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The SST Gradient between the Southwestern Pacific and the Western Pacific
Warm Pool: A New Factor Controlling the Northwestern Pacific
Tropical Cyclone Genesis Frequency
RUIFEN ZHAN
Laboratory of Typhoon Forecast Technique, Shanghai Typhoon Institute of China Meteorological Administration,
Shanghai, China
YUQING WANG
International Pacific Research Center, and Department of Meteorology, School of Ocean and Earth
Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii
MIN WEN
State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China
(Manuscript received 14 November 2012, in final form 14 January 2013)
ABSTRACT
The sea surface temperature gradient (SSTG) between the southwestern Pacific Ocean (408–208S, 1608E–
1708W) and the western Pacific warm pool (08–168N, 1258–1658E) in boreal spring has been identified as a new
factor that controls the interannual variability of tropical cyclone (TC) frequency over the western North
Pacific Ocean (WNP). This SSTG can explain 53% of the total variance of the WNP TC genesis frequency
during the typhoon season for the period 1980–2011. The positive SSTG anomaly produces an anomalous
cross-equatorial pressure gradient and thus anomalies in low-level southward cross-equatorial flow and
tropical easterlies over the central-western Pacific. The anomalous easterlies further produce local equatorial
upwelling and seasonal cooling in the central Pacific, which in turn maintains the easterly anomalies
throughout the typhoon season. These dynamical/thermodynamical effects induced by the positive SSTG
anomaly lead to a reduced low-level cyclonic shear, increased vertical wind shear, and weakened monsoon
trough over the WNP, greatly suppressing WNP TC genesis during the typhoon season. This implies that the
spring SSTG could be a good predictor for WNP TC genesis frequency.
1. Introduction
In 2004, the western North Pacific Ocean (WNP) experienced a very busy typhoon season with 29 named
tropical cyclones (TCs), while in 2010 the WNP was unusually quiet with only 14 TCs forming. This indicates
strong interannual variability in TC genesis frequency
over the WNP (Landsea 2000; Ying et al. 2011). Over the
past 2–3 decades, many efforts have been made to understand the factors controlling the interannual variability
of WNP TC genesis frequency and the involved physical
mechanisms. For example, El Niño–Southern Oscillation
Corresponding author address: Dr. Yuqing Wang, IPRC/SOEST,
University of Hawaii at Manoa, RM POST 409G, 1680 East-West
Road, Honolulu, HI 96822.
E-mail: [email protected]
DOI: 10.1175/JCLI-D-12-00798.1
Ó 2013 American Meteorological Society
(ENSO) (Chan 2000; Wang and Chan 2002), the quasibiennial oscillation (QBO) in the equatorial stratosphere
(Chan 1995), and the Antarctic Oscillation (AAO) (Ho
et al. 2005) have been identified as factors affecting the
interannual variability of WNP TC genesis frequency.
Recently, a causal relationship between the eastern Indian
Ocean (EIO) sea surface temperature (SST) anomaly
(SSTA) and WNP TC genesis frequency has also been
reported (Zhan et al. 2011a,b; Tao et al. 2012).
Based on the identified relationships between TC
activity and climate modes/variations, skillful seasonal
forecasts for TC frequency have been issued by some
major meteorological agencies around the world
(Camargo et al. 2010). Unfortunately, the prediction
errors for annual TC numbers over the WNP have remarkably increased in recent years (Zhan et al. 2012).
1 APRIL 2013
ZHAN ET AL.
The increasing errors might be related to two facts: 1) the
statistical relationships between TC activity and various
large-scale climate modes are not stable with time and
2) the predictors identified could not fully explain the
interannual variability of TC frequency. Take the QBO–
WNP TC relationship, for instance: a recent reexamination
has shown that the significant relationship found in earlier
studies is no longer present in recent years (Camargo and
Sobel 2010). Thus, it is necessary to discover any new
factors and to understand how they contribute to the interannual variability of the WNP TC genesis frequency.
In the Atlantic Ocean, the SST gradient (SSTG) between the North and South Atlantic [Atlantic meridional
mode (AMM)] is shown to modulate seasonal hurricane
activity (Kossin and Vimont 2007; Vimont and Kossin
2007). Previous studies have also revealed that the SST
contrast between the Northern and Southern Hemispheres exerts a significant impact on the tropical Pacific
climate (Chiang et al. 2008; Chiang and Friedman 2012).
