Variations in the Frequency of Stratospheric Sudden Warmings in CMIP5 and CMIP6 and Possible Causes

2020 ◽  
Vol 33 (23) ◽  
pp. 10305-10320
Author(s):  
Zheng Wu ◽  
Thomas Reichler
Keyword(s):  
Climate Models ◽  
Lower Stratosphere ◽  
Polar Vortex ◽  
Past Research ◽  
Wave Activity ◽  
Cold Bias ◽  
Dynamical Characteristic ◽  
Vertical Resolution ◽  
Upper Lid ◽  
Extratropical Tropopause

AbstractThe climatological frequency of stratospheric sudden warming events (SSWs) is an important dynamical characteristic of the extratropical stratosphere. However, modern climate models have difficulties in simulating this frequency, with many models either considerably under- or overestimating the observational estimates. Past research has found that models with a higher upper lid tend to simulate a higher and more realistic number of SSWs. The present study revisits this issue and investigates causes for biases in the simulated SSW frequency from the CMIP5 and CMIP6 models. It is found that variations in the frequency are closely related to 1) the strength of the polar vortex and 2) the upward-propagating wave activity in the stratosphere. While it is difficult to explain the variations in the polar vortex strength from the available model output, the stratospheric wave activity is influenced by different aspects of the climatological mean state of the atmosphere in the lower stratosphere. We further find that models with a finer vertical resolution in the stratosphere are overall more realistic: vertical resolution is associated with a smaller cold bias above the extratropical tropopause, more upward-propagating wave activity in the lower stratosphere, and a higher frequency of SSWs. We conclude that not only a high model lid but also a fine vertical resolution in the stratosphere is important for simulating the dynamical variability of the stratosphere.

Variations in The Frequency of Sudden Stratospheric Warmings in CMIP5 and CMIP6

2020 ◽  
Author(s):  
Zheng Wu ◽  
Thomas Reichler
Keyword(s):  
Climate Models ◽  
Model Performance ◽  
Polar Vortex ◽  
Specific Model ◽  
Wave Activity ◽  
Index Of Refraction ◽  
Highly Sensitive ◽  
Important Regulator ◽  
Extratropical Tropopause ◽  
Tropopause Temperature

<p>The frequency of sudden stratospheric warming events (SSWs) is an essential characteristic of the coupled stratosphere-troposphere system. This study is motivated by the fact that many of the CMIP5 and CMIP6 climate models considerably over- or underestimate the observed SSW frequency. The goal is to understand the causes for the large intermodel spread in the number of SSWs and relate it to specific model configurations. To this end, various dynamical quantities associated with the simulation of SSWs are investigated. It is found that variations in the SSW frequency are closely related to the strength of the polar vortex and the stratospheric wave activity. While it is difficult to explain the variations in the strength of the polar vortex, the stratospheric wave activity is strongly influenced by the background state (i.e., zonal wind and index of refraction) of the lower stratosphere. An important regulator for the background is the extratropical tropopause temperature, which in turn is associated with the vertical model resolution. Low-resolution models tend to have large biases in simulating the location and temperature of the extratropical tropopause. The results indicate that the simulated SSW frequency is a useful metric for model performance, as the frequency is highly sensitive to a number of stratospheric and tropospheric factors.</p>

scholarly journals Assessing stratospheric transport in the CMAM30 simulations using ACE-FTS measurements

2017 ◽  
Author(s):  
Felicia Kolonjari ◽  
David A. Plummer ◽  
Kaley A. Walker ◽  
Chris D. Boone ◽  
James W. Elkins ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Climate Models ◽  
Middle Atmosphere ◽  
Polar Vortex ◽  
Sampling Technique ◽  
Dynamics Simulation ◽  
Global Circulation Models ◽  
Extratropical Tropopause ◽  
Hemisphere Winter ◽  
June July

