Key points at a glance:
The Arctic Ocean plays a key role in Earth’s climate because it is the northernmost part of the Atlantic Meridional Overturning Circulation (AMOC). The Arctic Ocean is almost enclosed, with only a few openings (i.e. straits) where seawater can flow in and out. Warm, salty water from the Atlantic enters mainly through the Barents Sea Opening and Fram Strait. Once inside the Arctic Ocean, this water cools down, mixes with fresher water, and interacts with sea ice, which changes its temperature and salt content. On the Pacific side, fresher water flows in through the Bering Strait and in addition, a lot of riverine water from the large Siberian and North American rivers provide freshwater to the Arctic Ocean. Fresh and colder seawater as well as sea ice leaves the Arctic mainly through the East Greenland Current and Davis Strait.
In the four main Arctic gateways – Fram Strait, Barents Sea Opening, Bering Strait and Davis Strait – arrays of moored instruments that measure physical properties such as ocean velocity, temperature and salinity have been in place since the late 1990s or early 2000s (see map, right). Temperature and salinity are also measured by research vessels when they collect hydrographic sections (lines of measurements taken from the sea surface down to the seafloor, showing how ocean properties change with depth and distance).
The EPOC team interpolated the ocean observations onto a grid in each gateway and ran these through an inverse model. This is a method to slightly adjust the observed velocities in order to obtain a balance between the volume of water flowing into and out of the Arctic.
From these adjusted fields, net heat and freshwater transport through the Arctic gateways were derived. Earlier studies looked at short periods from one to five years, but now we have output from 2004 to 2022. This long record allows us to determine seasonal changes, year-to-year differences, and even long-term trends.
From this long-term record, we find that the Arctic Ocean transports on average about 162 terawatts of heat in and 168 millisverdrups of freshwater out through its main gateways (see figure below). Over the period from 2004 to 2022, both heat and freshwater transports show significant downward trends, with heat decreasing by about 1.3 terawatts per year and freshwater by about 2.2 millisverdrups per year. Seasonal variations are also clear: heat transport is highest in November and lowest in May, with an average seasonal amplitude of about 102 terawatts, while freshwater transport peaks in March and reaches its minimum in June, varying by about 164 millisverdrups. Most of this exchange is driven by ocean currents, but sea ice also contributes, especially in winter, with its own seasonal cycle peaking in March and dipping in August.
EPOC Deliverable 1.2 Heat and freshwater transport variability through Arctic gateways (2004-2020). Report by Hege-Beate Fredriksen & Laura de Steur, October 2025.
The full report will be available for download when the EC review process is complete.