http://acce.scar.org/w/index.php?title=Atmospheric_circulation_changes_over_the_21st_century&feed=atom&action=historyAtmospheric circulation changes over the 21st century - Revision history2024-03-28T09:58:38ZRevision history for this page on the wikiMediaWiki 1.23.2http://acce.scar.org/w/index.php?title=Atmospheric_circulation_changes_over_the_21st_century&diff=359&oldid=prevTonyp: Changed book section reference to a page link2014-08-22T16:11:10Z<p>Changed book section reference to a page link</p>
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<td colspan='2' style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan='2' style="background-color: white; color:black; text-align: center;">Revision as of 16:11, 22 August 2014</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Observed changes in weather systems are consistent with the trends in the dominant modes of variability, although these results too are strongly dependent on the quality of reanalysis products. In the Southern Hemisphere, cyclonic activity is strikingly different between the ERA-40 and NRA re-analyses (Bromwich et al., 2007<ref name="Bromwich et al, 2007">Bromwich, D.H., Fogt, R.L., Hodges, K.I. and Walsh, J.E. 2007. A tropospheric assessment of the ERA-40, NCEP, and JRA-25 global reanalyses in the polar regions, J. Geophys. Res., 112, Art. No. D10111.</ref>). Nevertheless, consistent signals include a decrease in the frequency and increases in the size and intensity of extratropical cyclones in recent decades (e.g. Simmonds and Keay, 2000<ref name="Simmonds and Keay, 2000">Simmonds, I. and Keay, K. 2000. Variability of Southern Hemisphere extratropical cyclone behaviour, 1958-97, ''J. Climate'', '''13'''(3), 550-561.</ref>) with a moderate increase in frequency over the Southern Ocean (e.g. Fyfe, 2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>).</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Observed changes in weather systems are consistent with the trends in the dominant modes of variability, although these results too are strongly dependent on the quality of reanalysis products. In the Southern Hemisphere, cyclonic activity is strikingly different between the ERA-40 and NRA re-analyses (Bromwich et al., 2007<ref name="Bromwich et al, 2007">Bromwich, D.H., Fogt, R.L., Hodges, K.I. and Walsh, J.E. 2007. A tropospheric assessment of the ERA-40, NCEP, and JRA-25 global reanalyses in the polar regions, J. Geophys. Res., 112, Art. No. D10111.</ref>). Nevertheless, consistent signals include a decrease in the frequency and increases in the size and intensity of extratropical cyclones in recent decades (e.g. Simmonds and Keay, 2000<ref name="Simmonds and Keay, 2000">Simmonds, I. and Keay, K. 2000. Variability of Southern Hemisphere extratropical cyclone behaviour, 1958-97, ''J. Climate'', '''13'''(3), 550-561.</ref>) with a moderate increase in frequency over the Southern Ocean (e.g. Fyfe, 2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>).</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>A consistent result that has emerged recently from Twenty First Century projections is a tendency for a poleward shift of several degrees latitude in mid-latitude storm tracks (e.g. Fischer-Bruns et al., 2005<ref name="Fischer-Bruns et al, 2005">Fischer-Bruns, I., Von Storch, H., Gonzalez-Rouco, J.F. and Zorita, E. 2005. Modelling the variability of midlatitude storm activity on decadal to century time scales, ''Clim. Dyn.'', '''25''', 461-476.</ref>; Bengtsson et al., 2006<ref name="Bengtsson et al, 2006">Bengtsson, L., Hodges, K. and Roeckner, E. 2006. Storm tracks and climate change, ''J. Climate'', '''19''', 3518-3543.</ref>). Consistent with these shifts, Lynch et al. (2006<ref name="Lynch et al, 2006">Lynch, A., Uotila, P. and Cassano, J.J. 2006. Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic, ''Int. J. Climatol.'', '''26''', 1181-1199.</ref>) demonstrated increasing cyclonicity and stronger westerlies in high southern latitudes in a 10-member multi-model ensemble simulation of the Twenty First Century. One study (Fyfe, 2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>) has suggested a reduction in sub-Antarctic cyclones of more than 30% by the end of the century. Fyfe (2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>) did not definitively identify a poleward shift of storm tracks, but the relatively coarse grid of the CCCma climate model (T32, or approximately 600 km grid spacing) may not have been able to detect such a shift. These changes have been related to a simulated circumpolar signal of increased precipitation off the coast of Antarctica (Lynch et al., 2006<ref name="Lynch et al, 2006">Lynch, A., Uotila, P. and Cassano, J.J. 2006. Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic, ''Int. J. Climatol.'', '''26''', 1181-1199.</ref>; see also <del class="diffchange diffchange-inline">Section 5.2.5</del>), which perhaps, though loosely, argues against the frequency trend being related to increased data availability, as suggested by Hines et al. (2000<ref name="Hines et al, 2000">Hines K.M., Bromwich, D.H. and Marshall, G.J. 2000. Artificial surface pressure trends in the NCEP&ndash;NCAR reanalysis over the Southern Ocean and Antarctica, ''J. Climate'', '''13''', 3940-3952.</ref>).</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>A consistent result that has emerged recently from Twenty First Century projections is a tendency for a poleward shift of several degrees latitude in mid-latitude storm tracks (e.g. Fischer-Bruns et al., 2005<ref name="Fischer-Bruns et al, 2005">Fischer-Bruns, I., Von Storch, H., Gonzalez-Rouco, J.F. and Zorita, E. 2005. Modelling the variability of midlatitude storm activity on decadal to century time scales, ''Clim. Dyn.'', '''25''', 461-476.</ref>; Bengtsson et al., 2006<ref name="Bengtsson et al, 2006">Bengtsson, L., Hodges, K. and Roeckner, E. 2006. Storm tracks and climate change, ''J. Climate'', '''19''', 3518-3543.</ref>). Consistent with these shifts, Lynch et al. (2006<ref name="Lynch et al, 2006">Lynch, A., Uotila, P. and Cassano, J.J. 2006. Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic, ''Int. J. Climatol.'', '''26''', 1181-1199.</ref>) demonstrated increasing cyclonicity and stronger westerlies in high southern latitudes in a 10-member multi-model ensemble simulation of the Twenty First Century. One study (Fyfe, 2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>) has suggested a reduction in sub-Antarctic cyclones of more than 30% by the end of the century. Fyfe (2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>) did not definitively identify a poleward shift of storm tracks, but the relatively coarse grid of the CCCma climate model (T32, or approximately 600 km grid spacing) may not have been able to detect such a shift. These changes have been related to a simulated circumpolar signal of increased precipitation off the coast of Antarctica (Lynch et al., 2006<ref name="Lynch et al, 2006">Lynch, A., Uotila, P. and Cassano, J.J. 2006. Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic, ''Int. J. Climatol.'', '''26''', 1181-1199.</ref>; see also <ins class="diffchange diffchange-inline">[[Precipitation changes over the 21st century]]</ins>), which perhaps, though loosely, argues against the frequency trend being related to increased data availability, as suggested by Hines et al. (2000<ref name="Hines et al, 2000">Hines K.M., Bromwich, D.H. and Marshall, G.J. 2000. Artificial surface pressure trends in the NCEP&ndash;NCAR reanalysis over the Southern Ocean and Antarctica, ''J. Climate'', '''13''', 3940-3952.</ref>).</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==Impacts on accumulation==</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==Impacts on accumulation==</div></td></tr>
</table>Tonyphttp://acce.scar.org/w/index.php?title=Atmospheric_circulation_changes_over_the_21st_century&diff=209&oldid=prevMaintenance script: Importing text file2014-08-06T14:33:15Z<p>Importing text file</p>
<p><b>New page</b></p><div>:''This page is part of the topic [[Atmospheric change over the next 100 years]]''<br />
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The atmospheric circulation over Antarctica and the Southern Ocean is critical for the future evolution of global climate in a number of ways. Perhaps most important is the role of circulation in defining accumulation over the Antarctic ice sheet. Other aspects of importance include the role of circulation in the warming of the Antarctic Peninsula, the distribution of sea ice, and the seasonal to interannual variability of the Southern Hemisphere.<br />
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==The role of modes of circulation variability==<br />
