Fig. 2

Isoguchi Jet strength. a Regression map (shade; °C) between SST anomaly (December, January, February) derived from OISST²⁰ and normalized time series of Isoguchi Jet Index (IJI), where the normalization is made with respect to its standard deviation. Here the IJI is defined by the difference between the satellite derived SSH anomaly⁴⁵ in the southeastern box and that in the northwestern box, where definition of each box is given in Methods. White contours denote significance level greater than 95%. Solid lines denote the SST front latitude\(\overline {\Phi (x)} + {\rm{EOF}}1(x) \cdot \sigma\) derived from OISST, where σ = 0 (thin line) and σ = 1 (thick line). b Monthly time series of IJI (black), SSH anomaly in the southeastern box (SSH-S; red) and that in the northwestern box (SSH-N; blue). Thick lines denote 7-year low-pass filtered time series. c Correlation map between low-pass filtered (>7 years) SSH anomaly and normalized IJI time series. White contours denote significance level greater than 95%