GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L14803, doi:10.1029/2011GL047711, 2011

Recent historically low global tropical cyclone activity
Ryan N. Maue1
Received 14 April 2011; revised 6 June 2011; accepted 7 June 2011; published 20 July 2011.

[1] Tropical cyclone accumulated cyclone energy (ACE)
has exhibited strikingly large global interannual variability
during the past 40‐years. In the pentad since 2006,
Northern Hemisphere and global tropical cyclone ACE has
decreased dramatically to the lowest levels since the late1970s. Additionally, the global frequency of tropical
cyclones has reached a historical low. Here evidence ispresented demonstrating that considerable variability intropical cyclone ACE is associated with the evolution of thecharacter of observed large‐scale climate mechanisms
including the El Niño Southern Oscillation and Pacific
Decadal Oscillation. In contrast to record quiet North Pacifictropical cyclone activity in 2010, the North Atlantic basinremained very active by contributing almost one‐third of theoverall calendar year global ACE. Citation: Maue, R. N.
(2011), Recent historically low global tropical cyclone activity, Geophys. Res. Lett., 38, L14803, doi:10.1029/2011GL047711.

1. Introduction[2] During the past four decades, integrated and secularfrequency measures of global tropical cyclone (TC) activity
have undergone considerable interannual variability. Since
2006, global and Northern Hemisphere (NH) TC accumulated cyclone energy (ACE) has decreased dramatically,
nearly cut in half from the previous peak in 2005 to thelowest levels since the 1970s. While the annual global frequency of tropical storm TCs has not exhibited a long‐term
trend (∼87 per year), the calendar year total of 69 observedin 2010 represented a decades’ low. Indeed while the very
active North Atlantic (NATL) basin generated 19 named
storms in 2010, the rest of the global tropics produced only
50 TCs including the fewest Western North Pacific (WNP)
typhoons and Northeast Pacific (NEP) hurricanes counted in
the reliable historical record.
[3] The strikingly large global interannual variation in TC
frequency and ACE (which also includes duration andintensity) is shown here to be a function of observed large‐
scale low frequency modes of ocean‐atmosphere climate
variability mainly on interannual and longer time scales.
Cursory analysis here demonstrates that low frequency
variability in global TC ACE and hurricane‐force (HF) TC
frequency is associated with changes or evolution in thecharacteristics of the El Niño Southern Oscillation (ENSO)
and Pacific Decadal Oscillation (PDO). This current andmost recent historical downturn in global TC ACE andfrequency is consistent with similar past periods of overallcolder Pacific large‐scale climate conditions. However, anotable difference is the observed doubling of the NATL
basin contribution of TC ACE to the global total coinciding
with the onset of the so‐called active era in 1995 [Lander
and Guard, 1998; Bell and Chelliah, 2006].

2. Data and Methodology[4] Global TC lifecycle data is obtained from the complete IBTrACS (version v03r02) database [Knapp et al.,2010] with six hourly best‐track positions and intensity
estimates from 1970–2010 (operational center advisory estimates are used for TCs observed through the end of May2011). Only TCs designated as in a tropical phase with one‐minute maximum sustained surface wind speeds exceeding
34‐knots are included in this analysis. Until a comprehensive global TC reanalysis is conducted [Chu et al., 2002],
considerable uncertainty will remain in interagency TC
intensity estimates especially during the 1970s, prior to thesatellite era [Knapp and Kruk, 2010]. Detailed informationon the construction of the dataset used in the following
analysis is provided in the auxiliary material.1[5] The accumulated cyclone energy (ACE) metric [Bellet al., 2000], analogous to the power dissipation index(PDI) [Emanuel, 2005] convolves intensity and duration
information for each individual TC observed in the global
tropical basins. ACE is calculated by squaring the 6‐hourly
intensity estimates reported in the best‐track database andintegrating over individual lifecycles or seasons partitioned
according to basin or hemisphere. The ACE metric has anadvantage of including considerable information from theentire TC lifecycle complimenting the familiar secular categorization according to the maximum lifecycle wind speed[e.g., Webster et al., 2005; Klotzbach, 2006]. On the individual storm scale, the correlation between duration andmaximum wind speed is a variable function of basin, genesis location, track, time of year, and a host of other large‐
scale environmental factors.