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Mean sea level pressure changes associated with the long-term variability of circulation of the Southern Hemisphere have been reported for many decades (e.g. van Loon, 1967<ref name="Loon, 1967">Van Loon, H. 1967. The half yearly oscillations in middle and high southern latitudes and the coreless winter, ''J. Atmos. Sci.'', '''24''', 472-486.</ref>; Hurrell and van Loon, 1994<ref name="Hurrell and Loon, 1994">Hurrell, J.W. and Van Loon, H. 1994. A Modulation of the Atmospheric Annual Cycle in the Southern Hemisphere, ''Tellus'', '''46A''', 325-338.</ref>). More recently, analyses of sea level pressure have revealed secular decreases over the Antarctic, associated with increases in mid-latitude westerlies, a poleward displacement of the polar front jet stream, and a more zonal circulation. The robustness of these observed changes is open to question, given that spurious trends are evident in reanalysis products (Bromwich et al., 2007<ref name="Bromwich et al, 2007">Bromwich, D.H., Fogt, R.L., Hodges, K.I. and Walsh, J.E. 2007. A tropospheric assessment of the ERA-40, NCEP, and JRA-25 global reanalyses in the polar regions, J. Geophys. Res., 112, Art. No. D10111.</ref>). Further, there are significant differences between the ERA-40 and NRA re-analyses in the Southern Hemisphere, in which the ERA-40 has a tendency towards more intense cyclones in all seasons (Wang et al., 2006<ref name="Wang et al, 2006">Wang, X.L., Swail, V.R. and Zwiers, F.W. 2006. Climatology and changes of extratropical storm tracks and cyclone activity: Comparison of ERA-40 with NCEP/NCAR Reanalysis for 1958-2001, J. Climate, 19, 3145&minus;3166.</ref>). Nevertheless, tendencies consistent with these changes in circulation have been demonstrated in station-based data and climate model realizations of the Twentieth and Twenty First Centuries.<br />
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Given the large inter-annual variability of the high southern latitudes, secular trends must be put in the context of responses to changes in the modes of variability that are dominant in the region, namely the El Ni&ntilde;o Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) (Turner, 2004<ref name="Turner, 2004">Turner, J. 2004. The El Ni&ntilde;o-Southern Oscillation and Antarctica, ''International Journal of climatology'', '''24''', 1-31.</ref>; Sen Gupta and England, 2007<ref name="Gupta and England, 2007">Sen Gupta, A. and England, M.H. 2007. Coupled ocean-atmosphere feedback in the Southern Annular Mode, ''J. Climate'', '''20''', 3677-3692.</ref>). The long-term changes forced by increases of GHG concentrations have been found to possess a similar spatial pattern to the observed short-term (month to month) variability (Brandefelt and K&auml;ll&eacute;n, 2004<ref name="Brandefelt and K&auml;ll&eacute;n, 2004">Brandefelt, J. and K&auml;ll&eacute;n, E. 2004: The response of the Southern Hemisphere atmospheric circulation to an enhanced greenhouse gas forcing, ''J. Climate'', '''17''', 4425-4442.</ref>). Of less significance, due largely to its contested nature (Park Y. et al., 2004), is the Antarctic Circumpolar Wave (ACW) (White and Peterson, 1996<ref name="White and Peterson, 1996">White, W.B. and Peterson, R. 1996. An Antarctic Circumpolar Wave in surface pressure, wind, temperature, and sea ice extent, ''Nature'', '''380''', 699-702.</ref>), a postulated pattern of variability with an approximately four-year period characterised by the eastward propagation of anomalies in sea ice extent.<br />
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The Southern Oscillation Index has a negative trend over recent decades, corresponding to a tendency towards more frequent El Ni&ntilde;o conditions in the equatorial Pacific. This trend is associated with negative sea ice cover anomalies in the Ross and Amundsen Sea and positive sea ice anomalies in the Bellingshausen and Weddell Seas (Kwok and Comiso, 2002<ref name="Kwok and Comiso, 2002">Kwok, R. and Comiso, J.C. 2002. Southern ocean climate and sea ice anomalies associated with the Southern Oscillation, ''J. Climate'', '''15''', 487-501.</ref>).<br />
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The SAM index has a positive trend over recent decades during the summer and winter, which reflects trends in the zonally averaged mid-latitude westerlies. The increases in the SAM index are associated with strong warming on the eastern side of the Antarctic Peninsula and low pressure west of the Peninsula (Orr et al., 2004<ref name="Orr et al, 2004">Orr, A., Cresswell, D., Marshall, G.J., Hunt, J.C.R., Sommeria, J., Wang, C.G. and Light, M. 2004. A &lsquo;low-level&rsquo; explanation for the recent large warming trend over the western Antarctic Peninsula involving blocked winds and changes in zonal circulation, ''Geophys. Res. Lett.'', '''31''', L06204, doi:10.1029/2003GL019160.