Satellite Observations

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Satellite Observations of Near-Equatorial
Symmetric Instability Gad Levy and Tim J.
Dunkerton NorthWest Research Associates http//w
ww.nwra.com/resumes/levy/papers/ November 30,
2004 PORSEC - Univesidad de Concepcion, Chile
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Background

• Observational
• Young, 1987 (Western Indian Ocean)
• Wallace et al., 1989 (eastern Pacific)
• Levy and Battisti, 1995
• Thomas and Webster, 1997 (suggest instability
  causes off equatorial convection)

• Theoretical
• Stevens 1983 Zonal flow is symmetrical across
  the equator meridional overturning will restore
  symmetric stability.
• Dunkerton 1981 Vertical mixing by inertial
  instability may be responsible for maintaining
  PBL.

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Background

• Modeling
• Thomas et al., 1999 Dissipation has a
  stabilizing effect sufficient.

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Objectives

• Take advantage of the remote sensing capabilities
  to monitor climate processes and
• Document the symmetric stability of the
  equatorial flow based on 5 years of Quikscat data
• Document OLR (remotely sensed convection proxy
• Is there a relationship?

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The criterion for symmetric instability

• In a nearly barotropic (tropical) atmosphere a
  sufficient condition for linear symmetric
  instability, is the same as that for inertial
  instability
• f ( f relative vorticity) lt 0
• Scatterometer observation allow one to evaluate
  this criterion globally near the surface

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Equatorial absolute vorticity Atlantic
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OLR Observations
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Observations Atlantic

• Seasonal cycle retreat in boreal winter
• Convection over Amazon basin and Western Pacific
  in boreal winter
• Symmetric instability in SH in Western Atlantic
  for most of the year.
• Main climatological features seen in OLR is
  mirrored in vorticity

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Observations Pacific

• Double ITCZ
• Instability migrates south of the equator in
  western Pacific in boreal winter
• Smaller amplitude and less convective activity in
  central Pacific
• Main climatological features seen in OLR is
  mirrored in vorticity

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Correlation E-C Atlantic .57 (explain 33 Epac
.2 (explain 4)
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Modeled Vertical Structure
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Possible Feedback Mechanism

• A significant monsoonal/SST asymmetrical forcing
  causes cross equatorial flow leading to unstable
  region north of the equator.
• Northward advection (v gt 0) of low angular
  momentum air, which crosses the equator
  decelerates westerly zonal flow.
• Easterly momentum advected across equator is
  turned to the north by the Coriolis force,
  reinforcing the northward flow.
• A vertical component to the motion caused by a
  two dimensional instability may drive the system
  back towards neutral stability by turbulent
  mixing, which maintains the PBL in the latitudes
  between the equator and the ITCZ.

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Summary

• Remote sensors allow monitoring the low level
  symmetric stability of the equatorial flow and
  convective activity.
• The two appear to be related.
• Remote sensing of the stress field (friction)
  would allow proper modeling of the equatorial
  PBL, which would allow testing hypothesized
  feedback mechanism.