Climate Variability of Tropical Cyclones: Past, Present and Future Christopher W. Landsea

Climate variability of tropical cyclones: Past, Present and Future. Storms, 2000 edited by R. A. Pielke, Sr. and R. A Pielke, Jr,

Routledge, New York, 220-241

I. Introduction

Worldwide, tropical cyclones are the deadliest and costliest natural disasters, 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 the 1992 Hurricane Andrew in the Southeast United States

can attest (Holland 1993, Hebert et al. 1997). Pielke and Pielke (1997) show that U.S.

hurricane damages - which exceed those due to earthquakes by a factor of four - accounted

for 40% of all insured property losses for 1984 to 1993. Understanding how tropical cyclone

activity has varied in the past and will vary in the future is a topic of great interest to

meteorologists, policymakers and the general public. Some have expressed concern about the

possibility that anthropogenic climate change due to increases in "greenhouse" gases may

alter the frequency, intensity and areal occurrence of tropical cyclones. A review of the

interannual variations of tropical cyclones, their causes and seasonal predictability has been

covered by Landsea (1999). This chapter, as documented from instrumental records and the

emerging field of paleotempestology, will focus instead on what have been the long-term

variations in global tropical cyclone activity, what may be responsible for such variability, and

what might occur in future decades through both natural fluctuations and man-made causes.

II. Definitions and environmental conditions needed for tropical cyclogenesis and

development

"Tropical cyclone" is the generic term for a non-frontal synoptic scale "warm-core" low-

pressure system that develops over tropical or sub-tropical waters with organized convection

and a well-defined cyclonic surface wind circulation. It derives its energy primarily by

evaporation of water and sensible heat flux from the sea enhanced by high winds and lowered

surface pressure. These energy sources are tapped through condensation in convective

clouds concentrated near the cyclone's center (Holland 1993). Tropical cyclones with

maximum sustained surface winds of less than 18 ms-1 are called "tropical depressions". Once

tropical cyclones reach winds of about 18 ms-1 they are typically called a "tropical storm" and

assigned a name. If winds reach 33 ms-1, they are called: a "hurricane" (the North Atlantic

Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of

160 E); a "typhoon" (the Northwest Pacific Ocean west of the dateline); a "severe tropical

cyclone" (the Southwest Pacific Ocean west of 160 E or Southeast Indian Ocean east of 90

E); a "severe cyclonic storm" (the North Indian Ocean); or a "tropical cyclone" (the Southwest

Indian Ocean) (Neumann 1993). Additionally, the category of "intense (or major) hurricane"

has been utilized for the Atlantic basin for those tropical cyclones obtaining winds of at least 50

ms-1, which corresponds to a category 3, 4 or 5 on the Saffir-Simpson hurricane intensity scale

(Simpson 1974, Hebert et al. 1997).

Before tropical cyclogenesis and further development can occur, several necessary

environmental conditions must be met (Gray 1968, 1979):

1. Warm ocean waters (of at least 26.5 C) throughout a sufficient depth (unknown how deep,

but at least on the order of 50 m) - are necessary to fuel the tropical cyclone heat engine1.

2. An atmosphere in which temperatures decrease fast enough with height such that it is

potentially unstable to moist convection. It is the precipitating convection typically in the form of

thunderstorm complexes that allows the heat stored in the ocean waters to be liberated for the

tropical cyclone development.

3. A relatively moist mid-troposphere. Dry middle levels are not conducive for allowing the

continuing development of widespread thunderstorm activity.

4. A minimum distance of around 500 km from the equator. For tropical cyclogenesis to occur,

there is a requirement for sufficient amounts of the Coriolis force to provide for near gradient

wind balance to occur.

5. A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical

cyclones cannot be generated spontaneously. To develop, they require a weakly organized

system with sizable spin and low level inflow.

6. Low magnitudes (less than about 10 ms-1) of vertical wind shear between the ocean's

surface and the upper troposphere. Vertical wind shear is the horizontal wind change with

height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can

prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken

or destroy the tropical cyclone by interfering with the organization of deep convection around

the cyclone center.

These six conditions are necessary, but not sufficient, as many disturbances that appear to

have favorable conditions do not develop. Recent work (Velasco and Fritsch 1987, Chen and

Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale

convective complexes [MCC]) often produce an inertially stable, warm core vortex in the

trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of

approximately 100 to 200 km, are strongest in the mid-troposphere and have no appreciable

signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs

in two stages: stage one occurs when the MCC produces a mesoscale vortex and stage two

occurs when a second blow up of convection at the mesoscale vortex initiates the

intensification process of lowering central pressure and increasing swirling winds.

Variations of the above broad-scale factors on the order of days, months, years and multi-

decades determine how changes in tropical cyclone activity have occurred in the past and will

be manifested in the future.

