Analysis and attribution of total column ozone changes over the Tibetan Plateau during 1979–2017
2020
Abstract. Various observation-based datasets have confirmed
positive zonal mean column ozone trends at midlatitudes as a result of the
successful implementation of the Montreal Protocol. However, there is still
uncertainty about the longitudinal variation of these trends and the
direction and magnitude of ozone changes at low latitudes. Here, we use the
extended Copernicus Climate Change Service (C3S) dataset (1979–2017) to
investigate the long-term variations in total column ozone (TCO) over the
Tibetan Plateau (TP) for different seasons. We use piecewise linear trend
(PWLT) and equivalent effective stratospheric chlorine loading (EESC)-based
multivariate regression models with various proxies to attribute the
influence of dynamical and chemical processes on the TCO variability. We
also compare the seasonal behaviour of the relative total ozone low (TOL)
over the TP with the zonal mean at the same latitude. Both regression models show that the TP column ozone trends change from
negative trends from 1979 to 1996 to small positive trends from 1997 to 2017,
although the later positive trend based on PWLT is not statistically
significant. The wintertime positive trend starting from 1997 is larger than that in
summer, but both seasonal TP recovery rates are smaller than the zonal means
over the same latitude band. For TP column ozone, both regression models
suggest that the geopotential height at 150 hPa (GH150) is a more suitable
and realistic dynamical proxy compared to a surface temperature proxy used
in some previous studies. Our analysis also shows that the wintertime GH150
plays an important role in determining summertime TCO over the TP through
persistence of the ozone signal. For the zonal mean column ozone at this
latitude, the quasi-biennial oscillation (QBO) is nonetheless the dominant dynamical proxy. We also use a 3-D chemical transport model to diagnose the contributions of
different proxies for the TP region. The role of GH150 variability is
illustrated by using two sensitivity experiments with repeating dynamics of
2004 and 2008. The simulated ozone profiles clearly show that wintertime TP
ozone concentrations are largely controlled by tropics to midlatitude
pathways, whereas in summer variations associated with tropical processes
play an important role. These model results confirm that the long-term
trends of TCO over the TP are dominated by different processes in winter and
summer. The different TP recovery rates relative to the zonal means at the
same latitude band are largely determined by wintertime dynamical processes.
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