• Introduction
• MIMOSA simulations
• Chemically activated air estimate
• Conditions for activation and transport
• Conclusions
• Acknowledgements
• References

FIGURE

• Figure 1

Transport of air from the Northern Hemisphere polar stratospheric vortex is an important mechanism for ozone reduction at middle latitudes. Polar Stratospheric Clouds (PSCs) lidar measurements performed with the ALOMAR R/M/R lidar (69°N, 16°E) show the presence of PSCs outside the polar vortex in the coldest period of the 1994/1995 and 1996/1997 winters. Meteorological analyses confirm that temperatures below PSCs formation point can be reached in the stratosphere outside the polar vortex. This is observed mainly above Northern Europe. This work is aimed to study the transport of the PSC chemically activated. We have used a transport model with high horizontal resolution to quantify the area covered by chemically activated air in the northern hemisphere and to evaluate the potential impact on the middle latitude chemistry. MIMOSAmodel has been adapted for advection of tracers simulating the behavior of activated radicals as ClO, BrO and OH. This approach allow us to analyse the activation outside the vortex and to estimate the timelife of the tracer inside the filaments. The main features of the tropospheric circulation allowing the PSCs formation outside the polar vortex is adressed

Introduction

During last decade, after the discovery of the Antarctic stratospheric ozone hole, the behavior of the stratospheric ozone at middle latitudes has been deeply investigated. Recent analyses (Solomon, 1999 for a complete review) have shown an ozone reduction at middle latitudes of 4 % per decade in the Southern hemisphere and of 3 % per decade in the northern hemisphere. Several works were devoted to the estimate of the relative influence on mid-latitude stratospheric ozone decrease of the processes in the Polar regions and the in-situ processes.

Solomon et al. (1998) have shown that the decrease can be attributed mainly to the heterogeneous chemistry on the in-situ stratospheric aerosols. Polar air transport simulations during 1991 to 1993 winters indicates that inter-annual variability of transport is not correlated with the interannual variability of the middle latitude ozone total column (Norton and Chipperfield, 1995).

Nevertheless, simulations presented by Hadjinicolau et al. indicates that the influence of the chemically activated at high latitudes contributes up to 50 % to the ozone budget at the middle latitudes.

In this work we quantify the amount of Aerosol measurements

In this work we estimate the role of the Polar Stratospheric Clouds outside or at the edge of the Northern hemisphere polar stratospheric vortex . ALOMAR R/M/R lidar measurements show the presence of PSC outside the core of the polar vortex using an equivalent latitude definition of the surf zone.

We used a theoretical approach in order to evaluate the transport of air masses containing active chlorine toward middle latitudes. Horizontal resolution currently used in CTM's simulations do not allow the description of sub-grid features due to the numerical horizontal diffusion. Spurious diffusive effects reduce the concentration of chemical species. This effect is considered as responsible for an underestimate of the efficiency of the chemical processes involved in ozone depletion (Edouard et al. 1999). Our approach is based on an high resolution transport model that allow to reduce numerical diffusion, coupled with a simplified chemical scheme in order to perform long time simulations.

MIMOSA simulations

In order to reproduce filamentary structures that can be responsible to a large extent for the transport extra vortex of the PSCs processed air, it is necessary to simulate the horizontal transport with an adequate resolution. The advection scheme is based on the isentropic domain-filling model MIMOSA (Hauchecorne et al. 1999). The horizontal resolution is 20 km. We have chosen a simplified photochemical radicals parametrisation of radicals activation and deactivation due to the presence of NO2. The activation takes place on PSCs surface. The presence of PSCs is defined with a simple temperature threshold criterion. PSCs form below the temperature where the average volume of Supercooled Ternary Solutions (Tsts) increases, say 194 K for 50 mb level. The choice to simulate only STS instead of NAT and ice is motivated by the experimental finding that STS PSC formation is more usual than type II or NAT PSC during Arctic winter.

So it is necessary to define the highest temperature at which PSCs can be formed. The PSCs formation scheme validated by laboratory studies and measurements assume that STS are the first step in aerosol condensation with decreasing temperature (Peter 1997) and NAT PSCs are formed only from warming of type II PSC.

We will not calculate the concentration of chemical species. Tracer representative of the degree of activation is set to 1 in presence of PSCs and decay exponentially with time constant varying from 10 days to 5 days for an air mass inside the polar vortex and at middle latitudes respectively. The model is driven by temperature and wind fields from the European Center for Medium Range Weather Forecasting (ECMWF) analysis with an horizontal resolution of 2.5° and temporal resolution of 24 hours. The comparison between tracer fields and CTM active species simulations (reported in Fierli et al, 2000) indicates that tracer values greater than 0.5 corresponds remarkably well to ClOx concentrations greater than 1 ppbv.

