Multidecadal Variability of Tropical Cyclones Affecting Florida

Jenni L. Evans The Pennsylvania State University

A report to the

Florida Commission on Hurricane Loss Projection Methodology

May 20061

1 Figure is tracks of the 2004 tropical cyclones in the vicinity of the southeast U.S.

1

Multidecadal Variability of Tropical Cyclones Jenni L. Evans

Department of Meteorology The Pennsylvania State University

jle7@psu.edu

Executive Summary

The recent upturn in Atlantic hurricane activity raises the question as to whether

there: is a trend in hurricane activity or whether this variation is attributable to natural

variability. The answer provides the framework for planning in the hurricane-affected

community and is helpful in the Commission’s planning process.

Recent papers have examined long-term variations in the number of tropical

cyclones around the globe, including possible changes in frequencies across storm

categories. These findings have also raised questions about the quality of the historical

datasets. A comprehensive evaluation of the overall quality of the HURDAT and other

tropical cyclone databases is necessary before definitive statements about long-term

variations can appropriately be made. Such reanalyses have begun for HURDAT, but

are in their early stages. For example, while almost all tropical cyclones in the satellite

era were identified, changes in satellite technology through that 30 year period raise

questions about the intensity records even in “modern” times. Satellite reconnaissance

of hurricanes tend to successfully eliminate sea surface measurements from vessels

caught in an event. The record is less problematic where observations are available at

landfall, but often the instrumentation doesn’t survive intense hurricane passage, so

even in populated areas the “observations” may not be complete. Finally, recent insights

into the vertical structure of tropical cyclones have led to the upgrading of Hurricane

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Andrew (1992; Landsea et al. 2004). Other historical hurricane records may also be

adjusted based on this reasoning. The HURDAT reanalysis is ongoing.

In spite of these concerns regarding the historical tropical cyclone archives,

decisions must be made about how to model hurricanes and their impacts.

1) What can be said with confidence from the current HURDAT?

2) Is this global warming or a multidecadal cycle?

3) If it is a cycle, are we on the increasing or decreasing part?

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Table of Contents

1. Introduction 5

2. Background on Available Data Sources 6

3. Issues with Determining Tropical Cyclone Variability from the Record 8

4. Multidecadal Variability or Climate Change? ?

5. Summary and Implications for the Commission ?

References ?

4

1. Introduction

The 2004 and 2005 Atlantic hurricane seasons had much higher storm activity

than most of the previous thirty years. Moreover, 1933 was the last time three strong

hurricanes (category ≥ 3) made US landfall in the same year. This raises the question

“Is there a trend in hurricane activity or is this variation in activity merely natural

variability?” For planning purposes it is important to understand whether this is a long-

term shift in activity or a temporary – and brief – anomaly. To understand this hurricane

variation, we consider the following five questions:

(1) Is there any obvious trend or cycle in global tropical cyclone activity over the

last 100 or so years?

(2) Is there any trend or cycle in Atlantic hurricane activity over the same period?

(3) Based on the quality of the storm database, how much credence can we place

on these signals in storm frequency?

(4) What is the history of landfall frequency in the US and specifically in Florida

and how reliable is this database?

(5) Based on the past, how credible are the current theories on global warming

influences on hurricanes versus explanations for multidecadal variability?

To address these questions, we must first look at history. Thus, in Section 2 we

review the available storm datasets and their compilation.

In this report we consider long-term records from around the globe. The term

“tropical cyclones” refers to both tropical storms and hurricanes in the Atlantic. Tropical

cyclones of hurricane strength are called typhoons in the western Pacific and severe

tropical cyclones in the other tropical oceans. Long-term variations in tropical cyclone

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numbers around the globe and possible changes in frequencies across storm

categories are discussed in Section 3. Results from recent studies have raised

questions about the datasets described in Section 2. These concerns will also be

reviewed in Section 3, leading to an evaluation of the overall quality of the database.

Having established the patterns of storm activity in the available Atlantic

database and compared them with other basins, it is helpful to consider the various

methods (mainly statistical analyses and theory) used to extrapolate history to infer

future storm activity. These studies are reviewed in Section 4 and overall conclusions

for the Commission are summarized in Section 5.

