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 Niño-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 Niño events and coolings are La Niña events.

In some basins, El Niña 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 Niñas 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 Niña (El Niño) 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 Niño 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, Fernández-Partagás 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 Niño events during the late 1970s through the early 1990s than earlier. If more frequent El Niño 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 Niño 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 Niño 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 Niño 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 Niño 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 26.5°C) 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 Niño 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 Niña events became more prevalent. As described earlier, El Niño 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 Niño 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 Niño 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 Niño events would continue, then a continued low number of tropical cyclones would be expected for this area. However, the mechanism for such El Niño 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 Niño-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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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