Homogenization of the Observatoire de Haute Provence ECC ozonesonde data record: comparison with lidar and satellite observations

The Observatoire de Haute Provence (OHP) weekly Electrochemical Concentration Cell (ECC) ozonesonde data have been homogenized for the time period 1991–2020 according to the recommendations of the Ozonesonde Data Quality Assessment (O3S-DQA) panel. The assessment of the ECC homogenization benefit has been carried out using comparisons with ground based instruments also measuring ozone at the same station (lidar, surface measurements) and with collocated satellite observations of the O3 vertical profile by Microwave Limb Sounder (MLS). The major differences between uncorrected and homogenized ECC are related to a change of ozonesonde type in 1997, removal of the pressure dependency of the ECC background current and correction of internal ozonesonde temperature. The 3–4 ppbv positive bias between ECC and lidar in the troposphere is corrected with the homogenization. The ECC 30-years trends of the seasonally adjusted ozone concentrations are also significantly improved both in the troposphere and the stratosphere when the ECC concentrations are homogenized, as shown by the ECC/lidar or ECC/surface ozone trend comparisons. A −0.29 % per year negative trend of the normalization factor (NT) calculated using independent measurements of the total ozone column (TOC) at OHP disappears after homogenization of the ECC. There is however a remaining −5 % negative bias in the TOC which is likely related to an underestimate of the ECC concentrations in the stratosphere above 50 hPa as shown by direct comparison with the OHP lidar and MLS. The reason for this bias is still unclear, but a possible explanation might be related to freezing or evaporation of the sonde solution in the stratosphere. Both the comparisons with lidar and satellite observations suggest that homogenization increases the negative bias of the ECC up to 10 % above 28 km.

Global total ozone recovery trends derived from five merged ozone datasets

We report on updated trends using different merged zonal mean total ozone datasets from satellite and ground-based observations for the period from 1979 to 2020. This work is an update from the trends reported in Weber et al. (2018) using the same datasets up to 2016. Merged datasets used in this study include NASA MOD v8.7 and NOAA Cohesive Data (COH) v8.6, both based on data from the series of Solar Backscatter UltraViolet (SBUV), SBUV-2, and Ozone Mapping and Profiler Suite (OMPS) satellite instruments (1978–present) as well as the Global Ozone Monitoring Experiment (GOME)-type Total Ozone (GTO-ECV) and GOME-SCIAMACHY-GOME-2 (GSG) merged datasets (both 1995–present), mainly comprising satellite data from GOME, SCIAMACHY, OMI, GOME-2A, -2B, and TROPOMI. The fifth dataset consists of the annual mean zonal mean data from ground-based measurements collected at the World Ozone and UV Radiation Data Center (WOUDC). Trends were determined by applying a multiple linear regression (MLR) to annual mean zonal mean data. The addition of four more years consolidated the fact that total ozone is indeed on slowly recovering in both hemispheres as a result of phasing out ozone depleting substances (ODS) as mandated by the Montreal Protocol. The near global ozone trend of the median of all datasets after 1996 was 0.5 ± 0.2 (2σ) %/decade, which is in absolute numbers roughly a third of the decreasing rate of 1.4 ± 0.6 %/decade from 1978 until 1996. The ratio of decline and increase is nearly identical to that of the EESC (equivalent effective stratospheric chlorine or stratospheric halogen) change rates before and after 1996 which confirms the success of the Montreal Protocol. The observed trends are also in very good agreement with the median of 17 chemistry climate models from CCMI (Chemistry Climate Model Initiative) with current ODS and GHG (greenhouse gas) scenarios. The positive ODS related trends in the NH after 1996 are only obtained with a sufficient number of terms in the MLR accounting properly for dynamical ozone changes (Brewer-Dobson circulation, AO, AAO). A standard MLR (limited to solar, QBO, volcanic, and ENSO) leads to zero trends showing that the small positive ODS related trends have been balanced by negative trend contributions from atmospheric dynamics resulting in nearly constant total ozone levels since 2000.

Short term stratospheric ozone trend over Dumdum and its relation with Flare Index of northern hemisphere

This paper presents an analysis of the effect of flare index (Q) on the stratospheric ozone concentration over Dumdum station, Kolkata for the period 1979-95. As Dumdum is situated in northern hemisphere we have considered the flare index values for the northern hemisphere only.

