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Volume 36, issue 2 | Copyright

Special issue: Space weather connections to near-Earth space and the...

Ann. Geophys., 36, 381-404, 2018
https://doi.org/10.5194/angeo-36-381-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Regular paper 16 Mar 2018

Regular paper | 16 Mar 2018

Variability and trend in ozone over the southern tropics and subtropics

Abdoulwahab Mohamed Toihir1, Thierry Portafaix1, Venkataraman Sivakumar2, Hassan Bencherif1,2, Andréa Pazmiño3, and Nelson Bègue1 Abdoulwahab Mohamed Toihir et al.
  • 1Laboratoire de l'Atmosphère et des Cyclones, Université de La Réunion, St-Denis, Réunion Island, France
  • 2Discipline of Physics, School of Chemistry and Physics, University of KwaZulu-Natal, Durban, South Africa
  • 3Laboratoire Atmosphères, Milieux, Observations Spatiales, CNRS, Université Versailles Saint Quentin, Guyancourt, France

Abstract. Long-term variability in ozone trends was assessed over eight Southern Hemisphere tropical and subtropical sites (Natal, Nairobi, Ascension Island, Java, Samoa, Fiji, Reunion and Irene), using total column ozone data (TCO) and vertical ozone profiles (altitude range 15–30km) recorded during the period January 1998–December 2012. The TCO datasets were constructed by combination of satellite data (OMI and TOMS) and ground-based observations recorded using Dobson and SAOZ spectrometers. Vertical ozone profiles were obtained from balloon-sonde experiments which were operated within the framework of the SHADOZ network. The analysis in this study was performed using the Trend-Run model. This is a multivariate regression model based on the principle of separating the variations of ozone time series into a sum of several forcings (annual and semi-annual oscillations, QBO (Quasi-Biennial Oscillation), ENSO, 11-year solar cycle) that account for most of its variability.

The trend value is calculated based on the slope of a normalized linear function which is one of the forcing parameters included in the model. Three regions were defined as follows: equatorial (0–10°S), tropical (10–20°S) and subtropical (20–30°S). Results obtained indicate that ozone variability is dominated by seasonal and quasi-biennial oscillations. The ENSO contribution is observed to be significant in the tropical lower stratosphere and especially over the Pacific sites (Samoa and Java). The annual cycle of ozone is observed to be the most dominant mode of variability for all the sites and presents a meridional signature with a maximum over the subtropics, while semi-annual and quasi-biannual ozone modes are more apparent over the equatorial region, and their magnitude decreases southward. The ozone variation mode linked to the QBO signal is observed between altitudes of 20 and 28km. Over the equatorial zone there is a strong signal at  ∼ 26km, where 58% ±2% of total ozone variability is explained by the effect of QBO. Annual ozone oscillations are more apparent at two different altitude ranges (below 24km and in the 27–30km altitude band) over the tropical and subtropical regions, while the semi-annual oscillations are more significant over the 27–30km altitude range in the tropical and equatorial regions. The estimated trend in TCO is positive and not significant and corresponds to a variation of  ∼ 1.34±0.50% decade−1 (averaged over the three regions). The trend estimated within the equatorial region (0–15°S) is less than 1% per decade, while it is assessed at more than 1.5% decade−1 for all the sites located southward of 17°S. With regard to the vertical distribution of trend estimates, a positive trend in ozone concentration is obtained in the 22–30km altitude range, while a delay in ozone improvement is apparent in the UT–LS (upper troposphere–lower stratosphere) below 22km. This is especially noticeable at approximately 19km, where a negative value is observed in the tropical regions.

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