Atlantic Basin and eastern Pacific they are called hurricanes



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Extreme Weather Lecture Packet #5:

Tropical Cyclones
I. Hurricanes, typhoons and “cyclones” are exactly the same (strong tropical cyclones),

they are just called different names in different parts of the world (Fig. 5.1)

A. In the Atlantic Basin and eastern Pacific they are called hurricanes

1. Only about 10% of global tropical cyclones

B. In the western Pacific they are called typhoons

1. Most common location for tropical cyclones with about 33% of global

number of tropical cyclones

C. In the Indian Ocean and near Australia they are just called cyclones or

tropical cyclones

1. The northern Indian Ocean basin is the least common location for

global tropical cyclones (5%) but is home to the most devastating landfalling tropical cyclones (Bangladesh)
II. What are they and how do they differ from midlatitude (extratropical) cyclones?

A. Surface low pressure systems with cyclonic (counter-clockwise in the NH)



circulation

1. This is the same as extratropical cyclones

B. These storms are bigger than thunderstorms but generally somewhat smaller

than extratropical cyclones

1. Generally....hundreds of kilometers in diameter

C. These storms have important differences from extratropical cyclones

1. Extratropical cyclones feed off the energy provided by strong

temperature contrasts (and the strong winds (jet streams) that result from this contrast)

2. Tropical cyclones form in tropical latitudes where there is very little



temperature contrast

a. The energy source for tropical cyclones is the warm ocean



water that is released as vast amounts of latent heat

generated by the heavy precipitation in the core of these storms

b. Tropical cyclones are “heat engines” with a strong positive

feedback mechanism that enables them to become much stronger (lower pressure, stronger winds) than extratropical cyclones

3. Because tropical cyclones do not have temperature contrasts, they are

generally round and “symmetrical” (the stronger they are, the more

symmetrical they become) and do not have the “fronts” associated with

extratropical cyclones (Fig. 5.2)

4. These are, by far, the most destructive storms on Earth

a. Even though winds are frequently not as strong as a tornado,

their larger size and, in particular, their association with oceans results in more widespread mayhem. More on this later.....
III. Structure (Figs. 5.3, 5.4, 5.5 and 5.6)

A. Differ from extratropical cyclones in that there is low pressure at the surface

and relatively (compared with surrounding pressures at the same level of the atmosphere) high pressure aloft. Also, instead of needing to be tilted in the vertical to strengthen (extratropical cyclone) tropical cyclones need to be vertically oriented in order to strengthen. This is why vertical wind shear acts to weaken a tropical cyclone

B. Because there is high pressure aloft at the center of the cyclone (in the eye)

the air sinks (disruption of hydrostatic balance between gravity and PGF)

1. Sinking air warms and dries (dec. RH) which is the reason skies are



clear in the eye of a hurricane or typhoon

2. This (adiabatic) warming of the core helps to deepen surface low

pressure as well (think pressure lines....)

3. The strong “outflow” or divergence from the PGF at upper-levels also

enables the surface low to continue to deepen

C. The most intense updrafts surround the eye in the eye wall

1. The horizontal winds are also the strongest here due to the strongest

PGF located in this region

2. Winds are calm in the eye because, even though pressure is the lowest,

it is the same throughout the eye, so there is no PGF

3. The reason that the strong cyclonic circulation ends at the eye wall is

because of what is called inertial instability (don’t worry I’m not going there). Basically, the balance of forces cannot be maintained inside where the eye wall sets up because of the extreme centripetal force created by such strong winds in such a tight circle


IV. Formation: How and where?

A. Environmental requirements for formation

1. Sea surface temperatures (SSTs) must be greater than 26.5°C (80° F)

(Fig. 5.7)

a. Warmer water results in elevated SVP of the air next to it which

leads to increased evaporation (due to the ability of this air to hold more water vapor)

b. The water must be this warm in order to create enough latent

heat to generate the vast amounts of energy which are necessary

to power a tropical cyclone

c. Because of this SST requirement, tropical cyclones can only

form in certain areas of the tropical oceans and usually only in summer and fall

2. The warm surface water must extend to depths of at least 60 meters

a. The winds of the tropical cyclone “stir” the water, which causes

upwelling of water from below the surface. The warm water must be deep enough so that the deeper cool water does not make it to the surface

