Abstract
AbstractA 15‐year (2004–2018) record of mooring observations from the upper 50 m of the ocean in the eastern Eurasian Basin reveals increased current speeds and vertical shear, associated with an increasing coupling between wind, ice, and the upper ocean over 2004–2018, particularly in summer. Substantial increases in current speeds and shears in the upper 50 m are dominated by a two times amplification of currents in the semidiurnal band, which includes tides and wind‐forced near‐inertial oscillations. For the first time the strengthened upper ocean currents and shear are observed to coincide with weakening stratification. This coupling links the Atlantic Water heat to the sea ice, a consequence of which would be reducing regional sea ice volume. These results point to a new positive feedback mechanism in which reduced sea ice extent facilitates more energetic inertial oscillations and associated upper‐ocean shear, thus leading to enhanced ventilation of the Atlantic Water.
Highlights
Throughout much of the Arctic Ocean layers of colder water isolates the sea surface from warm and salty water of Atlantic origin (Atlantic Water, AW), which is transported throughout the Arctic Ocean at intermediate depths (~150–900 m) as topographically steered boundary currents (e.g., Aagaard, 1989; Rudels et al, 1994)
Amplification of Upper‐Ocean Currents and Shear in the Eastern EB The original hourly ADCP records of total current speed (|U|) and shear (|Uz|) in the upper ~30–50 m layer are shown in supporting information Figure S1
Measurements of currents from a 15‐year duration mooring record in the eastern EB of the Arctic Ocean demonstrate that the previously identified weakening of stratification in the halocline (e.g., Polyakov et al, 2017, 2018) has been accompanied by increased upper‐ocean current speeds and associated current shear. Most of this increased energy and shear is in the semidiurnal band, which includes baroclinic tides and wind‐driven inertial oscillations, with little change of mean along‐slope water transport (Pnyushkov et al, 2018)
Summary
Throughout much of the Arctic Ocean layers of colder water isolates the sea surface from warm (temperature >0°C) and salty water of Atlantic origin (Atlantic Water, AW), which is transported throughout the Arctic Ocean at intermediate depths (~150–900 m) as topographically steered boundary currents (e.g., Aagaard, 1989; Rudels et al, 1994). The AW holds enough heat to melt Arctic sea ice several times over but is separated from the near‐freezing, and relatively fresh water in the Arctic Ocean surface mixed layer, by a cold halocline layer which has a negligible vertical temperature gradient but a large salinity gradient. The associated density gradient impedes vertical mixing of AW heat upward to the sea ice (e.g., Fer, 2009; Rudels et al, 1996). Double diffusion is driven by the different molecular diffusivities of heat and salt and is evident in vertical hydrographic profiles as multiple layers of near‐uniform temperature and salinity that are separated by strong‐gradient, thin POLYAKOV ET AL
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