Signature of the positive AO phase in the stratospheric ozone and temperature

1. Signature of the positive AO phase in the • stratospheric ozone and temperature • during boreal winter • E. Rozanov1,2, T. Egorova1,2, W. Schmutz1 , V. Zubov3 • Physical-Meteorological Observatory Davos, Switzerland, • (2) Institute of Climate and Atmospheric, ETHZ, Switzerland, • (3) Main Geophysical Observatory, St.-Petersburg, Russia

2. Introduction The aim of this study is to validate the imprint of the Arctic Oscillation (AO) in the stratospheric ozone and temperature during boreal winter. It is well known [e.g., Thompson and Wallace, 1998] that the positive phase of the AO is characterized by deeper vortex and more intensive Polar Night Jet. Therefore, it is expected that for the positive AO phase a weaker meridional circulation will result in an elevated ozone mixing ratio and warmer temperature in the tropical lower stratosphere [Kodera and Kuroda, 2002].

3. Approach To analyze this process we divided the simulated data into two groups according AO phase and compared their difference against observational data processed using the same procedure. The observations we used are NMC data (1978-1998) and SAGE ozone density (1979-2001) compiled by Randel et al. [acd.ucar.edu/user/randel]. The simulated data are from the output of SOCOL CCM (25-year long unforced steady-state run and 26-year long transient simulation driven by SST/SI, GG and ODS time dependant forcing) and UIUC CCM (15-year long unforced steady-state run).

4. AO in the Sea Level Pressure From Thompson and Wallace, 1998

5. From Kodera and Kuroda, 2002

6. Zonal mean geopotential height (m) Positive – Negative AO phase

7. Zonal mean zonal wind (m/s) Positive – Negative AO phase

8. Zonal mean temperature (K) Positive – Negative AO phase

9. Zonal mean ozone (%) Positive – Negative AO phase

10. Conclusions • During boreal winter a signature of the positive AO phase is clearly visible in all data sets, therefore the proposed approach can be used for the CCM validation • All considered models reasonably reproduce AO-like pattern of the inter-annual variability, deepening of the polar vortex and acceleration of the PNJ in the Northern Hemisphere during positive AO phase. • Theoretically expected deceleration of the meridional (BD) circulation, subsequent warming and ozone increase in the lower tropical stratosphere have been detected in the observational data as well as pronounced dipole-like temperature changes over the northern high-latitudes. All these features are reasonably simulated by all considered models.

11. However, • the simulated warming in the tropical lower stratosphere is less intensive; • observed warming in the upper stratosphere is not reproduced by the models; • simulated dipole-like temperature changes in the southern hemisphere are shifted up in comparison with observed temperature changes; • observed ozone changes in the tropical lower stratosphere are confined to the lowermost stratosphere while simulated ozone changes extend to the middle stratosphere; • ozone changes over the northern high-latitudes completely disagree with observed ozone changes. • To make more definite conclusions additional observation and simulation data should be analyzed.