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Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Satellite Observationsof Near-Equatorial Symmetric Instability
Gad Levy and Tim J. Dunkerton
NorthWest Research Associates
http://www.nwra.com/resumes/levy/papers/
November 30, 2004: PORSEC - Univesidad de Concepcion, Chile
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Background
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.
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)
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Background
Modeling Thomas et al., 1999: Dissipation has a
stabilizing effect sufficient.
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
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?
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
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) < 0
Scatterometer observation allow one to evaluate this criterion globally near the surface
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Equatorial absolute vorticity: Atlantic
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
OLR Observations
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
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
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
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
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Time Series of Symmetric Instability/OLR minima by sector
-6
-4
-2
0
2
4
6
8
10
12
14
16
Jul-99Nov-99Mar-00Jul-00Nov-00Mar-01Jul-01Nov-01Mar-02Jul-02Nov-02
Month
Latitude
10W-30W II (ext)lat.
E-C Atlantic OLR Lat.
80W-120W II Lat
Epac OLR Lat.
Correlation: E-C Atlantic .57 (explain 33%; Epac .2 (explain 4%)
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Modeled Vertical Structure
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
Possible Feedback Mechanism
A significant monsoonal/SST asymmetrical forcing causes cross equatorial flow leading to unstable region north of the equator.
Northward advection (v > 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.
Satellite Observations of Equatorial Symmetric Instability - PORSEC 2004
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.