3. Weak vertical wind shear (Fig. 5.8)

a. Stronger winds aloft will tilt the vortex

b. For surface low to deepen, latent heat must be focused vertically

over a small area

c. Stronger winds aloft actually “tear apart” the vortex

4. Must be at least 5 degrees latitude from the equator (Fig. 5.1)

a. Because there is no Coriolis Force near the equator, it is not

possible to generate a cyclonic circulation

b. Near the equator, the PGF is virtually unopposed so that air will

flow directly into the center of low pressure causing pressure to rise and the low pressure center to weaken

B. Tropical cyclones usually originate as clusters of thunderstorms within or



near the Intertropical Convergence Zone (ITCZ) (Fig. 5.9)

1. The convection within these clusters is usually enhanced due to the

presence of equatorial waves

a. For example, tropical cyclones in the Atlantic basin most

frequently originate from African Easterly Waves (AEWs)

(Fig. 5.10)

1. These are organized areas of convection (MCSs) that

form over central Africa during monsoon season

a. These systems are the essential water source for

agriculture in this region

C. The general theory of tropical cyclone formation goes like this:

1. Latent heat generated within the cluster of thunderstorms creates a low

pressure center at low levels of the atmosphere

2. If there is enough Coriolis force (> 5 deg. latitude) a low-level cyclonic



circulation will develop

3. The deepening surface low and its cyclonic circulation results in



convergence at the center of the low

4. This convergence creates “lift” and thus upward motion in the center



of the low which enhances the precipitation and latent heat release

5. The increase in latent heat release leads to even lower pressure at the

surface which leads to greater convergence which leads to greater latent

heat release and on and on..... (this is a positive feedback process)

6. Adding to this positive feedback process is the fact that as the pressure

decreases, the winds will increase. This increased wind leads to more evaporation from the warm ocean surface (think of wind blowing across wet skin) which provides more moisture for subsequent latent heat release

7. The central surface low pressure will tend to deepen even further due

to upper level divergence from the upper level high pressure and

warming of the core from sinking air in the eye

8. In this way, a tropical cyclone will develop and continue to deepen



(lower pressure) and strengthen (stronger winds and heavier rain)

unless some other process interferes

D. It is not entirely clear why all clusters of tropical thunderstorms over warm

enough water do not develop into tropical cyclones

1. One reason is that there is too much vertical wind shear in the

environment in some areas

a. If the wind increases in speed with height this can disrupt the

symmetrical three-dimensional circulation that is critical to the aforementioned positive feedback process

2. However, even in environments with very low shear only some clusters

of thunderstorms develop into tropical cyclones????
V. Life Cycle (Fig. 5.11)

A. Tropical disturbances

1. Any organized cluster of thunderstorms

a. AEWs, other types of equatorial waves

2. No well defined low pressure center or cyclonic circulation

B. Tropical depression

1. When a disturbance develops a low pressure center and cyclonic

circulation

2. Only about one in ten disturbances become depressions

a. This is due in part to how warm the SSTs are and how low the

vertical wind shear is, but not entirely

C. Tropical storm

1. Some depressions will intensify with the central pressure decreasing

(this leads to an increase in the pressure gradient and PGF and thus an increase in wind speed)

2. When the sustained wind speeds reach 35 knots (39 mph) it becomes a

tropical storm

3. At this point it receives a name (Fig. 5.11)

D. Hurricanes, typhoons and cyclones

1. About half of tropical storms continue to intensify (central pressure

falls and winds increase)

2. When sustained winds reach 65 knots (74 mph) the tropical cyclone is

classified as a hurricane or typhoon (depending on the ocean basin)

3. With further intensification the eye will become visible on satellite

images

4. If sustained winds reach 150 mph in the Pacific basin a typhoon is



classified as a “super-typhoon”

E. Tropical cyclone tracks (Figs. 5.1 and 5.12)

1. Tracks of cyclones tends to follow the anticyclonic flow around the

semi-permanent subtropical high pressure systems

2. Majority of tropical cyclones remain “harmlessly” at sea and do not



make landfall

F. Extratropical transition

1. Some tropical cyclones will migrate into middle latitudes

2. As they reach the latitude of the jet stream and the strong temperature



contrasts associated with it, they begin to feed of the energy of the jet stream and transition from a warm core structure to “cold core”

