Climate Variability of Tropical Cyclones around the World
Dr. Christopher W. Landsea
Mr. Stan Goldenberg
Dr. Lixion Avila (NOAA/NWS/National Hurricane Center)
Dr. Gerry Bell (NOAA/Climate Prediction Center)
Dr. David Enfield (NOAA/AOML/Physical Oceanography Division)
Mr. Jon Gill (Bureau of Meteorology, Australia)
Prof. William M. Gray, Colorado State University
Mr. Todd Kimberlain (Colorado State University)
Dr. Neville Nicholls (Bureau of Meteorology Research Centre, Australia)
Dr. Roger Pielke, Jr. (National Center for Atmospheric Research)
Dr. Lloyd Shapiro (Colorado State University)
Mr. John Sheaffer (Colorado State University)
OBJECTIVE OF THE WORK
To analyze past tropical cyclone variability and its accompanying
climate and to quantify how changes have occurred on the timescales of
intra-seasonal, interannual, interdecadal and century ranges.
Tropical cyclones are the costliest and deadliest natural disasters
around the world, as the approximate 300,000 death toll in the infamous
Bangladesh Cyclone of 1970 and the $26.5 billion (U.S.) in damages due to
Hurricane Andrew can attest. In the United States over a recent ten year
period, hurricane property damages - exceeding that due to earthquakes by
a factor of four - account for 40% of all insured losses. Understanding
how tropical cyclone activity has varied in the past and will vary in the
future is a topic of great interest to meteorologists, public and private
decisionmakers and the general public alike. Even of more concern is the
possibility that anthropogenic climate change due to increases in
"greenhouse" gases may alter the frequency, intensity and area of
occurrence of tropical cyclones.
Current work has been focussing upon tropical cyclone variations on
four distinct timescales: intraseasonal,interannual, multidecadal
and "greenhouse gases" impact. To examine fluctuations on the
intraseasonal (month-to-month) scale, monthly and even weekly
large-scale fields are analyzed to better understand the
inter-relationships between early, peak, and late-season
activity. On the interannual variability, understanding is gained
on why individual years are very active with multiple, strong tropical
cyclones and other seasons can be quite quiet. This is accomplished in
part by analyzing anomalies of relevant large-scale fields such as sea
surface temperature, tropospheric vertical shear, ENSO phase, lower tropospheric vorticity, deep layer
mean flow, etc. Much of the work for interannual variability is enhanced by
utilizing the NCAR/NCEP reanalysis effort currently underway.
Multidecadal variation research has centered on describing low-period
modes of preferred global sea surface temperatures - especially with
regards to Atlantic changes - and relating these modes to Atlantic basin
hurricane activity through direct thermodynamic modulations of the
hurricane environment and by indirect changes of the atmospheric
circulation overlaying the ocean. Finally, investigations into possible
changes caused by anthropogenic "greenhouse gas" increases have been
carried out by reviews and critiques of numerical global model
simulations of possible future climates.
- One recent project examining causes for fluctuations in
intraseasonal variability has been completed. Two additonal
projects are ongoing. The completed project (in collaboration
with Gerry Bell and Bill Gray) was undertaken as part of the
examination of the 1995 hurricane season (see interannual
projects below). Though the season as a whole was extremely
active, with above normal activity during August and October,
the activity was actually below average during September, the
month that is climatologically the peak of the season. It was
found that this variability was due to near-normal vertical
shear in September. In addition, another key factor may have
been the enhanced activity in August producing a temporary
suppression of the above normal SSTs in the tropical and
subtropical Atlantic. Ongoing projects are looking at ways to
enchance intraseasonal predicition of Atlantic tropical cyclone
activity. Previous research has concluded that early season
activity (i.e. June-July) is unrelated to peak season activity
(i.e. August-September-October). The first projecct has
documented that, in some cases, proper geographical
stratification of early season activity is able to resolve a
predicitive element for peak season activity. In particular,
early season activity in the central and eastern portions of the
deep tropics (e.g. 1996) is almost always indicative of average
or above average activity for the remainder of the season, while
even a high amount of early activity farther north can be
followed, in some cases, by very low activity in the following
months (e.g. 1997).
The second ongoing project deals with activity at the end of the
peak three months. Results have ellucidated the existence of a
necessary (but not sufficient) condition, the presence of very
favorable climatological factors related in overall activity,
for October major hurricane activity to take place in the
- Four recent projects are being completed for the interannual
timescales. Two additional projects are just being initiated.
