Long-term changes in the cave atmosphere air temperature as a result of periodic heliophysical processes

Climatic trends connected with short- and long-period variations of the solar activity occur as a reaction even in such conservative media as the air volumes of karst caves. The yearly mean air temperatures in the zone of constant temperatures of four show caves in Bulgaria were studied for a period of 36 years (1968-2003). The examination was made by everyday noon measurements in Ledenika, Saeva dupka, Snezhanka and Uhlovitsa cave. The caves are situated at different altitudes and geographic latitude. Seasonal fluctuations of the yearly mean air temperature in the ZCT of the explored caves have been identified by Fourier analysis. The same analysis has been applied for the Sunspot number and Apmax indices, which are representatives of the solar and geomagnetic activity, for the same period of data available. Autocorrelograms have been used for examination of the seasonal patterns of the air temperatures in the ZCT in every cave and in Sunspot number and Apmax indices. Cross-spectrum analysis has been applied for retrieving the correlations between air ZCT temperatures in the caves and solar and geomagnetic activity. It has been found that the correlation between ZCT temperature time series and sunspot number is better than that between the cave air temperature and Apmax. indices. It has been found that t(ZCT)(o) is rather connected with the first peak in geomagnetic activity, which is associated with transient solar activity, i.e., coronal mass ejections (CMEs) than with the second one, which is higher and connected with the recurrent high speed streams from coronal holes (Webb, D.F., 2002. CMEs and the solar cycle variation in their geoeffectiveness. In: Wilson, A. (Ed.), Proceedings of the SOHO 11 Symposium on From Solar Min to Max: Half a Solar Cycle with SOHO, 11-15 March 2002, Davos, Switzerland. ESA Publications Division, Noordwijk, 2002, ISBN 92-9092-818-2, pp. 409-419). This work can contribute to studying the mechanisms of atmospheric circulation changes and calibration of long-period climatic data read from cave speleothems and deposits. (C) 2006 Elsevier Ltd. All rights reserved.