III. Current Climate - how tropical cyclones have varied during the instrumental record

a. Databases and climatology

Understanding tropical cyclone variability on interannual to interdecadal timescales is

hampered by the relatively short period over which accurate records are available. Figure 1

presents the various observational platforms available for analyzing tropical cyclone

occurrences in the Atlantic basin. Changes in the tropical cyclone databases due to

observational platform improvements (and sometime degradations) can often be mistaken as

true variations in tropical cyclone activity. For the Atlantic basin (including the North Atlantic

Ocean, the Gulf of Mexico and the Caribbean Sea), aircraft reconnaissance has helped to

provide a nearly complete record back to the mid-1940s. The Northwest Pacific basin (i.e. the

Pacific north of the equator and west of the dateline, including the South China Sea) also has

had extensive aircraft surveillance giving valid records going back to at least the late 1950s2,

though this aircraft reconnaissance program was discontinued in 1987. However, for the

remaining basins (the North Indian, the Southwest Indian, the Australian/Southeast Indian, the

Australian/South Pacific and the Northeast Pacific), routine aircraft reconnaissance has not

been available and reliable estimates of tropical cyclones only exist for the satellite era

beginning in the mid-1960s. Thus, with the instrumental record so limited, it is difficult to make

extensive analyses of trends and of the physical mechanisms responsible for the tropical

cyclone variability on a global basis. Because of this limitation, most studies on long-term

changes in tropical cyclone activity have focused upon the Atlantic and Northwest Pacific.

However, even with these limitations, some conclusions can be drawn about past variations in

all of the basins.

The averages and standard deviations over the last few decades for each tropical cyclone

basin are given in Table 1. For example, the Atlantic basin averages around 10 tropical

cyclones reaching tropical storm strength and, of these, about 6 reach hurricane strength,

comprising only about 12% of the global total. By far, the most active region is the Northwest

Pacific with 27 tropical storms, of which 17 becoming typhoons - over 30% of the global total.

Overall, the global average number of tropical cyclones reaching 18 m s-1 averages 86 with a

range of (+ one standard deviation) from 78 to 94. Global hurricane-force tropical cyclones

average 47 yearly with a typical range from 41 to 54. Of particular interest are the tropical

cyclones with winds of at least 50 m s-1, as these intense tropical cyclones comprise a much

larger proportion of the tropical cyclone-caused fatalities and destruction. In the Atlantic, for

example, intense hurricanes account for only 21% of all U. S. landfalling tropical cyclones, yet

cause over 82% of the tropical cyclone caused damage (Pielke and Landsea 1998). Intense

hurricane-force tropical cyclones are most common in the Northwest and Northeast Pacific

basins, making up nearly two-thirds of the average of 20 around the globe.

b. Interannual variability of tropical cyclones - a review

Seasonal variations of tropical cyclone activity depend upon changes in one or more of the

parameters discussed in section II. Many studies have focused upon the variations in these

values both before and during the tropical cyclone season. While the bulk of these studies has

been centered upon the Atlantic basin, the interannual fluctuations in all of the global basins

have been analyzed to some degree. A detailed survey paper of the interannual variations of

tropical cyclones, their causes and seasonal predictability has been covered by Landsea

(1999). What follows is a brief review of the topic.

Globally, tropical cyclones are affected dramatically by the El Nio-Southern Oscillation

(ENSO). ENSO is a fluctuation on the scale of a few years in the ocean-atmospheric system

involving large changes in the Walker and Hadley Cells throughout the tropical Pacific Ocean

region (Philander 1989). The state of ENSO can be characterized, among other features, by

the SST anomalies in the eastern/central equatorial Pacific: warmings in this region are

referred to as El Nio events and coolings are La Nia events.

In some basins, El Nia events bring increases in tropical cyclone formation (e.g. the South

Pacific [Revell and Goulter 1986] and the Northwest Pacific between 160 E and the dateline

[Chan 1985]) while others see decreases (e.g. the North Atlantic [Gray 1984a], the Northwest

Pacific west of 160 E [Lander 1994], the Australian region [Nicholls 1979]). Las Nias typically

bring opposite conditions. These alterations in tropical cyclone activity are due to a variety of

ENSO effects: by modulating the intensity of the local monsoon trough, by repositioning the

location of the a monsoon trough and by altering the tropospheric vertical shear.

In addition to ENSO, three basins (the Atlantic [Gray 1984a], Southwest Indian [Jury 1993],

and Northwest Pacific [Chan 1995]) show systematic alterations of tropical cyclone frequency

by the stratospheric Quasi-Biennial Oscillation (QBO), an east-west oscillation of stratospheric

winds that encircle the globe near the equator (Wallace 1973). These relationships may be due

to alterations in the static stability and dynamics near the tropopause. Given the robustness of

these alterations in tropical cyclone activity that match the QBO phases, it appears unlikely

that the associations are purely chance correlations. More research is needed, however, to

provide a thorough explanation of these relationships.

Interannual tropical cyclone variations have also been linked to more localized, basin-specific

features such as sea level pressures, local SST, monsoon strength and rainfall, sea level

pressures and tropospheric vertical shear changes.

Sea level pressures changes in the Atlantic (Shapiro 1982; Gray 1984b) and Australian

(Nicholls 1984) basins can force alterations in tropical cyclogenesis frequency. Lower (higher)

pressures are associated with less (more) vertical wind shear, weakened (enhanced)

subsidence drying, and a stronger (weaker) intertropical convergence zone [ITCZ]/monsoon

promoting increased (decreased) tropical cyclone activity (Knaff 1997).

Sea surface temperatures in the genesis regions have both a direct thermodynamic and

dynamic effect on tropical cyclones. In general, warmer than average waters are accompanied

by decreased moist static stability, lower than average surface pressures, and reduced shear.

