Decreasing Midlatitude Ozone:
The Role of the North Atlantic Oscillation

Stephen J. Reid*†, Adrian F. Tuck† & George N. Kiladis†

*†Cooperative Institute for Research in Environmental Science, 
University of Colorado, Boulder, U.S.A.
†NOAA Aeronomy Laboratory, Boulder, Colorado, U.S.A.

We link the decreasing trend in northern hemisphere ozone to an increased frequency of intrusions of air from the UT/LS region in the tropics. The increased number of these intrusions makes a substantial contribution to the downward ozone trend over the last 14 years, and is enhanced by mixing, which supplies an irreversible component. 

A correlation with the North Atlantic Oscillation is found, accounting for a high fraction of the variance. Using potential vorticity as a surrogate for ozone on the 380K isentrope, we have found a dynamical trend in northern hemisphere mid-latitudes. 

Figure 1, reproduced from the 1998 WMO report on ozone depletion, shows that ozone changes occur primarily in two altitude regions: around 35-45 km, and between ~12km (the tropopause) and ~20 km. The lower layer accounts for most of the change in the total ozone column.

Figure 1.

In figure 2, we see how ozone minima appear in ozonesonde profiles. Here, the red curve shows a profile for 7th March 1997 at Aberystwyth, Wales (52.4oN, 4.1oW), with an ozone minimum is present in the lower stratosphere. The red shading the deviation from the climatological mean for March. 
The ozone concentrations inside this minimum, as with all such event seen in the mid-latitude lower stratosphere, are comparable to those seen in profiles recorded at subtropical latitudes. The green curve shows an example taken from the Japanese station Kagoshima (31.4oN, 130.4oE) on 2nd March 1995. This is a very obvious indication of the origin of minima: they exist purely because of advection transporting ozone-poor air from subtropical to higher latitudes.

Figure 2.

Because the ozone field is not measured globally in the lowermost part of the stratosphere, a region often beyond the reach of satellite profilers, it is necessary to use the dynamical substitute, potential vorticity to study minima in a non-local sense. The blue curve is a PV profile PV (in units of 10-6Km2kg-1s-1) at the co-ordinates of Aberystwyth,  derived from the NCEP meteorological analyses. PV captures the presence of the ozone minimum reasonably well, and can therefore be used as a surrogate for ozone.

Figure 3.

Figure 3 presents an example of how well PV captures not only the presence of the ozone minimum, shown here in cross-section along the Greenwich meridian at 12Z on 7th March1997, but also reflects the height of the tropopause. The ozone minimum is following in the wake of a ridge travelling north-east, drawing the ozone-poor air along in its wake. (The tropopause height has been determined by use of radiosonde profiles obtained close to the Greenwich meridian between 10UT and 13UT on this day).
This particular PV minimum extended from around 47o to 62oN and spanned ~5 km (60K) in height. It also extended from ~10oE to ~40oW (not shown),  occupying an area ~4.3 x 106 km2. 
Figure 4.

In figure 4, the light blue and red shading indicate, respectively, the positive and negative interannual variations in the North Atlantic Oscillation index. The dark blue curve represents interannual variations in the frequency of  occurrence of ozone minima above the European ozonesonde stations Hohenpeissenberg (47.8oN, 11.1oE) and Lindenberg (52.2oN, 14.1oE). 
After de-trending these two data sets and performing a linear regression to evaluate the degree of correspondence between their interannual variations, the result was found to be significant at the 99% level (using Pearson's Significance Test), with a coefficient of 0.86 and leaving just 28% of the variance unexplained by the regression.

Figure 5.

The histogram above shows how the annual frequency of PV minima at 380K (NCEP reanalyses) between 10oE and 80oW, 40o and 60oN, correlates with annual-mean values of the NAO, for the period 1968-2000. More PV minima are observed during years when the NAO is in its positive phase than when the phase is negative.

Conclusions

•	Ozonesondes have demonstrated that the largest decline in ozone concentrations occurs in the lower stratosphere, where the natural variability in the ozone layer is greatest. This is well below altitudes where heterogeneous chemistry is effective in mid-latitudes.

•	The high correlation between interannual variations in the transport from subtropical to middle latitudes and the phase of the North Atlantic Oscillation suggest that some of the ozone trend measured over Europe is produced by dynamical changes, and accounts for at least some of the observed trend in ozone.