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Hurricane Intensity Changes over the Past 100 Years and Future Projections

Hurricane Intensity Changes over the Past 100 Years and Future Projections. Andrew Condon University of Miami Rosenstiel School of Marine and Atmospheric Science 11-16-2005. Outline . Introduction The Hurricane Record The Past 10 years Climate Model Simulations Remaining Questions

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Hurricane Intensity Changes over the Past 100 Years and Future Projections

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  1. Hurricane Intensity Changes over the Past 100 Years and Future Projections Andrew Condon University of Miami Rosenstiel School of Marine and Atmospheric Science 11-16-2005

  2. Outline • Introduction • The Hurricane Record • The Past 10 years • Climate Model Simulations • Remaining Questions • Summary

  3. Introduction • A tropical cyclone (TC) is defined as a non-frontal synoptic scale low pressure system originating over tropical or subtropical waters with organized convection and definite cyclonic surface wind circulation • Tropical storm: sustained winds of at least 17 ms-1 and less than 33 ms-1 • Hurricane: sustained winds of greater than 33 ms-1 • A TC with winds greater than 50 ms-1 is considered a major hurricane.

  4. Introduction • Hurricanes are warm core systems that derive their energy mainly from evaporation from the ocean and condensation in convective clouds concentrated near their center • 87% form between 20° N and 20° S with approximately two thirds of all tropical cyclones occurring in the Northern Hemisphere.

  5. Frequency and Intensity Factors The dynamic potential for cyclone development is governed by: (i) large values of low-level relative vorticity (ii) Coriolis parameter (at least a few degrees poleward of the equator) (iii) a weak vertical shear of the horizontal winds • The thermodynamic potential consists of: (iv) high sea surface temperatures (SST) exceeding 26°C and a deep thermocline (v) conditional instability through a deep atmospheric layer (vi) large values of relative humidity in the lower and middle troposphere

  6. The observational Record • Reliable observational records for the Atlantic and Northeast Pacific date back to 1944 with aircraft reconnaissance • Atlantic has somewhat reliable records from ship and land reports from the 1800’s • Late 1960’s the first Visible and Infra-Red satellite observations allowed global coverage, but limited quantity of images • Modern network of ships, buoys, satellites and aircraft offer complete coverage Neumann 1993

  7. The Observational Record • The average global range of tropical cyclones is 76 to 92 with a mean of 84 • The North Atlantic averages 9-10 TC’s a year which counts for about 12% of the world total. Of those 9-10 TC’s, a typical season will see 5-6 hurricanes • Globally the average number of TC’s that reach hurricane force is 45 with a range of plus and minus one standard deviation from 39 to 51

  8. Non-Atlantic Basin Records • Downward trend in the number of TC’s in the Australian region, although there has been very little change in the occurrence of intense storms • The northeast Pacific has experienced a notable upward trend • The north Indian features a distinct downward trend • No appreciable long-term variation in the southwest Indian and southwest Pacific • The Northwest Pacific Ocean is currently in an environment that is conducive to sustaining tropical systems. Since about 1980 the basin has experienced above normal tropical cyclone activity. However, the basin experienced a decrease of nearly identical magnitude in the 20 years preceding 1980

  9. Atlantic Records • Substantial year to year variability in number of storms • No clear trend in the number of storms Landsea 1996

  10. Atlantic Records Goldenberg et al. 2001 • Number of intense hurricanes is much more cyclic in nature • Above average 1940s-1960s, below average 1970s-1994 • Abrupt shift in hurricane record in 1995

  11. Shift in 1990s Atlantic Basin • 1991-1994 saw unprecedented low numbers of tropical cyclones (lowest number of TS’s, Hurricanes and Major Hurricanes of any 4 year period on record) • In 1995 there were 19 tropical storms of which 11 were hurricanes and five reached major hurricane status • The combination of the end of the El Nino event, warmer SSTs, lower sea level pressures, and extremely low vertical wind shear ushered in new period of activity

