Tracer and observationally-derived constraints on horizontal and
diapycnal diffusivities in ocean models
Abstract
Mixing parameters can be inaccurate in ocean data assimilation systems,
even if there is close agreement between observations and mixing
parameters in the same modeling system when data are not assimilated. To
address this, we investigate whether there are additional observations
that can be assimilated by ocean modeling systems to improve their
representation of mixing parameters and thereby gain knowledge of the
global ocean’s mixing parameters. Observationally-derived diapycnal
diffusivities–using a strain-based parameterization of finescale
hydrographic structure–are included in the Estimating the Circulation
& Climate of the Ocean (ECCO) framework and the GEOS-5 coupled Earth
system model to test if adding observational diffusivities can reduce
model biases. We find that adjusting ECCO-estimated and
GEOS-5-calculated diapycnal diffusivity profiles toward profiles derived
from Argo floats using the finescale parameterization improves agreement
with independent diapycnal diffusivity profiles inferred from
microstructure data. Additionally, for the GEOS-5 hindcast, agreement
with observed mixed layer depths and temperature/salinity/stratification
(i.e., hydrographic) fields improves. Dynamic adjustments arise when we
make this substitution in GEOS-5, causing the model’s hydrographic
changes. Adjoint model-based sensitivity analyses suggest that the
assimilation of dissolved oxygen concentrations in future ECCO
assimilation efforts would improve estimates of the diapycnal
diffusivity field. Observationally-derived products for horizontal
mixing need to be validated before conclusions can be drawn about them
through similar analyses.