Interannual variation of cold frontal activity in spring in Mongolia

In Mongolia and northern China, most of the dust storms in spring occur in association with a passage of cold front formed at the leading edge of cold air outbreak. In this study, we propose an index to evaluate a cold frontal activity by identifying "cooling days". These are days when there is a strong decrease in daily mean surface air temperature of greater than 5 K, using the European Centre for Medium-Range Weather Forecasts 45-year reanalysis data for 1967 through 2002. The geographical distribution of seasonal mean cooling day frequencies showed that Mongolia is the most frequent area over Eurasia with the highest frequency of cooling days in spring. Interannual variations in the area-averaged frequency of cooling days in Mongolia showed that most of the years with less-than-average frequencies in spring occurred in the last 20 years. To determine the main cause(s) for the differences in cooling day frequencies, we conducted a composite study for the 8 years with the highest area-averaged spring cooling day frequencies (active years), and the 8 years with the lowest (inactive years). This study revealed that there were no apparent differences between the active and inactive years with regard to geographical route of cold air, or total number of cooling events. In contrast, the cooling intensity in the inactive years was approximately 20% below that in the active years. These results suggest that recent weakening of cooling intensity over eastern Mongolia has resulted from warming over Siberia.