By: Christopher W. Landsea
Department of Atmospheric Science
Colorado State University
Fort Collins, Colorado 80523
Monthly Weather Review, Vol.121, pg. 1703-1713, (1993)
(Manuscript received 23 March 1992, in final form 12 October 1992)
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The variability of intense (or major) hurricanes in the Atlantic basin is investigated on both intraseasonal and interannual timescales. Differences are highlighted in characteristics between intense hurricanes and the weaker minor hurricanes/tropical storms. Intense hurricanes show a much more peaked annual cycle than do weaker tropical cyclones. Ninety-five percent of all intense hurricane activity occurs during August to October. In addition, over eighty percent of all intense hurricanes originate from African easterly waves, a much higher proportion than is observed for weaker cyclones. Of all classes of Atlantic basin tropical cyclones, the intense hurricanes display the greatest year-to-year variability. The incidence of intense hurricanes also has decreased during the last two decades. A small portion of this decreased activity appears to be due to an overestimation of hurricane intensity during the period spanning the 1940s through the 1960s. However, after adjusting for this bias, a substantial downward trend in intense hurricane activity during recent years is still apparent. Given that intense hurricanes are responsible for more than seventy percent of all destruction caused by tropical cyclones in the United States, an understanding is needed of the physical mechanisms for these observed variations of intense hurricane activity.
Climate change due to an increase of anthropogenic ``greenhouse gases'' may warm tropical sea surface temperatures although the magnitude, timing, and spatial variations of such a warming are very uncertain (IPCC 1990). One hypothesis is that increased sea surface temperatures will cause ``a higher frequency and greater intensity of hurricanes" (AMS Council and UCAR Board of Trustees 1988). Gray (1989), however, has suggested that warmer sea surface temperatures do not necessarily mean increased hurricane activity. He stated that ``scenarios can just as easily be made for reductions in hurricane activity due to greenhouse influences.''
In either case, before extrapolations to future frequencies of intense hurricanes can be attempted, it is essential that there be an understanding of the current annual cycle and interannual variability of intense hurricane occurrence. Intense (or major) hurricanes (IH) are defined as those tropical cyclones with maximum sustained (1 min) surface winds of 50 m s-1 during some part of their lifetimes (Hebert and Taylor 1978). These are the Saffir-Simpson (Simpson 1974) category 3, 4, and 5 hurricanes (Table 1).
Previous climatologies of Atlantic basin (i.e., North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico) tropical cyclones by Mitchell (1924), Cry (1965), and Neumann et al. (1987) have primarily focused on the seasonal numbers of named storms (i.e., storms with maximum sustained wind speeds of 18 m s-1) and hurricanes (33 m s-1). Hebert and Taylor (1978), Hebert and Case (1990), and Hebert et al. (1992) discussed IH which have affected the United States, including details of U.S. IH frequency and U.S. IH spawned death and destruction. However, no work has been done on basinwide IH activity for either the intrannual or interannual timescales.
This paper partly fills this void by providing discussions of the annual cycle of IH activity, the interannual climatology of Atlantic basin IH since the mid-1940s and the interannual variations in the origins of IH, as well as providing updates on the U.S. IH frequency and on the IH contribution to U.S. tropical cyclone spawned damage.
Most of the data sets utilized for this study are detailed in Landsea and Gray (1992). The Atlantic basin ``Best Track" (Jarvinen et al. 1984) data set includes the six-hour positions, estimated maximum sustained surface winds, and any measured central pressures of tropical cyclones from 1886 to 1991. The second data set which includes U.S. landfalling hurricanes since 1899 categorized by intensity with the Saffir-Simpson Scale was first documented by Hebert and Taylor (1975). Circulation features associated with the origins of all Atlantic basin tropical cyclones since 1967 were provided by Avila (personal communication). In addition, variations in dollar values for tropical cyclone spawned damage in the United States were studied utilizing the yearly summaries appearing in the Monthly Weather Review from 1944 to 1991. Changes in some of the damage figures are given in Hebert and Case (1990).
