Dynamics of Synoptic Eddy and Low-Frequency Flow Interaction. Part III: Baroclinic Model Results

2006 ◽  
Vol 63 (7) ◽  
pp. 1709-1725 ◽  
Author(s):  
L-L. Pan ◽  
F-F. Jin ◽  
M. Watanabe
Keyword(s):  
Low Frequency ◽  
Equation System ◽  
The Self ◽  
The Arctic ◽  
Mean Flow ◽  
Synoptic Eddy ◽  
Model Study ◽  
Baroclinic Model ◽  
The Arctic Oscillation ◽  
Extratropical Circulation

Abstract In this three-part study, a linear closure has been developed for the synoptic eddy and low-frequency flow (SELF) interaction and demonstrated that internal dynamics plays an important role in generating the leading low-frequency modes in the extratropical circulation anomalies during cold seasons. In Part III, a new linearized primitive equation system is first derived for time-mean flow anomalies. The dynamical operator of the system includes a traditional part depending on the observed climatological mean state and an additional part from the SELF feedback closure utilizing the observed climatological properties of synoptic eddy activity. The latter part relates nonlocally all the anomalous eddy-forcing terms in equations of momentum, temperature, and surface pressure to the time-mean flow anomalies. Using the observational data, the closure was validated with reasonable success, and it was found that terms of the SELF feedback in the momentum and pressure equations tend to reinforce the low-frequency modes, whereas those in the thermodynamic equation tends to damp the temperature anomalies to make the leading modes equivalent barotropic. Through singular vector analysis of the linear dynamical operator, it is highlighted that the leading modes of the system resemble the observed patterns of the Arctic Oscillation, Antarctic Oscillation, and Pacific–North American pattern, in which the SELF feedback plays an essential role, consistent with the finding of the barotropic model study in Part II.

Dynamics of Synoptic Eddy and Low-Frequency Flow Interaction. Part I: A Linear Closure

2006 ◽  
Vol 63 (7) ◽  
pp. 1677-1694 ◽  
Author(s):  
F-F. Jin ◽  
L-L. Pan ◽  
M. Watanabe
Keyword(s):  
Atmospheric Circulation ◽  
Low Frequency ◽  
Equation Model ◽  
The Self ◽  
Quasi Equilibrium ◽  
Mean Flow ◽  
Linear Dynamic ◽  
Synoptic Eddy ◽  
Linear Framework ◽  
Low Frequency Variability

Abstract The interaction between synoptic eddy and low-frequency flow (SELF) has been recognized for decades to play an important role in the dynamics of the low-frequency variability of the atmospheric circulation. In this three-part study a linear framework with a stochastic basic flow capturing both the climatological mean flow and climatological measures of the synoptic eddy flow is proposed. Based on this linear framework, a set of linear dynamic equations is derived for the ensemble-mean eddy forcing that is generated by anomalous time-mean flows. By assuming that such dynamically determined eddy-forcing anomalies approximately represent the time-mean anomalies of the synoptic eddy forcing and by using a quasi-equilibrium approximation, an analytical nonlocal dynamical closure is obtained for the two-way SELF feedback. This linear closure, directly relating time-mean anomalies of the synoptic eddy forcing to the anomalous time–mean flow, becomes an internal part of a new linear dynamic system for anomalous time–mean flow that is referred to as the low-frequency variability of the atmospheric circulation in this paper. In Part I, the basic approach for the SELF closure is illustrated using a barotropic model. The SELF closure is tested through the comparison of the observed eddy-forcing patterns associated with the leading low-frequency modes with those derived using the SELF feedback closure. Examples are also given to illustrate an important role played by the SELF feedback in regulating the atmospheric responses to remote forcing. Further applications of the closure for understanding the dynamics of low-frequency modes as well as the extension of the closure to a multilevel primitive equation model will be given in Parts II and III, respectively.

scholarly journals Empirical Mode Reduction in a Model of Extratropical Low-Frequency Variability

2006 ◽  
Vol 63 (7) ◽  
pp. 1859-1877 ◽  
Author(s):  
D. Kondrashov ◽  
S. Kravtsov ◽  
M. Ghil
Keyword(s):  
Numerical Study ◽  
Singular Spectrum Analysis ◽  
Low Frequency ◽  
The Arctic ◽  
Reduced Model ◽  
The North ◽  
Low Frequency Variability ◽  
Intraseasonal Oscillations ◽  
The Arctic Oscillation ◽  
The North Atlantic Oscillation

