ALBERT

Description: The Southern Ocean plays a fundamental role in the global carbon cycle. Physical and biogeochemical processes, including primary production and the upwelling of carbon-rich water masses, govern carbon exchange between the atmosphere and ocean carbon reservoirs. To study this region, we configured a regional East-Antarctic simulation derived from ECCO-Darwin, a global-ocean biogeochemistry model that assimilates both physical and biogeochemical observations. Our regional ocean model extends from the Antarctic Continent to 60°S and from 100°E to 150°E with horizontal grid spacing of 3–4 km. The model domain includes the Shackleton, Conger, Totten, Moscow University, Holmes, Dibble, and Mertz ice shelves. Since the biogeochemical component of ECCO-Darwin is optimized to best fit global observations, model-data agreement for the East Antarctic region requires further adjustments. For example, (1) simulated upper-100 m nutrient fields are biased high and typical Circumpolar-Deep-Water characteristics with nutrient-rich waters are not clearly simulated and (2) plankton types in the ECCO-Darwin do not include Phaeocystis, an abundant type that plays a key role in the Southern Ocean climate system. In this study, we adjust a small number of physical and biogeochemical model parameters and lateral boundary conditions to achieve improved model-data agreement. We define the cost function as a sum of weighted model-data differences based on both novel in-situ observations and further optimize our simulation using a Green's Functions approach. This work demonstrates downscaling methods for developing regional cutouts from the global-ocean ECCO-Darwin model, which allows for high-resolution coastal studies that include optimized sea ice, ocean physics, and biogeochemistry.

Description: The system of oceanic flows constituting the Atlantic Meridional Overturning Circulation (AMOC) moves heat and other properties to the subpolar North Atlantic, controlling regional climate, weather, sea levels, and ecosystems. Climate models suggest a potential AMOC slowdown towards the end of the 21〈sup〉st〈/sup〉 century due to anthropogenic forcing, which would accelerate coastal sea level rise along the western boundary and dramatically increase coastal flood risk. While the slowdown has not been observed to date, we show here that the AMOC-induced intrinsic changes in gyre-scale heat content, superimposed on the global mean sea level rise, are already influencing the frequency of floods along the United States southeastern seaboard. For the South Atlantic Bight and Gulf of Mexico coasts, using observations and an ocean state estimate, we have established a strong link between coastal sea level, the associated flood frequency, and gyre-scale dynamic sea level and oceanic heat content variability, which are largely controlled by AMOC-driven ocean heat convergence. We find that ocean heat convergence, being the primary driver for interannual sea level changes in the subtropical North Atlantic, has led to an exceptional gyre-scale warming and associated dynamic sea level rise since 2010, accounting for 30-50% of flood days in 2015-2020. The results of this study highlight the importance of accounting for natural, large-scale sea level variability in order to improve coastal sea level projections and to better assess coastal flood risk.