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

2. Introduction Motivation Research Objectives
Socio-economic impacts and changing risk with climate change.
Impacts on the climate system, removing heat and moisture from the ocean affecting large scale circulation.
Research Objectives
Investigate the changes in TC activity (location, frequency, intensity and structure) with climate change.
Investigate a change of natural variability mechanisms on TC activity e.g. ENSO.
Investigate the impact of atmospheric resolution on TC activity with climate change.
Understanding ENSO and different types
HadGAM – N km
HiGEM - N144 1/3o ocean. 90km.
NUGAM - N km

3. GCM
HiGEM
UK’s new High-Resolution Global Environmental Model (Shaffrey et al, 2009)
HiGEM 4xCO2 30 yrs
HiGEM Transient 2% CO2/yr 70 yrs
1.25ox0.83o, ∆x50N = 90 km
HiGEM 2xCO2 30 yrs
HiGEM CTRL 150 yrs
1/3o ocean model
HiGEM CTRL 150 yrs

4. Tracking algorithm (TRACK; Bengstton et al, 2007)
Vorticity values… T63 ξ850 > 6x10-5 s-1, ξ850 – ξ200 > 6x10-5 s-1 , for at least 1 day (4 x 6hr).
warm core between p levels , hPa (+ ξ value)
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. Climate Change Simulations
Track density difference
2xCO2 - CTRL
NAtl basin is diffuse.
Also looked at RH_700A, omegaA, pptA, hadleyA
4xCO2 - CTRL
Stippling if outside 5x30yr CTRL variability
Storms/month/106km2

6. Sea Surface Temperature difference
Jul-Oct
2xCO2 - CTRL
4xCO2 - CTRL
Not so much the average T which controls TCs much more the distribution
Tang and Neelin (2004) troposphere warms. Inc. Stability. Warm free trop T dynamics. (weak horizontal T gradient)
Vecchi and Soden (2007) lower PI
Sea Surface Temperature difference (°C)
Tongue of relatively less warm water compared to the rest of the tropics (NAtl)
Decreases TC frequency in the NAtl.
Leads to increased vertical wind shear (VWS) via thermal wind balance

7. Vertical Wind Shear difference
Jul-Oct
2xCO2 - CTRL
4xCO2 - CTRL
Stippling if outside 5x30yr CTRL variability
Vertical Wind Shear difference (m/s)
VWS spreads to the NEPac
Detrimental affect on TCs

8. Walker Circulation anomaly
Jul-Oct 0-10N°
2xCO2 - CTRL
4xCO2 - CTRL
‘Walker circulation leads to a weakening of the Trades over most of the Pacific. However, the Trade Winds are primarily caused by the Hadley circulation, and are only modulated by the Walker circulation, so it is more precise to think of this result as indicating a change in strength of the Walker circulation.’
C.C. strong increase of BL water vapor content with temperature (7%/K)
Ppt increases slower than this at 2-3%/K
Latent heat release must be balanced by infrared radiative cooling of the troposphere.
This radiative constrain limits the ppt increase.
The radiative constrain increases with increasing CO2.
Less circulation is need to transport more moisture content. P=Mq
Inc dry static stability in the models as air follows moist adibats in the tropics and more warming occurs higher up.
Radiadive cooling balances adiabatic warming associated with subsidence Q=w*dtheta/dp.
Moist adiabat dtheta/dp~Lq
The radiative cooling does not increase as much as the stability and the subsidence weakens.
Weakening of circ by 5%/K
divU = d(VP)/dx ?
-ω (Pa/s) and divU anomaly (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

9. Conclusion Future work
TCs become less frequent in the CO2 increased experiments globally.
A tongue of relatively less warm water in the tropical North Atlantic increases VWS and decreases upward motion which reduces the frequency.
A weaker Walker circulation suppresses activity in the NW Pac and enhances activity in the NC Pac.
Future work
Understand ENSO variability, as well as the two types of El Niño.
Investigate how different atmospheric resolutions of the model simulates gradients and the global circulation, and their impact on changing TC activity with climate change.
TCs become more intense
2 types of el nino are...

