scholarly journals Surface Control of the Frequency of Stratospheric Sudden Warming Events

2019 ◽  
Vol 32 (15) ◽  
pp. 4753-4766 ◽  
Author(s):  
Zheng Wu ◽  
Thomas Reichler
Keyword(s):  
Climate Models ◽  
Nonlinear Function ◽  
Vertical Wind Shear ◽  
Polar Vortex ◽  
Transient Wave ◽  
Stratospheric Polar Vortex ◽  
Stratospheric Sudden Warming ◽  
Surface Control ◽  
Wide Range ◽  
Range Of Values

AbstractThe frequency of stratospheric sudden warming events (SSWs) is an important characteristic of the coupled stratosphere–troposphere system. However, many modern climate models are unable to reproduce the observed SSW frequency. A previous study suggested that one of the reasons could be the momentum damping at the surface. The goal of the present study is to understand what determines the climatological SSW frequency and how the surface damping comes into play. To this end, we conduct a parameter sweep with an idealized model, using a wide range of values for the surface damping. It is found that the SSW frequency is a strong and nonlinear function of the surface damping. Various tropospheric and stratospheric factors are identified to link the surface damping to the SSW frequency. The factors include the magnitude of the surface winds, the meridional and vertical wind shear, the synoptic eddy activity in the troposphere, the transient wave activity flux at the lower stratosphere, and the strength of the stratospheric polar vortex. Mathematical–statistical modeling, informed by the parameter sweep, is used to quantify how the different factors relate to each other. This successfully reproduces the complex variations of the SSW frequency with the surface damping seen in the parameter sweep. The results may help in explaining some of the difficulties that climate models have in simulating the observed SSW frequency.

Planetary Wave Breaking and Tropospheric Forcing as Seen in the Stratospheric Sudden Warming of 2006

2009 ◽  
Vol 66 (2) ◽  
pp. 495-507 ◽  
Author(s):  
Lawrence Coy ◽  
Stephen Eckermann ◽  
Karl Hoppel
Keyword(s):  
North Atlantic ◽  
Wave Breaking ◽  
Potential Temperature ◽  
Polar Vortex ◽  
Stratospheric Polar Vortex ◽  
Stratospheric Sudden Warming ◽  
Advanced Level ◽  
Earth Observing System ◽  
The North ◽  
Temperature Surface

Abstract The major stratospheric sudden warming (SSW) of January 2006 is examined using meteorological fields from Goddard Earth Observing System version 4 (GEOS-4) analyses and forecast fields from the Navy Operational Global Atmospheric Prediction System–Advanced Level Physics, High Altitude (NOGAPS-ALPHA). The study focuses on the upper tropospheric forcing that led to the major SSW and the vertical structure of the subtropic wave breaking near 10 hPa that moved low tropical values of potential vorticity (PV) to the pole. Results show that an eastward-propagating upper tropospheric ridge over the North Atlantic with its associated cold temperature perturbations (as manifested by high 360-K potential temperature surface perturbations) and large positive local values of meridional heat flux directly forced a change in the stratospheric polar vortex, leading to the stratospheric subtropical wave breaking and warming. Results also show that the anticyclonic development, initiated by the subtropical wave breaking and associated with the poleward advection of the low PV values, occurred over a limited altitude range of approximately 6–10 km. The authors also show that the poleward advection of this localized low-PV anomaly was associated with changes in the Eliassen–Palm (EP) flux from equatorward to poleward, suggesting an important role for Rossby wave reflection in the SSW of January 2006. Similar upper tropospheric forcing and subtropical wave breaking were found to occur prior to the major SSW of January 2003.

scholarly journals Eurasian Cold Air Outbreaks under Different Arctic Stratospheric Polar Vortex Strengths

2019 ◽  
Vol 76 (5) ◽  
pp. 1245-1264 ◽  
Author(s):  
Jinlong Huang ◽  
Wenshou Tian
Keyword(s):  
Climate Models ◽  
Sensible Heat Flux ◽  
Coupled Model ◽  
Surface Air Temperature ◽  
Polar Vortex ◽  
Northern Eurasia ◽  
Reanalysis Dataset ◽  
Stratospheric Polar Vortex ◽  
Cold Air ◽  
Cold Air Outbreaks