Abstract. Stratospheric transport in global circulation models and chemistry-climate models is an important component in simulating the recovery of the ozone layer as well as changes in the climate system. The Brewer-Dobson circulation is not well constrained by observations and further investigation is required to resolve uncertainties related to the mechanisms driving the circulation. This study has assessed the specified dynamics mode of the Canadian Middle Atmosphere Model (CMAM30) by comparing to the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) profile measurements of CFC-11 (CCl3F), CFC-12 (CCl2F2), and N2O. In the CMAM30 specified dynamics simulation, the meteorological fields are nudged using the ERA-Interim Reanalysis and a specified tracer was used for each species, with hemispherically-defined surface measurements used as the boundary condition. A comprehensive sampling technique along the line-of-sight of the ACE-FTS measurements has been employed to allow for direct comparisons between the simulated and measured tracer concentrations. The model consistently overpredicts tracer concentrations in the lower stratosphere, particularly in the Northern Hemisphere winter and spring seasons. The three mixing barriers investigated, including the polar vortex, the extratropical tropopause, and the tropical pipe, show that there are significant inconsistencies between the measurements and the simulations. In particular, the CMAM30 simulation exhibits too little isentropic mixing in the June-July-August season.

scholarly journals A Study of Multiple Tropopause Structures Caused by Inertia–Gravity Waves in the Antarctic

2015 ◽  
Vol 72 (5) ◽  
pp. 2109-2130 ◽  
Author(s):  
Ryosuke Shibuya ◽  
Kaoru Sato ◽  
Yoshihiro Tomikawa ◽  
Masaki Tsutsumi ◽  
Toru Sato
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Circulation Model ◽  
Cold Bias ◽  
Syowa Station ◽  
Dynamical Mechanism ◽  
The Antarctic ◽  
Inertia Gravity Waves

Abstract Multiple tropopauses (MTs) defined by the World Meteorological Organization are frequently detected from autumn to spring at Syowa Station (69.0°S, 39.6°E). The dynamical mechanism of MT events was examined by observations of the first mesosphere–stratosphere–troposphere (MST) radar in the Antarctic, the Program of the Antarctic Syowa MST/Incoherent Scatter (IS) Radar (PANSY), and of radiosondes on 8–11 April 2013. The MT structure above the first tropopause is composed of strong temperature fluctuations. By a detailed analysis of observed three-dimensional wind and temperature fluctuation components, it is shown that the phase and amplitude relations between these components are consistent with the theoretical characteristics of linear inertia–gravity waves (IGWs). Numerical simulations were performed by using a nonhydrostatic model. The simulated MT structures and IGW parameters agree well with the observation. In the analysis using the numerical simulation data, it is seen that IGWs were generated around 65°S, 15°E and around 70°S, 15°E, propagated eastward, and reached the region above Syowa Station when the MT event was observed. These IGWs were likely radiated spontaneously from the upper-tropospheric flow around 65°S, 15°E and were forced by strong southerly surface winds over steep topography (70°S, 15°E). The MT occurrence is attributable to strong IGWs and the low mean static stability in the polar winter lower stratosphere. It is also shown that nonorographic gravity waves associated with the tropopause folding event contribute to 40% of the momentum fluxes, as shown by a gravity wave–resolving general circulation model in the lower stratosphere around 65°S. This result indicates that they are one of the key components for solving the cold-bias problem found in most climate models.

scholarly journals Meteorological implementation issues in chemistry and transport models

2006 ◽  
Vol 6 (10) ◽  
pp. 2895-2910 ◽  
Author(s):  
S. E. Strahan ◽  
B. C. Polansky
Keyword(s):  
Climate Models ◽  
Lower Stratosphere ◽  
Model Performance ◽  
Transport Processes ◽  
The Arctic ◽  
Integration Time ◽  
Vertical Resolution ◽  
Transport Models ◽  
Sensitivity Experiments ◽  
The Antarctic

Abstract. Offline chemistry and transport models (CTMs) are versatile tools for studying composition and climate issues requiring multi-decadal simulations. They are computationally fast compared to coupled chemistry climate models, making them well-suited for integrating sensitivity experiments necessary for understanding model performance and interpreting results. The archived meteorological fields used by CTMs can be implemented with lower horizontal or vertical resolution than the original meteorological fields in order to shorten integration time, but the effects of these shortcuts on transport processes must be understood if the CTM is to have credibility. In this paper we present a series of sensitivity experiments on a CTM using the Lin and Rood advection scheme, each differing from another by a single feature of the wind field implementation. Transport effects arising from changes in resolution and model lid height are evaluated using process-oriented diagnostics that intercompare CH4, O3, and age tracer carried in the simulations. Some of the diagnostics used are derived from observations and are shown as a reality check for the model. Processes evaluated include tropical ascent, tropical-midlatitude exchange, poleward circulation in the upper stratosphere, and the development of the Antarctic vortex. We find that faithful representation of stratospheric transport in this CTM is possible with a full mesosphere, ~1 km resolution in the lower stratosphere, and relatively low vertical resolution (>4 km spacing) in the middle stratosphere and above, but lowering the lid from the upper to lower mesosphere leads to less realistic constituent distributions in the upper stratosphere. Ultimately, this affects the polar lower stratosphere, but the effects are greater for the Antarctic than the Arctic. The fidelity of lower stratospheric transport requires realistic tropical and high latitude mixing barriers which are produced at 2°×2.5°, but not lower resolution. At 2°×2.5° resolution, the CTM produces a vortex capable of isolating perturbed chemistry (e.g. high Cly and low NOy) required for simulating polar ozone loss.