Cooler than average waters are usually found in conjunction with a stable troposphere, higher

pressure, and increased shear. Somewhat surprisingly, interannual SST variations have

relatively small or negligible contributions toward increasing the tropical cyclone frequency in

most basins. Only the Atlantic, Southwest Indian and Australian regions have significant

though small, positive associations in the months directly before the tropical cyclone seasons

begin (Raper 1992, Shapiro and Goldenberg 1997). In the Atlantic basin, however, Saunders

and Harris (1997) provide substantial evidence that both preceding and during the hurricane

season that SSTs in the "main development region" (i.e. between 10 and 20 N from North

Africa to Central America - Goldenberg and Shapiro 1996) contribute a large percentage of the

variance explained (over 30% during the height of the season) with the number of hurricanes

generated in that area.

One aspect recently uncovered is the association of a tropical cyclone basin with its generating

(or nearby) monsoon trough. Evans and Allen (1992) identified that variations in the Australian

monsoonal flow can be associated with changes in tropical cyclone activity such that a strong

(weak) monsoon circulation during La Nia (El Nio) events is accompanied by many (few)

tropical cyclones. Over the Atlantic basin, June through September monsoonal rainfall in

Africa's Western Sahel has shown a very close association with intense hurricane activity

(Gray 1990). Wet years in the Western Sahel (e.g. 1988 and 1989) are accompanied by

dramatic increases in the incidence of intense hurricanes, while drought years (e.g. 1990

through 1993) are accompanied by a decrease in intense hurricane activity. Variations in

tropospheric vertical shear and African easterly wave intensity have been hypothesized as the

physical mechanisms that link the two phenomena (Landsea and Gray 1992), although

Goldenberg and Shapiro (1996) have demonstrated that changes in the vertical shear probably

dominate.

Some of this work has led to real-time seasonal forecasting efforts. The Atlantic basin has

generated the most interest with predictions methods described in Gray (1984a, 1984b), Gray

(1992, 1993, 1994), Elsner and Schmertmann (1993), Hess et al. (1995) and Lehmiller et al.

(1997). Nicholls (1979, 1984, 1992) has developed forecasts for the Australian basin as well.

Currently, no other group has issued real-time forecasts for basinwide tropical cyclones based

upon peer reviewed research.

c. Interdecadal variability of tropical cyclones

Among the basins with relatively short reliable records, Nicholls (1992) identified a downward

trend in the numbers of tropical cyclones occurring in the Australian region from 105-160 E,

primarily from the mid-1980s onward. However, a portion of this trend is likely artificial as the

forecasters in the region no longer classify weak (greater than 990 hPa central pressure)

systems as "cyclones" if the systems do not possess the traditional tropical cyclone inner-core

structure, but have the band of maximum winds well-removed from the center (Fig. 2a -

Nicholls et al. 1998). These changes in methodology around the mid-1980s have been

prompted by improved access to and interpretation of digital satellite data, the installation of

coastal and off-shore radar, and an increased understanding of the differentiation of tropical

cyclones from monsoonal depressions (McBride 1987) and subtropical storms (Neumann et al.

1993). By considering only the moderate and intense (less than or equal to 990 hPa) tropical

cyclones, this artificial bias in the cyclone record can be overcome.Figure 2b shows that even

with the removal of this bias in the weak tropical cyclones that the frequency of the remaining

moderate and strong tropical cyclones has been reduced substantially over the years 1969/70-

1995/96. (The intense tropical cyclones with minimum central pressure dropping below 970

hPa has a very slight upward trend - not shown.) Nicholls et al. (1998) attribute the decrease in

moderate cyclones to the occurrence of more frequent El Nio occurrences during the 1980s

and 1990s. However, the relatively small trend in the intense tropical cyclones implies that

while ENSO modulates the total frequency of cyclones in the region, ENSO does not exert a

control on the intensity of the systems after formation.

For the remaining short record basins based upon data from the late 1960s onwards, the

Northeast Pacific has experienced a significant upward trend in tropical cyclone frequency, the

North Indian a significant downward trend, and no appreciable long-term variation was

observed in the Southwest Indian and Southwest Pacific (east of 160 E) for the total number

of tropical storm strength cyclones (from Neumann 1993). However, whether these represent

longer term ( > 30 years) or shorter term (on the scale of tens of years) variability is completely

unknown because of the lack of a long, reliable record.

For the Northwest Pacific basin, Chan and Shi (1996) found that both the frequency of

typhoons and the total number of tropical storms and typhoons have been increasing since

about 1980 (Fig. 3). This recent trend holds true whether the curvilinear fit is utilized for the

years 1972-1994 or on the whole 1959-1994 time series. However, the increase was preceded

by a nearly identical magnitude of decrease from about 1960 to 1980. It is unknown currently

what has caused these decadal-scale changes. Additionally, no analysis has been done as of

yet on the numbers of intense typhoons (winds at least 50 m s-1) because of an unremoved

overestimation bias in the intensity of such storms in the 1950s and 1960s (Bouchard 1990,

Black 1993).