Abstract This study analyzes the differences and similarities of Eurasian cold air outbreaks (CAOs) under the weak (CAOW), strong (CAOS), and neutral (CAON) stratospheric polar vortex states and examines the potential links between the polar vortex and Eurasian CAOs. The results indicate that the colder surface air temperature (SAT) over Europe in the earlier stages of CAOW events is likely because the amplitude of the preexisting negative North Atlantic Oscillation pattern is larger in CAOW events than in CAON and CAOS events. Marked by the considerably negative stratospheric Arctic Oscillation signals entering the troposphere, the SAT at midlatitudes over eastern Eurasia in CAOW events is colder than in CAON events. A larger diabatic heating rate related to a positive sensible heat flux anomaly in CAOW events likely offsets, to some degree, the cooling effect caused by the stronger cold advection and makes the differences in area-averaged SAT anomalies over northern Eurasia between the CAOW and CAON events look insignificant in most stages. Massive anomalous waves from the low-latitude western Pacific merge over northeastern Eurasia, then weaken the westerly wind over this region to create favorable conditions for southward advection of cold air masses in the earlier stages of all three types of CAOs. This study further analyzes the interannual relationship between the stratospheric polar vortex strength and the intensity of Eurasian CAOs and finds that climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) relative to the reanalysis dataset tend to underestimate the correlation between them. The relationship between them is strengthening under representative concentration pathway 4.5 (RCP4.5) and 8.5 (RCP8.5) scenarios over the period 2006–60. In addition, the intensity of Eurasian CAOs exhibits a decreasing trend in the past and in the future.

scholarly journals Stratosphere–Troposphere Coupling in a Relatively Simple AGCM: The Importance of Stratospheric Variability

2009 ◽  
Vol 22 (8) ◽  
pp. 1920-1933 ◽  
Author(s):  
Edwin P. Gerber ◽  
Lorenzo M. Polvani
Keyword(s):  
General Circulation ◽  
Polar Vortex ◽  
Circulation Model ◽  
Stationary Waves ◽  
Stratospheric Polar Vortex ◽  
Stratospheric Sudden Warming ◽  
Annular Modes ◽  
Dynamical Coupling ◽  
The Impact ◽  
Stratospheric Variability

Abstract The impact of stratospheric variability on the dynamical coupling between the stratosphere and the troposphere is explored in a relatively simple atmospheric general circulation model. Variability of the model’s stratospheric polar vortex, or polar night jet, is induced by topographically forced stationary waves. A robust relationship is found between the strength of the stratospheric polar vortex and the latitude of the tropospheric jet, confirming and extending earlier results in the absence of stationary waves. In both the climatological mean and on intraseasonal time scales, a weaker vortex is associated with an equatorward shift in the tropospheric jet and vice versa. It is found that the mean structure and variability of the vortex in the model is very sensitive to the amplitude of the topography and that Northern Hemisphere–like variability, with a realistic frequency of stratospheric sudden warming events, occurs only for a relatively narrow range of topographic heights. When the model captures sudden warming events with fidelity, however, the exchange of information both upward and downward between the troposphere and stratosphere closely resembles that in observations. The influence of stratospheric variability on variability in the troposphere is demonstrated by comparing integrations with and without an active stratosphere. A realistic, time-dependent stratospheric circulation increases the persistence of the tropospheric annular modes, and the dynamical coupling is most apparent prior to and following stratospheric sudden warming events.

scholarly journals A New Look at Stratospheric Sudden Warmings. Part I: Climatology and Modeling Benchmarks

2007 ◽  
Vol 20 (3) ◽  
pp. 449-469 ◽  
Author(s):  
Andrew J. Charlton ◽  
Lorenzo M. Polvani
Keyword(s):  
Numerical Model ◽  
Zonal Wind ◽  
Polar Vortex ◽  
Identification Algorithm ◽  
Event Type ◽  
Stratospheric Polar Vortex ◽  
Stratospheric Sudden Warming ◽  
Model Simulations ◽  
Stratospheric Sudden Warmings ◽  
Dynamical Coupling

