1. Mid-latitude and Tropical Storms Simulated Changes in Atmospheric Drivers of Extreme Sea Levels Ruth McDonald Hadley Centre, Met Office Understanding Sea-level Rise and Variability WCRP Workshop, Paris, June 2006 Ruth.McDonald@metoffice.gov.uk formerly Ruth Carnell

2. Page 2© Crown copyright 2005 Outline  Introduction  Model predictions of future changes in the frequency and intensity of mid-latitude storms  Model predictions of future changes in the frequency of tropical storms  Why are cyclones important for sea-level?  Storm surges  1953 flooding around North Sea coast  Hurricane Katrina New Orleans 2005

3. Page 3© Crown copyright 2005 Introduction to tropical and extra-tropical cyclones Tropical CycloneExtra-tropical cyclone colours = SST blue=15°C, blue/green = 20°C, green =25°C solid lines = wind speed Figure from Merrill (1993), downloaded from Hurricanes FAQ by Chris Landsea at http://www.aoml.noaa.gov/hrd/tcfaq/A7.html Storm descriptions based on Hurricanes FAQ Section A7 (by Stan Goldenberg) No temperature contrast at surface Energy from latent heat Baroclinic Energy from horizontal temperature contrast Contours= pressure surfaces Warm core Strongest winds near surface Cold core Strongest winds near tropopause Scale of 100s kmScale of 1000s km

4. Page 4© Crown copyright 2005 Impacts of global warming on storms  It is often assumed that global warming will cause an increase in the frequency and intensity of extra-tropical and tropical cyclones  Not necessarily the case  Mid-latitude storms  Changes occur as a result of competing effects due to changes in atmospheric temperature and moisture  Decrease in low level temperature contrast between pole and equator  Less energy for storms  Increases in upper level temperature gradient  More energy for storms  Local temperature contrasts are also important  More moisture  More latent heating. More intense storms  Fewer storms are required to give the same energy flux between equator and poles  Tropical storms  Linked to enhanced sea surface temperatures and increased moisture  Changes to large-scale circulation, wind shear  Changes to modes of variability like NAO and ENSO also important

5. Page 5© Crown copyright 2005 Analysis of storms in climate models  Apply an objective technique to locate and in some studies track storms  Local centres of geopotential height, mean sea level pressure or relative vorticity  Last for at least 2 days and be non-stationary (if tracking)  Tropical cyclones also test for warm cores  Use parameter diagnostics  Band pass filter storm track to look at synoptic variability  Eady parameter to look at baroclinicity  Seasonal genesis parameter for tropical storms  Lots of techniques, makes it hard to compare results

6. Page 6© Crown copyright 2005 Examples of model cyclone tracks Tropical Storms for 15 years Mid-Latitude Storms for a single winter Organised into tracks: Atlantic, Pacific, Mediterranean Tracks look sensible, despite low resolution and poor simulation of individual cyclones Few events

7. Mid-latitude Storms

8. Page 8© Crown copyright 2005 Simulation of mid-latitude storms by models:- Cyclone density AMIP 13 Model Mean ERA Difference Lambert et al. (2002) Climate Dynamics number of cyclone events per 145,000 km 2 accumulated over nine 120 day winter periods for the Northern Hemisphere Summary All models have systematic errors in the simulation of cyclone tracks Cyclones tend to be too weak Errors often larger than climate change signal Too many Too few Too far north

9. Page 9© Crown copyright 2005 Future changes in mid-latitude storms:- Cyclone and track density ECHAM5-OM A1B ECHAM4/ OPYC3 IS92a Bengtsson et al.(2006) J Climate, inpress, thanks to KHodgesTracks permonth per10 6 km 2 Leckebusch et al.(2006)Climate Research,in press, thanks to GLeckebuschTrack density Carnell and Senior(1998)Climate DynamicsCyclones perseason per10 6 km 2 HadCM2 IS92a JMA Geng and Sugi (2003) J Climate Cyclones per 4.5°x4.5° per season North Atlantic HadAM3P A2 ECHAM5 NE Pacific SH Summary: Little consensus of local changes in frequency of storms amongst climate models

10. Page 10© Crown copyright 2005 Future changes in the frequency of winter mid- latitude storms ReferenceModel Experiments NH ChangeSH Change Carnell and Senior 1998 HadCM2 N48 IS95a 3x30y Fewer Geng and Sugi 2003 JMA T10620y OBS 2050s Fewer Poleward and eastward Fewer Fyfe 2003 CCCma3xIS92a 500y Ctrl Sub-Antarctic 30% fewer Lambert 2004 CGCM1 CGCM2 T32 1% 1850-2100 Fewer Watterson 2006 CSIRO Mk2 R21, Mk3 T63 30y A2 Fewer Lambert and Fyfe 2006 IPCC 4AR GCMs 20yFewer No shift Fewer No shift Bengtsson et al. 2006 ECHAM5 OM T63 3x30y A1B No change Poleward shift No change Poleward shift Summary: There are fewer mid-latitude storms in winter in both hemispheres in the future simulations