3. The cyclone then develops “fronts” and takes on the structure of an



extratropical cyclone

a. Can weaken or remain intense (e.g. Sandy) depending on the

structure at upper levels of the troposphere

VI. Intensification

A. Warm SSTs

1. In general, the warmer the SSTs, the greater energy that can be



derived from the ocean and the greater the chance of intensification

2. Rapid intensification is a more complicated issue but in some cases it

has been noted to occur when a tropical cyclone passes over a pool of particularly warm SSTs

a. This occurred with Katrina (Fig. 5.13)

b. However, rapid intensification has also been noted to occur in

the absence of a warm pool

B. Weak vertical wind shear

1. Low wind shear conditions preserve the symmetry of the tropical

cyclones structure and circulation which permits the positive feedback process to thrive and the intensification process to proceed unimpeded

C. Satellite and radar clues to intensification/weakening

1. Although not a strict rule, the eye tends to become smaller and more

symmetric as the hurricane/typhoon intensifies

a. Tropical cyclones thrive on symmetry

b. Look for the most intense convection to surround the eye in a

circle
VII. Weakening and demise of a tropical cyclone

A. The following three mechanisms cause the disruption of the tropical cyclone

heat engine and lead to its demise

1. Vertical wind shear

2. Cool SSTs

3. Land (Fig. 5.14)

a. Loss of moisture (absence of warm ocean waters) and thus loss



of energy source (latent heat)

b. Friction

1. Slows winds

2. Causes increased convergence in the center of the low at

the surface

a. Leads to a “filling” of the low pressure center


VIII. Seasonal Cycle for tropical cyclone formation

A. Tropical cyclone season follows the seasonal cycle of SSTs (Figs. 5.7, 5.15

and 5.16)

1. Tropical cyclone season generally occurs in late summer and fall



when SSTs are the warmest

a. Solar energy gains exceed losses for most of the summer and it

takes time for ocean waters to heat up due to the high specific heat of water

2. In the western Pacific (north of the equator) tropical cyclones

(typhoons) develop throughout the year due to the persistence of warm SSTs during winter and spring (Fig. 5.15)

3. “Hurricane season” in the Atlantic basin goes from June 1 to

November 30 with the peak in mid-September (Fig. 5.16) but tropical cyclones can occur on any day of any season if the SSTs are warm enough and vertical shear is low enough

a. There were tropical storms in the Atlantic in December and

January during the 2005 – 2006 season

B. Atlantic basin storms form in different locations at different times of the season

1. Cape Verde tropical cyclones form from easterly waves that form over

Africa and move out over the eastern Atlantic

a. Most Atlantic basin storms originate in this way during August

and September when the African monsoon is most active

b. 85% of Category 3 – 5 hurricanes in the Atlantic develop from



easterly waves

1. Storms have a long “fetch” of warm SSTs to intensify

2. Early and late season tropical cyclones usually develop in the Gulf of

Mexico and Carribean

a. SSTs warm early and stay warm longer due to the “contained”



seas that are less exposed to ocean currents

b. Not monsoon season in Africa

1. The easterly waves that do develop are less vigorous ant

tend to be too close to the equator

c. Tend to be less intense

1. SSTs not as warm and more vertical shear since jet

stream is further south
IX. Modes of Destruction

A. Wind


1. This is how hurricanes are categorized by intensity (Fig. 5.17) and

causes the greatest damage (Saffir – Simpson Scale)

a. Direct effects not as “deadly” as the storm surge but the wind

causes the storm surge

b. There are exceptions, such as Hurricane Andrew (1992)

2. Important to note that the strongest winds (and more importantly, the

strongest storm surge) are to the right (in relation to storm motion) of

the eye

a. This is because the wind “felt” in any location is a



combination of the actual wind speed (rotational velocity) and

the speed of motion of the storm (translational velocity) (Fig.

