Large freshwater anomalies clearly exist in the Arctic Ocean. For example, liquid freshwater has accumulated in the Beaufort Gyre in the decade of the 2000s compared to 1980-2000, with an extra ≈ 5000 km 3-about 25%-being stored. The sources of freshwater to the Arctic from precipitation and runoff have increased between these periods (most of the evidence comes from models).

Fresh water from summer ice melt and the total freshwater content of the Arctic Ocean water column above the thermocline are estimated from vertical profiles of temperature and salinity observed on the I/B Oden 1991 cruise. The seasonal ice melt ranges from 0.5 m to slightly above 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater content is calculated relative to the salinity just above the thermocline north of the Barents Sea. The freshwater content increases toward the interior of the Arctic Ocean, showing that fresh water is advected from other regions into the observation area. Regions of different freshwater content are separated by fronts over the Nansen‐Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between waters derived from the Eurasian and Canadian Basins. Denser water, homogenized north of the Barents Sea, is recognized by a temperature minimum layer. The absence of the temperature minimum near the Nansen‐Gakkel Ridge indicates that heat is transferred from the Atlantic Layer over a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic Layer, i.e., toward the east, and returns above the Nansen‐Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water formed north of the Barents Sea becomes covered by low‐salinity shelf water. The increased freshwater content limits the winter convection, so it no longer reaches the thermocline and an intermediate halocline is formed. The halocline in the Eurasian Basin consists of water originating from winter convection in the Arctic Ocean north of the Barents Sea, which then circulates around the basin. Such a formation mechanism also explains the observed distribution of low NO water. The strong density increase limits vertical exchange, and the vertical diffusion coefficient in the halocline is small (∼1 × 10−6 m2 s−1). The increased temperature of the halocline shows that the heat lost upward by the Atlantic Layer, mainly by double‐diffusive convection, is trapped below the mixed layer.

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Deep-water formation in the northern North Atlantic Ocean and the Arctic Ocean is a key driver of the global thermohaline circulation and hence also of global climate. Deciphering the history of the circulation regime in the Arctic Ocean has long been prevented by the lack of data from cores of Cenozoic sediments from the Arctic's deep-sea floor. Similarly, the timing of the opening of a connection between the northern North Atlantic and the Arctic Ocean, permitting deep-water exchange, has been poorly constrained. This situation changed when the first drill cores were recovered from the central Arctic Ocean. Here we use these cores to show that the transition from poorly oxygenated to fully oxygenated ('ventilated') conditions in the Arctic Ocean occurred during the later part of early Miocene times. We attribute this pronounced change in ventilation regime to the opening of the Fram Strait. A palaeo-geographic and palaeo-bathymetric reconstruction of the Arctic Ocean, together with a physical oceanographic analysis of the evolving strait and sill conditions in the Fram Strait, suggests that the Arctic Ocean went from an oxygen-poor 'lake stage', to a transitional 'estuarine sea' phase with variable ventilation, and finally to the fully ventilated 'ocean' phase 17.5 Myr ago. The timing of this palaeo-oceanographic change coincides with the onset of the middle Miocene climatic optimum, although it remains unclear if there is a causal relationship between these two events.