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.