Assessment of the z~ time-filtered Arbitrary
Lagrangian-Eulerian coordinate in a global eddy-permitting ocean model
Abstract
A recognized deficiency of ocean models with a constant-depth vertical
coordinate is for truncation errors in the advection scheme to result in
spurious numerical mixing of tracers, which can be substantial larger
than that prescribed by the model’s mixing scheme. The
z~ vertical coordinate allows vertical levels to
displace in a lagrangian fashion on time scales shorter than a few days,
but reverts to fixed levels on longer timescales, and is intended to
reduce numerical mixing from transient vertical motions such as internal
waves and tides. An assessment of z~ in a ¼° global
implementation of the NEMO model is presented. It is shown that, in the
presence of near-inertial gravity waves in the North Atlantic,
z~ significantly reduces eulerian vertical velocities
with respect to those in a control simulation with the default z*
vertical coordinate; that the vertical coordinate approaches an
isopycnal, or adiabatic, surface on short timescales; and that both
tendences are enhanced when the z~ timescale parameters
are lengthened with respect to the default settings. Evaluation of an
effective diapycnal diffusivity, based on density transformation rates,
shows that numerical mixing is consistently reduced as the
z~ timescales are lengthened. The realism of the model
simulation with different timescale parameters is assessed in the global
domain, and it is shown that drifts in temperature and salinity, and the
spindown in z*of the Antarctic Circumpolar Current, are reduced with
z~, without incurring significant penalties in other
metrics such as the strength of the overturning circulation or sea ice
cover.