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Title: Hurricane Intensity Changes over the Past 100 Years and Future Projections


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 26C 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 1800s
  • Late 1960s 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 TCs a year
    which counts for about 12 of the world total.
    Of those 9-10 TCs, a typical season will see 5-6
    hurricanes
  • Globally the average number of TCs 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 TCs 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 TSs,
    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
Trenberth 2005
Goldenberg et al. 2001
  • 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 0.1C
  • Positive anomaly in the Atlantic Multidecadal Mode

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 10S to 65N and fixed, two inner domains
    moved with storm
  • Upper tropospheric temperature anomalies ranging
    from 2.5C to -2.5C 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 1.5C can be offset by upper
    tropospheric warming of 3-4C 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 5C 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 2.2C for SSTs and 2.5C 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 0.8C and 2.4C

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 1-2C
    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 SSTs 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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