What contributes to the interannual variability in tropical lower
stratospheric temperatures?
Abstract
The interannual variability in mid and lower stratospheric temperatures
for the period 1984–2019 is decomposed into dynamical and radiative
contributions using a radiative calculation perturbed with changes in
dynamical heating, trace gases and aerosol optical depth. The
temperature timeseries obtained is highly correlated with the
de-seasonalised ERA5 temperature (r2>0.6 for 1995 to 2019
in the region 15 to 70 hPa). Contributions from ozone and dynamical
heating are found to be of similar importance, with water vapor,
stratospheric aerosols, and carbon dioxide playing smaller roles.
Prominent aspects of the temperature timeseries are closely reproduced,
including the 1991 Pinatubo volcanic eruption, the year-2000 water
vapour drop, and the 2016 Quasi-biennial oscillation (QBO) disruption.
Ozone below 20 hPa is primarily controlled by transport and is
positively correlated to the upwelling. This ozone-transport feedback
acts to increase the temperature response to a change in upwelling by
providing an additional ozone-induced radiative temperature change. This
can be quantified as an enhancement of the dynamical heating of about
20% at 70 hPa. A principle oscillation pattern (POP) analysis is used
to estimate the contributionof the ozone QBO (±1 K at 70 hPa). The
non-QBO ozone variability is also shown to be significant. Using the QBO
leading POP timeseries as representative of the regular QBO signal, the
QBO 2016 disruption is shown to have an anomalously large radiative
impact on the temperature due to the ozone change (>3 K at
70 hPa).