Title: Hurricane Intensity Changes over the Past 100 Years and Future Projections
1Hurricane 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
2Outline
- Introduction
- The Hurricane Record
- The Past 10 years
- Climate Model Simulations
- Remaining Questions
- Summary
3Introduction
- 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.
4Introduction
- 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.
5Frequency 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
6The 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
7The 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
8Non-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
9Atlantic Records
- Substantial year to year variability in number of
storms - No clear trend in the number of storms
Landsea 1996
10Atlantic 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
11Shift 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
12Accumulated 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
13Atlantic 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
14Other 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
15Model 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
16Model 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
17Model 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
18Model 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
19Model 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
20Model 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
21Model Simulations Knutson and Tuleya 2004
22Model Simulations Knutson and Tuleya 2004
23Model 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.
24Remaining 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
25Summary
- 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
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