Quasi-decadal variability of the Tropical Lower Stratosphere: The role of extratropical wave forcing

Column ozone and satellite-derived temperature records with lengths >20 yr are consistent with the existence of a long-term, quasi-decadal oscillation (QDO) of the tropical lower stratosphere. Using a one-dimensional model for the quasi-biennial oscillation (QBO) of ozone and temperature, it is found that decadal variability of the QBO can account for, at most, only a minor fraction of the tropical lower-stratospheric QDO. One additional 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-yr 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 the Northern Hemisphere with an amplitude that increases with increasing altitude. In the extratropical Southern Hemisphere, a decadal variation is also present but is less regular, possibly owing to reduced radiosonde data coverage. 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 may be sufficient to explain much of the amplitude and the phase of the observed QDO of the tropical lower stratosphere.