99 research outputs found
Deep ocean influence on upper ocean baroclinic instability saturation
In this paper we extend earlier results regarding the effects of the lower
layer of the ocean (below the thermocline) on the baroclinic instability within
the upper layer (above the thermocline). We confront quasigeostrophic
baroclinic instability properties of a 2.5-layer model with those of a 3-layer
model with a very thick deep layer, which has been shown to predict spectral
instability for basic state parameters for which the 2.5-layer model predicts
nonlinear stability. We compute and compare maximum normal-mode perturbation
growth rates, as well as rigorous upper bounds on the nonlinear growth of
perturbations to unstable basic states, paying particular attention to the
region of basic state parameters where the stability properties of the 2.5- and
3-layer model differ substantially. We found that normal-mode perturbation
growth rates in the 3-layer model tend to maximize in this region. We also
found that the size of state space available for eddy-amplitude growth tends to
minimize in this same region. Moreover, we found that for a large spread of
parameter values in this region the latter size reduces to only a small
fraction of the total enstrophy of the system, thereby allowing us to make
assessments of the significance of the instabilities.Comment: To appear \emph{in} O. U. Velasco-Fuentes et al. (eds.),
\textit{Nonlinear Processes in Geophysical Fluid Dynamics}, Kluwer Academi
Enduring Lagrangian coherence of a Loop Current ring assessed using independent observations
Ocean flows are routinely inferred from low-resolution satellite altimetry
measurements of sea surface height assuming a geostrophic balance. Recent
nonlinear dynamical systems techniques have revealed that surface currents
derived from altimetry can support mesoscale eddies with material boundaries
that do not filament for many months, thereby representing effective transport
mechanisms. However, the long-range Lagrangian coherence assessed for mesoscale
eddy boundaries detected from altimetry is constrained by the impossibility of
current altimeters to resolve ageostrophic submesoscale motions. These may act
to prevent Lagrangian coherence from manifesting in the rigorous form described
by the nonlinear dynamical systems theories. Here we use a combination of
satellite ocean color and surface drifter trajectory data, rarely available
simultaneously over an extended period of time, to provide observational
evidence for the enduring Lagrangian coherence of a Loop Current ring detected
from altimetry. We also seek indications of this behavior in the flow produced
by a data-assimilative system which demonstrated ability to reproduce observed
relative dispersion statistics down into the marginally submesoscale range.
However, the simulated flow, total surface and subsurface or subsampled
emulating altimetry, is not found to support the long-lasting Lagrangian
coherence that characterizes the observed ring. This highlights the importance
of the Lagrangian metrics produced by the nonlinear dynamical systems tools
employed here in assessing model performance.Comment: In press in nature.com/Scientific Report
Zonal Jets as Transport Barriers in Planetary Atmospheres
The connection between transport barriers and potential vorticity (PV)
barriers in PV-conserving flows is investigated with a focus on zonal jets in
planetary atmospheres. A perturbed PV-staircase model is used to illustrate
important concepts. This flow consists of a sequence of narrow eastward and
broad westward zonal jets with a staircase PV structure; the PV-steps are at
the latitudes of the cores of the eastward jets. Numerically simulated
solutions to the quasigeostrophic PV conservation equation in a perturbed
PV-staircase flow are presented. These simulations reveal that both eastward
and westward zonal jets serve as robust meridional transport barriers. The
surprise is that westward jets, across which the background PV gradient
vanishes, serve as robust transport barriers. A theoretical explanation of the
underlying barrier mechanism is provided. It is argued that transport barriers
near the cores of westward zonal jets, across which the background PV gradient
is small, are found in Jupiter's midlatitude weather layer and in the Earth's
summer hemisphere subtropical stratosphere.Comment: Accepted for publication in JA
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