Northward ocean heat transport and its variability influence the Arctic sea ice cover, contribute to surface warming or cooling, or simply warm or cool the Arctic Ocean interior. A simulation with the forced global ocean model NorESM20CR, aided by hydrographic observations since 1900, show large decadal fluctuations in the ocean heat transport, with the largest variations in the Atlantic sector. The simulated net poleward ocean heat transport over the last century is about 68 TW, and 88% of this occurs in the Barents Sea Opening (45 TW) and the Fram Strait (15 TW). Typical variations are 40 TW over time scales between 5 and 10 years, related to thermohaline and wind stress forcings. The mean heat transport in the Davis Strait is about 10 TW, and less than 5 TW flows north in the Bering Strait. The core temperature of the Atlantic Water (AW) entering the Arctic Ocean has increased in recent decades, consistent with an ongoing expansion of the Atlantic domain (Atlantification), but earlier warm events are also documented. The temperature of the northward‐flowing AW thus plays a vital role, with decadal variations of around 0.5 ∘C. The Nordic Seas atmosphere contributes with thermodynamic forcing, dampening the advected heat anomalies. In the Barents Sea, variations in the inflow volume flux dominate, while variations in temperature dominate the heat transport in the Fram Strait. There are significant trends over recent decades, also dominated by the Barents Sea that presently has 1 Sv higher volume transport and +1.0 ∘C warmer AW than the long‐term mean.

Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.