Composite Analysis of Polar Mesocyclones over the Western Part of the Sea of Japan

Polar mesocyclones occur frequently over the Sea of Japan during winter in association with cold air outbreaks from the Eurasian continent. In this study, the general characteristics of polar mesocyclones over the western part of the Sea of Japan are examined using composite analysis. The synoptic-scale environment associated with these mesocyclones is characterized by a negative sea level pressure anomaly to the east that causes a cold air outbreak at low levels. There is also a geopotential height trough moving eastward at upper levels. In the cold air outbreak, a convergence zone known as the Japan Sea polar airmass convergence zone (JPCZ), collocated with a thermal ridge, develops on the lee side of the mountains at the root of the Korean Peninsula. These polar mesocyclones are generated when the upper-level trough approaches the JPCZ from the west. However, the behavior of the JPCZ and the movement of the polar mesocyclones differ depending on the location of the upper-level trough. A piecewise potential vorticity inversion analysis revealed that the circulation associated with the upper-level trough modifies the low-level winds, which affects the direction of extension of the JPCZ as well as the genesis location and the movement of the polar mesocyclones.