A drying–wetting variation of the Northern Hemisphere
monsoon and a south–north shift of the intertropical
convergence zone (ITCZ) are especially pronounced
(Chiang and Friedman 2012). Since WNP TC genesis is
directly related to the tropical Pacific’s thermodynamical
and dynamical conditions, the interhemispheric thermal
gradient may also affect WNP TC genesis. In this study,
we will focus on the possible effect of the cross-equatorial
thermal gradient between the southwestern Pacific
Ocean (SWP) and the western Pacific warm pool (WWP)
on WNP TC activity. We will show that this thermal
gradient in boreal spring is strongly correlated with the
WNP TC genesis frequency during the typhoon season
and could be a new predictor in statistical or hybrid
statistical–dynamical models for seasonal prediction of
WNP TC activity.
2. Data and methodology
Three best-track TC datasets over the WNP, from the
Shanghai Typhoon Institute of the China Meteorological Administration (CMA), the Joint Typhoon Warning
Center (JTWC), and the Regional Specialized Meteorological Center of the Japan Meteorological Agency
(JMA), for the period 1980–2011 were used. TCs with at
least tropical storm intensity (maximum sustained 10-m
wind speed $17 m s21) over both the South China Sea
(SCS) and the WNP (hereinafter referred to simply as
WNP TCs) were considered in this study. Since about
80% of WNP TCs occur from June to October (JJASO),
we defined JJASO as the typhoon season.
The monthly mean extended reconstructed SST
(ERSST) analyses with a 2.58 3 2.58 horizontal resolution from the National Oceanic and Atmospheric
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Administration (NOAA) (Smith and Reynolds 2003),
outgoing longwave radiation (OLR) from NOAA
(Liebmann and Smith 1996), and wind and sea level
pressure (SLP) fields from the National Centers for
Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses (Kalnay
et al. 1996) for the period 1980–2011 were also used.
According to the key regions where the SST is significantly correlated with the WNP TC genesis frequency in
the correlation map (not shown), we defined the thermal
gradient (SSTG) between the SWP and WWP as the
difference between the SST averaged over 408–208S,
1608E–1708W (SWP) and that averaged over 08–168N,
1258–1658E (WWP) (as marked in Fig. 3a, introduced
below). In the following discussion, in terms of seasons
we refer to the seasons in the Northern Hemisphere.
3. Relationship between the SSTG and the WNP
TC frequency
Figure 1a shows the correlations of the WNP TC
genesis frequency during the typhoon season from the
CMA best-track data with the SSTG indices from the
preceding October–August. Before March, the correlation coefficients are low. A negative correlation becomes
significant at the 99% confidence level from March and
reaches the highest in April. The significant correlation is
maintained until July. Since the negative correlation is
the highest in spring (March–May), we will focus only on
the influence of the spring SSTG in our following analyses. Figure 1b shows the time series of the normalized
WNP TC genesis frequency during the typhoon season
and the spring SSTG. An out-of-phase relationship is
prominent with a temporal correlation coefficient of
20.73. That means that the variation of the spring SSTG
can explain as much as 53% of the total variance of the
variability in the WNP TC genesis frequency.
To confirm this relationship, Table 1 shows the correlations of the WNP TC frequency during the typhoon
season from three best-track datasets with the spring
SSTG, and the individual SSTs averaged in the SWP and
the WWP in spring. All of the three original best-track
datasets show significant negative correlations of the
WNP TC genesis frequency with the SWP SST and the
SSTG, well above the 99% confidence level. In particular, the correlation coefficients with the SSTG reach
20.73, 20.64, and 20.61 for the CMA, JMA, and JTWC
datasets, respectively. The correlation between the TC
genesis frequency and the WWP SST, however, is quite
weak. Results with the interdecadal variability removed
based on a 9-point Gaussian filter (Table 1) suggest that
the SWP (WWP) SSTA mainly modulates the interdecadal (interannual) variability of the WNP TC genesis
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TABLE 1. Correlation matrix among the WNP TC genesis frequency (TC_Fr) during the typhoon season (June–October) for the
period 1980–2011 derived from the CMA/JMA/JTWC best-track
data, SSTA indices in the SWP and the WWP, and the SSTG index
in boreal spring on the original data and on the data (on interannual time scales) with the interdecadal variations removed
based on a 9-point Gaussian filter. The significant coefficients over
the 99% confidence level (60.45) are in boldface.
SWP
WWP
SSTG
TC_Fr (original)
(CMA/JMA/JTWC)
TC_Fr (interannual)
(CMA/JMA/JTWC)
20.61/20.53/20.54
20.11/20.10/20.14
20.73/20.64/20.61
20.33/20.22/20.25
0.60/0.58/0.50
20.69/20.57/20.62
4. Possible physical mechanism
FIG. 1. (a) Correlations between the WNP TC genesis frequency
during the typhoon season (June–October) and monthly SSTG
between the SWP (408–208S, 1608E–1708W) and the WWP (08–168N,
1258–1658E) from the preceding October–August during 1980–2011.
