Notes and Correspondence
Are Tropical Cyclones Less Effectively Formed by Easterly Waves in the
Western North Pacific than in the North Atlantic?
Tsing-Chang Chen1, Shih-Yu Wang1, Ming-Cheng Yen2, and Adam J. Clark1
1. Atmospheric Science Program
Department of Geological and Atmospheric Sciences
Iowa State University
Ames, Iowa
2. Department of Atmospheric Science
National Central University
Chung-Li, Taiwan
Submitted to Monthly Weather Review
Submitted: January 2007
Revised: December 2007
Corresponding author address: Tsing-Chang (Mike) Chen, Atmospheric Science Program, Department of Geological
and Atmospheric Sciences, 3010 Agronomy Hall, Iowa State University, Ames, IA, 50011. e-mail:
tmchen@iastate.edu
Abstract
It has been observed that the percentage of tropical cyclones originating from easterly
waves is much higher in the North Atlantic (~60%) than in the western North Pacific (10-20%).
This disparity between the two ocean basins exists because the majority (71%) of tropical
cyclogeneses in the western North Pacific occur in the favorable synoptic environments evolved
from monsoon gyres. Because the North Atlantic does not have a monsoon trough similar to the
western North Pacific which stimulates monsoon gyre formation, a much larger portion of
tropical cyclogeneses than in the western North Pacific are caused directly by easterly waves.
This study also analyzed the percentage of easterly waves that form tropical cyclones in the
western North Pacific. By carefully separating easterly waves from the lower tropospheric
disturbances generated by upper-level vortices that originate from the tropical upper tropospheric
trough (TUTT), it is observed that 25% of easterly waves form tropical cyclones in this region.
Because TUTT-induced lower tropospheric disturbances often become embedded in the trade
easterlies and resemble easterly waves, they have likely been mistakenly identified as easterly
waves. Inclusion of these “false” easterly waves in the “true” easterly wave population would
result in an underestimation of the percentage of easterly waves that form tropical cyclones,
because the TUTT-induced disturbances rarely stimulate tropical cyclogenesis.
However, an analysis of monsoon gyre formation mechanisms over the western North
Pacific reveals that 82% of monsoon gyres develop through a monsoon trough-easterly wave
interaction. Thus, it can be inferred that 58% (= 82% x 71%) of tropical cyclones in this region
are an indirect result of easterly waves. Including the percentage of tropical cyclones that form
directly from easterly waves (~25%), it is found that tropical cyclones formed directly and
indirectly from easterly waves account for over 80% of tropical cyclogeneses in the western
North Pacific. This is more than the percentage that has been documented by previous studies in
the North Atlantic.
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1. Introduction
It is well known that westward-propagating easterly disturbances in the tropics can be a
potential forcing mechanism for tropical cyclogenesis. As documented by Landsea (1993) for
the 1967-91 period, approximately 60% of tropical storms, 60% of minor hurricanes, and 80% of
intense hurricanes in the North Atlantic evolved from African easterly waves (AEWs). In
contrast, tropical cyclogenesis associated with easterly waves in the western North Pacific has
been observed to be much less effective. For example, Frank (1988) reported that in the western
North Pacific approximately 80% of disturbances that go on to become tropical cyclones form in
the monsoon trough and the rest (~10%) develop from easterly waves. Exploring mesoscale
aspects of tropical cyclogenesis for 1990-92, Ritchie (1995) showed that 10%-15% of all tropical
cyclones formed from easterly waves in this ocean basin. Later, compiling tropical cyclogenesis
related to monsoon gyres (Lander 1994) in the western North Pacific, Chen et al. (2004a) found
that 25% of geneses originated from easterly waves. The purpose of this study will be to
investigate what causes this pronounced disparity in the occurrence frequency of tropical
cyclogenesis from easterly waves between the North Atlantic and the western North Pacific.
Upper-level vortices resulting from instability of the tropical upper tropospheric trough
(TUTT lows, hereafter; Sadler 1976, 1978; Chen et al. 2001) may penetrate to the lower
troposphere and become embedded in the trade easterlies, thus, it is likely that they have been
mistakenly identified as easterly waves. Because the cold and dry downdraft core of these
easterly wave-like disturbances rarely stimulates tropical cyclogenesis, the inclusion of these
disturbances in the population of easterly waves over the western North Pacific would lower the
apparent effectiveness of easterly waves in generating tropical cyclones. Thus, a careful
separation between TUTT lows and easterly waves is also important to diagnose the
effectiveness of easterly waves in stimulating tropical cyclogenesis in the western North Pacific.
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Previous studies (e.g., Lander 1994; Chen et al. 2004a) have shown that monsoon gyres,
which often form from a southeast extension of the western North Pacific monsoon trough, play
a major role in many tropical cyclogeneses that occur in this region. This large role of monsoon
gyres in western North Pacific tropical cyclogeneses likely results in the reduced percentage of
tropical cyclones relative to the North Atlantic that are directly stimulated by easterly waves,
because the western North Atlantic does not have a monsoon trough like that of the western
North Pacific. However, it has been observed (e.g., Holland 1995) that some monsoon gyres
form through an interaction between easterly waves and the western North Pacific monsoon
trough. Thus, tropical cyclones forming from monsoon gyres stimulated by this interaction are
an indirect result of easterly waves. To fully diagnose the effectiveness of easterly waves in
stimulating tropical cyclogeneses and to compare the effectiveness to that of the North Atlantic,
tropical cyclones in the western North Pacific forming both directly and indirectly from easterly
waves should be considered.
