Winter marine atmospheric conditions over the Japan Sea

Four basic types of synoptic‐scale conditions describe the atmospheric structure and variability observed over the Japan Sea during the 1999/2000 winter season: (1) flow of cold Asian air from the northwest, (2) an outbreak of very cold Siberian air from the north and northeast, (3) passage of a weak cyclone over the southern Japan Sea with a cold air outbreak on the backside of the low, and (4) passage of a moderate cyclone along the northwestern side of the Japan Sea. In winter, the Russian coastal mountains and a surface‐air temperature inversion typically block cold surface continental air from the Japan Sea. Instead, the adiabatic warming of coastal mountain lee‐side air results in small air‐sea temperature differences. Occasional outbreaks of very cold Siberian air eliminate the continental surface‐based inversion and stability, allowing very cold air to push out over the Japan Sea for 1–3 days. During these outbreaks, the 0°C surface air isotherm extends well southward of 40°N, the surface heat losses in the center of the Japan Sea can exceed 600 W m−2, and sheet clouds cover most of the Japan Sea, with individual roll clouds extending from near the Russian coast to Honshu. During December through February, 1991–2002, these strong cold‐air outbreak conditions occur 39% of the time and contribute 43% of the net heat loss from the Japan Sea. The average number of strong cold‐air events per winter (November–March) season is 13 (ranging from 5 to 19); the 1999/2000 winter season covered in our measurements was normal.