Spatial and Transient Behavior of the South Pacific Convergence Zone

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Spatial and Transient Behavior of the South
Pacific Convergence Zone
Presented by, Matthew Widlansky Peter J.
Webster, Advisor Hai-Ru Chang Carlos
Hoyos School of Earth and Atmospheric
Sciences, Georgia Institute of Technology November
14, 2008
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Literary Review
Cloud Cover

• The SPCZ is a region of widespread cloud cover
  and precipitation extending southeastward from
  New Guinea into Southern Hemisphere (SH)
  mid-latitudes. (Streten 1973 Trenberth 1976)
• Tropical convection is oriented zonally and
  highly correlated with the warmest SSTs. (Vincent
  1994)
• Baroclinic-type disturbances influence the
  diagonal region. (Kiladis et al. 1989)
• Orientation changes during different phases of
  the El Niño-Southern Oscillation (ENSO).
  (Trenberth 1997 Karoly and Vincent 1999)

Focus of Study
Why does the SPCZ veer southward away from the
ITCZ?
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Motivation
While basin-scale climate studies point to the
southwest Pacific as a region pivotal to decadal
climate variability, neither its oceanic or
atmospheric features specifically the South
Pacific Convergence Zone have been properly
depicted by models or observations (Ganachaud
et al. 2007, CLIVAR SPICE Project)
Stated Goals of SPICE

• Improve dynamical explanations for why the SPCZ
  forms.
• Understand why General Circulation Models (GCMs)
  misdiagnose the southward veering of the SPCZ.
  (Double ITCZ problem)
• Determine where new meteorological observations
  are necessary to better constrain atmospheric
  GCMs.

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Data and Methods
Upper-troposphere (200 hPa) zonal winds (u)
diagnose regions of negative zonal stretching
deformation (s-1)
Nomenclature
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SPCZ Seasonal Cycle
JJA Climatology OLR and Zonal Stretching
Deformation (200 hPa)

• Regions with OLR values less than 240 W m-2
  (Vincent 1994) are experiencing deep atmospheric
  convection.
• Seasonal convection patterns are driven by the
  meridional shift of the West Pacific Warm Pool.
• SPCZ reaches strongest intensity during the
  austral summer months (DJF).

DJF Climatology OLR and Zonal Stretching
Deformation (200 hPa)
Contours Negative Zonal Stretching Deformation
(200 hPa)
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Fundamental Questions

• What dynamical processes amplify convection in
  the mid-latitude (diagonal) SPCZ?
• Why do many GCMs fail to simulate convection in
  the diagonal portion of the SPCZ?
• Why does the SPCZ veer southward away from the
  ITCZ?

NCEP Reanalysis
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Mid-latitude Wave Trains
2-8 day bandpass filtered OLR linear regression
Base Point 35S, 195E
OLR (unfiltered)- Shaded contours ?u/?x
(unfiltered)- Values greater (less) than 4x10-7
s-1 (-4x10-7 s-1) are shown.
Solid contours depict negative
anomalies, 2x10-7 s-1 interval.
Methods adapted from (Serra, Kiladis, Cronin
2008)
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Wave Energy Accumulation
NCEP Reanalysis
Upper Troposphere

• Mean zonal winds create a band of
  upper-tropospheric negative stretching
  deformation near the subtropical and mid-latitude
  SPCZ.

• Group velocities (Cgd) of Rossby waves slow
  down in these regions leading to an accumulation
  of wave energy (e). (Webster and Chang 1997)

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Baroclinic Instability
Do mid-latitude cyclones influence the SPCZ?

• Disturbance activity measured by eddy
  perturbation kinetic energy (PKE)
• Amplification near sub-tropical jet stream exit
  region
• Maximum PKE from east coast of Australia to
  mid-latitude SPCZ

(e.g., Webster 1985 Webster 1989 Matthews and
Kiladis 1999)
NCEP Reanalysis
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Baroclinic Instability
2006 Case Study
NCEP Reanalysis
Latitude-Time PKE Chart

• Bursts of PKE in sub-tropical and mid-latitude
  SPCZ.
• No clear latitudinal propagation into tropical
  SPCZ.

NCEP Reanalysis
Do disturbances accumulate near the SPCZ?
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Mid-latitude Wave Accumulation
2006 Case Study
OLR and Zonal Stretching Deformation
B
A
C
Hovmoller (Longitude- Time) Diagram

• Case Study (DJF 2006)
• Meridional average (20S-35S)

Time (days)

• Three regions of enhanced convection
• South Indian Convergence Zone
• SPCZ
• South Atlantic Convergence Zone

Pronounced eastern boundary of the SPCZ.
Zonal Stretching Deformation (s-1)
OLR (W m-2)
A
B
C
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Mid-latitude Wave Accumulation
2006 Case Study
2-8 day Filtered OLR and Zonal Stretching
Deformation
Two Propagation Regimes 1) Fast 1,500
km day-1 2) Slow 600 km day-1
Time (days)

• Case Study Observations
• Many disturbances propagate slowly through the
  SPCZ.
• Convective anomalies increase.

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Wave Energy Accumulation
Review of hypothesis for wave accumulation near
the SPCZ
Boundary Layer (Previous Work)
Tropical modes may accumulate in lower
troposphere bands of
(e.g., Webster and Chang 1988)
Upper Troposphere (Current Focus)
Mid-latitude Rossby waves may accumulate in upper
troposphere region of
(Schematic based on concepts in Webster and Chang
1997)
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SST Forcing on Zonal Winds
Zonal SST Gradient

• Strong basin-scale zonal SST gradient.
• Greatest SST gradient found near SPCZ eastern
  boundary.

90 Confidence Bounds
Correlations Zonal Stretching and OLR
Zonal Stretching Deformation (s-1)
OLR (W m-2)
90 Confidence Bounds
?u/?x
OLR
Increased Convection
More Negative
SST Gradient (C)
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Intra-seasonal Forcing
Intensity Changes
El Niño events (e.g., 1998)
?u/?x gt 0
OLR
Convection decreases in mid-latitude SPCZ (black
box)
(Standardized Indices)
Spatial Behavior
Correlation of Seasonal Averages OLR and Zonal
Stretching Deformation
El Niño (NE Shift)
La Niña (SW Shift)
Correlations statistically significant (95
level) for r gt0.4
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Review of Dynamical Processes

• Sub-tropical jet stream enhances baroclinic
  instability in diagonal SPCZ. (Kiladis et al.
  1989)
• Negative zonal stretching deformation cause
  synoptic disturbances to slow down and accumulate
  in the mid-latitudes. (Webster and Chang 1988
  and 1997)
• Wave energy accumulation may enhance convection
  in the diagonal SPCZ. (Current Work)
• Zonal stretching deformation may be forced by
  basin-scale SST gradients which are influenced by
  the phase of ENSO. (Current Work)

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Remaining Questions and Future Work

• Do correlations between wave energy accumulation
  and convection exist on synoptic timescales?
• What causal relationships exist between SST,
  ?u/?x, and convection?

Large-scale and high resolution modeling
experiments
6hr WRF simulation of the SPCZ
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