Abstract This paper constructs and analyzes a reduced nonlinear stochastic model of extratropical low-frequency variability. To do so, it applies multilevel quadratic regression to the output of a long simulation of a global baroclinic, quasigeostrophic, three-level (QG3) model with topography; the model's phase space has a dimension of O(104). The reduced model has 45 variables and captures well the non-Gaussian features of the QG3 model's probability density function (PDF). In particular, the reduced model's PDF shares with the QG3 model its four anomalously persistent flow patterns, which correspond to opposite phases of the Arctic Oscillation and the North Atlantic Oscillation, as well as the Markov chain of transitions between these regimes. In addition, multichannel singular spectrum analysis identifies intraseasonal oscillations with a period of 35–37 days and of 20 days in the data generated by both the QG3 model and its low-dimensional analog. An analytical and numerical study of the reduced model starts with the fixed points and oscillatory eigenmodes of the model's deterministic part and uses systematically an increasing noise parameter to connect these with the behavior of the full, stochastically forced model version. The results of this study point to the origin of the QG3 model's multiple regimes and intraseasonal oscillations and identify the connections between the two types of behavior.

An Eddy-Feedback Model for Propagating Annular Modes

2020 ◽  
Author(s):  
Sandro Lubis ◽  
Pedram Hassanzadeh
Keyword(s):  
Low Frequency ◽  
Wave Activity ◽  
Mean Flow ◽  
Wave Source ◽  
Annular Mode ◽  
Annular Modes ◽  
The North ◽  
Feedback Model ◽  
Low Frequency Variability ◽  
Extratropical Circulation

<p>Some types of extreme events<span> in the extratropics are often associated with anomalous jet behaviors. A well-known example is the annular mode, wherein its variation e.g., the meandering in the north-south direction of the jet, disrupts the normal eastward migration of troughs and ridges.</span> <span>Since the seminal works of Lorenz and Hartmann, the annular mode has been mostly analyzed based on single EOF mode. However, a recent study showed that the first and second leading EOFs are strongly correlated at long lags and are manifestations of a single oscillatory decaying-mode. This means that the first and second leading EOF modes interact and exert feedbacks on each other. The purpose of this study is to develop an eddy-feedback model for the extratropical low-frequency variability that includes these cross-EOF feedbacks to better isolate the eddy momentum/heat flux changes with time- and/or zonal-mean flow. Our results show that, in the presence of the poleward-propagation regime, the first and second leading EOF modes interact and exert positive feedbacks at lags ~10 (~20) days about ~0.07 (~0.16) day</span><span><sup>-1</sup></span><span> in the reanalysis (idealized GCM). This feedback is often ignored in the previous studies, and in fact, the magnitude is nearly double the feedback exerted by the single EOF mode. We found that this apparent positive eddy feedback is a result of the effect of jet pulsation (strengthening and weakening) in zonal flow variability (z</span><span><sub>2</sub></span><span>) on the eddy momentum flux due to the meandering in the north-south direction of the jet (m</span><span><sub>1</sub></span><span>). A finite-amplitude eddy-mean flow interaction diagnostic has been performed to demonstrate the dynamics governing the positive feedback in the propagating regime of the annular modes. It is shown that the poleward propagation is caused by an orchestrated combination of equatorward propagation of wave activity (baroclinic process), nonlinear wave breaking (barotropic processes), and radiative relaxation. The latter two processes follow the first one, and as such, the meridional propagation of Rossby wave activity (likely generated by an enhanced baroclinic wave source at a low level) is the central mechanism. Finally, our model calculations suggest the rule of thumb that the propagating annular modes (i.e., when EOF1 and EOF2 together represent quasi-periodic poleward propagation of zonal-mean flow anomalies) exist if the ratio of the fractional variance and decorrelation time-scale of EOF2 to that of EOF1 exceeds 0.5 or the two leading PCs showing maximum correlations at larger lags. These criteria can be used to assess the predictability of preferred modes of extratropical circulation in GCMs. The present study advances and potentially transforms the state of our understanding of the low-frequency variability of the extratropical circulation.</span></p>

scholarly journals Atmospheric Circulation Regimes in a Nonlinear Quasi-Geostrophic Model

