TC Activity in WNP, murphreenps'edu, Oct08

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Long Term Changes in Tropical Cyclone Activity
in the Western North Pacific Tom Murphree and
David Meyer Naval Postgraduate School
(NPS) murphree_at_nps.edu and dwmeyer_at_nps.edu
33rd Annual Climate Diagnostics and Prediction
Workshop 21 October 2008
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity

• Overview
• Assessment of changes in tropical cyclone (TC)
  formations and
• intensities in western North Pacific during
  1970-2006
• Intraseasonal to multidecadal changes
• Long term net changes
• Relationships between TC changes and known large
  scale environmental factors (LSEFs)
• Relationships between TC changes and global
  warming (GW)

Hypothesis Assessments of GW impacts on TCs
need to account for all LSEFs that affect TCs,
and do so at time and space scales at which LSEFs
affect TCs.
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity

• Prior Studies
• LSEFs strongly affect TC formations (e.g.,
  McBride 1995)
• SST
• Vertical shear
• Vertical velocity
• Absolute vorticity
• Relative humidity
• ENLN impacts on TC activity (e.g., Chan 2000,
  Ford 2000)
• Impacts on formation sites, intensities, tracks
• GW impacts on TCs so far (e.g., Chan and Liu
  2004, Emanuel 2005, Webster and Holland 2005,
  Klotzbach 2006, Landsea 2007)
• Little or no increase in TC numbers
• Increase in TC intensities via SST increases
• Impacts from interannual-multidecadal variations
  dominate
• TC data inadequate to determine GW impacts
• Limitations of prior studies
• Coarse resolution basin and seasonal averages

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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity

• Data
• TC activity JTWC best track data (no
  adjustments)
• LSEFs NCEP reanalysis 1 and 2
• Period 1970-2006
• Main TC season May-December
• Region western North Pacific (WNP) 0-40 N,
  115185E
• Main development region (MDR) 0-20 N,
  115165E
• Temporal resolution one week
• Spatial resolution 5x5
• Number of TCs 989

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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
Selection of analysis period
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity

• Methods
• Identify major temporal and spatial patterns of
  changes in
• TCs formations and accumulated cyclone energy
  (ACE)
• LSEFs
• Develop regression models that relate TC
  formation probability and ACE to local LSEFs at
  weekly and 5x5 resolutions
• Validate models using independent TC and LSEF
  data
• Use models to assess sensitivity of TC formations
  and ACE to LSEF variations
• Relate GW to LSEF changes, and, thus, to TC
  changes
• By-product Models for sub-seasonal forecasting
  of TC formations and general tropical convection

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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
Number of TC Formations, 1970-2006
Main development region (MDR) 0-20N, 115-165E
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
Changes in TC Numbers and ACE, 1970-2006

• Large interannual-multidecadal variations (linked
  to ENLN)
• Long term net increases in TC formations and ACE

Can changes in LSEFs explain these long term
changes in TC activity?
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
LSEF Changes SST in MDR
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
LSEF Changes SST

• Overall SST increase in MDR (0.4 C) is
  favorable for increases in TC formations and
  intensities, and consistent with GW.
• Some SST decreases in TC active areas of WNP.

SST increases consistent with long term increases
in TC numbers and intensities.
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
LSEF Changes Vertical Velocity in MDR
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
LSEF Changes Vertical Velocity
Vertical Velocity Change, JASO, 1980-2006 (-Pa/s)

• Overall decrease in MDR is unfavorable for TC
  formation and ACE increases.
• Other LSEFs (shear, vorticity, RH) show no
  pronounced long term changes.

Have vertical velocity decreases countered
impacts of SST increases on TC formations?
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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
Regression Modeling of TC LSEF
Relationships 1. TC formation probability
log(p/(1-p)) a bSST cVertVel - dShear
eShear2 fAbsVort gEquat 2. ACE A
BSST CVertVel DShear - EShear2
FAbsVort GRH Sensitivity to LSEF Changes
Increase in each LSEF by one standard
deviation leads to following changes in formation
probability and ACE

• TC formation relatively sensitive (insensitive)
  to vertical velocity (SST).
• ACE moderately sensitive to SST and relatively
  insensitive to vertical velocity.

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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
Validation of TC Formation Probability Model

• Week-block LSEF values used to calculate ACE,
  given a TC had formed or was present in that
  week-block.
• If model ACE within 25 of analyzed scored as a
  hit, then model hit over 90 of the time,
  including the data from the 1970s and 2006 which
  were not used in model formation.

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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity
Validation of TC Formation Probability Model

• Modeled TC formation probabilities for individual
  week at 5x5 degree resolution (calculated using
  independent LSEF data)
• Developing sub-seasonal TC formation forecasts
  based on model

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TC Activity in WNP, murphree_at_nps.edu, Oct08
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Long Term Changes in WNP Tropical Cyclone Activity

• Summary
• Shortcomings of TC and LSEF data sets are
  problematic.
• TC formations and intensities have undergone long
  term increases since 1970.
• SST and vertical velocity are the only LSEFs that
  experienced net long term changes in WNP during
  1970-2006.
• SST (vertical velocity) changes were favorable
  (unfavorable) for increases in TC formation and
  intensity.
• Impacts of SSTs on TC formations may have been
  minimized by vertical velocity decreases.
• GW may have contributed to increases in TC
  formations and ACE, by way of SST increases but
  mitigated by vertical velocity decreases.
• But attribution is problematic, given
• Shortcomings of data sets
• Large interannual to multidecadal variations
• Uncertainties about LSEF-TC relationships and GW
  signatures

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TC Activity in WNP, murphree_at_nps.edu, Oct08