sea Ice Mass balance from In Situ Measurements. – OPTIMISM

The most conspicuous manifestations of the ongoing climate warming are found in the Arctic. Changes in this region outpace the most pessimistic climate model predictions. Summer sea ice extent has shrunk very substantially (ice cover 40% smaller in 2007 than the 1980-2000 median). Multiyear ice declines at an accelerated pace: sea ice aged 5 years or more has declined by 60% since the 80's, while the oldest sea ice has nearly entirely disappeared. Sea ice is not only an indicator of the ongoing global warming but is also a major actor of the climate machinery. It bounces back to space 80% of the incoming solar radiation, hence the consequences on the Earth's global radiative budget of a vanishing ice cover (albedo feedback). It is also a poor thermal conductor which efficiently limits heat transfers from the ocean to the atmosphere. A removal of this insulator would contribute to the warming of the polar troposphere with potentially severe consequences: melting of the snow covering the neighboring continents, thawing of the permafrost inducing methane outgassing, accelerated melting of the Greenland Ice shelf (contributing to sea level rise). Deep waters, which ventilate the ocean interior, are formed in the periphery of the Arctic. These convection processes fuel the global thermohaline circulation, an important component of the Earth's climate system. The release by ice melting of much fresher, lighter water, capping the ocean could inhibit deep water formation. Given the consequences of a melting of Arctic sea ice for the evolution of climate, there is a great need to enhance our ability to monitor polar oceans not only to assess the magnitude of ongoing changes, but also to improve our understanding and representation of physical processes that govern heat exchanges between the ocean, the sea ice, and the atmosphere, in order to improve the predictive capabilities of climate models. Reaching a correct representation of the sea ice mass balance is particularly complex as it requires a correct representation of thermodynamics and thermal properties of sea ice (in particular its brine content), of thermal exchanges between the ocean mixed-layer and the ice (for which internal high frequency plays a key role), as well as turbulent and radiative heat fluxes at the surface, which are particularly spatially heterogeneous, and poorly known in some cases (eg albedo of thin ice). There is a real need to document and analyse such processes from observations. While sea ice extent is now routinely monitored from space, remote sensing of ice-thickness is still in its early stage. Dedicated missions have or will be launched (Ice2at 2003, Cryosat-2, 2009) but in-situ time series are needed to develop and validate algorithms necessary to relate raw measurements to geophysical parameters (current ice-thickness estimates with IceSat have an accuracy of ~70cm). Automated in-situ ice thickness measurements, but also measurements of snow load and heat flux at the interfaces rank high on the list of priorities for both Arctic and Southern Ocean Observing Systems. Polar surface instruments face new constraints with the decline of perennial ice, and it becomes critical to develop floating devices adapted to seasonal ice. A main objective of the current OPTIMISM project is to develop an automated, reasonable coast, system providing real-time measurement not only of ice thickness and fluxes in the ice, but also of heat and freshwater fluxes at the ocean-ice-atmosphere interfaces, which are needed to assess the sea ice mass balance. This challenging technological development will build upon the Ice-T prototype, floating instrument, adapted to both thin and thick ice conditions. The instrument will be deployed for process-oriented studies, motivated or supplemented with numerical modelling works. These works, devoted to the study of coastal polynia of the Arctic, will focus on the role of ocean mixing and surface energy budget on sea ice evolution and dense water formation. Studies will also focus on wave-ice interactions. These processes will also be studied in the multiyear ice pack, where deployments will also aim at demonstrate the interest of the instrument in the frame of an observational network of polar regions, providing as well a validations platform for satellite-derived sea ice thickness estimates. Snow layer thickness measurements from the instrument will also be used to validate satellite-derived estimates of this parameter based on the analysis of the Ku-band radar echo from CryoSat-2.