The first completed project (in collaboration with Gerry Bell
and Bill Gray) investigated the physical mechanisms for the
hyperactive 1995 Atlantic hurricane season. We found that
1995's extreme activity was due to a juxtaposition of very low
vertical wind shear, warm local sea surface temperatures, high
amounts of total precipitable water, low sea level pressures, a
west phase of the stratospheric quasi-biennial oscillation and a
cold phase of the ENSO.
The second (in collaboration with Neville Nicholls and Jon
Gill) analyzed the trends of Australian tropical cyclones and
the ENSO. We found a strong downward trend in the cyclones (a
substantial portion of which was likely artificial) that can be
primarily explained by decreases in the ENSO Index.
The third (in collaboration with Roger Pielke, Jr.)
normalized United States hurricane damages back several decades
by population changes, inflation and changes in wealth. The
resultingtime series provides realistic measurements of how
damaging previous hurricanes would be if they would strike today.
- from (Pielke and Landsea 1998).
Top 30 Damaging Hurricanes
Normalized to 1995 dollars by inflation,
personal property increases, and coastal county population changes.
|1.||SE Florida/Alabama|| 1926|| 4||$72,303,000,000
|2.||ANDREW (SE FL/LA)|| 1992|| 4||33,094,000,000
|3.||N Texas (Galveston)||1900|| 4||26,619,000,000
|4.||N Texas (Galveston)||1915|| 4||22,602,000,000
|5.||SW Florida|| 1944|| 3||16,864,000,000
|6.||New England||1938|| 3||16,629,000,000
|7.||SE Florida/Lake Okeechobee|| 1928|| 4||13,795,000,000
|8.||BETSY (SE FL/LA)|| 1965|| 3||12,434,000,000
|9.|| DONNA (FL/Eastern U.S.)||1960|| 4||12,048,000,000
|10.||CAMILLE (MS/LA/VA)|| 1969|| 5||10,965,000,000
|11.||AGNES (NW FL, NE U.S.)|| 1972|| 1||10,705,000,000
|12.||DIANE (NE U.S.)||1955|| 1||10,232,000,000
|13.||HUGO (SC)|| 1989|| 4|| 9,380,000,000
|14.||CAROL (NE U.S.)||1954|| 3|| 9,066,000,000
|15.||SE Florida/Louisiana/Alabama|| 1947|| 4|| 8,308,000,000
|16.||CARLA (N & Central TX)|| 1961|| 4|| 7,069,000,000
|17.||HAZEL (SC/NC)|| 1954|| 4|| 7,039,000,000
|18.||NE U.S.||1944|| 3|| 6,536,000,000
|19.||SE Florida|| 1945|| 3|| 6,313,000,000
|20.||FREDERIC (AL/MS)|| 1979|| 3|| 6,293,000,000
|21.||SE Florida|| 1949|| 3|| 5,838,000,000
|22.||S Texas||1919|| 4|| 5,368,000,000
|23.||ALICIA (N TX)|| 1983|| 3|| 4,056,000,000
|24.||CELIA (S TX)|| 1970|| 3|| 3,338,000,000
|25.||DORA (NE FL)|| 1964|| 2|| 3,108,000,000
|26.||OPAL (NW FL/AL)||1995|| 3|| 3,000,000,000
|27.||CLEO (SE FL)|| 1964|| 2|| 2,435,000,000
|28.||JUAN (LA)||1985|| 1|| 2,399,000,000
|29.||AUDREY (LA/N TX)|| 1957|| 4|| 2,396,000,000
|30.||KING (SE FL)|| 1950|| 3|| 2,266,000,000
The fourth project (in collaboration with Lloyd Shapiro)
attempted to isolate the physical mechanism reponsible for the
long accepted relationship between interannual fluctuations in
Atlantic SSTs and fluctuations in Atlantic basin tropical
cyclone activity. The method of singular value decomposition
(SVD) was used to demonstrate that although large-scale SSTs are
of secondary importance to vertical shear in modulating
hurricane formation, explaining ~10% of interannual
variablity in hurricane frequency over the ~50% explained by
vertical shear, the warmest SSTs directly enhace development.
- A new line of investigation (in collaboration with Dave
Enfield) will analyze the contributions of tropical cyclones
toward total rainfall and extreme rain events from a
climatological and interannual perspective.
A second new research topic (in collaboration with Todd Kimberlain) is
to thoroughly document the effect of ENSO on tropical cyclones
throughout the global tropics.
- On the multidecadal timescales, two projects have been
completed and one is work in progress. The first (in
collaboration with Neville Nicholls, Bill Gray and Lixion Avila)
involves analyzing trends of Atlantic hurricane activity. We
show that - instead of the popular notion that intense hurricane
activity has been on the rise - Atlantic hurricanes actually
decreased during the 1970s through the early 1990s.
