Variation of the sea-ice cover in the Arctic Ocean and the abnormal weather in mid-latitudes

In contrast to the recent trend of global warming, air temperature in northern Japan have often lowered in the warm season to cause very cold summers and poor rice crops after the year of 1980. This seems to be deeply related with abnormal condition of the atmospheric general circulation. A study has been made to find relationship between the cold northeasterly winds, called Yamase circulation, and the general circulation of the Northern Hemisphere. The Yamase has been found to blow over northern Japan when the Baiu Front (low pressure zone to the south of Japanese islands) extends towards the east and part of the Pacific High extends northwestwards to form the Okhotsk High. At the same time, the westerlies (jet stream) at the 500hPa level splits into two branches and forms a blocking high between them (to the west of the Okhotsk Sea). A detailed study has shown that the split of jet stream takes place when the lower polar jet (-700hPa) develops over the central Siberia Plain while the higher jet stream (-200hPa) remains at subtropical latitudes. Satellite data of sea-ice concentration in the Arctic Ocean have been analyzed in order to examine possible relation between the sea-ice cover and the general circulation. It has been found that Kara and Barents Seas, open seas of the highest latitude, show the maximum variability in sea ice concentration in summertime (from May to August). By making composite maps of pressure fields in terms of sea-ice concentration in Kara and Barents Seas, it has been found that when the sea ice cover is more extensive towards the low latitudes than the normal year, the jet stream at 500 hPa level splits in to two branches and the jet stream at 200hPa is located at more southern latitudes. The above results strongly suggest that the sea ice cover in Kara and Barents Seas may be responsible for generating the lower polar jet over the Siberia, for the split of the jet stream, and for the cold northeasterly winds over northern Japan.