235 research outputs found
Cloud/climate sensitivity experiments
A study of the relationships between large-scale cloud fields and large scale circulation patterns is presented. The basic tool is a multi-level numerical model comprising conservation equations for temperature, water vapor and cloud water and appropriate parameterizations for evaporation, condensation, precipitation and radiative feedbacks. Incorporating an equation for cloud water in a large-scale model is somewhat novel and allows the formation and advection of clouds to be treated explicitly. The model is run on a two-dimensional, vertical-horizontal grid with constant winds. It is shown that cloud cover increases with decreased eddy vertical velocity, decreased horizontal advection, decreased atmospheric temperature, increased surface temperature, and decreased precipitation efficiency. The cloud field is found to be well correlated with the relative humidity field except at the highest levels. When radiative feedbacks are incorporated and the temperature increased by increasing CO2 content, cloud amounts decrease at upper-levels or equivalently cloud top height falls. This reduces the temperature response, especially at upper levels, compared with an experiment in which cloud cover is fixed
Optimally coherent sets in geophysical flows: A new approach to delimiting the stratospheric polar vortex
The "edge" of the Antarctic polar vortex is known to behave as a barrier to
the meridional (poleward) transport of ozone during the austral winter. This
chemical isolation of the polar vortex from the middle and low latitudes
produces an ozone minimum in the vortex region, intensifying the ozone hole
relative to that which would be produced by photochemical processes alone.
Observational determination of the vortex edge remains an active field of
research. In this letter, we obtain objective estimates of the structure of the
polar vortex by introducing a new technique based on transfer operators that
aims to find regions with minimal external transport. Applying this new
technique to European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40
three-dimensional velocity data we produce an improved three-dimensional
estimate of the vortex location in the upper stratosphere where the vortex is
most pronounced. This novel computational approach has wide potential
application in detecting and analysing mixing structures in a variety of
atmospheric, oceanographic, and general fluid dynamical settings
Generalized Quasilinear Approximation: Application to Zonal Jets
Quasilinear theory is often utilized to approximate the dynamics of fluids exhibiting significant interactions between mean flows and eddies. We present a generalization of quasilinear theory to include dynamic mode interactions on the large scales. This generalized quasilinear (GQL) approximation is achieved by separating the state variables into large and small zonal scales via a spectral filter rather than by a decomposition into a formal mean and fluctuations. Nonlinear interactions involving only small zonal scales are then removed. The approximation is conservative and allows for scattering of energy between small-scale modes via the large scale (through nonlocal spectral interactions). We evaluate GQL for the paradigmatic problems of the driving of large-scale jets on a spherical surface and on the beta plane and show that it is accurate even for a small number of large-scale modes. As GQL is formally linear in the small zonal scales, it allows for the closure of the system and can be utilized in direct statistical simulation schemes that have proved an attractive alternative to direct numerical simulation for many geophysical and astrophysical problems
The backbone of the climate network
We propose a method to reconstruct and analyze a complex network from data
generated by a spatio-temporal dynamical system, relying on the nonlinear
mutual information of time series analysis and betweenness centrality of
complex network theory. We show, that this approach reveals a rich internal
structure in complex climate networks constructed from reanalysis and model
surface air temperature data. Our novel method uncovers peculiar wave-like
structures of high energy flow, that we relate to global surface ocean
currents. This points to a major role of the oceanic surface circulation in
coupling and stabilizing the global temperature field in the long term mean
(140 years for the model run and 60 years for reanalysis data). We find that
these results cannot be obtained using classical linear methods of multivariate
data analysis, and have ensured their robustness by intensive significance
testing.Comment: 6 pages, 5 figure
Influence of turbulent advection on a phytoplankton ecosystem with nonuniform carrying capacity
In this work we study a plankton ecosystem model in a turbulent flow. The plankton model we consider contains logistic growth with a spatially varying background carrying capacity and the flow dynamics are generated using the two-dimensional (2D) Navier-Stokes equations. We characterize the system in terms of a dimensionless parameter, γ TB / TF, which is the ratio of the ecosystem biological time scales TB and the flow time scales TF. We integrate this system numerically for different values of γ until the mean plankton reaches a statistically stationary state and examine how the steady-state mean and variance of plankton depends on γ. Overall we find that advection in the presence of a nonuniform background carrying capacity can lead to very different plankton distributions depending on the time scale ratio γ. For small γ the plankton distribution is very similar to the background carrying capacity field and has a mean concentration close to the mean carrying capacity. As γ increases the plankton concentration is more influenced by the advection processes. In the largest γ cases there is a homogenization of the plankton concentration and the mean plankton concentration approaches the harmonic mean, 1/K -1. We derive asymptotic approximations for the cases of small and large γ. We also look at the dependence of the power spectra exponent, β, on γ where the power spectrum of plankton is k-β. We find that the power spectra exponent closely obeys β=1+2/γ as predicted by earlier studies using simple models of chaotic advection
