Examining the ENSOtyphoon hypothesis

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• Examining the ENSO-typhoon hypothesis
• (Elsner and Liu, 2003)
• 2005. 4. 20.
• ? ? ?, ? ? ?, ? ? ?

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1. Introduction

• ENSO plays a role in the frequency and
  distribution of WNP typhoons(Li, 1986). While the
  effect of ENSO on the overall number of typhoons
  appears to be minor, it tends to create
  conditions favorable for shifts in the main
  genesis region.
• During El Nino events, tropical cyclones are
  relatively more likely to form east of about
  160E(Lander 1994).
• These refer to as the ENSO-typhoon hypothesis.
• The corollary is that typhoons are relatively
  more likely to affect China during La Nina years.
• Mordern observational records of tropical
  cyclones are generally too short to reliably
  associate climate factors with landfall
  probabilities over specific regions. -
  paleotempestology, abundance of historical
  documentary records from southern China
• We analyze the mordern typhoon record from WNP
  with annual typhoon counts extrcted from
  Guangdong(GD) historical documents in an attempt
  to better understand the relationships of climate
  to catastropic typhoons. The modern record allows
  us to group typhoon tracks and relate track types
  to ENSO.

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2. Data

• 1) Mordern typhoon track data for th entire WNP
  from Unisys co. web. 6h positions and intensities
  by JTWC
• 2) Instrumental records of climate indices CTI,
  PDOI, NAOI data from ICODAS(1849-1997),
  NCEP(1998-2002)
• CTI(cold tongue index) average SST anomalies
  over the region bounded by 6N to 6S and 90 to
  180E
• PDOI(Pacific decadal oscillation index) is
  defined as the leading principal component of
  North Pacific monthly SST variability.
• NAOI(North Atlantic Oscillation Index- the
  normalized pressure difference btween Azores and
  Iceland) correlated with the frequency of
  tropical cyclones(hurricane)
• 3) Historical typhoon records from GD
• A portion(16001909) of the reconstructed 1000yr
  time-series of typhoon landfalls from GD provides
  a key data set
• 4) Paleoclimatic proxy records of ENSO
• Reconstructions of past climate condions from
  tree-ring chronologies from NOAA paleoclimatology
  program.

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3.1 Typhoons in the morden record(1945-2002)-stat
istics

• Typhoon tropical cyclone that has sustained 1
  min wind speed of at least 64knot.

• The summary statistics based on 1002 typhoons
  over the WNP for 58yr
• From pre(19451970) and post(19712002)
  satellite eras, typhoon position-no difference
  but maximum intensities are higher 4.4knot
• Most typhoons originate between 525N and
  120160E and develop within the low-level monsoon
  trough. During El Nino years the monsoon
  trough extends farther east, allowing typhons to
  form farther east whereas during La Nina stronger
  easterly trades dominate the low latitude of WNP
  keep the monsoon through and typhoon genesis
  region to the west.

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3. 2 Typhoon track types
The mean tracks for the 3 typhoon groups(SM,R,NO)
based on average latitude and longitude
positions at initial, maximum, and final typhoon
intensities.-k-mean cluster analysis

• distinguish feature SM-remain on a general
  westerly course, R and NO-move toward higher
  latitude
• Average MI of SM typhoons is less than for R
  typhoons
• Average time at typhoon intensity is shortest for
  SM because they reach cost quicker
  

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3. 2 Typhoon track types

3.7/a year
13(1958)
6.5/a year
2(1975,7)
17(1964)
7.1/a year
0(2002)

• Anti-correlation between the annual frequencies
  of SM and R typhoons(r-0.37).
• Anti-correlation hints at a large scale
  mechanism for changes in landfall probability -
  1) mean position of the subtropical high, 2)
  changes in SST
• No association between annual counts of SM and
  NO typhoons or between R and NO typhoons

• Annual average number of WNP typhoons over
  1945-2002
• Straight-moving typhoons are most frequent

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3. 3 Terminal positions of typhoons

• Annual number of Japan and south China
  landfalling typhoons vs R and SM typhoons
• weak or no relationship between R and south
  China landfalls and between SM and Japan
  landfalls
• positive relationship between R and Japan
  landfalls and between SM and south China
  landfalls
• indices of landfall threat

• Terminal positions of all 1002 typhoons (1945
  2002) by track type
• SM - Philippines, Vietnam, southern China
• R - Korea, Japan
• NO remain offshore

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3. 4 Relationships between track type and climate

(La Nina)
(El Nino)

• Typhoon activity relative to the 3 climate
  indices. ENSOvalue of the CTI(SSTs over the
  equatorial Pacific region). NAOvalue of the
  pressure differencs between Iceland and
  Gibralter. PDOlead principal component of SSTs
  over the extra-tropical regions of the North
  Pacific Ocean. Index values are averaged over the
  3 mo of AugOct.
• The relationship between track frequency and
  ENSO is opposite for R and NO typhoons
• R typhoons are more common during El Nino years
  consistent with an eastward displacement of
  tropical cyclone origins during El Nino years.
  During La Nina years, typhoons developing
  farther west tend to move straight in direction
  of southern China.
• activity is minor difference with NAO index,
  unrelate to PDO index

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4. Historical typhoon record

Grand mean 1.5

• Rate difference in GD typhoon activity between
  extermes of the ENSO cycle translates into
  difference in the annual probability of activity.
• Assumed poisson distribution.
• Multiple typhoon hits are more likely during
  strong La Nina events.

• GD typhoons by year. Years are 20 warmest and
  coldest based on annual tree-ring chronologies as
  an index for ENSO SST over 16001909. proxy
  values Dec-Feb average placed on the December
  year.
• grand mean 1.5 typhoons per year,
• El Nino(La Nina) events decrease(increase) the
  typhoon rate below(above) climatology

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5. Summary and Conclusion

• Mordern typhoon data and historical documents GD
  are analyzed and found to support the
  ENSO-typhoon hypothesis.
• The hypothesis states that tropical cyclone
  formation during an El Nino event shifts eastward
  with typhoons tending to recurve north.
• Typhoon tracks are grouped into 3 distint
  cluster(SM, R, NO) based on geographic position
  at maximum and terminal typhoon intensities.
• Major typhoon origin 110-170E and 8-25N.
• SMwesterly path-threaten Philippines, southern
  China, Vietnman, Rwest-northwesterly
  path-threaten Korea Japan, NOkeeps them out to
  sea.
• SM typhoons is positively correlated with the
  number of landfalls over southern China. Thus
  the abundance of SM typhoons is a good indicator
  of the typhoon threat to southern China.
• SM typhoons is correlated with the ENSO cycle.
• A long annual time-series(16001909) of typhoon
  landfall counts from GD, extracted from
  historical documents together with tree-ring
  proxy records of the ENSO cycle, provide data
  that independently support this relationship.

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6. References

• Lander MA(1994) An exploratory analysis of the
  relationship hetween tropical storm formation in
  the western north Pacific and ENSO. Mon weather
  Rev 122636-651.
• Elsner JB and Liu Kam-biu (2003) Examining the
  ENSO-typhoon hypothesis. Clim Res 2543-54.

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• The k-means clustering method is based on iterave
  relocation in which data values are moved from
  one group to another until there is no additional
  improvement in the sum-of-squares criterion.
• Four variables are used in the cluster analysis
  1)latitude, 2)longitude of the typhoon at maximum
  intensity, and 3)latitude, 4)longitude of the
  typhoon at final intensity.
• Three unique characteristics of the present
  calssification exclusion or tropical storms,
  inclusion of typhoons over the South China Sea,
  and use of an objective classification
  scheme(cluster analysis)