Abstract Stratospheric sudden warmings are the clearest and strongest manifestation of dynamical coupling in the stratosphere–troposphere system. While many sudden warmings have been individually documented in the literature, this study aims at constructing a comprehensive climatology: all major midwinter warming events are identified and classified, in both the NCEP–NCAR and 40-yr ECMWF Re-Analysis (ERA-40) datasets. To accomplish this a new, objective identification algorithm is developed. This algorithm identifies sudden warmings based on the zonal mean zonal wind at 60°N and 10 hPa, and classifies them into events that do and do not split the stratospheric polar vortex. Major midwinter stratospheric sudden warmings are found to occur with a frequency of approximately six events per decade, and 46% of warming events lead to a splitting of the stratospheric polar vortex. The dynamics of vortex splitting events is contrasted to that of events where the vortex is merely displaced off the pole. In the stratosphere, the two types of events are found to be dynamically distinct: vortex splitting events occur after a clear preconditioning of the polar vortex, and their influence on middle-stratospheric temperatures lasts for up to 20 days longer than vortex displacement events. In contrast, the influence of sudden warmings on the tropospheric state is found to be largely insensitive to the event type. Finally, a table of dynamical benchmarks for major stratospheric sudden warming events is compiled. These benchmarks are used in a companion study to evaluate current numerical model simulations of the stratosphere.

scholarly journals Diversity of the Wintertime Arctic Oscillation Pattern among CMIP5 Models: Role of the Stratospheric Polar Vortex

2019 ◽  
Vol 32 (16) ◽  
pp. 5235-5250 ◽  
Author(s):  
Hainan Gong ◽  
Lin Wang ◽  
Wen Chen ◽  
Renguang Wu ◽  
Wen Zhou ◽  
...  
Keyword(s):  
North Atlantic ◽  
North Pacific ◽  
Arctic Oscillation ◽  
Climate Models ◽  
Polar Vortex ◽  
Cmip5 Models ◽  
Stratospheric Polar Vortex ◽  
The North ◽  
The North Atlantic ◽  
The North Pacific

AbstractThe wintertime Arctic Oscillation (AO) pattern in phase 5 of the Coupled Model Intercomparison Project (CMIP5) climate models displays notable differences from the reanalysis. The North Pacific center of the AO pattern is larger in the ensemble mean of 27 models than in the reanalysis, and the magnitude of the North Pacific center of the AO pattern varies largely among the models. This study investigates the plausible sources of the diversity of the AO pattern in the models. Analysis indicates that the amplitude of the North Pacific center is associated with the coupling between the North Pacific and North Atlantic, which in turn is primarily modulated by the strength of the stratospheric polar vortex. A comparative analysis is conducted for the strong polar vortex (SPV) and weak polar vortex (WPV) models. It reveals that a stronger stratospheric polar vortex induces more planetary waves to reflect from the North Pacific to the North Atlantic and more wave activity fluxes to propagate from the North Pacific to the North Atlantic in the SPV models than in the WPV models. Thus, the coupling of atmospheric circulation between the North Pacific and North Atlantic is stronger in the SPV models, which facilitates more North Pacific variability to be involved in the AO variability and induces a stronger North Pacific center in the AO pattern. The increase in vertical resolution may improve the simulation of the stratospheric polar vortex and thereby reduces the model biases in the North Pacific–North Atlantic coupling and thereby the amplitude of the North Pacific center of the AO pattern in models.

scholarly journals The role of Arctic sea ice loss in projected polar vortex changes

2020 ◽  
Author(s):  
Marlene Kretschmer ◽  
Giuseppe Zappa ◽  
Theodore G. Shepherd
Keyword(s):  
Sea Ice ◽  
Climate Models ◽  
Causal Effect ◽  
Regional Climate ◽  
Polar Vortex ◽  
Internal Variability ◽  
Arctic Sea Ice ◽  
Stratospheric Polar Vortex ◽  
Ensemble Mean

