scholarly journals Zonal Asymmetry of the Annular Mode and Its Downstream Subtropical Jet: An Idealized Model Study

2011 ◽  
Vol 68 (9) ◽  
pp. 1946-1973 ◽  
Author(s):  
Jie Song ◽  
Wen Zhou ◽  
Xin Wang ◽  
Chongyin Li
Keyword(s):  
Storm Track ◽  
Uniform Structure ◽  
Annular Mode ◽  
Idealized Model ◽  
Synoptic Eddy ◽  
Subtropical Jet ◽  
Model Study ◽  
Zonal Asymmetry ◽  
Variance Explained ◽  
Tropical Heating

Abstract This study investigates linkages between the zonal asymmetry of the annular mode (AM) zonal pattern and the subtropical jet (STJ) over its downstream regions of the storm track by using an idealized model. Observational analyses show that the AM zonal patterns are more zonally asymmetric during days when the STJ downstream of the storm track is unusually strong, and vice versa. In the idealized model, the STJ downstream of the storm track is varied by introducing an additional zonally localized tropical heating. The model’s AM variability exhibits a nearly zonally uniform structure when there is no or only weak tropical heating. However, the signatures of the AM are locally strengthened in the heating sector; thus, the AM zonal pattern is zonally asymmetric when the tropical heating is large enough to create a strong STJ. The model results also show that the percentage of the variance explained by the AM, the persistence of the AM index, and the intensity of eddy feedback are also increased when the tropical heating becomes stronger. It is argued herein that the zonal asymmetry of the AM pattern is caused by the zonal asymmetry of the anomalous synoptic eddy forcing projecting on the AM, which is primarily due to the zonal asymmetry of the variations of the storm track between the nonheating and heating sectors.

scholarly journals Seasonal Variations of Subtropical Precipitation Associated with the Southern Annular Mode

2014 ◽  
Vol 27 (9) ◽  
pp. 3446-3460 ◽  
Author(s):  
Harry H. Hendon ◽  
Eun-Pa Lim ◽  
Hanh Nguyen
Keyword(s):  
Seasonal Variation ◽  
Seasonal Variations ◽  
Meridional Circulation ◽  
Storm Track ◽  
Phase Speed ◽  
Southern Annular Mode ◽  
Subtropical Climate ◽  
Flux Divergence ◽  
Annular Mode ◽  
Subtropical Jet

Abstract Seasonal variations of subtropical precipitation anomalies associated with the southern annular mode (SAM) are explored for the period 1979–2011. In all seasons, high-polarity SAM, which refers to a poleward-shifted eddy-driven westerly jet, results in increased precipitation in high latitudes and decreased precipitation in midlatitudes as a result of the concomitant poleward shift of the midlatitude storm track. In addition, during spring–autumn, high SAM also results in increased rainfall in the subtropics. This subtropical precipitation anomaly is absent during winter. This seasonal variation of the response of subtropical precipitation to the SAM is shown to be consistent with the seasonal variation of the eddy-induced divergent meridional circulation in the subtropics (strong in summer and weak in winter). The lack of an induced divergent meridional circulation in the subtropics during winter is attributed to the presence of the wintertime subtropical jet, which causes a broad latitudinal span of eddy momentum flux divergence due primarily to higher phase speed eddies breaking poleward of the subtropical jet and lower speed eddies not breaking until they reach the equatorward flank of the subtropical jet. During the other seasons, when the subtropical jet is less distinctive, the critical line for both high and low speed eddies is on the equatorward flank of the single jet and so breaking in the subtropics occurs over a narrow range of latitudes. The implications of these findings for the seasonality of future subtropical climate change, in which a shift to high SAM in all seasons is expected to be promoted, are discussed.

Observed Patterns of Month-to-Month Storm-Track Variability and Their Relationship to the Background Flow*

2010 ◽  
Vol 67 (5) ◽  
pp. 1420-1437 ◽  
Author(s):  
Justin J. Wettstein ◽  
John M. Wallace
Keyword(s):  
Northern Hemisphere ◽  
Geopotential Height ◽  
Storm Track ◽  
Meridional Wind ◽  
Storm Tracks ◽  
Mean Flow ◽  
Annular Mode ◽  
The North ◽  
Jet Variability ◽  
The North Atlantic Oscillation

Abstract Month-to-month storm-track variability is investigated via EOF analyses performed on ERA-40 monthly-averaged high-pass filtered daily 850-hPa meridional heat flux and the variances of 300-hPa meridional wind and 500-hPa height. The analysis is performed both in hemispheric and sectoral domains of the Northern and Southern Hemispheres. Patterns characterized as “pulsing” and “latitudinal shifting” of the climatological-mean storm tracks emerge as the leading sectoral patterns of variability. Based on the analysis presented, storm-track variability on the spatial scale of the two Northern Hemisphere sectors appears to be largely, but perhaps not completely, independent. Pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy-forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability. The North Atlantic Oscillation (NAO)–Northern Hemisphere annular mode (NAM) is associated with a pulsing of the Atlantic storm track and a meridional displacement of the upper-tropospheric jet exit region, whereas the eastern Atlantic (EA) pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper-tropospheric jet. Analogous patterns of storm-track and upper-tropospheric jet variability are associated with the western Pacific (WP) and Pacific–North America (PNA) patterns. Wave–mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere. The Southern Hemisphere annular mode (SAM) is associated with a latitudinally shifting storm track over the South Atlantic and Indian Oceans and a pulsing South Pacific storm track. The patterns of storm-track variability are shown to be related to simple distortions of the climatological-mean upper-tropospheric jet.

