GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L05805, doi:10.1029/2008GL035946, 2009

Northern Hemisphere tropical cyclone activity
Ryan N. Maue1
Received 8 September 2008; revised 9 December 2008; accepted 5 January 2009; published 4 March 2009.

[1] Recent historical Northern Hemisphere (NH) tropical
cyclone (TC) inactivity is compared with strikingly large
observed variability during the past three decades. Yearly
totals of the combined active-basin NH accumulated cyclone
energy (ACE) are highly correlated with boreal spring seasurface temperature (SST) in the North Pacific Ocean and are
representative of an evolving dual-gyre, trans-hemisphericcorrelation pattern throughout the calendar year. The observed
offsetting nature of Eastern Pacific and North Atlantic basin
ACE during the past three decades and a strong dependenceof combined Pacific TC activity upon the El Niño-Southern
Oscillation reflect the interrelated modulation of overall
NH integrated TC energy by large-scale modes of climate
variability. Thus, the quiescent period of overall integrated
NH TC ACE continuing throughout 2008 is not unexpectedin the context of previous periods of colder Pacific SSTs.
Citation: Maue, R. N. (2009), Northern Hemisphere tropical
cyclone activity, Geophys. Res. Lett., 36, L05805, doi:10.1029/
2008GL035946.

1. Introduction[2] During 2007, the major active basins of the Northern
Hemisphere (NH) combined for historically low levels of integrated tropical cyclone (TC) energy (Figure 1) as measured
by the accumulated cyclone energy index (ACE) [Bell andChelliah, 2006]. This remarkable NH TC inactivity has motivated an investigation into potential explanations for the strikingly large variability observed during the past three decades.
[3] Emanuel [2005] utilized the TC power dissipationindex (PDI) metric, analogous to ACE, and discovered
highly-significant correlations with low-frequency local
summer SST during the past 30 years, mainly in the NA. In
addition to the potential role of global warming for the observed TC power increases in the NA [e.g., Emanuel, 2005],
other potential climate factors posited have included theAtlantic Multidecadal Oscillation (AMO) [e.g., Goldenberg
et al., 2001], the Atlantic Meridional Mode (AMM) [Kossin
and Vimont, 2007], which is the leading mode of coupledocean atmosphere variability in the Atlantic, as well as the
El Niño-Southern Oscillation (ENSO) [e.g., Gray, 1984;
Landsea et al., 1998; Bell and Chelliah, 2006]. In theWestern North Pacific (WP) basin, both ENSO and thePacific Decadal Oscillation (PDO) have been shown tomodulate intense typhoon activity on multidecadal timescales [Chan, 2007].
[4] Here, the long-term climatology of NH TC activity is
examined as a whole and established inter-basin TC relationships [Frank and Young, 2007] are incorporated to link NH
1

Center for Ocean-Atmospheric Prediction Studies, Department ofMeteorology, Florida State University, Tallahassee, Florida, USA.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2008GL035946

ACE with large-scale climate variability. Throughout the
study, NH ACE is defined as the combined total of eachindividual active basin during the calendar year. It is discovered that NH ACE is closely related to the evolution of SST
structure in the Pacific and especially well-correlated withboreal spring Gulf of Alaska SST. However, in the NA, aconclusion that changes in North Pacific (NP) climate variability are directly impacting hurricane behavior cannot be
firmly established. Nevertheless, there is evidence to suggestthat the recent uptick in NA activity primarily since 1995
corresponds with a concomitant decrease in Eastern Pacific(EP) integrated hurricane energy. The downturn in EP and
WP ACE is consistent with previous periods of overall colder
Pacific SSTs and generally shorter-lived, weaker storms. If
combined NH TC ACE variability simply serves as a proxy
for NP SST and its attendant large-scale, low-frequency climate variability in a similar way as MDR SST serves as aproxy for NA ACE, then new insight may be gained into therespective role of tropical cyclones in climate.

