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  • 1
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    Abrupt Stratospheric Vortex Weakening Associated With North Atlantic Anticyclonic Wave Breaking (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The sudden stratospheric warming (SSW) of 12 February 2018 was not forecast by any extended‐range model beyond 12 days. From early February, all forecast models that comprise the subseasonal‐to‐seasonal (S2S) database abruptly transitioned from indicating a strong stratospheric polar vortex (SPV) to a high likelihood of a major SSW. We demonstrate that this forecast evolution was associated with the track and intensity of a cyclone in the northeast Atlantic, with an associated anticyclonic Rossby wave break, which was not well forecast. The wave break played a pivotal role in building the Ural high, which existing literature has shown was a precursor of the 2018 SSW. The track of the cyclone built an anomalously strong sea level pressure dipole between Scandinavia and Greenland (termed the S‐G dipole), which we use as a diagnostic of the wave break. Forecasts that did not capture the magnitude of this event had the largest errors in the SPV strength and did not show enhanced vertical wave activity. A composite of 49 similarly strong wintertime (November–March) S‐G dipoles in reanalysis shows associated anticyclonic wave breaking leading to significantly enhanced vertical wave activity and a weakened SPV in the following days, which occurred in 35% of the 15‐day periods preceding observed major SSWs. Our results indicate a particular transient trigger for weakening the SPV, complementing existing results on the importance of tropospheric blocking for disruptions to the Northern Hemisphere extratropical stratospheric circulation.
    Print ISSN: 2169-897X
    Electronic ISSN: 2169-8996
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Trends in Austral jet position in ensembles of high- and low-top CMIP5 models (2012)
    Wiley
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    Publication Date: 2012-07-13
    Description: Trends in the position of the DJF Austral jet have been analyzed for multimodel ensemble simulations of a subset of high- and low-top models for the periods 1960–2000, 2000–2050, and 2050–2098 under the CMIP5 historical, RCP4.5, and RCP8.5 scenarios. Comparison with ERA-Interim, CFSR and the NCEP/NCAR reanalysis shows that the DJF and annual mean jet positions in CMIP5 models are equatorward of reanalyses for the 1979–2006 mean. Under the RCP8.5 scenario, the mean jet position in the high-top models moves 3 degrees poleward of its 1860–1900 position by 2098, compared to just over 2 degrees for the low-top models. Changes in jet position are linked to changes in the meridional temperature gradient. Compared to low-top models, the high-top models predict greater warming in the tropical upper troposphere due to increased greenhouse gases for all periods considered: up to 0.28 K/decade more in the period 2050–2098 under the RCP8.5 scenario. Larger polar lower-stratospheric cooling is seen in high-top models: −1.64 K/decade compared to −1.40 K/decade in the period 1960–2000, mainly in response to ozone depletion, and −0.41 K/decade compared to −0.12 K/decade in the period 2050–2098, mainly in response to increases in greenhouse gases. Analysis suggests that there may be a linear relationship between the trend in jet position and meridional temperature gradient, even under strong forcing. There were no clear indications of an approach to a geometric limit on the absolute magnitude of the poleward shift by 2100.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    The nature of Arctic polar vortices in chemistry–climate models (2012)
    Wiley
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    Publication Date: 2012-04-15
    Description: The structure of the Arctic stratospheric polar vortex in three chemistry–climate models (CCMs) taken from the CCMVal-2 intercomparison is examined using zonal mean and geometric-based methods. The geometric methods are employed by taking 2D moments of potential vorticity fields that are representative of the polar vortices in each of the models. This allows the vortex area, centroid location and ellipticity to be determined, as well as a measure of vortex filamentation. The first part of the study uses these diagnostics to examine how well the mean state, variability and extreme variability of the polar vortices are represented in CCMs compared to ERA-40 reanalysis data, and in particular for the UMUKCA-METO, NIWA-SOCOL and CCSR/NIES models. The second part of the study assesses how the vortices are predicted to change in terms of the frequency of sudden stratospheric warmings and their general structure over the period 1960–2100. In general, it is found that the vortices are climatologically too far poleward in the CCMs and produce too few large-scale filamentation events. Only a small increase is observed in the frequency of sudden stratospheric warming events from the mean of the CCMVal-2 models, but the distribution of extreme variability throughout the winter period is shown to change towards the end of the twentyfirst century. Copyright © 2012 Royal Meteorological Society and British Crown Copyright, the Met Office
    Print ISSN: 0035-9009
    Electronic ISSN: 1477-870X
    Topics: Geography , Physics
    Published by Wiley
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  • 4
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    The nature of Arctic polar vortices in chemistry–climate models (2012)
    Wiley
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    Publication Date: 2012-03-08
    Description: The structure of the Arctic stratospheric polar vortex in three chemistry–climate models (CCMs) taken from the CCMVal-2 intercomparison is examined using zonal mean and geometric-based methods. The geometric methods are employed by taking 2D moments of potential vorticity fields that are representative of the polar vortices in each of the models. This allows the vortex area, centroid location and ellipticity to be determined, as well as a measure of vortex filamentation. The first part of the study uses these diagnostics to examine how well the mean state, variability and extreme variability of the polar vortices are represented in CCMs compared to ERA-40 reanalysis data, and in particular for the UMUKCA-METO, NIWA-SOCOL and CCSR/NIES models. The second part of the study assesses how the vortices are predicted to change in terms of the frequency of sudden stratospheric warmings and their general structure over the period 1960–2100. In general, it is found that the vortices are climatologically too far poleward in the CCMs and produce too few large-scale filamentation events. Only a small increase is observed in the frequency of sudden stratospheric warming events from the mean of the CCMVal-2 models, but the distribution of extreme variability throughout the winter period is shown to change towards the end of the twentyfirst century. Copyright © 2012 Royal Meteorological Society and British Crown Copyright, the Met Office
    Print ISSN: 0035-9009
    Electronic ISSN: 1477-870X
    Topics: Geography , Physics
    Published by Wiley
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  • 5
