Abstract
We investigated liquid freshwater content (FWC) in the upper 100 m layer of the Arctic Ocean using oceanographic observations covering the period from 1990 through 2018. Our analysis revealed two opposite tendencies in freshwater balance—the freshening in the Canada Basin at the mean rate of 2.04 ± 0.64 m/decade and the salinization of the eastern Eurasian Basin (EB) at the rate of 0.96 ± 0.86 m/decade. In line with this, we found that the Arctic Ocean gained an additional 19,000 ± 1000 km3 of freshwater over the 1990–2018 period. FWC changes in the EB since 1990 demonstrate an intermittent pattern with the most rapid decrease (from ~5.5 to 3.8 m) having occurred between 2000 and 2005. The 1990–2018 FWC changes in the upper ocean were concurrent with prominent changes of the thermohaline properties of the intermediate Atlantic Water (AW)—the main source of salt and heat for the Arctic Basin. In the eastern EB, we found a 50 m rise of the upper AW boundary accompanied by a ~0.5 °C increase in the AW core temperature. The close relationship (R > 0.7 ± 0.2) between available potential energy in the layer above the AW and FWC in the eastern EB suggests a positive feedback mechanism that links the amount of freshwater with the intensity of vertical heat and salt exchange in the halocline and upper AW layers. Together with other mechanisms of Atlantification, this feedback creates a complex picture of interactions behind the observed changes in the hydrological and ice regimes of the Eurasian sector of the Arctic Ocean.
Highlights
The Arctic Ocean is one of the most significant freshwater reservoirs accumulating about 11% of the global continental runoff [1–3]
The only difference we found is the positive FWCMLD trend (0.22 ± 0.16 m/decade) in the western sector of the Eurasian Basin (EB) compared to the insignificant freshwater content (FWC) trend of −0.07 ± 0.25 m/decade within the upper 100 m layer
The trend estimated for the Canada Basin is persistent and reflects a general tendency towards freshening that has been typical for the Arctic Ocean over the last three decades
Summary
The Arctic Ocean is one of the most significant freshwater reservoirs accumulating about 11% of the global continental runoff [1–3]. Along with the continental runoff, the Arctic Ocean’s freshwater/salt balance is strongly influenced by inflows from the adjacent sectors of the Pacific and Atlantic Oceans [4–8]. One example illustrating the impact of freshwater on ocean stratification involves the increase in the melting of sea ice at the ocean surface that results in the suppressed mixing and reducing of ocean heat fluxes from the intermediate ocean layers to the upper ocean and the bottom of sea ice [16,17]. The loss of surface fresh layer provides favorable conditions for deeper winter ventilation and is associated with larger vertical heat fluxes (the “halocline catastrophe”), which may have irreversible consequences for the hydrological regime of some regions of the Arctic Ocean [15,17,18]
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