As stated previously, no climatology has adequately described basinwide IH variability. Part of this neglect may be due to a lack of confidence in analyzed tropical cyclone intensity (as measured by maximum sustained surface winds or by the lowest central pressure) until recent years. The U.S. Air Force and Navy began the practice of routine reconnaissance flights into the storms in 1944 (see Sumner 1944). Before then, unless a tropical cyclone went directly over a ship or a coastal station, the exact intensity of the cyclone was unknown. Of course, not every tropical cyclone in the Atlantic basin since 1944 has received aircraft monitoring every six hours of its existence. However, as described by Clark (1960), tropical cyclones which may threaten coastal areas are kept under nearly continuous aircraft surveillance. This is still true, as was the case for 1991's Hurricane Bob (McAdie and Rappaport 1991).
There is also the question of incidence of unobserved storms during the early days of hurricane forecasting. Certainly, before 1944 a number of short-lived tropical cyclones in the eastern and central Atlantic were likely completely missed. The monitoring situation improved greatly with the advent of aircraft reconnaissance. Nonetheless, while synoptic flights were sent out regularly into the central Atlantic before reports of tropical cyclones were received, some tropical cyclones may still have gone undetected in the pre-satellite, post-aircraft era. However, these are likely few in number and weak in intensity.
Because of these uncertainties, Neumann et al. (1987) suggest utilizing tropical cyclone statistics since 1944 as this period probably best represents the frequencies and intensities of tropical cyclones in the Atlantic. For this reason, the Atlantic basin interannual variability study includes only Best Track data after 1943, a severe limitation for analysis of daily incidence of activity because of the relative scarcity of IH in the Atlantic basin relative to the more active Northwest Pacific basin. The intraseasonal statistics, however, are based upon the entire Best Track data set from 1886 to 1991. Although the data from 1886 to 1943 are incomplete because of unobserved and misclassified tropical cyclones, there is likely no systematic intraseasonal bias induced in using the longer term record.
Reliable data are available, however, for hurricanes which have affected the United States since 1899. There are a few minor inconsistencies, though, between this data set and the Best Track because of slight differences in classification methodology [see Hebert and Taylor (1975) and Jarvinen et al. (1984)]. However, these small differences do not affect the conclusions in this paper.
Before continuous satellite coverage in the mid-1960s, identification of the storm formation characteristics of individual tropical cyclones is unreliableexcept for a few storms. Starting in 1967, the National Hurricane Center has maintained records on the type of cyclogenesis occurring for every Atlantic basin tropical cyclones (eg., Avila 1991). While some terminology has changed since 1967, these summaries are the most complete set of information regarding the interannual variations of the origins of tropical cyclogenesis.
Statistics on tropical storm and hurricane-caused destruction in the United States were obtained from the yearly summaries. Landsea (1991) tabulated this information. Other recent studies (eg., Sheets (1990); Hebert et al. (1992)) normalize long term damage values to current dollars using the Monthly Department of Commerce Implicit Price Deflator for Construction (U.S. Department of Commerce 1991) to account for inflation.
However, to perform a comparative assessment of interannual variations in U.S. damage, the large increases in population along the U.S. coastal zones must also be considered. Sheets (1990) noted a 60% rise in the number of Atlantic coastal inhabitants, a 250% increase in coastal Texas, and an enormous 400% increase in coastal Florida between 1950 and 1985. Obviously, an adjustment only for inflation severely underestimates the potential for damage in areas where populations have increased substantially.
To overcome this deficiency of comparative studies which adjust only for inflation, this paper utilizes both the Implicit Price Deflator for Construction as well as population changes for specific locations. Hurricane King, which struck Miami in 1950 and caused $28 million, provides a good example of the normalization procedure. Since 1950, construction costs have increased by a factor of 5.7 and population along coastal southeast Florida has increased by a factor of 6.0. This combined factor of 34.2 normalizes the damage to $957 million in 1990 dollars.
Atlantic basin tropical cyclones show a very pronounced seasonal cycle in the level of activity. Neumann et al. (1987) have provided the most recent analysis of the intraseasonal variation of named storms and hurricanes. Atlantic basin tropical cyclone activity either before 1 June or after 30 November is nearly negligible. Both named storms and hurricanes show a strong maximum in mid-September with most cyclones occurring between 1 August and 31 October. This late summer-early fall maximum corresponds to the time of the largest areal extent of warm SSTs and low tropospheric vertical shear in the tropical Atlantic (Gray 1979). However, it is difficult to determine whether named storms and hurricanes have the same peak in terms of percent of annual activity per day.
