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					                                                         The Tropopause at High Northern Latitudes: Trends and Influence of Atmospheric Dynamics
                                                                                                               Georg Hansen and Kerstin Stebel, Norwegian Institute for Air Research (NILU), Tromsø and Kjeller, Norway

                                       The tropopause is the important transition interface between the lower and the middle atmosphere with a profound impact on atmospheric chemistry                         Trends in Tropopause Pressure
                                       both below and above. In particular, it has been found that the altitude of the tropopause as the lower boundary of the stratospheric ozone layer strongly
                                       influences the total ozone amount on a global scale (e.g., Steinbrecht et al., 1998). Systematic changes in tropopause altitude will thus contribute to
                                       trends in total ozone. The aims of this study were to investigate:                                                                                                                                                                                                  Figure 5.
                                                                                                                                                                                                                                                                                                           Linear trends in monthly mean tropopause pressure (1979-
                                       • Trends and inter-annual variability in tropopause pressure/altitude and temperature since the late 1970s at 60º, 70º and 80º N                                                                                                                                    2002) at Ny-Ålesund (79ºN, 11ºE), Andøya (69ºN, 16ºE)
                                       • Regional European deviations from the zonal mean and parameters influencing the year-to-year variability                                                                                                                                                          and Oslo (60ºN, 10ºE) in January (left panels) and June
                                                                                                                                                                                                                                                                                                           (right panels), compared with zonal means at 80º, 70º and
                                       • Coupling and exchange processes between the stratosphere and the troposphere in the European Arctic and their relation to the polar stratospheric
                                                                                                                                                                                                                                                                                                           60º N (lower panels):
                                         vortex                                                                                                                                                                                                                                                            - trends between the three European stations diverge
                                                                                                                                                                                                                                                                                                             stronger than between zonal avergaes
                                       Data sets used:                                                                                                                                                                                                                                                     - station trends significantly deviate significantly from
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                                                                                                                                                                                                                                                                                                             zonal averages in all seasons
                                       • Re-analysed meteorological data from the European Centre for Medium-Range Weather Forecast (ECMWF – ERA 40)
                                       • Radiosonde data from Sodankylä, Bodø, Bjørnøya, Ny-Ålesund for the years 2000-2004
                                       • MST radar data from Andøya, Kiruna and Adventfjorden (not presented here)

                                       Analysis:
                                       Calculation of ERA40 thermal tropopause pressure and temperature according to Reichler et al. (GRL, 2003), comparison with radiosondes for selected sites and
                                       years


                                                                                                                                             Figure 1.
                                                                                                                                             Monthly means of tropopause pressure (left panel) and
                                                                                                                                             temperature and temperature (right panel) in 2000 and 2001
                                                                                                                                             derived from radiosonde soundings (bold lines; with
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                                                                                                                                             standard deviations) at Bodø (red) and Ny-Ålesund (blue),
                                                                                                                                             and at the closest ERA 40 gridpoints (dotted lines; red:
                                                                                                                                             Andøya grid point). Typically, ERA 40 derived tropopause
                                                                                                                                                                                                                Seasonal trends in tropopause pressure [hPa/yr]
                                                                                                                                             pressure values are slightly larger than the values derived
                                                                                                                                                                                                                Values in parantheses: correlation coefficients. Values marked bold red are significant   Figure 6.
                                                                                                                                             from radiosondes due to the coarse vertical gridding. For
                                                                                                                                                                                                                at 95% confidence level (normal red: 90% confidence level)
                                                                                                                                             the Bodø-Andøya comparison, the latitude offset                                                                                                              Monthly mean tropopause pressure at Andøya in four
                                                                                                                                             contributes, too. It is assumed that the offset has no             At all latitudes, there are significant trends only in autumn: decreasing tropopause      selected months. Note the widely differing patterns of year-
                                                                                                                                             temporal trend.                                                    pressure/ increasing altitude.                                                            to-year variability in the spring months!