</ref>; Lefebvre et al., 2004<ref name="Lefebvre et al, 2004">Lefebvre, W., Goosse, H., Timmermann, R. and Fichefet, T. 2004. Influence of the Southern Annular Mode on the sea ice-ocean system, ''J. Geophys. Res.'', '''109''', C09005, doi:10.1029/2004JC002403.</ref>) &ndash; this reflects increased poleward flow, resulting in both the eastern Peninsula warming and reduced sea ice in the region (Liu et al., 2004<ref name="Liu et al, 2004">Liu, J., Curry, J.A. and Martinson, D.G. 2004. Interpretation of recent Antarctic sea ice variability, ''Geophys. Res. Lett.'', '''31''', L02205, doi:10.1029/2003GL018732.</ref>). The SAM index trends are also related to the observed and projected (in the near term) East Antarctic surface cooling (Shindell and Schmidt, 2004<ref name="Shindell and Schmidt, 2004">Shindell, D.T. and Schmidt, G.A. 2004. Southern hemisphere climate response to ozone changes and greenhouse gas increases, ''Geophys. Res. Lett.'', '''31''', L18209, doi:10.1029/2004GL020724.</ref>; Marshall, 2007<ref name="Marshall, 2007">Marshall, G.J. 2007. Half-century seasonal relationships between the Southern Annular Mode and Antarctic temperatures, ''Int. J. Climatol.'', '''27''', 373-383.</ref>). However, changes in the SAM are not thought to be responsible for the large winter season warming on the western side of the Antarctic Peninsula.<br />
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Progress in simulating ENSO variability has led to significant improvements in representing the spatial pattern of sea surface temperatures in the equatorial Pacific (Randall et al., 2007<ref name="Randall et al, 2007">Randall, D.A., Wood, R.A., Bony, S., Colman, R., Fichefet, T., Fyfe, J., Kattsov, V., Pitman, A., Shukla, J., Srinivasan, J., Stouffer, R.J., Sumi, A. and Taylor, K.E. 2007. Climate Models and Their Evaluation. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref>). Uncoupled models have demonstrated similar ENSO variability to that observed (e.g. Marshall et al., 2007<ref name="Marshall et al, 2007">Marshall, D. P. and Naveira Garabato A. C. 2007. A conjecture on the role of bottom-enhanced diapycnal mixing in the parameterization of geostrophic eddies, J. Phys. Oceanogr., in press.</ref>). However, serious discrepancies remain in the attempts of coupled models to represent the ENSO (Joseph and Nigam, 2006<ref name="Joseph and Nigam, 2006">Joseph, R. and Nigam, S. 2006. ENSO evolution and teleconnections in IPCC's twentieth-century climate simulations: Realistic representation?, ''J. Climate'', '''19''', 4360-4377.</ref>). Atmosphere-ocean interaction leads to inaccuracies in the sea surface temperature and in the structure of the thermocline (Cai et al., 2003<ref name="Cai et al, 2003">Cai, W., Whetton, P.H. and Karoly, D.J. 2003. The response of the Antarctic Oscillation to increasing and stabilized atmospheric CO<sub>2</sub>, ''J. Climate'', '''16''', 1525-1538.</ref>; Davey et al., 2002<ref name="Davey et al, 2002">Davey, M.K., Huddleston, M., Sperber, K., Braconnot, P., Bryan, F., Chen, D., Colman, R., Cooper, C., Cubasch, U., Delecluse, P., Dewitt, D., Fairhead, L., Flato, G., Gordon, C., Hogan, T., Ji, M., Kimoto, M., Kitoh, A., Knutson, T., Latif, M., Le Treut, H., Li, T., Manabe, S., Mechoso, C., Meehl, G., Power, S., Roeckner, E., Terray, L., Vintzileos, A., Voss, R., Wang, B., Washington, W., Yoshikawa, I., Yu, J., Yukimoto, S. and Zebiak, S. 2002. STOIC: A study of coupled model climatology and variability in tropical ocean regions, ''Clim. Dyn.'', '''18''', 403-420.</ref>). Furthermore, the timescale of variability in the coupled system is generally too short (van Oldenborgh et al., 2005<ref name="Oldenborgh et al, 2005">Van Oldenborgh, G.J., Philip, S.Y. and Collins, M. 2005. El Nino in a changing climate: a multi-model study, ''Ocean Sci.'', '''1''', 81-95.</ref>), although in some models a peak at around 7 years is observed (Marshall et al., 2007<ref name="Marshall et al, 2007">Marshall, D. P. and Naveira Garabato A. C. 2007. A conjecture on the role of bottom-enhanced diapycnal mixing in the parameterization of geostrophic eddies, J. Phys. Oceanogr., in press.</ref>). The interaction between climate change and ENSO variability is also subject to substantial uncertainty, with no coupled model consensus on the likelihood of a relationship between more frequent El Ni&ntilde;o conditions and increasing GHG concentrations (van Oldenborgh et al., 2005<ref name="Oldenborgh et al, 2005">Van Oldenborgh, G.J., Philip, S.Y. and Collins, M. 2005. El Nino in a changing climate: a multi-model study, ''Ocean Sci.'', '''1''', 81-95.</ref>; Collins et al., 2005<ref name="Collins et al, 2005">Collins, M. and The CMIP Modelling Groups. 2005. El Ni&ntilde;o- or La Ni&ntilde;a-like climate change? Climate Dynamics, 24, 89-104.</ref>; Wang, 2007<ref name="Wang, 2007">Wang, F.M. 2007. Investigating ENSO sensitivity to mean climate in an intermediate model using a novel statistical technique, ''Geophys. Res. Lett.'', '''34''', L07705.</ref>).<br />