(b) Time series of the normalized WNP TC genesis frequency (TCF)
during the typhoon season from the CMA best-track data and the
SSTG in spring for 1980–2011. The solid transverse line in (a) indicates correlations significant at the 99% confidence level based on
a Student’s t test.
frequency. Note that the significant correlation of the
TC genesis frequency with the SSTG remains regardless
of whether their interdecadal variability is removed. In
this sense, the spring SSTG is a better predictor for
seasonal prediction of the WNP TC genesis frequency
than the individual SWP and WWP SSTs. To further
examine the effects of interdecadal variations, we extended our analysis back to 1951, the earliest time when
the best-track data are available from all three agencies.
The results show that there is a strong interdecadal
modulation of this relationship, with correlations before
1975 being very weak (not shown). This indicates the
unsteady nature of the statistical relationship between
predictors and predictand and could explain why the
errors in predicting the seasonal TC frequency over the
WNP increased greatly in recent years because none of
the evaluated statistical schemes has used the SSTG as
a predictor for the basin (Zhan et al. 2012). The reasons
for this interdecadal modulation will be examined in our
future work.
Figure 2 shows the regressed seasonal mean 850-hPa
wind, OLR, SLP, 850-hPa relative vorticity, vertical wind
shear between 200 and 850 hPa, and 500-hPa omega
during the typhoon season with respect to the spring
SSTG. Over the tropical western Pacific, the positive
spring SSTG anomaly raises SLP; induces low-level
anomalous tropical easterlies and southward crossequatorial flows centered at 1258 and 1508E, respectively;
increases local vertical wind shear; and induces anomalous midtropospheric subsidence over the WNP. The
tropical easterly anomalies with anticyclonic shear reduce
the low-level cyclonic vorticity over the main WNP TC
genesis region. These dynamical responses to the spring
SSTG anomaly would suppress WNP TC genesis.
In addition, we also calculated the correlation between
the spring SSTG and the WNP summer monsoon index
(Wang and Fan 1999) during 1980–2011. The correlation
coefficient reaches 20.43, which is significant at the 95%
confidence level, suggesting that the positive SSTG
anomaly in spring can lead to the weakened WNP summer monsoon. Since the WNP summer monsoon trough
contributes significantly to the WNP TC genesis (Ritchie
and Holland 1999; L. Wu et al. 2012), the weakened
WNP summer monsoon associated with the positive
SSTG anomaly would further suppress TC genesis over
the WNP.
The positive spring SSTG anomaly also induces
anomalous diabatic cooling in the central equatorial
Pacific as inferred from the regressed OLR field in
Fig. 2a. The simple model from Gill (1980) suggests
that an equatorial cooling source in the midtroposphere
can produce anomalous low-level easterlies to its west
with anticyclonic circulations on its northwestern and
southwestern flanks as a result of the equatorial Rossby
wave response. Therefore, the anomalous diabatic cooling
in the central equatorial Pacific may enhance the anomalous tropical easterlies induced by the positive SSTG
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FIG. 2. Regression patterns of (a) the seasonal mean 850-hPa winds (m s21; vectors), OLR
(W m21; shaded), and SLP (hPa; black contour) and (b) 850-hPa vorticity (Vor; 1025 s21; green
dashed contour), vertical wind shear between 200 and 850 hPa (VWS; m s21; shaded), and
500-hPa omega (1022 Pa s21; black contour) during the typhoon season with respect to the
spring SSTG. Only areas where the difference is statistically significant at the 95% confidence
level based on the F test are shown. In the OLR field, the shaded values are significantly
positive. In the VWS field, the significantly positive (negative) values are shaded in red (blue).
anomaly and the anomalous anticyclone over the main
WNP TC genesis region, which is unfavorable for TC
genesis. That means that the combined dynamical and
thermodynamical effect associated with the positive spring
SSTG anomaly greatly suppresses WNP TC geneses.
To address how the spring SSTG affects the dynamical
and thermodynamical conditions over the main WNP
TC genesis region during the typhoon season, we show
in Figs. 3c and 3e the regressed seasonal mean 850-hPa
winds and SLP in spring with respect to the spring SSTG.
