Tropical Cyclones and Climate Change in a High Resolution General Circulation Model, HiGEM

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 (Shaffreyet al, 2009) HiGEM 4xCO230 yrs HiGEM Transient 2% CO2/yr 70 yrs N144 HiGEM 2xCO230 yrs 1.25ox0.83o, ∆x50N = 90 km HiGEM CTRL ~5x30 yrs HiGEM 1.1 CTRL 150 yrs 1/3o ocean model 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. Tracking algorithm (TRACK; Bengsttonet al, 2007) A 20 year time-slice of GCM simulated tropical storms 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

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

6. Climate Change Simulations Track density difference 2xCO2 – CTRL 4xCO2 – CTRL Stippling if outside 5x30yr CTRL variability

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

8. Sea Surface Temperature Difference JASO 2xCO2 – CTRL 4xCO2 – CTRL Stippling not shown significant everywhere • 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 Zhao et al (2009)

9. Walker CirculationDifference JASO 0-10N° CTRL 2xCO2 - CTRL 4xCO2 - CTRL -ω difference (Pa/s) and divU difference (m/s) • 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)

10. Vertical Wind Shear Difference JASO 2xCO2 - CTRL 4xCO2 - CTRL • VWS spreads to the NEPac especially in the 4xCO2 • Detrimental affect on TCs. • Reduced VWS in CPac favours development Vecchi and Soden(2007)

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

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 4xCO2 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) Differences in TC counts as we substitute observed SSTs with SSTs generated by HiGEM 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 Jane Strachan HiGEM-AMIPII SST bias (DJFM)

18. TRACK Hodges (1995); Bengstssonet al. (2007) • T42 ξ850 – 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 (0-30oN over ocean) • Coherent vertical structure and warm core • Max T63 vor at each level from 850hPa to 250hPa • Intensity threshold T63 ξ850 > 6x10-5 s-1, ξ850 – ξ200 > 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

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 Hurricane Mitch 200hPa 200hPa 200hPa 500hPa 500hPa 500hPa 850hPa 850hPa 850hPa 135km resolution 60km HadGAM-HiGAM-NUGAM tangential wind

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 RH700 Vecchi and Soden (2007)

30. Change in –ω500

31. Change in ppt

32. Large scale tropical change

33. Held and Zhao (2011)