scholarly journals The Storm-Track Response to Idealized SST Perturbations in an Aquaplanet GCM

2008 ◽  
Vol 65 (9) ◽  
pp. 2842-2860 ◽  
Author(s):  
David James Brayshaw ◽  
Brian Hoskins ◽  
Michael Blackburn
Keyword(s):  
Large Scale ◽  
Storm Track ◽  
Hadley Cell ◽  
Atmospheric Response ◽  
Subtropical Jet ◽  
Poleward Shift ◽  
Sst Anomaly ◽  
Hadley Centre ◽  
Sst Gradient ◽  
Sst Anomalies

Abstract The tropospheric response to midlatitude SST anomalies has been investigated through a series of aquaplanet simulations using a high-resolution version of the Hadley Centre atmosphere model (HadAM3) under perpetual equinox conditions. Model integrations show that increases in the midlatitude SST gradient generally lead to stronger storm tracks that are shifted slightly poleward, consistent with changes in the lower-tropospheric baroclinicity. The large-scale atmospheric response is, however, highly sensitive to the position of the SST gradient anomaly relative to that of the subtropical jet in the unperturbed atmosphere. In particular, when SST gradients are increased very close to the subtropical jet, then the Hadley cell and subtropical jet is strengthened while the storm track and eddy-driven jet are shifted equatorward. Conversely, if the subtropical SST gradients are reduced and the midlatitude gradients increased, then the storm track shows a strong poleward shift and a well-separated eddy-driven jet is produced. The sign of the SST anomaly is shown to play a secondary role in determining the overall tropospheric response. These findings are used to provide a new and consistent interpretation of some previous GCM studies concerning the atmospheric response to midlatitude SST anomalies.

scholarly journals The Connection between the Southern Annular Mode and a Feature-Based Perspective on Southern Hemisphere Midlatitude Winter Variability

2020 ◽  
Vol 33 (1) ◽  
pp. 115-129 ◽  
Author(s):  
Clemens Spensberger ◽  
Michael J. Reeder ◽  
Thomas Spengler ◽  
Matthew Patterson
Keyword(s):  
Storm Track ◽  
Principal Component ◽  
Southern Annular Mode ◽  
Strong Correlations ◽  
Sea Level Pressure ◽  
Annular Mode ◽  
Rossby Wave Breaking ◽  
The Pacific ◽  
Feature Based ◽  
Consistent Picture

AbstractThis article provides a reconciling perspective on the two main, but contradictory, interpretations of the southern annular mode (SAM). SAM was originally thought to characterize meridional shifts in the storm track across the entire hemisphere. This perspective was later questioned, and SAM was interpreted as a statistical artifact depending on the choice of base region for the principal component analysis. Neither perspective, however, fully describes SAM. We show that SAM cannot be interpreted in terms of midlatitude variability, as SAM merely modulates the most poleward part of the cyclone tracks and only marginally influences the distribution of other weather-related features of the storm track (e.g., position of jet axes and Rossby wave breaking). Instead, SAM emerges as the leading pattern of geopotential variability due to strong correlations of sea level pressure around the Antarctic continent. As SAM correlates strongly both with the pan-Antarctic mean temperature and the meridional heat flux through 65°S, we hypothesize that SAM can be interpreted as a measure of the degree of the (de)coupling between Antarctica and the southern midlatitudes. As an alternative way of characterizing southern midlatitude variability, we seek domains in which the leading EOF patterns of both the geopotential and storm-track features yield a dynamically consistent picture. This approach is successful for the South Pacific. Here the leading variability patterns are closely related to the Pacific–South America pattern and point toward an NAO-like variability.

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 The Structure and Dynamics of the Stratospheric Northern Annular Mode in CMIP5 Simulations

2014 ◽  
Vol 28 (1) ◽  
pp. 86-107 ◽  
Author(s):  
Yun-Young Lee ◽  
Robert X. Black
Keyword(s):  
Rossby Wave ◽  
Planetary Wave ◽  
Regional Analysis ◽  
Storm Track ◽  
Polar Vortex ◽  
Wave Activity ◽  
Flux Analysis ◽  
Annular Mode ◽  
Northern Annular Mode ◽  
Structure And Dynamics

