Shallow open-ocean convection in the Weddell Sea: A case study using observations and modelling techniques

Water mass transformations in the Southern Ocean serve as a lynchpin in the global overturning circulation. Among them, the transformation of Circumpolar Deep Water into Antarctic Winter and Surface Water is uniquely critical for the export of Intermediate Waters north of the Polar Front, the exchange of carbon dioxide between the atmosphere and the subsurface ocean, and the upwards flux of oceanic heat, which inhibits sea ice growth. However, our understanding of the processes responsible for the upwelling of Circumpolar Deep Water and its destruction remains incomplete. We hypothesize that shallow open-ocean convective plumes, only extending into or just below the pycnocline, are underrepresented in both the observational record and in global Earth System Models (ESMs), due to their elusive spatial and temporal scales and the hydrostatic approximation made by all ESMs. Therefore, they play a hitherto undervalued role in setting the water mass structure of the Southern Ocean. We present evidence from a unique year-round upper ocean mooring in the Weddell Sea of a shallow open-ocean convective plume extending into the pycnocline during winter 2021. Using the MITgcm ocean model, we simulate an analogous plume in both hydrostatic and non-hydrostatic configurations. Preliminary results suggest that the conditions necessary to form such plumes can be expected with some regularity in the Weddell Sea. We also note differences between the non-hydrostatic and hydrostatic simulations, highlighting the expected biases associated with the hydrostatic approximation in ESMs.