While global stratospheric O

The slowdown in the O

Ozone at 10 hPa over the equator is particularly sensitive to catalytic
cycles involving the odd nitrogen (NO

In addition to long-term anthropogenically driven changes, events such as
the eruption of Mt. Pinatubo may alter the chemistry and dynamics of the
stratosphere for extended periods. Aquila et al. (2013) compared a reference model with a
model which simulated the effect of the volcanic aerosols on both chemistry
and dynamics. They calculated an increase in O

Previous observational work has correlated O

The present study extends the previous observational studies with a
combination of 21 years of ozone data from the UARS (Upper Atmosphere Research Satellite) HALOE and the Aura
Microwave Limb Sounder (MLS) measurements, plus nitrogen species data from
HALOE, MLS and the Atmospheric Chemistry Experiment (ACE). Our results
confirm the existence of the 10 hPa tropical ozone trend anomaly and link it
to a correspondingly consistent local change in the nitrogen species which
affect O

We make use of measurements from the HALOE, MLS, and the Fourier transform
spectrometer measurements from ACE. HALOE measurements of O

MLS measurements of O

Since 2004 ACE has been measuring O

In cases where species are affected by the solar cycle, one of the
challenges in interpreting decadal scale trends in the stratosphere is
separating these trends from solar cycle induced variations. Model studies
provide some guidance as to the expected solar cycle variations in the
species of interest. Egorova et al. (2005) used the SOCOL Chemistry Climate Model (CCM)
and found that at 30 km O

Throughout this study we will calculate trends based on a function including
terms to fit the annual, semi-annual, QBO (quasi-biennial oscillation), plus a constant term and a linear
trend term. The QBO terms were calculated using the Center for Climate
Prediction 30 and 50 hPa winds anomalies obtained from

The MLS measurement time series used here extends from 2004 to 2014, and
therefore clearly does not extend over a full solar cycle. The linear trend
calculations from MLS measurements which will be shown cover the period
August 2004 to May 2013. Because solar cycle 24 is particularly weak, the Mg
II values in 2013 are comparable to those in 2004, so solar effects are
unlikely to cause a trend in the MLS data set used here. In order to provide
an estimate of the uncertainty in the trend, which is introduced by the
presence of a solar cycle, we will show some MLS results both with and
without the inclusion of a solar cycle term. We will show that in the region
of greatest interest, near the tropics at

In Fig. 1 we show the annual median O

Annual median ozone anomalies at 10 hPa
5

An estimate of the uncertainty in these annual medians can be obtained from
the standard deviation of the individual anomalies. The average value of

In Fig. 2 we show the linear trend in the global HALOE ozone measurements
from 1991 to 2005. Remsberg (2008) show a quite similar figure (their Fig. 13) for linear trends in HALOE O

The calculated linear trend in HALOE ozone for 1991–2005. The HALOE data
has been sorted into 11 individual 10

HALOE measurements ceased in 2005, and Aura MLS has been providing O

The O

While Fig. 1 shows a general decrease in O

Just as the HALOE and MLS trends show clear differences away from the
tropics, they also show clear differences in the tropics at other levels.
The 1991–2005 HALOE trend shows an increase in tropical O

As noted above, O

Correlation coefficients between N

This positive correlation between N

Monthly median N

As indicated in Sect. 2.1, the MLS instrument has been operational for
less than a full solar cycle; hence for tropical trend calculations we show
results both with and without the inclusion of a solar cycle term. In Fig. 7 we show the calculated profiles as a function of pressure as derived from
eight (constant term, two annual terms, two semi-annual terms, two QBO
terms, and a linear trend) and nine (including a solar cycle) parameter fits
to the monthly median MLS measurements. Figure 7 shows the profiles (the
constant terms from the fits) in addition to the linear trend and the net
effect of 8 years of such a trend (2004–2005 vs. 2012–2013). The O

ACE measurements of O

Left hand panel: annual average MLS profiles of O

As was shown in Fig. 2, HALOE measurements showed a decrease in O

In Fig. 8 we show annual median HALOE anomalies in O

Figure 8 shows that NO

Annual median HALOE O

In Fig. 9 we show the calculated linear trends in the HALOE O

In order to better understand the changes in the observed species we have employed the two-dimensional chemical transport model (CHEM2D; Bacmeister et al., 1998). The model includes parameterized gravity wave and planetary wave drag and is ideal for understanding tracer transport and the response of the global middle atmospheric circulation to external forcings. Compared with those earlier studies, the present model has an improved vertical resolution (1 instead of 2 km). CHEM2D's most recent applications have included simulating the solar cycle variations of polar mesospheric clouds (Siskind et al., 2005) and studying the response of stratospheric ozone to both the solar cycle and the tropical quasi-biennial oscillation (McCormack et al., 2007).

Left hand panel: annual average HALOE profiles of O

We will show results from two model runs, each of which has been integrated
for 12 years to ensure stability from year-to-year. Since the goal of the
model was to test whether dynamical changes would affect N

Figure 10 shows the change in N

Annual average altitude profiles of O

The calculated equatorial N

While the model runs do support the suggestion that the changes in O

Ozone measurements from HALOE and MLS show a long-term decrease in O

A feature of particular interest for future work is the increase in O

This project was funded by NASA under the Upper
Atmosphere Research Program, by the Naval Research Laboratory, and by the
Office of Naval Research. Work at the Jet Propulsion Laboratory, California
Institute of Technology, was carried out under a contract with the National
Aeronautics and Space Administration. MLS and HALOE data are available from
the NASA Goddard Earth Science Data Information and Services Center
(