The typical point of view connects variations of the atmosphere's global mean surface temperature with the fluctuations of the heat fluxes only. Diabatic heating or cooling (connected, for example, with changes in planetary albedo or the greenhouse effect of the large-scale cloud fields) are the most important causes of such variability. However, more than half of the extra-tropical mean air surface temperature variability is dynamically contributed and associated with the COWL (Cold Ocean Warm Land) structures (Walles et al., 1995). Strong variability of the large-scale atmospheric circulation and a multiplicity of circulation regimes allows the hypothesis that an additional mechanism of global air surface temperature fluctuation exists. On the other hand it is extremely important to distinguish between dynamically contributed and «real» diabatic (radiative) variations of the global air surface temperature. Such kind of research in turn will allows more precise conclusions about anthropogenic and natural global changes.
We put forward a hypothesis that short-term variations of the global surface air temperature may be partly a result of adiabatic vertical mixing. For the purpose of research a run of a global atmospheric model, T10-L14 (Main Geophysical Observatory, St.-Petersburg, Russia) in the regime of perpetual January for 200 years' was carried out. An additional set of numerical 14-year runs with the T30-L14 MGO model, using annual cycle of incoming solar radiation and the sea surface temperature, has also been performed.
Using Walles et al's (1995) method of analysis and the results of the long-term MGO model integration, we have revealed the structures of the temperature field at the AT-1000 level, and outgoing long-wave radiation fields at the top boundary of the atmosphere. Using monthly data, we have demonstrated the so-called COWL structures (Walles 1995, 1996) in the temperature field; these are well correlated with the global mean surface temperature (r=0.7). On the other hand, we have shown the Hadly structures in the outgoing long-wave radiation fields. These structures describe approximately 25% of the global mean temperature variability. The enhancement of the Hadly cell means (besides the incoming and outgoing radiation redistribution) an increase of vertical large-scale mixing, which may lead to the adiabatic growth of temperature in the lower troposphere. We have found good agreement between the theoretical findings (sensitivity parameters of the air surface temperature to lapse rate for the dry adiabatic processes) and the results of the GCM long-term integration. In the moderate and high latitudes such agreement is especially well pronounced. Integration of a higher-resolution version of the MGO model (T30-L14) with the observed sea surface temperature as a bottom boundary condition has demonstrated that adiabatic mixing is a significant part of the atmosphere global mean surface temperature variability. An adiabatic mechanism for the air surface temperature variations has been hypothesized. The hypothesis is based on an assumption that the formation of COWL structures is not only connected to the heat fluxes from ocean to atmosphere, but is also due to an intensification of the vertical mixing over the continents.
The work presented here was supported in part by the Russian Fund of Basic Research under Grant number: 96 05 - 64960.