The second (in collaboration with Bill Gray and John Sheaffer)
provides a hypothesis - oceanic conveyor belt variability - to
explain the observed multidecal changes in Atlantic sea surface
temperature, Sahel rainfall and Atlantic hurricanes. Lastly (in
collaboration with Lloyd Shapiro and Bill Gray), we are
attempting to determine whether we have re-entered an era of
increased Atlantic intense hurricane activity. The implications
of which - if it does turn out to be the case - are staggering
for coastal communities in both the United States and the Caribbean.
- Most of the years 1970-94, the North Atlantic hurricane basin
had experienced a relative lull in overall activity. These
years were floowed by the "hyper-active" years 1995 and 1996,
each containing more overall activity than any year since the
1960s. The chief issue being addressed in the current study is
whether or not the activity of these two years were from
short-term variablity or possibly the result of multi-decadal
climate shifts. Preliminary results from analyses of Atlantic
SSTs, vertical shear, and tropical cyclone activity itself, do
seem to indicate the presence of the long-term shift toward
- The question of man-made changes of our climate is also a
topic that has been investigated as well in two projects nearing
completion. The first of which is a critique of a global
modeling run that suggested a doubled carbon world would have
fewer tropical cyclones than in pre-industrialized times.
However, my analysis indicates that this result is fundamentally
flawed because of an incompatibility in the downscaling
technique utilized. The second project was in participation in
a World Meteorological Organization commissioned panel to evaluate
current evidence for how tropical cyclones will change in the future.
Our assessment is that we have no conclusive evidence for changes
(either upward or downward) in frequency or mean intensity,
though the maximum potential intensity may rise by 10%. We also
conclude that the genesis regions of tropical cyclones will not
change substantially in a doubled carbon dioxide world.
- Pielke, Jr., R. A., and Landsea, C. W., 1998: Normalized
U. S. hurricane damage, 1925-1995. To be submitted to Wea.
- Landsea, C. W., 1998: Climate variability of tropical cyclones:
Past, Present and Future. Submitted as a chapter for the book Storms
edited by R. A. Pielke, Sr. and R. A. Pielke, Jr.
- Shapiro, L.J. and S.B. Goldenberg, 1998; Atlantic sea
surface temperatures and tropical cyclone formation.
J.Climate, 11, in press.
- Henderson-Sellers, A., H. Zhang, G. Berz, K. Emanuel, W. Gray,
C. Landsea, G. Holland, J. Lighthill, S-L. Shieh, P. Webster,
K. McGuffie, 1998: Tropical cyclones and global climate change: A
post-IPCC assessment. Submitted to Bull. Amer. Meteor. Soc..
- Nicholls, N., C. W. Landsea, and J. Gill, 1998: Australian tropical
cyclones: Trends and seasonal predictability. Accepted to Met. Atmos.
- Landsea, C. W., W. M. Gray, G. A. Bell, and S. B. Goldenberg,
1998: The extremely active 1995 Atlantic hurricane season:
Environmental conditions and verification of seasonal forecasts.
In press in Mon. Wea. Rev..
- Landsea, C. W., 1997: Comments on "Will greenhouse gas-induced
warming over the next 50 years lead to higher frequency and greater
intensity of hurricanes?". In press in Tellus.
- Landsea, C. W., N. Nicholls, W. M. Gray, and L. A. Avila, 1997:
Reply to Comments on "Downward trends in the frequency of intense Atlantic
hurricanes during the past five decades". In press in Geo. Res.
- Gray, W. M., J. D. Sheaffer, and C. W. Landsea, 1997: Climate
trends associated with multidecadal variability of Atlantic hurricane
activity. Hurricanes: Climate and Socioeconomic Impacts.
H.F. Diaz and R.S. Pulwarty, Eds., Springer-Verlag, New York, 15-53.
- Landsea, C. W., N. Nicholls, W. M. Gray, and L. A. Avila, 1996:
Downward trends in the frequency of intense Atlantic hurricanes
during the past five decades. Geo. Res. Letters, 23, 1697-1700.
- Landsea, C. W., 1993: A climatology of intense (or major) Atlantic
hurricanes. Mon. Wea. Rev., 121, 1703-1713.
- Landsea, C.W., W.M. Gray, P.W. Mielke, Jr., and K.J. Berry, 1992:
Long-term variations of Western Sahelian monsoon rainfall and intense U.S.
landfalling hurricanes. J. Climate, 5, 1528-1534.
- Landsea, C. W. and W. M. Gray, 1992: The strong association between
western Sahelian monsoon rainfall and intense Atlantic hurricanes.
J. Climate, 121, 1703-1713.