Atlantic Water Heat Transport Variability in the 20th Century Arctic Ocean From a Global Ocean Model and Observations

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 68TW, and 88% of this occurs in the Barents Sea Opening (45TW) and the Fram Strait (15TW). Typical variations are 40TW over time scales between 5 and 10years, related to thermohaline and wind stress forcings. The mean heat transport in the Davis Strait is about 10TW, and less than 5TW 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 degrees 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 1Sv higher volume transport and +1.0 degrees C warmer AW than the long-term mean. Plain Language Summary Ocean currents bring heat northward into the Arctic Ocean. This ocean heat influences the sea ice cover regionally and warms the Arctic Ocean surface and interior. Warm and salty Atlantic Water that enters the Arctic Ocean through the Fram Strait and the Barents Sea dominates the oceanic heat contribution to the Arctic Ocean in the 20th century. In this study, we have investigated how this ocean heat transport has varied over time and which physical mechanisms that influence these variations. Understanding these natural variations is important for explaining ongoing Arctic change today. Our main tool for investigation is a global ocean model driven by observed atmospheric variations, and we compare this model with observations of temperature and salinity in the Nordic Seas. We find that the ocean heat transport has varied on different time scales and that these variations are linked to changes in the wind systems in the Nordic Seas, atmosphere-ocean heat exchanges, and, to some extent, local winds in the straits. Overall, the ocean heat transports and Atlantic Water temperatures have increased during recent decades. These changes are especially large at the Barents Sea entrance where the water has warmed about 1 degrees C.