Okhotsk Index is a quantitative measure of intensity for the seasonal area of high atmospheric pressure over the Okhotsk Sea usual for May-August. It could be seen on climatic charts of atmospheric pressure at the sea level as the ridge of the Hawaiin High spreading to the Okhotsk Sea. However, as was studied in details by O.K. Ilyinsky (1959), the anticyclone forming over the Okhotsk Sea is not connected directly with the Hawaiin High but is caused by specific structure of thermobaric field in the atmosphere when a geopotential trough is formed in the middle troposphere, well seen on AT-500 maps. Cold air masses, mostly of Arctic origin, are transferred to the Okhotsk Sea in the rear of this trough, as small synoptic anticyclones, and maintain the high pressure center over the Okhotsk Sea (Fig. 1).
The Okhotsk High influence on atmospheric circulation and weather in the Russian Far East is principally different than the Hawaiin High and its ridges influence. The active Okhotsk High provides advection of cold and humid air from the Okhotsk Sea to Sakhalin, Japan and Kuril Islands, and coastal areas of Asia. Moreover, the Okhotsk High blocks eastward propagation of cyclones, that’s why quasi-stationary area of low pressure forms in the Amur basin in summer.
Fig. 1. Typical tracks of synoptic anticyclones to the Okhotsk Sea area (Ilyinsky, 1959)
The Okhotsk Index values are calculated as the mean month values of atmospheric pressure at the sea level over the whole area of the Okhotsk Sea determined by re-analysis NCEP/NCAR for knots of regular 2.5°x2.5° grid since 1948. Interannual variability of this index is distinguished by easy visible long-term tendencies (Fig. 2). A tendency to weakening of the Okhotsk High is observed recently.
Fig. 2. Interannual variations of Okhotsk Index for June and July
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