Effects of a semienclosed ocean on extratropical cyclogenesis: The dynamical processes around the Japan Sea on 23-25 January 2008

Based on a case study of an extratropical cyclone that developed rapidly near Japan on 23-25 January 2008, this paper proposes a new type of atmospheric response to a surface warming/cooling anomaly in the semienclosed Japan Sea, which is approximately half a synoptic-scale baroclinic wavelength in size. The thermal effects of the Japan Sea during early cyclogenesis spread to the upper levels via an upward wind, when the trough of the baroclinic wave is located west of the Japan Sea. In the case of a relatively cold Japan Sea, the lower level air temperature decreases over the sea, and the baroclinic wave becomes weak around Japan in the early stage. As the cyclone progressively develops and travels east, the positive and negative geopotential height anomalies induced by the sea surface cooling increase over time in the lower level trough and the upper level ridge outside the Japan Sea, respectively. The temperature anomaly spreads to upper levels through the development of a warm conveyer belt. As the cyclone becomes fully developed, a geopotential height anomaly pattern similar to the west Pacific teleconnection forms as a result of the S-shaped trough-ridge structure in the middle and upper troposphere. The cooling/warming effect of the sea influences surface heat fluxes and precipitation in the Pacific region outside the Japan Sea via a synoptic-scale atmospheric bridge caused by the traveling and developing cyclone and cold air outbreak. Key Points Atmospheric responses to a semienclosed ocean in extratropical cyclogenesisFormation of short-term WP-like pattern forced by the warm/cool Japan SeaSynoptic-scale atmospheric bridge from the Japan Sea to Pacific via cyclogenesis