Beaufort Gyre Isopycnal Structure Produces a Steady Mesoscale Eddy Field Modulated by Sea Ice Drag

Abstract

The Beaufort Gyre (BG) mean state is set by a balance between surface wind stress, sea ice, and subsurface eddy transport. This study addresses the existence and generation of the eddies. We show that underice baroclinic instability of the observed BG mean state produces an eddy field that is both consistent with observations and sufficient to maintain the mean state. We use high-resolution simulations forced by the BG mean state to spin up a steady eddy field. The model includes a representation of sea ice with active rheology but inactive thermodynamics. Each simulation is initialized with different sea ice concentrations that remain constant throughout each integration, thus imparting varying amounts of surface drag. We find that these simulations are characterized by two regimes: a static ice regime with strong surface drag and a motile ice regime with low surface drag. Simulations in the motile ice simulations exhibit high eddy energy with near-surface maximum eddy velocities of about $30$ cm s⁻¹ and eddy diffusivities of about $4000$ m² s⁻¹. Eddy energy is lower in the static ice regime but nevertheless maintains significant strength at the halocline depth (150 m) with eddy velocity maxima exceeding $10$ cm s⁻¹. Eddy diffusivity under static ice ($O\left(1000\right)$ m² s⁻¹)) peaks just below the surface and decays with depth, which is consistent with the upper end of previous analyses. This work both suggests the presence of locally generated halocline-intensified eddies and provides insight into the future behavior of the BG as we slowly transition to an ice-free Arctic.