Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Modulation of the southern polar vortex and tropospheric variability by forcing in the tropical stratosphere(contrast with Northern Hemisphere) DavidNoone*, Yuk Yung, and Run-Lie ShiaDivision of Geological and Planetary SciencesCalifornia Institute of Technology, Pasadena, California Also John Farrara and Carol Hsu, UCLA
Overview • 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? Dynamics or chemistry or coupled?Stratosphere or troposphere?
Recent thoughts 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”?
Pinatubo and ozone Late winter 1991 Nimbus7 TOMS column O3 anomaly (GSFC) DU 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)
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 circulation 5K 4 m/s H8m/s Heating at tropics, increase overturning, increase heat transport. Not SH!
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.
Conclusions 2 • 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.