2. Background on Available Data Sources

To address the questions central to this report, we must first look to history, so

the available storm datasets and the data and methods used in their compilation are

briefly reviewed here. Since we will consider long-term records from around the globe,

we will use the generic term “tropical cyclones” to refer to both tropical storms and

hurricanes in the Atlantic. Tropical cyclones of hurricane strength are called typhoons in

the western Pacific and severe tropical cyclones in the other tropical oceans (see

definitions).

The National Hurricane Center/Tropical Prediction Center (NHC/TPC) of the

National Weather Service has U.S. responsibility for issuing watches and warnings for all

tropical cyclones in the Western Hemisphere. Post-season analysis of each storm

provides an opportunity to incorporate information about the event that either (1) was not

available to the forecasters in realtime, (2) was generated using non-operational products,

or (3) related to societal impacts of the event. Based on these analyses, summaries of

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each storm are written and posted on the NHC website. These summaries provide the

data incorporated into NHC’s annual operational updates to HURDAT, and into the

archive of tropical cyclones in the North Atlantic Ocean. All of the NHC storm summaries,

as well as archives of track and intensity data (HURDAT) and (for recent years) the

complete set of operational advisories for each storm are available online.

Figure 1: Tropical cyclone basins and relative activity (courtesy UNEP). While the tropical cyclone experts in these offices make every endeavor to

ensure the quality of the databases they compile, they are not the only sources for

these tropical cyclone records. Every country in the affected region (Fig. 1) has their

own national forecast office and, while these offices coordinate their information during

and after each event, inconsistencies and even professional disagreements can arise

between the various keepers of the archival data. This can be especially important for

historical storm records, when the convention of distributing meteorological data across

national boundaries had not yet been established. Much of this data has been shared

between countries in the recent hurricane reanalysis project, although some data may

have been lost in the passage of time. This informal process may not be the ideal way

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to compile a historical record, however it is the way that has evolved through the

eventual cooperation of nations that recognize that meteorology and natural disasters

do not heed political boundaries.

The same broad approach to tropical cyclone reporting and archiving exists

across the rest of the globe, although the quality of the database is somewhat

dependent on the ability of the governments to sustain such expenses in the countries

impacted. Thus, the important historical record of tropical cyclone activity and intensity

has evolved haphazardly, with uneven data archiving and quality control.

3. Issues with Determining Tropical Cyclone Variability from the Record

In this section, long-term variations in tropical cyclone numbers around the globe

and possible changes in frequencies across storm categories are reviewed.

Annual frequencies for each tropical cyclone basin are plotted in Figure 2. Clearly

there is much year-to-year variation in all basins, with some suggestion of longer term

variations in the North Atlantic, western North Pacific and North Indian Ocean basins.

Note that the length of record differs for each basin.

Figure 2: Annual tropical cyclone activity in each tropical ocean basin from 1900-1995. Data are from NHC and JTWC archives. Basin names are given in Fig. 1, with the exception of the Australian region which covers the South Pacific and the eastern South Indian Oceans.

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1887 19/11 (4) 1936 16/7 (3) 1995 19/11 (2) 2005 28/15 (6)

1933 21/10 (5) 1969 18/12 (2) 2003 16/9 (2)

Table 1: Years (bold) with over 15 tropical storms and their storm/hurricane (landfall) count.

Figure 3: Annual Atlantic tropical cyclone frequency histogram for the period 1850–2004: number of named systems (yellow), hurricanes (hatched green), and intense hurricanes (Cat 3–5; red). Lines marking ten (dashed) and 15 (dotted) cyclones per year are marked. This is an annotated version of an image provided by NHC.

A more detailed perspective on the historical variation of tropical cyclone activity in the

North Atlantic is presented in Fig. 3. The occurrence of more than 15 tropical cyclones in

a year is a rare event, occurring only seven times between 1850 and 2005. Five of these

years with over 15 tropical storms also had at least ten hurricanes (Table 1); however the

link between annual frequency (the number of storms of all categories) and the number of

hurricanes is not straightforward. Periods of relatively more major hurricanes (four per

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season at least every five years) are marked with ovals on Fig. 3. These three periods are

very different, with relatively low recorded activity in the first period (1915-1934),

“average” activity (typically ten named storms per season) in the second period (1950-

1969) and elevated storm frequencies in the third period (1995-2004).

