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; in
practice, however, these methods can be challenging to implement and
interpret, and therefore remain 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 to large data sets 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
Generalized 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 region of a Baltic Sea regional
configuration of the Modular Ocean Model v6 (MOM6). We verify the
convergence of our WMT diagnostics by brute-force, comparing
time-averaged diagnostics on various vertical grids to timestep-averaged
diagnostics on the native model grid. Finally, we briefly describe a
stack of xarray-enabled Python packages for evaluating WMT budgets in
FV-GVC models, which is intended to be model-agnostic and available for
community use and development.