Physical Properties of Summer Sea Ice in the Pacific Sector of the Arctic During 2008–2018

Abstract

Sea ice physical properties were determined at 21 first‐year ice (FYI) and 20 multiyear ice (MYI) stations in the Pacific sector of the Arctic during summer in 2008–2018. The bulk ice temperature was between −2.7 and −0.3 °C for FYI and between −1.7 and −0.2 °C for MYI. The bulk salinity was 0.4–3.2 practical salinity unit (psu) for FYI and 0.4–2.4 psu for MYI. A low‐salinity layer and an almost fresh layer were always present at the top of FYI and MYI, respectively. The bulk density was 600.3–900.1 kg/m ³ for FYI and 686.1–903.3 kg/m ³ for MYI. The upper layer density was less than the lower layer density due to higher gas content. Salinity and density were less than previously reported values of summer sea ice, and their parameterization formulae were updated. The brine and gas volume fractions were determined based on the measured ice temperature, salinity, and density. The average bulk brine and gas volume fractions were 11.3 ± 6.2% and 14.9 ± 7.9% for FYI and 9.5 ± 11.0% and 13.5 ± 12.7% for MYI. Typical brine and gas volume fraction profiles were identified and parameterized using regression analysis. The large brine and gas content of sea ice affects the sea ice melting. The energy used to melt a unit volume of summer Arctic sea ice was estimated as (2.26 ± 0.29) × 10 ⁵ kJ for FYI and (2.36 ± 0.24) × 10 ⁵ kJ for MYI. This study shows that the current physical state of the Arctic summer sea ice is different from earlier sea ice climatology, and the results will guide further research and modeling of Arctic sea ice.

Plain Language Summary

Arctic sea ice has undergone dramatic changes during the recent decades. Both ice thickness and extent have declined. However, it has remained unclear whether sea ice physical properties, such as temperature, salinity, and density, have changed in response to the Arctic warming. In this study, we present an analysis of measurements of sea ice physical properties in the late summer over the past 10 years in the Pacific sector of the Arctic Ocean. We found that the salinity and density of summer Arctic sea ice were lower than in earlier studies, and thus, their previous parameterizations need to be updated, and therefore, new parameterizations are proposed here. With the enhanced melt of the summer Arctic sea ice, the fractions of the sea ice phase components (pure ice, brine, and gas) have changed. The vertical profiles of the brine and gas volume fractions with depth are also quantified for future research and modeling efforts. The findings will help to improve the accuracy of existing sea ice models.

Key Points

• New parameterizations of salinity and density for late summer first‐year ice (FYI) and multiyear ice (MYI) are proposed

• Mean bulk brine and gas volume fractions are 11.3 ± 6.2% and 14.9 ± 7.9% for FYI and 9.5 ± 11.0% and 13.5 ± 12.7% for MYI in late summer

• The energy used to melt a unit volume of sea ice is (2.26 ± 0.29) × 10 ⁵ kJ for FYI and (2.36 ± 0.24) × 10 ⁵ kJ for MYI in late summer