Concealed Layering in the Upper Arctic

Abstract Fully developed thermohaline staircases and intrusions represent the most dramatic and recognizable manifestations of double-diffusive convection in the ocean. These structures are common in the Arctic halocline, where they can be readily identified by their distinct vertical temperature and salinity patterns. This investigation draws attention to a related but subtler phenomenon described here as concealed layering. Although concealed layers have a limited impact on background stratification, they can be highly persistent and laterally coherent over large distances. Our inquiry into their dynamics is motivated by echosounder measurements collected in the upper Arctic. These observations reveal ubiquitous, predominantly horizontal reflectivity patterns in diffusively stratified parts of the water column. However, the vertical temperature and salinity profiles from these locations lack well-defined steps typically associated with staircases and intrusions. The present study is based on a series of direct numerical simulations that illustrate the spontaneous formation of concealed layers in moderately sheared, doubly stratified flows. We argue that concealed layering is a widespread yet unexplored phenomenon that affects high-frequency sound propagation in polar oceans. Concurrently, our simulations support the hypothesis that layers, both concealed and manifest, can be generated by subcritical shear even when double diffusion alone does not lead to layering.