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Abstract. Various observation-based datasets have confirmedpositive zonal mean column ozone trends at midlatitudes as a result of thesuccessful implementation of the Montreal Protocol. However, there is stilluncertainty about the longitudinal variation of these trends and thedirection and magnitude of ozone changes at low latitudes. Here, we use theextended Copernicus Climate Change Service (C3S) dataset (1979–2017) toinvestigate the long-term variations in total column ozone (TCO) over theTibetan Plateau (TP) for different seasons. We use piecewise linear trend(PWLT) and equivalent effective stratospheric chlorine loading (EESC)-basedmultivariate regression models with various proxies to attribute theinfluence of dynamical and chemical processes on the TCO variability. Wealso 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 fromnegative trends from 1979 to 1996 to small positive trends from 1997 to 2017,although the later positive trend based on PWLT is not statisticallysignificant. The wintertime positive trend starting from 1997 is larger than that insummer, but both seasonal TP recovery rates are smaller than the zonal meansover the same latitude band. For TP column ozone, both regression modelssuggest that the geopotential height at 150 hPa (GH150) is a more suitableand realistic dynamical proxy compared to a surface temperature proxy usedin some previous studies. Our analysis also shows that the wintertime GH150plays an important role in determining summertime TCO over the TP throughpersistence of the ozone signal. For the zonal mean column ozone at thislatitude, the quasi-biennial oscillation (QBO) is nonetheless the dominant dynamical proxy. We also use a 3-D chemical transport model to diagnose the contributions ofdifferent proxies for the TP region. The role of GH150 variability isillustrated by using two sensitivity experiments with repeating dynamics of2004 and 2008. The simulated ozone profiles clearly show that wintertime TPozone concentrations are largely controlled by tropics to midlatitudepathways, whereas in summer variations associated with tropical processesplay an important role. These model results confirm that the long-termtrends of TCO over the TP are dominated by different processes in winter andsummer. The different TP recovery rates relative to the zonal means at thesame latitude band are largely determined by wintertime dynamical processes.

Analysis and attribution of total column ozone changes over the Tibetan Plateau during 1979–2017