Winter thermohaline variability of the upper central Arctic Ocean: observational results from the MOSAiC Distributed Network and Central Observatory in Leg 1

Abstract:

We analyzed hydrographic data from several autonomous oceanographic buoys within the MOSAiC Distributed Network together with regular CTD casts from the MOSAiC Central Observatory during the 2019/20 winter in the Amundsen Basin. These data augment sparse wintertime observations below the Arctic multiyear ice and are used to determine the (sub)mesoscale dynamics, in particular lateral thermohaline gradients in the upper 100 m. Full-depth CTD profiles yielded the first baroclinic Rossby radii (R₁) of ~7.5 km, which is consistent with previous studies based on climatology. Compensated fronts, where temperature and salinity partially cancel out their contributions in density, were only found at the depth of the halocline (~100 m) for scales smaller than R₁. At shallower depths, or for fronts with length scales longer than R₁, salinity dominated the density field. Near-surface layers shallower than the halocline were not always mixed. Restratification was commonly observed, suggesting the onset of baroclinic instabilities and/or eddies emanating from the lateral fronts. After a strong wind event with speeds > 20 m s^-1, the wavenumber spectra of density increased the energy level by ~30% and changed the scaling factor for length scales < R₁ at shallow depths (< ~20 m). We identified one submesoscale surface-layer eddy with a radius of ~5 km. The observed geostrophic shears relative to 200 dbar suggest that baroclinic instabilities could be active at depths shallower than ~80 m due to the lateral thermohaline gradients. We conclude that eddies penetrating into the halocline can explain why compensated fronts are only found at ~100 m. This underlines the role of surface-layer baroclinic eddies in lateral dispersion in the winter halocline.