1. Tropical Cyclones and Climate Change in a High Resolution General Circulation Model, HiGEM Ray Bell Supervisors: Prof. P.L. Vidale, Dr. Kevin Hodges and Dr. Jane Strachan

2. Outline Research Objective Investigate the changes in TC activity with climate change focusing on: - location, frequency, - intensity, structure and duration HiGEM model TRACKing algorithm TC changes Large scale forcing changes Conclusions

3. Idealised GCM simulations HiGEM UK’s High-Resolution Global Environmental Model (Shaffrey et al, 2009) 1.25 o x0.83 o, ∆x 50N = 90 km 1/3 o ocean model HiGEM Transient 2% CO 2 /yr 70 yrs HiGEM 1.1 CTRL 150 yrs HiGEM 2xCO 2 30 yrs HiGEM 4xCO 2 30 yrs HiGEM CTRL ~5x30 yrs N144 HadGAM N48 (~270km) HiGAM is part of our HIERARCHY OF AGCMs HadGAM N96 (~135km) HiGAM N144 (~90km) NUGAM N216 (~60km) HadGEM3H N216 (~60km) HadGEM3H N320 (~40km) HadGEM3H N512 (~25km) New Hadley Centre Model with double vertical resolution and higher model top (85km)

4. 1) Locate and track all centres of high relative vorticity  35000/yr 2) Apply a 2-day filter to the tracks  8000 storms / yr 3) Analyse vertical structure of storm for evidence of warm-core (tropical storm structure)  120 storms / yr Tracking algorithm (TRACK; Bengstton et al, 2007) A 20 year time-slice of GCM simulated tropical storms

5. Validating the model At this model resolution we are able to realistically capture location and frequency Strachan et al (2012) in review Atmosphere only Coupled (TRACK) AMIP

6. Climate Change Simulations Track density difference 2xCO 2 – CTRL 4xCO 2 – CTRL Stippling if outside 5x30yr CTRL variability

7. Climate Change Simulations TC frequency difference Error bars show 5x30yr CTRL variability CTRL

8. Sea Surface Temperature Difference 2xCO 2 – CTRL JASO Tongue of relatively less warm water compared to the rest of the tropics Grave results of TCs in this vicinity (NAtl). Leads to increased vertical wind shear (VWS) via thermal wind balance 4xCO 2 – CTRL Zhao et al (2009) Stippling not shown significant everywhere

9. Weakening of the tropical circulation inline with other studies (Vecchi and Soden, 2007) Favours development in the CPac and reduces TC frequency is the NWPac (Li et al, 2010; Murakami et al, 2011) Walker Circulation Difference JASO 0-10N° -ω difference (Pa/s) and divU difference (m/s) 2xCO 2 - CTRL 4xCO 2 - CTRL CTRL

10. Vertical Wind Shear Difference JASO VWS spreads to the NEPac especially in the 4xCO 2 Detrimental affect on TCs. Reduced VWS in CPac favours development 2xCO 2 - CTRL 4xCO 2 - CTRL Vecchi and Soden (2007)

11. Large scale forcing % change % change NAtl NEPac Similar to Held and Zhao (2011) CTRL

12. Conclusions HiGEM realistically captures the geographical location and NH TC frequency compared to those identified in reanalyses and observations. HiGEM simulates a decrease of TC frequency in most regions except for the North Indian basin and North Central Pacific region. A weaker Walker circulation suppresses activity in the North West Pacific and enhances activity in the North Central Pacific. An increase in VWS in the 4xCO 2 over the North Atlantic spreads to the North East Pacific and decreases TC frequency.

13. Future work Investigate different types of El Niño in the control simulation and how these may change with climate change. Investigate TC changes with varying model resolutions with a focus on intensity. Investigate TC changes in a transient forcing vs. a stabilised forcing

15. Validating the model At this model resolution we are able to realistically capture location and frequency Strachan et al (2012) in rev

16. AMIPII SSTs (1979-2002) JASO mean (for HiGAM) HiGEM SSTs (30 years) JASO mean HiGEM-AMIPII SST bias (JASO) AMIPII SSTs (1979-2002) DJFM mean (for HiGAM) HiGEM SSTs (30 years) DJFM mean HiGEM-AMIPII SST bias (DJFM) Differences in TC counts as we substitute observed SSTs with SSTs generated by HiGEM Jane Strachan

18. T42 ξ 850 – Reduce noise. Comparison of different spatial resolution data Minimum lifetime of 2 days and no constraint on the minimum displacement distance. Capture more of TC lifecycle Cyclogenesis (0-30 o N over ocean) Coherent vertical structure and warm core Max T63 vor at each level from 850hPa to 250hPa Intensity threshold T63 ξ 850 > 6x10 -5 s -1, ξ 850 – ξ 200 > 6x10 -5 s -1, for at least 1 day (4 x 6hr). Search for warm core between p levels 850-500, 500-200hPa (+ ξ value) Statistical packages TRACK Hodges (1995); Bengstsson et al. (2007)

19. Validation of TRACK against obs Robert Lee

20. Are the modelled TCs really warm core storms ? Is the near-core circulation correct; do we see changes with resolution ? Had-Hi-NUGAM, MIROC, ECHAM: 100 most intense TCs in 25 yrs HadGAM-HiGAM-NUGAM tangential wind Hurricane Mitch 850hPa 500hPa 200hPa 850hPa 500hPa 200hPa 850hPa 500hPa 200hPa 135km60kmresolution

21. Validation of TC frequency (AVG)

22. Absolute track densities and NAtl changes

23. Climate Change Simulations

24. HiGEM absolute SST change and SH SST change

25. Absolute Walker circulation changes

26. NAtl Hadley cell changes

27. HiGEM absolute vws and SH change

28. NH diff in winds

29. Change in RH 700 Vecchi and Soden (2007)

30. Change in –ω 500

31. Change in ppt

32. Large scale tropical change

33. Held and Zhao (2011)