In spring, the positive SSTG anomaly gives rise to a crossequatorial pressure gradient west of about 1708E with low
pressure over the warmed southern subtropics and high
pressure over the cooled northern subtropics. Previous
studies have suggested that the cross-equatorial pressure
gradient is important for driving the cross-equatorial flow
(Mahrt 1972; Hoskins and Wang 2006). Thus, such an
anomalous pressure distribution in response to the spring
SSTG anomaly drives the anomalous southward crossequatorial flow and equatorial easterlies in the lower
troposphere (Fig. 3c). This suggests that the dynamical
environment during the typhoon season shown in Fig. 2
has been preconditioned in spring.
To confirm the above result, several numerical experiments were conducted using the University of Hawaii
International Pacific Research Center (IPRC) Regional
Atmospheric Model (iRAM). The details of the model
can be found in Wang et al. (2007). The model has been
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FIG. 3. The seasonal mean 850-hPa winds (m s21) in the spring of 2004 from (a) the NCEP reanalysis and (b) the
CTRL experiment; the regressed seasonal mean (c) 850-hPa winds (m s21) and (e) SLP (hPa) during 1980–2011 with
respect to the spring SSTG; and differences in the seasonal mean (d) 850-hPa winds (m s21) and (f) SLP (hPa)
between the WS_2004 and the CS_2004 runs for the spring of 2004. In (a), the solid box indicates the domain of the
SWP, and the dashed one indicates that of the WWP.
used in the studies of dynamically downscaled WNP TC
activity and has shown good skill (Zhan et al. 2011b; C.-C.
Wu et al. 2012). The model settings are the same as those
in Zhan et al. (2011b), except that the model domain is
508S–57.58N, 1008E–1608W with a grid spacing of 0.58 in
this study. We conducted a control simulation (CTRL)
and two sensitivity experiments (CS_2004 and WS_2004)
for 2004, a year in which both the TC frequency during the
typhoon season and the spring SSTG are nearly normal.
In the CS_2004 run, a negative SSTG anomaly of 18C was
imposed by adding 10.38C to the SSTs over the WWP and
20.78C to the SSTs over the SWP onto the observed SSTs
in the CTRL run, while in the WS_2004 run a positive
SSTG anomaly of 18C was imposed by adding 20.38C to
the SSTs over the WWP and 10.78C to the SSTs over the
SWP. The above negative and positive SSTG anomalies
were chosen based on observations, which shows a greater
anomaly in the SWP than in the WWP. The model was
initialized at 0000 UTC 21 February 2004 and integrated
through the end of May 2004.
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ZHAN ET AL.
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FIG. 4. Lag correlations between the SST gradient in spring and monthly mean (a) SLP gradient between
(108–258N, 1058–1658E) and (258–108S, 1058–1658E); (b) cross-equatorial flow averaged over 58S–08, 1008–1308E;
(c) equatorial westerly averaged over 58S–58N, 1408E–1808; and (d) OLR averaged over 58S–58N, 1608E–1808 from
April to October during 1980–2011. The solid (dashed) transverse line indicates correlations significant at the 95%
(99%) confidence level.
Figures 3a and 3b show the seasonal mean 850-hPa
winds in the spring of 2004 from the NCEP reanalysis
and the CTRL experiment, respectively. The low-level
circulation in the reanalysis is characterized by the
Australian high and the WNP subtropical high. These
main features are well captured by the model, although
the WNP subtropical high is slightly weaker and the
equatorial westerlies west of 1708E are stronger in the
CTRL simulation than in the reanalysis. Nevertheless,
overall, the iRAM forced by the reanalysis captures
reasonably well the large-scale features in the region.
Figures 3d and 3f show the differences in the simulated
seasonal mean 850-hPa winds and SLP between the
WS_2004 and the CS_2004 runs. When the SSTG is increased, a cross-equatorial pressure gradient occurs in the
region west of 1508E, with low pressure over the warmed
southern subtropics and high pressure over the cooled
northern subtropics, and southward cross-equatorial flow
and the equatorial easterlies are triggered in the lower
troposphere. The simulated dynamical response to the
SSTG anomaly is very similar to the regressed pattern
based on the reanalysis as shown in Figs. 3c and 3e, although the cross-equatorial pressure gradient and equatorial easterlies are located more westward in the former
than in the latter. This suggests that it is in spring that the
SSTG anomaly produces the dynamical environments
similar to those during the typhoon season.
Since the response of the dynamical environments to
the SSTG anomaly is established in spring while the
SSTG anomaly weakens significantly toward the typhoon
season, a question arises as to how the preconditioned
dynamical environments are maintained and intensified
throughout the upcoming typhoon season. This is discussed by examining the lagged correlations between the
spring SSTG and several key dynamical and thermodynamical variables in the following months as shown in
Fig. 4. The key factors are selected based on the regressed
patterns in spring and during the typhoon season as
shown in Figs. 2 and 3, including the SLP gradient measured as the difference between SLP averaged over 108–
258N, 1058–1658E and that averaged over 258–108S,
1058–1658E; cross-equatorial flow averaged over 58S–08,
1008–1308E; an equatorial westerly averaged over 58S–
58N, 1408E–1808; and OLR averaged over 58S–58N,
1608E–1808.