In view of tropical cyclogeneses directly linked to easterly waves, the potential
impact of TUTT lows on the easterly wave population, and tropical cyclogeneses indirectly
associated with easterly waves through monsoon gyres, this study aims to clarify the
effectiveness of easterly waves in tropical cyclogenesis over the western North Pacific relative to
the North Atlantic, and is organized as follows: In Section 2, data sources and procedures are
presented. In Section 3, the differences in the large-scale summer circulation between the North
Atlantic and western North Pacific Ocean basins, as well as the potential impact of misidentified
TUTT lows on the easterly wave population are considered. Also in Section 3, the formation of
monsoon gyres and the effect of monsoon gyres on tropical cyclogeneses are analyzed.
Subsequently, the numbers of tropical cyclones that form directly and indirectly from easterly
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waves are determined. Concluding remarks and a summary are provided in Section 4.
2. Data and procedures
During the period of 1979-2002, the 6-hr ERA-40 reanalyses (Källberg et al. 2004) and the
best-track records issued by the Joint Typhoon Warning Center (JTWC in Hawaii; JTWC 1991
and Chu et al. 2002) are used for the circulation depiction and tropical cyclone/depression track
analyses, respectively. Infrared (IR) images shown in Fig. 1 were derived from Geostationary
Meteorological Satellite (GMS) observations from the Japan Meteorological Agency (JMA).
Using ERA-40 horizontal wind and vertical motion, easterly perturbations in the western North
Pacific were tracked and classified into two types: conventional easterly wave (e.g. Reed and
Recker 1971) and easterly disturbance generated by the downward penetration of a TUTT low.
These perturbations were identified in terms of the low-level flow patterns and categorized
through their upper-level structure using three steps:
1) Identification: A clear cyclonic perturbation or an open trough embedded in the tropical
easterlies in the 850mb streamline charts must have a life cycle longer than 3 days.
2) Classification: A conventional easterly wave is identified if this perturbation occurs below an
upper-level anticyclonic circulation. An easterly wave-like disturbance is induced by the
downward penetration of a TUTT low and overlaid by a well-defined closed vortex of TUTT
low in the 200-mb streamline charts.
3) Verification: Grid-scale upward motion in the ERA-40 reanalyses appears west (east) of the
trough of the identified easterly wave (TUTT low-induced easterly disturbance) for more than
half of this perturbation’s life cycle. Thus, the easterly wave/disturbance is coupled with an
east-west circulation (as indicated by thick shafted green line in Fig. 2 and will be discussed
later). Upward motion induced by easterly disturbances is also verified by GMS IR images as
shown in Fig. 1.
The life cycle of a perturbation is defined in terms of 850-mb vorticity. The mature stage of an
easterly wave is defined as the period when the maximum vorticity of this wave reaches
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1.5x10-5s-1 and is coupled with a well-organized east-west circulation. In addition to the eastwest circulation, the mature phase of the TUTT low-induced easterly disturbance is defined
when the vorticity of this disturbance reaches 10-5s-1. To identify the origin of individual easterly
waves, a backtracking approach was adopted. When a tropical cyclone that formed from an
easterly wave was identified, the easterly wave was tracked backwards until the wave was no
longer detectable. In this approach, the easterly waves were detected using 850-mb streamlines
superimposed with 850-mb vorticity, ∆OLR (≡235Wm2−OLR), ω(500mb) vertical motion, and
available infrared satellite images from the JMA Geostationary Meteorological Satellite.
As previously pointed out, to fully diagnose the effectiveness of easterly waves in
stimulating tropical cyclogenesis, tropical cyclones forming indirectly from easterly waves
through monsoon gyres must be considered (Chen et al. 2004a). This requires a careful analysis
of different monsoon gyre formation mechanisms and a determination of the percentage of
monsoon gyres formed through some type of easterly waves/monsoon trough interaction. To
accomplish this analysis, monsoon gyres (MGs) are identified in terms of the following criteria:
1) Closed cyclonic streamlines, with an east-west dimension of 1,500-3,000 km and a life cycle
of 5 days−3 weeks, appear prior to or during tropical cyclogenesis.
2) Intense upward motion exists over the confluence region east of the gyre, while broader weak
downward motion covers the rest of the gyre west of the confluence region.
3) The vertical structure of streamfunction in the short-wave (waves 3-15) regime containing the
MG exhibits a vertical phase reversal with a cyclonic cell in the lower troposphere and an
anticyclonic cell in the upper troposphere.
4) At least one tropical cyclogenesis occurs along the southeast-east periphery of the gyre.
Note that Criteria 1 and 4 are not as restrictive as those used in some past studies (e.g. Lander
1994; Chen et al. 2004a). For example, Holland (1995) pointed out that the MG life cycle can be
as short as five days and is involved with only one tropical cyclogenesis. However, Criteria 2
and 3 imposed here make it nearly impossible for other types of cyclonic disturbances (e.g.
tropical depressions) to be classified as MGs.
