Variability and trend in ozone over the southern tropics and subtropics

Abstract

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 15–30 km) 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 28 km. Over the equatorial zone there is a strong signal at ∼26 km, 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 24 km and in the 27–30 km altitude band) over the tropical and subtropical regions, while the semi-annual oscillations are more significant over the 27–30 km 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 22–30 km altitude range, while a delay in ozone improvement is apparent in the UT–LS (upper troposphere–lower stratosphere) below 22 km. This is especially noticeable at approximately 19 km, where a negative value is observed in the tropical regions.

Stratospheric ozone trends for 1985–2018: sensitivity to recent large variability

“…In particular, these 6 years encompass periods of both negative/easterly and positive/westerly phases of ENSO/QBO. These modes are major contributors to stratospheric variability (Zerefos et al, 1992;Tweedy et al, 2017;Toihir et al, 2018;Garfinkel et al, 2018;Diallo et al, 2018Diallo et al, , 2019, and any sensitivity of the end year to the state of these drivers should be encapsulated in the set of spatial responses depending on the end year only (Fig. 1), particularly if these modes were not well-captured by DLM predictors.…”

“…The peaks (or troughs) in the SH are 2-3 years apart. The QBO has a similar periodicity and is known to have the largest interannual dynamical impact on ozone in the stratosphere (see Gray and Pyle, 1989;Zerefos et al, 1992, andToihir et al, 2018, and references therein).…”

“…Composites are subject to artefacts from merging different observing platforms. Multiple papers (Tummon et al, 2015;Harris et al, 2015;Steinbrecht et al, 2017;Ball et al, 2017Ball et al, , 2018) and a SPARC report (Petropavlovskikh et al, 2019) review, discuss, and attempt to account for the artefacts in the uncertainty budget. Ball et al (2018) integrated ozone over the whole stratosphere, i.e.…”

Stratospheric ozone trends for 1985–2018: sensitivity to recent large variability

“…In particular, these six years encompass periods of both negative/Easterly and positive/Westerly phases of ENSO/QBO. These modes are major contributors to stratospheric variability (Zerefos et al, 1992;Tweedy et al, 2017;Toihir et al, 2018;Garfinkel et al, 2018), and any sensitivity of the end year to the state of these drivers should be encapsulated in this 195 group of spatial responses, particularly if these modes were not well-captured by DLM predictors (Fig. 1).…”

Surface Temperature Trend Estimation over 12 Sites in Guinea Using 57 Years of Ground-Based Data

“…As we know that the Trend-Run model is an additive model based on a linear regression fitting, an important factor is that the dependent parameter (temperature) and nondependent parameters (forcings) are all quantitative variables. Thus, AO and SAO have been taken into account and, for their phase construction, a sinusoidal function is used, as detailed in the work of Toihir et al [60] and Sivakumar et al [61].…”