THE QUANTIFICATION OF THE IMPORTANCE OF THE SEA ICE BUDGET IN THE CLIMATE SYSTEM - Final report

The overall objective is to quantify the importance of a better knowledge of sea ice budget on relevant climate processes such as freshwater budget, thermohaline circulation and deep water formation. The retrieval of the key parameter sea ice thickness from remote sensing techniques poses a major challenge compared with the observation of sea ice extent and ice concentration. The specific objectives are:
• To examine the sensitivity of state of the art dynamic-themodynamic sea ice models to changes in sea ice thickness.
• To derive observation requirements (e.g. requirements for accuracy, repeatability and temporal-spatial coverage) senarios for the estimation of sea ice thickness in the Arctic.
• Perform a sensitivity study of the importance of sea ice volume fluxes for freshwater budgets, thermohaline circulation and deep water formation in high latitude regions. Based on the results the observation requirements for ice thickness the analysis shall be updated and refined.
In order to meet these objectives five tasks have been conducted.

In Task 1 the most important data sets are ice area and concentration from satellite passive microwave data and ice velocity data from drifting buoys and satellite data. Retrieval of ice thickness data from satellite altimeter is discussed based on recent analysis of ERS data carried out at UCL. The results of the Sea Ice Model Intercomparison Project (SIMIP) are reivewed where the performance of different ice rheologies are assessed. Finally, satellite data assimilation in ice models is mentioned, reflecting that only limited work has been done in this field.

Task 2 used ice velocity data from the Arctic Ocean Buoy Programme and satellite data from NOAA  to compare with model simulations. Extensive model validation has been done for ice extent and concentration, where merged SMMR and SSMI data have been used to assess the basic model scenario. UCL has also made a comparison of ERS altimeter thickness with the model simulations at University of Kiel and the Hadley Centre showing promising results of the altimeter-derived thickness estimates. Parallel model experiments with perturbed freshwater discharge in to the Arctic Ocean showed that increased freshwater discharge leads to reduced sea surface salinity and increased sea surface temperature in the central Arctic, leading to increased melting of ice and intensified Beaufort Gyre, which in turn increases the volume transport through the Arctic Archipelago. To compensate for this southward transport of mass, more warm and saline Atlantic Water is carried northward, leading to an increased heat transport to the Nordic Seas. The increased transport of salt to the North Atlantic and the Nordic Seas thus counteract the impact of the increased freshwater runoff in the Arctic, and tends to stabilize the thermohaline circulation.

Task 3 is focused on developing a sea ice simulation model for a beam-limited altimeter planned to be used on CryoSat. Synthetic observations of sea ice have been generated using a number of mission scenarios combined with models describing the statistical and surface properties of sea ice that may affect the quality of the observations. Three key elements to the generation of an instrument surface model include: (1) generation of a realistic Floe Geometry Model for the two dimensional geometry of an ice field which describes the horizontal distribution of ice and water within the instrument field of view. (2) Superimposing realistic vertical topography based on a given mean snow and ice thickness. (3) Attribute backscatter characteristics to the ice and water surfaces within the footprint depending on snow cover and the state of surface melt. The simulations provided the first comprehensive and quantitative view of likely CryoSat performance under a variety of different ice conditions. The data sets generated could provide the basis for the development and optimization of more sophisticated processing systems required for CryoSat level 1.5 and 2  processing.