Theories of the Beaufort Gyre (BG) dynamics commonly represent the halocline as a single layer with a thickness depending on the Eulerian-mean and eddy-induced overturning. However, observations suggest that the isopycnal slope increases with depth, and a theory to explain this profile remains outstanding. Here we develop a multilayer model of the BG, including the Eulerian-mean velocity, mesoscale eddy activity, diapycnal mixing, and lateral boundary fluxes, and use it to investigate the dynamics within the Pacific Winter Water (PWW) layer. Using theoretical considerations, observational data, and idealized simulations, we demonstrate that the eddy overturning is critical in explaining the observed vertical structure. In the absence of the eddy overturning, the Ekman pumping and the relatively weak vertical mixing would displace isopycnals in a nearly parallel fashion, contrary to observations. This study finds that the observed increase of the isopycnal slope with depth in the climatological state of the gyre is consistent with a Gent–McWilliams eddy diffusivity coefficient that decreases by at least 10%–40% over the PWW layer. We further show that the depth-dependent eddy diffusivity profile can explain the relative magnitude of the correlated isopycnal depth and layer thickness fluctuations on interannual time scales. Our inference that the eddy overturning generates the isopycnal layer thickness gradients is consistent with the parameterization of eddies via a Gent–McWilliams scheme but not potential vorticity diffusion. This study implies that using a depth-independent eddy diffusivity, as is commonly done in low-resolution ocean models, may contribute to misrepresentation of the interior BG dynamics.@en

The Beaufort Gyre is driven by the prevailing anticyclonic wind stress curl over the western Arctic Ocean, which pumps relatively low-salinity water into the gyre interior and deepens the halocline. Storage/release of freshwater occurs during periods of anomalous wind stress curl, which has considerable implications for the Atlantic Meridional Overturning Circulation and thus the global climate. Current theories portray the halocline as a buoyant layer with a thickness that is governed by a balance (or lack thereof) between the Ekman downwelling and mesoscale eddy transport. In equilibrium, the isopycnal slopes are predicted to be proportional to the strength of the surface stress and inversely proportional to the eddy diffusivity. However, eddy diffusivity is often parameterized to be proportional to the isopycnal slope. Climatological observations of the gyre suggest that its isopycnal slope significantly increases with depth, while the mooring-derived isopycnal eddy diffusivities decrease with depth, suggesting that the current theory is insufficient. Here, we develop a simple new theory that reconciles these seemingly contradictory statements.
We extend the single-layer halocline paradigm to a multi-layer model in which the isopycnal interfaces are coupled with each other. The strength and vertical profile of the eddy diffusivities are determined from the baroclinic instability characteristics of the geostrophic currents, which are linked to isopycnal slopes via thermal wind balance. Using this basic model of eddy-mean flow interactions, we identify the critical processes that set the vertical structure of the gyre. Specifically, we prove that potential vorticity sources at the gyre boundaries (over continental slopes) are essential for a realistic vertical distribution of the isopycnal slopes. In the absence of the boundary fluxes, the gyre attracts to a state with depth-invariant isopycnal slope, regardless of the initial conditions. Our results further justify the need for observational constraints on eddy diffusivity and boundary fluxes.@en