Seasonal Variations of Air Masses in the Arctic and in the Middle and High Latitudes of the Eastern Part of the Eurasian Continent

An air mass is a large body of air with relatively homogeneous properties across its horizontal extent. Since the air mass causes weather, such as cold polar outbreaks in winter or hot, humid days in summer, it is thought to be one of the important elements of climate. Since Bergeron(1930) presented the concept of the air mass, many studies have been conducted. However, the vertical structure and seasonal variation of the air mass, which are the important elements in the relation between the air mass and the general circulation of the atmosphere, have not been fully examined. This work investigates the stratification and distribution of air masses and their seasonal varia-tions in the Arctic and in the middle and high latitudes of the eastern part of the Eurasian Conti-nent.<br> First, the Arctic region (A air mass), the eastern part of the Eurasian Continent (Pc air mass), the Sea of Okhotsk (Pm air mass), and the Bering Sea (Pm air mass), were identified as the air mass source regions, using the monthly normal surface air temperatures in January and July (Figs. 1 and 2).<br> Next, the properties of each air mass were examined by the use of time sequences of potential temperature, mixing ratio, and equivalent potential temperature (Fig. 3). The time variations of potential temperature and mixing ratio were similar in the Arctic. On the other hand, the mixing ratio was large in the summer half-year in the eastern part of the Eurasian Continent and the Sea of Okhotsk. Because of the cyclonic activity, a large amount of mixing ratio was seen in the winter half-year in the Bering Sea.<br> Since the stratifications of each air mass source region changed nearly simultaneously (Figs. 3 and 4), the year 1985 was divided into five stages: I (October 21 to February 28), II (March 1 to April 15), III (April 16 to June 10), IV (June 11 to August 31), and V (September 1 to October 20). The differences of θ and θe between the eastern part of Eurasian Continent and the Arctic in dicate that the air mass over the Continent is colder from mid-November to late February, and that the vapor amount on the Continent is larger in stage IV (Fig. 5).<br> The vertical profiles of temperature and θe show that the stratification of the A air mass is sta-ble through the year, and the upper limits of this air mass are defined as 700mb in stages I to III and 850mb in stage IV (Fig. 6). The stratification of the Pc air mass is stable in stages I, II and V, and the stratification is unstable in stages III and IV. The upper limit of the Pc air mass is about 700mb in. stages I, III and IV, and 850mb in stage II. The vertical extents of the A and Pc air mass cannot be identified in stage V. The Pm air mass formed under 850mb in stage IV, whereas the air mass over the Sea of Okhotsk was hotter and more stable than that in the Bering Sea. On the other hand, stable stratification is seen under 850 mb in stage III in the Sea of Okhotsk, which suggests that the Pm air mass also existed in this stage. Over the Bering Sea, however, such stable stratification cannot be found in stage III.<br> Lastly, the correspondence between the seasonal variations of air mass boundaries and stages suggests that the air mass stratifications are related to the distributions of air masses (Figs. 7-10). The southern limit of the A air mass was around 70-75°N except for stage II and V (Figs. 7 and 8). In stage II, the southern limit of the A air mass advanced southward around 60-65°N, and it could not be defined in stage V. The southern limit of the Pc air mass could be seen around 45° N throughout the year (Figs. 7 and 8). Especially in stage I, this air mass was divided into two masses: very cold (Pc₁) and cold (Pc₂). On the other hand, warm and moist air characterizes the Pc air mass around 50-60°N in stage IV.