2. Northern Hemisphere Tropical Cyclone
Accumulated Cyclone Energy[5] The ACE integrated TC activity metric [Bell andChelliah, 2006] convolves frequency, duration, and intensity
as provided in the historical best-track datasets (auxiliarymaterial1), and all three components co-vary in each basin
and are not independent. Klotzbach [2006] reported that therecord of NH and global ACE during the 20-year period of
1986– 2005 failed to exhibit a significant trend with large
variability. Yet, the remarkable correlations between ACE
and tropical SST [Emanuel, 2005] or relative SST [Swanson,
2008] in the main development region (MDR) of the NAsuggested that ACE (or PDI) reflects a more fundamental
metric of global climate than its individual components separately [Emanuel, 2007].
[6] The large NH TC ACE variability is a synthesis of theindividual basin contributions. Figure 1 demonstrates throughstacked bars calendar year sums of WP+Northern Indian
Ocean (NIO) (bottom), EP (middle), and NA (top) ACE.
NH ACE for the calendar year 2007 amounted to a value of
386 (x104 knots2), less than half of 2004’s output when the
WP basin itself exceeded 2007’s overall total. Preliminary
operationally available TC tracks through December 2008
show similar levels of continuing inactivity with NH ACE
totals at 400. The specific ACE and hurricane days (HDAYS)
breakdown per basin and yearly mean values according toclimatology (1981 – 2006) are highlighted in Table 1. Comparison with the box-and-whisker inset of Figure 1 clearlyshows that 2007 and 2008 will reside in the lowest quartileof the NH ACE distribution since 1981. Sharply lower
1
Auxiliary materials are available in the HTML. doi:10.1029/
2008GL035946.

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L05805Figure 1. Raw values of Northern Hemisphere and individual basin ACE from 1981 – 2007 (shaded bar chart). The stacked
bars include ACE for the WP+NIO (bottom), EP (middle), and NA (top; darkest) basins. The inset box and whisker plot shows
the distribution of NH ACE from 1981 –2006 with the median separator between the two rectangles representing the areas
encompassed by the first and third quartiles. The total distribution is indicated by the barbed endpoints. Ratio of NH ACE
occurring in the EP+NA in percent.
NH ACE was experienced in 1977 (250), with admittedly
more uncertain data prior to the launch of Japan’s GMS-1
geostationary satellite in July 1977 (prior temporal sampling
was less frequent with only polar orbiters).
[7] Since yearly ACE is the integration of individual storm
events and their respective intensity and duration, long-lived(weeks) major TCs (Category 3+) with trans-basin tracks,
while less frequent than weaker storms, contribute disproportionately more to the ACE calculation. Indeed, the distributionof NH ACE per storm in the record examined (1981 – 2008)
clearly shows a long tail toward high-ACE storms (Figure S1).
For the 2007 – 2008 period, the median (75th percentile) ACE
per storm was 3.0 (10.5) compared to the climatologicalvalue of 5.2 (13.8). Thus, further examination is warranted
for understanding not only the evolution of TC maximum
winds throughout the data record, but also individual ACE
per storm (Figure S1), which can be significantly influenced
by ENSO especially in the WP [Camargo and Sobel, 2005]
and the AMM in the NA [Kossin and Vimont, 2007].
[8] The development of a strong La Niña event during
2007 – 2008 clearly affected Pacific basin TC activity [Chan,
2000]. For example, TC genesis locations in the WP areshifted southeastward towards the dateline during El Niñoevents which generally portend longer-lived and more intenseTCs with some originating in the Central Pacific (i.e., Ioke
in 2006). Modified steering currents are also more readilyfavorable for these lower-latitude storms to recurve into theextratropics and garner considerable ACE during long lifecycles [Camargo and Sobel, 2005].

3. Atlantic and Pacific Basin Relationships[9] Understandably, less attention has been focused upon
the dramatic decline in EP hurricane activity and ACE during
the past decade [e.g., Klotzbach, 2006; Kossin et al., 2007]
compared to the NA, which experiences more numerous
storms with significant societal impact. Indeed, average combined EP+NA ACE (Figure S2) in two separate 13 years
periods (1982 –1994) and (1995 – 2007) is nearly identicaland exhibits no significant trend. Figure 1 shows the percentage contribution of EP+NA ACE to the NH total in simple
ratio terms without any temporal filtering. It should be notedthat there is considerable variability in this quantity and it is areflection of the WP basin ACE activity since it is the residualof the full NH total. The NIO basin contributes on average
3% of NH ACE but its coastal nations experience a disproportionate frequency of devastating, intense cyclones like
Sidr in 2007 and Nargis in 2008.
[10] The relative stability of the EP+NA/NH ratio has notbeen previously identified and seems hardly a coincidence.
Indeed, it appears to be a consequence of well-understood
ENSO and slower PDO variability in the Pacific basin and

