352 research outputs found
Low-frequency variability and heat transport in a low-order nonlinear coupled ocean-atmosphere model
We formulate and study a low-order nonlinear coupled ocean-atmosphere model
with an emphasis on the impact of radiative and heat fluxes and of the
frictional coupling between the two components. This model version extends a
previous 24-variable version by adding a dynamical equation for the passive
advection of temperature in the ocean, together with an energy balance model.
The bifurcation analysis and the numerical integration of the model reveal
the presence of low-frequency variability (LFV) concentrated on and near a
long-periodic, attracting orbit. This orbit combines atmospheric and oceanic
modes, and it arises for large values of the meridional gradient of radiative
input and of frictional coupling. Chaotic behavior develops around this orbit
as it loses its stability; this behavior is still dominated by the LFV on
decadal and multi-decadal time scales that is typical of oceanic processes.
Atmospheric diagnostics also reveals the presence of predominant low- and
high-pressure zones, as well as of a subtropical jet; these features recall
realistic climatological properties of the oceanic atmosphere.
Finally, a predictability analysis is performed. Once the decadal-scale
periodic orbits develop, the coupled system's short-term instabilities --- as
measured by its Lyapunov exponents --- are drastically reduced, indicating the
ocean's stabilizing role on the atmospheric dynamics. On decadal time scales,
the recurrence of the solution in a certain region of the invariant subspace
associated with slow modes displays some extended predictability, as reflected
by the oscillatory behavior of the error for the atmospheric variables at long
lead times.Comment: v1: 41 pages, 17 figures; v2-: 42 pages, 15 figure
Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin
Transport and mixing properties of surface currents can be detected from
altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with
Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have
the advantage of exploiting both spatial and temporal variability of the
velocity field and are in principle able to unveil subgrid filaments generated
by chaotic stirring. However, one may wonder whether this theoretical advantage
is of practical interest in real-data, mesoscale and submesoscale analysis,
because of the uncertainties and resolution of altimetric products, and the
non-passive nature of biogeochemical tracers. Here we compare the ability of
standard Eulerian diagnostics and the finite-size Lyapunov exponent in
detecting instantaneaous and climatological transport and mixing properties. By
comparing with sea-surface temperature patterns, we find that the two
diagnostics provide similar results for slowly evolving eddies like the first
Alboran gyre. However, the Lyapunov exponent is also able to predict the
(sub-)mesoscale filamentary process occuring along the Algerian current and
above the Balearic Abyssal Plain. Such filaments are also observed, with some
mismatch, in sea-surface temperature patterns. Climatologies of Lyapunov
exponents do not show any compact relation with other Eulerian diagnostics,
unveiling a different structure even at the basin scale. We conclude that
filamentation dynamics can be detected by reprocessing available altimetric
data with Lagrangian tools, giving insight into (sub-)mesoscale stirring
processes relevant to tracer observations and complementing traditional
Eulerian diagnostics
Lagrangian Data-Driven Reduced Order Modeling of Finite Time Lyapunov Exponents
There are two main strategies for improving the projection-based reduced
order model (ROM) accuracy: (i) improving the ROM, i.e., adding new terms to
the standard ROM; and (ii) improving the ROM basis, i.e., constructing ROM
bases that yield more accurate ROMs. In this paper, we use the latter. We
propose new Lagrangian inner products that we use together with Eulerian and
Lagrangian data to construct new Lagrangian ROMs. We show that the new
Lagrangian ROMs are orders of magnitude more accurate than the standard
Eulerian ROMs, i.e., ROMs that use standard Eulerian inner product and data to
construct the ROM basis. Specifically, for the quasi-geostrophic equations, we
show that the new Lagrangian ROMs are more accurate than the standard Eulerian
ROMs in approximating not only Lagrangian fields (e.g., the finite time
Lyapunov exponent (FTLE)), but also Eulerian fields (e.g., the streamfunction).
We emphasize that the new Lagrangian ROMs do not employ any closure modeling to
model the effect of discarded modes (which is standard procedure for
low-dimensional ROMs of complex nonlinear systems). Thus, the dramatic increase
in the new Lagrangian ROMs' accuracy is entirely due to the novel Lagrangian
inner products used to build the Lagrangian ROM basis
Statistical and dynamical properties of covariant lyapunov vectors in a coupled atmosphere-ocean modelâmultiscale effects, geometric degeneracy, and error dynamics
We study a simplified coupled atmosphere-ocean model using the formalism of covariant Lyapunov vectors (CLVs), which link physically-based directions of perturbations to growth/decay rates. The model is obtained via a severe truncation of quasi-geostrophic equations for the two fluids, and includes a simple yet physically meaningful representation of their dynamical/thermodynamical coupling. The model has 36 degrees of freedom, and the parameters are chosen so that a chaotic behaviour is observed. There are two positive Lyapunov exponents (LEs), sixteen negative LEs, and eighteen near-zero LEs. The presence of many near-zero LEs results from the vast time-scale separation between the characteristic time scales of the two fluids, and leads to nontrivial error growth properties in the tangent space spanned by the corresponding CLVs, which are geometrically very degenerate. Such CLVs correspond to two different classes of ocean/atmosphere coupled modes. The tangent space spanned by the CLVs corresponding to the positive and negative LEs has, instead, a non-pathological behaviour, and one can construct robust large deviations laws for the finite time LEs, thus providing a universal model for assessing predictability on long to ultra-long scales along such directions. Interestingly, the tangent space of the unstable manifold has substantial projection on both atmospheric and oceanic components. The results show the difficulties in using hyperbolicity as a conceptual framework for multiscale chaotic dynamical systems, whereas the framework of partial hyperbolicity seems better suited, possibly indicating an alternative definition for the chaotic hypothesis. They also suggest the need for an accurate analysis of error dynamics on different time scales and domains and for a careful set-up of assimilation schemes when looking at coupled atmosphere-ocean models
Hyperbolic Covariant Coherent Structures in two dimensional flows
A new method to describe hyperbolic patterns in two dimensional flows is
proposed. The method is based on the Covariant Lyapunov Vectors (CLVs), which
have the properties to be covariant with the dynamics, and thus being mapped by
the tangent linear operator into another CLVs basis, they are norm independent,
invariant under time reversal and can be not orthonormal. CLVs can thus give a
more detailed information on the expansion and contraction directions of the
flow than the Lyapunov Vector bases, that are instead always orthogonal. We
suggest a definition of Hyperbolic Covariant Coherent Structures (HCCSs), that
can be defined on the scalar field representing the angle between the CLVs.