3. Global Tropical Cyclone ACE

1Center for Ocean and Atmosphere Studies, Department of Earth,
Ocean and Atmospheric Science, Florida State University, Tallahassee,
Florida, USA.

[6] Twenty‐four month running sums of global ACE(units of 104 knots2) are partitioned according to hemispheresince 1970 (Figure 1a). The top (bottom) time series represents
the global (NH) ACE for the preceding 24‐months, while
the shaded area in between describes the SH ACE. Quantitatively, keeping in mind inherent quality issues with dataprior to 1980, recent global ACE has decreased to the lowest
levels since 1977 coincident with the great Pacific climate
shift of 1976–1977 [Yeh et al., 2011]. Preliminary data
through May 2011 shows a continuation of overall global

Copyright 2011 by the American Geophysical Union.
0094‐8276/11/2011GL047711

1
Auxiliary materials are available in the HTML. doi:10.1029/
2011GL047711.

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Figure 1. Global tropical cyclone Accumulated Cyclone Energy (ACE units 104 knots2) from 1970–2011. (a) 24‐monthrunning or accumulated sums, with the top time series (blue boxes) representing total global ACE, and the bottom (grayboxes) as the Northern Hemisphere ACE, with the shaded area in between corresponding to the Southern HemisphereACE. (b) Annual totals of global TC ACE (calendar year totals; red boxes), NH TC ACE (calendar year; blue boxes),
and SH TC ACE. Note that totals defined for the SH are for July – June due to the offset tropical cyclone season straddlingthe calendar year change. Global ACE data is included through May 31, 2011.

and NH below average TC ACE, which initially began in2007 [Maue, 2009].
[7] The most prolific calendar year totals of global ACE
occurred during the 1990s peaking with the exceptional
1997–98 El Niño event, and again during 2002–2005(Figure 1b). The recent four years of global ACE haveremained near or below 600 units and are closer to values
observed in 1981, 1988, and 1999–2001. From the end of
the most recent peak in 2005–06, global ACE has decreasedby almost half and is currently at a three decades’ low. Thisdecrease resulted from four consecutive years of below
average NH ACE from 2007 to 2010 combined with average to below‐average SH ACE during the same period
including the most recent season ending May 2011. Similar
conclusions arise when 12‐month running sums of TC ACE
are examined globally (Figure S1).
[8] Table 1 records the global ACE for each hemisphereand active TC basins for the 32‐years inclusive between
1979 and 2010. During the entire period, the NH (SH)supplied 72% (28%) of total global ACE with the WNP
contributing the largest proportion at 39%. Two separate
16‐year periods are then subjectively chosen (1979–94 and1995–2010) to coincide with the onset of the NATL active

period [Bell and Chelliah, 2006] in 1995 typically associated with a transition to the warm phase of the Atlantic
Multidecadal Oscillation (AMO). Remarkably, a comparison of each respective period’s ACE for both hemispheresshows nearly identical quantities (Table 2).
[9] While the hemisphere ACE totals for each period
have remained nearly the same, significant differences areobserved in the contribution of the NATL to the NH and
Table 1. Global TC ACE Apportioned According to Basin or
Hemisphere and the Percentage Compared to the Global or Northern
Hemisphere Totala

GlobalNorth PacificWestern North PacificEastern North PacificNorth Atlantic
North IndianSouthern HemisphereNorthern Hemisphere

Total ACE

% of Global% of NH

24422
13694
9622
4072
3355
547
6826
17596

100
56.1
39.4
16.7
13.7
2.2
28.0
72.0

‐
77.8
54.7
23.1
19.1
3.1
‐
100

a
Units: 104 kts2. ACE values are tabulated for the 32‐year period ofJanuary 1979 – December 2010.

