scholarly journals On Annular Modes and Zonal Jets

2008 ◽  
Vol 21 (9) ◽  
pp. 1963-1978 ◽  
Author(s):  
Adam H. Monahan ◽  
John C. Fyfe
Keyword(s):  
Southern Hemisphere ◽  
Empirical Orthogonal Function ◽  
Zonal Wind ◽  
Domain Size ◽  
Annular Mode ◽  
Annular Modes ◽  
Orthogonal Function ◽  
Zonal Index ◽  
Zonal Jets ◽  
Analytic Expressions

Abstract This study considers the relation of the annular mode to the kinematics of a fluctuating jet in zonal-mean zonal wind and to the zonal index, using an idealized model of fluctuations in the eddy-driven jet. When the sphericity of the domain is accounted for, observed and numerically simulated annular modes for the Southern Hemisphere summertime are found to be in excellent agreement. In particular, the annular mode and zonal index mode are shown to be related but distinct. Although the annular mode is strongly (but not identically) related to fluctuations in jet position, fluctuations in jet strength and width are shown to also be important for its simulation. When the sphericity of the domain is neglected, analytic expressions for the leading empirical orthogonal function (EOF) modes of zonal-mean geopotential for the cases of individual fluctuations in jet strength, position, and width can be obtained. None of these EOF modes have the characteristics of the annular mode. In the presence of simultaneous fluctuations in jet strength and position, the leading zonal-mean geopotential EOF mode (strongly resembling the annular mode) is shown to mix the zonal index mode of zonal-mean zonal wind with other EOF modes, demonstrating why the annular mode and zonal index mode are related but distinct. The greater sensitivity to domain size of EOF modes of geopotential relative to the EOF modes of zonal-mean zonal wind is also discussed. This study focuses on the Southern Hemisphere summertime, which is characterized by a single, eddy-driven jet; the generality of the results presented suggest that the conclusions should be qualitatively unchanged in the presence of both subtropical and eddy-driven jets.

scholarly journals The Vertical Structure of Annular Modes

2018 ◽  
Vol 75 (10) ◽  
pp. 3507-3519 ◽  
Author(s):  
Aditi Sheshadri ◽  
R. Alan Plumb ◽  
Erik A. Lindgren ◽  
Daniela I. V. Domeisen
Keyword(s):  
Time Series ◽  
Empirical Orthogonal Function ◽  
Zonal Wind ◽  
Vertical Structure ◽  
Zonal Flow ◽  
Annular Modes ◽  
Orthogonal Function ◽  
Linearized System ◽  
Set Up ◽  
Stratospheric Variability

Stratosphere–troposphere interactions are conventionally characterized using the first empirical orthogonal function (EOF) of fields such as zonal-mean zonal wind. Perpetual-winter integrations of an idealized model are used to contrast the vertical structures of EOFs with those of principal oscillation patterns (POPs; the modes of a linearized system governing the evolution of zonal flow anomalies). POP structures are shown to be insensitive to pressure weighting of the time series of interest, a factor that is particularly important for a deep system such as the stratosphere and troposphere. In contrast, EOFs change from being dominated by tropospheric variability with pressure weighting to being dominated by stratospheric variability without it. The analysis reveals separate tropospheric and stratospheric modes in model integrations that are set up to resemble midwinter variability of the troposphere and stratosphere in both hemispheres. Movies illustrating the time evolution of POP structures show the existence of a fast, propagating tropospheric mode in both integrations, and a pulsing stratospheric mode with a tropospheric extension in the Northern Hemisphere–like integration.

scholarly journals Southern Hemisphere Annular Mode Variability and the Role of Optimal Nonmodal Growth

2005 ◽  
Vol 62 (6) ◽  
pp. 1947-1961 ◽  
Author(s):  
Harun A. Rashid ◽  
Ian Simmonds
Keyword(s):  
Southern Hemisphere ◽  
Zonal Wind ◽  
Positive Feedback ◽  
General Model ◽  
Initial Perturbation ◽  
Low Frequency ◽  
Southern Annular Mode ◽  
Feedback Mechanism ◽  
Annular Mode ◽  
Positive Feedback Mechanism

Abstract The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large amplitudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.

scholarly journals Preferred Modes of Variability and Their Relationship with Climate Change

2006 ◽  
Vol 19 (10) ◽  
pp. 2063-2075 ◽  
Author(s):  
Seok-Woo Son ◽  
Sukyoung Lee
Keyword(s):  
Recent Trend ◽  
Equation Model ◽  
Mean Flow ◽  
Primitive Equation ◽  
Climate Predictability ◽  
Annular Mode ◽  
Annular Modes ◽  
Modes Of Variability ◽  
Orthogonal Function ◽  
Flow Response

Abstract Spatial structure of annular modes shows a remarkable resemblance to that of the recent trend in the observed circulation (Thompson et al.). This study performs a series of multilevel primitive equation model simulations to examine the extent to which the annular mode is capable of predicting changes in the zonal-mean flow response to external heat perturbations. Each of these simulations represents a statistically steady state and differs from each other in the values of the imposed tropical heating (ℋ) and high-latitude cooling (𝒞). Defining the annular mode as the first empirical orthogonal function (EOF1) of zonal-mean tropospheric zonal wind, it is found that the “climate predictability” is generally high in the small 𝒞–large ℋ region of the parameter space, but is markedly low in the large 𝒞–small ℋ region. In the former region, EOF1 represents meridional meandering of the midlatitude jet, while in the latter region, EOF1 and EOF2 combine to represent coherent poleward propagation of zonal-mean flow anomalies. It is also found that the climate predictability tends to be higher with respect to changes in 𝒞 than to changes in ℋ. The implications of these findings for the Southern Hemisphere climate predictability are also presented.