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Model outputs submitted to the IPCC AR4 simulate the SAM with a high degree of accuracy (e.g., Miller et al., 2006<ref name="Miller et al, 2006">Miller, R.L., Schmidt, G.A. and Shindell, D.T. 2006. Forced annular variations in the 20<sup>th</sup> century Intergovernmental Panel on Climate Change Fourth Assessment Report models, J. Geophys. Res., 111, doi:10.1029/2005JD006323.</ref>), generally with spatial correlations greater than 0.95 (Randall et al., 2007<ref name="Randall et al, 2007">Randall, D.A., Wood, R.A., Bony, S., Colman, R., Fichefet, T., Fyfe, J., Kattsov, V., Pitman, A., Shukla, J., Srinivasan, J., Stouffer, R.J., Sumi, A. and Taylor, K.E. 2007. Climate Models and Their Evaluation. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref>). The SAM signature in the surface warming anomaly over the Antarctic Peninsula is also captured by some models (e.g. Delworth et al., 2006<ref name="Delworth et al, 2006">Delworth, T.L., Broccoli, A.J. , Rosati, A., Stouffer, R.J., Balaji, V., Beesley, J.A., Cooke, W.F., Dixon, K.W., Dunne, J., Dunne, K.A., Durachta, J.W., Findell, K.L., Ginoux, P., Gnanadesikan, A., Gordon, C.T., Griffies, S.M., Gudgel, R., Harrison, M.J., Held, I.M., Hemler, R.S., Horowitz, L.W., Klein, S.A., Knutson, T.R., Kushner, P.J., Langenhorst, A.R., Lee, H.C., Lin, S.J., Lu, J., Malyshev, S.L., Milly, P.C.D., Ramaswamy, V., Russell, J., Schwarzkopf, M.D., Shevliakova, E., Sirutis, J.J., Spelman, M.J., Stern, W.F., Winton, M., Wittenberg, A.T., Wyman, B., Zeng, F. and Zhang, R. 2006. GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics, ''J. Climate'', '''19''', 643-674.</ref>). Other features, including the zonal structure and the temporal signal, exhibit large variance between outputs even in a single model (Miller et al., 2006<ref name="Miller et al, 2006">Miller, R.L., Schmidt, G.A. and Shindell, D.T. 2006. Forced annular variations in the 20<sup>th</sup> century Intergovernmental Panel on Climate Change Fourth Assessment Report models, J. Geophys. Res., 111, doi:10.1029/2005JD006323.</ref>; Raphael and Holland, 2006<ref name="Raphael and Holland, 2006">Raphael, M.N. and Holland, M.M. 2006. Twentieth century simulation of the Southern Hemisphere climate in coupled models. Part 1: Large scale circulation variability, ''Clim. Dyn.'', '''26''', 217-228, doi:10.1007/s00382-005-0082-8.</ref>). Hence, the extent of discrepancies in the simulated SAM due to model shortcomings alone is difficult to gauge. As an added complexity, new evidence has emerged that ENSO variability can influence SAM variability in the southern summer (L&rsquo;Heureux and Thompson, 2006<ref name="L&rsquo;Heureux and Thompson, 2006">L&rsquo;Heureux, M.L. and Thompson, D.W.J. 2006. Observed relationships between the El-Ni&ntilde;o/Southern Oscillation and the extratropical zonal mean circulation, ''J. Climate'', '''19''', 276-287.</ref>).<br />
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==Projected changes in modes of circulation variability==<br />
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[[File:Figure 5.4 - Change in average tropical Pacific SSTs and El Nino variability simulated by AOGCMs.png|thumb|'''5.4''' Base state change in average tropical Pacific SSTs and change in El Ni&ntilde;o variability simulated by AOGCMs. The base state change (horizontal axis) is denoted by the spatial anomaly pattern correlation coefficient between the linear trend of SST in the 1%/yr CO<sub>2</sub> increase climate change experiment and the first Empirical Orthogonal Function (EOF) of SST in the control experiment over the area 10&ordm;S to 10&ordm;N, 120&ordm;E to 80&ordm;W (reproduced from Yamaguchi and Noda, 2006<ref name="Yamaguchi and Noda, 2006">Yamaguchi, K. and Noda, A. 2006. Global Warming Patterns over the North Pacific: ENSO versus AO, ''J. Meteor. Soc. Japan'', '''84''', 221-241.</ref>). The change in El Ni&ntilde;o variability (vertical axis) is denoted by the ratio of the standard deviation of the first EOF of sea level pressure (SLP) between the current climate and the last 50 years of the IPCC A2 experiments (2051&ndash;2100), in the region 30&ordm;S to 30&ordm;N, 30&ordm;E to 60&ordm;W (reproduced from van Oldenborgh et al., 2005<ref name="Oldenborgh et al, 2005">Van Oldenborgh, G.J., Philip, S.Y. and Collins, M. 2005. El Nino in a changing climate: a multi-model study, ''Ocean Sci.'', '''1''', 81-95.</ref>). Error bars indicate the 95% confidence interval; from IPCC (2007<ref name="IPCC, 2007">IPCC 2007. Climate Change 2007: The Physical Science Basis. Contribution of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.</ref>).]]<br />