  12. Accumulated Cyclone Energy (ACE) • The sum of the squares of the estimated 6-hourly maximum sustained wind speeds for all named systems while they are at least tropical storm strength • During the 1995-2004 period the basin averaged 13.4 storms, with 7.8 hurricanes, 3.8 major hurricanes, and an ACE index value of 169% of the median • This contrasts sharply with an ACE value of 70% of the median during the 1970-1994 period Trenberth 2005

  13. Atlantic SST changes • Nonlinear upward trend in SSTs over the 20th century • Despite the multidecadal fluctuations that are evident, the last decade (1995-2004) features the highest decadal average on record by > 0.1°C • Positive anomaly in the Atlantic Multidecadal Mode Trenberth 2005 Goldenberg et al. 2001

  14. Other factors in Atlantic Shift • An amplified high pressure ridge in the upper troposphere across the central and eastern North Atlantic • Reduced vertical wind shear over the central North Atlantic • Low level easterly African winds that favor the development of hurricanes from tropical disturbances moving westward off the African coast. • Since 1988 the amount of total column water vapor over the global oceans has increased by 1.3% per decade (From SSM/I data) • This coupled with the higher SSTs creates more convective available potential energy (CAPE) and a more conducive environment for storm growth

  15. Model Simulations – Walsh and Ryan • For the Australian region • The standard deviations are quite large and the statistical significance is not that great • Under the 2 x CO2 conditions the average pressure drops by over 15 hPa and the corresponding wind speed increase is about 13% • Enhanced greenhouse conditions should bring slightly more intense storms to the Australian region

  16. Model Simulations – Shen, Tuleya, and Ginis • Used GFDL hurricane model to focus on atmospheric stability and SST changes on intensity of hurricanes due to global warming • 3 meshed models with the outermost domain ranging from 10°S to 65°N and fixed, two inner domains moved with storm • Upper tropospheric temperature anomalies ranging from 2.5°C to -2.5°C lead to hurricane minimum surface pressure changes by about 15 hPa and maximum surface wind speed changes of about 8 ms-1 • Any SST increase of 1.5°C can be offset by upper tropospheric warming of 3-4°C relative to the surface temperature due to the stabilizing effect of raising the upper tropospheric temperature

  17. Model Simulations – Knutson and Tuleya 1999 • In another experiment using the GFDL model Knutson and Tuleya looked at 51 northwest Pacific storm cases and some Atlantic scenarios as well as trends for all basins • The warming in the upper troposphere is greater than 5°C larger than near the surface • The high CO2 case is shifted toward higher intensities than the control by 5 ms-1 and the surface pressure is 6.6 hPa lower • The maximum intensity of these high CO2 cases has a positive trend of 6 ms-1 per decade • The mean of high CO2 storm precipitation is 28% higher than against the control

  18. Model Simulations – Knutson and Tuleya 1999 • A statistically significant tendency for more intense storms under high CO2 conditions for all basins except the South Indian • Overall their model simulates large scale changes of about 2.2°C for SSTs and 2.5°C in the lower troposphere with a warming of about twice as much in the upper troposphere • Surface wind speed increases of 3-7 ms-1 extending out about 2 to 3% larger in radius, about a 28% increase in near-storm precipitation, and a decrease of central pressure of 7 to 24 hPa is expected in the 2 x CO2 environments • This all correlates to a roughly 5-11% increase in the intensity of strong hurricanes. Knutson and Tuleya 1999

  19. Model Simulations – Knutson and Tuleya 2004 • Took parameterizations from 9 different change scenarios and used the results as input to the idealized hurricane model • For the study a control run was compared to an 80 year +1% per year CO2 scenario resulting in raising CO2 levels by a factor of 2.22 • All simulations run show a substantial tropical SST increase of between +0.8°C and +2.4°C

  20. Model Simulations – Knutson and Tuleya 2004 • The temperature change in the upper troposphere will exceed the change in the lower troposphere, leading to increased atmospheric stability • This agrees with most simulations

