W4400: Survival Guide – Hurricanes
Prepared by Bali White and Emily Firth
December 2006
I
Background information
Tropical cyclones, also called hurricanes and typhoons, are intense low pressure disturbances that
form and migrate over the tropical ocean regions and are associated with intense winds and a very
strong convective activity, which brings thunderstorms and large amounts of rainfall. They have
the potential to cause major damage and loss of life when they make landfall.
These massive disturbances that grow in a time
frame of a week or so, need specific and
favorable conditions to occur, including high sea
surface temperatures (at least 26°C) and weak
vertical wind shears. Once they do, they spread
over a radius of a few hundred kilometers.
Hurricanes are surrounded by rings of towering
thunder clouds spiraling up to a small circle at the center of the storm, with a radius of 30-40 km.
Here the winds can reach a speed of 100 km/hour and more and the most intense rainfall occurs.
Inside this ring lies the eye of the storm, where the air is still and the convection is suppressed by
slow downward motion (subsidence). As illustrated in the diagram the clouds form a banded
structure and the eye at the center is approximately 30-40 km wide.
Recent hurricane events include:
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10 landfalling tropical cyclones in Japan in 2004
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Cyclone Gafilo in Madagascar, 2004
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Extremely active – record setting – 2004/2005 Atlantic tropical cyclone seasons including
Hurricane Katrina, 2005
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5 tropical cyclones affecting the Cook Islands in a 5-week period in 2005
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Cyclone Larry in Australia, 2006
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Typhoon Saomai in China, 2006
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Typhoon Durian in Philippines, 2006
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Cyclone Catarina in Brazil, March 2004 – the first and only (to date) recorded Category 1
hurricane in the South Atlantic.
II
Regional formation and migration of hurricanes
Hurricanes are active in the "trade wind" belts - the
regions just north or south of the equator where the winds
blow quite steadily from east to west (easterlies).
Here tropical disturbances generally form, initiated by
weak pressure perturbations that exist all the time in the
tropics. They move west with the trade winds in a steady,
relatively slow motion (10-20 km/hour). During this
phase they intensify mainly through the release of latent heat in the surrounding clouds and a
small percentage reach full hurricane intensity. The hurricanes tracks curve poleward and they
speed up north of ~30°N.
Hurricane development is predominantly in the following ocean basins:
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North Atlantic Ocean
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East Pacific Ocean
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West Pacific Ocean
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South West Pacific Ocean
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North Indian Ocean
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South Indian Ocean
Figure 1: Tracks of Tropical cylcones which formed worldwide
Category
Wind
speed
mph
(km/h)
Storm
surge
ft
(m)
5
=156
(=250)
>18 (>5.5)
4
131–155
(210– 249)
13– 18
(4.0–5.5)
3
111–130
(178– 209)
9–12
(2.7–3.7)
2
96– 110
(154– 177)
6–8
(1.8–2.4)
1
74– 95
(119– 153)
4–5
(1.2–1.5)
Tropical
storm
39– 73
(63–117)
0–3
(0–0.9)
Tropical
depression
0–38
(0–62)
0
(0)
from 1985 to 2005. The points show the locations of the storms at
six-hourly intervals and correspond to the color scheme shown to
the right from the Saffir-Simpson Hurricane Scale (source:
Wikipedia.org).
III
Global warming and future trends of hurricane activity
“Though there is evidence both for and against the existence of a detectable anthropogenic signal
in the tropical cyclone climate record to date, no firm conclusion can be made on this point”.
World Meteorological Organization (WMO) International Workshop on Tropical Cyclones,
Costa Rica, November 2006.
Despite extensive hurricane-related research conducted in the past few decades, a significant level
of uncertainty still exists in the link, if any, between global warming and future hurricane
projections. In recent times there has been increasing media attention and public interest in this
discussion, most likely due to the number of recent high-impact tropical cyclone events around
the globe (see Section I) which have left devastating socio-economic impacts in their wake but
also due to a general increase in public awareness of human-induced global warming.
There is now significant evidence that tropical sea surface temperatures (SSTs) over most tropical
basins have increased in magnitude by between 0.25 – 0.5 degrees C during the past several
decades (WMO, 2006) and more substantial future warming is anticipated due to greenhouse
forcing (IPCC, 2001).
