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K. Bhattacharya
,
M. Ghil
, and
I. L. Vulis

Abstract

We present a simple, deterministic energy-balance model with possible relevance to climatic variations on the time scale of glaciation cycles. The lag between ice-sheet extent and zonally-averaged temperature is modeled as a time delay in the ice-albedo feedback. The model exhibits self-sustained oscillations which are quasi-periodic or aperiodic in character. Fourier spectra of solutions have the features of many paleo-climatic records: peaks of variable height and width superimposed on a continuous, red-noise type background.

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P. Bernardet
,
A. Butet
,
M. Déqué
,
M. Ghil
, and
R. L. Pfeffer

Abstract

Experiments were performed in a rotating, differentially heated annulus, with and without bottom topography of azimuthal wavenumber 2. Both water and a viscous glycerol-water mixture were used as a working fluid. In one series of experiments, measurements of azimuthal velocity u were carded out by Doppler-laser velocimetry at midradius and at ⅓ and ⅔ depth. In the other, temperature measurements were made by a set of thermistors at three different heights and three different radii. Results were analyzed by Fourier transformation, separately in space and in time, and in terms of complex empirical orthogonal functions (CEOFs).

In the experiments with topography, a standing wave 2 is generated, with larger amplitude at the upper level and a tilted wave structure. The two leading CEOFs contain a very large fraction of the variance, and give an excellent picture of the spatial modulation of the traveling baroclinic waves. The dominant baroclinic wave has azimuthal wavenumber 4, 5 or 6, according to the nondimensional parameters of the given experiment, and pronounced sidebands due to the topography. The modulation of this wave is such that its largest amplitude occurs at the lower level upstream of the two topographic ridges. At the upper level, the modulation is weaker, with the maximum wave amplitude located downstream of the ridges. Partial decoupling of the two wave trains attached to the two ridges is evident in one experiment.

Low-frequency vacillation of the entire flow pattern is apparent; this vacillation has a period of about 50 annulus rotations in the viscous mixture. The possible relevance of this topographically induced vacillation to the extratropical 30–60 day oscillation is discussed.

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A. Deloncle
,
R. Berk
,
F. D’Andrea
, and
M. Ghil

Abstract

Two novel statistical methods are applied to the prediction of transitions between weather regimes. The methods are tested using a long, 6000-day simulation of a three-layer, quasigeostrophic (QG3) model on the sphere at T21 resolution.

The two methods are the k nearest neighbor classifier and the random forest method. Both methods are widely used in statistical classification and machine learning; they are applied here to forecast the break of a regime and subsequent onset of another one. The QG3 model has been previously shown to possess realistic weather regimes in its northern hemisphere and preferred transitions between these have been determined. The two methods are applied to the three more robust transitions; they both demonstrate a skill of 35%–40% better than random and are thus encouraging for use on real data. Moreover, the random forest method allows one, while keeping the overall skill unchanged, to efficiently adjust the ratio of correctly predicted transitions to false alarms.

A long-standing conjecture has associated regime breaks and preferred transitions with distinct directions in the reduced model phase space spanned by a few leading empirical orthogonal functions of its variability. Sensitivity studies for several predictors confirm the crucial influence of the exit angle on a preferred transition path. The present results thus support the paradigm of multiple weather regimes and their association with unstable fixed points of atmospheric dynamics.

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S. L. Marcus
,
M. Ghil
, and
J. O. Dickey

Abstract

Intraseasonal oscillations in a 3-yr, perpetual-January simulation are examined using a version of the UCLA GCM that produces no self-sustained Madden–Julian oscillation in the Tropics. A robust, 40-day oscillation is found to arise in the model's Northern Hemisphere (NH) extratropics when standard topography is present. Part I of this study addressed the zonally averaged component of the GCM oscillation, manifested in wind- and pressure-induced variations in atmospheric angular momentum (AAM). The focus here is on the spatial features of the oscillation as manifested in the variability of the 500-mb height field.