2015 ◽  
Vol 2015 ◽  
pp. 1-19 ◽  
Author(s):  
Henriette Labsch ◽  
Dörthe Handorf ◽  
Klaus Dethloff ◽  
Michael V. Kurgansky
Keyword(s):  
Atmospheric Circulation ◽  
Arctic Oscillation ◽  
Significant Contribution ◽  
Decadal Variability ◽  
Low Frequency ◽  
The Arctic ◽  
Satisfactory Model ◽  
Low Frequency Variability ◽  
Interannual And Decadal Variability ◽  
The Arctic Oscillation

Atmospheric low-frequency variability and circulation regime behavior are investigated in the context of a quasi-geostrophic (QG) three-level T63 model of the wintertime atmospheric circulation over the Northern Hemisphere (NH). The model generates strong interannual and decadal variability, with the domination of the annular mode of variability. It successfully reproduces a satisfactory model climatology and the most important atmospheric circulation regimes. The positive phase of the Arctic Oscillation is a robust feature of the quasi-geostrophic T63 model. The model results based on QG dynamics underlie atmospheric regime behavior in the extratropical NH and suggest that nonlinear internal processes deliver significant contribution to the atmospheric climate variability on interannual and decadal timescales.

scholarly journals Coupling Modes among Action Centers of Wave–Mean Flow Interaction and Their Association with the AO/NAM

2012 ◽  
Vol 25 (2) ◽  
pp. 447-458 ◽  
Author(s):  
Nan Zhao ◽  
Sujie Liang ◽  
Yihui Ding
Keyword(s):  
Northern Hemisphere ◽  
Three Dimensional ◽  
Polar Vortex ◽  
The Arctic ◽  
Mean Flow ◽  
Annular Mode ◽  
Flow Interaction ◽  
Flow Interactions ◽  
Local Wave ◽  
The Arctic Oscillation

Abstract The Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM) is attributed to wave–mean flow interaction over the extratropical region of the Northern Hemisphere. This wave–mean flow interaction is closely related to three atmospheric centers of action, corresponding to three regional oscillations: the NAO, the PNA, and the stratosphere polar vortex (SPV), respectively. It is then natural to infer that local wave–mean flow interactions at these three centers of action are dynamically coupled to each other and can thus explain the main aspects of the three-dimensional coherent structure of the annular mode, which also provides a possible way to understand how the local NAO–PNA–SPV perspective and the hemispheric AO/NAM perspective are interrelated. By using a linear stochastic model of coupled oscillators, this study suggests that two coupling modes among the PNA, NAO, and SPV are related to the two-dimensional pattern in sea level pressure of the AO. Although both of them may contribute to the AO/NAM, only one is related to the three-dimensional equivalent barotropic structure of the NAM, while the other one is mainly restricted to the troposphere. So the equivalent barotropic structure of the NAM, as usually revealed by the regression of the zonal wind against the AO index, is the manifestation of just one coupling mode. Another coupled mode is a baroclinic mode that resembles the NAM only in the troposphere. However, this similarity in spatial structures does not imply that the total variability of the AO/NAM index can be explained by those of the NAO–PNA–SPV or their coupling modes, because of the existence of the variability that may contribute to the AO/NAM, produced outside of these three regions. It is estimated that the coupling modes can jointly explain 44% of the variance of the AO/NAM index.

scholarly journals Dynamics of Synoptic Eddy and Low-Frequency Flow Interaction. Part II: A Theory for Low-Frequency Modes

2006 ◽  
Vol 63 (7) ◽  
pp. 1695-1708 ◽  
Author(s):  
F-F. Jin ◽  
L-L. Pan ◽  
M. Watanabe
Keyword(s):  
Storm Track ◽  
Low Frequency ◽  
Prototype Model ◽  
Mean Flow ◽  
Zonal Scale ◽  
Synoptic Eddy ◽  
The Pacific ◽  
Storm Track Activity ◽  
Recurrent Patterns ◽  
The Antarctic