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Table 2. Global and NH ACE Contribution (%) From the North
Atlantic, Eastern North Pacific, Western North Pacific, and North
Indian Ocean Basins for Two 16‐Year Periods: 1979–1994 and1995–2010a

NATL % of global
NATL % of NH
EPAC % of global
EPAC % of NH
WPAC % of global
WPAC % of NH
NIO % of global
NIO % of NH
Global Total
NH Total

1979–1994

1995–2010

9.0
12.5
20.5
28.6
40.6
56.5
1.7
2.4
12349
8872

18.6
25.7
12.7
17.6
38.2
52.8
2.7
3.8
12073
8723

a

The global and NH ACE quantities (units: 104 kts2) are provided for thetwo periods.

global ACE totals, which underwent an almost exact doubling in proportion. This coincides with a nearly inverse
decrease in the NEP and a smaller reduction in the WNP.
Maue [2009] discussed the observed inverse nature of TC
frequency and ACE between the neighboring NEP and
NATL basins positing that the large‐scale influences of
ENSO and the Atlantic Meridional Mode affected African
easterly wave lifecycles through upper‐level vertical wind
shear modulation [e.g., Kossin et al., 2010]. Indeed 2010 was
an excellent example of a very active or prolific NATL hur-

L14803ricane season against an inactive or meager NEP and WNP
TC year as previously observed during intense and/or prolonged La Niña events [Lander and Guard, 1998].
[10] While the NATL accounted for about 14% of global
ACE on average from 1979–2010, the calendar year proportion was highly variable (standard deviation of 8.5%)
ranging between 2% and 32% (Figure S2). Indeed, the
NATL contribution during 2010 of nearly one‐third ofglobal ACE clearly beats out 1996 and 1999 for thehighest percentage since at least 1979. Much of thisobserved dramatic increase in NATL ACE is associatedwith a strong upward trend in the frequency of deep
tropical systems called Cape Verde hurricanes during thepast two decades [Kossin et al., 2010]. Cape Verde origin
storms typically have long lifecycles and peak at highintensities, thus contributing considerable ACE to theseasonal totals.

4. Global Tropical Cyclone Frequency[11] A total of 69 TCs were counted during calendar year
2010, the fewest observed in the past 40‐years with reliablefrequency data (Figure 2a). In that historical period, 12‐monthrunning‐sums of the number of global TCs of at leasttropical storm force has averaged 87 with an interquartile
range of 82–92 (median 87). The minimum number of
64 TCs was recently tallied through May 2011, as a strong
La Niña event waned. Conversely, the most recent peak of

Figure 2. Global tropical cyclone frequency, monthly from 1970–2011: (a) 12‐month running sums including all TCs (toptime series), and only hurricane force TCs (bottom time series). (b) Same as Figure 2a except for 24‐month running sums.
Red linear trend lines are drawn on each time series. Global frequency data is included through May 31, 2011.
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L14803Figure 3. Relationship between ENSO indices and TC ACE, monthly 1979–2010. (a) Standardized 24‐month runningsums of North Pacific TC ACE (blue), PDO (green), and MEI (black). (b) Same as Figure 3a but for overall global TC
ACE (blue), cold tongue ENSO index NCT (black), and the warm‐pool ENSO index NWP (red).

108 observed during the 12‐months ending February 1997
corresponded with the onset of the record El Niño event.
There is no significant linear trend in the frequency of
global TCs (of at least tropical storm strength), which inagreement with previous analysis [e.g., Webster et al., 2005;
Wang et al., 2010].
[12] When only considering TCs of hurricane force (HF)
and above (maximum wind speed >64 knots; Figure 2abottom time‐series) the variability is also large with amean (standard deviation) of 48 (+7) TCs. A minimum of
29 was reached both in 12‐months ending in September
1977 and more recently from August 2008 to July 2009.
Using 24‐month running sums (Figure 2b), a record low
number of global HF TCs of 69 was recently recorded inJuly 2010. A 24‐month running sum total of 145 TCs is
the historical record low reached during the period ending
May 2011.
[13] The NATL produced 19 (12) tropical storms (hurricanes) in 2010, which was a well‐above average number.
However, the WNP and ENP basins were both historicallyquiet with a combined total of 22 TCs of which 12 exceeded
HF, nearly 50% below the previous 40‐year average. With
the inclusion of the Northern Indian Ocean basin, theNH during generated 46 TCs in 2010, the fewest since
1977.
[14] Elucidating upon aspects of the ACE metric, TCs thatfail to reach HF tend to be of comparatively short durationand contribute significantly less to the global ACE total in agiven year [e.g., Villarini et al., 2011]. It is noteworthy thatwhen 24‐month running sums of global ACE and the fre-quency of HF TCs are compared (Figure S3), the two time‐series are closely correlated (r = 0.90).