scholarly journals An unusual stratospheric ozone decrease linked to isentropic air-mass transport as observed over Irene (25.5° S, 28.1° E) in mid-May 2002

2005 ◽  
Vol 5 (6) ◽  
pp. 12617-12639
Author(s):  
N. Semane ◽  
H. Bencherif ◽  
B. Morel ◽  
A. Hauchecorne ◽  
R. D. Diab
Keyword(s):  
Total Ozone ◽  
Planetary Wave ◽  
Lower Stratosphere ◽  
Polar Vortex ◽  
Ozone Layer ◽  
Wave Activity ◽  
Transport Processes ◽  
Diagnostic Tools ◽  
Air Masses ◽  
Middle Stratosphere

Abstract. A prominent ozone minimum of less than 240 Dobson Units (DU) was observed over Irene (25.5° S, 28.1° E) by the Total Ozone Mapping Spectrometer (TOMS) during May 2002 with extremely low ozone value of less than 219 DU recorded on 12 May, as compared to a climatological mean of 249 DU for May between 1999 and 2005. In this study, the vertical structure of this ozone minimum is examined using ozonesonde measurements performed over Irene on 15 May 2002, when the total ozone (as given by TOMS) was about 226 DU. Indeed, it is found that the ozone minimum is of Antarctic polar origin with a low-ozone layer in the middle stratosphere above 625 K and of tropical origin with low-ozone layer between 400-K and 450-K isentropic levels in the lower stratosphere. The upper and lower depleted parts of the ozonesonde profile for 15 May, are respectively attributed to equatorward and poleward transport of low-ozone air toward the subtropics. The tropical air moving over Irene and the polar one passing over the same area associated with enhanced planetary-wave activity are simulated successfully using a high-resolution advection contour model (MIMOSA) of Potential Vorticity. Indeed, in mid-May 2002, MIMOSA maps show a polar vortex filament in the middle stratosphere above the 625-K isentropic level and they show also tropical air-masses moving southward (over Irene) in the lower stratosphere between 400-K and 450-K isentropic levels. The winter stratospheric wave driving and its associated localized isentropic mixing leading to the ozone minimum are investigated by means of two diagnostic tools: the Eliassen-Palm flux and the effective diffusivity computed from the European Center for Medium-range Weather Forecasts (ECMWF) fields. The unusual distribution of ozone over Irene during May 2002 in the middle stratosphere is closely connected to the anomalously pre-conditioned structure of the polar vortex at that time of the year. Indeed, the perturbed vortex was typically predisposed for easy erosion by dynamical transport processes, which have been driven by strong planetary wave activity and have eventually resulted in a very large latitudinal advection of polar air masses towards the subtropics. The exceptional presence of polar vortex air over the subtropics during May 2002 can be considered as the first sign of the particular polar vortex disturbances, which after being well reinforced, contributed to the unprecedented behavior of the Antarctic spring ozone hole observed during September 2002.

Stratospheric circulation trends in ERA5 reanalysis compared to climate models over 1960-2020

2021 ◽  
Author(s):  
Roland Eichinger ◽  
Mohamadou Diallo ◽  
Fernando Iglesias-Suarez ◽  
Petr Pisoft
Keyword(s):  
Time Series ◽  
Zonal Wind ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Climate Model ◽  
Polar Vortex ◽  
Reanalysis Data ◽  
Striking Difference ◽  
Stratospheric Circulation