5.18)

3. Stronger “mini-vortices” within the larger hurricane can cause the most



damage (This was the case with Andrew – Florida 1992)

B. Storm Surge (Figs. 5.18 and 5.19)

1. Acts like a tidal wave

2. Winds “push” the ocean water, “piling water up” in front of it as it

moves forward (Fig. 5.19A)

a. Due to the stronger winds to the right of the storm motion, the



storm surge is considerably greater to the right of the path

of the eye

3. Sea level also rises due to the surface pressure gradient (like sucking

on a straw) (Fig. 5.19B)

4. As the tropical cyclone hits the coastline this “wave” of water is pushed

inland flooding the coastline

a. This storm surge is enhanced by shallow inland bays (Fig.

5.19E) (Lake Pontchatrain) like waves breaking on the shallow shoreline

b. Enhanced if occurs during high tide (Fig. 5.19D)

5. This surge of ocean water is far more deadly than the winds themselves

and is responsible for 90% of hurricane related deaths

a. 8000 deaths in Galveston (1900) and hundreds of thousands in

Bangladesh where 20% of country and the vast majority of

population is less than one meter above sea level

C. Rainwater Flooding

1. Flooding from the large area of very heavy rain causes the most

deaths related to all tropical cyclone activity

2. Tropical cyclones weaken in intensity (winds) when they move inland

but still have copious moisture associated with them. In addition, many weaker tropical storms that never attain hurricane status still have voluminous amounts of rainfall (sometimes measured in feet)

3. This is particularly true in poorer countries with weaker



infrastructure and mountainous and muddy terrain

a. For example, Hurricane Stan caused as many deaths from

rainwater flooding in Guatemala (October 1, 2005) as the storm surge flooding in New Orleans from Katrina earlier in the season

b. Hurricane Mitch (Focus 23.1) in late October 1998 weakened

to a Category 1 hurricane before making landfall in Honduras

1. Although weak in terms of wind, Mitch moved slowly

and dropped up to 75 inches of rainfall in Honduras and Nicaragua

2. Catastrophic flooding and landslides resulted in 11,000



deaths

a. Rainfall enhanced by orographic (mountains)



lifting

b. Flooding and landslides exacerbated by “slash

and burn” agriculture which limited the rainwater absorption of forests

X. Tropical Cyclone Forecasting

A. Forecast Centers

1. National Hurricane Center (part of National Weather Service)

a. Issues forecasts for the Atlantic and Eastern Pacific basins

2. Joint Typhoon Warning Center

a. Located at Pearl Harbor

b. Under purview of U.S. Department of Defense (Navy)

c. Issues forecasts for Pacific and Indian Ocean basins

3. Various Pacific and Indian Ocean countries’ meteorological services

a. Australian Bureau of Meteorology, etc.

B. Forecast Tracks and Intensity

1. Numerous tropical cyclone computer models are integrated to generate

zones of probablility as well as a “best track” forecast (Fig. 5.20)

2. These forecasts have steadily and dramatically improved over the past

several decades, particularly longer range forecasts (Fig. 5.21)

3. Intensity forecasts have a far greater degree of uncertainty due to a

poorer understanding of the physical processes involved in tropical cyclone intensification and weakening (e.g., eye wall replacement cycles)

C. Watches and Warnings (U.S. government issued) (Fig. 5.22)

1. Tropical Cyclone Watch

a. Tropical storm or hurricane that pose a potential threat

within 36 hours

2. Warning

a. When tropical storm or hurricane conditions will be present in

an area within 24 hours

D. Must balance evacuation “risks” with risks of landfalling tropical cyclone

1. Do not want too many false positive forecasts

a. Hurricane Rita (2005) following Katrina

1. Track was off, ending up going east of where forecast

2. Evacuations from Houston area resulted in massive

traffic jams

a. Caused major societal disruption and even a few

deaths (dehydration)

b. People won’t evacuate next time? (boy who cried wolf)


XI. Case Study: Hurricane Katrina

A. Introduction

Hurricane Katrina made landfall in coastal Louisiana, as almost a

direct hit on the city of New Orleans, on August 29, 2005. The inundation of the city that followed was a realization of the nightmare scenario that scientists had feared and warned about for many decades. The location of a major city along the Gulf Coast below sea-level (Fig. 5.24) made this disaster possible. The failure of the levee system built to protect the city and the failure of the local and federal governments’ evacuation of the city’s residents made the disaster a reality.