Abstract Amidst stormy atmospheric circulation, there are prominent recurrent patterns of variability in the planetary circulation, such as the Antarctic Oscillation (AAO), Arctic Oscillation (AO) or North Atlantic Oscillation (NAO), and the Pacific–North America (PNA) pattern. The role of the synoptic eddy and low-frequency flow (SELF) feedback in the formation of these dominant low-frequency modes is investigated in this paper using the linear barotropic model with the SELF feedback proposed in Part I. It is found that the AO-like and AAO-like leading singular modes of the linear dynamical system emerge from the stormy background flow as the result of a positive SELF feedback. This SELF feedback also prefers a PNA-like singular vector as well among other modes under the climatological conditions of northern winters. A model with idealized conditions of basic mean flow and activity of synoptic eddy flow and a prototype model are also used to illustrate that there is a natural scale selection for the AAO- and AO-like modes through the positive SELF feedback. The zonal scale of the localized features in the Atlantic (southern Indian Ocean) for AO (AAO) is largely related to the zonal extent of the enhanced storm track activity in the region. The meridional dipole structures of AO- and AAO-like low-frequency modes are favored because of the scale-selective positive SELF feedback, which can be heuristically understood by the tilted-trough mechanism.

scholarly journals Eddy-Induced Instability for Low-Frequency Variability

2010 ◽  
Vol 67 (6) ◽  
pp. 1947-1964 ◽  
Author(s):  
F-F. Jin
Keyword(s):  
Dynamic Instability ◽  
Low Frequency ◽  
Basic Flow ◽  
Mean Flow ◽  
Eddy Activity ◽  
Planetary Scale ◽  
Synoptic Eddy ◽  
Low Frequency Variability ◽  
Eddy Structures ◽  
Scale Perturbation

Abstract Synoptic eddy–mean flow interaction has been recognized as one of the key sources for extratropical low-frequency variability. In this paper, the underlying dynamics of this interaction are examined from the perspective of a synoptic eddy-induced dynamic instability. To delineate this instability, a barotropic model is used that is linearized with respect to a stochastic basic flow prescribed with both climatologic-mean flow and synoptic eddy statistics. This linear model captures the dynamics of feedback between synoptic eddy and low-frequency flow through a dynamic closure that relates the anomalous eddy vorticity forcing to low-frequency flow anomalies. After reducing this dynamic closure to its fundamental components, this stability is elucidated with analytical results under the most idealized consideration of basic flow. It is shown that through systematic alteration of the synoptic eddy structures in the basic flow, a low-frequency planetary-scale perturbation generates anomalous eddy vorticity forcing positively proportional to the vorticity of the perturbation. Such a perturbation amplifies itself; the energy source for its growth comes from the reservoir residing in the basic synoptic eddy activity. Thus, the growth rate of the synoptic eddy-induced dynamic instability depends primarily on the kinetic energy level of the basic synoptic eddy activity. Moreover, this instability is scale selective with preference for zonal symmetric and asymmetric planetary-scale modes, whose meridional and zonal scales are roughly in the range of those of the observed leading low-frequency patterns. Analysis of this synoptic eddy-induced instability provides insight into the origin of extratropical low-frequency variability.

scholarly journals Classification of Polar-Night Jet Oscillations and Their Relationship to Fast and Slow Variations in a Global Mechanistic Circulation Model of the Stratosphere and Troposphere

2010 ◽  
Vol 23 (23) ◽  
pp. 6438-6444 ◽  
Author(s):  
Masashi Kohma ◽  
Seiya Nishizawa ◽  
Shigeo Yoden
Keyword(s):  
Low Frequency ◽  
Polar Vortex ◽  
Circulation Model ◽  
Negative Temperature ◽  
The Arctic ◽  
Eof Analysis ◽  
Polar Night ◽  
Low Pass ◽  
Cold Events ◽  
The Arctic Oscillation

Abstract Polar-night jet oscillation (PJO), which is a low-frequency intraseasonal oscillatory variation in the winter stratosphere, is analyzed statistically with a 14 000-yr-long dataset obtained with an idealized global mechanistic circulation model of the stratosphere and troposphere. After performing an empirical orthogonal function (EOF) analysis on the low-pass-filtered time series of the northern polar temperature, 10 647 PJO events are identified and classified into four groups. About 80% of them are two groups of warm events while the rest are two groups of cold events, which are newly identified variations with opposite sign from the warm events by the same EOF analysis. All of them show slow downward propagations of a positive or negative temperature anomaly, with a relatively short or long lifetime. Composite analysis with such a large number of samples shows that each group has its own typical relationship to unfiltered relatively fast variations in the polar stratosphere known as stratospheric sudden warming and polar vortex intensification and to the slow variation in the troposphere known as the Arctic Oscillation. Statistically significant evidence of the downward dynamical influence of PJO on the surface is obtained for a group of warm events with a longer lifetime.

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