Near-Bottom Baroclinic and Submesoscale Instabilities at the Chukchi Shelf Break

Abstract We present novel observations of submesoscale instability along a sloping bottom in the Arctic Ocean where the Chukchi Sea meets the Beaufort Sea. Highly spatially resolved cross-shelf sections of hydrography and velocity suggest that baroclinic instability may have been active in the bottom boundary layer, generating 2–4-km-scale boluses. A downslope Ekman transport may have contributed to weakening stratification in the bottom layer and created the flow conditions for instability to occur. Additionally, potential vorticity analyses revealed that the flow was susceptible to symmetric and gravitational instability. Regions of the flow susceptible to symmetric instability were coincident with the most coherent eddy features we measured. Regions favorable to gravitational instability were observed within less coherent eddy features and at separation scales similar to the eddy widths and were associated with elevated turbulence ( ϵ = 1 × 10 −8 –1 × 10 −7 W kg −1 ), weak stratification, and an anomalously strong baroclinic component of potential vorticity, typically associated with symmetric instability. The observed plumes of gravitational instability are consistent with localized density inversions that could arise from shear-driven processes like Kelvin–Helmholtz instability or secondary instabilities associated with sharp frontal gradients at eddy edges. This collection of instability processes may play an important role in water mass modification and evolution at this crucial gateway to the Arctic Ocean and along sloping shelves subject to downslope Ekman transport in other parts of the World Ocean. Significance Statement This study describes a previously unknown pathway that may play an important role in how heat originating from Pacific water masses is mixed and transported into the Arctic Ocean. As water in the lower layer flows along the sloping shelf separating the Chukchi Sea from the deeper Arctic, bottom friction causes it to slow and veer toward deeper depths. The offshore motion can lead to horizontal density gradients that promote vertical shear in horizontal velocity which can cause the flow to go unstable. We observed patterns in temperature and velocity that were consistent with the patterns expected for baroclinic instability, which derives its energy from the potential energy associated with horizontal density gradients. These processes have not previously been accounted for when considering mixing and potential heat transport in this region of the Arctic.