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 A Numerical Sensitivity Study of the Influence of Siberian Snow on the Northern Annular Mode

2012 ◽  
Vol 25 (2) ◽  
pp. 592-607 ◽  
Author(s):  
Y. Peings ◽  
D. Saint-Martin ◽  
H. Douville
Keyword(s):  
General Circulation ◽  
Arctic Oscillation ◽  
Polar Vortex ◽  
Circulation Model ◽  
Sensitivity Study ◽  
The Arctic ◽  
Annular Mode ◽  
Northern Annular Mode ◽  
Model Free ◽  
The Arctic Oscillation

Abstract The climate version of the general circulation model Action de Recherche Petite Echelle Grande Echelle (ARPEGE-Climat) is used to explore the relationship between the autumn Siberian snow and the subsequent winter northern annular mode by imposing snow anomalies over Siberia. As the model presents some biases in the representation of the polar vortex, a nudging methodology is used to obtain a more realistic but still interactive extratropical stratosphere in the model. Free and nudged sensitivity experiments are compared to discuss the dependence of the results on the northern stratosphere climatology. For each experiment, a positive snow mass anomaly imposed from October to March over Siberia leads to significant impacts on the winter atmospheric circulation in the extratropics. In line with previous studies, the model response resembles the negative phase of the Arctic Oscillation. The well-documented stratospheric pathway between snow and the Arctic Oscillation operates in the nudged experiment, while a more zonal propagation of the signal is found in the free experiment. Thus, the study provides two main findings: it supports the influence of Siberian snow on the winter extratropical circulation and highlights the importance of the northern stratosphere representation in the models to capture this teleconnection. These findings could have important implications for seasonal forecasting, as most of the operational models present biases similar to those of the ARPEGE-Climat model.

scholarly journals The impact of wave-mean flow interaction on the Northern Hemisphere polar vortex after tropical volcanic eruptions

2016 ◽  
Vol 121 (10) ◽  
pp. 5281-5297 ◽  
Author(s):  
Matthias Bittner ◽  
Claudia Timmreck ◽  
Hauke Schmidt ◽  
Matthew Toohey ◽  
Kirstin Krüger
Keyword(s):  
Northern Hemisphere ◽  
Polar Vortex ◽  
Volcanic Eruptions ◽  
Mean Flow ◽  
Flow Interaction ◽  
The Impact

Variability in the Position and Strength of Winter Jet Stream Cores Related to Northern Hemisphere Teleconnections

2008 ◽  
Vol 21 (3) ◽  
pp. 584-592 ◽  
Author(s):  
Courtenay Strong ◽  
Robert E. Davis
Keyword(s):  
North America ◽  
Northern Hemisphere ◽  
Three Dimensional ◽  
Analysis Data ◽  
The Arctic ◽  
Jet Stream ◽  
Arctic Oscillation Index ◽  
The Pacific ◽  
Height Fields ◽  
The Arctic Oscillation

Abstract Numerous teleconnections have been identified based upon spatial variability in sea level pressure or lower-tropospheric geopotential height fields. These teleconnections, which are commonly strongest in winter when the mean meridional temperature gradient is large, typically are neither derived from nor linked to changes in the jet stream. Here, winter tropospheric jet stream cores over the Northern Hemisphere (NH) are recovered from 6-hourly gridded data and interannual variability in winter jet core position, speed, and pressure are investigated in the context of NH teleconnections. Common methods for researching jet stream speed and position variability may yield unrepresentative results because jet core pressure variability is ignored (only one isobaric surface is evaluated) or pressure variability effects are smoothed (values are vertically averaged across several isobaric surfaces). In this analysis, data are extracted at the surface of maximum wind, thus controlling for jet core pressure variability and allowing for a more representative tracking of three-dimensional jet core variations. In the extratropics, the leading pattern of variability in jet core frequency is correlated with the Arctic Oscillation index (AOI) and appears as an oscillation about the spiral-shaped mean configuration of the winter jet stream. In contrast to previous research, the authors find no evidence of Pacific jet deceleration during positive AOI. The second leading mode of variability appears as a split (merged) winter-mean jet stream in the east Pacific together with a merged (split) winter-mean jet stream over North America, a pattern of change that correlates with the Pacific–North American pattern and is reflected in the amplitude of the long-wave ridge over western North America.

scholarly journals Volcanic activity sparks the Arctic Oscillation

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Weizheng Qu ◽  
Fei Huang ◽  
Jinping Zhao ◽  
Ling Du ◽  
Yong Cao
Keyword(s):  
Volcanic Activity ◽  
Volcanic Eruption ◽  
Arctic Oscillation ◽  
Polar Vortex ◽  
Volcanic Eruptions ◽  
The Arctic ◽  
Vortex System ◽  
Significant Fluctuation ◽  
The Arctic Oscillation