<p>The recent release of the long-term ERA5 reanalysis data spanning from 1950 to present offers new opportunities for analysing trends and variability of stratospheric dynamics. For the first time, a 60 year period (1960-2020) can be analysed in reanalysis data and compared with chemistry-climate model simulations. The analyses of stratospheric circulation trends and seasonalities over this long time period can help us to better understand the long-term evolution of the Brewer-Dobson circulation (BDC), and the related inter-model differences and model dependencies. Therefore, this way an improved credibility in future projections of the BDC can be obtained.<br />We find that the global trend patterns of the temperature, zonal wind and residual vertical velocity agrees well between ERA5 and the multi model mean. However, differences occur in the width and altitude of the maximum trend. The tropical upwelling mass flux time series in the lower stratosphere of models and reanalysis disagrees at the beginning of the period, but they converge after around 1980. The agreement of the time series increases with altitude, where the QBO dominates the signal. Moreover, we find a generally good agreement in the zonal wind trends, although some differences are detected in the subtropical jet strength and upward shift, as well as in the polar vortex region where the models exhibit larger changes than ERA5. Another striking difference is the temperature trend in the tropical upper troposphere/lower stratosphere, where models show a more extended warming trend into the lower stratosphere. In this presentation, we show these results, put them in relation to what had been shown in previous studies for other time periods and discuss possible explanations for the differences as well as implications for the further evolution of the BDC.</p>

scholarly journals Assessing stratospheric transport in the CMAM30 simulations using ACE-FTS measurements

2018 ◽  
Vol 18 (9) ◽  
pp. 6801-6828 ◽  
Author(s):  
Felicia Kolonjari ◽  
David A. Plummer ◽  
Kaley A. Walker ◽  
Chris D. Boone ◽  
James W. Elkins ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Climate Models ◽  
Lower Stratosphere ◽  
Middle Atmosphere ◽  
Polar Vortex ◽  
Sampling Technique ◽  
Dynamics Simulation ◽  
Mixing Efficiency ◽  
Global Circulation Models ◽  
June July

Abstract. Stratospheric transport in global circulation models and chemistry–climate models is an important component in simulating the recovery of the ozone layer as well as changes in the climate system. The Brewer–Dobson circulation is not well constrained by observations and further investigation is required to resolve uncertainties related to the mechanisms driving the circulation. This study has assessed the specified dynamics mode of the Canadian Middle Atmosphere Model (CMAM30) by comparing to the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) profile measurements of CFC-11 (CCl3F), CFC-12 (CCl2F2), and N2O. In the CMAM30 specified dynamics simulation, the meteorological fields are nudged using the ERA-Interim reanalysis and a specified tracer was employed for each species, with hemispherically defined surface measurements used as the boundary condition. A comprehensive sampling technique along the line of sight of the ACE-FTS measurements has been utilized to allow for direct comparisons between the simulated and measured tracer concentrations. The model consistently overpredicts tracer concentrations of CFC-11, CFC-12, and N2O in the lower stratosphere, particularly in the northern hemispheric winter and spring seasons. The three mixing barriers investigated, including the polar vortex, the extratropical tropopause, and the tropical pipe, show that there are significant inconsistencies between the measurements and the simulations. In particular, the CMAM30 simulation underpredicts mixing efficiency in the tropical lower stratosphere during the June–July–August season.

scholarly journals Implementation issues in chemistry and transport models

2005 ◽  
Vol 5 (5) ◽  
pp. 10217-10258
Author(s):  
S. E. Strahan ◽  
B. C. Polansky
Keyword(s):  
Climate Models ◽  
Lower Stratosphere ◽  
Model Performance ◽  
Transport Processes ◽  
The Arctic ◽  
Integration Time ◽  
Vertical Resolution ◽  
Transport Models ◽  
Single Feature ◽  
The Antarctic

Abstract. Offline chemistry and transport models (CTMs) are versatile tools for studying composition and climate issues requiring multi-decadal simulations. They are computationally fast compared to coupled chemistry climate models, making them well-suited for integrating sensitivity experiments necessary for understanding model performance and interpreting results. The archived meteorological fields used by CTMs can be implemented with lower horizontal or vertical resolution than the original meteorological fields in order to shorten integration time, but the effects of these shortcuts on transport processes must be understood if the CTM is to have credibility. In this paper we present a series of CTM experiments, each differing from another by a single feature of the implementation. Transport effects arising from changes in resolution and model lid height are evaluated using process-oriented diagnostics that intercompare CH4, O3, and age tracer carried in the simulations. Some of the diagnostics used are derived from observations and are shown as a reality check for the model. Processes evaluated include the tropical ascent, tropical-midlatitude exchange, the poleward circulation in the upper stratosphere, and the development of the Antarctic vortex. We find that faithful representation of stratospheric transport in this CTM using Lin and Rood advection is possible with a full mesosphere, ~1 km resolution in the lower stratosphere, and relatively low vertical resolution (>4 km spacing) in the middle stratosphere and above, but lowering the lid from the upper to lower mesosphere leads to less realistic constituent distributions in the upper stratosphere. Ultimately, this affects the polar lower stratosphere, but the effects are greater for the Antarctic than the Arctic. The fidelity of lower stratospheric transport requires realistic tropical and high latitude mixing barriers which are produced at 2°×2.5°, but not lower resolution. At 2°×2.5° resolution, the CTM produces a vortex capable of isolating perturbed chemistry (e.g. high Cly and low NOy) required for simulating polar ozone loss.