There has been an extensive analysis of the North Atlantic basin due in part to the reliable

record for both the entire basin (back to 1944) and U. S. landfallings (back to 1899)3. Similar to

the problems with the Northwest Pacific data, the all-basin data also has had a bias in the

measurement of strong hurricanes: during the mid-1940s through the late 1960s, the intensity

of strong hurricanes was likely overestimated by 2.5-5 m s-1 (Landsea 1993). This bias has

been crudely removed to provide estimates of the true occurrence of intense (or major)

hurricanes. No estimate of the true occurrence of all-basin intense hurricanes is attempted for

the era before the mid-1940s because of the lack of reliable data on the strong inner core of

the hurricanes except for very infrequent measurements conducted by unlucky ships' crews.

The U.S. landfalling hurricane records back to the turn of the century are very reliable as

opposed to open-water storms because of the use of actual central pressure measurements at

landfall (Jarrell et al. 1992).

Examination of the record for the Atlantic numbers of tropical storms (including those

designated as subtropical storms4 1968 onward) shows substantial yearly variability, but no

significant trend (Fig. 4). In contrast, the numbers of intense hurricanes have gone through

pronounced multidecadal changes: active during the late 1940s through the mid-1960s, quiet

from the 1970s through the early 1990s, and then a shift again to busy conditions again during

the extraordinarily active years 1995 and 1996 (Fig. 5). Concurrent with these frequency

changes, there have been periods of strong mean intensity of the Atlantic tropical cyclones

(mid-1940s-1960s and 1995-1996) and weak mean intensity (1970s-early 1990s), though

there has been no significant change in the peak intensity reached by the strongest hurricane

each year (Landsea et al. 1996a).

These trends for the entire Atlantic basin are mirrored by those intense hurricanes striking the

U. S. East Coast, from the Florida peninsula through New England (Fig. 6). The quiet period of

the 1970s to the early 1990s is similar to a quiescent regime in the first two decades of this

century. A more active regime began in the mid-1920s and continued into the 1960s, with a

peak in landfalling intense hurricanes from the 1940s through the mid-1960s. During two

particularly busy periods, the Florida peninsula and the Carolinas to New England each

experienced seven intense hurricane landfalls in seven years (1944-1950 and 1954-1960,

respectively). Other regions within the Atlantic basin - such as the Caribbean Sea and

surrounding land masses - also have experienced these multidecadal changes with even

greater amplitude (Fig. 7). In contrast, a subset of the Atlantic basin consisting of the U. S. Gulf

Coast from Texas to the Florida panhandle (Fig. 8) has observed much weaker multidecadal

variability in intense hurricane strikes. Going back even farther into the historical records,

Fernndez-Partags and Diaz (1996) estimate that the overall Atlantic tropical storm and

hurricane activity for the years 1851-1890 was 12% lower than the corresponding forty year

period of 1951-1990, though little can be said regarding the intense hurricanes.

Finally, hurricane-caused damage in the United States - when properly normalized - can also

provide an independent indication of multiyear changes in tropical cyclone activity. Pielke and

Landsea (1998) standardized the amount of U. S. destruction from tropical cyclones by taking

into account inflation, coastal county population changes and trends in personal property

amounts. Figure 9 shows the time series of normalized damage amounts when these three

factors are taken into account. Note the extreme destruction in 1926 (due to the near worst

case scenario of a large Category 4 hurricane striking first the Miami-Ft. Lauderdale region in

Florida, then the Florida panhandle and Alabama as a Category 3 hurricane), lowered values

of damage in the early and mid-1930s followed by $3-7 billion damage per year for nearly

every five year period from the late 1930s until the late 1960s. During the 1970s and 1980s,

the normalized damage in the United States was substantially smaller ($1-3 billion per year)

than in earlier decades. During the first five years of the 1990s, damage again returned to

higher levels due primarily to the destructiveness of Hurricane Andrew in 1992.

Gray (1990) and Gray et al. (1997) have attributed these multidecadal variations in intense

hurricane activity to changes in the Atlantic SST structure. Warmer (cooler) than average

conditions in the Atlantic north of the equator coupled with cooler (warmer) than average SSTs

in the South Atlantic favor increased (decreased) intense hurricane activity. Such a dipole

structure of the Atlantic SSTs also forces drought and wet periods in the North Africa's

Western Sahel (Fig. 10, Folland et al. 1986), which at least partially explains why there is a

strong concurrent link between the year-to-year Sahel rainfall variations and intense Atlantic

hurricanes (Reed 1988, Gray 1990, Landsea and Gray 1992, Landsea et al. 1992). The SST

dipole pattern appears to alter the overlaying tropospheric circulation such that warm

North/cold South Atlantic conditions correspond to reduced vertical wind shear in the main

development region favoring the formation and intensification of tropical cyclones. In contrast,

a cool North/warm South Atlantic acts in concert with enhanced tradewind easterlies and upper

tropospheric westerlies and thus increased tropospheric vertical wind shear (Gray et al. 1997).

Additionally, these SST variations likely play a direct role in providing changes of the heat input

available to the incipient tropical cyclone by changing the boundary layer moist enthalpy values

(Saunders and Harris 1997, Landsea et al. 1998).

