Transcript Hurricane Intensity Changes over the Past 100 Years

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
Summary
Introduction
A tropical cyclone (TC) is defined as a nonfrontal 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.
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.
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
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
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
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
Atlantic Records
Substantial year to
year variability in
number of storms
No clear trend in the
number of storms
Landsea 1996
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
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
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
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 > 0.1°C
Positive anomaly in the Atlantic Multidecadal Mode
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
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
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
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
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
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
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
Model Simulations – Knutson and
Tuleya 2004
Model Simulations – Knutson and
Tuleya 2004
Model Simulations
Most global models indicate large mid- to uppertropospheric 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.”
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
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
References for Paper and
Presentation
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