On the Link between Arctic Sea Ice Decline and the Freshwater Content of the Beaufort Gyre: Insights from a Simple Process Model

Abstract

Recent satellite and hydrographic observations have shown that the rate of freshwater accumulation in the Beaufort Gyre of the Arctic Ocean has accelerated over the past decade. This acceleration has coincided with the dramatic decline observed in Arctic sea ice cover, which is expected to modify the efficiency of momentum transfer into the upper ocean. Here, a simple process model is used to investigate the dynamical response of the Beaufort Gyre to the changing efficiency of momentum transfer, and its link with the enhanced accumulation of freshwater. A linear relationship is found between the annual mean momentum flux and the amount of freshwater accumulated in the Beaufort Gyre. In the model, both the response time scale and the total quantity of freshwater accumulated are determined by a balance between Ekman pumping and an eddy-induced volume flux toward the boundary, highlighting the importance of eddies in the adjustment of the Arctic Ocean to a change in forcing. When the seasonal cycle in the efficiency of momentum transfer is modified (but the annual mean momentum flux is held constant), it has no effect on the accumulation of freshwater, although it does impact the timing and amplitude of the annual cycle in Beaufort Gyre freshwater content. This suggests that the decline in Arctic sea ice cover may have an impact on the magnitude and seasonality of the freshwater export into the North Atlantic.

On the Interplay between the Circulation in the Surface and the Intermediate Layers of the Arctic Ocean

“…Following the approach of Yang (2005), Spall (2013), and Davis et al (2014), the domain consists of a circular, semienclosed basin with a 1500-km diameter, connected to a sponge region by a 375-km-wide channel that allows inflow and outflow to the basin. The depth in the interior of the domain is 1500 m, with a slope all around the boundary over a 150-km-wide region ( Fig.…”

“…It uses a grid with a horizontal resolution of 15 km and 30 evenly spaced vertical levels of 50 m. Following Spall (2013) and Davis et al (2014), the internal Rossby deformation radius based on an average salinity of 32.3 in the surface layer and 35.0 in the AW layer (resulting in a reduced gravity g 0 of 0.02 m s 22 ) and a surface layer thickness of 400 m is L d 5 ffiffiffiffiffiffiffiffiffiffi (g 0 h) p /f 0 ' 20 km. This is also consistent with the results of Nurser and Bacon (2014), who used a climatology and estimated a ;15-km Rossby radius in the interior of the Arctic basin.…”

“…Here, our model setup does not include a sea ice component in order to keep our configuration as simple as possible. We thus follow the approach detailed in Davis et al (2014), and we prescribe an idealized anticyclonic surface stress centered over the circular basin and directly applied at the ocean surface (Fig. 2).…”

A Theory of the Wind-Driven Beaufort Gyre Variability

“…A recently proposed hypothesis points to mesoscale eddy transport being a sufficient mechanism to oppose the Ekman pumping (Davis et al 2014;Marshall 2015;Lique et al 2015;Manucharyan and Spall 2016, hereinafter MS16;Yang et al 2016). The eddies are generated via baroclinic instabilities of the large-scale gyre circulation that releases available potential energy associated with the halocline deepening.…”

“…Taking into account the eddy transport mechanism, Davis et al (2014) have used a low-resolution, shallowwater model (with eddies parameterized as horizontal diffusion) to explore seasonal gyre dynamics. Yang et al (2016) explored the potential vorticity budget of the gyre and reached the conclusion that eddies are necessary to close it.…”