The strong sensitivity of Atlantic intense hurricanes to these changes while the frequency of

named storms remains relatively constant is likely due the formation differences between the

two. The vast majority of Atlantic intense hurricanes develop from easterly waves exiting the

North African coast and moving across the tropical North Atlantic (Landsea 1993). Conditions

throughout the main development region are usually unfavorable for any tropical cyclone to

form and intensify, so typically the most that is realized is a tropical storm or a weak hurricane

(Gray et al. 1993). In active intense hurricane years such as 1995 and 1996, the vertical shear

is lowered and the SSTs are warmer along the main development region allowing a few

easterly waves to develop up to intense hurricanes (Goldenberg and Shapiro 1996, Saunders

and Harris 1997). In contrast during quiet seasons for intense hurricanes such as 1991 through

1994, tropical storms can occur in relative abundance in the subtropical latitudes (20-40 N)

forming from upper level lows, stationary frontal boundaries and easterly waves that survive

the hostile tropical latitudes.

The lack of a distinct multidecadal variation of intense hurricanes in the Gulf of Mexico is likely

due to local conditions that dominate over these basinwide SST changes (Landsea et al.

1992). Since 1967 (when satellite monitoring made it possible), only intense hurricanes that

were spawned from easterly waves have made landfall along the U.S. East Coast, while mid-

latitude systems (e.g. stationary frontal boundaries or upper-tropospheric cutoff lows) can

occasionally form an intense hurricane that makes landfall along the U.S. Gulf Coast.

Hurricane Alicia, which struck the Texas coast in 1983, is a notable example of this latter

phenomena. Additionally, vertical shear changes in the Gulf are not correlated highly with

variations of ENSO or West Sahel rainfall, unlike the main development region (Goldenberg

and Shapiro 1996).

IV. Paleotempestology - the prehistoric record of tropical cyclones

The study of pre-historic tropical cyclones, or "paleotempestology" as it could be called, may

be a way to extend back these records to provide measures of longer-term tropical cyclone

climate variability. Recent efforts to address this issue with a variety of creative methodologies

include examining: shallow coastal lake bed cores to locate storm surge sand layers (Liu and

Fearn 1993), cyclone-produced sediment deposits in shallow offshore waters (Keen and

Slingerland 1993), pollen changes recorded in coastal forest floors due to canopy blow down

(Bravo et al. 1997) and oxygen isotope variations found in coastal cavern stalactites5

(Malmquist 1997). Liu and Fearn's (1993) work suggests that at one particular location in

coastal Alabama, U.S., there were strikes by Saffir-Simpson Category 4 or 5 hurricanes at

around 3400, 2800, 2200, 1300 and 700 years ago (Fig. 11) implying an annual probability of

occurrence of about 0.17%. Before about 3400 years ago, there is no evidence in the sediment

record for Category 4 or 5 hurricanes, meaning that either the climate did not allow for such

strong hurricanes to occur, that the tracks of such hurricanes were altered away from this Gulf

of Mexico location, and/or the geomorphology of the region changed so that the technique

could not provide an accurate measure of such strong hurricanes before this time. These

methods provide promise in extending records of tropical cyclones well beyond the current few

decades of reliable standard historical data, provided that they are able to be calibrated

accurately against hurricanes that occurred in the instrumental record.

V. Future Climate - how tropical cyclones may change in coming years

a. Extrapolation of past variations

As a first approximation of tropical cyclone activity in the next decade or two, one can simply

extrapolate past variations in the data - that is assuming that such trends are not artificially-

induced and that a quasi-periodicity actually exists in the cyclone activity. Three basins - the

Australian, the Northwest Pacific and the Atlantic - have been examined in enough detail

possibly to allow some suggestions for what the late 1990s and first decade of the 21st century

may bring.

In the Australian basin as detailed earlier, Nicholls et al. (1997) identified an substantial

downward trend in the numbers of tropical cyclones over the period of 1969/70 through

1995/96, the non-artificial portion of which is linked to having more frequent El Nio events

during the late 1970s through the early 1990s than earlier. If more frequent El Nio events

were to continue in the coming decades, then the Australian region would likely continue to

receive fewer than normal tropical cyclones. The continuation in the trend in ENSO is

dependent upon its cause. One possibility is that the increased El Nio activity is due to natural

variability of the ocean-atmosphere system (e.g. Gray et al. 1997). However, Trenberth and

Hoar (1996) suggest that the extremely long-running El Nio event of late 1990 through early

1995 was not due to natural fluctuations, but instead may be due to climate changes

associated with increases in greenhouse gases. Such a statement is not supported by general

circulation model (GCM) simulations because GCMs characterize ENSO variability poorly.

Thus until the cause of the trend in ENSO is known, suggestions that there will be a

continuation of frequent El Nio events resulting in fewer Australian tropical cyclones for the

next decade or two is probably not very prudent.

Chan and Shi (1996) uncovered a decrease in Northwest Pacific typhoons and total number of

tropical storms from the late 1950s through the late 1970s, followed by a nearly comparable

increase from the early 1980s until the mid-1990s. However, since the mechanism for these

variations is unknown, a further extrapolation of the increase in the 1980s and 1990s into the

future would also be unfounded.