5. Large‐Scale Climate Modulation[15] The relationship between TC ACE and modes oflarge‐scale climate variability including the PDO and ENSO
is shown with standardized time series of 24‐month runningsums or accumulated quantities (Figure 3). The following
analysis discusses the relationship between TC ACE, thePDO, and three different ENSO indices since 1979. It isreasonable to expect that the collective global distribution ofTCs, their genesis locations, and tracks are temporally andspatially modulated by these large‐scale climate modes,
which directly relate to changes in lifecycle duration andintensity [e.g., Gray, 1984; Camargo and Sobel, 2005;
Kossin et al., 2010]. Moreover, global tropical storm days
[Wang et al., 2010] and WNP intense TC frequency fluctuations [Chan, 2008] have been shown to be regulated by
ENSO and PDO.
[16] Considerable research has focused on the various
spatiotemporal characteristics of ENSO, which has been
observed to undergo variations on decadal time scales. Two
distinct forms of El Niño have been described including the
canonical El Niño (cold tongue, CT) warming of the eastern
Pacific [Rasmusson and Carpenter, 1982] and a central
Pacific warming with major SST anomalies shifted westward near the warm‐pool (WP) edge [Ashok et al., 2007;
Yeh et al., 2009]. During the past two decades, the occurrence of central Pacific warming episodes referred to as El

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Niño Modoki events has noticeably increased beginningwith the prolonged 1990–1994 El Niño event while CT
events have become less frequent [Lee and McPhaden,
2010].
[17] To differentiate between the cold tongue (CT) andthe central Pacific warm‐pool (WP) flavors of El Niño, Ren
and Jin [2011] devised a piecewise‐linear combination of
the familiar Niño3 and Niño4 indices. Their definitions of
NCT and NWP capture the ENSO regime change of 1976–77
as well as the increasing prevalence of WP or El Niño
Modoki events since 1990. NCT = Niño3 − a Niño4 and
NWP = Niño4 − a Niño3, where a = 2/5 when Niño3*Niño4 > 0 and a = 0, otherwise. The two new indices are
not well correlated, and were devised to be independent (r =0.07), an important difference between Niño3 and Niño4,
which tend to conflate CT and WP events. The Multivariate
ENSO Index (MEI) [Wolter, 1987] is well correlated withNCT since 1979 on monthly time scales (r = 0.83), yet
poorly correlated with NWP (r = 0.06).
[18] The relationship between TC ACE and modes oflarge‐scale climate variability including the PDO and ENSO
indices is shown with standardized time series of 24‐monthrunning sums or accumulated quantities (Figure 3). After theobserved Pacific climate shift of 1989 [Yeh et al., 2011],
combined North Pacific ACE (WNP+ENP), which represents on average 56% of global ACE (Table 2), isremarkably well‐correlated with MEI (Figure 3a). However,during the 1980s, the correspondence is clearly less apparent
with North Pacific ACE undergoing much less variation
than either the PDO or the MEI.
[19] When compared to the North Pacific ACE timeseries (Figure 3a, blue line), the entire global TC ACE ismuch higher after 1999 corresponding to heightened
NATL basin TC activity (Figure 3b, blue line). Indeed, the
NWP representing the El Niño Modoki (red line) remains
strongly positive as the global TC ACE most recently
peaked in 2004–05. The cooling in the tropical Pacificshown in both the NCT and NWP indices has correspondedwith the decrease in overall global TC ACE during the last
pentad.
[20] Recent research has also identified the influence of El
Niño Modoki on TC activity in the NATL [Kim et al., 2009]
and the entire North Pacific basin [Chen and Tam, 2010;
Kim et al., 2011], yet the observed number of WP (Modoki)
events is still small due to the short data record. In related
research, Yeh et al. [2010] found that the correlation
between WNP TC frequency and tropical Pacific SST
indices varied greatly between two separate time periods,
1979–89 and 1990–2000, associated with the evolving
relationship between the Niño3 and Niño4 ENSO indices.
[21] On decadal time scales, the Pacific climate shifts of
1976–77 and 1988–89 have been related to the concomitantevolution of the two leading EOF modes of SST variability,
the PDO and North Pacific Gyre Oscillation (NPGO) [Yehet al., 2011]. Decadal variations in the NPGO, which has
been enhanced since 1989, have been linked to SST
anomaly patterns that closely resemble El Niño Modoki
events [DiLorenzo et al., 2010]. The cursory analysis presented in Figure 3 is in accord with the aforementioned
studies but includes Pacific and global ACE comparisons to
the PDO, MEI, and two new ENSO indices [Ren and Jin,2011], which were formulated to highlight the increasingly prevalent El Niño Modoki events.