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The ventilated pool: a model of subtropical mode water
An analytical model of subtropical mode water is presented, based on ventilated thermocline theory and on numerical solutions of a planetary geostrophic basin model. In ventilated thermocline theory, the western pool is a region bounded on the east by subsurface streamlines that outcrop at the western edge of the interior, and in which additional dynamical assumptions are necessary to complete the solution. Solutions for the western pool were originally obtained under the assumption that the potential vorticity of the subsurface layer was homogenized. In the present theory, it is instead assumed that all of the water in the pool region is ventilated, and therefore that all the Sverdrup transport is carried in the uppermost, outcropping layer. The result is the formation of a deep, vertically homogeneous, fluid layer in the northwest corner of the subtropical gyre that extends from the surface to the base of the ventilated thermocline. This ventilated pool is an analog of the observed subtropical mode waters. The pool also has the interesting properties that it determines its own boundaries and affects the global potential vorticity-pressure relationship. When there are multiple outcropping layers, ventilated pool fluid is subducted to form a set of nested annuli in ventilated, subsurface layers, which are the deepest subducted layers in the ventilated thermocline.KEYWORDS: Ventilated thermocline, Mode water, Ocean circulation, Planetary geostroph
The roles of latent heating and dust in the structure and variability of the northern Martian polar vortex
The winter polar vortices on Mars are annular in terms of their potential
vorticity (PV) structure, a phenomenon identified in observations, reanalysis
and some numerical simulations. Some recent modeling studies have proposed that
condensation of atmospheric carbon dioxide at the winter pole is a contributing
factor to maintaining the annulus through the release of latent heat. Dust and
topographic forcing are also known to be causes of internal and interannual
variability in the polar vortices. However, coupling between these factors
remains uncertain, and previous studies of their impact on vortex structure and
variability have been largely limited to a single Martian global climate model
(MGCM). Here, by further developing a novel MGCM, we decompose the relative
roles of latent heat and dust as drivers for the variability and structure of
the northern Martian polar vortex. We also consider how Martian topography
modifies the driving response. By also analyzing a reanalysis dataset we show
that there is significant dependence in the polar vortex structure and
variability on the observations assimilated. In both model and reanalysis, high
atmospheric dust loading (such as that seen during a global dust storm) can
disrupt the vortex, cause the destruction of PV in the low-mid altitudes (> 0.1
hPa), and significantly reduce spatial and temporal vortex variability. Through
our simulations, we find that the combination of dust and topography primarily
drives the eddy activity throughout the Martian year, and that although latent
heat release can produce an annular vortex, it has a relatively minor effect on
vortex variability.Comment: 16 pages, 14 figures, The Planetary Science Journa
Inferring directed climatic interactions with renormalized partial directed coherence and directed partial correlation
ACKNOWLEDGMENTS This work was supported in part by Spanish MINECO/FEDER (FIS2015-66503-C3-2-P) and ITN LINC (FP7 289447). C.M. also acknowledges partial support from ICREA ACADEMIAPeer reviewedPublisher PD
Generation and Structure of Solitary Rossby Vortices in Rotating Fluids
The formation of zonal flows and vortices in the generalized
Charney-Hasegawa-Mima equation is studied. We focus on the regime when the size
of structures is comparable to or larger than the deformation (Rossby) radius.
Numerical simulations show the formation of anticyclonic vortices in unstable
shear flows and ring-like vortices with quiescent cores and vorticity
concentrated in a ring. Physical mechanisms that lead to these phenomena and
their relevance to turbulence in planetary atmospheres are discussed.Comment: 3 pages in REVTeX, 5 postscript figures separately, submitted to
Phys. Rev.
Fluctuation Properties of Steady-State Langevin Systems
Motivated by stochastic models of climate phenomena, the steady-state of a
linear stochastic model with additive Gaussian white noise is studied.
Fluctuation theorems for nonequilibrium steady-states provide a constraint on
the character of these fluctuations. The properties of the fluctuations which
are unconstrained by the fluctuation theorem are investigated and related to
the model parameters. The irreversibility of trajectory segments, which
satisfies a fluctuation theorem, is used as a measure of nonequilibrium
fluctuations. The moments of the irreversibility probability density function
(pdf) are found and the pdf is seen to be non-Gaussian. The average
irreversibility goes to zero for short and long trajectory segments and has a
maximum for some finite segment length, which defines a characteristic
timescale of the fluctuations. The initial average irreversibility growth rate
is equal to the average entropy production and is related to
noise-amplification. For systems with a separation of deterministic timescales,
modes with timescales much shorter than the trajectory timespan and whose noise
amplitudes are not asymptotically large, do not, to first order, contribute to
the irreversibility statistics, providing a potential basis for dimensional
reduction.Comment: 8 pages, to be published in Physical Review
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