Global Climate Models (GCMs) have been used to simulate hurricane activity under various
conditions and with specific parameters to explore the relationship between hurricane activity and
atmospheric and ocean physics.
Projections of future hurricane trends have been made by
comparing the results with observations and datasets of hurricanes over the past few decades, yet
the confidence placed in the results is limited by a number of inherent uncertainties.
The uncertainties and limitations that make future projections difficult include:
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Strong year-to-year and multi-decadal variability in SSTs and hurricane activity. It is not
clear whether observed SST variability is attributable to natural physical phenomena such
as the El-Nino Southern Oscillation (ENSO) and the Atlantic Multi-decadal Oscillation
(AMO) which both have significant impacts on SST and cyclone activity, or due to
anthropogenic activities and increases in greenhouse gas emissions, or resulting from a
combination of both (WMO, 2006).
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Significant changes in tropical cyclone wind-speed monitoring over the last few decades.
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Variations in the methods used to monitor and record tropical cyclone events, both
historically and regionally.
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Temporal limitations, with reliable data from satellite information available only for the
past 30 years.
Taking these points into consideration, the research and modeling that have been conducted to
date suggest:
1.
A likely increase in hurricane intensity with rising Tropical SSTs
Recently published papers report a significant positive correlation between tropical SST
and tropical cyclone intensity over the past 30 years. Using a satellite derived dataset,
Webster et al (2005) articulated this trend as a two-fold global increase in the number and
proportion of hurricanes reaching categories 4 and 5 since 1970; while Emanuel (2005)
reported a near doubling of Power Dissipation Index (PDI)1 in the North Atlantic and
North Pacific over the past 30 years.
It is expected that increases in intensity will be reflected in both maximum wind speeds
and storm-related precipitation (IPCC, 2001; Webster et al, 2005; Emanuel, 2005; Curry
et al, 2006).
2.
Regions of hurricane origin likely to remain unchanged
Based on model simulations and projected climate scenarios under increased greenhouse
gas concentrations, the regions of hurricane formation and impact are not expected to
change significantly (WMO, 2006).
3.
Uncertainty surrounding impacts of rising SSTs on hurricane frequency
In its Third Assessment Report, the IPCC stated that there is currently no published
evidence for significant long term increase in hurricane frequency. This is supported by
the conclusion reached by Webster et al (2005) that ‘careful analysis of global hurricane
data shows that, against a background of increasing SST, no global trend has yet emerged
in the number of tropical storms and hurricanes’.
With the current level of uncertainty surrounding future hurricane trends based on global
warming projections, the general consensus is that a more extensive global data record and a
stronger understanding of the role of hurricanes in atmospheric and ocean physics is required. As
articulated by Curry et al (2006) in their review of the politics and scientific debate surrounding
hurricane activity and global warming, ‘progress on this topic requires multidisciplinary
1
PDI is a measure of tropical cyclone intensity and is related to the time integral of wind velocity cubed.
collaboration that includes hurricane researchers and forecasters, climate researchers and
modelers, and oceanographers to address this complex scientific problem’.
IV
Useful websites and resources
HURDAT: Atlantic basin hurricane database www.aoml.noaa.gov/hrd/hurdat
Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report. Climate Change
2001: The Scientific Basis. http://www.grida.no/climate/ipcc_tar/wg1/index.htm
Wikipedia: The Free Encyclopedia www.wikipedia.org/wiki/Hurricanes
References:
Curry, J. A., P. J. Webster, and G. J. Holland, 2006. Mixing politics and science in testing the
hypothesis that greenhouse warming is causing a global increase in hurricane intensity, Bull.
Amer. Met. Soc., 87, 1025-1037.
Emanuel, KA, 2005. Increasing destructiveness of tropical cyclones over the past 30 years,
Nature, 433: 686-688.
IPCC, 2001. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the
Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y.
Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, and C. A. Johnson
(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
881pp.
Knutson, TR, RE Tuleya and Y Kurihara, 1998. Simulated increase of hurricane intensities in a
CO2-warmed climate, Science, 279 (5353): 1018-1020.
Webster, P.J., G. J. Holland, J. A. Curry, H.-R. Chang, 2005. Changes in tropical cyclone
number, duration and intensity in a warming environment, Science, 309: 1844-1846.