AbstractThe parasol effect of volcanic dust and aerosol caused by volcanic eruption results in the deepening and strengthening of the Arctic vortex system, thus stimulating or strengthening the Arctic Oscillation (AO). Three of the strongest AOs in more than a century have been linked to volcanic eruptions. Every significant fluctuation of the AO index (AOI = ΔH_middle latitudes − ΔH_Arctic) for many years has been associated with a volcanic eruption. Volcanic activity occurring at different locations in the Arctic vortex circulation will exert different effects on the polar vortex.

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.

Barotropic and Baroclinic Eddy Feedbacks in the Midlatitude Jet Variability and Responses to Climate Change–Like Thermal Forcings

2016 ◽  
Vol 74 (1) ◽  
pp. 111-132 ◽  
Author(s):  
D. Alex Burrows ◽  
Gang Chen ◽  
Lantao Sun
Keyword(s):  
Climate Change ◽  
Time Scale ◽  
Climate Models ◽  
Atmospheric Model ◽  
Finite Amplitude ◽  
Mean Flow ◽  
Annular Mode ◽  
Flow Interactions ◽  
Jet Variability ◽  
Vertically Propagating

Abstract Studies have suggested that the persistence in the meridional vacillation of the midlatitude jet (i.e., annular mode time scale) in comprehensive climate models is related to the model biases in climatological jet latitude, with important implications for projections of future climates and midlatitude weather events. Through the use of the recently developed finite-amplitude wave activity formalism and feedback quantifying techniques, this paper has quantified the role of barotropic and baroclinic eddy feedbacks in annular mode time scales using an idealized dry atmospheric model. The eddy–mean flow interaction that characterizes the persistent anomalous state of the midlatitude jet depends on processes associated with the lower-tropospheric source of vertically propagating Rossby waves, baroclinic mechanisms, and processes associated with upper-tropospheric wave propagation and breaking, barotropic mechanisms. A variety of climate change–like thermal forcings are used to generate a range of meridional shifts in the midlatitude eddy-driven jet. The idealized model shows a reduction in annular mode time scale associated with an increase in jet latitude, similar to comprehensive climate models. This decrease in time scale can be attributed to a similar decrease in the strength of the barotropic eddy feedback, which, in the positive phase of the annular mode, is characterized by anomalous potential vorticity (PV) mixing on the equatorward flank of the climatological jet. The decrease in subtropical PV mixing is, in turn, associated with a stronger subtropical jet as the eddy-driven jet is more distant from the subtropics. These results highlight the importance of subtropical eddy–mean flow interactions for the persistence of an eddy-driven jet.

Internal variability of the Arctic Oscillation and its projections

2020 ◽  
Author(s):  
Annalisa Cherchi ◽  
Paolo Oliveri ◽  
Aarnout van Delden
Keyword(s):  
Northern Hemisphere ◽  
Arctic Oscillation ◽  
Internal Variability ◽  
The Arctic ◽  
Positive Phase ◽  
Cmip5 Models ◽  
Modes Of Variability ◽  
The Future ◽  
The Arctic Oscillation

<p>The Arctic Oscillation (AO) is one of the main modes of variability of the Northern Hemisphere winter, also referred as Northern Annular Mode (NAM). The positive phase of the AO is characterized by warming/cooling over Northern Eurasia and the United States and cooling over Canada, especially over eastern Canada. Its positive phase is also characterized by very dry conditions over the Mediterranean and wet conditions over Northern Europe. A positive trend of the AO is observed for the period 1951-2011 and it is captured in CMIP5 models only when GHG-only forcing are included. In CMIP5 models the change expected is mostly mitigated by the effects of the aerosols. When considering AR5 scenarios, the AO is projected to become more positive in the future, though with a large spread among the models.</p><p>Overall the spread in the representation of the AO variability and trend is large also in experiments with present-day conditions, likely associated with the large internal variability. Unique tools to identify and measure the role of the internal variability in the model representation of the large-scale modes of variability are large ensembles where multiple members are built with different initial conditions.</p><p>Here we use the NCAR Community Model Large Ensemble (CESM-LE) composing the historical period (1920-2005) to the future (2006-2100) in a RCP8.5 scenario to measure the role of the internal variability in shaping AO variability and changes. Potential predictability of the AO index is quantified in the historical and future periods, evidencing how the members spread remain large without specific trends in these characteristics. Preliminary results indicate that the internal variability has large influence on the AO changes and related implications for the Northern Hemisphere climate.</p>