The annual cycle of IH occurrence in Figure 1 illustrates the "noisy" aspect of the unfiltered data (light curve), reflecting the relatively low climatological incidence of these cyclones. Following Neumann et al. (1987), a nine day running mean was applied to the data, as shown by the solid dark curve. With this filter, the main features of the IH annual cycle appear similar to that identified by Neumann et al. (1987) of the weaker cyclones: namely, a sharp peak of occurrence during mid-September; most activity occurring between 1 August and 31 October; and almost no events before 1 July or after 30 November. The secondary peak in IH activity in late August is likely not physically meaningful.
Figure 2 gives a more quantitative comparison of the annual cycles for various categories of tropical cyclones. This figure shows the daily incidence of tropical storms (maximum sustained wind speed greater than 17 m s-1 but less than 33 m s-1), minor hurricanes (Saffir-Simpson categories 1 and 2), and IH. The total incidence of IH is much less than that of minor hurricanes and tropical storms.
Although this figure indicates that the three categories peak at about the same time, it does not assess the relative strengths of the three maxima. Normalization by percent of annual activity per day accentuates differences in activity among the three groups, as shown in Fig. 3. Intense hurricanes have a much sharper peak during the height of the season. During mid-September, IH experience a maximum of 2.5% of their total annual activity per day, as compared to 1.9% for minor hurricanes and 1.4% for tropical storms. During 1 to 20 September, the average annual occurrence of IH, minor hurricanes, and tropical storms is 43%, 34%, and 25%, respectively. The rankings are reversed for the early and late portions of the hurricane season. In particular, on or before 31 July, the historical mean activity is 13% for tropical storms, 7% for the minor hurricanes, and only 2% for IH. Comparing the variances of the IH with the tropical storm activity, we find that the two distributions are significantly different (at the 0.05 level) using a two-tailed F distribution test (Mendenhall 1979).
The small range of dates during which IH form is likely indicative of the limited duration of favorable environmental conditions available for deep intensification of Atlantic basin tropical cyclones into IH. The more marginal conditions for tropical cyclogenesis (Gray 1979), which occur in this basin from late May to late July and from early November to early December, allow only the occasional tropical storm and infrequent minor hurricane.
For climatological studies of IH which strike the United States, it is instructive to stratify the coastline into two regions. As shown in Fig. 4 (Landsea et al. 1992), these two areas are the Gulf Coast, from Texas to the Florida panhandle, and the East Coast, from the Florida peninsula up to Maine. The entire peninsula of Florida has been included in the East Coast category because cyclones which struck the peninsula were more characteristic, both intraseasonally and interseasonally, of the remainder of the East Coast than the Gulf Coast.
Figure 5 shows that the two geographical regions delineated in Fig. 4 are susceptible to IH strikes at differing times of the year. While the timing of the overall activity is similar to that shown for IH in the entire Atlantic basin in Fig. 1, the Gulf Coast seems to experience IH somewhat earlier in the calendar year than does the East Coast. These differences are explicitly delineated in Table 2 in terms of the first, median, and last date of occurrence.
Figure 6 portrays all IH tracks from
1944 to 1991 by individual
months. The regional variations in Figs.
5 and 6 and in
Table 2 are
likely related to the favorable areas for genesis of tropical
cyclones, as well as variations of the general circulation steering.
From mid-June to July, IH are infrequent, tend to form in the Gulf of
Mexico or in the western Caribbean Sea and generally track toward the
west or north. The preferred area of formation and direction of
motion favor landfall along the Gulf Coast. From August to early
October, intensification of cyclones into IH can occur nearly
anywhere in the central and western Atlantic basin between
10°N and 35°N. But by mid-October, IH form almost
exclusively in the western Caribbean or the Atlantic Ocean between
the Bahamas and Bermuda and typically begin taking a
due north or northeast course, though occasionally moving into the
East Coast.
Table 3 summarizes the intraseasonal
variability by month. Note that
this table suggests that, in general, the amount of August activity
is equivalent to that in October. This, at first glance, appears to
contradict Table 4 from Neumann et al. (1987), which tabulates
the average number of tropical cyclones that form each month. They
found that more tropical cyclones form in August than October.
However, Table 3 here measures total
occurrence, not formation. The
different findings for occurrence and formation are reconciled in
that while more tropical cyclones form in August than October, more
cyclones form in late September and continue into October than form
in late July and continue into August.
Year-to-year variations of Atlantic basin IH since 1944 are
depicted in Fig. 7. A large range of
activity is apparent, varying from no IH occurrences in 1968,
1972, and 1986, to eight major cyclones during 1950.
