DavidDavid Noone*Noone*, , Yuk Yung, and Run-Lie ShiaYuk Yung, and Run-Lie Shia

Division of Geological and Planetary SciencesDivision of Geological and Planetary SciencesCalifornia Institute of Technology, Pasadena, CaliforniaCalifornia Institute of Technology, Pasadena, California

Modulation of the southern polar vortex Modulation of the southern polar vortex and tropospheric variability and tropospheric variability

by forcing in the tropical stratosphereby forcing in the tropical stratosphere((contrast with Northern Hemispherecontrast with Northern Hemisphere))

Also John Farrara and Carol Hsu, UCLAAlso John Farrara and Carol Hsu, UCLA

• Importance of stratospheric forcing on troposphere(“Pinatubo-like” tropical heating in a GCM)

• Model response (winter) – particularly the downward influence at mid/high latitudes

• Change in mean state, role of eddies(contrast SH with NH)

• Wave forcing feedback on stratosphere

• Implication for tracer transport and ozone

• Who leads? Who leads? Dynamics or chemistry or coupled?Dynamics or chemistry or coupled?Stratosphere or troposphere?Stratosphere or troposphere?

Overview

Observed trends in Antarctic temperature (cooling) empirically coincident with trend in Southern Annular Mode and ozone loss due to anthropogenic chlorine. (Thompson and Solomon 2002)

GCM studies suggest radiative cooling with ozone loss influences the dynamics of the polar vortices i.e., phase of Annular Modes (e.g., Shindell et al., 2002)

Unusually early breakup of the Antarctic polar vortex 2002. Signal of “regime shift” with less ozone, or just “weather”?

Recent thoughts

Pinatubo and ozone

Following the Pinatubo eruption (15 June 1991)… “Startling decreases in ozone abundance and in the rates of ozone destruction were also observed over Antarctica in 1991 and 1992” Self and Mouginis-Mark (Rev. Geophys, 1995)

Nimbus7 TOMS column O3 anomaly (GSFC)

Late winter 1991

DU

Model experiment

• NCAR-CCM3 T31L26

• 26 levels (13 in stratosphere, 2 in mesosphere)

• “perpetual aerosol thermal forcing”

• Radiatively “symmetric” w.r.t. season and hemisphere

20 year simulation (10 years results)

0.5 K/day

(UARS, Eluszkiewicz et al., 1997)

Winter geopotential height

Aerosol-heating experiment minus control

Enhanced stratospheric vortex slightly colder vortex core

Zonal wintertime circulation5K

4 m/sH8m/s

Heating at tropics, increase overturning, increase heat transport. Not SH!

Eddy heat transport

Eddy momentum transport

Tropospheric circulation

Stratospheric wave forcing

EP flux divergence (density weighted)

Proportional to meridional circulation

8 km/m2/s /day

negative anomalyincreased drag on mean flow increased overturning

Ozone response to “dynamics”

DU

2d CTM – transport prescribed. Modified by EP flux (see Noone et al. 2003)

Conclusions• With increased heating in the stratosphere,

generally weaker circulation in troposphere (both eddy activity and mean meridional flow).

• Substantial changes in stratosphere (different in SH and NH). Changes in the troposphere more subtle.

• Reduced extraction of heat from tropics by eddies, decreases the intensity of the Hadley cell (c.f. Ferrell cell NH)

• Transient wave activity response in SH limits possible “wave forcing” feedback on stratospheric dynamics. NH has longer (stationary) waves leading to a transient “wobble” in the stratosphere.

• Thus, while NH has increased meridional circulation in the stratosphere (temperature and ozone transport) this does not occur in SH.

• Colder Antarctic stratosphere: PSCs more likely, efficient destruction by heterogeneous chemistry (reverse case in Arctic).

• Although downward control occurs, lack of topography in SH give reduced dynamic wave-pumping feedback. Instead, larger influence on (polar) climate via radiative effects associated with chemical composition.

• Ozone loss (or otherwise) potentially more important driver.

Conclusions 2