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3. Formation of tropical cyclones related to easterly waves
3.1 Summer circulation
Vorticity and large scale ascent within the environmental flow are crucial factors in tropical
cyclogenesis (Gray 1978).
Thus, significant differences in the large-scale summer
environmental flows between the two ocean basins may affect the effectiveness of easterly
waves in tropical cyclogenesis.
The most relevant features of the summer circulation are
highlighted below:
1) The Asian monsoon consists of the Tibetan high and the Indian monsoon trough, while the
North American monsoon includes the Mexican high and the North American continental
thermal low. These circulation features are basically formed by the wavenumber-1 and -2
components along longitudinal circles.
The vertical structure of the wavenumber-1
component is in phase with the Asian monsoon, but out of phase with the North American
monsoon (Chen 2003). Because of this vertical phase relationship between the wavenumber1 component and the two monsoons, the development of the North American monsoon is
limited by this wave, which may explain its very limited role in North Atlantic tropical
cyclogenesis. The difference in intensity and scale of these two monsoons is also reflected by
the existence of the upper-level oceanic trough and the low-level monsoon trough in the
western tropical Pacific formed by the strong monsoon westerlies and the trade easterlies of
the North Pacific anticyclone.
These circulation features are formed primarily by the
wavenumber-1 and -2 components which are spatially in quadrature in the North Pacific, but
out of phase in the North Atlantic (Krishnamurti 1971). The equatorial side of the upper-level
oceanic trough is a breeding region for TUTT lows which may generate “false” easterly
waves (Chen et al. 2001), while large vorticity along the monsoon trough facilitates tropical
cyclogenesis (Gray 1978).
2) The large-scale east-west differential heating between the warming caused by latent heat
released from cumulus convection over the Asian monsoon region and radiative cooling in the
upper troposphere over North Africa maintains a planetary-scale east-west circulation with a
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strong and extensive upward branch over the western tropical Pacific. A similar regionalscale east-west differential heating between warming over the western tropical AtlanticCaribbean Sea region and cooling over the eastern tropical Pacific forms a smaller-scale and
less intense east-west circulation with the upward branch over the former region. A factor
conducive to tropical cyclogenesis is upward motion generated by the large-scale circulation
(Zehr 1992). The intensity difference between these two east-west circulations should affect
the occurrence frequency of tropical cyclogenesis in the western tropical regions of the two
ocean basins.
3.2 Effect of TUTT lows
The formation of TUTT lows by the instability of the North Pacific oceanic trough often
induces a low-level vortex with a descending cold core from the convergence of upper-level cold
air.
The downward penetration of a cold-core vortex may be weakened by the lower-
tropospheric North Pacific anticyclone and often no signal of the cold-core vortex is detected in
the lower troposphere. It was observed by Chen et al. (2001) that one third of TUTT lows in the
North Pacific may drift across 15oN to the tropics (Fig. 1a) and penetrate down to the lower
troposphere (Fig. 1c) to form low-level disturbances (Fig. 1b) along the southern periphery of the
North Pacific anticyclone. The spatial structure of these disturbances in the lower troposphere
resembles easterly waves (Fig. 1e).
If these TUTT low-induced easterly disturbances are
mistakenly identified as easterly waves, the population of easterly waves would be
overestimated. In fact, the characteristics of TUTT low-induced easterly disturbances differ
from easterly waves in many ways. To distinguish these two types of low-level disturbances,
some differences in their basic characteristics revealed from Fig. 1 are highlighted:
(1) Vertical Structure: It is revealed from Figs. 1a and b that the TUTT low-induced easterly
disturbance occurs below an upper-level cyclonic vortex. The vertically uniform structure
of a TUTT low emerges from the east-west cross-section of the eddy component of
geopotential height Z E (15 o N) (Fig. 1c). In contrast, a low-level easterly wave (Fig. 1e)
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occurs below an upper-level anticyclonic vortex (Fig. 1d). A vertical phase reversal of the
easterly wave can be inferred from the contrast between the upper- and lower-level flows
shown in Figs. 1d and e, respectively. This vertical phase change in an easterly wave is
confirmed by the east-west cross-section of Z E (12.5 o N) (Fig. 1f).
(2) Cumulus convection/east-west circulation: Cumulus clouds appear east of (behind) the
trough line of the TUTT low-induced easterly disturbance (Fig. 1b), but exist west of
(ahead) the trough line of the easterly wave (Fig. 1e). Cumulus convection is maintained
by upward motion. Based on the spatial relationship between cumulus clouds and trough
lines, it is expected that upward motion generated by TUTT low-induced easterly
disturbances and easterly waves is opposite in location with respect to their trough lines.
Dynamically, the surface divergence generated by the cold-core downdraft of TUTT lows is
blocked by the environmental airmass to produce upward motion around the periphery of
these upper-level lows.