Table 1. Ratio of Each Basin ACE to the NH Total is Calculated for 1981 – 2006 Climatology From the Historical Best-Track Dataa

NH
NATL
WPAC
EPAC
NIO

2007 Year ACE

1981 – 2006 Year Mean ACE(% of NH Total)

2007 HDAYS(*0.25)

1981 – 2006 Mean HDAYS

386
72
215
53
46

585
105 (18%)322 (55%)
142 (24%)
16 (3%)

327
49
212
35
31

526
97
288
131
10

a
ACE for NH, Northern Hemisphere; NATL, North Atlantic; WPAC, North Western Pacific; EPAC, North Eastern Pacific; NIO, Northern Indian basins.
HDAYS are equivalent to each 6-hourly tropical cyclone observation of maximum sustained winds greater than 64 knots.

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the somewhat less understood relationships between quasicyclical decadal variability, ENSO, AMM, and other interannual fluctuations in the Atlantic. The offsetting nature of
the yearly ACE between the basins reasonably suggests thatthe recent NA upward trend has not occurred in a vacuum
isolated from the rest of the NH and global large-scale climate
system but may have significant and as yet poorly understoodinterplay.
[11] Even though the EP and NA are not physically linked
via the ocean, ENSO teleconnections, local coupled meridional modes [e.g., Chiang and Vimont, 2004; Kossin andVimont, 2007], and the quasi-monthly Madden Julian Oscillation (MJO) [Aiyyer and Molinari, 2008] can impact thegeographic distribution of TC genesis locations and the environmental favorability for intensification through the modification of atmospheric vertical shear and steering flow [Belland Chelliah, 2006]. The evolution of upper-level circulation
patterns associated with ENSO events directly influences theeffects of vertical shear on both the EP and NA basins[Landsea et al., 1998; Goldenberg et al., 2001]. Furthermore,
the convective phase of the MJO may enhance the cyclonicshear of the low-level zonal wind aiding in the reversal of the
background potential vorticity gradient crucial for instability
and wave energy growth [Molinari et al., 1997; Aiyyer andMolinari, 2008].
[12] Through satellite imagery, African easterly wave
(AEW) disturbances in the EP have been traced backwards
for many TC genesis events to the African coast [Frank andRoundy, 2006]. If these TC seedlings developed into NA
storms, had their circulation disrupted by vertical shear, orwere deflected out of the lower-latitude MDR intensification
zone, then the AEWs would be unavailable to the EP basin
for development. Even if the waves progress into the EP, there
is still no guarantee of development. In addition to in situ
environmental conditions, non-local modification of vertical
shear and many other aspects of the coupled atmosphereocean system by larger-scale climate will also directly impact
AEW development [Goldenberg et al., 2001; Vecchi andSoden, 2007a].
[13] The compensatory or offsetting nature of ACE during
the past three decades between the basins at least suggests
a hypothesis that the modulation of AEW activity by largescale climate variability is critically important in both basins[e.g., Bell and Chelliah, 2006; Kossin and Vimont, 2007].
However, the exact role of the various mechanisms and interbasin feedback remains to be quantified.
[14] Additional analysis finds high monthly correlations
between 24-month backwards sums of the multivariate
ENSO index (MEI) [Wolter and Timlin, 1998] and combined
EP+WP ACE especially since the late 1980s (Figure S3). As
ENSO events are locked to the seasonal cycle, they peak in
the winter after the TC active period in autumn. The utilization of 24-month sums allows for integration of TC activity
on scales closer to ENSO (2– 7 years). This has the distinct
advantage of capturing the winter and spring WP activity thatis associated with the previous or ongoing ENSO event[Camargo and Sobel, 2005]. This robust relationship between monthly ACE and MEI provides further evidence that
integrated metrics of TC activity may represent fundamental
and important aspects of the climate state, and, in this case,
Pacific ACE on ENSO.