Improvement in Prediction of the Arctic Oscillation with a Realistic Ocean Initial Condition in a CGCM

2015 ◽  
Vol 28 (22) ◽  
pp. 8951-8967 ◽  
Author(s):  
Hae-Jeong Kim ◽  
Joong-Bae Ahn
Keyword(s):  
Arctic Oscillation ◽  
Polar Vortex ◽  
Polar Front ◽  
Forecast Skill ◽  
Sea Surface ◽  
The Arctic ◽  
Initial Condition ◽  
Stratospheric Polar Vortex ◽  
The Arctic Oscillation ◽  
Polar Front Jet

Abstract This study verifies the impact of improved ocean initial conditions on the Arctic Oscillation (AO) forecast skill by assessing the one-month lead predictability of boreal winter AO using the Pusan National University (PNU) coupled general circulation model (CGCM). Hindcast experiments were performed on two versions of the model, one does not use assimilated ocean initial data (V1.0) and one does (V1.1), and the results were comparatively analyzed. The forecast skill of V1.1 was superior to that of V1.0 in terms of the correlation coefficient between the predicted and observed AO indices. In the regression analysis, V1.1 showed more realistic spatial similarities than V1.0 did in predicted sea surface temperature and atmospheric circulation fields. The authors suggest the relative importance of the contribution of the ocean initial condition to the AO forecast skill was because the ocean data assimilation increased the predictability of the AO, to some extent, through the improved interaction between tropical forcing induced by realistic sea surface temperature (SST) and atmospheric circulation. In V1.1, as in the observation, the cold equatorial Pacific SST anomalies generated the weakened tropical convection and Hadley circulation over the Pacific, resulting in a decelerated subtropical jet and accelerated polar front jet in the extratropics. The intensified polar front jet implies a stronger stratospheric polar vortex relevant to the positive AO phase; hence, surface manifestations of the reflected positive AO phase were then induced through the downward propagation of the stratospheric polar vortex. The results suggest that properly assimilated initial ocean conditions might contribute to improve the predictability of global oscillations, such as the AO, through large-scale tropical ocean–atmosphere interaction.

scholarly journals Influence of the Arctic Oscillation on the Vertical Distribution of Wintertime Ozone in the Stratosphere and Upper Troposphere over the Northern Hemisphere

2017 ◽  
Vol 30 (8) ◽  
pp. 2905-2919 ◽  
Author(s):  
Jiankai Zhang ◽  
Fei Xie ◽  
Wenshou Tian ◽  
Yuanyuan Han ◽  
Kequan Zhang ◽  
...  
Keyword(s):  
Vertical Distribution ◽  
Northern Hemisphere ◽  
Arctic Oscillation ◽  
Stratospheric Ozone ◽  
The Arctic ◽  
Upper Troposphere ◽  
Eddy Transport ◽  
The Arctic Oscillation ◽  
The Vertical Distribution ◽  
Middle Stratosphere

The influence of the Arctic Oscillation (AO) on the vertical distribution of stratospheric ozone in the Northern Hemisphere in winter is analyzed using observations and an offline chemical transport model. Positive ozone anomalies are found at low latitudes (0°–30°N) and there are three negative anomaly centers in the northern mid- and high latitudes during positive AO phases. The negative anomalies are located in the Arctic middle stratosphere (~30 hPa; 70°–90°N), Arctic upper troposphere–lower stratosphere (UTLS; 150–300 hPa, 70°–90°N), and midlatitude UTLS (70–300 hPa, 30°–60°N). Further analysis shows that anomalous dynamical transport related to AO variability primarily controls these ozone changes. During positive AO events, positive ozone anomalies between 0° and 30°N at 50–150 hPa are related to the weakened meridional transport of the Brewer–Dobson circulation (BDC) and enhanced eddy transport. The negative ozone anomalies in the Arctic middle stratosphere are also caused by the weakened BDC, while the negative ozone anomalies in the Arctic UTLS are caused by the increased tropopause height, weakened BDC vertical transport, weaker exchange between the midlatitudes and the Arctic, and enhanced ozone depletion via heterogeneous chemistry. The negative ozone anomalies in the midlatitude UTLS are mainly due to enhanced eddy transport from the midlatitudes to the latitudes equatorward of 30°N, while the transport of ozone-poor air from the Arctic to the midlatitudes makes a minor contribution. Interpreting AO-related variability of stratospheric ozone, especially in the UTLS, would be helpful for the prediction of tropospheric ozone variability caused by the AO.

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