HCCSs can be defined for every time instant and could be useful to understand
the long term behaviour of particle tracers.
We consider three examples: a simple autonomous Hamiltonian system, as well
as the non-autonomous "double gyre" and Bickley jet, to see how well the angle
is able to describe particular patterns and barriers. We compare the results
from the HCCSs with other coherent patterns defined on finite time by the
Finite Time Lyapunov Exponents (FTLEs), to see how the behaviour of these
structures change asymptotically
Exploring the Lyapunov instability properties of high-dimensional atmospheric and climate models
The stability properties of intermediate-order climate models are investigated by computing their Lyapunov exponents (LEs). The two models considered are PUMA (Portable University Model of the Atmosphere), a primitive-equation simple general circulation model, and MAOOAM (Modular Arbitrary-Order Ocean-Atmosphere Model), a quasi-geostrophic coupled oceanâatmosphere model on a ÎČ-plane. We wish to investigate the effect of the different levels of filtering on the instabilities and dynamics of the atmospheric flows. Moreover, we assess the impact of the oceanic coupling, the dissipation scheme, and the resolution on the spectra of LEs.
The PUMA Lyapunov spectrum is computed for two different values of the meridional temperature gradient defining the Newtonian forcing to the temperature field. The increase in the gradient gives rise to a higher baroclinicity and stronger instabilities, corresponding to a larger dimension of the unstable manifold and a larger first LE. The KaplanâYorke dimension of the attractor increases as well. The convergence rate of the rate function for the large deviation law of the finite-time Lyapunov exponents (FTLEs) is fast for all exponents, which can be interpreted as resulting from the absence of a clear-cut atmospheric timescale separation in such a model.
The MAOOAM spectra show that the dominant atmospheric instability is correctly represented even at low resolutions. However, the dynamics of the central manifold, which is mostly associated with the ocean dynamics, is not fully resolved because of its associated long timescales, even at intermediate orders. As expected, increasing the mechanical atmosphereâocean coupling coefficient or introducing a turbulent diffusion parametrisation reduces the KaplanâYorke dimension and KolmogorovâSinai entropy. In all considered configurations, we are not yet in the regime in which one can robustly define large deviation laws describing the statistics of the FTLEs.
This paper highlights the need to investigate the natural variability of the atmosphereâocean coupled dynamics by associating rate of growth and decay of perturbations with the physical modes described using the formalism of the covariant Lyapunov vectors and considering long integrations in order to disentangle the dynamical processes occurring at all timescales
Variability and Predictability of a reduced-order land atmosphere coupled model
This study delves into the predictability of atmospheric blocking, zonal, and
transition patterns utilizing a simplified coupled model. Initially, we
comprehensively scrutinize the model's responses to environmental parameters
like solar radiation, surface friction, and atmosphere-ground heat exchange.
Subsequently, employing Gaussian mixture clustering, we successfully delineate
distinct blocking, zonal, and transition flow regimes, unveiling their
dependencies on surface friction. To gauge predictability and persistence, we
compute the averaged local Lyapunov exponents for each regime. Our
investigation uncovers the presence of zonal, blocking, and transition regimes,
particularly under conditions of reduced surface friction. As surface friction
increases further, the system transitions to a state characterized by two
blocking regimes and a transition regime. Intriguingly, periodic behavior
emerges under specific surface friction values, returning to patterns observed
under low friction coefficients. Model resolution increase impacts the system
in a way that only two regimes are then obtained with the clustering: the
transition phase disappears and the predictability drops to roughly 2 days for
both of the remaining regimes. In accordance with previous research findings,
our study underscores that when all three regimes coexist, zonal patterns
exhibit a more extended predictability horizon compared to blocking patterns.
Remarkably, transition patterns exhibit reduced predictability when coexisting
with the other regimes. In addition, within a specified range of surface
friction values where two blocking regimes are found, it is observed that
blocked atmospheric situations in the west of the applied topography are marked
by instabilities and reduced predictability in contrast to the blockings
appearing on the eastern side of the topography.Comment: 26 pages, 15 figures, 1 table, Submitted to ES
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