A Theory of the Wind-Driven Beaufort Gyre Variability

Abstract

The halocline of the Beaufort Gyre varies significantly on interannual to decadal time scales, affecting the freshwater content (FWC) of the Arctic Ocean. This study explores the role of eddies in the Ekmandriven gyre variability. Following the transformed Eulerian-mean paradigm, the authors develop a theory that links the FWC variability to the stability of the large-scale gyre, defined as the inverse of its equilibration time. The theory, verified with eddy-resolving numerical simulations, demonstrates that the gyre stability is explicitly controlled by the mesoscale eddy diffusivity. An accurate representation of the halocline dynamics requires the eddy diffusivity of 300 6 200 m 2 s 21 , which is lower than what is used in most low-resolution climate models. In particular, on interannual and longer time scales the eddy fluxes and the Ekman pumping provide equally important contributions to the FWC variability. However, only large-scale Ekman pumping patterns can significantly alter the FWC, with spatially localized perturbations being an order of magnitude less efficient. Lastly, the authors introduce a novel FWC tendency diagnostic-the Gyre Index-that can be conveniently calculated using observations located only along the gyre boundaries. Its strong predictive capabilities, assessed in the eddy-resolving model forced by stochastic winds, suggest that the Gyre Index would be of use in interpreting FWC evolution in observations as well as in numerical models.

Impact of the Surface Stress on the Volume and Freshwater Transport Through the Canadian Arctic Archipelago From a High‐Resolution Numerical Simulation

“…Jahn et al () and Houssais and Herbaut () also found a correlation between large‐scale atmospheric patterns (i.e., North Atlantic Oscillation) and the transport through the CAA. Linking the CAA throughflow to large‐scale atmospheric circulation is consistent with a recent study done by Manucharyan et al (), which proposed that the large‐scale wind forcing over the Beaufort Gyre leads to the accumulation or release of freshwater, based on the direction of the winds.…”

“…SSH in Beaufort Gyre is associated with the accumulation of mass caused by Ekman transport. A decrease in surface stress in this region will decrease Beaufort Gyre SSH and then flatten the baroclinic gradient between the Gyre and Baffin Bay, driving less transport through the CAA straits (e.g., Manucharyan & Spall, ; Manucharyan et al, ). The transports through Prince of Wales Strait (Figure i) and the strait between Prince Patrick and Melville Islands (Figure d) are positively correlated to surface stress in Beaufort Gyre.…”

A 4.5 km resolution Arctic Ocean simulation with the global multi-resolution model FESOM 1.4

“…10i-k). Recent research indicates that mesoscale eddy fluxes counteract Ekman pumping, thus playing a crucial role in Beaufort Gyre FW content variability (e.g., Manucharyan et al, 2016;Yang et al, 2016). In model simulations, eddy parameterization (applied on coarse meshes) and the effect of implicit numerical mixing will certainly influence the dynamical balance and the Beaufort Gyre FW content.…”

Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman‐Driven Beaufort Gyre

“…However, within this basic paradigm, a steady state cannot be achieved for any nonzero time‐mean Ekman pumping, and thus, a stabilizing process must exist to counteract the Ekman‐driven halocline deepening. Near its stable equilibrium, the halocline evolution under relatively small perturbations obeys h t = w Ek − h / T eq , where T eq > 0 is a halocline equilibration timescale, as introduced in Manucharyan and Spall () and Manucharyan et al (). Note that T eq is an equilibrium state characteristic, and hence, it could change if halocline depth perturbations are sufficiently large for the omitted nonlinearities to become significant.…”