This large year-to-year variability is reflected in a high coefficient of
variation (i.e., the ratio of the standard deviation to the mean) of
0.71 which is much larger than that observed for total named storm
activity (0.30). This implies that the interannual variability for
IH is much greater than weaker cyclones.
As presented in Landsea and Gray (1992), intense hurricane days (IHD)
are an additional measure of the seasonal incidence of strong
cyclones. One unit of IHD (see Fig. 8)
represents four six-hour
periods during which an IH was in existence. This type of measure is
intrinsically biased to those IH which persist for several days.
Historically, long lasting IH originate as strong easterly waves over
West Africa and develop in the central North Atlantic. These are
termed `Cape Verde' hurricanes (Dunn and Miller 1960), of which
Donna-1960, Allen-1980, and Hugo-1989 are notable examples.
Figure 8 details the yearly variations of
the Atlantic basin IHD.
The seasonal incidence of IHD fluctuates from 0 to 24.5; the
coefficient of variation is 0.93 which is even higher than that observed for IH.
Both IH and IHD show a substantial decrease in activity with time.
This trend may account for a portion of the high coefficients of
variation observed.
It is possible that artificial differences of
IH activity related to changes in measurement technology are
responsible for the apparent trend in the seasonal numbers of IH and
IHD. To test for bias between the earlier versus
the later decades, the relationship between wind speed and surface
pressure was examined. A simplified but useful empirical relationship between the maximum
sustained wind speeds and lowest surface pressures of Atlantic basin tropical cyclones was given by Kraft
(1961).
Vmax in this relation is the maximum sustained wind
speed in m s-1 and Pc is the minimum sea level pressure in
mb.
The Best Track data set contains wind speeds at 6 hourly intervals
for the lifetime of each tropical cyclone (even if an estimation was
required). The data set does not always include surface pressures
which were estimated and archived only from pressure observations and
not from the winds (Jarvinen et al. 1984). There have been no
significant changes in the measurement of actual or extrapolated
surface pressure in tropical cyclones. The methodologies of
extrapolating aircraft reconnaissance height data to surface
pressures developed in the 1940s and early 1950s (see Jordan 1958)
are still being utilized. Hence, we may deduce that any changes in
the wind-pressure relationship are due to alterations in the way
sustained wind speeds were measured or estimated.
The Best Track data set reports the maximum sustained wind speeds in
5 kt (2.5 m s-1) intervals. A finer tuned estimate of the
strength of the cyclones is not feasible with the uncertainties of
the various observational platforms. Table
4 presents means and standard
deviations of pressure readings stratified for the decades under
consideration. For specific wind speed categories, there has been a
shift to lower observed pressures, a possible indication that the
decades of the 1940s to the 1960s had overestimated wind speeds as
compared to later years. In addition, the higher standard deviations
for the decades from the 1940s to the 1960s suggest that there was
more uncertainty as to what wind speed to assign to strong
hurricanes.
To better delineate this bias, Table 5
presents a combined decadal
classification which contains at least ten cases per wind division.
The corresponding wind speed suggested by Kraft's relationship is
included next to the observed minimum pressure. As the criterion used to
define IH and IHD in the Best Track data set is a threshold of 100 kt
(51 m s-1), it is important to understand the decadal
differences (or bias)
at that intensity. Hurricanes in the 51 m s-1 category during
the years 1970 to 1991 have a mean minimum sea level pressure of 962
mb, almost precisely the pressure required for such a storm by
equation (1). However, the mean minimum sea level pressure for the
51 m s-1 hurricane in the 1940s to the 1960s was 968 mb which
corresponds to an equivalent wind speed from Kraft of only 48 m s-1. Rather, it is the 54 ms -1 hurricanes in the 1940s to the 1960s that are associated with mean sea level pressure of 963 mb, the highest pressure allowed for IH sta
tus by Kraft's relationship.
A larger bias of about 5 m s-1 is observed for stronger
(Category 4 and 5) hurricanes. However, since the threshold in this
study for IH categorization is at 51 m s-1 by the construction
of the Best Track data, that higher bias does not affect IH and IHD
calculations.
In an attempt to remove the 2.5 m s-1 bias at the IH threshold,
the criterion for IH status was increased to 54 m s-1 for the years
1944 to 1969. This effectively reduces the numbers of IH by at most three
storms and the numbers of IHD by at most 5 days in this
earlier period. The resulting time series of IH and IHD are depicted
in Figs. 7 and 8.
