Lin Ching1

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An Analysis of Multi-scale Nature of Tropical
Cyclone activities in June 2004

• Lin Ching1
• Chung-Hsiung Sui2,1
• Ming-Jen Yang1,2
• 1Department of Atmospheric Sciences, National
  Central University, Taiwan
• 2Institute of Hydrological and Oceanic Science,
  National Central University, Taiwan

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Introduction
A record-breaking five tropical cyclones (TC)
formed in the northwestern Pacific (NWP) Ocean in
June 2004 (climatological value 1.8) and two of
them made landfall over Japan. In this study, we
analyze the weather and climate oscillations of
this particular month in relation to other years
from 1982 to 2006 to investigate the possible
causes of this unusual event.
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Table 1. Numbers of Western North Pacific
Tropical Cyclones in June. Red (blue) boxes
represent ENSO warm (cold) years based on Oceanic
Niño Index (ONI). The TC numbers in red
represent TC-active years (more than 3 TCs
occur). Those for TC-inactive years (no TC) are
denoted in blue. The average number of TC in
June from 1982 to 2006 is 1.8. The MJO-active
and inactive June are marked act in red and
inact in blue along with measures of MJO
strength (numbers below) based on RMM indices,
i.e. the summation of amplitude of active phase
(5, 6, 7) or inactive phase (1, 2, 3) in June
larger than 2/3 of mean MJO amplitude in June
(19.58).
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TC tends to be more active in El Niño developing
June and suppressed in La Niña.
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TC-active years 1982, 1990, 1997, 2002,
2004 TC-inactive years 1996, 1998, 2000,
2005 Normal years 1983, 1984, 1985, 1986, 1987,
1988, 1989, 1991, 1992, 1993, 1994, 1995, 1999,
2001, 2003, 2006 El Niño 1982, 1987, 1991,
1994, 1997, 2002, 2004, 2006 La Niña 1983,
1984, 1998, 1995, 1998, 1999, 2000
1.8
3.4
5
0
2.625
1.75
1.143
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In TC-active June, equatorial SST is warmer in
central Pacific and colder in western Pacific.
Correspondingly, westerly anomaly prevails at low
level and an anomalous cyclonic circulation forms
to the north, providing a favorable large-scale
circulation for Cyclogenesis.
TC active Year
High level
Low level
Eq
warm
cold
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Total number of MJO active days is 268 Number of
TCs is 28
Total number of MJO inactive days is 303 Number
of TCs is 12
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Cyclogenesis Mechanism In June 2004
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TC A Warning 2004/06/04 06UTC _at_ 15.7N,
116.6E Formed in Monsoon trough Affected by
easterly wave
Monsoon trough
Easterly wave
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-3 day
warming
-1 day
Tropical strom
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TC B Warning 2004/06/05 12UTC _at_ 6.7N,
141.7E Affected by Rossby wave energy dispersion
and easterly wave
Monsoon confluence zone
Easterly wave
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A
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TC C Warning 2004/06/11 18UTC _at_ 6.4N,
140.0E Formed in Monsoon shear line Affected by
TD-type disturbance
Monsoon shear line
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TD-type disturbance
TC B
TD-type disturbance
TC B
TC C
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TC D Warning 2004/06/21 18UTC _at_ 13.0N,
147.3E Formed in Monsoon confluence
zone Affected by MRG waves and Rossby wave energy
dispersion
Monsoon confluence zone
Mixed Rossby-Gravity wave
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C
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TC E Warning 2004/06/24 18UTC _at_ 11.6N,
153.8E Formed in Monsoon shear line Affected by
Rossby wave energy dispersion
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D
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Discussions

• A mechanism with long-term variations in SST in
  the tropical central Pacific and westerly wind
  anomalies associated with the monsoon trough
  provides a favorable large-scale circulation for
  TC genesis.
• The composite for TC-active (TC-inactive) years
  resemble the El Niño developing (La Niña)
  condition, i.e. warm (cold) SSTA occurring in
  central equatorial Pacific with westerly
  (easterly) anomaly and the monsoon trough and
  shear line extending eastward (westward).
• The SST and low-level winds in June 2004 is quite
  similar to the June composite of TC-active years
  but with much stronger amplitude.
• MJO provides a more favorable condition for TC
  genesis when its convective phase arrives at the
  western Pacific, and a less favorable condition
  when its suppressed phase resides over the
  western Pacific.
• The results indicate that the high TC genesis in
  June 2004 is a combined result of favorable
  large-scale environment provided by a developing
  El Niño warming condition and an unusually strong
  MJO that is coupled with high tropical waves-TC
  activities.
• Rossby wave energy dispersion is the main
  contribution of the clustering of TC genesis.

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Thank you
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The Oceanic Niño Index (ONI) has become the
de-facto standard that NOAA uses for identifying
El Niño (warm) and La Niña (cool) events in the
tropical Pacific.  It is the running 3-month mean
SST anomaly for the Nino 3.4 region (i.e.,
5oN-5oS, 120o-170oW).  Events are defined as 5
consecutive months at or above the 0.5o anomaly
for warm (El Nino) events and at or below the
-0.5 anomaly for cold (La Nina) events.  These
are further broken down into Weak (with a 0.5 to
0.9 SST anomaly), Moderate (1.0 to -1.4) and
Strong ( 1.5) events.
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The genesis mechanisms of TS B
Easterly wave and Rossby wave
A
A
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Phase speed is about 5-6 m/s