The physical structure of the upper ocean is an important control on ocean-atmosphere exchange of momentum, heat, freshwater and gases such as carbon dioxide. It also regulates the distribution of nutrients and their delivery to the sunlit upper ocean, and therefore the production of plant biomass, which ultimately supports nearly all marine life and mediates key carbon fluxes. Determining the mechanisms structuring upper ocean dynamics is critical to understanding how the physical climate system and biogeochemical cycles function. Moreover, climate change is expected to strongly impact these processes; thus, it is crucial to develop a capacity to predict their evolution under altered forcing regimes. This work cannot be done with coarse resolution numerical models because at large scales ocean circulation is constrained mostly to the horizontal plane by the rotation of the Earth. This constraint is lessened at smaller scales and vertical flows become increasingly more energetic. This project will develop a sound, high-resolution numerical framework for evaluating climate change impacts on upper ocean processes globally and regionally. The tools and workflow for conducting such experiments will be codified in the Community Earth System Model framework and thus be available to other researchers. The study will provide an assessment of the importance of scales not normally included in climate scale models and projections. The results of this and any follow-on studies, will therefore be helpful in putting bounds on the reliability of current projections and in the design of future studies. The project also includes the training and mentoring of a graduate student and a postdoc, who will gain skills in model design, numerical experimentation, and analysis that are vitally important in future studies of societally relevant science.
Climate change projections to date have been done with coarse-resolution ocean models. Recent research, however, has revealed that submesoscale processes, at scales where the constraint by the rotation of the Earth is lessened, play an important role in determining upper ocean stratification, mixed layer depths, and surface-to-depth exchange. Critically, changes to the ocean mean-state under a warming climate are very likely to impact submesoscale activity, with implications for the lateral and vertical fluxes mediated by these processes. This project will use a combination of numerical simulations and observations to quantify the role of submesoscale dynamics in controlling net primary production and export production under present-day climate conditions; it will also examine the mechanisms driving changes in these processes under climate forcing representative of future warming. This is made possible by a novel experimental design that enables computationally feasible climate-change experiments conducted at high-resolution. This involves integrating an eddy-resolving (1/10 degree) global ocean model with biogeochemistry under present-day and future conditions, using anomalies extracted from fully-coupled ensemble integrations to define the climate perturbation. A regional model, nested within the global eddy-resolving domain, will be used to investigate submesoscale dynamics in distinct oceanic regions under different climate conditions. There will be a number of important outcomes of this research relating to changes to the ocean system under a warmer climate. The changing role of mesoscale dynamics in mediating biogeochemical processes globally will be determined. The submesoscale activity in a number of contrasting regions in present-day and warmer climate conditions will be determined. Submesoscale events, their underlying physics and associated heat fluxes as a function of season in both present day and warmer climate conditions will be quantified. The impact of submesoscale activity on ocean biogeochemistry under varied conditions will be determined. The degree to which the changes in submesoscale activity in warmer climate conditions impacts the overall change to the upper ocean physics and biogeochemistry will be determined.
