Indices of cool summer climate in northern Japan: Yamase indices

This study examined seven indices of the cool summer climate in northern Japan in terms of spatial representativeness and interannual variability using weather observation station data and reanalysis data from the Japanese 25-year Reanalysis/Japan Meteorological Agency Climate Data Assimilation System in June-August for 1979-2010. The indices are constructed using sea level pressure (SLP) and surface air temperature (SAT): area-average SLP over the Okhotsk Sea; north-south SLP difference in northern Japan; east-west SLP difference along the Tsugaru Strait; east-west SLP difference along the Soya Strait; SAT anomaly from climatology at a location along the coasts of the Pacific and the Okhotsk Sea; diurnal variance of SAT at a location along the coasts of the Pacific and the Okhotsk Sea; and time coefficient of the east-west oscillation mode of SAT in northern Japan. The last two are newly proposed. The atmospheric fields represented by the indices commonly show the following features: the developed Okhotsk high at the surface and a mid-troposphere ridge to its northwest; southward extensions of low SAT, high SLP, low specific humidity, and high cloud water content along the Pacific coast of northern Japan and along the Japan Sea coast of the Eurasian continent; and strong easterly/northeasterly winds to the east and west of northern Japan. Furthermore, these indices show consistent interannual variabilities and clearly detect cool summers in northern Japan in the past. Meanwhile, differences between the indices lie in the location of the ridge in the mid-troposphere and the vertical structure of the Okhotsk high, the center locations of low SAT and enhanced easterly/northeasterly surface winds, and the degree of the southward extensions of the cool air along the Pacific coast of northern Japan and along the Japan Sea coast of the Eurasian continent. Based on these characteristics, we can choose a suitable index for the intended use.