Abstract
Using various satellite-based observations, a linear ozone transport model (LOTM), a chemistry-climate model (WACCM3) and an offline chemical transport model (SLIMCAT), zonally asymmetric trends of the total column ozone (TCO) in the northern middle latitudes during winter for the period 1979–2015 are analyzed and factors responsible for the trends are diagnosed. The results reveal that there are significant negative TCO trends over the North Pacific and positive TCO trends over the northwestern North America. The zonally asymmetric TCO trends are mainly contributed by the trends in partial column ozone in the upper troposphere and lower stratosphere (UTLS) which are closely related to the long-term changes of geopotential height in the troposphere. Furthermore, the trends of geopontential height in the UTLS are mainly modulated by pattern changes in the Arctic Oscillation (AO), the Cold Ocean–Warm Land (COWL) and the North Pacific (NP) index. Accordingly, the zonally asymmetric TCO trends can be largely reconstructed by the trends of the above three teleconnection patterns. Sea surface temperature (SST) changes over the Pacific Ocean and the Atlantic Ocean can also exert a significant contribution to the zonally asymmetric TCO trends through their influence on the COWL and NP patterns. In addition, chemical ozone loss partially offsets the positive trends in zonal TCO anomalies over Central Siberia and enhances the positive TCO trends over northwestern North America. However, the contribution of chemical processes to the zonally asymmetric TCO trends is relatively smaller than that of dynamical transport effects. Interpreting the zonally asymmetric TCO trends and their responsible factors would be helpful for accurately predicting the stratospheric ozone return date in the northern middle latitudes.
Highlights
The implementation of the Montreal Protocol and its amendments has effectively stopped the continuous stratospheric ozone depletion caused by anthropogenic ozone depleting substances, and global total column ozone (TCO) is expected to show a positive trend in the coming decades (e.g. Newchurch et al 2003; Angell and Free 2009; Eyring et al 2010; Krzyścin 2010; Zhang et al 2014; Chipperfield et al 2017)
The Arctic Oscillation (AO), Cold Ocean–Warm Land (COWL) and North Pacific (NP) patterns reflect most of the tropospheric climate variability (Wu and Straus 2004a); the main feature of the zonal asymmetric ozone (ZAO) trends are well captured by the TCO changes associated with the three patterns, for the COWL and NP patterns, suggesting that tropospheric climate change may exert a significant influence on the ZAO trend in the northern middle latitudes
It is found that there are maximum decreasing TCO trends over the North Pacific and relatively weaker decreasing TCO trend over the northern Atlantic Ocean and the northern Caspian Sea in winter, while increasing TCO trends are noted over northwestern North America and Central Siberia, suggesting that the ozone trends in northern mid-latitude winter are longitude-dependent
Summary
The implementation of the Montreal Protocol and its amendments has effectively stopped the continuous stratospheric ozone depletion caused by anthropogenic ozone depleting substances, and global TCO is expected to show a positive trend in the coming decades (e.g. Newchurch et al 2003; Angell and Free 2009; Eyring et al 2010; Krzyścin 2010; Zhang et al 2014; Chipperfield et al 2017). Peters and Entzian 1999; Peters et al 2008; Efstathiou et al 2003; Hio and Yoden 2004; Grytsai et al 2005; Gabriel et al 2011; Ialongo et al 2012; Zhang et al 2015) and have proposed that the planetary waves of tropospheric origin could affect TCO through tropopause height modulations and stratosphere-troposphere exchange processes. We will analyze ZAO variations over the past three decades and diagnose the relative importance of various processes which dominate the zonally asymmetric TCO trend over the whole northern middle latitudes during winter (December–January–February, DJF) when the ZAO structure is more pronounced (Hood and Zaff 1995; Gabriel et al 2011).
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