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.

Through analysis of remotely sensed sea surface temperature (SST) and sea ice concentration data, we investigate the impact of winds related to the Southern Annular Mode (SAM) on sea ice extent around Antarctica. We show that positive SAM anomalies in the austral summer are associated with anomalously cold SSTs that persist and lead to anomalous ice growth in the following autumn, while negative SAM anomalies precede warm SSTs and a reduction in sea ice extent during autumn. The largest effect occurs in April, when a unit change in the detrended summertime SAM is followed by a 1.8±0.6 ×105 km2 change in detrended sea ice extent. We find no evidence that sea ice extent anomalies related to the summertime SAM affect the wintertime sea ice extent maximum. Our analysis shows that the wind anomalies related to the negative SAM during the 2016/2017 austral summer contributed to the record minimum Antarctic sea ice extent observed in March 2017.

Observations of ocean currents in the Arctic interior show a curious, and hitherto unexplained, vertical and temporal distribution of mesoscale activity. A marked seasonal cycle is found close to the surface: strong eddy activity during summer, observed from both satellites and moorings, is followed by very quiet winters. In contrast, subsurface eddies persist all year long within the deeper halocline and below. Informed by baroclinic instability analysis, we explore the origin and evolution of mesoscale eddies in the seasonally ice-covered interior Arctic Ocean. We find that the surface seasonal cycle is controlled by friction with sea ice, dissipating existing eddies and preventing the growth of new ones. In contrast, subsurface eddies, enabled by interior potential vorticity gradients and shielded by a strong stratification at a depth of approximately 50 m, can grow independently of the presence of sea ice. A high-resolution pan-Arctic ocean model confirms that the interior Arctic basin is baroclinically unstable all year long at depth. We address possible implications for the transport of water masses between the margins and the interior of the Arctic basin, and for climate models’ ability to capture the fundamental difference in mesoscale activity between ice-covered and ice-free regions.