Studies over Sudden Stratospheric Warmings (SSW)

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Martijn
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Studies over Sudden Stratospheric Warmings (SSW)

Berichtdoor Martijn » Wo 30 Okt 2013, 13:41

Bron en volledige tekst: http://journals.ametsoc.org/doi/full/10.1175/JCLI3996.1

Charlton, Andrew J., Lorenzo M. Polvani, 2007: A New Look at Stratospheric Sudden Warmings. Part I: Climatology and Modeling Benchmarks. J. Climate, 20, 449–469.
doi: http://dx.doi.org/10.1175/JCLI3996.1

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

Andrew J. Charlton*
Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York

Lorenzo M. Polvani
Department of Applied Physics and Applied Mathematics, and Department of Earth and Environmental Sciences, Columbia University, New York, New York

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.

Keywords: General circulation models, Climatology, Stratosphere

Received: October 13, 2005; Final Form: March 28, 2006

* Current affiliation: Department of Meteorology, University of Reading, Reading, United Kingdom

Corresponding author address: Andrew J. Charlton, Department of Meteorology, University of Reading, Reading, Berkshire, RG6 6BB, United Kingdom. Email: a.j.charlton@reading.ac.uk


Deel 2 van de studie:

Bron en volledige tekst: http://journals.ametsoc.org/doi/full/10.1175/JCLI3994.1

Charlton, Andrew J., and Coauthors, 2007: A New Look at Stratospheric Sudden Warmings. Part II: Evaluation of Numerical Model Simulations. J. Climate, 20, 470–488.
doi: http://dx.doi.org/10.1175/JCLI3994.1

A New Look at Stratospheric Sudden Warmings. Part II: Evaluation of Numerical Model Simulations

Andrew J. Charlton,*,@@ Lorenzo M. Polvani,+ Judith Perlwitz,# Fabrizio Sassi,@ Elisa Manzini,& Kiyotaka Shibata,** Steven Pawson,++ J. Eric Nielsen,++ and David Rind##

a*Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York
+Department of Applied Physics and Applied Mathematics, and Department of Earth and Environmental Sciences, Columbia University, New York, New York
#Cooperative Institute for Research in Environmental Sciences, Climate Diagnostics Center, University of Colorado, and Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado
@National Center for Atmospheric Research, Boulder, Colorado
&Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy
**Meteorological Research Institute, Tsukuba, Ibaraki, Japan
++Global Modeling and Assimilation Office, NASA GSFC, Greenbelt, Maryland
##NASA Goddard Institute for Space Studies, New York, New York

Abstract

The simulation of major midwinter stratospheric sudden warmings (SSWs) in six stratosphere-resolving general circulation models (GCMs) is examined. The GCMs are compared to a new climatology of SSWs, based on the dynamical characteristics of the events. First, the number, type, and temporal distribution of SSW events are evaluated. Most of the models show a lower frequency of SSW events than the climatology, which has a mean frequency of 6.0 SSWs per decade. Statistical tests show that three of the six models produce significantly fewer SSWs than the climatology, between 1.0 and 2.6 SSWs per decade. Second, four process-based diagnostics are calculated for all of the SSW events in each model. It is found that SSWs in the GCMs compare favorably with dynamical benchmarks for SSW established in the first part of the study.

These results indicate that GCMs are capable of quite accurately simulating the dynamics required to produce SSWs, but with lower frequency than the climatology. Further dynamical diagnostics hint that, in at least one case, this is due to a lack of meridional heat flux in the lower stratosphere. Even though the SSWs simulated by most GCMs are dynamically realistic when compared to the NCEP–NCAR reanalysis, the reasons for the relative paucity of SSWs in GCMs remains an important and open question.

Keywords: General circulation models, Climatology, Stratosphere

Received: October 13, 2005; Final Form: March 28, 2006

@@ Current affiliation: Department of Meteorology, University of Reading, Reading, United Kingdom

Corresponding author address: Andrew J. Charlton, Department of Meteorology, University of Reading, Reading, Berkshire, RG6 6BB, United Kingdom. Email: a.j.charlton@reading.ac.uk

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