Environment for Explosive Development of Extratropical Cyclones around Japan

A.Yoshida* and Y.Asuma

(8th Scientific Assembly of IAMAS, S06.2-2 11:13.July.2001)

To investigate the characteristics of explosively developed extra tropical cyclones around Japan, we analyzed their tracks and atmospheric environment for a year, from April 1994 to March 1995, using the global objectively analyzed data set (GANAL). We classified cyclones into three types according to their places of origin and development.

As most cyclones intensify over bodies of water, we classified them into two types according to where they developed most strongly: the Pacific Ocean (PO) type and the Sea of Okhotsk and Sea of Japan (OJ) type. Although all of the OJ type cyclones originated over the eastern Asian Continent, we found that the PO type had two regions of their outbreak. One type originates over the southern China Sea (PO-O type) and the other over the eastern Asian Continent (PO-L type). We found 30 explosively developed cyclones in total, according to the definition of Sanders and Gyakum (1980). All of these cyclones occurred during cold seasons. The PO-O type most frequently occurred (16 cases), followed by the OJ type (8 cases). The PO-L type occurred the least (6 cases). The OJ type tended to occur in the autumn while the PO-O and the PO-L type tended to occur in mid-winter.

Through composite analyses with 400hPa and 850hPa GANAL data, we found different characteristics among the three types. From the 400hPa PV analyses, we noticed that the OJ type had a weak positive PV anomaly over Hokkaido, the PO-O type had a strong positive PV anomaly and the PO-L type had a positive PV anomaly in the northeastern part of Japan over the Pacific Ocean. On the 850hPa surface, the OJ type had a strong baroclinicity over the Sea of Japan and the northern Sea of Okhotsk. The PO-O type had a strong zonal baroclinicity along the southern coast of Japan and the PO-L type had a weak baroclinicity in the eastern part of Japan over the Pacific Ocean.

We also estimated the terms of the development equation followed by Zwack and Okossi (1986). From these kinds of analyses we conclude that the tracks and developing mechanism of explosively developed extratropical cyclones around Japan were affected by seasonal variations of large scale atmospheric environment. These analyses can also contribute to a better understanding of the weather system and climate over the northwestern Pacific Ocean region.