In April, significant correlations exist between the
spring SSTG and the SLP gradient and the crossequatorial flow, which can persist throughout the typhoon
season with some changes in magnitude only. From May,
the negative correlation between the spring SSTG and
the equatorial westerlies becomes significant. The correlation coefficient between the spring SSTG and the OLR
over the equatorial central Pacific increases from April,
reaches the 95% confidence level in July, and then remains above this level until October. These correlations
indicate a pathway for the spring SSTG to control the
WNP TC genesis frequency during the upcoming typhoon season. In spring, the positive SSTG anomaly
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FIG. 5. Schematic diagram showing the response of the atmospheric fields and the equatorial
upper-ocean dynamics to the positive SSTG anomaly. ‘‘H’’ indicates high pressure located in
the Northern Hemisphere, and ‘‘D’’ indicates depression in the Southern Hemisphere, showing
a cross equatorial pressure gradient induced by the positive SSTG anomaly. See text for details.
produces the cross-equatorial pressure gradient, which
forces both the anomalous southward cross-equatorial
flow and equatorial easterly anomalies. The equatorial
easterlies then act to cool the central equatorial Pacific
by inducing local upwelling and raising the thermocline
to the east, which, in turn, maintains and intensifies
the equatorial easterly anomalies and hence the crossequatorial pressure gradient in the following months
during the typhoon season.
5. Conclusions
We have discovered a new factor: namely, the spring
SST gradient between the SWP and the WWP, which is
significantly negatively correlated with the WNP TC
genesis frequency during the typhoon season. The correlations of the spring SSTG with frequencies of different
intensity categories (weak TCs, intense TCs, and typhoons) and the power dissipation index (PDI) are also
statistically significant (not shown). This suggests that the
statistical model uncovered in this study might be valid
not only for TC genesis but also for TC intensity.
This SSTG–WNP TC relationship has been physically
explained by the atmospheric response to the SSTG
anomaly in spring and the air–sea interaction during
the typhoon season triggered by the spring SSTG
anomaly, as shown schematically in Fig. 5. In spring, the
positive SSTG anomaly leads to a cross-equatorial
pressure gradient, which forces anomalous southward
cross-equatorial flow and tropical easterlies over the
WNP. This observed response was further demonstrated
by several sensitivity experiments using a regional climate model. After being triggered, the equatorial easterlies act to cool the equatorial central Pacific by inducing
local upwelling and raising the thermocline in the east,
which, in turn, maintains and intensifies the equatorial
easterlies and hence the cross-equatorial pressure gradient during the upcoming typhoon season. As a result, the
combined dynamical and thermodynamical effect triggered by the spring SSTG anomaly produces the lowlevel negative vorticity anomaly, the strong vertical wind
shear, and the weak WNP summer monsoon throughout
the typhoon season, greatly suppressing the WNP TC
genesis. Conversely, the negative SSTG anomaly would
play a role opposite to the positive spring SSTG anomaly
and will produce dynamical and thermodynamical conditions favorable for TC genesis over the WNP in summer.
As indicated in section 1, a similar relationship between the interhemispheric SSTG and TC activity has
also been documented for the Atlantic. Both the SSTG in
the Pacific revealed here and the AMM in the Atlantic
are associated with climatic variations in the individual
basins via both the teleconnection and the air–sea interaction that all coherently control TC activity. However, the evolution of the effect is somewhat different.
The AMM variance can persist through the hurricane
season and contribute directly to Atlantic hurricane activity, while the SSTG anomaly in the Pacific peaks in
spring and weakens rapidly afterward and thus modulates
WNP TC activity via the persistence of the preconditioned atmospheric circulation and the subsequent air–
sea interaction in summer.
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ZHAN ET AL.
Acknowledgments. The authors are grateful to three
anonymous reviewers and the editor Dr. Kevin Walsh
for their helpful comments. This study has been supported in part by the National Basic Research Program
‘‘973’’ (Grant 2012CB956003) and in part by the National Natural Science Foundation of China (NSFC)
Grants 40921003 and 41075071. YW has been partly
supported by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) through its sponsorship of the International Pacific Research Center in
the School of Ocean and Earth Science and Technology
at the University of Hawaii at Manoa.
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