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

与最近全球变暖的趋势相反，1980年以后，日本北部的气温经常在温暖的季节降低，导致夏季非常寒冷，水稻作物歉收。这似乎与大气环流异常状况有很大关系。一项研究发现了被称为Yamase环流的寒冷东北风与北半球大气环流之间的关系。当白云锋(日本岛屿以南的低压区)向东延伸，部分太平洋高压向西北延伸形成鄂霍次克高压时，Yamase被发现吹过日本北部。同时，500hPa层的西风(急流)分裂成两支，并在它们之间(鄂霍次克海以西)形成阻挡高压。研究表明，低空急流(-700hPa)在西伯利亚平原中部发展，而高空急流(-200hPa.)仍在副热带地区时，急流发生分裂。分析了北冰洋海冰集中的卫星资料，以探讨海冰覆盖与大气环流之间的可能关系。研究发现，最高纬度的开阔海域卡拉和巴伦支海的海冰密度在夏季(5月至8月)变化最大。通过对喀拉海和巴伦支海海冰浓度场合成图的分析，发现当海冰覆盖范围比常年更大时，500hPa急流分裂成两支，200hPa急流位于更南纬。上述结果有力地表明，喀拉海和巴伦支海的海冰覆盖可能是西伯利亚低空急流、急流分裂和日本北部冷风东北风的主要原因。

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.