11. Page 11© Crown copyright 2005 Future changes in the frequency of intense Northern Hemisphere winter mid-latitude storms ReferenceModelExperimentIntensity measure Change in frequency of intense cyclones Carnell and Senior 1998 HadCM2 N48 IS95a 3x30y Central MSL pressure More intense Geng and Sugi 2003 JMA T10620y OBS, 2050s Central MSL pressure gradient More intense Lambert 2004 CGCM1 CGCM2 T32 1% 1850-2100 Central MSL pressure More intense Watterson 2006 CSIRO Mk2 R21, Mk3 T63 30y A2 Various dynamical measures & precipitation Little change in dynamical intensity, more precipitation Lambert and Fyfe 2006 IPCC 4AR GCMs 20yCentral MSL pressure More intense Bengtsson et al. 2006 ECHAM5 OM T63 3x30y A1B Central Relative vorticity Fewer weak Summary: There is some evidence of an increase in the frequency of the deepest storms in the future simulations

12. Page 12© Crown copyright 2005 Summary of other studies on changes in mid- latitude storms RefModelExpChanges Lionello et al. 2002ECHAM430y 2xCO2 Fewer tracks in Mediterranean region Fyfe 2003CCCmaIS92a x3 1850-2100 Sub-Antarctic fewer cyclones Lozano et al 2004ECHAM4 AGCM T106 2xCO2 30yCoastal storms Fewer but more intense in region covering Ireland and Scotland Leckebusch and Ulbrich 2004 HadCM3A1, B2 30y More North Atlantic cyclone tracks, and more intense Yin 2005IPCC 4AR GCMsA1B 20y BPF EKE Poleward and upward shift intensification Inatsu and Kimoto 2005 CCSR/NIES/FRC GC AGCM 20y x7 2xCO2 timeslice EKE shows W Pacific tracks stronger and W Hemisphere track weaker Fischer-Bruns et al. 2006 ECHAM4- HOPEG T30 A2, B2Max wind speed events, poleward shift, more N Atl, SO, fewer Pacific ocean Leckebusch et al. 2006 5 GCMsA1 and IS92a 30y Fewer in N Atlantic, Oct-Mar More intense

13. Tropical Storms

14. Page 14© Crown copyright 2005 Simulation of tropical storms by models:- cyclone genesis density N144 HadAM3 McDonald et al. (2005) Climate Dynamics Cyclone genesis per 0.83°x1.25° x per 17y T106 JMA Yoshimura and Sugi (2006) SOLA Observations 10y Model CLIM1 10y Models simulate cyclones in S Atlantic Too few in NE Pacific Summary: Simulation of TC genesis is realistic but there are some errors

15. Page 15© Crown copyright 2005 Future changes in the frequency of tropical storms McDonald et al. (2005) Climate Dynamics Cyclone genesis per 0.83°x1.25° x per 17y N144 HadAM3 Oouchi et al. (2006) J Met Soc Japan T106 JMA Sugi et al. J Met Soc Japan 20km MRI/JMA N Atlantic NW Pacific

16. Page 16© Crown copyright 2005 Future changes in the frequency of tropical storms Ocean basin modelreferenceGlobalNAWNPENPNISISWP T106 JMA 10y Sugi et al. 2002 6616134331094369 T42 NCAR CCM2 10y Tsutsui 2002 102861119111612499 N144 HadAM3 15y McDonald et al. 2005 94757018014211082 T106 CCSR/NIES/FRCGC Hasegawa and Emori 2005 96 T106 JMA 10y Yoshimura & Sugi 05 fewer T63 ECHAM5-OM Bengtsson et al. 2006 94 20km MRI/JMA Oouchi et al. 2006 701346266487257 Ratio (%) of number of storms in global warming experiment to number in control experiment Red = significantly more tropical storms in the future simulation Blue = significantly fewer tropical storms in the future simulation Summary: fewer tropical cyclones globally in the future simulations, sign of regional changes varies between model and basin

17. Page 17© Crown copyright 2005 Summary  IPCC TAR: simulated future changes in storms are inconclusive  There are still large uncertainties in the future changes of mid-latitude and tropical storms  Mid-latitude storms  Models are tending to agree on there being fewer Northern and Southern Hemisphere extra-tropical storms in winter  Models do not agree on regional changes in frequency of storms  Some models show poleward shifts in the storm tracks  There is some agreement on increased frequency of extra-tropical storms with central pressure below 970 hPa  Tropical storms  Models are tending to show reduced frequency of tropical storms, but the sign of the changes varies with region

