Operational sea ice charts: An integrated data product suitable for observing long-term changes in Arctic sea ice?

Passive microwave data has been the primary source for observing large-scale trends and variability in Arctic sea ice. This provides a consistent record of ice conditions since 1978. However, there are known deficiencies in the passive microwave data, including low spatial resolution that limits the precision of the ice edge location and an underestimation of summer ice area due to surface melt water. Operational ice analyses aim to provide the most accurate estimate of ice conditions using the best available information, including high spatial resolution satellite data. The manual analyses used to produce the charts provide consistent integration of the various sources as well as quality control of the final products. The National Snow and Ice Data Center, in collaboration with the U.S. National Ice Center (NIC), has updated the NIC ice chart climatology through 2004 and released hemispheric field in a format that is easy to access and analyze. This climatology provides a 30-year record of ice conditions and also yields information on different ice types (e.g., fast ice) that cannot be easily obtained from passive microwave data. However, the quality and quantity of data that is used to produce the ice charts has varied over time, leading to inconsistencies in the timeseries. For example, since 1995 the NIC charts have relied significantly on high resolution synthetic aperture radar imagery from Radarsat-1. This has resulted in higher area and extent estimates in the ice charts compared to earlier years that need to be accounted for if long-term trends are to be estimated from the ice chart climatology. On the other hand, ice charts from the Radarsat-1 era can provide a useful comparison for possible changes in the passive microwave data due to more extreme melt over the past ten years. Here we examine the new NIC ice chart climatology to investigate the consistency of the timeseries through statistical analysis and comparison with the passive microwave record.