Abstract. The Northern Hemisphere stratospheric polar vortex (SPV) plays a key role for mid-latitude weather and climate. However, in what way the SPV will respond to global warming is not clear, with climate models disagreeing on the sign and magnitude of projected SPV strength change. Here we address the role of Barents and Kara (BK) sea ice loss in this. We provide evidence for a non-linear response of the SPV to global mean temperature change, dependent on the time the BK Seas become ice-free. Using a causal network approach, we demonstrate that climate models show some partial support for the previously proposed link between low BK sea ice in autumn and a weakened winter SPV, but that this effect is plausibly very small relative to internal variability. Yet, given the expected dramatic decrease of sea ice in the future, a small causal effect can explain all of the projected ensemble-mean SPV weakening, approximately one-half of the ensemble spread at the middle of the 21st century, and one-third of the spread at the end of the century. Finally, we note that most models have unrealistic amounts of BK sea ice, meaning that their SPV response to ice loss is unrealistic. Bias-adjusting for this effect leads to pronounced differences in SPV response of individual models at both ends of the spectrum, but has no strong consequences for the overall ensemble mean and spread. Overall, our results indicate the importance of exploring all plausible implications of a changing Arctic for regional climate risk assessments.

scholarly journals Characteristics of Baroclinic Wave Packets during Strong and Weak Stratospheric Polar Vortex Events

2010 ◽  
Vol 67 (10) ◽  
pp. 3190-3207 ◽  
Author(s):  
Ian N. Williams ◽  
Stephen J. Colucci
Keyword(s):  
Zonal Wind ◽  
Lower Stratosphere ◽  
Baroclinic Instability ◽  
Phase Speed ◽  
Vertical Wind Shear ◽  
Polar Vortex ◽  
Vertical Shear ◽  
Stratospheric Polar Vortex ◽  
The Pacific ◽  
Observational Record

Abstract This study tested a numerical and theoretical prediction that the stratosphere and troposphere are coupled through the effect of stratospheric vertical wind shear on baroclinic waves. Wavelengths, phase speeds, and background quasigeostrophic potential vorticity gradients were analyzed over the Pacific and Atlantic during strong and weak stratospheric polar vortex events and interpreted in terms of the counterpropagating Rossby wave perspective on baroclinic instability. Effects of zonal variations in the background flow were included in the analysis of phase speeds. Observed changes in wave packet average wavelength and phase speed support the vertical shear hypothesis for stratosphere–troposphere coupling; however, changes in the intrinsic phase speed contradict the hypothesis. This inconsistency was resolved by considering the change in zonal wind speed in the lower stratosphere, which accounts for most of the change in phase speeds during strong and weak vortex events. Changes in the average wavelengths and meridional wave activity flux are also consistent with this modified hypothesis involving the stratospheric zonal wind. The results demonstrate that a simple mechanism for stratosphere–troposphere coupling can be found in the observational record.

scholarly journals The role of Barents–Kara sea ice loss in projected polar vortex changes

2020 ◽  
Vol 1 (2) ◽  
pp. 715-730
Author(s):  
Marlene Kretschmer ◽  
Giuseppe Zappa ◽  
Theodore G. Shepherd
Keyword(s):  
Sea Ice ◽  
Climate Models ◽  
Causal Effect ◽  
Regional Climate ◽  
Polar Vortex ◽  
Internal Variability ◽  
Stratospheric Polar Vortex ◽  
Global Mean Temperature ◽  
Ensemble Mean

Abstract. The Northern Hemisphere stratospheric polar vortex (SPV) plays a key role in mid-latitude weather and climate. However, in what way the SPV will respond to global warming is not clear, with climate models disagreeing on the sign and magnitude of projected SPV strength change. Here we address the potential role of Barents and Kara (BK) sea ice loss in this. We provide evidence for a non-linear response of the SPV to global mean temperature change, which is coincident with the time the BK seas become ice-free. Using a causal network approach, we demonstrate that climate models show some partial support for the previously proposed link between low BK sea ice in autumn and a weakened winter SPV but that this effect is plausibly very small relative to internal variability. Yet, given the expected dramatic decrease in sea ice in the future, even a small causal effect can explain all of the projected ensemble-mean SPV weakening, approximately one-half of the ensemble spread in the middle of the 21st century, and one-third of the spread at the end of the century. Finally, we note that most models have unrealistic amounts of BK sea ice, meaning that their SPV response to ice loss is unrealistic. Bias adjusting for this effect leads to pronounced differences in SPV response of individual models at both ends of the spectrum but has no strong consequences for the overall ensemble mean and spread. Overall, our results indicate the importance of exploring all plausible implications of a changing Arctic for regional climate risk assessments.

