Diagnosing scale-dependent energy cycles in a high-resolution isopycnal
ocean model
Abstract
Energy exchanges between large-scale ocean currents and mesoscale eddies
play an important role in setting the large-scale ocean circulation but
are not fully captured in models. To better understand and quantify the
ocean energy cycle, we apply along-isopycnal spatial filtering to output
from an isopycnal 1/32$^\circ$ primitive equation
model with idealized Atlantic and Southern Ocean geometry and
topography. We diagnose the energy cycle in two frameworks: (1) a
non-thickness-weighted framework, resulting in a Lorenz-like energy
cycle, and (2) a thickness-weighted framework, resulting in the Bleck
energy cycle. This paper shows that (2) is the more useful framework for
studying energy pathways when an isopycnal average is used. Next, we
investigate the Bleck cycle as a function of filter scale. Baroclinic
conversion generates mesoscale eddy kinetic energy over a wide range of
scales, and peaks near the deformation scale at high latitudes, but
below the deformation scale at low latitudes. Away from topography, an
inverse cascade transfers kinetic energy from the mesoscales to larger
scales. The upscale energy transfer peaks near the energy-containing
scale at high latitudes, but below the deformation scale at low
latitudes. Regions downstream of topography are characterized by a
downscale kinetic energy transfer, in which mesoscale eddies are
generated through barotropic instability. The scale- and flow-dependent
energy pathways diagnosed in this paper provide a basis for evaluating
and developing scale- and flow-aware mesoscale eddy parameterizations.