Scientific Background 03-51-5380

The increase in global surface air temperature during the last century has been attributed predominantly to the increasing atmospheric concentration of greenhouse gases. In the upper atmosphere, the radiative effects of greenhouse gases become more pronounced and produce a cooling, rather than a warming effect. At these altitudes, greenhouse gases are optically thin and are unable to contain outgoing infrared radiation, so that thermal energy is lost to space via infrared radiation. As with global change near the Earth’s surface, the challenge facing upper atmospheric climate scientists is to detect long-term trends and attribute them to their primary causes. Still, time series of many middle and upper atmosphere parameters are relatively short when information of long-term trends should be derived from these. Nevertheless, information on long-term trends in various parameters, based upon the last 3-4 decades, has accumulated to a level such that a global pattern of upper atmospheric climate change is beginning to emerge:

The observed decline of temperatures in the mesosphere and neutral density in the thermosphere, and the electron density trends in the ionosphere are qualitatively consistent with thermal contraction of the mesosphere as a consequence of the mesospheric greenhouse cooling, with some contribution from stratospheric ozone depletion-induced cooling.
Near the mesopause the observed long-term temperature trend is weaker, which is also supported by model results.

However, there are still some inconsistencies, weaknesses and gaps in our knowledge of upper atmospheric and MLT region trends. The agreement between model and observational trends is primarily qualitative rather than quantitative. Reasons for that are that some observational results are limited by data quality and homogeneity problems, as well as spatial and temporal coverage. There are two groups of quantities with problems and uncertainties in observational trends in the MLT region: (1) winds and waves, (2) water vapour, noctilucent clouds and polar mesospheric clouds. We deal with the first group, winds and waves.

Firstly, modelling the mesosphere and above layers of the atmosphere requires parameterisation of a variety of processes that are still unknown in strength, variability and global morphology. One of the most crucial ones among them is the global distribution of gravity waves, which basically force the mesospheric vertical zonal wind decrease and lower thermosphere wind reversal. Available middle atmosphere models usually use parameterisations that are also used to tune the mean model climatology to empirical models and therefore not necessarily include regional details of the circulation. Also, the vertical distribution of CO2 in the MLT region is uncertain to a certain degree.

Secondly, measured mesospheric and lower thermospheric wind and wave activity does not yet provide a clear and stable trend pattern. Long-term data sets are especially available from radar wind measurements at heights between 70 and 100 km (mesosphere/lower thermosphere, MLT). Recent results of MLT mean wind and tidal amplitude analysis from radar winds show that these trends seem to change, reverse or at least level off in the late 1980s and 1990s. Planetary wave activity in the MLT region inferred from ionospheric measurements displays a similar pattern of levelling off. These changes of trends are likely owing to the interplay among changes in lower atmospheric sources of waves, the stratospheric propagation of these waves, and mesospheric trends themselves.

Thirdly, to date trends in the MLT winds have often been estimated on a base of the simplest one-factor linear regression without taking into account existing autocorrelations. In addition, observed correlations with the solar flux, QBO, NAO/AO index or others have only been partly included into these analyses. A new approach for a correct trend assessment leads to the necessity of studying correlations between the MLT wind and atmospheric indices on the new background.

Fourthly, the MLT wind data sets are still relatively short (few decades maximum) to define a unique model of long-term tendencies in wind parameters. Therefore a comparison with other time series of parameters physically linked to the MLT wind parameters is a necessary part of the trend assessment and a selection of a proper trend model. The majority of relevant parameters in the upper atmosphere is available over even shorter periods, therefore only lower atmosphere parameters may be used in this connection.

There are other atmospheric parameters that also show a change in long-term trends. This is first of all tendency towards a recovery of the ozone layer (visible in the total ozone content as well as in upper stratospheric ozone) which possibly began around 1996. The total ozone and ozone laminae trend patterns indicate a corresponding change in the sign of trends in the mid-1990s, its origin being probably changes in trends in stratospheric dynamics (for example, the mid-latitude winter heat flux at 100 hPa increases since mid-1990s) and in a decrease of chlorine loading. An assessment of three alternatives to linear trends allows to find a climate regime shift for tropospheric temperature data. The shift is a structural change in the mean value that is a partial case of possible structural changes in trends.

Questions arising from the discussion above thus are:

Can we already detect significant changes in the structure of middle and upper (mainly MLT region) atmosphere trends and the type of these changes? Can we, and to what degree, more precisely than to date define the time when these changes occur ?
Are there other atmospheric (lower atmosphere circulation, climate gases, planetary and gravity waves) or extraterrestrial (solar, ionospheric, magnetospheric) parameters that show a concurrent behaviour?
Can we define sources of the upper atmosphere trend changes, e.g. tropospheric/stratospheric mean circulation and planetary wave activity changes, and can we model upper atmosphere trends using this information?