Sea-ice cover timing in the Pacific Arctic: The present and projections to mid-century by selected CMIP5 models

摘要

With the sea-ice cover in the Arctic fast declining, changes to the timing of sea-ice break-up and freeze-up is an urgent economic, social, and scientific concern. Based on daily sea-ice concentration data we assess three variables: the dates of sea-ice break-up and freeze-up, and the annual sea-ice duration in the Pacific Arctic. The simulation results from the coupled Atmosphere-Ocean General Circulation Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are the source for this study. Compared with observations, CMIP5 models simulate all three variables well. The length of sea-ice duration is shrinking, with the strongest trend occurring for the period 1990–2014; this downward trend is projected to continue at least through mid-century by the CMIP5 models. Comparisons made at eight Chukchi Sea mooring sites and eight Distributed Biological Observatory (DBO) regions show consistent results. The 30-year averaged trend for annual sea-ice duration in the southern Chukchi Sea is projected to be −0.68 (−0.74) days/year to −1.20 (−1.17) days/year for 2015–2044 under RCP8.5 (RCP4.5) emissions scenarios. This is equivalent to a reduction of 20–36 days in the annual sea-ice duration. A similar negative trend is also found at all eight DBO regions. The reduction in annual sea-ice duration will include both earlier break-up dates and later freeze-up dates. However, models project that a later freeze-up contributes more than earlier break-up to the overall shortening of annual sea-ice duration. Around the Bering Strait area, future changes are the smallest, with less than 20 days change in duration during the next 30 years. In contrast, up to a 60-day reduction of the sea-ice duration in the East Siberian, Chukchi and Beaufort Seas is projected near the middle of the 21st century, when averaged over the period of 2030–2044.