Table 6 gives the means and standard
deviations of
the IH activity for the original and adjusted data. Note that the
removal of this bias reduces the IH coefficient of variation down to
0.66, still much larger than that observed for named storms.
Removal of the early period bias in IH affects only a portion of the
downward trend in IH and IHD. The trend in the bias-removed IH
accounts for 13% of the variance in the data (significant at the
0.01 level, from Spiegel (1988)) and explains 8% of the variance in
the corrected IHD data (significant at the 0.05 level).
Additionally, removal of these trends still leaves a much higher
coefficient of variation for IH than named storms (0.62 to 0.30).
However, this reduced activity in recent years might not best be
characterized as a linear trend. It appears that IH activity
decreased dramatically from 1961 to 1970 and has remained nearly
constant through the present time at a much lower frequency than
occurred during the 1940s and 1950s. Table
7 contrasts the two long
term periods of 1944 to 1960 and 1971 to 1991. The differences in IH
and IHD between the time periods are significant at the 0.01 level
using the student t test with an adjustment for serial correlation
(Reid et al. 1989).
The time series of Atlantic basin IH which struck
the U.S. coastline between 1899 and 1991 is presented in
Fig. 9 (from Landsea et al.
1992). Hebert and Taylor (1978) and Sheets
(1990) have previously identified the notable lack of IH affecting
the East Coast during the late 1960s to the mid 1980s. This is
likely a reflection of the overall basinwide reduction in IH activity
during the last two decades as discussed in the previous section.
The only other period which incurred a similar (but smaller)
reduction in East Coast activity was during the early 1900s to the
late 1910s.
Over the same 92 year time frame, the Gulf Coast had nearly the same
number of IH as the East Coast but experienced much less multidecadal
variation as indicated by the smaller coefficient of variation in
Table 8.
The main advantages in using this landfalling cyclone data set are
the length of record and consistency of analysis throughout the
period (see Hebert and Taylor (1975) for description). No systematic
bias would be expected or
is found. Additionally, the IH which strike the U.S. mainland can be
used, to some degree, as a proxy estimate of variability for IH
activity in the entire Atlantic basin. The linear correlation
coefficients between landfalling versus total cyclone activity for
the years 1944 to 1991 are 0.55 for the original Best Track data set
and 0.59 for the bias-removed data set (both significant at the 0.01
level). Thus, while year-to-year variations of landfall activity
explain only 35% of the total basin activity, the longer-term
decadal activity of the Atlantic basin IH can be approximately
represented by the decadal numbers of IH which affect the United
States.
The time series of normalized values for damage
(in millions of 1991 dollars) in the United States is presented in
Fig. 10. Note the dependence of large
damage values on the
occurrence of IH. The mean annual damage caused by all tropical
cyclones is $1,905 in millions of 1991 dollars. The East Coast
suffers more damage on average than the Gulf Coast (59 to 41%,
respectively) of the total U.S. damage.
Table 9 presents a summary of the mean
and median amounts of damage
caused by tropical/subtropical storms and the various Saffir-Simpson categories
of hurricanes which affected the United States. Note that median values
are more representative of the data set because the means are highly
skewed by extremely large outliers. The ``potential damage'' is an
idealized normalization of median damage relative to the category 1
hurricanes. This concept illustrates the fact that hurricane damage
tends to increase exponentially with intensity. While Hebert and
Taylor's (1978) study clearly showed that IH are responsible for most
of the U.S. tropical cyclone spawned damage, it is startling to note
that category 4 and 5 hurricanes typically do 100 to 250 times the
damage of a minimal hurricane. Note that the tropical/subtropical
storms are given a `0' for potential damage. Many cyclones striking
the U.S. coast with tropical storm or minimal hurricane strength
supply enough beneficial rainfall that offsets any minor storm surge
or wind-caused damage (Sugg 1968). Of course, occasionally these
weaker storms are responsible for major rainfall-induced flooding
(eg., Diane in 1955 and Agnes in 1972).