11. Tracking algorithm IBTrACS (’79-’02) ERA-Interim (’79-’10)
HiGEM CTRL 150 years
Track density

12. Climate Change Simulations
NAtl basin in diffuse

13. Climate Change Simulations

15. Climate Change Simulations

16. Change in duration Largest decrease of short lived storms.
Warm core – Future may look at TCs which meet TC criteria (~ 850hPa wind speed > 20m/s)

17. Regional Change in duration
Largest decrease in the Atl
Incrase in EPAC at 2xCO2
NIND inc.
WPAC step wise change
Warm core – Future may look at TCs which meet TC criteria (~ 850hPa wind speed > 20m/s)

18. Change in structure 100 most intense TCs at most intense
Earth relative winds
Below 35oN NH
Avg 30 yr ctrl
4xCO2
2xCO2
Rad and Tan winds.
10o radius
850hPa

19. TRACK Hodges (1995); Bengstsson et 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 , hPa (+ ξ value)
Wind speed must attain 20m/s at 850hPa (change in slightly more intense TCs) [att20 dataset]
Statistical packages
Tracking storms differently leads to differences in results

20. Change in SST
Zhao et al 2009 – CMIP 3 A1B
Zhao et al
(2009)

21. Klotzbach and Gray (2011)
AMO ~= AMOC

22. Change in vws Vecchi et al (2007)
Comparing decades in the transient runs? Compare 50 strongest storms. Location – track densities, intensities – strongest storms, lifetime – may be linked to intensity, could do ACE, PDI?
Regress TC count in a area with the area average parameter e.g. Wind shear and SST.
Bengstton et al (2007) found need higher res to pick a change in activity in a warming world
Ocean coupling - AGCM with AOGCM runs and AGCM runs forced by AOGCM SSTs and AOGCM runs in which the coupling is performed over a range of frequencies
Look at ENSO indices from the transient run and correlate to TC numbers/activity in the Atlantic region.
Can use IBTraCS and Re-analysis to compare the model present day climate in representing the El Nino teleconnections
Vecchi et al (2007)

23. Change in RH700 Vecchi et al (2007)
Comparing decades in the transient runs? Compare 50 strongest storms. Location – track densities, intensities – strongest storms, lifetime – may be linked to intensity, could do ACE, PDI?
Regress TC count in a area with the area average parameter e.g. Wind shear and SST.
Bengstton et al (2007) found need higher res to pick a change in activity in a warming world
Ocean coupling - AGCM with AOGCM runs and AGCM runs forced by AOGCM SSTs and AOGCM runs in which the coupling is performed over a range of frequencies
Look at ENSO indices from the transient run and correlate to TC numbers/activity in the Atlantic region.
Can use IBTraCS and Re-analysis to compare the model present day climate in representing the El Nino teleconnections
Vecchi et al (2007)

24. Change in –ω500
Omega = - rho * g * w

25. Change in ppt
Comparing decades in the transient runs? Compare 50 strongest storms. Location – track densities, intensities – strongest storms, lifetime – may be linked to intensity, could do ACE, PDI?
Regress TC count in a area with the area average parameter e.g. Wind shear and SST.
Bengstton et al (2007) found need higher res to pick a change in activity in a warming world
Ocean coupling - AGCM with AOGCM runs and AGCM runs forced by AOGCM SSTs and AOGCM runs in which the coupling is performed over a range of frequencies
Look at ENSO indices from the transient run and correlate to TC numbers/activity in the Atlantic region.
Can use IBTraCS and Re-analysis to compare the model present day climate in representing the El Nino teleconnections

26. 2 types of ENSO in HiGEM Kug and Ham (2011)
Kug and Ham (2011) – Where NINO3 > NINO4
Kug and Ham (2011)
Investigate change in TC activity with the 2 types of El Nino.
Investigate the change in types of ENSO in HiGEM inc. CO2 and the TC relation

30. Large scale tropical change
Comparing decades in the transient runs? Compare 50 strongest storms. Location – track densities, intensities – strongest storms, lifetime – may be linked to intensity, could do ACE, PDI?
Regress TC count in a area with the area average parameter e.g. Wind shear and SST.
Bengstton et al (2007) found need higher res to pick a change in activity in a warming world
Ocean coupling - AGCM with AOGCM runs and AGCM runs forced by AOGCM SSTs and AOGCM runs in which the coupling is performed over a range of frequencies
Look at ENSO indices from the transient run and correlate to TC numbers/activity in the Atlantic region.
Can use IBTraCS and Re-analysis to compare the model present day climate in representing the El Nino teleconnections