In contrast, the east-west circulation of easterly waves is
maintained by the vertical motion induced by the differentiation of vorticity advection. To
substantiate our inference, the maximum and minimum ω (500mb) values of easterly
disturbances/waves along the abscissa in Fig. 2 and their distance from the trough line
along the ordinate during the mature stage (defined in Section 2) are plotted in scatter
diagrams (Fig. 2); upward (downward) motion is denoted by red dots (blue triangles). The
schematic east-west circulations coupled with these two types of easterly disturbances are
also added in this figure. It was pointed out that convective clouds appear east of the
trough line of TUTT low-induced easterly disturbances (as the east half of Fig. 2a) and
sometimes stratus clouds may encircle the TUTT low [as indicated by the distribution of
ω (500mb) in Fig. 2a and the superimposed east-west circulation]. In contrast, upward
(downward) motion in a mature easterly wave concentrates west (east) of the wave’s trough
line (Fig. 2b).
(3) Tropical cyclogenesis
As pointed out in Item (2), upward motion around the periphery of a TUTT low may
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facilitate TC/TD formation. The genesis of Typhoon Cary (July 5, 1984) will be used as an
example to illustrate this possibility. The three-dimensional structure of the TUTT low
associated with this typhoon is shown in Fig. 3. Convective clouds around the eastnortheast periphery of the TUTT low are revealed from ∆OLR (≡ 235 Wm-2-OLR) (Fig. 3a).
The east-west cross-section of the TUTT low indicated by vorticity ζ(15oN) (Fig. 3b) is
superimposed with vertical motion ω . The downdraft core of the TUTT low is surrounded
by the updraft. An easterly wave-like disturbance overlaid by the vortex of this TUTT low
is apparent from the 850-mb streamline chart (Fig. 3c). During the 24 year period of 19792002, it was found that only 9 TCs/TDs formed from TUTT lows as Typhoon Cary
contributing to only 1.9% of the TC/TD geneses analyzed in this study. Thus, it is possible
for a TUTT low to stimulate tropical cyclogenesis, but this rarely occurs.
(4) Daily locations
Daily locations of easterly waves (blue dots) and TUTT low-induced easterly disturbances
(red dots) for all identified cases, superimposed on 200- and 850-mb summer-mean
streamline charts, are shown in Fig. 4a and b, respectively.
Locations of the latter
disturbances are restricted to within the North Pacific west of approximately 150oW, while
some of the easterly waves can be traced to the eastern tropical Pacific and West Africa
[not shown, because residual disturbances of AEWs contribute to only 7% of the population
of easterly waves]. The mean latitudes of easterly waves and TUTT-induced easterly
disturbances west of 150oW are at 7.5oN and 17.5oN, respectively. As inferred from
streamlines at both levels, these tropical weather disturbances are transported westward by
easterlies. Trajectories of easterly waves are usually located further south of the North
Pacific oceanic trough than the TUTT low-induced easterly disturbances.
The average population of easterly waves during summer (MJJAS) for the period of 19792002 is 18.4, while that of TUTT low-induced easterly disturbances is 8.8. Thus, the population
of easterly perturbations in the western tropical Pacific (27.2 per summer) contributed to by the
former is 67% and by the latter is 33%, respectively. Regardless of distinctive features between
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easterly waves and TUTT low-induced easterly disturbances depicted above, the latter
perturbations may be included in the population of the former, because of their similar spatial
structure in the lower troposphere (shown in Figs. 1b and e). Consequently, the population of
easterly waves in the western tropical Pacific is overestimated by including TUTT low-induced
easterly disturbances. Approximately 25% of easterly waves formed tropical cyclones in the
western North Pacific (the South China Sea is not included) for the 1979-2002 period. If the
population of TUTT low-induced easterly disturbances is mistakenly included in easterly waves,
the percentage of easterly waves forming tropical cyclones in the western North Pacific would be
reduced to slightly below 17%.
3.3 Formation and effect of monsoon gyres
From the analysis of monsoon gyres linked to tropical cyclones over the study period, it
was found that all monsoon gyres generally form through one of four common large-scale flow
patterns, two of which involve easterly waves. Different phases in the evolution of these four
patterns are described using 850-mb streamline and vorticity charts for examples of individual
cases corresponding to each flow type (Fig. 5).
1) Easterly waves (Type 1— merging case; Fig. 5a)
The formation of a MG coupled with an easterly wave is depicted in terms of four
different synoptic steps for a case that occurred on 8-13 August 2001 (Fig. 5a).
Step 1: The South/Southeast Asian monsoon circulation is characterized by a subtropical
monsoon trough juxtaposed meridionally with an equatorial anticyclone which is
separated from the trough by the monsoon westerlies. Over the western tropical Pacific,
the flow along the southern rim of the North Pacific anticyclone is driven by the trade
easterlies. An easterly wave (indicated by thick-dashed line in the top panel in Fig. 4a)
is embedded in the trade easterlies at 0000 UTC 8 August 2001. This disturbance is
trailed by a weak anticyclonic perturbation to the east and also accompanied by another
anticyclonic perturbation to the west forming a synoptic-scale short-wave train, as
depicted by previous studies (e.g. Liebmann and Hendon 1990, Takayabu and Nitta
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1993). The northwestward propagation of the cyclonic disturbance in the wave train
follows the northward progression of the monsoon trough (explained below).
Step 2: The anticyclonic perturbation west of the easterly wave dissipates and merges with the
equatorial anticyclone, while the easterly wave reaches the monsoon trough one day later
on 9 August 2001. The easterly wave eventually merges with the monsoon trough,
extending the trough eastward beyond 150°E.