scholarly journals Seasonal prediction of the boreal winter stratosphere

2021 ◽  
Author(s):  
Alice Portal ◽  
Paolo Ruggieri ◽  
Froila M. Palmeiro ◽  
Javier García-Serrano ◽  
Daniela I. V. Domeisen ◽  
...  
Keyword(s):  
Southern Oscillation ◽  
Polar Vortex ◽  
Seasonal Prediction ◽  
Wave Activity ◽  
Boreal Winter ◽  
Quasi Biennial Oscillation ◽  
Stratospheric Polar Vortex ◽  
Eddy Heat Flux ◽  
Prediction Systems ◽  
Model Database

AbstractThe predictability of the Northern Hemisphere stratosphere and its underlying dynamics are investigated in five state-of-the-art seasonal prediction systems from the Copernicus Climate Change Service (C3S) multi-model database. Special attention is devoted to the connection between the stratospheric polar vortex (SPV) and lower-stratosphere wave activity (LSWA). We find that in winter (December to February) dynamical forecasts initialised on the first of November are considerably more skilful than empirical forecasts based on October anomalies. Moreover, the coupling of the SPV with mid-latitude LSWA (i.e., meridional eddy heat flux) is generally well reproduced by the forecast systems, allowing for the identification of a robust link between the predictability of wave activity above the tropopause and the SPV skill. Our results highlight the importance of November-to-February LSWA, in particular in the Eurasian sector, for forecasts of the winter stratosphere. Finally, the role of potential sources of seasonal stratospheric predictability is considered: we find that the C3S multi-model overestimates the stratospheric response to El Niño–Southern Oscillation (ENSO) and underestimates the influence of the Quasi–Biennial Oscillation (QBO).

On the role of vertical and horizontal model resolution for the simulation of stratospheric transport

2021 ◽  
Author(s):  
Hella Garny ◽  
Simone Dietmüller ◽  
Roland Eichinger ◽  
Aman Gupta ◽  
Marianna Linz
Keyword(s):  
Climate Models ◽  
Horizontal Resolution ◽  
Climate Forcing ◽  
Numerical Dispersion ◽  
Mixing Efficiency ◽  
Vertical Resolution ◽  
Model Resolution ◽  
Residual Circulation ◽  
Year 2000

<p>The stratospheric transport circulation, or Brewer-Dobson Circulation (BDC), is often conceptually seperated into advection along the residual circulation and two-way mixing. In particular the latter part has recently been found to exert a strong influence on inter-model differences of mean age of Air (AoA), a common measure of the BDC. However, the precise reason for model differences in two-way mixing remains unknown, as many model<br>components in multi-model projects differ. One component that likely plays an important role is model resolution, both vertically and horizontally. To analyse this aspect, we carried out a set of simulations with identical and constant year 2000 climate forcing varying the spectral horizontal<br>resolution (T31,T42,T63,T85) and the number of vertical levels (L31,L47,L90). We find that increasing the vertical resolution leads to an increase in mean AoA. Most of this change can be attributed to aging by mixing. The mixing efficiency, defined as the ratio of isentropic mixing strength and the diabatic circulation, shows the same dependency on vertical resolution. While horizontal resolution changes do not systematically change mean AoA, we do<br>find a systematic decrease in the mixing efficiency with increasing horizontal resolution. Non-systematic changes in the residual circulation partly compensate the mixing efficiency changes, leading to the non-systematic mean AoA changes. The mixing efficiency changes with vertical and horizontal resolution are consistent with expectations on the effects of numerical dispersion on mean AoA. To further investigate the most relevant regions of mixing differences, we analyse height-resolved mixing efficiency differences. Overall, this work will help to shed light on the underlying reasons for the large biases of climate models in simulating stratospheric transport.</p>

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