Abstract The structure and dynamics of stratospheric northern annular mode (SNAM) events in CMIP5 simulations are studied, emphasizing (i) stratosphere–troposphere coupling and (ii) disparities between high-top (HT) and low-top (LT) models. Compared to HT models, LT models generally underrepresent SNAM amplitude in stratosphere, consistent with weaker polar vortex variability, as demonstrated by Charlton-Perez et al. Interestingly, however, this difference does not carry over to the associated zonal-mean SNAM signature in troposphere, which closely resembles observations in both HT and LT models. Nonetheless, a regional analysis illustrates that both HT and LT models exhibit anomalously weak and eastward shifted (compared to observations) storm track and sea level pressure anomaly patterns in association with SNAM events. Dynamical analyses of stratosphere–troposphere coupling are performed to further examine the distinction between HT and LT models. Variability in stratospheric planetary wave activity is reduced in LT models despite robust concomitant tropospheric variability. A meridional heat flux analysis indicates relatively weak vertical Rossby wave coupling in LT models consistent with the excessive damping events discussed by Shaw et al. Eliassen–Palm flux cross sections reveal that Rossby wave propagation is anomalously weak above the tropopause in LT models, suggesting that weak polar vortex variability in LT models is due, at least in part, to the inability of tropospheric planetary wave activity to enter the stratosphere. Although the results are consistent with anomalously weak vertical dynamical coupling in LT models during SNAM events, there is little impact upon attendant tropospheric variability. The physical reason behind this apparent paradox represents an important topic for future study.

Intraseasonal Modulation of the North Pacific Storm Track by Tropical Convection in Boreal Winter

2011 ◽  
Vol 24 (4) ◽  
pp. 1122-1137 ◽  
Author(s):  
Yi Deng ◽  
Tianyu Jiang
Keyword(s):  
North Pacific ◽  
Storm Track ◽  
Tropical Convection ◽  
Boreal Winter ◽  
Mean Flow ◽  
Central Pacific ◽  
Synoptic Eddy ◽  
The North ◽  
The North Pacific ◽  
North Pacific Storm Track

Abstract The modulation of the North Pacific storm track by tropical convection on intraseasonal time scales (30–90 days) in boreal winter (December–March) is investigated using the NCEP–NCAR reanalysis and NOAA satellite outgoing longwave radiation (OLR) data. Multivariate empirical orthogonal function (MEOF) analysis and case compositing based upon the principal components (PCs) of the EOFs reveal substantial changes in the structure and intensity of the Pacific storm track quantified by vertically (925–200 mb) averaged synoptic eddy kinetic energy (SEKE) during the course of a typical Madden–Julian oscillation (MJO) event. The storm-track response is characterized by an amplitude-varying dipole propagating northeastward as the center of the anomalous tropical convection moves eastward across the eastern Indian Ocean and the western-central Pacific. A diagnosis of the SEKE budget indicates that the storm-track anomaly is induced primarily by changes in the convergence of energy flux, baroclinic conversion, and energy generation due to the interaction between synoptic eddies and intraseasonal flow anomalies. This demonstrates the important roles played by eddy–mean flow interaction and eddy–eddy interaction in the development of the extratropical response to MJO variability. The feedback of synoptic eddy to intraseasonal flow anomalies is pronounced: when the center of the enhanced tropical convection is located over the Maritime Continent (western Pacific), the anomalous synoptic eddy forcing partly drives an upper-tropospheric anticyclonic (cyclonic) and, to its south, a cyclonic (anticyclonic) circulation anomaly over the North Pacific. Associated with the storm-track anomaly, a three-band (dry–wet–dry) anomaly in both precipitable water and surface precipitation propagates poleward over the eastern North Pacific and induces intraseasonal variations in the winter hydroclimate over western North America.

An Idealized Model Study Relevant to the Dynamics of the Midwinter Minimum of the Pacific Storm Track

2005 ◽  
Vol 62 (4) ◽  
pp. 1209-1225 ◽  
Author(s):  
Yi Deng ◽  
Mankin Mak
Keyword(s):  
Nonlinear Model ◽  
Baroclinic Instability ◽  
Storm Track ◽  
Simultaneous Increase ◽  
Synoptic Variability ◽  
Model Study ◽  
The Pacific ◽  
Reference Potential ◽  
Dynamical Nature ◽  
Instability Growth

Abstract The synoptic variability of a two-level quasigeostrophic flow in response to plausible changes in the forcing of a localized baroclinic jet is investigated in the context of the midwinter minimum of the Pacific storm track (MWM). The changes in the model forcing are introduced in terms of a reference potential vorticity field that is associated with plausible changes in the global baroclinicity, zonal variation of the baroclinicity, and horizontal deformation over the Pacific from early winter to midwinter conditions. It is found that the modal instability growth rate of perturbation in such a localized jet is significantly reduced in spite of an increase in the local baroclinicity. The dynamical nature of such an effect can be interpreted as a generalized barotropic governor effect on localized baroclinic instability. The existence of three instability regimes is established on the basis of energetics characteristics. The intensity of the nonlinear model storm track is reduced by about 30% in response to a change in the forcing condition from early to midwinter. The characteristics of the linear model storm track and nonlinear model storm track are compared. The overall results support a hypothesis that MWM could stem from a sufficiently large increase in the stabilizing influence of the local barotropic process in spite of a simultaneous increase in its local baroclinicity in the Pacific jet from early to midwinter.

Sign in / Sign up

Export Citation Format

Share Document