An important consideration in understanding the
interseasonal variability of the Atlantic basin tropical cyclones is
in tracing their origins. The circulation features constituting the
largest contributor to tropical cyclogenesis in the Atlantic basin
are the West African easterly waves (Avila 1991). On an interannual
time scale, Avila showed that the number of easterly waves observed
each year is relatively steady; a mean of 60 waves per year with a
standard deviation of only 6 is observed. Avila also points out that
the number of waves each year has no correlation with the number of
Atlantic tropical cyclones. What is noteworthy, however, is the
difference in the relative frequency at which easterly waves
contribute to the genesis of each of the various tropical cyclone
categories. These associations are summarized in Table 10. Note
that tropical storms and minor hurricanes (Category 1 and 2
hurricanes) are almost identical (i.e., 57 and 58%) in their
relatively moderate rate of easterly wave origins whereas the large
majority (83%) of IH form from easterly waves.
Intense hurricanes have substantially
different climatological characteristics as compared to the weaker
tropical cyclones in the Atlantic basin. On intraseasonal time scales, IH
activity experiences a much sharper temporal peak than that of weaker
storms with over half of all IH activity occurring during September
alone. Interseasonal variability of IH is also much greater than
that of tropical storms and minor hurricanes. On multidecadal time
scales, basinwide IH activity during the 1970s and 1980s was much
reduced from that experienced during the 1940s and 1950s. Note that
this reduced level of activity remains apparent in the data after a
small bias to overestimation of cyclone intensity during the earlier
era is removed. The recent multidecadal pattern of basinwide IH
activity as also been reflected in higher amounts of IH strikes in
the 1940s and 1950s and a much reduced frequency of IH strikes during
the 1970s and 1980s along the U.S. East Coast (Hebert and Taylor
(1978) and Sheets (1990)). Also, whereas IH occur less frequently
than tropical storms and minor hurricanes, IH are much more dependent
upon easterly waves for their formation.
The importance of basic knowledge of IH climatology is underscored by
the potential for damage along the U.S. coastal zones which these
cyclones can produce. After normalization to 1991 dollars, IH are
responsible for an average of $1.3 billion damage per year ($1.9
billion per IH); this is more than 70% of all U.S. tropical
cyclone-spawned damage even though they comprise only 20% of all
landfalling cyclones. It is likely that a similar ratio also applies
in other countries in the Atlantic basin which are affected by IH.
The interannual and interdecadal variability of IH and IHD discussed
in sections 4.1 and 4.2 is strongly mirrored by rainfall variations
in the Sahel region of West Africa (Gray 1990; Landsea 1991; Landsea
and Gray 1992; Landsea et al. 1992). Abundant rainfall in the
Western Sahel during the late 1940s through the mid 1960s was
concurrent with larger than normal numbers of IH in the Atlantic
basin. Conversely, a severe two-decade long Sahel drought occurred
with fewer than normal numbers of IH. However, the current Sahel
drought is likely a temporary phenomena that will eventually abate
(Nicholson 1989; Gray 1990). A return of rainfall in the Sahel,
though beneficial for that region, will likely also bring increased
IH activity to the Atlantic. The possibility that tropical
cyclone-spawned damage will rise dramatically is further accentuated
by the large increases of U.S. coastal population during recent
decades. Note that these inferences are independent of any potential
greenhouse gas warming effects. This climatology is an essential
starting point for extrapolations into the future. Further research
is needed on the variations of the general circulation which are
responsible for the intra- and interseasonal fluctuations that have
been described here.
Essential programming assistance has been provided
by Richard Taft and William Thorson. Barbara Brumit, Laneigh
Walters, and Judy Sorbie-Dunn have contributed help in manuscript and
figure drafting preparation. Roger Pielke, John Sheaffer, Pat
Fitzpatrick, Ray Zehr, and John Knaff of Colorado State University,
Robert Sheets of the National Hurricane Center, Paul Hebert of NWS
WSFO in Miami, and Stan Goldenberg of the Hurricane Research Division
of NOAA/ERL have made useful comments on an earlier draft of this
paper. Additionally, two anonymous reviewers gave very useful
critical comments that helped the paper to its final form.
And, of course, William Gray, my adviser at Colorado State University
has provided invaluable guidance in this work. This research has
been supported by NSF and NOAA climate research grants with
supplemental support from a NASA Global Change Fellowship.
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Interseasonal Variability
a. Atlantic Basin Cyclones
1) Original Best Track Data Set
b. Bias Corrected Best Track Data Set
b. U.S. Landfalling Cyclones
c. U.S. Damage
d. Variation of Genesis
Summary and Discussion
Acknowledgements
Bibliography