Annular Mode–Like Variation in a Multilayer Quasigeostrophic Model

2012 ◽  
Vol 69 (10) ◽  
pp. 2940-2958 ◽  
Author(s):  
Yang Zhang ◽  
Xiu-Qun Yang ◽  
Yu Nie ◽  
Gang Chen
Keyword(s):  
Zonal Wind ◽  
Channel Model ◽  
Phase Speed ◽  
Surface Friction ◽  
Zonal Flow ◽  
Lower Phase ◽  
Annular Mode ◽  
Annular Modes ◽  
Low Level ◽  
Different Response

Abstract Eddy–zonal flow interactions in the annular modes are investigated in this study using a modified beta-plane multilayer quasigeostrophic (QG) channel model. This study shows the different response of high- and low-phase-speed (frequency) eddies to the zonal wind anomalies and suggests a baroclinic mechanism through which the two eddies work symbiotically maintaining the positive eddy feedback in the annular modes. Analysis also indicates that the different roles played by these two eddies in the annular modes are related to the differences in their critical line distributions. Eddies with higher phase speeds experience a low-level critical layer at the center of the jet. They drive the zonal wind anomalies associated with the annular mode but weaken the baroclinicity of the jet in the process. Lower-phase-speed eddies encounter low-level critical lines on the jet flanks. While their momentum fluxes are not as important for the jet shift, they play an important role by restoring the lower-level baroclinicity at the jet center, creating a positive feedback loop with the fast eddies that extends the persistence of the jet shift. The importance of the lower-level baroclinicity restoration by the low-phase-speed eddies in the annular modes is further demonstrated in sensitivity runs, in which surface friction on eddies is increased to selectively damp the low-phase-speed eddies. For simulations in which the low-phase-speed eddies become inactive, the leading mode of the zonal wind variability shifts from the position fluctuation to a pulsing of the jet intensity. Further studies indicate that the response of the lower-level baroclinicity to the zonal wind anomalies caused by the low-phase-speed eddies can be crucial in maintaining the annular mode–like variations.

scholarly journals Southern “Annular Modes” Simulated by a Climate Model—Patterns, Mechanisms, and Uses

2007 ◽  
Vol 64 (9) ◽  
pp. 3113-3131 ◽  
Author(s):  
I. G. Watterson
Keyword(s):  
Zonal Wind ◽  
Climate Model ◽  
Southern Annular Mode ◽  
Momentum Equation ◽  
Mean Flow ◽  
Annular Mode ◽  
Annular Modes ◽  
Daily Data ◽  
Physically Based ◽  
Mean Climate

Abstract Both high-latitude (HLM) and low-latitude modes (LLM) of variability of zonal wind in the Southern Hemisphere have been identified. Through an analysis of a simulation for 1871–2200 by the CSIRO Mark 3 climate model, the extent to which these might both be described as “annular modes,” based on their statistical patterns, physical mechanisms, and usefulness in climate study, is assessed. The modes are determined as EOF1 and EOF2 of vertically integrated zonal and monthly mean zonal wind, for 1871–1970. These match well those from ECMWF Re-Analysis (ERA) data and also from the earlier Mark 2 model. The mode index time series relate to largely annular patterns of local wind and surface pressure anomalies [with HLM giving the familiar southern annular mode (SAM)], and other simulated quantities. While modes calculated from 90° sectors are only moderately correlated (mostly in the polar region) for HLM, the link increases with time scale. There is little such relationship for LLM. A momentum equation analysis using daily data confirms that both zonal modes are driven by eddies, but only HLM features a positive eddy–mean flow feedback. Variation in feedback and surface damping through the seasonal cycle relate well to that in index autocorrelation, with the HLM being more persistent in summer. Stratospheric winds feature a long-lived component that tends to lead the HLM. The HLM drives sea surface temperature anomalies that persist for months, and coupling with the ocean increases variability on longer time scales. The annular variability in the warmer climate of the twenty-second century is barely changed, but the mean climate change in the far south projects strongly on the HLM. The LLM features some statistical annularity and may have some uses. However, only the HLM can be considered to be a physically based mode—the zonal-wind equivalent to the one southern annular mode.

Observations, Simulations, and Dynamics of Jet Stream Variability and Annular Modes

2010 ◽  
Vol 23 (23) ◽  
pp. 6186-6199 ◽  
Author(s):  
Joseph Kidston ◽  
D. M. W. Frierson ◽  
J. A. Renwick ◽  
G. K. Vallis
Keyword(s):  
Wind Speed ◽  
Zonal Wind ◽  
Source Region ◽  
Baroclinic Instability ◽  
Jet Stream ◽  
Mean Flow ◽  
Annular Mode ◽  
Annular Modes ◽  
Real Atmosphere ◽  
The Mean

Abstract The characteristics of the dominant pattern of extratropical variability (the so-called annular modes) are examined in the context of the theory that eddy-driven jets are self-maintaining. It is shown that there is genuine hemispheric symmetry in the variation of the zonal wind in the Southern Hemisphere but not the Northern Hemisphere. The annular mode is shown to be baroclinic in nature; it is associated with changes in the baroclinic eddy source latitude, and the latitude of the eddy source region is organized by the mean flow. This behavior is expected if there is a baroclinic feedback that encourages the maximum baroclinic instability to be coincident with the maximum zonal wind speed, and discourages the meridional vacillation of the eddy-driven jet stream. It is shown that the strength of the thermally indirect circulation that gives rise to the baroclinic feedback appears to influence the time scale of the annular mode. When the thermally indirect circulation is stronger the annular mode has a longer e-folding time in a simplified GCM. Preliminary results indicate that the same dynamics are important in the real atmosphere.

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