In the range of studies reported by the IPCC&rsquo;s AR4, it has been demonstrated that the ENSO response of climate system models in the Twenty First Century is highly model dependent (see [[:File:Figure 5.4 - Change in average tropical Pacific SSTs and El Nino variability simulated by AOGCMs.png|Figure 5.4]]; (Meehl et al., 2007<ref name="Meehl et al, 2007">Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver, and Z.-C. Zhao, 2007: Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 749-845.</ref>). This outcome has changed little in the most recent analyses (e.g. Yeh and Kirtman, 2007<ref name="Yeh and Kirtman, 2007">Yeh, S.-W. and Kirtman, B.P. 2007. ENSO amplitude changes due to climate change projections in different coupled models, ''J. Climate'', '''20''', 203-217.</ref>), and represents a major challenge in projecting Antarctic variability. Currently, there is some consensus that there will be little change in the magnitude of ENSO variability in the Twenty First Century, although some of the models that simulated Twentieth Century ENSO variability well do indicate Twenty First Century increases in the amplitude of El Ni&ntilde;o events (Meehl et al., 2007<ref name="Meehl et al, 2007">Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, A.T. Gaye, J.M. Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson, A.J. Weaver, and Z.-C. Zhao, 2007: Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 749-845.</ref>). If such a trend is manifest, it would contribute to changes in sea ice cover and circulation over Antarctica of a similar sense to those observed in the last few decades, but the reliability of such a projection is confounded by the apparent decadal variability in the system (e.g. Fogt and Bromwich, 2006<ref name="Fogt and Bromwich, 2006">Fogt, R.L. and Bromwich, D.H. 2006. Decadal Variability of the ENSO Teleconnection to the High-Latitude South Pacific Governed by Coupling with the Southern Annular Mode, ''J. Climate'', '''19''', 979-997.</ref>).<br />
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[[File:Figure 5.5 - Mean regression of the leading EOF of ensemble mean SH sea level pressure.png|thumb|'''5.5''' Multi-model mean of the regression of the leading EOF of ensemble mean Southern Hemisphere sea level pressure. The time series of regression coefficients has zero mean between year 1900 and 1970. The thick red line is a 10-year low-pass filtered version of the mean with ozone forcing; the blue line is without ozone forcing. The grey shading represents the inter-model spread at the 95% confidence level and is filtered. A filtered version of the observed SLP from the Hadley Centre (HadSLP1) is shown in black. Adapted from (Miller et al., 2006<ref name="Miller et al, 2006">Miller, R.L., Schmidt, G.A. and Shindell, D.T. 2006. Forced annular variations in the 20<sup>th</sup> century Intergovernmental Panel on Climate Change Fourth Assessment Report models, J. Geophys. Res., 111, doi:10.1029/2005JD006323.</ref>). From IPCC (2007<ref name="IPCC, 2007">IPCC 2007. Climate Change 2007: The Physical Science Basis. Contribution of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.</ref>).]]<br />
The future trend in the SAM, as characterized by the leading Empirical Orthogonal Function (EOF) of sea level pressure, has been reported from a number of model projections (e.g. GISSII - Shindell and Schmidt, 2004<ref name="Shindell and Schmidt, 2004">Shindell, D.T. and Schmidt, G.A. 2004. Southern hemisphere climate response to ozone changes and greenhouse gas increases, ''Geophys. Res. Lett.'', '''31''', L18209, doi:10.1029/2004GL020724.</ref>; CCSM - Arblaster and Meehl, 2006<ref name="Arblaster and Meehl, 2006">Arblaster, J.M. and Meehl G.A. 2006. Contributions of external forcings to Southern Annual Mode trends, ''Journal of Climate'', '''19''', 2896-2905.</ref>). Most models support a continuing positive trend in the SAM index, as manifest by a strong intensification of the polar vortex. The observed SAM index trend has been related to stratospheric ozone depletion (Sexton, 2001<ref name="Sexton, 2001">Sexton, D.M.H. 2001. The effect of stratospheric ozone depletion on the phase of the Antarctic Oscillation, ''Geophys. Res. Lett.'', '''28''', 3697-3700.