12th Conference on Middle Atmosphere

Monday, 4 November 2002: 4:20 PM
Long-Term Variability of ozone and temperature in the tropical lower stratosphere: The role of extratropical wave forcing
Lon L. Hood, University of Arizona, Tucson, AZ; and B. E. Soukharev
Satellite remote sensing records with lengths > 20 years are consistent with the existence of a long-term, decadal variation of ozone and temperature in the tropical lower stratosphere. Using a one-dimensional model for the quasi-biennial oscillation (QBO) of ozone and temperature, it is shown that decadal variability of the QBO can account for, at most, only a minor fraction of the tropical lower stratospheric decadal variation. One additional possible source of long-term variability in the tropics is extratropical wave forcing, which is an important driver of the Brewer-Dobson circulation. To investigate possible long-term variability of extratropical wave forcing, daily and monthly mean meridional eddy heat fluxes are calculated at a series of lower stratospheric pressure levels over a 23-year period using National Centers for Environmental Prediction (NCEP) Reanalysis data. A decadal variation of the low-pass filtered extratropical eddy heat flux is present in both hemispheres with an amplitude that increases with increasing altitude. A simplified model of the contribution of extratropical wave forcing to long-term variations in tropical lower stratospheric ozone and temperature is then formulated based on the ozone chemical continuity and thermodynamic energy equations. Using this model together with empirically derived regression relationships between short-term changes in extratropical eddy heat flux and tendencies in both tropical column ozone and lower stratospheric temperature, it is found that decadal variations of extratropical wave forcing in both hemispheres are sufficient to explain approximately both the amplitude and the phase of the observed decadal variation of the tropical lower stratosphere. A possible external source of the extratropical eddy heat flux variation is 11-year solar UV variability, which may influence the selection of preferred internal modes in the winter stratosphere.

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