Figure 1: Number of detected Polar Lows in the North Pacific per Polar Low season (October to April). Top curves: the detected number of Polar Lows without NS criterion (marked with ■). The solid line represents the trend from 62 PLSs, from 1948/1949 to 2009/2010, which is 0.17 cases/year; the dashed line represents the trend from only 60 years, from 1949/1950 to 2008/2009, which is 0.09 cases/year. Bottom curves: the detected number of Polar Lows with NS criterion (marked with ◯). The solid line represents the trend from 1948/1949 to 2009/2010, which is 0.2 cases/year; the dashed line represents the trend from 1949/1950 to 2008/2009, which is 0.14 cases/year. 