B. Timeline and Meteorology

1. Occurred during the most active Atlantic hurricane season of modern



times.......2005

a. Records set that season (old record in parentheses)

1. 27 named storms (21, 1933)

2. 14 hurricanes (12, 1969)

3. 7 major [Category 3 or higher] hurricanes (7, 1950)

4. 4 Category 5 hurricanes (2, 1960 and 1961)

5. 7 landfalling U.S. storms (8, 2004 and 1916)

2. Formed as Tropical Depression twelve over the Bahamas on August 23

a. Due to interaction of a tropical wave with a prior depression

3. Became a tropical storm on the morning of August 24

4. Became a hurricane two hours before landfall in southern Florida

5. Weakened over land then regained hurricane status shortly after moving

over the Gulf of Mexico

6. Rapidly intensified as moved over warm waters of the loop current

7. Reached Category 3 on Saturday August 27 and then intensified to

Category 5 by the next morning, reaching peak intensity at 1:00 p.m. on Sunday August 28

8. At peak intensity, sustained winds were 175 mph and pressure was 902

mb

a. This was the strongest hurricane ever recorded in the Gulf of



Mexico until Rita broke the record several weeks later

9. Weakened rapidly during the several hours before landfall

a. Combination of land encroaching on this very large cyclone and

the occurrence of an “eyewall replacement cycle”

10. Made landfall just to the east of New Orleans at 6:00 a.m. on Monday

August 29 as a Category 3 hurricane

a. Unfortunately, despite this weakening, and the path crossing

land to the east of New Orleans, the storm surge in the city was still enormous

1. Long fetch over water by a large Category 5 storm

a. Weakening only just before landfall

2. Funneling of the surge by the narrow inlet into Lake

Pontchartrain (bay to the north of New Orleans)(Fig.

5.23)

3. Once the ocean water inundated Lake Pontchartrain, the



strong northwest wind after the eye passed piled water up against the levees further

11. During the afternoon of August 29 the levees began to fail

a. 53 different breaches left approximately 80% of the city flooded

C. New Orleans: Prelude to Landfall

1. On Friday night, 56 hours before landfall, the National Hurricane

Center (NHC) issued a “best track” forecast for Katrina that eventually verified as exactly correct (Note: At this time, and even into the following day, Accuweather ( a private forecasting company) was very loudly and publicly insisting that their forecast of landfall along the panhandle of Florida was more accurate)

2. On Saturday evening, approximately 36 hours before landfall, as

Katrina was a strengthening Category 3 storm, NHC director Max Mayfield called Governor Blanco and Mayor Nagin and personally warned them about the power of Katrina and the impending danger

3. At 10:00 a.m. on Sunday (21 hours before landfall), after Katrina had

strengthened to a Category 5 hurricane, the local National Weather Service office issued the following statement:

“ Devastating damage expected....Hurricane Katrina.....a most powerful hurricane with unprecedented strength.....rivaling the intensity of Hurricane Camille of 1969......Most of the area will be uninhabitable for weeks....perhaps longer.....Water shortages will make human suffering incredible by modern standards.”

4. Both Governor Blanco and Mayor Nagin made public pronouncements

(Blanco in a CNN interview) as early as Saturday evening that the levee system was only built to withstand the storm surge of a Category 3 hurricane and that Katrina would “most likely topple the levee system” and result in “intense flooding” and “waters as high as 15 or 20 feet”, rendering portions of the city uninhabitable

5. Federal officials (FEMA director Brown) were also aware that the

levees were only rated to withstand a Category 3 storm

6. Despite all this advance warning, combined with the long standing

knowledge that a direct hit of a major hurricane to New Orleans would be a catastrophe, Mayor Nagin did not order a “mandatory” evacuation order until 19 hours before landfall (as late as Saturday afternoon, Nagin was consulting city lawyers about legal liability to the city’s businesses for lost revenue from evacuating customers)

7. Jefferson Parish – the other major component of metropolitan New

Orleans – never declared a mandatory evacuation

8. Even after the mandatory evacuation was ordered it was apparent that

there was not a coherent plan in place to evacuate those without means to exit the city (the voluntary evacuation had proceeded very successfully)

a. The City of New Orleans did have a “Comprehensive



Emergency Management Plan” which “specifically addressed the issue of those without access to transportation” which numerous agencies were supposed to implement

1. The plan acknowledged that “approximately 100,000

Citizens of New Orleans do not have means of personal transportation”