Step 3: On 10 August, the anticyclonic perturbation east of the monsoon trough is trailed by a
new easterly wave located near 160°E.
One day later on 11 August 2001, this
anticyclonic perturbation became well-organized and was juxtaposed between the
monsoon trough (with a developing MG on its east tail) and the easterly wave (not
shown). The amplifying anticyclonic perturbation connects the equatorial anticyclone
and the North Pacific anticyclone.
Step 4: By 1200 UTC 13 August 2001, a MG on the east end of the monsoon trough had formed.
A slow northward progression of this monsoon trough/gyre was ongoing as a new shortwave train composed of the anticyclonic perturbation east of the MG and the new
easterly wave propagated northwestward (as reported by Liebmann and Hendon 1990,
and Takayabu and Nitta 1993). As expected by wave accumulation theory (Sobel and
Bretherton 1999), the interaction between this MG and this new short-wave train formed
in Step 3 eventually led to the genesis of Typhoon Pabuk.
2) Easterly waves (Type 2— self-development case; Fig. 5b)
Initially, the northwestward propagating easterly wave is embedded in a synoptic-scale
short-wave train and juxtaposed with anticyclonic perturbations to the west and east (Step 1).
The wave accumulation caused by the westward propagation of the easterly wave does not
amplify this disturbance, rather, the disturbance decays and tilts northwestward. The western
anticyclonic perturbation does not dissipate as in Step 2 of the Type-1 formation process, but
eventually merges with the equatorial anticyclone (Step 2). The easterly wave grows to form a
synoptic vortex, in accompany with the amplification of the eastern anticyclonic perturbation
which became part of the equatorial anticyclone (Step 3). Eventually, a MG formed from the
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synoptic vortex around 1200 UTC 15 September 1986, about 3 days before the genesis of
Typhoon Abby at 0000 UTC 9 September 1986 (Step 4).
3) Midlatitude trough (Fig. 5c)
The MG of July-August 1991 discussed in Lander (1994) illustrates the southward
intrusion of a midlatitude trough in the North Pacific and the subsequent formation of a MG. For
the corresponding synoptic development at upper levels, one may consult Lander (1994).
Step 1: The low-level North Pacific anticyclone was divided by the equatorward intrusion of a
midlatitude trough into two anticyclonic cells that followed the eastward propagation of
the upper level East-Asian anticyclone into the North Pacific (not shown). The eastward
extension of the upper-level anticyclone was separated from the East-Asian high by a
midlatitude trough, like the low-level anticyclone.
Step 2: Over only six hours (from 18Z on July 31 to 00Z on August 1, 1991), the low-level
western North Pacific anticyclone retreated slightly westward, while the midlatitude
trough deepened further forming a cutoff low. This newly developed low was overlaid
by the eastward extension of the upper-level anticyclone (not shown). Thus, a monsoonlike vertical structure with a phase reversal started to appear.
Step 3: The cutoff low eventually separated from the midlatitude trough and formed a MG.
4) Monsoon trough (Fig. 5d)
The monsoon trough with intensified monsoon westerlies along its southern rim extends
east to the Dateline (step 1; August 8, 1981). The strong shear flow across this trough is
perturbed by easterly waves forming multiple closed lows within the trough (step 2). Later, the
monsoon trough became SW-NE oriented, like Lander’s (1996) reverse-oriented monsoon
trough.
These closed lows within the trough developed into MGs and led to geneses of
Typhoons Vanessa, Thad, and Bill (step 3; August 11, 1981). The formation process of MGs in
this flow pattern is similar to that of the Type-1 easterly wave, except for the eastward extension
of the monsoon trough and intensified monsoon westerlies.
To facilitate a statistical analysis of MG formation mechanisms using the previously
mentioned criteria, schematic diagrams of the basic synoptic flow patterns discussed in the
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previous four examples are presented in Fig. 6. It is found that the Type-1 and Type-2 MG
formation mechanisms, which involve easterly waves, contribute to 54% and 28% of identified
gyres, respectively (Fig. 7). Populations of MGs formed by midlatitude troughs and perturbed
monsoon troughs are 13% and 5%, respectively. Although Type-3 MGs [similar to Lander
(1994)] can be prolific generators of tropical cyclones, their low occurrence frequency results in
a relatively small contribution to the total number of tropical cyclogeneses along with the Type-4
MGs. The majority (82%) of MGs form through the interaction between easterly waves and the
monsoon trough (Types 1 and 2) and 71% of tropical cyclogeneses are linked to MGs in the
western tropical Pacific (Chen et al. 2004). It can be easily inferred that approximately 58%
(=82%x71%) of tropical cyclogenesis in this region are an indirect result of easterly waves.
3.4 Impact of Easterly waves
The geographic locations of tropical cyclones formed directly by easterly waves (blue
tropical cyclone symbol) and indirectly by easterly waves through their stimulation of monsoon
gyres (red tropical cyclone symbol) with the seasonal-mean 850-mb streamline are shown in Fig.
8a. The monsoon gyre-related tropical cyclogeneses are concentrated over the monsoon trough
while those formed directly from easterly waves and TUTT low-induced easterly disturbances
are scattered over the western tropical-subtropical Pacific.