The one region where it may be possible to make a reasonable assessment of future climate

trends is the Atlantic, because of the multidecadal variations in hurricane activity that have

been described and to some extent understood. As described above, while the total number of

tropical storms and hurricanes do not vary greatly on a multidecadal time scale, the intense

hurricanes show a strong variation. More numerous intense hurricanes occur, such as in the

decades of the 1940s through the 1960s, while the North Atlantic is warmer than average and

the South Atlantic is cooler. Converse conditions of few intense hurricanes were observed in

the 1970s through the early 1990s while the North Atlantic was cool and the South Atlantic was

warm. It has been hypothesized (Gray et al. 1997) that these multidecadal oceanic

temperature and hurricane changes are regulated by the strength of the thermohaline

circulation and North Atlantic deep water formation - portions of the global "Great Ocean

Conveyor" (Broecker 1991). Given that the Sahel drought and wet regimes also occur in

conjunction with the Atlantic intense hurricane quiet and active periods, respectively (Gray

1990, Landsea et al. 1992), and that the Sahel has experienced several multidecadal periods

of wet and dry conditions over at least the last few hundreds of years (Nicholson 1989), it

stands to reason that these fluctuations are a natural manifestation of the ocean-atmospheric

system and that an end to the Sahel drought and Atlantic hurricane quiet period of the 1970s-

early 1990s would soon come to an end. In fact, Gray (1990) predicted as much:

"If these past variations are a reasonable indication of the future, then we should expect an

eventual recurrence of somewhat heavier Western Sahel precipitation, possibly during the

1990s and the early years of the 21st century. With such a rainfall increase, we should also

expect a return of more frequent intense hurricane activity in the Caribbean Basin and along

the U. S. coastline."

The hyperactive Atlantic hurricane seasons of 1995 and 1996 with a total of 32 named storms,

20 hurricanes and especially 11 intense hurricanes may indicate the start of such a return to

active conditions (Gray et al. 1996, Goldenberg et al. 1997). The 11 intense hurricanes over

two years represents a 450% increase over the frequency of intense hurricanes during 1991-

1994 and a 139% increase over the long-term (1950-1990) average of 2.2 intense hurricanes

per year. Along with the increase in hurricane activity, the West Sahel rainfall has returned to

near average conditions for 1994-1996, the first three year stretch of near to above normal

rainfall since 1965-1967 (Landsea et al. 1996b). Corresponding to, and most likely leading,

these changes in the Atlantic intense hurricane and West Sahel rainfall, are rather dramatic

increases in the North Atlantic SSTs from 5 N to 60 N and a cooling of the South Atlantic

SSTs from 5 N to 50 S (Gray et al. 1996). It is also possible that such changes were

beginning to occur in 1988-1989 - which were two years of high West Sahel rainfall and active

Atlantic hurricane seasons - but that the highly anomalous long-running El Nio event of late

1990 through early 1995 acted to mask the enhancing effects of the Atlantic SSTs

(Goldenberg et al. 1997). More in depth research is needed to better define if indeed this

change in the Atlantic SSTs with the attendant effects on Atlantic hurricanes and Sahel rainfall

has occurred; if it has switched, when the change took place; and how long would an active

intense hurricane regime stay in place.

b. The effects of anthropogenic global warming

Two impacts of anthropogenic climate change due to increasing amounts of "greenhouse"

gases that may occur (Houghton et al., 1990, 1992, 1996) are increased tropical sea surface

temperatures (Fig. 12) and increased tropical rainfall associated with a slightly stronger ITCZ

(Fig. 13). Note in these figures the 0.5 to 1.5 C warming of the tropical and subtropical SSTs

and an overall increase in the ITCZ precipitation near the equator, though the precipitation

changes show a "noisier" signal. Because of these possible changes, there have been many

suggestions based upon global circulation and theoretical modeling studies that increases may

occur in the frequency (AMS Council and UCAR Board of Trustees 1988; Houghton et al.

1990; Broccoli and Manabe 1990; Ryan et al. 1992; Haarsma et al. 1993), area of occurrence

(Houghton et al. 1990; Ryan et al. 1992), mean intensity (AMS Council and UCAR Board of

Trustees 1988; Haarsma et al. 1993), and maximum intensity (Emanuel 1987; AMS Council

and UCAR Board of Trustees 1988; Houghton et al. 1990; Haarsma et al. 1993; Bengtsson et

al. 1996) of tropical cyclones. In contrast, there have been some conclusions that decreases in

frequency may result (Broccoli and Manabe 1990; Bengtsson et al. 1996). Finally, one report

concluded that any changes in frequency or intensity due to increased greenhouse gases

would be "swamped" by the large natural variability (Lighthill et al. 1994). As discussed earlier,

there is currently no evidence at the present time that there have already been systematic

changes in the observed tropical cyclones around the globe.

