Two prominent signatures of the climate system response to increasing greenhouse gases are acceleration of the hydrologic cycle and sea level rise. The current formulation of the ocean component of the Community Earth System Model (CESM) makes it ill-suited to provide accurate or direct estimates of the oceanic signature of changes in evaporation and precipitation (i.e., changes in ocean salinity, of sea level change, and of the carbon cycle). These same limitations will bias the model response to freshwater input from melting ice sheets, and thus call into question any model based assessment of the potential for "tipping points" or abrupt change associated with interruption of deep water formation. We propose to address these shortcomings of the present ocean model by reformulating the model dynamics as well as by improving the representation of freshwater and carbon exchanges between the ocean and the other CESM components. The improved ocean model will have natural boundary conditions and a mass based vertical coordinate system without the Boussinesq approximation. In addition, we will re-evaluate the formulation and algorithms for the barotropic equation to improve its accuracy and scalability. Without these improvements, the CESM ocean component (Parallel Ocean Program) will not remain as one of the leading state-of-the-science models.

The overarching objectives of the proposed research are to:

• Improve the representation of the exchange of freshwater and carbon between the ocean and other climate system components and the effect of this exchange on tracers by replacing the "virtual flux" boundary condition formulation in CESM with the correct "natural boundary condition".
• Reformulate the model dynamics in a mass based vertical coordinate system, thereby circumventing the limitations of the Boussinesq approximation.
• Revisit the formulation and algorithm for the elliptic barotropic equation by exploring alternative approaches and solvers, thereby improving the accuracy and scalability of the solver.
• Investigate the sensitivity of climate length simulations of CESM to these changes in model formulation, in terms of regional and global sea level rise, water mass properties, tracers, and circulation.