Occurrence of discontinuities in the ozone concentration data from three reanalyses

Ozone is a very important trace gas in the stratosphere and thus we need to know its time evolution over the globe. The ground based measurements are rare, especially in the Southern Hemisphere. Satellite ozone data have broader coverage, but they are not available from everywhere. On the other hand, the reanalyse data have regular spatial and temporal structure, which is very good for trend analyses. But there are discontinuities in these data.These discontinuities may influence the result of trend studies. The aim of this paper is to detect the discontinuity occurrence (DO) in the following reanalyses: MERRA-2, ERA-5 and JRA-55 with the help of the Pettitt homogeneity test at all common layers above 500 hPa. The discontinuities are sorted according to their size to the significant and the insignificant ones; the former can affect the ozone trend studies. It was shown that DO for the significant discontinuities is the smallest in JRA-55. In the upper model layers, the discontinuity occurrence is the highest. The other area of high DO is the troposphere.

On ozone trend detection: using coupled chemistry–climate simulations to investigate early signs of total column ozone recovery

Total column ozone values from an ensemble of UM-UKCA model simulations are examined to investigate different definitions of progress on the road to ozone recovery. The impacts of modelled internal atmospheric variability are accounted for by applying a multiple linear regression model to modelled total column ozone values, and ozone trend analysis is performed on the resulting ozone residuals. Three definitions of recovery are investigated: (i) a slowed rate of decline and the date of minimum column ozone, (ii) the identification of significant positive trends and (iii) a return to historic values. A return to past thresholds is the last state to be achieved. Minimum column ozone values, averaged from 60° S to 60° N, occur between 1990 and 1995 for each ensemble member, driven in part by the solar minimum conditions during the 1990s. When natural cycles are accounted for, identification of the year of minimum ozone in the resulting ozone residuals is uncertain, with minimum values for each ensemble member occurring at different times between 1992 and 2000. As a result of this large variability, identification of the date of minimum ozone constitutes a poor measure of ozone recovery. Trends for the 2000–2017 period are positive at most latitudes and are statistically significant in the mid-latitudes in both hemispheres when natural cycles are accounted for. This significance results largely from the large sample size of the multi-member ensemble. Significant trends cannot be identified by 2017 at the highest latitudes, due to the large interannual variability in the data, nor in the tropics, due to the small trend magnitude, although it is projected that significant trends may be identified in these regions soon thereafter. While significant positive trends in total column ozone could be identified at all latitudes by ∼ 2030, column ozone values which are lower than the 1980 annual mean can occur in the mid-latitudes until ∼ 2050, and in the tropics and high latitudes deep into the second half of the 21st century.

Future changes in the stratosphere-to-troposphere ozone mass flux and the contribution from climate change and ozone recovery

Model simulations consistently project an increase in the stratosphere-troposphere exchange (STE) of ozone in the future. Both, a strengthened circulation and ozone recovery in the stratosphere contribute to the increased mass flux. In our study, we investigate with a state-of-the-art chemistry-climate model the drivers of future STE change as well as the change in the distribution of stratospheric ozone in the troposphere. Our focus is on the investigation of the changes on the monthly scale. The global mean influx of stratospheric ozone into the troposphere is projected to increase between the years 2000 and 2100 by 53 % under the RCP8.5 greenhouse gas scenario. We find the largest increase of STE in the NH in June due to increasing greenhouse gas (GHG) concentrations. In the SH the GHG effect is dominating in the winter months, while decreasing levels of ozone depleting substances (ODS) and increasing GHG concentrations contribute nearly equally to the increase in SH summer. A large ODS-related ozone increase in the southern hemisphere (SH) stratosphere leads to a change in the seasonal breathing term which results in a future decrease of the ozone mass flux into the troposphere in the SH in September and October. We find that the GHG effect on the STE change is due to circulation and stratospheric ozone changes, whereas the ODS effect is dominated by the increased ozone abundance in the stratosphere. The resulting distributions of stratospheric ozone in the troposphere for the GHG and ODS changes differ because of the different regions of ozone input (GHG: midlatitudes; ODS: high latitudes) and the larger increase of tropospheric ozone loss rates due to GHG increase. Thus, the model simulations indicate that stratospheric ozone is more efficiently mixed to lower levels if only ODS levels are changed. The increase of the stratospheric ozone column in the troposphere explains more than 80 % of the tropospheric ozone trend in NH spring and in the SH except for the summer months. The importance of the future stratospheric ozone contribution to tropospheric ozone burdens therefore depends on the season.