The link between total storm numbers and distribution of U.S. landfalls is also

complex. The landfall locations of all hurricanes hitting the mainland U.S. from 1950-2004

are plotted on Fig. 4. During the lull in total storm activity in 1970-1994, the landfall of

Hurricane Andrew (1992) alone was devastating. Comparison of the decade 1941-1950

and the most recent four years available (2001-2004) highlights the difficulty in relating

annual storm numbers and U.S. landfalls, especially those relating to major hurricanes,

the source of 50% of total hurricane related damages in the U.S. (Pielke and Landsea

1998). While 1941-1950 was a relatively quiet decade (Fig. 3, long overbar), ten major

hurricane U.S. landfalls occurred in this decade, with two years (1944 and 1950) having

multiple landfalls (Fig. 5a). Indeed, both of the 1950 landfalls (Easy and King) were

category 3 storms in Florida (Fig. 4). The most recent four years available (2001-2004;

Fig. 3, short overbar) continue the pattern of higher than average annual storm numbers

begun in around 1995, yet the frequency of major hurricane U.S. landfalls is comparable

to the 1940s at ~1 per year (Fig. 5b and most recent HURDAT).

In 2005, the pattern of higher than average annual storm numbers that had begun

in around 1995 continued, with 28 named storms and seven mainland U.S. landfalls. Four

of these (Dennis, Katrina, Rita and Wilma) were estimated Cat 3 at landfall. Florida was

impacted by four hurricanes (Cat 3 Dennis, Cat 1 Katrina, by-passing Cat 1 Rita and

Cat 3 Wilma), two of these major hurricanes (Dennis and Wilma) in 2005.

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Figure 4: US mainland hurricane landfall locations from 1950-2004. Image courtesy of the NOAA National Climatic Data Center (NCDC).

Figure 5: Landfalling U.S. major hurricanes (Cat 3–5) for (left) the ten year period 1941-1950; and (right) the four year period 2001-2004 (courtesy NHC).

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Figure 6: Tropical cyclone tracks for the 2005 Atlantic hurricane season (courtesy NHC).

Analysis of the total time series of Atlantic tropical cyclone activity from 1900 to

2005 (Fig. 7) provides some insights into the source of the long time variations in total

Atlantic tropical cyclone numbers. A linear trend line (green) highlights the consistent

increase in storms through the period of record. The 21-year running mean provides

evidence of a smoothed oscillation about this trend line. The relatively low storm

frequency (seven storms per year on average) prior to 1940 is likely at least partially

explained by the incomplete observing of storms at sea. The advent of the satellite era

in 1966 (when the first “geostationary” satellite that could send constant pictures of the

same location was launched) and the intermittent hurricane hunter flights until the 1970s

left much room for missed storm events. However, the active seasons still observed in

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this early part of the record and the relatively well observed lull in activity in the 1970s

and 1980s are suggestive of variations in hurricane activity over decades or longer.

However, given the constant increase in the globally averaged temperature over this

time, along with rising concentrations of carbon dioxide (CO2) and other “greenhouse”

gases, some researchers have proposed that the long time (term?) variations,

especially since the 1970s are due to man-made impacts on the climate. The literature

and recent conference presentations exploring these ideas are reviewed in the next

section.

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1905

1910

1915

1920

1925

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

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Figure 7: Annual Atlantic tropical cyclone numbers from 1900-2005 (red). A linear trend line (green) and 21-year running average (blue) are overlaid. The launch of the first U.S. weather satellite in 1966 is marked with an arrow. Periods of aircraft observations are marked with overbars. The dashed overbar represents a period of intermittent observing. More details on aircraft observations of hurricanes are given in Appendix A.

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4. Multidecadal Variability or Climate Change?

Having established the patterns of storm activity in the available Atlantic database

and compared it with other basins, it is helpful to consider the various methods (mainly

statistical analyses and theory) used to extrapolate history to future storm activity.

Conclusions drawn from these studies will likely inform models of hurricane risk.

The long-term variations in Atlantic tropical cyclone numbers reviewed in the

previous section inspired recent papers by Webster et al. (2005) on global tropical

cyclone frequency and intensity changes, Chan (2006) on long-term variations of

tropical cyclone activity in the North Pacific, and Emanuel (2005) on possible increases

in Atlantic hurricane damage potential. These studies have raised concerns about

increasing frequency and intensity of tropical cyclones around the world. They have also

raised questions about the quality of the historical tropical cyclone archives – as

discussed by Landsea (2006). These four authors formed a panel on climate effects on

tropical cyclones at the 27th AMS Conference on Hurricanes and Tropical Meteorology

in Monterey in April 2006, in which their published conclusions were re-iterated.

