Increasing influence of warm and salty Atlantic water on the cold season Arctic sea ice melting

The Arctic Ocean in the upper 100 – 200 m is typically characterized by a cold and fresh surface mixed layer and a layer of rapidly increasing salinity with depth, as known as halocline, separating the surface mixed layer from the warm and salty Atlantic water at depth. Due to large vertical density gradient and static stability of the halocline water, the surface mixed layer is mostly isolated from the influence of warm and salty intermediate-depth water originating from the North Atlantic Ocean. A new study published in the Journal of Climate analyzed mooring date in the eastern Eurasian Basin (77 – 80oN and 125 – 142oE) during 2003-2018 to report a gradual weakening of the halocline and a shoaling of the warm Atlantic water. Consistently, further analysis of the mooring data showed that the upward oceanic heat flux across the mixed layer base in the winter season (65 ~ 150 m) substantially increased from an average of 3 – 4 Wm-2 in 2007–08 to >10 Wm-2 in 2016–18. The study suggests that the increasing speed of the wind-driven upper ocean current (due to decreasing sea-ice and increasing exposure of surface ocean to wind) and associated shear-driven mixing at depth are the main causes of the increasing upward oceanic heat flux. An important implication of this report is that Arctic sea-ice in the winter season is increasingly melting from below due to the increasing upward heat release of the warm Atlantic water, potentially contributing the winter-time Arctic amplification of the lower atmospheric warming.

Figure 11 from Polyakov et al. (2020). Conceptual model of shift of the mixing regime in the eastern Eurasian Basin in recent years and associated suite of processes and state conditions including 1) thinner, more mobile ice, 2) warmer surface mixed layer (SML), 3) weakening and retreat of cold halocline (HC) layer, 4) increased Atlantic water vertical heat flux (red arrows) and horizontal currents and their vertical shear (blue arrows), 5) shoaling of upper Atlantic water boundary, and 6) replacement of double diffusion by shear instabilities as the fundamental mechanism of vertical flux.

Polyakov, I. V., and Coauthors, 2020: Weakening of cold halocline layer exposes sea ice to oceanic heat in the Eastern Arctic Ocean. J. Climate33, 8107–8123, https://doi.org/10.1175/JCLI-D-19-0976.1.

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