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Abstract

Does a warming world, where extremely hot summers are becoming more common, mean that cold summers will never occur in the future? It is important to know whether extremely cold summers will occur, especially to determine whether crops need to be adapted to cold summers. Japan, which has suffered from many extremely cold summers in the past, managed these agricultural disruptions with emergency rice imports. Here, we show that, in 2010, a climate regime shift associated with the positive phase shift of the Arctic Oscillation occurred in northeast Eurasia, making the occurrence of extremely cold summers highly unlikely as long as this new regime persists. In fact, Japan has not experienced a cold summer since 2010, while extremely hot summers have been frequent. Since 2010, a double jet structure with subtropical and polar jets has strengthened, and the polar jet has meandered further north of Japan, resulting in an upper tropospheric anticyclone. This anticyclone, which extends downwards and tilts southwards, reaches southern Japan and prevents cold advection of oceanic air over the cold Oyashio Current. The Okhotsk High, known as the main cause of cold summers, has occurred frequently in recent years, but cold summers have not occurred due to the tilting anticyclone. The recent warming of the Oyashio Current weakens cold advection. The Pacific-Japan pattern, known as a remote tropical influence, has been weakened. A better understanding of the regime shift will help us understand the tilting anticyclone and the associated extreme summers in northeast Eurasia.