Any changes in tropical cyclone activity are intrinsically tied in with large-scale changes in the

tropical atmosphere. One key feature that has been focused upon has been possible changes

in SSTs. However, SSTs by themselves cannot be considered without corresponding

information regarding the moisture and stability in the tropical troposphere. What has been

identified in the current climate as being necessary for genesis and maintenance for tropical

cyclones (e.g. SSTs of at least 26.5C) would likely change in a 2 x CO2 world because of

possible changes in the moisture or stability. It is quite reasonable that an increase in tropical

and subtropical SSTs would be also accompanied by an increase in the SST threshold value

needed for cyclogenesis because of compensating changes in the tropospheric most static

stability (Emanuel 1995). Such difficulties then make it problematic to address the issue as

Ryan et al. (1992) did in using Gray's (1979) "Genesis Parameter" to diagnose changes in

large-scale fields from GCM output for tropical cyclone frequency and area of occurrence

issues. Indeed, Watterson et al. (1995) found that Gray's Genesis Parameter, while quite

useful for diagnosis of the mean climatology of tropical cyclone frequency and area of

occurrence, was not able to correctly anticipate interannual fluctuations in tropical cyclone

activity and thus, probably would not be useful for analysis of future climate states.

Additionally, besides the thermodynamic variables, changes in the tropical dynamics will also

play a big role in determining changes in tropical cyclone activity. For example, if the vertical

wind shear over the tropical North Atlantic moderately increased (decreased) during the

hurricane season in a 2 x CO2 world, then we would see a significant decrease (increase) in

activity because this particular basin is marginal for tropical cyclone activity. Another large

unknown is how the monsoonal circulations may change. If the monsoons become more

active, then it is likely that more tropical cyclones in the oceanic monsoon regions would result.

In contrast to other GCM results (e.g. the "variable cloud" run in Broccoli and Manabe 1990,

Haarsma et al. 1993, etc.), Bengtsson et al. (1996) show that a GCM climate with doubled

carbon dioxide amounts compared with pre-industrialized values produces substantially fewer

tropical cyclones around the world because of a weakened ITCZ and monsoonal circulations.

However, the downscaling technique utilized (i.e. a high resolution atmospheric GCM run for

five years run from the SST boundary conditions from the 60th year of a low resolution GCM

run) appears to be flawed because the ITCZ response to increased carbon dioxide was

actually opposite in the low resolution model (Landsea 1997b), thus calling into question the

validity of Bengtsson et al.'s results.

One last final wild card in all of this is how ENSO may change in a 2 x CO2 world, as ENSO is

the largest single factor controlling year-to-year variability of tropical cyclones globally

(Landsea 1997a). If El Nio events occur more often or with more intensity, then the

inhabitants along the Atlantic basin and in Australia would likely have fewer tropical cyclones to

worry about, whereas people living in the South Central Pacific would have more storms to

prepare for. The reverse would be true if La Nia events became more prevalent. As described

earlier, El Nio events indeed have become more frequent in occurrence during the most

recent two decades, actually resulting in some of the changes noted above. It is currently

unknown whether this trend toward more El Nio events is simply natural variability or is due to

anthropogenic forcing.

Overall, it is difficult to assess globally how changes of tropical cyclone intensities (both the

mean and the maximum), frequencies, and area of occurrence may change in a 2 x CO2 world.

It is because of this uncertainty that the 1995 Intergovernmental Panel on Climate Change

assessment (Houghton et al. 1996) came out with this straightforward admission:

"The formation of tropical cyclones depends not only on sea surface temperature (SST), but

also on a number of atmospheric factors. Although some models now represent tropical storms

with some realism for present day climate, the state of the science does not allow assessment

of future changes."

Clearly, much more investigation is needed to narrow down the uncertainties that are currently

in this field of tropical cyclone climate change. Currently, there is no convincing evidence that

there will be a systematic increase in the tropical cyclone mean intensity, maximum intensity or

frequency due to increases in "greenhouse gases". Nor, for that matter, is there strong

evidence for decreases in hurricane, typhoons and tropical cyclones. It may turn out that

changes around the globe will not be consistent; some regions may experience more activity,

others less.

VI. Summary

Tropical cyclones - including hurricanes and typhoons - have been and continue to be

extremely disruptive events for inhabitants in the global tropics and subtropics. Knowledge of

how and why the characteristics of these coupled ocean-atmospheric systems have changed

in the past is a topic of much interest. With a more complete understanding, we will be better

prepared to answer the question: "How will tropical cyclone activity change in future years?".

Progress is being made in analyzing both the interannual fluctuations of tropical cyclone

activity (see review by Landsea 1997a) as well as the multidecadal facet. In this chapter, three

basins - the Australian, the Northwest Pacific and the Atlantic - have been examined in detail.

In the remaining basins, reliable records are too short to demonstrate reliable trends. For long-

term trends in total frequency of events, the Australian basin has shown a decline (since the

late 1960s), the Northwest Pacific is now showing an increase after experiencing a decrease in

frequency from the late 1950s through 1980, while the Atlantic has been fairly constant since

the mid-1940s. For mean intensity, the Australian basin has very little trend, the Northwest

Pacific has shown a downward trend during the 1960s and 1970s and an upward trend in

intensity of events since, and the Atlantic has been observed to have a quasi-cyclic

multidecadal regime that alternates between active and quiet phases of mean intensity on the

scale of 25-40 years each. Such variations in Atlantic hurricanes are mirrored by normalized

destruction amounts that occurred in the United States. For maximum intensity, only the

Atlantic has been examined revealing no substantial trend or consistent variation in the

strongest hurricane each year.

