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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 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:

• Trends and inter-annual variability in tropopause pressure/altitude and temperature since the late 1970s at 60º, 70º and 80º N

• Regional European deviations from the zonal mean and parameters influencing the year-to-year variability

• Coupling and exchange processes between the stratosphere and the troposphere in the European Arctic and their relation to the polar stratospheric vortex

Data sets used:• 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 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 pressure values are slightly larger than the values derived from radiosondes due to the coarse vertical gridding. For the Bodø-Andøya comparison, the latitude offset contributes, too. It is assumed that the offset has no temporal trend.

Figure 2. Monthly mean tropopause pressure fields based on ERA 40 north of 60º N. Examples: January 1996 (upper panel) and January 1997 (lower panel). The strong negative pressure anomaly in the European Atlantic sector is found in most years.

ERA40 Monthly Mean Tropopause Fields

Figure 3. Linear trends in monthly mean tropopause pressure (1979-2002) north of 60º N, shown here for January (upper panels) and March (lower panels).

Left panels: trend [hPa] , centre panels: standard deviations [hPa]; right panels: correlation coefficient (significant at 95% confidence level if r > 0.45)

Figure 4.Monthly averages (1990-2001) of tropopause pressure (bold) at Ny-Ålesund (red), Andøya (black) and Oslo (green), compared to zonal means (dotted) at 80º, 70º and 60º N. Thin lines: pressure level of 2 PVU surfaces.

- Significant ”phase shift” of maxima and minima at European stations relative to zonal means- Large offset of thermal tropopause relative to PV2 surface (20-50 hPa, depending on season and latitude).

Trends in Tropopause Pressure

Figure 5. Linear trends in monthly mean tropopause pressure (1979-2002) at Ny-Ålesund (79ºN, 11ºE), Andøya (69ºN, 16ºE) and Oslo (60ºN, 10ºE) in January (left panels) and June (right panels), compared with zonal means at 80º, 70º and 60º N (lower panels):- trends between the three European stations diverge stronger than between zonal avergaes - station trends significantly deviate significantly from zonal averages in all seasons

Figure 6.

Monthly mean tropopause pressure at Andøya in four selected months. Note the widely differing patterns of year-to-year variability in the spring months!

Seasonal trends in tropopause pressure [hPa/yr]Values in parantheses: correlation coefficients. Values marked bold red are significant at 95% confidence level (normal red: 90% confidence level)

At all latitudes, there are significant trends only in autumn: decreasing tropopause pressure/ increasing altitude.

Multiple Correlation Analysis of Tropopause pressure at Andøya and Ny-Ålesund

Input parameters:- local temperature at 30 hPa level- NOAA CPC tele-connection patterns- volcanic aerosols- solar flux- Quasi-biennal oscillation @30hPa

Results:

(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, especially in autumn

(3)Stronger signature of NAO and PNA at Ny-Ålesund than at Andøya

(4)PT dominant feature in June at both sites (also found in total ozone analysis)

(5) QBO influence less pronounced than in total ozone data analysis ; stratospheric aerosols and solar cycle of minor influence

Acknowledgements

This study was funded by the Research Council of Norway (contract no. 155978/S30) and partially through the centre funding of the Ministry of the Environment to NILU at the Polar Environmental Centre)

Contact address:Georg HansenNorwegian Institute for Air Research (NILU)Polar Environmental Centre N-9296 Tromsø, NorwayE-mail: ghh@nilu.no