9. People were instructed to report to checkpoints for bus service which

only transported them to the Superdome, not out of the city

10. 70,000 people, mostly the poor who did not own cars (80% of New

Orleans citizens routinely rode public transportation) remained in the

city


D. The Aftermath: The Failure of the Levees (Figs. 5.25 and 5.26)

1. The levees were only built to withstand a Category 3 hurricane

2. That being said, Katrina, although a Category 5 storm while in the Gulf,

made landfall as a Category 3

3. Although some of the failures were due to overtopping of the levees,

the majority were caused by portions of the levees collapsing due to poor construction and maintenance of the levees

a. “...failures were caused by weaknesses in the foundation soil

underlying the levees, the weakness in the soils used to construct the earthen levee embankments themselves, or weaknesses caused by vegetation growing along the levees.”

4. It should be noted that the levees are being rebuilt to, once again, only

withstand a Category 3 hurricane (Fig. 5.25)

a. Improvements have been made in the construction design but it

is unclear if these improvements will be satisfactory

E. The Aftermath: The People Left Behind

1. 80% of the city was flooded, with the water 20 feet deep in places

2. Approximately 1100 people died from multiple causes

a. Most of the fatalities were due to drowning or people being

trapped in their attics without water and very hot temperatures

(dehydration and heat stroke)

1. The coroner noted that cause of death for many was

uncertain due to the level of decomposition by the time the bodies were recovered

b. Many died of numerous causes while awaiting rescue on roofs

and in their houses

c. The water supply became contaminated and some died from

infectious diseases (e.g. E. coli)

3. The National Guard and Coast Guard rescued ~ 58,000 people pulled



out of floodwaters into boats or taken off roofs by helicopters

4. Thousands took refuge in the Superdome, Convention Center and high

ground areas such as “The Cloverleaf”

a. Conditions were deplorable with no power (no sewage disposal

or air conditioning) and limited water and food supplies

b. Despite initial reports, only 10 deaths were reported at the

Superdome and Convention center only one of which was a homicide

c. It took six days to evacuate these “shelters”

F. Long-term impact

1. Tens of thousands relocated

2. Businesses destroyed

3. New Orleans is slowly rebuilding but much of the city is still



vacant/uninhabitable

4. Many have returned but have no housing

a. Approximately 12,000 homeless
G. Could this happen again to a U.S. city?

1. New Orleans levee system is only being rebuilt to withstand a Category

3 hurricane

2. Population in coastal areas is increasing and many urban areas along the

U.S. coast are vulnerable, particularly with rising sea-levels with global

warming


a. Tampa, Miami

b. Long Island

1. Much of island is only a few feet above sea-level

2. Evacuation routes very limited

H. A Matter of Perspective: Bangladesh

a. The shape of the Bay of Bengal coastline funnels storm surge to

the Bangladesh coastline (Figs. 5.27, 5.28 and 5.31)

b. Millions of impoverished people in poorly constructed housing

live in the coastal lowlands

1. Large river megadelta

2. 20% of area of country, but a far greater percentage of

the population, is less than 3 feet above sea-level (Figs. 5.29, 5.30 and 5.31)

c. Tropical Cyclones are far more devastating than Katrina

1. 1970 Bhola Cyclone

a. 1/2 million deaths

2. 1991 Cyclone

a. 138 thousand deaths, 10 million homeless

3. 2007 Cyclone Sidr

a. ~ 7000 deaths

b. Many lives saved by cyclone shelters built after

the 1991 cyclone

3. Hurricane Katrina

a. 1833 deaths


XI. Tropical Cyclones and Global Warming

A. As with ECs and tornadoes, climate models do not have the spatial or temporal (space or time) resolution to simulate these relatively small features. For this reason, it is, once again, difficult to assess definitively whether there will be an increase in frequency and intensity of tropical cyclones with global warming. There are still gaps in our knowledge of how tropical cyclones form and intensify. However, our level of understanding is considerably better than it is for tornadoes. Therefore, as we did with ECs, in addition to assessing the recent change in tropical cyclones, we can look at the conditions necessary for tropical cyclones to form and intensify (essentially, sea surface temperatures (SSTs), the depth of warm water, and vertical wind shear) and assess whether global warming should make these conditions more or less likely.