</ref>) and to greenhouse gas increases (Hartmann et al., 2000<ref name="Hartmann et al, 2000">Hartmann, D.L., Wallace, J.M., Limpasuvan, V., Thompson, D.W.J. and Holton, J.R. 2000. Can ozone depletion and global warming interact to produce rapid climate change?, ''Proc. Natl. Acad. Sci. U.S.A.'', '''97''', 1412-1417.</ref>). Specifically, a larger positive trend was projected during the late Twentieth Century by models that included stratospheric ozone changes (e.g. Cai and Cowan, 2007<ref name="Cai and Cowan, 2007">Cai, W.J. and Cowan, T. 2007. Trends in Southern Hemisphere circulation in IPCC AR4 models over 1950-99: Ozone depletion versus greenhouse forcing, ''J. Climate'', '''20''', 681-693.</ref>; see [[:File:Figure 5.5 - Mean regression of the leading EOF of ensemble mean SH sea level pressure.png|Figure 5.5]]). Further evidence for this relationship is found in the fact that the signal is largest in the lower stratosphere in austral spring through summer (Arblaster and Meehl, 2006<ref name="Arblaster and Meehl, 2006">Arblaster, J.M. and Meehl G.A. 2006. Contributions of external forcings to Southern Annual Mode trends, ''Journal of Climate'', '''19''', 2896-2905.</ref>). Though somewhat uncertain, it is expected that ozone will continue to slow its decline in the Twenty First Century, as has been observed since 1997 (Yang et al., 2006<ref name="Yang et al, 2006">Yang, E.-S., Cunnold, D.M., Salawitch, R.J., McCormick, M.P., Russell III, J., Zawodny, J.M., Oltmans, S., and Newchurch, M.J. 2006. Attribution of recovery in lower-stratospheric ozone, ''J. Geophys. Res.'', '''111''', D17309, doi:10.1029/2005JD006371.</ref>). Hence, in future projections, the SAM index trends for simulations with and without ozone are comparable. However, the increase in greenhouse gases is also an important factor that supports a continued increase in the SAM index on an annual basis, forced by trends in the meridional temperature gradient (Brandefelt and K&auml;ll&eacute;n, 2004<ref name="Brandefelt and K&auml;ll&eacute;n, 2004">Brandefelt, J. and K&auml;ll&eacute;n, E. 2004: The response of the Southern Hemisphere atmospheric circulation to an enhanced greenhouse gas forcing, ''J. Climate'', '''17''', 4425-4442.</ref>). Like the uncertainties surrounding the sources of model error in simulating the SAM in the Twentieth Century, the confounding element of future trajectories in stratospheric ozone concentration makes precise projection of the SAM more problematic than other elements of Antarctic climate; nevertheless the general trend is clear.<br />
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==Impacts on synoptic climate==<br />
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Observed changes in weather systems are consistent with the trends in the dominant modes of variability, although these results too are strongly dependent on the quality of reanalysis products. In the Southern Hemisphere, cyclonic activity is strikingly different between the ERA-40 and NRA re-analyses (Bromwich et al., 2007<ref name="Bromwich et al, 2007">Bromwich, D.H., Fogt, R.L., Hodges, K.I. and Walsh, J.E. 2007. A tropospheric assessment of the ERA-40, NCEP, and JRA-25 global reanalyses in the polar regions, J. Geophys. Res., 112, Art. No. D10111.</ref>). Nevertheless, consistent signals include a decrease in the frequency and increases in the size and intensity of extratropical cyclones in recent decades (e.g. Simmonds and Keay, 2000<ref name="Simmonds and Keay, 2000">Simmonds, I. and Keay, K. 2000. Variability of Southern Hemisphere extratropical cyclone behaviour, 1958-97, ''J. Climate'', '''13'''(3), 550-561.</ref>) with a moderate increase in frequency over the Southern Ocean (e.g. Fyfe, 2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>).<br />