L05805

4. Relationship With SST[15] In the NA, filtered seasonal totals of ACE are poorly
related with ENSO as shown by weak correlations of August–October Hadley Centre SST [Rayner et al., 2003] in thetropical Pacific with NA ACE (Figure 2 (top)). It is apparent
that the global warming trend in low-frequency North Atlantic
SSTs is exceptionally well correlated with the upward trending
NA ACE from 1981– 2007, which has been demonstratedby Emanuel [2005] for the NA MDR. However, the similarly high correlations in austral winter in the South Pacificdemonstrate the difficultly in attributing NA integrated TC
metrics to coincident trends in local SST [Kossin and Vimont,
2007; Vecchi and Soden, 2007b; Swanson, 2008].
[16] The entire NH ACE and global SST correlation map(Figure 2 (bottom)) for the period 1981 – 2007 exhibits astriking trans-hemispheric scale, double gyre structure withvery high correlations in the North Pacific during boreal
spring (April – June). The structure of the correlations
evolves throughout the calendar-year with maximum values
during boreal spring (Figures S4a– S4j). Remarkably, during
the previous three decades, NH TC ACE has reflected Gulf of
Alaska SST during April – June to a highly significant degree(R > 0.95). It should be noted that the strong correlations
remain irrespective of low-frequency filtering procedures,
yet the degrees of freedom are still few.
[17] As expected from Figure S3, correlations between
WP+EP ACE and SST show strong tropical Pacific sensitivity associated with ENSO (Figure S5). From an evaluation ofeach individual basin’s ACE against SST during April – June
(Figures S6a –S6d) and the calendar year (Figures S7a– S7d),
only the WP basin (which on average contributes 55% of
NH ACE) demonstrates a similar correlation pattern to thatof the overall NH ACE (Figure 2 (bottom)). Since the NA
and EPACE time series exemplify strong but opposing trends(Figure S2), relationships with global SST on long time scales
will indubitably be dominated by the observed warming trend
during the past three decades especially in the tropical North
Atlantic [Emanuel, 2005].

5. North Pacific SST Variability[18] The striking relationship between NH TC ACE andextratropical NP SST appeals to an inherent and overarching mechanism of climate variability linking the tropics andextratropics on seasonal and longer time scales. The leading
tropical mode of variability is well-established to be ENSO,
but how it manifests itself spatiotemporally with respect toNorth Pacific climate is still the subject of considerable
research [e.g., Newman, 2007]. Thus, it is hypothesized thatthe correlation pattern between NH ACE and boreal spring
NP SST arises from the slow evolution of the large-scalemodes of interannual climate variability. While the TCs
themselves directly influence in situ SST anomaly patterns,
there is currently little evidence to suggest that TCs represent
a causal mechanism for non-local, extratropical Pacific SST
variability.
[19] Previous studies have elucidated upon the dominantmodes of North Pacific climate variability including the
PDO [Mantua et al., 1997], interdecadal Pacific signal (IPS)[Frauenfeld et al., 2005], North Pacific Oscillation (NPO)[Rogers, 1981; Linkin and Nigam, 2008], and its oceanic

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L05805Figure 2. (top) Correlation map between NA ACE and August – October monthly mean SST for 1981 – 2007 with a 1-2-1
filter applied once. (bottom) Correlation map between NH ACE and April – June SST (1981 – 2007).

expression, the recently observed North Pacific Gyre Oscillation (NPGO) [Di Lorenzo et al., 2008]. The latter NPO/NPGO was posited as a quasi-decadal coupled oscillation
which is significantly correlated with fluctuations of salinity,
nutrients, and fish stocks in the Northeast Pacific. Initially
identified as the Victoria Mode [Bond et al., 2003], the
NPGO is the second EOF of North Pacific climate variability(PDO is first EOF). Indeed, the correlation pattern between
the NPGO and SST anomaly map [Di Lorenzo et al., 2008,
Figure 4b] closely resembles the structure of Figure 2 (bottom),which implicated tropical coupled dynamics in the forcing of
the NPGO and associated ecosystem changes.
[20] The NPO [Rogers, 1981; Linkin and Nigam, 2008]
is primarily driven by internal, stochastic dynamics during
boreal winter that affect the intensity of the subtropical highand therefore modulate the strength of prevailing trade winds.
In the NP, the peak variability of the atmospheric NPO mode
occurs in boreal winter through an accumulation of storm
tracks and the modulation of the semi-permanent AleutianLow.
[21] If the NP acts as a climate integrator [Newman et al.,
2003; Newman, 2007] of previous ENSO phases, then it
follows that some manifestation of the integrated TC memory
is retained in the NP SST in a proxy sense. That is to say theatmosphere-ocean conditions which favor enhanced or decreased TC activity are retained in patterns of SST variability.
Obviously the TCs themselves are not responsible for thepatterns of SST on large-scales, especially in the extratropics
during winter.