Webster et al. (2005) explore trends in the number of intense tropical cyclones (Cat

3–5 hurricanes or typhoons) globally. Although there was no trend in the annual number of

tropical cyclones globally since 1970, they concluded that there was an upward trend in the

most intense tropical cyclones. This result is very controversial since (1) it focuses on the

same time period for which other authors (e.g. Landsea et al 1998; Chan 2006) have

argued correspond to long time variations in tropical cyclone activity and have shown

mechanisms for these variations related to changes in the base climate; and (2) the

underlying data used in the paper have been questioned as flawed.

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In a recent computer modeling study Knutson and Tuleya (2004) attempted to

separate out the effects of societally driven climate change (which they refer to as

“CO2-induced”):

“An important issue is whether and when any CO2-induced increase of tropical cyclone intensity is likely to be detectable in the observations. The magnitude of the simulated increase in our experiments is about +6% for maximum tropical cyclone surface winds … The SST changes observed for the past 50 yr in the Tropics imply that the likely SST-inferred intensity change for the past half century is small, relative to both the limited accuracy of historical records of storm intensity and to the apparently large magnitude of interannual variability of storm intensities in some basins. This further implies that CO2-induced tropical cyclone intensity changes are unlikely to be detectable in historical observations and will probably not be detectable for decades to come.”

Prior to his most recent paper, Emanuel (2004) concurred with Knutson and

Tuleya (2004) in maintaining that expected changes in tropical cyclone intensity (based

on theory) should not be expected to be detected in the observational record:

“Can one detect an actual increase in global tropical cyclone Intensity? … Since 1950 … one would expect to have observed an average increase in intensity of around 0.5 ms-1 or 1 knot. Because tropical cyclone maximum wind speeds are only reported at 5 knot intervals and are not believed to be accurate to better than 5 to 10 knots, and given the large interannual variability of tropical cyclone activity, such an increase would not be detectable. Thus any increase in hurricane intensity that may have already occurred as a result of global warming is inconsequential compared to natural variability.”

In his latest paper, Emanuel (2005a, b) argues that another measure of intensity (PD, or

power dissipation2) does display evidence of increasing intensity over time since 1970.

He also notes a correlation between ocean temperatures and changes in PD over this

time. However, this is the time period that Landsea et al. (1998) foreshadowed as a time

of increased major hurricane frequency based analysis of the historical records by

Landsea (1993).

2The storm kinetic energy times wind speed per unit mass, ∫≈τ

oMAX dtVPD 3

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Potential weaknesses in the current historical records must also be considered

when examining trends (Landsea 2005, 2006; Pielke 2006). While the data are

adequate to provide a reasonable record of US landfall frequency and location in the

20th century, it is possible that many storms that remained at sea throughout their

lifecycle went undetected in the pre-satellite era (i.e. before about 1975). Thus, since

only a small fraction of storms in a season make landfall, extrapolations of trends in

tropical cyclone frequency and intensity in any ocean basin should be treated with

caution. The present HURDAT is suggestive of a very active period in the early-mid 20th

century, followed by a period of relatively few landfalls and quieter seasons – and then a

return to increased activity recently. Such variations have been tied to fluctuations in the

African monsoon, the source of many of the disturbances that ultimately become

hurricanes in the Atlantic (Landsea 1993). This link to the African monsoon is supported

in both African rainfall records and analyses of surface pressure and winds

(observations and computer models). Finally, the theory relating intensity and climate

change applies only to a theoretical maximum possible intensity of a tropical cyclone

(e.g. Holland 1997; Tonkin et al. 2000), not actual, observed intensity. Evans (1993)

showed that the observed intensities of the vast majority of tropical cyclones do not

achieve this theoretical maximum threshold. Consistent with this, DeMaria and Kaplan

(1994) use this threshold as an upper bound on their forecast storm intensity.

Even for the period in which we have acceptable landfall records for the US

(arguably since 1900), there is some question relating to the intensity records:

communities that reported being “hit” by a hurricane may not have experienced its peak

winds – or their observations may have been either based on personal estimates of

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wind speeds or on instruments that did not survive the storm. All of these caveats on the

historical records lead to the conclusion that an evaluation of the overall quality of the

database is warranted. This is the purpose of the HURDAT reanalysis project being

coordinated by Dr. Chris Landsea at TPC/NHC

(http://www.aoml.noaa.gov/hrd/data sub/re anal.html).

