Western Arctic Shelfbreak Eddies: Formation and Transport

Abstract

The mean structure and time-dependent behavior of the shelfbreak jet along the southern Beaufort Sea, and its ability to transport properties into the basin interior via eddies are explored using high-resolution mooring data and an idealized numerical model. The analysis focuses on springtime, when weakly stratified winter-transformed Pacific water is being advected out of the Chukchi Sea. When winds are weak, the observed jet is bottom trapped with a low potential vorticity core and has maximum mean velocities of O(25 cm s Ϫ1 ) and an eastward transport of 0.42 Sv (1 Sv ϵ 10 6 m 3 s Ϫ1 ). Despite the absence of winds, the current is highly time dependent, with relative vorticity and twisting vorticity often important components of the Ertel potential vorticity. An idealized primitive equation model forced by dense, weakly stratified waters flowing off a shelf produces a mean middepth boundary current similar in structure to that observed at the mooring site. The model boundary current is also highly variable, and produces numerous strong, small anticyclonic eddies that transport the shelf water into the basin interior. Analysis of the energy conversion terms in both the mooring data and the numerical model indicates that the eddies are formed via baroclinic instability of the boundary current. The structure of the eddies in the basin interior compares well with observations from drifting ice platforms. The results suggest that eddies shed from the shelfbreak jet contribute significantly to the offshore flux of heat, salt, and other properties, and are likely important for the ventilation of the halocline in the western Arctic Ocean. Interaction with an anticyclonic basin-scale circulation, meant to represent the Beaufort gyre, enhances the offshore transport of shelf water and results in a loss of mass transport from the shelfbreak jet.

Lateral Coupling in Baroclinically Unstable Flows

“…Although very idealized, these results may explain the previous finding that a baroclinic boundary current can be stabilized by a weakening of the bottom slope (Spall et al 2008) through a lateral shifting of regions of nonzero pv gradient so that they do not overlap.…”

“…However, the neglect of lateral shear in the mean flow allows us to focus on baroclinic instability as the source of the waves and to isolate the effects of the relative positions of the mean potential vorticity gradient regions from other influences that would arise in a more general problem formulation. Baroclinic conversion is often the dominant source of energy for growing perturbations even in cases that do have significant lateral shear, as found for the observational and modeling study that motivated this work (Spall et al 2008).…”

“…For a boundary current of width equal to the internal deformation radius, this decoupling occurs when the bottom slope is less than f/N, where f is the Coriolis parameter and N is the Brunt-Väisälä frequency. The topographic stabilization found numerically by Spall et al (2008) occurs near this value of bottom slope, and suggests that the lateral offset of the regions of differing ⌸ y resulting from the sloping bottom may be the mechanism causing the increased stability.…”

“…The approach follows the usual linear stability analysis, as can be found in, for example, Pedlosky (1987). The nondimensional equation for the perturbation potential vorticity q n may be written as ) from a mooring array along the north slope of Alaska, described further in Spall et al (2008).…”

“…Our interest in the relative positioning of regions of opposite sign potential vorticity gradient is motivated by an observational and modeling study of a boundary current along the northern slope of Alaska (Spall et al 2008). The boundary current is composed of low potential vorticity water that is flowing eastward in a bottomintensified jet over sloping topography (Fig.…”