                                       ERA40 Monthly Mean Tropopause Fields
                                                                                                                                                                                                               Multiple Correlation Analysis of Tropopause pressure at Andøya and Ny-Ålesund
                                                                                                                                                                      Figure 3.
                                                                                                                                                                      Linear trends in monthly mean
                                                                                                                                                                                                                                                                                                                          Input parameters:
                                                                                                                                                                      tropopause pressure (1979-2002)
                                                                                                                                                                                                                                                                                                                            - local temperature at 30 hPa level
                                                                                                                                                                      north of 60º N, shown here for
                                                                                                                                                                                                                                                                                                                            - NOAA CPC tele-connection
                                                                                                                                                                      January (upper panels) and
                                                                                                                                                                                                                                                                                                                              patterns
                                                                                                                                                                      March (lower panels).
                                                                                                                                                                                                                                                                                                                            - volcanic aerosols
                                                                                                                                                                      Left panels: trend [hPa] , centre                                                                                                                     - solar flux
                                                                                                                                                                      panels: standard deviations [hPa];                                                                                                                    - Quasi-biennal oscillation @30hPa
                                                                                                                                                                      right panels: correlation
                                                                                                                                                                      coefficient (significant at 95%                                                                                                                     Results:
                                                                                                                                                                      confidence level if r > 0.45)                                                                                                                       (1)At both sites strong impact of local mid-
                                                                                                                                                                                                                                                                                                                            stratosphere conditions: at Ny-Ålesund
                                                                                                                                                                                                                                                                                                                            more limited to winter/spring months, at
                                                                                                                                                                                                                                                                                                                            Andøya most pronounced in September;
                                                                                                                                                                                                                                                                                                                            in most cases with time delay of 1-2
                                                                                                                                                                                                                                                                                                                            months (especially at Ny-Ålesund)
                                                                                                                                                                                                                                                                                                                          (2)At both sites strong signature of Polar-
                                                                                                                                                                                                                                                                                                                            Eurasian pattern in mid-winter, otherwise
                                                                                                                                                                                                                                                                                                                            Scandinavia pattern most persistent,
                                                                                                                                                                   Figure 4.                                                                                                                                                especially in autumn
                                                                                                                                                                   Monthly averages (1990-2001) of                                                                                                                        (3)Stronger signature of NAO and PNA at
                                                                                                                                                                   tropopause pressure (bold) at Ny-                                                                                                                        Ny-Ålesund than at Andøya
                                                                                                                                                                   Ålesund (red), Andøya (black) and
                                                                                                                                                                   Oslo (green), compared to zonal                                                                                                                        (4)PT dominant feature in June at both
                                                                                                                                                                   means (dotted) at 80º, 70º and 60º                                                                                                                       sites (also found in total ozone analysis)
                                                                                                                                                                   N. Thin lines: pressure level of 2                                                                                                                     (5) QBO influence less pronounced than in
                                                                                                                                                                   PVU surfaces.                                                                                                                                            total ozone data analysis ; stratospheric
                                                                                                                                                                   - Significant ”phase shift” of                                                                                                                           aerosols and solar cycle of minor
                                       Figure 2.                                                                                                                     maxima and minima at European                                                                                                                          influence
                                       Monthly mean tropopause pressure fields based on
                                                                                                                                                                     stations relative to zonal means
                                       ERA 40 north of 60º N. Examples: January 1996
                                       (upper panel) and January 1997 (lower panel). The                                                                           - Large offset of thermal tropopause                                                                                                                   Contact address:
                                       strong negative pressure anomaly in the European                                                                              relative to PV2 surface (20-50                                                                                                                       Georg Hansen
                                       Atlantic sector is found in most years.                                                                                       hPa, depending on season and                                                                                                                         Norwegian Institute for Air Research (NILU)
                                                                                                                                                                     latitude).                            Acknowledgements
                                                                                                                                                                                                                                                                                                                          Polar Environmental Centre
                                                                                                                                                                                                           This study was funded by the Research Council of Norway (contract no. 155978/S30) and partially through        N-9296 Tromsø, Norway
                                                                                                                                                                                                           the centre funding of the Ministry of the Environment to NILU at the Polar Environmental Centre)               E-mail: ghh@nilu.no

				
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