While concerns about some aspects of HURDAT remain, some investigators

argue that it is unwise to attribute any perceived trends to climate change (e.g. Pielke et

al. 2005).

5. Summary and Implications for the Commission

The increases in coastal population and wealth over the last century have made

US society more susceptible to tropical cyclones and the damages continue to increase

as these trends continue upward (Pielke and Landsea 1998).

While some recent publications (Webster et al. 2005; Emanuel 2005a,b) have

noted changes in tropical cyclone activity since 1970 and have attributed these changes

to global warming (see also Hoyos et al. 2005), there are also compelling physical

arguments that relate these same changes to long-term fluctuations in the global

climate system (e.g. Landsea 1993; Chan 2006). There is general agreement that the

overall number of tropical cyclones is not increasing (Frank and Young 2006). Thus, any

discussions of climate influence on tropical cyclones should presently focus on natural

variability.

A focus on natural variability brings one to the question of the reliability of the

historical database known as HURDAT. Of all ocean basins affected by tropical

cyclones, the North Atlantic record is likely to be the most complete since (1) the basin

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is small and has been a major trading route for almost 400 years giving a reasonable

likelihood (although no guarantee) of detecting storms at sea; (2) it was the first basin to

have reliable satellite coverage; and (3) the Atlantic has had the longest history of

aircraft observations. Even with these positive attributes, it is likely that a number of

storms at sea were missed in the pre-satellite era and that even some US landfalls went

unrecorded in the 19th century. In spite of these caveats, the HURDAT database is

complete enough to show long-term swings in tropical cyclone frequency that are

consistent with other, related, changes in the tropical climate. Thus, HURDAT has value

as a reference for calibrating statistical models, but these models should not be

expected to match HURDAT, as reanalyses of historical storms presently underway will

likely cause modifications to this database.

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References

Chan, J. C. L., 2006: Comment on “Changes in tropical cyclone number, duration, and

intensity in a warming environment.” Science, 311, 1713.

DeMaria, M., and J. Kaplan 1994: Sea surface temperature and the maximum intensity

of Atlantic tropical cyclones. J. Climate, 7, 1324-1334.

Emanuel, K. A., 2005a: Increasing destructiveness of tropical cyclones over the past 30

years. Nature, 436, 686-688.

Emanuel, K. A., 2005b: Emanuel replies. Nature, 438, E13.

Evans, J. L., 1993: Sensitivity of tropical cyclone intensity to sea surface temperature. J.

Climate, 6, 1133-1140.

Frank, W. M., and G. S. Young, 2006: The 80 cyclones myth. 27 AMS Conference on

Hurricanes and Tropical Meteorology, 24–28 April 2006, Monterey, CA

th

.

Holland, G. J., 1997: The maximum potential intensity of tropical cyclones. J. Atmos.

Sci., 54, 2519-2541.

Hoyos, C. D., P. A. Agudelo, P. J. Webster, and J. A. Curry, 2006: Deconvolution of the

Factors Contributing to the Increase in Global Hurricane Intensity. Sciencexpress

/ www.sciencexpress.org / 16 March 2006 / Page 1 / 10.1126/science.1123560.

Hoyos, C. D., P. A. Agudelo, P. J. Webster, and J. A. Curry, 2006: Supporting online

material for: Deconvolution of the Factors Contributing to the Increase in Global

Hurricane Intensity. www.sciencemag.org/cgi/content/full/1123560/DC1.

Landsea, C. W., 1993: A climatology of intense (or major) Atlantic hurricanes. Mon.

Wea. Rev., 121, 1703-1713.

Landsea, C. W., 2005: Hurricanes and global warming. Nature, 438, E11-E12.

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Landsea, C. W., 2006: Hurricanes and global warming: Expectations versus observations.

18th AMS Conference on Climate Variability and Change, 30 January-2 February

2006, Atlanta, GA.

Landsea, C. W., G. D. Bell, W. M. Gray, and S. B. Goldenberg 1998: The extremely

active 1995 Atlantic hurricane season: Environmental conditions and verification

of seasonal forecasts. Mon. Wea. Rev., 126, 1174-1193.