The percentage of tropical
cyclogeneses induced directly by easterly waves is 25% and indirectly through monsoon gyres is
58%. Thus, the total contribution of easterly to tropical cyclone formation in the western North
Pacific is approximately 83% (=58+25%). TUTT lows and other processes combined contribute
to only 4% of tropical cyclogeneses (Fig. 8b).
Recall that the western tropical Atlantic does not have a monsoon trough like the western
tropical Pacific.
Thus, most hurricanes are formed directly from AEWs (Landsea 1993).
Because of the existence of an extended monsoon trough and its interaction with monsoon
westerlies and trade easterlies, the role played by easterly waves in the tropical cyclogenesis over
the western tropical Pacific is different from that over the tropical Atlantic. Tropical cyclones
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are directly formed from easterly waves more often in the North Atlantic than in the North
Pacific.
On the other hand, tropical cyclogenesis in the western North Pacific can be
accomplished indirectly by easterly waves through the formation/development of monsoon gyres
which are either stimulated by these easterly perturbations or grown directly from the easterly
waves. Evidently, easterly waves in the western North Pacific are not less effective in forming
tropical cyclones than easterly waves in the tropical Atlantic if this formation through the
stimulation of monsoon gyres is considered.
The contribution to tropical cyclogenesis by
easterly waves in the western North Pacific that stimulate monsoon gyres, which in turn lead to
tropical cyclogenesis, has not been considered in past studies.
Because of an understanding of the role of AEWs in tropical cyclogenesis in the North
Atlantic [the majority of tropical cyclones in the North Atlantic evolve from AEWs (Landsea
1993)], as well as the preferred genesis region for AEWs [south of the African easterly jet
(Burpee 1972) and over the Saharan thermal low north of the African easterly jet (Chen 2006)],
the National Hurricane Center rigorously tracks AEWs from their genesis locations during the
hurricane season. In the western North Pacific, however, the weather services in this region (e.g.
JTWC, JMA, and other agencies) have not operationally tracked easterly waves, which is likely
because the indirect role easterly waves have in the majority of tropical cyclones has not been
recognized in the past. In addition, plots of genesis locations of easterly waves (Figs 4c and d)
linked directly (solid blue circles) and indirectly (solid red triangles) to tropical cyclogeneses in
the western North Pacific do not show a preferred genesis region, which may be caused by the
large east-west extent of barotropic instability in the Pacific trader easterlies that may lead to a
majority of easterly wave geneses (Yanai and Nitta 1968; Nitta and Yanai 1969). Note that a
small portion (~7%) of tropical Pacific easterly waves actually originated as AEWs which are
generated by Charney-Stern instability (Burpee 1972) and Chang-Thorncroft instability (Chen
2006) over north and west Africa. The lack of a preferred genesis region would likely make it
difficult, if not impossible, to operationally track easterly waves in the tropical Pacific, like the
AEWs in the North Atlnatic.
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4. Concluding remarks
This study explored whether tropical cyclones formed by easterly waves in the western
North Pacific occur less frequently than in the western North Atlantic, as observed by previous
studies (Frank 1988, Ritchie 1995, Chen et al. 2004a). A majority of tropical cyclones in the
western North Pacific are formed directly by easterly waves and indirectly by easterly waves that
stimulate monsoon gyre formation providing a favorable breeding region for tropical cyclones.
Both formation mechanisms are related to special features of the North Pacific summer
circulation:
(1) From a planetary-scale perspective, the zonal wavenumber-1 and -2 components of the
summer circulation are spatially in quadrature over the North Pacific, but out of phase over
the North Atlantic.
Because of the spatial relationship between these two wave
components, intensities of the upper-level oceanic trough and the low-level anticyclone
over the North Pacific are stronger in the North Pacific than the North Atlantic.
(2) In the lower troposphere, a monsoon trough is formed over the southeast Asia-western
tropical Pacific region by the strong monsoon westerlies in South Asia and the trade
easterlies of the North Pacific anticyclone.
The North Pacific summer circulation provides a favorable environment for TUTT lows
that form along the equatorial side of an oceanic trough.
These upper-level vortices can
resemble easterly waves when they penetrate to lower levels and become embedded in the trade
easterlies. A careful separation of easterly waves and lower tropospheric disturbances generated
by these upper-level vortices reveals that about 25% of western North Pacific easterly waves
form tropical cyclones, a number that would have been reduced to 17% if the lower tropospheric
disturbances generated by the upper-vortices were included in the easterly wave count.
On the other hand, the two elements of the North Pacific summer circulation form a
favorable environment for monsoon gyres of which approximately 82% form through monsoon
trough/easterly wave interaction. Without the existence of a monsoon trough in the tropical
15
Atlantic, tropical cyclones formed directly from easterly waves occur more frequently in the
North Atlantic than in the western North Pacific.
However, because 71% of tropical
cyclogeneses have been observed to be related to monsoon gyres and 82% of monsoon gyres are
formed through an interaction with easterly waves, it can be inferred that tropical cyclones
indirectly formed by easterly waves contribute to about 58% (= 71% x 82%) of tropical cyclones
in the western North Pacific. Including the percentage of tropical cyclones that form directly
(~25%) and indirectly (~58%) from easterly waves, over 80% of tropical cyclogeneses in the
western North Pacific are the direct or indirect result of easterly waves.
