The MJO signal in tropical cyclone activity A study using a genesis potential index

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The MJO signal in tropical cyclone activity A
study using a genesis potential index

• Suzana J. Camargo
• Lamont-Doherty Earth Observatory, Columbia
  University, Palisades, NY
• Matthew C. Wheeler
• Centre for Australian Weather and Climate
  Research, Melbourne, Australia
• Adam H. Sobel
• Columbia University, New York, NY

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• The convective phase of the MJO brings
  increased TC activity in many basins. By what
  mechanism does this occur?
• We examine this using an empirical genesis
  potential index. To the extent that the GP index
  captures the
• MJO signal in TC activity, it gives us an
  objective
• way to compare quantitatively the influence of
• different environmental factors (humidity, wind
  shear
• etc.) all of which are modulated by the MJO.

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We use the Emanuel-Nolan genesis potential index.
The idea follows Gray (1979)(but e.g. no SST
threshold)

• GP 105 ?3/2 (H/50)3 (Vpot/70)3 (10.1Vshear)-2
• ? absolute vorticity at 850hPa (s-1)
• H relative humidity at 700hPa ()
• Vpot potential intensity (m/s) as per
  Emanuel.
• Vshear magnitude of the vertical wind shear
  between 200 and 850hPa (m/s).

K.A. Emanuel and D. Nolan, 26th AMS Conf.
Hurricanes Tropical Meteorology,
2004. Camargo, Emanuel and Sobel, J. Climate, 20,
4819-4834.
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• We use the real-time multivariate MJO index of
  Wheeler and Hendon (2004) to define the MJO phase
  and amplitude.

Composite OLR anomalies (shading,
hatchedpositive) and 850 hPa wind
anomalies associated with 8 phases of the MJO
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We compute MJO anomaly composites for the GPI
(computed from NCEP Reanalysis data similar
results from ECMWF) and for observed TC genesis
density and track density (computed from best
track data sets).
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GPI Anomaly composites - JFM
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GPI and OLR Anom. Composites - JFM
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Track Density Anom. composites - JFM
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Having shown that the GPI captures the MJO signal
in TC activity, we use it to assess which factor
is most important in producing that signal
potential intensity, low-level absolute vorticity,
vertical wind shear, or midlevel relative
humidity? We do this by constructing a new GPI
in which 3 of 4 factors are set to climatology
and the 4th is allowed to fluctuate. We do this
for all 4 factors. This gives us a set of
anomalies in each individual factor which is
weighted appropriately to compare its influence
on TC activity to that of the others.
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Relative Humidity varying - JFM
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Vorticity varying - JFM
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Vertical Shear varying - JFM
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Potential Intensity Varying - JFM
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Relative Humidity varying - ASO
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Conclusions

• The GP index is able to capture the observed TC
  activity variations with the MJO, qualitatively
  and quantitatively.
• Parsing the GP into its various factors, relative
  humidity makes the largest contribution to the
  MJO enhancement to TC activity. Vorticity also
  contributes, vertical shear goes the wrong way,
  PI (which incorporates column stability and SST)
  is relatively unimportant.

Paper in review for J. Climate, available at
www.columbia.edu/ahs129/pubs.html
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GPI Anom. Composites - ASO
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GPI OLR Anom. Composites ASO
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Track Density Anom. Composites ASO
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Calculating GPI - MJO composites

• Data
• NCEP reanalysis daily data
• Reynolds WEEKLY SST data ( available Nov. 1981
  to present)
• Period 1982 - 2007
• Calculate daily PI, vertical shear and absolute
  vorticity.
• Weekly running means of PI, relative humidity,
  absolute vorticity and vertical shear
• Calculate daily GPI from smoothed fields
• Daily GPI climatology
• Daily GPI anomalies
• Composites of GPI using MJO index

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Other composites

• Outgoing longwave radiation (OLR)
• Daily data - Liebmann Smith (1982-2007)
• Composites calculated similarly to GPI
• Track density
• NHC and JTWC best track data (1982-2007)
• Count number of occurrences in each 2.5 degrees
  square - same resolution as other data.
• Composites calculated similarly to GPI.

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Genesis Potential Index Climatology - September