Landsea, C. W., J. L. Franklin, C. J. McAdie, J. L. Beven II, J. M. Gross, B. R. Jarvinen,

R. J. Pasch, E. N. Rappaport, J. P. Dunion and P. P. Dodge. 2004: A reanalysis

of Hurricane Andrew's intensity. Bulletin of the American Meteorological Society,

85, 1699–1712.

Pielke, R. A., Jr., 2005: Are there trends in hurricane destruction? Nature, 438, E11.

Pielke, R. A., Jr., and C. W. Landsea 1998: Normalized hurricane damages in the

United States: 1925-95. Wea. Forc., 13, 621-631.

Pielke, Jr., R. A., C. W. Landsea, M. Mayfield, J. Laver, and R. J. Pasch, 2005:

Hurricanes and global warming. Bull. Amer. Meteor. Soc., 86, 1571–1575.

Tonkin, H., G. J. Holland, N. Holbrook, and A. Henderson-Sellers, 2000: An evaluation

of thermodynamic estimates of climatological maximum potential tropical cyclone

intensity. Mon. Wea. Rev., 128, 746-762.

Webster, P. J., G. J. Holland, J. A. Curry, and H.-R. Chang, 2005: Changes in tropical

cyclone number, duration, and intensity in a warming environment. Science, 309,

1844-1846.

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Appendix A: A brief history of U.S. hurricane flights

53rd Air WX Squadron • July 1943 Lt. Col Joe Duckworth and navigator Ralph O'Hair intentionally fly an AT-6 aircraft into a hurricane in the Gulf of Mexico • 7 August 1944

3rd WX Reconnaissance Squadron created: precursor to the 53rd • 1946

Ex-bomber flight into hurricane at 22,000ft (cloud tops 36,000ft) • 1947 – 1969

WX Squadron discontinued and reconstituted a number of times; based in Bermuda, Britain and Puerto Rico at times

• 1963 WC-130 (Hercules) planes began flying

• 1973 As a result of Hurricane Camille (1969) the 53rd Air WX Squadron “Hurricane Hunters” is reborn at Keesler Air Force Base, MI

• 1975 815th Reserve WX Reconnaissance Squadron, "Storm Trackers" created

NOAA Corps Aircraft Operations Center (AOC) • 1961

U.S. Weather Bureau's National Hurricane Research Project creates a Research Flight Facility (RFF), acquiring 2 Douglas DC-6 aircraft, a B-57A and a DC-4 to support its research programs

• 1961-1992 AOC based at Miami International airport – facilities badly damaged in Andrew

• Project Stormfury Joint U.S. Weather Bureau and Department of Defense effort to determine the feasibility of decreasing hurricane intensity through cloud seeding

• October 1970 NOAA created to predict changes in the oceans, atmosphere and living marine resources

• 1973 Two WP-3D Orion turbine powered aircraft ordered from the Lockheed

• 1993 AOC moves to MacDill AFB in Tampa in January

• 1996 Gulfstream GIV-SP high altitude research aircraft acquired

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Appendix B: Definitions and Acronyms

AMS: American Meteorological Society

Gale force: wind speed of 39 miles per hour (17 ms-1).

Hurricane: A severe tropical cyclone in the Atlantic or eastern North Pacific Ocean.

JTWC: Joint Typhoon Warning Center, Pearl Harbor (joint between the US Navy and Air

Force)

Major hurricane: A hurricane with over 110 mph 1-minute sustained 10-meter winds.

This corresponds to at least a Category 3 on the Saffir Simpson scale.

NHC/TPC: National Hurricane Center/Tropical Prediction Center – also known as NHC!

Severe tropical cyclone: a tropical cyclone with peak near surface wind speeds

exceeding 74 miles per hour (33 ms-1).

Tropical cyclone: generic term for a cyclonic weather system in the tropics with

thunderstorms generally circling the center and no cold or warm fronts.

Tropical storm: a tropical cyclone with peak near surface wind speeds exceeding

39 miles per hour (17 ms-1), but weaker than 74 miles per hour (33 ms-1).

Typhoon: severe tropical cyclones in the western North Pacific Ocean.

World Meteorological Organisation (WMO): The United Nations organization for

coordination of meteorological studies worldwide. Coordination activities are

channeled through national meteorological services.

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