To confirm the association between PC1s and SSTA in the equatorial Pacific Ocean, the correlation analysis for −11- to +11-month lags was performed.
An important activity in the Pacific Ocean, El Niño, has been measured and monitored through SSTA variation [37]. The Niño3.4 index, which is averaged SSTA over 5°S–5°N, 170°–120°W, was used for the analysis.
For the importance of the Indian Ocean, the influence of SST anomalies in the tropical Indian Ocean (TIO) is measured by the index (hereafter, referred to as the TIO index) introduced by [8], which is determined by the average of SSTAs over the TIO area (40°–90°E, 10°S–10°N). It was used to confirm the association between PC1s and SSTAs in the Indian Ocean. Three-month running means were performed on SSTA to reduce the influence of seasonal means before the analysis, as suggested by a previous study [38]. The result is shown in Figure 7.