Dirk Offermann

Measurements of atmospheric temperatures show a variety of long-term oscillations. These can be simulated by computer models and exhibit multi-annual, decadal, and even centennial periods. They extend from the ground up to the lower thermosphere. Recent analyses have shown that they exist in the models even if the model boundaries are kept constant with respect to influences of the sun, ocean, and greenhouse gases. Therefore, these parameters appear not to be responsible for the excitation of these oscillations, i.e. the oscillations might be rather self-excited. However, influences of land surface and vegetation changes had not been entirely excluded. This is studied in the present analysis. It turns out that such influences might be active in the lowermost atmospheric levels. Long-term trends of atmospheric parameters such as the temperature are important for the understanding of the ongoing climate change. Their study is mostly based on data sets that are 1 to a few decades long. The trend values are generally small and so are the amplitudes of the long-period oscillations. It can therefore be difficult to disentangle these structures, especially if the interval of trend analysis is comparable to the period of the oscillations. If the oscillations are self-excited, there may be a non-anthropogenic contribution to the climate change which is difficult to determine. Long-term changes of the cold-point tropopause are analysed here as an example.

Multi-annual oscillations have been observed in measured atmospheric data. These oscillations are also present in general circulation models. This is the case even if the model boundary conditions with respect to solar cycle, sea surface temperature, and trace gas variability are kept constant. The present analysis contains temperature oscillations with periods from below 5 up to more than 200 years in an altitude range from the Earth's surface to the lower thermosphere (110 km). The periods are quite robust as they are found to be the same in different model calculations and in atmospheric measurements. The oscillations show vertical profiles with special structures of amplitudes and phases. They form layers of high or low amplitudes that are a few dozen kilometres wide. Within the layers the data are correlated. Adjacent layers are anticorrelated. A vertical displacement mechanism is indicated with displacement heights of a few 100 m. Vertical profiles of amplitudes and phases of the various oscillation periods as well as their displacement heights are surprisingly similar. The oscillations are related to the thermal and dynamical structure of the middle atmosphere. These results are from latitudes and longitudes in central Europe.

Self-generated (self-sustained) oscillations have been observed in measured atmospheric data at multi-annual periods. These oscillations are also present in General Circulation Models even if their boundary conditions with respect to solar cycle, sea surface temperature, and trace gas variability are kept constant. The present analysis contains temperature oscillations with periods from below 5 yr up to 341 yr in an altitude range from the Earth’s surface to the lower thermosphere (110 km). The periods are quite robust as they are found to be the same in different model calculations and in atmospheric measurements. The oscillations show vertical profiles with special structures of amplitudes and phases. They form layers of high/low amplitudes that are a few dozen km wide. Within the layers the data are correlated. Adjacent layers are anticorrelated. A vertical displacement mechanism is indicated with displacement heights of a few 100 metres. Vertical profiles of amplitudes and phases of the various oscillation periods as well as their displacement heights are surprisingly similar. The oscillations are related to the thermal and dynamical structure of the middle atmosphere. These results are from latitudes/longitudes in Central Europe.

Mesospheric and stratospheric temperatures and winds from several stations in Germany are analyzed for long‐term trends in 1988–2008. Emphasis is on upper mesosphere (87 km) hydroxyl (OH) temperatures at Wuppertal (51°N, 7°E) that agree favorably with satellite‐borne observations from Sounding of the Atmosphere Using Broadband Emission Radiometry and a twin OH instrument at Hohenpeißenberg (48°N, 11°E) that is operational since 2003. The two twin stations yield a combined data set with 80% time coverage suitable for high time resolution analyses. Annual mean temperatures at Wuppertal show a long‐term trend of −0.23 K/yr and a solar flux sensitivity of 0.035 K/solar flux unit. Trend analysis of monthly mean temperatures yields substantial variations from one month to another, between 0 and −0.6 K/yr, hence questioning the value of seasonal mean trends. The OH temperatures have a well‐known characteristic form of seasonal variation. This form changes during the 21 years of observation. The changes are compared to modifications of the summer length in the stratosphere and are interpreted as dynamics/circulation changes extending to the uppermost parts of the middle atmosphere.

Climatologies of gravity waves, quasi‐stationary planetary waves, and tides are compared in the upper stratosphere, mesosphere, and lower thermosphere. Temperature standard deviations from zonal means are used as proxies for wave activity. The sum of the waves is compared to directly measured total temperature fluctuations. The resulting difference is used as a proxy for traveling planetary waves. A preliminary climatology for these waves is proposed. A ranking of the four wave types in terms of their impact on the total wave state of the atmosphere is achieved, which is dependent on altitude and latitude. At extratropical latitudes, gravity waves mostly play a major role. Traveling planetary waves are found to play a secondary role. Quasi‐stationary planetary waves and tides yield a lesser contribution there. Vertical profiles of total temperature fluctuations show a sharp vertical gradient change (“kink” or “bend”) in the mesosphere. This is interpreted in terms of a change of wave damping, and the concept of a “wave turbopause” is suggested. The altitude of this wave turbopause is found to be mostly determined by the relative intensities of gravity waves and planetary waves. The turbopause is further analyzed, including earlier mass spectrometer data. It is found that the wave turbopause and the mass spectrometer turbopause occur rather close together. The turbopause forms a layer about 8 km thick, and the data suggest an additional 3 km mixing layer on top.

Trace gas mixing ratios were measured in the upper troposphere and lower stratosphere during the second mission of the Cryogenic Infrared Spectrometers and Telescopes (CRISTA) experiment in August 1997. Water vapor data at high vertical and horizontal resolution are presented here. Respective data for CFC 11 (CFCl₃) are given in a future paper by V. Kuell et al. (Tropopause region temperatures and CFC 11 mixing ratios from CRISTA 2, submitted to Journal of Geophysical Research, 2004). Some ozone data are also available. Zonal mean mixing ratios of water vapor are compared to several other measurements, and good agreement is obtained. Data variance of all three gases is high and is mostly attributable to atmospheric fluctuations rather than instrument noise. Diffusive (“eddy”) fluxes were derived from these fluctuations for all three gases. They were converted to meridional eddy coefficients K_yy by means of respective mixing ratio gradients. K_yy values obtained from CFC 11 and H₂O show a close correlation (r = 0.62). This suggests that water vapor may be a usable tracer at medium latitudes. (This does not include the tropics, where our K_yy values appear to be unreliable.) Our K_yy values are substantially higher than those frequently used in the literature. Another difference is that our summer values are larger on average than the winter ones (by a factor of 2). A similar result, however, is seen in the effective diffusivities K_eff of Haynes and Shuckburgh (2000a, 2000b) (hereinafter referred to as HS a,b). There are other agreements of the two data sets K_yy and K_eff: HS a,b discuss a number of transport barriers (equator and tropopause). These are found in a very similar way in our data. The latitude structures of the K_yy and K_eff coefficients can be compared at three altitude levels. These patterns are also found to be quite similar.