Arctic Sea Ice Variability in the 20th and Early 21st Centuries: Synthesis of Observations and Models

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Much of the uncertainty in the pre-satellite sea ice record stems from the difficulty establishing a consistent record of sea ice variability that covers the entire Arctic from historical observations such as ship logs, reconnaissance flights, etc. These observations are scattered in space and time and the primary challenge in constructing a representative picture of sea ice variability from these records is to adequately sample the strong seasonal cycle and high spatial variability of sea ice in the Arctic. Sea ice reconstructions must therefore rely on interpolation in time and space, which is made difficult by the dynamic nature of sea ice. For some critically important climate variables, such as sea ice thickness and total arctic sea ice volume, no historical data exist. The PIs will conduct a Century-scale Arctic Sea Ice Reanalysis (CASIR) that synthesizes observations and models from 1872 to the present to produce a long-term record of arctic sea ice extent, concentration, thickness, and volume. Advances in atmospheric reanalysis, sea ice modeling, and recovery of historical data make CASIR possible. To construct CASIR, they will recover and collect new and existing historical sea ice observations as far back as the late 1800s and integrate them into a model validation and assimilation framework based on the Pan-arctic Ice-Ocean Modeling and Assimilation System (PIOMAS). This will include the extraction of sea ice variables from historical Revenue Service/Coast Guard ship logs that are now being transcribed and promise to fill important gaps in the Pacific sector of the Arctic. Through data assimilation and model calibration and validation, the observational record will be integrated into PIOMAS to generate CASIR. An ensemble of atmospheric forcing data for CASIR will be extracted from atmospheric reanalysis projects for the 20th century that are completed or currently underway. This ensemble, in addition to comparisons with observations, will be used to determine uncertainties in CASIR sea ice variables. The PIs will analyze CASIR ice extent, concentration, thickness, and volume, to characterize 20th century temporal and spatial variability, improve understanding of the mechanisms driving this variability, and place the recent sea ice decline in a longer term context.