Understanding historical and projected compound change on the Northwest Atlantic shelf

Samantha A. Siedlecki , Felipe Soares, Zhuomin Chen, Enrique Curchitser, Hung Nguyen, Shannon Meseck, Michael A. Alexander , Sang-Ik Shin, Catherine Matassa, Halle M. Berger 

Increasing atmospheric carbon dioxide concentrations are accompanied by ocean acidification, oxygen loss, and warming of the global ocean. However, in coastal environments, local processes that occur on small spatial scales can moderate or exacerbate these trends. These processes are not well represented in global climate models. Therefore, downscaled tools are useful to decipher carbonate system drivers and predict conditions. Here we describe the application of a ROMS based regional model of the northwest Atlantic shelf, stretching from Florida to Newfoundland, with ~7 km horizontal resolution. The biogeochemical model relies on the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) model in combination with regional empirical models to reconstruct the carbon variables. Using a 30-year historical simulation, model results are evaluated against in situ observations and then used to estimate anthropogenic carbon inventories for the region. Historical trends differ between surface and bottom conditions with bottom trends identified as more severe. Circulation and changes in the water column metabolism amplify local rates of change historically, while warming and water mass changes act to dampen these changes. Regional locations of accelerated carbon storage and accumulation are identified and described to be modified by coastal processes. A time-varying dynamic delta forced future projection out to 2098 under SSP5-8.5 projects how these trends will continue and indicates future acceleration of trends. Observing compound change, or multiple stressors changing in concert or closely, requires not only over-constraint on the carbon cycle parameters, but also multiple co-existing biogeochemical observations to refine the mechanisms responsible for local climate variability.