A standing, wavenumber-two pattern is found in the NH extratropics, which undergoes tilted-trough vacillation in conjunction with the model's AAM oscillation. High (low) values of AAM are associated with low (high) 500-mb heights over the northeast Pacific and Atlantic Oceans; the two centers' of action slightly different frequencies give rise to a long-period modulation (of about 300 days) in the amplitude of the intraseasonal oscillation. Global correlations with the leading empirical orthogonal functions of the NH extratropical 500-mb height field show northeast–southwest teleconnection patterns extending into the Tropics, similar to those found in observational studies. The zonally averaged latent heating in the Tropics exhibits no intraseasonal periodicity, but a 39-day oscillation is found in cumulus precipitation over the western Indian Ocean. The latter shows significant coherence with EOF 1 but is absent in three shorter no-mountain experiments (see Part I), indicating that it may be remotely forced by the intraseasonal oscillation that arises in the model's NH extratropics only in the standard-topography experiment.

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Nathan Paldor
,
Ching-Hwang Liu
,
Michael Ghil
, and
Roger M. Wakimoto

Abstract

A short-wave instability theory is applied to secondary waves on a narrow cold-front rainband observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). The basic mean state is approximated by the parabolic, geostrophically balanced interface between two layers of homogeneous density. The observed wavelength of perturbations along the ERICA cold front is about 20–30 km and their doubling time is about 2 hours. The observed wavelength is well within the short-wave regime of the theory, which yields a growth rate in good agreement with the ERICA observations. The spatial patterns of both the horizontal and vertical velocity components observed during ERICA are consistent with the model-derived patterns.

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S. L. Marcus
,
M. Ghil
, and
J. O. Dickey

Abstract

Variations in atmospheric angular momentum (AAM) are examined in a three-year simulation of the large-scale atmosphere with perpetual January forcing. The simulation is performed with a version of the UCLA general circulation model that contains no tropical Madden-Julian Oscillation (MJO). In addition, the results of three shorter experiments with no topography are analyzed. The three-year standard topography run contains no significant intraseasonal AAM periodicity in the tropics, consistent with the lack of the MJO, but produces a robust, 42-day AAM oscillation in the Northern Hemisphere (NH) extratropics. The model tropics undergoes a barotropic, zonally symmetric oscillation, driven by an exchange of mass with the NH extratropics. No intraseasonal periodicity is found in the average tropical latent heating field, indicating that the model oscillation is dynamically rather than thermodynamically driven. The no-mountain runs fail to produce an intraseasonal AAM oscillation, consistent with a topographic origin for the NH extratropical oscillation in the standard model. The spatial patterns of the oscillation in the 500-mb height field, and the relationship of the extratropical oscillation to intraseasonal variations in the tropics, will be discussed in Part II of this study.

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Christopher M. Strong
,
Fei-Fei Jin
, and
Michael Ghil

Abstract

It has recently been suggested that oscillatory topographic instability could contribute to low-frequency variability over the Northern Hemisphere midlatitudes. A barotropic potential vorticity model, with a hierarchy of forcing and topography configurations on the sphere, is used to investigate the nature of low-frequency oscillations induced by such instabilities. Steady-state solutions of the model include multiple unstable equilibria that sustain oscillatory instabilities with periods of 10–15 days, 35–50 days, and 150–180 days, for a realistic forcing pattern.

Time-dependent solutions exhibit chaotic behavior with episodic oscillations, featuring both the intraseasonal (35–50 day) and biweekly (10–15 day) modes. The former is dominated by standing spatial patterns, the latter by traveling wave patterns. The phases of the intraseasonal oscillation are robust for all cases, exhibiting a clear oscillatory exchange of atmospheric angular momentum with the solid earth via mountain torque. It is demonstrated, through linear stability analysis on the sphere, that the intraseasonal oscillations are induced by topographic instabilities.

The role of the seasonal cycle is studied by prescribing an annual cycle in the forcing. In this case, the winter forcing is more favorable than the summer for the occurrence of episodic intraseasonal oscillations. Recent observations are consistent with this model result.

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