Satellite Observations of 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

Background Observational Theoretical: 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.

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

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?

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

Equatorial absolute vorticity: Atlantic

OLR Observations

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

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

Correlation: E-C Atlantic .57 (explain 33%; Epac .2 (explain 4%)

Modeled Vertical Structure

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.

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.