Acknowledgments
Part of this paper was done during T.-C. Chen’s visit to National Central University
(NCU) in Taiwan. This visit was partially supported by the NCU Development Program for
Top-Ranked University sponsored by the Ministry of Education in Taiwan, Grants NSC95-2811M-008-09 and NSC95-2111-M-008-007. The effort of M.-C. Yen was supported by Grants
NSC95-2111-M-008-016-MY3 and NSC95-2625-008-002. Comments offered by Dr. Chris
Landsea of National Hurricane Center and two anonymous reviewers are helpful in improving
this paper.
16
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19
Figure Captions
Fig. 1 Contrast of spatial structure between the tropical upper tropospheric trough (TUTT) loweasterly disturbance generated by the downward penetration of the TUTT low (left
column) and a conventional easterly wave (right column): GMS satellite imagery at 0000
UTC on 18 June 2000 superimposed with a) 200mb streamlines and b) 850mb
streamlines, c) and east-west vertical cross-section of the eddy component of geopotential
height at 15°N latitude, and (d)-(f) same as (a)-(c), except for a convectional easterly
wave.
Fig. 2 Scatter diagrams of maximum (red dots) and minimum (blue triangles) ω(500mb) vs. its
distance away from the trough line which is determined by 850mb streamline charts as
shown in Figs. 1b and 1e for (a) TUTT low-induced easterly disturbances, and (b)
conventional easterly waves. A schematic east-west circulation associated with both
TUTT low-induced easterly disturbances in (a) and easterly waves in (b) are depicted by
the green line with shaft. ω(500mb) is used when the vorticity (ζ) along the trough line
of an easterly disturbance/wave reaches its maximum (above 10-5s-1) over its lifetime.
Fig. 3 Three-dimensional structure of TUTT low and the lower-tropospheric easterly disturbance
induced by this TUTT low on July 5, 1984 when the genesis of Typhoon Cary occurred:
(a) 200-mb streamlines superimposed with ∆OLR (≡ 235 Wm-2 －OLR), (b) east-west
cross-section of (ζ, ω)(15oN), and (c) same as (a) except for 850-mb streamlines. The
genesis location (red typhoon symbol) is indicated in (c). ζ and ω are relative vorticity
(solid lines) and dp/dt (color areas; p=pressure, ω vertical motion). The contour interval
of ζ is 0.5×10-5s-1. Magnitude scales of OLR and ω are shown in the lower right of (a, c)
and (b), respectively.
Fig. 4 Daily locations of TUTT low-induced easterly disturbances (red dots) and easterly waves
(blue dots) superimposed on the seasonal-mean streamlines at (a) 200mb and (b) 850mb,
and (c) and (d), same as (a) and (b), respectively, except genesis locations of easterly
waves and TUTT low-induced easterly disturbances. Genesis locations of easterly waves
directly and indirectly linked to tropical cyclogenesis are marked by solid-blue dots and
20
solid-red squares, respectively, while TUTT low-induced easterly disturbances linked to
tropical cyclogenesis are indicated by golden triangles.
Fig. 5 Four different types of synoptic development for monsoon gyre formation portrayed by
850-mb streamlines and vorticity (shading): (a) the trough of Type-1 easterly wave
merged with the monsoon trough (8-13 August 2001), (b) the self-amplification of the
trough of Type-2 easterly wave (12-19 September 1986), (c) interaction of the
equatorward-intruding midlatitude trough with the monsoon trough (31 July–7 August
1991), and (d) deepening of the monsoon trough/intensification of monsoon westerlies
(8-17 August 1981). The date of each synoptic step is given in the upper-left corner.
Tropical cyclogeneses at the final synoptic step in the monsoon gyre formation are shown
by typhoon symbols. Trough lines of easterly waves are marked by dashed lines in (a)
and (b). The scale for vorticity is provided at the top right of each column.
Fig. 6 Schematic diagrams of streamline portraying the four different types of synoptic
development for monsoon gyre formation corresponding to Fig. 5. MT, EW, MG, H, and
L are acronyms for monsoon trough, easterly wave, monsoon gyre, anticyclonic and
cyclonic perturbations, respectively. Light-gray arrows indicate the transition of steps.
Fig. 7 Formation frequencies (NMG) of monsoon gyres by four different types of synoptic
developments analyzed during the typhoon season of June-October covering the period of
1979-2002. The percentage contribution from each type is given at the top of each
histogram.
Fig. 8 (a) Locations of tropical cyclones formed by easterly waves (blue tropical cyclone
symbol), monsoon gyres (red tropical cyclone symbol), and TUTT lows (golden tropical
cyclone symbol) superimposed on the 850-mb streamline, and (b) histogram of total
tropical cyclogeneses linked to monsoon gyres, easterly waves, TUTT lows, and other
processes.
21
TUTT-low induced
easterly disturbance
[V(200mb),IR]
[V(200mb),IR]
[V(850mb),IR]
[V(850mb),IR]
Easterly wave
Fig. 1 Contrast of spatial structure between the tropical upper tropospheric trough (TUTT) low-easterly
disturbance generated by the downward penetration of the TUTT low (left column) and a conventional
easterly wave (right column): GMS satellite imagery at 0000 UTC on 18 June 2000 superimposed with a)
200mb streamlines and b) 850mb streamlines, c) and east-west vertical cross-section of the eddy
component of geopotential height at 15°N latitude, and (d)-(f) same as (a)-(c), except for a convectional
easterly wave.
