1. Ozone stratospheric and tropospheric trends (1995-2015) at nine NDACC FTIR stations (79°N to 78°S)
Corinne Vigouroux M. De Mazière, Q. Errera (BIRA-IASB); E. Mahieu, B. Franco (Univ. of Liège); T. Blumenstock, F. Hase (IMK-ASF); O. García, M. Schneider (AEMET),
J. Mellqvist, G. Personn (Univ. of Chalmers); M. Palm, J. Notholt (Univ. of Bremen); J. Hannigan, I. Ortega (NCAR); D. Smale, B. Liley, J. Robinson (NIWA); N. Jones (Univ. of Wollongong); L. Thölix, R. Kivi (FMI).
Introduction
Spectral micro-window
Measurements of ground-based FTIR (Fourier transform infrared) high-resolution solar absorption spectra: day measurements, under clear-sky conditions, using primarily the Bruker 120M (or 125M) or Bruker 120HR (or 125HR), with a spectral resolution of and cm-1, respectively.
Retrieved products, using the algorithms PROFITT9 [Hase, 2000] or SFIT2/4 [Pougatchev et al., 1995]:
- ozone total columns with a precision of 2 %.
- low vertical resolution profiles are obtained using the Optimal Estimation Method (Rodgers, 2000): distinct information can be distinguished in about four altitude layers, with a precision of 5–6 %:
a priori
retrieved
The present work is an update, with 3 more years of data, of Vigouroux et al. (ACP, 2015) where we had reported the total column and the 4 layers trends ( Dec. 2012) at 8 FTIR stations, part of the Network for the Detection of Atmospheric Composition Change (NDACC). The major results of Vigouroux et al. (ACP, 2015):
- some signs of the onset of ozone mid-latitude recovery are observed at the Southern Hemisphere stations.
- a few more years seem to be needed to observe it at the northern mid latitude station (Jungfraujoch), +0.9±1.0 %/decade for period.
- the effect of the 11-year solar cycle not clear for shorter time-series starting in early 2000’s.
What’s going on 3 years later ?
- ground-9/11km (Izaña:13km): troposphere Trop
km (15-23): lower stratosphere LowS
km (23-32): middle stratosphere MidS
km (31-49): upper stratosphere UppS
Jungfraujoch Averaging Kernels
We avoid the 2 km layer where our smoothing error is the largest.
Tropospheric data are used in the upcoming Tropospheric Ozone Assessment Report TOAR.
Stratospheric data were used in Ozone Assessment reports (WMO 2010, WMO 2014), and the SI2N overview papers (Hassler et al., 2014; Harris et al., 2015).
Multiple regression model
Result 1: Proxies contribution to ozone variability
After the seasonal cycle, the main driver of ozone variability is the tropopause pressure TP.
Another short-term proxy that is often significant is the Equivalent Latitude, especially in the UppS layer, and for high- to mid-latitude stations.
The QBO proxy is mainly significant for the sub-tropical stations Izaña and Wollongong; and to a lesser extent to mid-latitude stations.
The 11-year solar cycle is commonly used in ozone trends studies, but its long cycle makes it difficult to be identified in short-term series. In Vigouroux et al. (2015), doubtful solar cycle influence were observed in the Izaña and Wollongong MidS layer. It has indeed vanished at Wollongong with 3 more years of data; while at Izaña more years seem needed to clarify it.
We apply a stepwise multiple linear regression model to the monthly means of total and partial ozone columns time-series Y(t):
Figures: individual contribution Cfrac of each proxy (retained by the stepwise regression model) to the coefficient of determination R2.
with A0 the intercept, A1 to A4 seasonal cycle parameters, A5 the annual trend, Xk the explanatory variables (proxies time-series), Ak their respective coefficient and e(t) the residuals.
Proxies Ak
Description
SOLAR
11-year solar cycle: described by the solar radio flux at 10.7 cm
QBO
Quasi-biennial Oscillation: described by a combination of zonal winds measured at Singapore at 30 and 10 hPa. If significant their seasonal contribution is also included in the model
ENSO
El Niño Southern Oscillation: described by the multivariate ENSO index
AO / AAO
Arctic / Antarctic Oscillation indices TP
Tropopause Pressure at the location of the station
EL(L/M/U)
Equivalent Latitude at 3 altitude levels ~ the middle of the 3 strato. layers HF
Heat Flux between 45° and 75° as a proxy for Brewer-Dobson circulation
VPSC
Volume of Polar Stratospheric Clouds; VPSC * EESC as a proxy for polar ozone destruction by chlorine
ENSO signal is significant only at the lowest latitude station Izaña.
The AO/AAO is significant only at the 28°/47°N stations.
The VPSC is significant at all polar stations and Jungfraujoch, which shows influence of transport of polar air masses.
The HF is significant mainly at the highest latitude stations 77-79°N.
To account for auto-correlation in the residuals for a correct estimation of trend uncertainties, we use Santer et al. (JGR, 2000).
Only the significant parameters are kept in the model: the final sets of parameters vary with the station and the partial columns.
Result 2: Annual trends
Trends
in % / decade
Start in
Trop.
LowS
MidS
UppS
Tot. Col.
Ny-Alesund
March-Sept.
1995
(1999)
+1.8±3.3
+0.9±3.9
+1.7±4.3
+5.7±3.6
(-0.0±3.6)
+2.2±2.3
Thule
1999
-2.6±4.4
+0.0±4.3
+2.6±5.5
-1.9±5.6
+1.7±2.7
Kiruna
Jan-Nov.
1996
-2.3±2.0
-1.5±2.1
+1.9±1.9
+5.0±2.3
+0.8±1.1
Harestua
Profiles still to be checked !
+0.8±1.7
Jungfraujoch
-0.8±2.3
+0.3±2.9
-0.0±1.0
+1.6±0.9
+0.3±1.1
Izaña
+2.2±2.7
-0.1±1.8
-0.7±1.3
-0.9±1.4
+0.0±0.9
Wollongong
-1.4±2.1
-0,2±3.3
+1.4±1,6
+0.7±0,9
+0.5±0,9
Lauder
- Profiles to be improved
- No profiles at present in (Bomem spectro.)
+6.4±2.5
-3.6±3.8
-4.9±2.9
+8.3±2.7
-1.1±1.4
+0.8±1.0
Arrival Heights Aug-Apr
1997
-0.8±3.6
+2.3±5.3
-1.7±5.9
+14.0±8.7
-1.3±2.3
Mid-latitude and sub tropical stations :
Total columns trends are all small and non significant.
All lower and middle stratospheric trends (12-30km) are non significant (except MidS at Lauder, to be checked).
Except at the 2 lowest latitude stations, upper stratospheric trends are significantly positive, in agreement with correlative data and model (WMO 2014): ozone recovery due to EESC decline is occurring at mid-latitude.
High latitude stations:
The total columns trends are all small and non significant.
All lower and middle stratospheric trends (12-30km) are non significant (except Harestua, but profiles need to be improved).
All upper stratospheric trends are significantly positive:
- Northern Hemisphere: due to the period, when EESC, still increasing in polar latitudes, cannot be responsible for this trend.
- Southern Hemisphere: more constant trend: could be EESC effect.
To be confirmed when profiles will be quality checked.
WMO 2014 report, Chapter 2
Conclusions
Consolidated results; agreement between stations.
Ozone recovery is observed in the upper stratosphere at mid-latitude stations and Antarctic.
Figures: monthly means, with seasonal cycle removed.