Water Mass Transformation Budgets in Finite-Volume Generalized Vertical Coordinate Ocean Models

Abstract

Water mass transformation theory provides conceptual tools that in principle enable innovative analyses of numerical ocean models but these tools, in practice, face multiple challenges to implementation and are therefore under-utilized. Most prior work evaluates only some of the simpler or more accessible terms in the water mass budget; meanwhile, the few full budget calculations in the literature are either limited to idealized model configurations and geometrically-simple domains or else have required heroic efforts that are neither scalable nor portable to other ocean models or research questions.We begin with a pedagogical derivation of key results of classical water mass transformation theory. We then describe best practices for diagnosing each of the water mass budget terms from the output of finite-volume general vertical coordinate (FV-GVC) ocean models, including the identification of a non-negligible remainder term as the spurious numerical mixing due to advection scheme discretization errors. We illustrate key aspects of the methodology through an example application to diagnostics from a polygonal sub-region of a Baltic Sea regional configuration of the Modular Ocean Model v6 (MOM6). Finally, we briefly describe a stack of xarray-enabled Python packages for evaluating water mass budgets in FV-GVC models, which is intended to be model-agnostic and available for community use and development.