  21. Model Simulations – Knutson and Tuleya 2004

  22. Model Simulations – Knutson and Tuleya 2004

  23. Model Simulations • Most global models indicate large mid- to upper-tropospheric warming (3°-6°) in a double CO2 world over the tropical oceans • There is a much smaller increase of about 1-2°C warming of the sea surface temperatures in the tropical basins • The official view of the Intergovernmental Panel on Climate Change (IPCC) is that “There is evidence that the peak intensity may increase by 5% to 10% and precipitation rates may increase by 20% to 30%. There is a need for much more work in this area to provide more robust results.”

  24. Remaining Questions • Most climate models that are currently run have an extremely course resolution of about 100 to 500 km grid spacing • There are mesoscale models that are driven off the coupled ocean-atmosphere general circulation models. However these models are parameterized with the output from the coarser resolution climate models • Exactly how the wind shear in the hurricane formation region will change in a warmer world has not been resolved by the models • It is not yet possible to say how El Nino and other factors that affect hurricane formation may change as the world warms • We do not know how the upper-ocean thermal structure will change

  25. Summary • There is evidence from the climate record that changes in hurricane intensity tend to be cyclic in nature • Global climate model simulations point towards more intense storms in a warmer world • Higher SST’s and more energy available for storms to develop and intensify will be somewhat offset by stronger warming in the upper troposphere which changes the stability of the atmosphere • Currently there is no model which can accurately simulate tropical cyclones in an enhanced CO2 environment • The resolution of the models is just not fine enough to give truly accurate and reliable results at this time

  26. References for Paper and Presentation • Chan, J. C., and J. Shi., 1996: An exploratory study of the relationship between annual frequency of invaded typhoons in Taiwan and El Nino/Southern Oscillation. Terr., Atmos. Oceanic Sci., 7, 83-105. • Evans, J. L., 1990: Envisaged impacts of enhanced greenhouse warming on tropical cyclones in the Australian region. CSIRO Division of Atmospheric Research Tech. Paper, 20, 31pp • Goldenberg, S. B. et al. 2001: The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474-479. • Gray, W. M., 1975: Tropical cyclone genesis. Dept. of Atmos. Sci. Paper No. 234, Colorado State University, Fort Collins, CO, 121 pp. • Henderson-Seller, A., and Coauthors, 1998: Tropical cyclones and global climate change: A post IPCC assessment. Bull. Amer. Meteo. Soc.,79, 19-38. • Houghton, J.T., and et al., 2001: Climate change 2001: The scientific basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. pp 881. • Landsea, C. W., 1993: A climatology of intense (or major) Atlantic hurricanes. Mon. Wea. Rev.,121, 1703-1713. • --------, et al., 1996: Downward trends in the frequency of intense Atlantic hurricanes during the past five decades. Geo. Phys. Res. Lett., 23, No 13, 167-1700. • Neumann, C. J., 1993: Global overview. Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Rep TCP-31 World Meteorological Organization, Geneva, Switzerland, 220 pp. • ---------, G. W. Cry, F. L. Caso and B. R. Jarvinen, 1985: Tropical Cyclones of the North Atlantic Ocean, 1871-1980. NOAA Special Publication, 174 pp. • Nicholls, N., 1992: Recent performance of a method for forecasting Australian seasonal tropical cyclone activity. Aust. Meteor. Mag.,40, 105-110. • Shen, W., Tuleya, R. E., and I. Ginis, 2000: A sensitivity study of thermodynamic environment on GFDL model hurricane intensity: Implications for global warming. Journal of Climate, 13, 109-121. • Trenberth, K., 2005: Uncertainty in Hurricanes and Global Warming. Science, 308, 1753-1754. • Walsh, K. J., E., and B. F. Ryan. 2000: Tropical cyclone intensity increase near Australia as a result of climate change. Journal of Climate, 13, 3029-3036.

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