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.
Variability and trend in ozone over the southern tropics and subtropics
