Ocean-to-Ice Heat Flux in the Central Arctic: Results from the MOSAiC Expedition (2019-2020)

 The Arctic is a hot spot of climate change. Sea ice and snow, in particular, act as an insulator that prevent heat exchange between the ocean and the atmosphere and have been an important factor in mitigating temperature increases in the Arctic. However, the reduction of sea ice over the past 40 years has led to an increase in ocean-atmosphere heat exchange, contributing to Arctic Amplification. Despite its importance, obtaining observational data beneath sea ice in the Arctic during winter has been challenging due to the unique conditions of ice coverage, especially in winter. Several studies have been able to make use of recent advances in autonomous instrumentation to calculate wintertime ocean to ice heat flux (OHF). However, there remain considerable discrepancies in OHF estimates, even when examining the same time periods and research areas, primarily due to variations in calculation methods.

 In this study, we used observational data from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) to calculate OHF from October 2019 to May 2020. The observations were made by Woods Hole Oceanographic Institution Ice-tethered Profilers and Microstructure profilers drifting with sea ice along the Transpolar Drift. Here, we assess the applicability of an OHF parameterization from observational data, relying on the temperature difference between the mixed layer and the freezing temperature.

 The results in winter predominantly show negative (downward) OHF. We consider those results physically implausible, and they seem to be related to the ubiquitous presence of supercooled water in the mixed layer. When applying near surface temperature rather than freezing temperature to assess the heat content in the boundary layer, the wintertime OHF values are closed to zero until mid-March 2020. This result is in line with direct (dissipation based) measurements of OHF from the stratified ocean into the mixed layer during the same period. This study, therefore, suggests limitations in the applicability of the OHF parameterization in supercooled conditions. By opting for ocean surface temperature observations from the Arctic winter of 2019-2020, which were consistently lower than the freezing temperature, we anticipate that these refined calculation methods will yield more accurate results for assessing heat flux in future Arctic winters.

 From mid-March to early May, the OHF increased significantly, and so did the upward heat flux into the mixed layer. Our results suggest this shift occurred once the sea ice had drifted southward across the Gakkel Ridge toward Fram Strait. Analyzing the hydrographic properties of the upper ocean, we conclude that not only seasonal but also regional changes contributed to this shift.