The Ice‐Ocean Governor: Ice‐Ocean Stress Feedback Limits Beaufort Gyre Spin‐Up

Abstract

The Beaufort Gyre is a key circulation system of the Arctic Ocean and its main reservoir of freshwater. Freshwater storage and release affects Arctic sea ice cover, as well as North Atlantic and global climate. We describe a mechanism that is fundamental to the dynamics of the gyre, namely, the ice-ocean stress governor. Wind blows over the ice, and the ice drags the ocean. But as the gyre spins up, currents catch the ice up and turn off the surface stress. This governor sets the basic properties of the gyre, such as its depth, freshwater content, and strength. Analytical and numerical modeling is employed to contrast the equilibration processes in an ice-covered versus ice-free gyre. We argue that as the Arctic warms, reduced sea ice extent and more mobile ice will result in a deeper and faster Beaufort Gyre, accumulating more freshwater that will be released by Ekman upwelling or baroclinic instability. Plain Language SummaryThe Beaufort Gyre, located north of Alaska and Canada, is a key circulation system of the Arctic Ocean. Changes in its depth and circulation influence the evolution of the Arctic sea ice cover, the North Atlantic circulation, and the global climate. The gyre is driven by persistent, ice-mediated winds, accumulating surface freshwater toward the center, deepening the gyre, and spinning up its currents. We describe a mechanism, dubbed here the ice-ocean governor, in which the interaction of surface currents with the ice regulates the depth of the Beaufort Gyre: The spinning up of the gyre reduces the relative speed between the ocean and the ice, and hence the freshwater accumulation. This competes with, and we argue is more important than, the release of freshwater by flow instability, which moves water from the center toward the periphery. In the current climate the depth and speed of the Beaufort Gyre are mainly set by the governor, but this may change in a warming world where reduced ice cover will render the ice-ocean governor less effective. The resulting deeper, swifter gyre will likely exhibit more variability in its freshwater storage and flow speeds.

A Three‐Way Balance in the Beaufort Gyre: The Ice‐Ocean Governor, Wind Stress, and Eddy Diffusivity

“…Retaining the sea ice concentration, α , in our analytical equation for isopycnal depth anomaly means that the equation is a nonlinear ordinary differential equation. The equilibrium solution to equation with u i ≥ u g is which collapses to the solution predicted by Meneghello, Marshall, Campin, et al () when α = 1 and κ = 0.…”

“…Following Meneghello, Marshall, Campin, et al (), we will henceforth approximate the surface ocean speed, u s , with the geostrophic speed of the gyre, u g = g ′ h / fR in which g ′ is the reduced gravity across the halocline and R is the radius of the gyre. Using the above idealizations means that equation can be written as where ξ ≥ 1 is a scaling factor that converts between a volume flux and the isohaline depth anomaly and depends on the initial shape of the isohaline (see Appendix ), u i is the ice speed, and u a is the wind speed.…”

“…The eddy‐induced overturning circulation opposes the wind‐driven overturning circulation, thereby halting the accumulation of freshwater and equilibrating the BG. However, an alternative mechanism for equilibrating the BG has recently been proposed: the Ice‐Ocean Governor (Meneghello, Marshall, Timmermans, et al, ; Meneghello, Marshall, Campin, et al, ). The Ice‐Ocean Governor relies on the fact that the relative velocity between the sea ice and the ocean controls the surface stress at the ice‐ocean interface; if the ocean and the ice are moving at the same speed, there is no transfer of momentum, and the gyre is equilibrated.…”

“…This mechanism is dubbed the Ice‐Ocean Governor by analogy with mechanical governors that regulate the speed of engines and other devices through dynamical feedbacks (see, e.g., Maxwell, ). The ocean velocity has long been included when calculating the ice‐ocean stress in numerical models (see, e.g., Hibler, ), but appreciation of the importance of this effect has been very recent (Dewey et al, ; Kwok & Morison, ; Meneghello et al, ; Meneghello, Marshall, Timmermans, et al, ; Meneghello, Marshall, Campin, et al, ; Zhong et al, ) following the publication of a data set that estimates sea surface height and surface geostrophic velocity in sea ice‐covered areas (Armitage et al, ).…”

“…However, mesoscale eddies are commonly found in the Arctic Ocean (Dmitrenko et al, ; Hunkins, ; Manley & Hunkins, ; Meneghello et al, ; Timmermans et al, ; Zhao et al, , ). It is therefore important to extend the theory of the Ice‐Ocean Governor presented by Meneghello, Marshall, Campin, et al () to include the effect of eddy diffusivity. A schematic of these three processes, wind stress, the Ice‐Ocean Governor, and eddy fluxes, is shown in Figure c.…”