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A consistent result that has emerged recently from Twenty First Century projections is a tendency for a poleward shift of several degrees latitude in mid-latitude storm tracks (e.g. Fischer-Bruns et al., 2005<ref name="Fischer-Bruns et al, 2005">Fischer-Bruns, I., Von Storch, H., Gonzalez-Rouco, J.F. and Zorita, E. 2005. Modelling the variability of midlatitude storm activity on decadal to century time scales, ''Clim. Dyn.'', '''25''', 461-476.</ref>; Bengtsson et al., 2006<ref name="Bengtsson et al, 2006">Bengtsson, L., Hodges, K. and Roeckner, E. 2006. Storm tracks and climate change, ''J. Climate'', '''19''', 3518-3543.</ref>). Consistent with these shifts, Lynch et al. (2006<ref name="Lynch et al, 2006">Lynch, A., Uotila, P. and Cassano, J.J. 2006. Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic, ''Int. J. Climatol.'', '''26''', 1181-1199.</ref>) demonstrated increasing cyclonicity and stronger westerlies in high southern latitudes in a 10-member multi-model ensemble simulation of the Twenty First Century. One study (Fyfe, 2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>) has suggested a reduction in sub-Antarctic cyclones of more than 30% by the end of the century. Fyfe (2003<ref name="Fyfe, 2003">Fyfe, J.C. 2003. Extratropical southern hemisphere cyclones: Harbingers of climate change? J. Climate, 16, 2802-2805.</ref>) did not definitively identify a poleward shift of storm tracks, but the relatively coarse grid of the CCCma climate model (T32, or approximately 600 km grid spacing) may not have been able to detect such a shift. These changes have been related to a simulated circumpolar signal of increased precipitation off the coast of Antarctica (Lynch et al., 2006<ref name="Lynch et al, 2006">Lynch, A., Uotila, P. and Cassano, J.J. 2006. Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, Part 2: Antarctic, ''Int. J. Climatol.'', '''26''', 1181-1199.</ref>; see also Section 5.2.5), which perhaps, though loosely, argues against the frequency trend being related to increased data availability, as suggested by Hines et al. (2000<ref name="Hines et al, 2000">Hines K.M., Bromwich, D.H. and Marshall, G.J. 2000. Artificial surface pressure trends in the NCEP&ndash;NCAR reanalysis over the Southern Ocean and Antarctica, ''J. Climate'', '''13''', 3940-3952.</ref>).<br />
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==Impacts on accumulation==<br />
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The Antarctic ice sheet constitutes the largest reservoir of freshwater on Earth, representing tens of metres of sea-level rise if it were to melt completely. Hence, the mass balance of the Antarctic ice sheet is an important contributor to the impacts of sea-level change over the next century. The circulation provides an important component of forcing, particularly through precipitation. The relationships between the major modes of variability and precipitation over the Antarctic continent have been studied, but while there seems to be a correlation between increased coastal precipitation and the SAM index (within the limits of the data), Noone and Simmonds (2002<ref name="Noone and Simmonds, 2002">Noone, D. and Simmonds, I. 2002. Annular variations in moisture transport mechanisms and the abundance of delta<sup>18</sup>O in Antarctic snow, ''J. Geophys. Res.'', '''107''', 4742, doi:10.1029/2002JD002262.</ref>) have demonstrated that, in at least one climate model, eddy moisture convergence (i.e. associated with depressions) represents a large fraction of net precipitation. Synoptic activity intensification over the Southern Ocean would suggest a potential for increase in accumulation along the coasts (Sinclair et al., 1997<ref name="Sinclair et al, 1997">Sinclair, M.R., Renwick, J.A. and Kidson, J.W. 1997. Low-frequency variability of Southern Hemisphere sea level pressure and weather system activity, ''Mon. Weather Rev.'', '''125''', 2531-2543.</ref>). However, significant correlations with the SOI appear to be intermittent (e.g. Bromwich et al., 2000<ref name="Bromwich et al, 2000">Bromwich, D.H., Rogers, A.N., Kallberg, P., Cullather, R.I., White, J.W.C. and Kreutz, K.J. 2000. ECMWF analyses and reanalyses depiction of ENSO signal in Antarctic precipitation, ''J. Climate'', '''13''', 1406-1420.</ref>; Genthon and Cosme, 2003<ref name="Genthon and Cosme, 2003">Genthon, C. and Cosme, E. 2003. Intermittent signature of ENSO in west-Antarctic precipitation, ''Geophys. Res. Lett.'', '''30''', 2081, doi:10.1029/2003GL018280.</ref>). More recently, a nonlinear interaction between the Southern Oscillation and the SAM that varies on decadal time scales has been identified as a possible reason for this irregularity (Fogt and Bromwich, 2006<ref name="Fogt and Bromwich, 2006">Fogt, R.L. and Bromwich, D.H. 2006. Decadal Variability of the ENSO Teleconnection to the High-Latitude South Pacific Governed by Coupling with the Southern Annular Mode, ''J. Climate'', '''19''', 979-997.</ref>).<br />
==References==<br />
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[[Category:The next 100 years]]<br />
[[Category:The Antarctic atmosphere]]</div>Maintenance script