Pa·s-1
Pa·s-1
km (away from trough line)
km (away from trough line)
Fig. 2 Scatter diagrams of maximum (red dots) and minimum (blue triangles) ω(500mb) vs. its distance
away from the trough line which is determined by 850mb streamline charts as shown in Figs. 1b and
1e for (a) TUTT low-induced easterly disturbances, and (b) conventional easterly waves. A schematic
east-west circulation associated with both TUTT low-induced easterly disturbances in (a) and easterly
waves in (b) are depicted by the green line with shaft. ω(500mb) is used when the vorticity (ζ) along
the trough line of an easterly disturbance/wave reaches its maximum (above 10-5s-1) over its lifetime.
Fig. 3 Three-dimensional structure of TUTT low and the lower-tropospheric easterly disturbance
induced by this TUTT low on July 5, 1984 when the genesis of Typhoon Cary occurred: (a) 200mb streamlines superimposed with ∆OLR (≡ 235 Wm-2－OLR), (b) east-west cross-section of (ζ,
ω)(15oN), and (c) same as (a) except for 850-mb streamlines. The genesis location (red typhoon
symbol) is indicated in (c). ζ and ω are relative vorticity (solid lines) and dp/dt (color areas;
p=pressure, ω vertical motion). The contour interval of ζ is 0.5×10-5s-1. Magnitude scales of
OLR and ω are shown in the lower right of (a, c) and (b), respectively.
Fig. 4 Daily locations of TUTT low-induced easterly disturbances (red dots) and easterly waves (blue dots)
superimposed on the seasonal-mean streamlines at (a) 200mb and (b) 850mb, and (c) and (d), same as (a)
and (b), respectively, except genesis locations of easterly waves and TUTT low-induced easterly
disturbances. Genesis locations of easterly waves directly and indirectly linked to tropical cyclogenesis
are marked by solid-blue dots and solid-red squares, respectively, while TUTT low-induced easterly
disturbances linked to tropical cyclogenesis are indicated by golden triangles.
(d) Deepening monsoon
trough
Fig. 5 Four different types of synoptic development for monsoon gyre formation
portrayed by 850-mb streamlines and vorticity (shading): (a) the trough of
Type-1 easterly wave merged with the monsoon trough (8-13 August 2001),
(b) the self-amplification of the trough of Type-2 easterly wave (12-19
September 1986), (c) interaction of the equatorward-intruding midlatitude
trough with the monsoon trough (31 July–7 August 1991), and (d)
deepening of the monsoon trough/intensification of monsoon westerlies (817 August 1981). The date of each synoptic step is given in the upper-left
corner. Tropical cyclogeneses at the final synoptic step in the monsoon gyre
formation are shown by typhoon symbols. Trough lines of easterly waves
are marked by dashed lines in (a) and (b). The scale for vorticity is provided
at the top right of each column.
(a) Easterly wave (Type 1) (b) Easterly wave (Type 2) (c) Midlatitude trough
step 3
step 2
step 1
MT
EW
H
step 4 MT
H
H
EW
H
H
MG
EW
H
MT
MT
step 3
MT
step 2
step 1
H
H
EW
H
EW
H
H
(b) Easterly wave (Type 2)
step 3
L
step 2
H
step 1
H
L
H
MG
H
L
L
H
H
EW
(c) Midlatitude trough
step 3
step 2
MG
rlies
Strong weste
H
H
step 1 Monsoon trough
MG
H
(d) Deepening monsoon tough
Fig. 6 Schematic diagrams of streamline portraying the four different types of synoptic development for monsoon gyre formation corresponding to Fig.
5. MT, EW, MG, H, and L are acronyms for monsoon trough, easterly wave, monsoon gyre, anticyclonic and cyclonic perturbations,
respectively. Light-gray arrows indicate the transition of steps.
H
MT
MT
(a) Easterly wave (Type 1)
NMG(JASO; 1979-2002)
l
ter
s
Ea
a
yw
(
ve
Ty
1
pe
l
ter
s
Ea
)
a
yw
(
ve
Ty
2
pe
)
u
tr o
gh
e
o
so
tt ud
i
n
dla
mo
g
Mi
in
en
p
e
De
gh
rt ou
n
Fig. 7 Formation frequencies (NMG) of monsoon gyres by four different types
of synoptic developments analyzed during the typhoon season of JuneOctober covering the period of 1979-2002. The percentage contribution
from each type is given at the top of each histogram.
Fig. 8
(a) Locations of tropical cyclones formed by
easterly waves (blue tropical cyclone symbol),
monsoon gyres (red tropical cyclone symbol),
and TUTT lows (golden tropical cyclone
symbol) superimposed on the 850-mb
streamline, and (b) histogram of total tropical
cyclogeneses linked to monsoon gyres,
easterly waves, TUTT lows, and other
processes.
e
av
low
W
T
T
o
rly
TU
nso
ste
o
a
E
M
ta
To
l
y
nG
re
he
Ot
rs