While the multidecadal variations for the Northwest Pacific currently have no explanation, there

exist plausible reasons for the changes in regimes in the Australian and Atlantic basins. The

decline in the Australian tropical cyclones are due to increasing El Nio events during the late

1970s through the early 1990s. The quiet decades of the 1970s to the early 1990s for intense

Atlantic hurricanes are likely due to changes in the Atlantic Ocean SST structure with cooler

than usual waters in the North Atlantic and warmer in the South Atlantic. The reverse situation

of a warm North Atlantic and a cool South Atlantic was present during the active 1940s through

the 1960s. A natural fluctuation of the Great Ocean Conveyer and the associated North

Atlantic deep water formation has been hypothesized as being responsible for such SST and

Atlantic hurricane changes.

Extrapolation of these multidecadal variations into the future is uncertain. Until an explanation

is found for the Northwest Pacific upward trend in frequency and intensity (as well as the

downward fall in both during the late 1950s through the late 1970s), a decadal forecast is

doubtful. For the Australian basin, if one could be assured that the more frequent El Nio

events would continue, then a continued low number of tropical cyclones would be expected

for this area. However, the mechanism for such El Nio changes over the past couple of

decades is not understood and thus, a decadal scale forecast of Australian cyclones would not

be prudent. The only basin that might be reliably forecast is the Atlantic because of the large,

apparently natural fluctuations of the Atlantic SSTs on a multidecadal timescale. It is possible

that the hyperactive seasons of 1995 and 1996 signal a return of the active regime (of an

unknown duration) to the Atlantic, though more research is needed to confirm or deny such a

hypothesis.

Over even longer timescales, the question has been raised as to the possible impact of

anthropogenic global warming on tropical cyclones around the world. Unfortunately, due to our

inability to simulate tropical cyclones on the scale needed within the context of a GCM,

because of conflicting model results, and due to our lack of knowledge about the processes of

tropical cyclogenesis and intensification, there is no convincing evidence for systematic

changes to occur in the frequency, mean intensity, maximum intensity, and area of occurrence

of tropical cyclones. Indeed, looking for a systematic global signal common to all tropical

cyclone basins is not the most reasonable approach. Because of strong links with global

phenomena such as the El Nio-Southern Oscillation, tropical cyclone activity in various basins

is not independent of one another. An increase in activity in one region may be instead be

accompanied by a decrease in tropical cyclone activity in another basin. It will take continued

efforts toward increasing our understanding before more definitive answers are available for

the global warming question.

VII. Acknowledgments I would like to acknowledge the helpful support and encouragement of

Hugh Willoughby and Stan Goldenberg here at the NOAA/AOML/Hurricane Research Division.

Bill Gray of Colorado State University has also sparked many useful and enlightening

discussions on the topic. Roger Pielke, Sr., Roger Pielke, Jr. and three anonymous reviewers

provided quite helpful comments that clarified and enhanced this review chapter. The author

thanks the Bermuda Biological Research Station's Risk Prediction Initiative for providing

financial support through a grant on the topic of interannual tropical cyclone variability.

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Footnotes:

1 However, documented cases exist (e.g. Atlantic Hurricane Karl in 1980 [Lawrence and

Pelissier 1981]) where this sea surface temperature (SST) threshold of 26.5 C was not

necessary. It may be instead that SSTs exceeding this amount are a general proxy for an

environment that is conditionally unstable to moist convection (see item 2). Conditions can -

and apparently do - set up on occasion to allow for conditional moist instability in waters cooler

than 26.5 C.

2 While formal U.S. armed forces aircraft reconnaissance was begun in the Northwest Pacific

in 1945 (Guard et al. 1992), the U.S. Joint Typhoon Warning Center (JTWC) considers data

only from 1959 onward as reliable (JTWC 1974). However, the aircraft data could and should

be utilized in a tropical cyclone "reanalysis" to extend the trustworthy records back as far as

possible for this basin.

3 While records are available for the entire Atlantic basin hurricanes back to the late 1800s

(Jarvinen et al. 1984) and for landfalling hurricanes along the United States coastline back to

the 16th Century (Ludlum 1989), reliably knowing the intensity of such systems extends for a

much briefer period of time. For the whole Atlantic basin, accurate intensity measures exist

back to 1944 at the commencement of routine aircraft reconnaissance (Neumann et al. 1993),

but even these data have been arbitrarily corrected to remove an overestimation bias in the

winds of intense hurricanes during the 1940s through the 1960s (Landsea 1993). For U.S.

landfalling hurricanes, observations of minimum central pressure provide accurate records

back to 1899 for nearly all hurricanes (Jarrell et al. 1992). Before this year, records of intensity

at landfall are incomplete and can only provide very rough estimates of the hurricanes'

strength.

4 "Subtropical storms" are non-frontal low pressure systems comprising initially baroclinic

circulations developing over subtropical waters with sustained one minute surface winds of at

least 18 ms-1 (NOAA 1997). Such nomenclature has been utilized since 1968, though it is likely

that these systems were designated as tropical storms previously. Thus, failure to include the

subtropical storms into the climate record examined would introduce an artificial bias into the

database (Neumann et al. 1993).

5 This curious measurement of past hurricanes can be obtained due to the characteristic of

primarily the most intense tropical cyclone rainfall having quite low oxygen-18 isotope

concentrations compared with other types of local rainfall (Lawrence and Gedzelman, 1996).

Created by. Noel J. Charles

Dec 10, 1999