Competing Effects of Elevated Vertical Mixing and Increased Freshwater Input on the Stratification and Sea Ice Cover in a Changing Arctic Ocean

Peter E D Davis,Camille Lique,Helen L Johnson,John D Guthrie

doi:10.1175/jpo-d-15-0174.1

Peter E D Davis, Camille Lique + Show 2 more

Open Access

https://doi.org/10.1175/jpo-d-15-0174.1

Copy DOI

Abstract

AbstractThe Arctic Ocean is undergoing a period of rapid transition. Freshwater input is projected to increase, and the decline in Arctic sea ice is likely to drive periodic increases in vertical mixing during ice-free periods. Here, a 1D model of the Arctic Ocean is used to explore how these competing processes will affect the stratification, the stability of the cold halocline, and the sea ice cover at the surface. Initially, stronger shear leads to elevated vertical mixing that causes the mixed layer to warm. The change in temperature, however, is too small to affect the sea ice cover. Most importantly, in the Eurasian Basin, the elevated shear also deepens the halocline and strengthens the stratification over the Atlantic Water thermocline, reducing the vertical heat flux. After about a decade this effect dominates, and the mixed layer begins to cool. The sea ice cover can only be significantly affected if the elevated mixing is sufficient to erode the stratification barrier associated with the cold halocline. While freshwater generally dominates in the Canadian Basin (further isolating the mixed layer from the Atlantic Water layer), in the Eurasian Basin elevated shear reduces the strength of the stratification barrier, potentially allowing Atlantic Water heat to be directly entrained into the mixed layer during episodic mixing events. Therefore, although most sea ice retreat to date has occurred in the Canadian Basin, the results here suggest that, in future decades, elevated vertical mixing may play a more significant role in sea ice melt in the Eurasian Basin.

Highlights

The vertical structure in the Eurasian Basin of the Arctic Ocean is characterized by a cold and fresh surface mixed layer overlying a deeper warm (T . 08C) and salty Atlantic Water layer (Fig. 1)
Arctic sea ice is sensitive to changes in the diffusive heat flux and the extent to which the mixed layer is isolated from the heat contained within the Atlantic Water layer
We have chosen to show the results from only the longest ice-free period, as the mechanisms that control mixed layer temperatures are independent of the length of the ice-free period and are applicable to the remainder of the parameter space

Summary

Introduction

The vertical structure in the Eurasian Basin of the Arctic Ocean is characterized by a cold and fresh surface mixed layer overlying a deeper warm (T . 08C) and salty Atlantic Water layer (Fig. 1). 08C) and salty Atlantic Water layer (Fig. 1). The heat contained within the Atlantic Water layer is sufficient to melt all sea ice in the Arctic within a few years (Turner 2010). This heat is isolated from the mixed layer by the cold halocline, which is characterized by the coincidence of near-freezing temperatures with a strong salinity gradient (Fig. 1; Aagaard et al 1981; Rudels et al 1996; Toole et al 2010). As salinity dominates density at low temperatures, the cold halocline creates a layer of strong stratification that limits the depth to which surfacegenerated mixing can penetrate, and the near-freezing temperatures ensure that any pycnocline water that is mixed up to the surface is devoid of excess heat. The only process by which the heat contained within the Atlantic Water layer can be mixed up to the surface is through diffusion, such as that associated with the breaking of internal waves or double diffusive processes

Objectives

Results

Discussion

Conclusion