Enhanced eddy activity in the Beaufort Gyre in response to sea ice loss

Thomas W K Armitage,Andrew F Thompson,Ron Kwok,Alek A Petty,Georgy E Manucharyan

doi:10.1038/s41467-020-14449-z

Abstract

The Beaufort Gyre freshwater content has increased since the 1990s, potentially stabilizing in recent years. The mechanisms proposed to explain the stabilization involve either mesoscale eddy activity that opposes Ekman pumping or the reduction of Ekman pumping due to reduced sea ice–ocean surface stress. However, the relative importance of these mechanisms is unclear. Here, we present observational estimates of the Beaufort Gyre mechanical energy budget and show that energy dissipation and freshwater content stabilization by eddies increased in the late-2000s. The loss of sea ice and acceleration of ocean currents after 2007 resulted in enhanced mechanical energy input but without corresponding increases in potential energy storage. To balance the energy surplus, eddy dissipation and its role in gyre stabilization must have increased after 2007. Our results imply that declining Arctic sea ice will lead to an increasingly energetic Beaufort Gyre with eddies playing a greater role in its stabilization.

Highlights

The Beaufort Gyre freshwater content has increased since the 1990s, potentially stabilizing in recent years
The first hypothesis relies on the fact that the ice–ocean surface stress depends on the vector difference between the sea ice and ocean surface velocities, such that an intensification of geostrophic currents due to surface freshwater accumulation and steepening of the dynamic ocean topography tends to reduce the sea ice–ocean surface stress, limiting further Ekman pumping in a negative feedback mechanism
The second hypothesis emphasizes that increased Ekman pumping, freshwater storage, and steepening of the Beaufort Gyre (BG) halocline slope is associated with generation of available potential energy (APE; the gravitational potential energy stored in a sloped halocline), and that this should be counteracted by increased production of eddies by baroclinic instabilities

Summary

Results and discussion

The BG can be thought of as existing in an “energetic governor regime”, where wind energy input was balanced by energy dissipation under sea ice and there was little change in APE storage This corresponded to a period of net Ekman downwelling of ~170 mSv (Fig. 2e; 1 mSv ≡ 103 m3/s) and an increase in BG FWC of almost 2000 km[3] over 4 years (Fig. 2d). Changes in the ice–ocean drag coefficient, due to, e.g., changes in form drag[30], or an increase in eddy density[26], will affect energy dissipation underneath sea ice. The spatial pattern of wind energy input to the BG (input along the southern edge, dissipation in the interior; Fig. 1f) implies a northward transport of energy from source to sink. Our method does not allow us to investigate this in detail and we can only speculate that the energy is transported by the mean geostrophic circulation as well as by preferential mesoscale eddy propagation away from the southern BG region, where strong baroclinic currents near the gyre boundaries facilitate eddy formation, towards the interior of the gyre where there is mechanical

Downwelling 2 Upwelling

Methods

Code availability