18. Page 18© Crown copyright 2005 References  Bengtsson L, Hodges K, Roeckner E (2006) Storm tracks and climate change. J Climate, in press  Carnell RE, Senior CA (1998) Changes in mid-latitude variability due to increasing greenhouse gases and sulphate aerosols. Clim Dynam 14: 369–383, DOI: 10.1007/s003820050229  Fischer-Bruns I., von Storch H, Gonzáles-Rouco JF and Zorita E (2005) Modelling the variability of midlatitude storm activity on decadal to century time scales. Clim Dynam 25: 461 – 476, DOI: 10.1007/s00382-005-0036-1  Fyfe JC (2003) Extratropical Southern Hemisphere cyclones: harbingers of climate change? J Clim 16: 2802-2805, DOI: 10.1175/1520- 0442(2003)016 2.0.CO;2  Geng Q and Sugi M (2003) Possible change of extratropical cyclone activity due to enhanced greenhouse gases and sulfate aerosols - study with a high-resolution AGCM. J Clim 16: 2262- 2274, DOI: 10.1175/1520-0442(2003)16 2.0.CO;2  Gray WM (1979) Hurricanes: their formation, structure and likely role in the tropical circulation. In Shaw DB, editor, Meteorology over the Tropical Oceans 155-218. Royal Meteor Soc  Hasegawa A and Emori S (2005) Tropical cyclones and associated precipitation over the Western North Pacific: T106 atmospheric GCM simulation for present-day and doubled CO2 climates. SOLA 1: 145-148, SOI:10.2151/sola.2005-038  Inatsu M, Kimoto M (2005) Two Types of Interannual Variability of the Mid-winter Storm-tracks and their Relationship to Global Warming. SOLA 1: 61-64, DOI: 10.2151/sola.2005-017  Lambert SJ (1995) The effect of enhanced greenhouse warming on winter cyclone frequencies and strengths. J Clim 8: 1447–1452, DOI: 10.1175/1520- 0442(1995)008 2.0.CO;2  Lambert SJ, Sheng J and Boyle J (2002) Winter cyclone frequencies in thirteen models participating in the Atmospheric Model Intercomparison Project (AMIP1). Clim Dynam 19: 1-16, DOI: 10.1007/s00382-001-0206-8  Lambert SJ (2004) Changes in winter cyclone frequencies and strengths in transient enhanced greenhouse warming simulations using two coupled climate models. Atmosphere-Ocean 42: 173-181  Lambert SJ, Fyfe JC (2006) Changes in winter cyclone frequencies and strengths simulated in enhanced greenhouse warming experiments: results from the models participating in the IPCC diagnostic exercise. Clim Dynam, 26: 713 - 728, DOI: 10.1007/s00382-006-0110-3  Leckebusch G, Ulbrich U (2004) On the relationship between cyclones and extreme windstorm events over Europe under climate change. Global and Planetary Change 44: 181–193, DOI 10.1016/j.gloplacha.2004.06.011  Leckebusch GC, Koffi B, Ulbrich U, Pinto JG, Spangehl T and Zacharias S (2006) Analysis of frequency and intensity of winter storm events in Europe on synoptic and regional scales from a multi-model perspective. Climate Research, in press  Lionello P, Dalan F, Elvini E (2002) Cyclones in the Mediterranean region: the present and the doubled CO2 climate scenarios. Clim Res 22: 147-159  Lozano I, Devoy RJN, May W, Anderson U (2004) Storminess and vulnerability along the Atlantic coastlines of Europe: analysis of storm records and of a greenhouse gases induced climate scenario. Marine Geology 210: 205-225  Merrill R T (1993) Tropical Cyclone Structure. Chapter 2, Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland  McDonald RE, Bleaken DG, Cresswell DR, Pope VD, Senior CA (2005) Tropical storms: representation and diagnosis in climate models and the impacts of climate change. Climate Dynamics 25: 19-36, DOI: 10.1007/s00382-004-0491-0  Oouchi K; Yoshimura J, Yoshimura H, Mizuta R, Kusunoki S and Noda A (2006) J Meteorol Soc Japan, 84, 259-276  Sugi M, Noda A, Sato N (2002) Influence of global warming on tropical cyclone climatology: an experiment with the JMA global model. J Meteorol Soc Japan 80: 249- 272  Tsutsui J (2002) Implications of anthropogenic climate change for tropical cyclone activity: a case study with the NCAR CCM2. J Meteorol Soc Japan 80: 45-65, DOI:10.2151/jmsj.80.45  Watterson IG (2006) The intensity of precipitation during extratropical cyclones in global warming simulations: a link to cyclone intensity? Tellus A 58: 82-97, DOI: 10.1111/j.1600-0870.2006.00147.x  Yin JH (2005), A consistent poleward shift of the storm tracks in simulations of 21st century climate, Geophys Res Lett 32: L18701, DOI:10.1029/2005GL023684  Yoshimura J and Sugi M (2005) Tropical Cyclone Climatology in a High-resolution AGCM - Impacts of SST Warming and CO2 Increase. SOLA 1: 133-136, DOI: 10.2151/sola.2005-035