Chemically activated air estimate

We have calculated the total amount of activated air with the MIMOSA scheme in order to evaluate the impact of the interannual difference. The condition of tracer greater than 0.5 as a threshold for the identification of the activated air has been used. The global amount of air accumulated outside the polar vortex has been estimated as the sum of the daily differences in surface of activated air. The sum has been calculated only during periods when surface was increasing using the approach given by Norton et al, 1995.

Both variables are reported in the figure 1 function of time for two isentropic levels, 475 K and 550 K where most of PSCs appeared during three winters. The interannual variability between 1994/1995, 1995/1996 winters and 1996/1997 winter is important. Up to 20 % of the vortex surface covered by activated air is present outside the polar vortex at both levels during the two first winters. The total amount of air activated is up to the 60 % of the average vortex surface during winter 1995/1996. We also observe that a filament formed during February 1996 at 475 (described in Fierli et al., 2000) is responsible for 60 % of the total surface of activated air. During winter 1996/1997 an isolate event during January at the 475 K levels leads to a total of 20 % of activated air. The periods of activation outside the polar vortex are in good agreement with PSCs measurements performed by the ALOMAR lidar.

Hadjinicolau et al. (1998) shown the role of PSCs activation on the extra vortex air during the same period studied here. Using a lower horizontal resolution model the highest contribution of activated air on ozone depletion to middle latitudes was evaluated at 20 % during 1995/1996.

Figure 1. Fraction of average polar vortex surface covered by chemically activated air (dashed line) and estimate of the activated air accumulated outside the polar vortex (plain line). The results are shown for three winters and two isentropic surfaces, 475 and 550 K

Conditions for activation and transport

The geopotential height at 500 mb and 30 mb has been studied during the period corresponding to the filamentary structure on February 1996. A geopotential ridge, located at 135°W is remarkably correlated on both height levels. In the stratosphere it corresponds to the elongation of the polar vortex. Such structure is related to high ground pressure above Iceland and subsequent adiabatic lift and hence cooling of the airmasses. So this meteorological pattern is related to the formation of PSC, and the transport of airmasses outside the polar vortex. In fact, the polar vortex is displaced northern of Iceland. Airmasses are activated outside the polar vortex, or at his edge and hence more subject to filamentary transport. This circulation pattern in the troposphere is identified as Arctic Oscillation anomaly, related to low frequency variability in the atmosphere (Appenzeller et al., 2000).

Conclusions

The proposed approach gives an estimate of the activated air outside the polar vortex. Strong inter-annual variability is observed. The simulations will be compared with ozone measurements in order to identify signatures corresponding to the interannual variability or singular strong events as large scale filaments in order to identify the effect of PSCs activation on stratospheric midlatitude ozone budget. Moreover a climatological study of the synoptic meteorological conditions related to the presence of PSCs outside the polar vortex is under way.

Acknowledgments

This work has been supported by EU DG-XII in the frame of the Environment and Climate programme

References

Edouard S., Legras B., Lefèvre F. and Eymard R.,The effect of mixing on ozone depletion in the Arctic, Nature, 16711-16788, 101-D1, 1996

Appenzeller, C, Weise, A.K, Stahelin, J., North Atlantic Oscillation modulates Total ozone winter trends, GRL, 27, 1131-1134, 2000

Fierli, F., Hauchecorne A., Sauvage, L., Lefevre F., Transport of chlorine activated air outside the northern hemisphere polar vortex during 1994/1997 winters, To appear in Air Pollution research report, European Commission, Proceedings of the European Workshop on arctic stratosphere, 1999

Hadjinicolau P. ,Pyle J.A., Chipperfield M.P., Kettleborough J.A., Effect on interannual meteorologic variability on mid latitude ozone, GRL, 24, 1993-1996, 1998

Hauchecorne A., Marchand M., Godin S., and Souprayen C., A high resolution advection model for the interpretation of ozone filaments observed in lower stratospheric ozone lidar profiles at mid-latitudes, Air Pollution research report 69, European Commission, Proceedings of the European Workshop on Mesoscale Processes in the Stratosphere, 1999

Norton W.A. and Chipperfield M.P., Quantification of the transport of chemically activated air from the northern hemisphere polar vortex, JGR, 100-D1, 25817-25840, 1995

Peter T., Microphysiscs and heterogeneous chemistry of polar stratospheric clouds, Annual Review of Physical Chemistry, 48, 785-822, 1997

Solomon S., Stratospheric ozone depletion : a review of concepts and history, Reviews of Geophysics, 37, 275-316, 1999