Climate Dependencies and Deterministic Variability in Stratospheric Dynamics and Ozone

Estimates of the ozone layer future evolution must consider both
climate dependencies and interannual variability. These
considerations imply analyses of transient-scenario realisations
with chemistry-climate models (CCM) under realistic boundary
conditions. In this context, investigations of ozone variability
usually involve multiple regression analysis (MRA), a statistically
efficient albeit complicated tool. However, a careful use of
advanced regression approaches may improve the variability
assessment considerably. The present study addresses climate
dependencies in ozone transport, and adopts an advanced regression
approach to both quantify deterministic ozone variability and trace
it back to the scenario boundary conditions; the investigations
refer to transient output of the CCM E39/C. Recent observations
show a cooling of the tropical lower stratosphere, and CCMs suggest
a spatial coincidence of the cooling with a stronger upward
advection of ozone-poor tropospheric air. This advection increase
appears to result from a currently unexplained strengthening of the
planetary-wave driven mean meridional transport, arguably relating
to the anthropogenic climate signal. The present study explores the
strengthening by comparing realisations of two different scenarios.
Both share the same boundary conditions including concentrations of
ozone-depleting substances (ODS), but differ in their climate
forcing via sea surface temperatures (SSTs) and well-mixed
greenhouse gas concentrations (GHG). In the summer hemisphere
tropics, higher SSTs for the warmer scenario amplify deep
convection and hence the convective excitation of internal
planetary waves. These waves travel upward through easterly winds
while dissipating, but still carry enough of the signal into the
lower stratosphere to intensify the mean meridional transport. The
transport change in turn strengthens the input rate into the
tropical lower stratosphere of ozone-poor tropospheric air,
ultimately weakening lower-stratospheric ozone concentrations via
higher tropical SSTs. The ozone variability assessment relates to
monthly-mean total columns from three independent realisations of a
60-year transient scenario with realistic boundary conditions. It
focuses on three latitudinal bands: southern/northern midlatitudes
(SH/NH) and tropics. Common ozone MRAs are linear and iterate to
account for auto-regression-induced nonlinearity. The present MRA
is nonlinear and the first to demonstrate the validity of such
iterations with respect to the least-squares surface: it detects
only a weak distortion of the surface associated with
autocorrelation, at least for the ozone time series examined. Also,
the present MRA is among the few to demonstrate sufficient
compliance with the regression requirements, particularly with that
of independent residuals. Additionally, the new approach of
response confidence bands permits a correct attribution of
individual anomalies to the scenario boundary conditions. As a
consequence, the present MRA is the first to explain the year 1985
SH low-ozone event, here reproduced by E39/C. The MRA further
captures, e.g., a similar anomaly for the year 1997, and verifies
the total-ozone response to stratospheric-transport modulating
boundary conditions: tropical-SST anomalies (ENSO) affect the
tropics and NH, but not the SH; or, the quasi-biennial oscillation
(QBO) causes a seasonally synchronised ozone response at SH and
more weakly at NH, but not in the tropics. While these features
have already been reported for E39/C data, the present study
establishes a firm statistical framework and discusses the physical
background. Other responses refer to the 11-year solar cycle (SSC),
to sulfate aerosols, and to ODS concentrations. The present
nonlinear regression approach provides ample potential for further
development. For instance, nonlinear deterministic regression terms
may examine the existence of interactions between the NH ENSO
response with long-term changes in the probability for northern
polar heterogenous ozone depletion. Last, accounting for
moving-average regression parameters may improve the compliance
with the inference requirements even further. In conclusion, the
E39/C boundary conditions modulate the ozone layer as well as
stratospheric mean meridional mass transport on long and short time
scales. In this respect, the most important result is the universal
significance of tropical SSTs controlling stratospheric transport
by governing the deep-convective production of internal and,
probably, external planetary waves. An important future research
task is whether increasing tropical SSTs can cause ENSO-like
changes in wintertime mid- and polar-latitude stratospheric
planetary-wave activity; such changes could disturb the northern
polar vortex against the effect of radiatively induced
stabilisation by higher GHG concentrations. E39/C and other CCMS
have certain weaknesses, one of which is an unrealistically
consistent QBO-related modulation of the northern polar vortex.
Keeping these weaknesses in mind, MRA may represent a helpful tool
as it improves the statistical efficiency.