L14803[22] The most recent El Niño of boreal summer 2009 tospring 2010 did not foster enhanced TC activity in the North
Pacific tropical basins, which remained anomalously inactive. The consequent transition to a strong La Niña in boreal
summer 2010 extending through spring 2011 has also
coincided with rather tepid SH TC activity and an absenceof southern Indian Ocean TCs away from the Australiancoast. The overall global TC activity drought has continued
unabated through May 2011 in terms of both TC energy andfrequency.

6. Discussion[23] During the past three decades, variability in global
TC ACE is shown to be related to the concomitant evolutionof tropical and North Pacific interannual and interdecadal
climate modes. As the PDO phase has turned decidedly
more negative (colder) and two strong La Niñas have
evolved since 2006, recent global TC ACE has fallen below
levels associated with the previous dramatic downturn in
1999, and the lowest since at least the late 1970s. The
increasing variance of NPAC climate explained by the
NPGO and related El Niño Modoki events during the past
two decades represents a change in overall Pacific climate[Yeh et al., 2011] that has modulated overall TC activity.
[24] While many overall global and basin ACE and frequency record lows have been set, the glaring anomaly in2010 was the extremely active NATL basin, which contributed more than 30% of the annual global ACE output. In
the context of overall NH ACE variability, even with the
doubling of the NATL to NH ACE proportion since 1995,
the ENP + NATL proportion has remained constant since
1979 with the WNP coincidentally reacting to the observed
fluctuations of ENSO and PDO [Chan, 2008].
[25] It is still a fundamental research question as to what
are the atmosphere and ocean mechanisms responsible forthe observed annual global TC frequency of ∼87 storms[Frank and Young, 2007]. With the upcoming IPCC AR5
assessment and associated CMIP5 climate simulations, it iscritical to have the best possible diagnosis of periods ofglobal TC inactivity and incorporate the recent pentad of
historical lows into the context of natural and anthropogenically forced climate variability [Knutson et al., 2010].
Furthermore, research must better explain the role of tropical
cyclones in the climate system especially during this current
period of record inactivity.
[26] Acknowledgments. The author graciously recognizes NOAA/
COD funding provided to Mark A. Bourassa at COAPS/FSU and NSF
grant 0842618 awarded to Robert E. Hart at FSU. This work was greatly
improved in clarity and content by two reviewers, as well as P. Klotzbach
and R. Hart.
[27] The Editor thanks the two anonymous reviewers for their assistance in evaluating this paper.

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R. N. Maue, Center for Ocean and Atmosphere Studies, Department ofEarth, Ocean and Atmospheric Science, Florida State University, PO Box
3064520, Tallahassee, FL 32306‐4520, USA. (rmaue@fsu.edu)

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