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

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 – N96. 135km HiGEM - N144 1/3o ocean. 90km. NUGAM - N216. 60km

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

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 850-500, 500-200hPa (+ 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

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

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

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

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

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...

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

Climate Change Simulations NAtl basin in diffuse

Climate Change Simulations

Climate Change Simulations

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)

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)

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

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 850-500, 500-200hPa (+ 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

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

Klotzbach and Gray (2011) AMO ~= AMOC

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)

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)

Change in –ω500 Omega = - rho * g * w

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

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

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