scholarly journals The Downward Influence of Uncertainty in the Northern Hemisphere Stratospheric Polar Vortex Response to Climate Change

2018 ◽  
Vol 31 (16) ◽  
pp. 6371-6391 ◽  
Author(s):  
Isla R Simpson ◽  
Peter Hitchcock ◽  
Richard Seager ◽  
Yutian Wu ◽  
Patrick Callaghan
Keyword(s):  
Northern Hemisphere ◽  
General Circulation ◽  
Polar Vortex ◽  
General Circulation Models ◽  
Cmip5 Models ◽  
Regression Analyses ◽  
Stratospheric Polar Vortex ◽  
Wide Range ◽  
Tropospheric Circulation ◽  
Downward Influence

Abstract General circulation models display a wide range of future predicted changes in the Northern Hemisphere winter stratospheric polar vortex. The downward influence of this stratospheric uncertainty on the troposphere has previously been inferred from regression analyses across models and is thought to contribute to model spread in tropospheric circulation change. Here we complement such regression analyses with idealized experiments using one model where different changes in the zonal-mean stratospheric polar vortex are artificially imposed to mimic the extreme ends of polar vortex change simulated by models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The influence of the stratospheric vortex change on the tropospheric circulation in these experiments is quantitatively in agreement with the inferred downward influence from across-model regressions, indicating that such regressions depict a true downward influence of stratospheric vortex change on the troposphere below. With a relative weakening of the polar vortex comes a relative increase in Arctic sea level pressure (SLP), a decrease in zonal wind over the North Atlantic, drying over northern Europe, and wetting over southern Europe. The contribution of stratospheric vortex change to intermodel spread in these quantities is assessed in the CMIP5 models. The spread, as given by 4 times the across-model standard deviation, is reduced by roughly 10% on regressing out the contribution from stratospheric vortex change, while the difference between models on extreme ends of the distribution in terms of their stratospheric vortex change can reach up to 50% of the overall model spread for Arctic SLP and 20% of the overall spread in European precipitation.

Analysis of interaction between parameterized orographic gravity wave drag and resolved dynamics in CMAM.

2021 ◽  
Author(s):  
Petr Šácha ◽  
Aleš Kuchař ◽  
Christoph Jacobi ◽  
Petr Pišoft ◽  
Roland Eichinger ◽  
...  
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Rossby Waves ◽  
Middle Atmosphere ◽  
Polar Vortex ◽  
Internal Gravity Waves ◽  
Wave Drag ◽  
Dynamical Analysis ◽  
Future Research ◽  
Stratospheric Polar Vortex

<div class="page" title="Page 1"> <div class="layoutArea"> <div class="column"> <p>In the extratropical atmosphere, Rossby waves (RWs) and internal gravity waves (GWs) propagating from the troposphere mediate a coupling with the middle atmosphere by influencing the dynamics herein. In current generation chemistry-climate models (CCMs), GWs are usually smaller than the model resolution and the majority of their spectrum therefore must be parameterized. From observations, we know that GWs are intermittent and asymmetrically distributed around the globe, which holds to some extent also for the parameterized GW drag (GWD) (in particular for orographic GWD (oGWD)). The GW parameterizations in CCMs are usually tuned to mitigate biases in the zonal mean climatology of particular quantities, but the complex interaction of parameterized GWs with the large- scale circulation and resolved waves in the models remains to date poorly understood.</p> <p>This presentation will combine observational evidence, idealized modeling and dynamical analysis of a CCM output to study both the short-term and long-term model response to the oGWD. Our results demonstrate that the oGW-resolved dynamics interaction is a complex two-way process, with the most prominent oGWD impact being the alteration of propagation of planetary-scale Rossby waves on a time-scale of a few days. The conclusions give a novel perspective on the importance of oGWD for the stratospheric polar vortex and atmospheric transport studies outlining potential foci of future research.</p> </div> </div> </div>

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