Abstract
Abstract. The neXtSIM-F (neXtSIM forecast) forecasting system consists of a stand-alone sea ice model, neXtSIM (neXt-generation Sea Ice Model), forced by the TOPAZ ocean forecast and the ECMWF atmospheric forecast, combined with daily data assimilation of sea ice concentration. It uses the novel brittle Bingham–Maxwell (BBM) sea ice rheology, making it the first forecast based on a continuum model not to use the viscous–plastic (VP) rheology. It was tested in the Arctic for the time period November 2018–June 2020 and was found to perform well, although there are some shortcomings. Despite drift not being assimilated in our system, the sea ice drift is good throughout the year, being relatively unbiased, even for longer lead times like 5 d. The RMSE in speed and the total RMSE are also good for the first 3 or so days, although they both increase steadily with lead time. The thickness distribution is relatively good, although there are some regions that experience excessive thickening with negative implications for the summertime sea ice extent, particularly in the Greenland Sea. The neXtSIM-F forecasting system assimilates OSI SAF sea ice concentration products (both SSMIS and AMSR2) by modifying the initial conditions daily and adding a compensating heat flux to prevent removed ice growing back too quickly. The assimilation greatly improves the sea ice extent for the forecast duration.
Highlights
Arctic sea ice has been in great decline in the last number of years (Meier, 2017)
Despite drift not being assimilated in our system, the sea ice drift is good throughout the year, being relatively unbiased, even for longer lead times like 5 d
Surface air temperatures in the Arctic continued to warm at twice the rate relative to the rest of the globe, and Arctic air temperatures for the past 5 years (2014– 2018) have exceeded all previous records since 1900 (Overland et al, 2018), which will contribute to future sea ice decline if it continues
Summary
Arctic sea ice has been in great decline in the last number of years (Meier, 2017). Perovich et al (2018) report that in 2018, the summer extent was the sixth lowest and the winter extent was the second lowest in the satellite record (1979– 2018). NeXtSIM-F entered into operations as part of the Copernicus Marine Environment Monitoring Services (CMEMS) on 7 July 2020 It was equipped with neXtSIM v1.0 based on the Maxwell elasto-brittle (MEB) rheology (Dansereau et al, 2016), which had been shown to reproduce Arctic sea ice drift and deformation well (Rampal et al, 2016, 2019). In September 2020 the core of the forecasting platform was replaced with a new model: neXtSIM v2.0 based on a preliminary version of the novel brittle Bingham–Maxwell (BBM) sea ice rheology (Ólason et al, 2021). This newer version of neXtSIM-F entered into operations in December 2020. We evaluate the neXtSIM-F forecast platform for the same period, when we assimilate concentration but use forecast forcing fields
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