6. Discussion[22] 1. NH ACE (Figure 1) continues through 2008 at lowlevels reminiscent of other cold periods of Pacific climate
including the late-1990s and the so-called climate shift of
1977: the previous lowest ACE year prior to 2007.
[23] 2. EP+NA ACE has largely compensated during thepast three decades and exhibits no significant trend (Figure S2).
The increase in NA TC frequency and intensity mainly since1995 has corresponded to a concomitant downturn in the
EP. The overall ratio of EP+NA ACE has remained stablethroughout the period examined.
[24] 3. Running sums of 24-month of ENSO and Pacificbasin ACE correspond very closely especially during the past
20-years (Figure S3).
[25] 4. Combined NH basin ACE is highly correlated with
SST across the Pacific (Figure 2 (bottom)). Individually, only
WP ACE is well-correlated with NH ACE. The NA andEP ACE correlations with SST are dominated (inversely so)
by regions of coincident long-term warming trends in SST(Figures S5 and S6).
[26] It is not clear why the number of global cyclones eachyear (80 – 90) has remained a long-term constant of nature.
Previous analysis showed that the interannual variability in

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large-scale climate patterns affected TC formation regions
and intensity in different ways [Frank and Young, 2007]; thisconclusion is clearly in accord with previous TC climatology
studies including the results presented here. Global and NH
ACE are not a constant of nature [Klotzbach, 2006], andundergo significant variability as exemplified by the tepid
totals of 2007 and 2008. Thus, the enhanced longevity andproclivity of intense TCs dominate in so-called active years,
which contribute considerable ACE, are the result of genesis
location shifts and the beneficial prevailing environmental
conditions such as weakened vertical shear modulated bylarge-scale climate variability.
[27] The structure of the Pacific correlation pattern of SST
with NH ACE suggests that considerable predictive information for the upcoming NH TC year is present in boreal
spring. Furthermore, when acknowledging the role of ENSO
in modulating WP+EP TC ACE, winter midlatitude storm
activity imprinting upon boreal spring NP SST, and theinterrelated basin climatology described above, NA ACE
activity falls out as a residual of the NH minus WP+EP ACE
total. This suggests an additional null hypothesis to the NA
global warming and hurricanes puzzle: the NA increase inactivity since 1995 is part of large-scale hemispheric climate
variability with clear association to the EP against a backdropof local and relative NA tropical SST increases.
[28] To understand the evolution of TCs in a futureclimate, an adequate understanding of the mechanisms controlling current and past TC activity is critical to identifying
exactly what features of climate will indeed change, especially in the Pacific basin [Vecchi and Soden, 2007a; Vecchi
et al., 2008]. The evolution of NP SST signals may provide
a fruitful area for further research into climate modulation of
global TC variability.
[29] Acknowledgments. The author acknowledges funding provided
by NOAA OCO to Mark A. Bourassa of the Center for Ocean-AtmosphericPrediction Studies (COAPS) at FSU. The paper benefited greatly fromfruitful discussions with Mark A. Bourassa, Lance F. Bosart, Allan J.
Clarke, Robert E. Hart, Chris W. Landsea, and two reviewers of the original
manuscript. This work originated from a term project in Clarke’s equatorial
dynamics course.

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R. N. Maue, Center for Ocean-Atmospheric Prediction Studies, Departmentof Meteorology, Florida State University, 404 Love Building, Tallahassee,
FL 32306, USA. (rmaue@met.fsu.edu)

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