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    Multimodel climate and variability of the stratosphere (2011)
    Wiley
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    Publication Date: 2011-03-03
    Description: The stratospheric climate and variability from simulations of sixteen chemistry-climate models is evaluated. On average the polar night jet is well reproduced though its variability is less well reproduced with a large spread between models. Polar temperature biases are less than 5 K except in the Southern Hemisphere (SH) lower stratosphere in spring. The accumulated area of low temperatures responsible for polar stratospheric cloud formation is accurately reproduced for the Antarctic but underestimated for the Arctic. The shape and position of the polar vortex is well simulated, as is the tropical upwelling in the lower stratosphere. There is a wide model spread in the frequency of major sudden stratospheric warnings (SSWs), late biases in the breakup of the SH vortex, and a weak annual cycle in the zonal wind in the tropical upper stratosphere. Quantitatively, “metrics” indicate a wide spread in model performance for most diagnostics with systematic biases in many, and poorer performance in the SH than in the Northern Hemisphere (NH). Correlations were found in the SH between errors in the final warming, polar temperatures, the leading mode of variability, and jet strength, and in the NH between errors in polar temperatures, frequency of major SSWs, and jet strength. Models with a stronger QBO have stronger tropical upwelling and a colder NH vortex. Both the qualitative and quantitative analysis indicate a number of common and long-standing model problems, particularly related to the simulation of the SH and stratospheric variability.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Stratospheric heating by potential geoengineering aerosols (2011)
    Wiley
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    Publication Date: 2011-12-28
    Description: A fixed dynamical heating model is used to investigate the pattern of zonal-mean stratospheric temperature change resulting from geoengineering with aerosols composed of sulfate, titania, limestone and soot. Aerosol always heats the tropical lower stratosphere, but at the poles the response can be either heating, cooling, or neutral. The sign of the change in stratospheric Pole-Equator temperature difference depends on aerosol type, size and season. This has implications for modeling geoengineering impacts and the response of the stratospheric circulation.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Stratospheric dynamics and midlatitude jets under geoengineering with space mirrors, and sulfate and titania aerosols (2014)
    Wiley
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    Publication Date: 2014-12-31
    Description: The impact on the dynamics of the stratosphere of three approaches to geoengineering by Solar Radiation Management is investigated using idealized simulations of a global climate model. The approaches are geoengineering with sulfate aerosols, titania aerosols and reduction in total solar irradiance (representing mirrors placed in space). If it were possible to use stratospheric aerosols to counterbalance the surface warming produced by a quadrupling of atmospheric carbon dioxide concentrations, tropical lower stratospheric radiative heating would drive a thermal-wind response which would intensify the stratospheric polar vortices. In the Northern Hemisphere this intensification results in strong dynamical cooling of the polar stratosphere. Northern Hemisphere stratospheric sudden warming events become rare (1 or 2 in 65 years for sulfate and titania respectively). The intensification of the polar vortices results in a poleward shift of the tropospheric midlatitude jets in winter. The aerosol radiative heating enhances the tropical upwelling in the lower stratosphere, influencing the strength of the Brewer-Dobson Circulation. In contrast, solar dimming does not produce heating of the tropical lower stratosphere so there is little intensification of the polar vortex and no enhanced tropical upwelling. The dynamical response to titania aerosol is qualitatively similar to the response to sulfate.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    The structure and evolution of the stratospheric vortex in response to natural forcings (2011)
    Wiley
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    Publication Date: 2011-08-10
    Description: The structure and evolution of the Arctic stratospheric polar vortex is assessed during opposing phases of, primarily, the El Niño–Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO), but the 11 year solar cycle and winters following large volcanic eruptions are also examined. The analysis is performed by taking 2-D moments of vortex potential vorticity (PV) fields which allow the area and centroid of the vortex to be calculated throughout the ERA-40 reanalysis data set (1958–2002). Composites of these diagnostics for the different phases of the natural forcings are then considered. Statistically significant results are found regarding the structure and evolution of the vortex during, in particular, the ENSO and QBO phases. When compared with the more traditional zonal mean zonal wind diagnostic at 60°N, the moment-based diagnostics are far more robust and contain more information regarding the state of the vortex. The study details, for the first time, a comprehensive sequence of events which map the evolution of the vortex during each of the forcings throughout an extended winter period.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
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    Stratospheric memory and skill of extended-range weather forecasts (2003)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2003-08-02
    Description: We use an empirical statistical model to demonstrate significant skill in making extended-range forecasts of the monthly-mean Arctic Oscillation (AO). Forecast skill derives from persistent circulation anomalies in the lowermost stratosphere and is greatest during boreal winter. A comparison to the Southern Hemisphere provides evidence that both the time scale and predictability of the AO depend on the presence of persistent circulation anomalies just above the tropopause. These circulation anomalies most likely affect the troposphere through changes to waves in the upper troposphere, which induce surface pressure changes that correspond to the AO.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Baldwin, Mark P -- Stephenson, David B -- Thompson, David W J -- Dunkerton, Timothy J -- Charlton, Andrew J -- O'Neill, Alan -- New York, N.Y. -- Science. 2003 Aug 1;301(5633):636-40.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Northwest Research Associates, 14508 NE 20th Street, Bellevue, WA, 98007, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/12893941" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 10
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    Correction to “Stratospheric heating by potential geoengineering aerosols” (2012)
    Wiley
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    Publication Date: 2012-05-22
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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