B. Recent tropical cyclone history

1. First, it must be understood that there is some uncertainty in the data

a. Satellite usage did not become routine until the 1970s

1. Therefore, hurricane frequency before this period must

be determined from extrapolation from landfalling tropical cyclones and ship reports

b. However, the frequency and intensity data is fairly reliable



since 1970

2. The frequency of tropical cyclones varies from basin to basin but the



overall global frequency has remained relatively constant since 1970

(Fig. 5.32)

3. Tropical cyclone intensity data tells a different story

a. There is evidence of a marked increase in tropical cyclone

intensity in all ocean basins since 1970 (Fig. 5.33)

1. This data has only been calculated through 2004 and



does not include the record 2005 North Atlantic hurricane season

b. It must be noted........a long term trend, as well as causation,

cannot be determined from a 30 year trend.

1. Need a longer record to separate this trend from the

signal of natural variability

a. El Niño

b. Variations in the poorly understood oceanic

global overturning thermohaline circulation

2. However, if we now look at the likely change in

conditions for tropical cyclone formation in a warming climate......

C. Likely change in atmospheric conditions necessary for tropical cyclone

formation and intensification in a warmer world

1. SSTs

a. There is a strong correlation between SSTs and maximum



tropical cyclone intensity and duration (not frequency)

b. Have been increasing steadily over all basins in last century



(Fig. 5.34)

1. Not only have the SSTs increased but the area of the so-

called “warm pool” (SSTs > 28°C) over which most tropical cyclones form has expanded markedly (Fig. 5.35)

2. As this “warm pool” expands, the depth of warm water

also increases

c. This increase is projected to continue and accelerate over the



next century (Figs. 5.36 and 5.37)

2. Vertical wind shear

a. Results are far more complicated and less conclusive than those

for SSTs


b. However, a decrease in wind shear is evident during the past

several decades over the Atlantic basin despite more frequent and stronger El Niños over the last several decades

1. This is probably due to a larger and stronger semi-

permanent subtropical high (Bermuda high) (Fig. 5.38)

c. Although model projections for wind shear in the tropics are

inconclusive, one could surmise that it will decrease as the subtropical highs expand and strengthen with global warming

1. Unfortunately, this prediction cannot be made with a

high level of confidence as vertical wind shear is complex and multifactorial in the tropics

D. Change in tropical cyclone impacts

1. Projected sea-level rise with global warming should increase the



storm surge impact of any given tropical cyclone

2. This impact will be even greater if there is an increase in Category 4



and 5 storms

3. This impact should be the greatest in low lying regions with poor

infrastructure such as the Asian megadelta regions (Fig. 5.39)

4. However, the ever increasingly populated U.S. East Coast (e.g. New

York City and Long Island) is an area of concern as well

E. Conclusions

1. There is evidence of a significant increase in intense tropical cyclones

over the past 35 years although global frequency of all tropical cyclones has remained stable

2. Overall, global warming is likely to result in an increase in the

conditions necessary for tropical cyclones to intensify

a. There is conclusive evidence for an increase in SSTs in all

ocean basins with global warming which is expected to continue

b. Evidence in regard to vertical wind shear is less conclusive but

shear likely will decrease with expansion and strengthening of the subtropical high pressure systems

3. The IPCC concludes that there will likely (>66% probability) be an

increase in tropical cylone intensity with future global warming

with an increase in the storm surge impact even more likely as sea

level rises


Fig. 5.1


Fig. 5.2


Fig. 5.3


Fig. 5.4 Fig. 5.5


Fig. 5.6

Fig. 5.7

Fig. 5.8

Fig. 5.9

Fig. 5.10



Fig. 5.11

Fig. 5.12


Fig. 5.13



Fig. 5.14


Fig. 5.15


Fig. 5.16



Fig. 5.17


Fig. 5.18



Fig. 5.19


Fig. 5.20


Fig. 5.21



Fig. 5.22


Fig. 5.23


Fig. 5.24



Fig. 5.25


Fig. 5.26


Fig. 5.27



Fig. 5.28



Fig. 5.29



Fig. 5.30


Fig. 5.31



Fig. 5.32


Fig. 5.33



Fig. 5.34




Fig. 5.35

Fig. 5.36



Fig. 5.37




Fig. 5.38




Fig. 5.39







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