The result shows significant negative correlations of PC1s to Niño3.4 index on previous months and following months, which starts from three months for PC1pri (Figure 7a) and five months for PC1sec (Figure 7b) before DJF (0), and both persist for three months later. Thus, the air-sea interaction over the equatorial Pacific Ocean affects the variability of NE monsoon over the IDP represented by PC1s. This result indicates that there is an association between NE monsoon variability and ENSO. The ENSO can trigger the monsoon over the IDP, and its relation persists a few months after the winter. The negative correlations between PC1s and the Niño3.4 index imply that the strengthened NE monsoon is associated with the negative phase of Niño3.4, which is considered as La Niña when the Niño3.4 values exceed −0.5 °C, consecutively, and vice versa for the weakened NE monsoon associated with El Niño.
For the TIO, there is a significant correlation between PC1pri and the TIO index before winter from January to June, but the PC1sec significant correlation is absent, as shown in Figure 7c,d. The result implies that the variability of SSTA in the TIO correlated to the variation of the NE monsoon over the IDP, due to the wind in the parallel direction to the eigenvector field of the leading mode, rather than that in the perpendicular direction. The relation of PC1s to the TIO index also shows early and less significant influence on the wintertime low-level winds over the IDP, as compared to the SSTA variation in the tropical Pacific Ocean. This agrees with a previous study, that suggested the SST anomalies in TIO are likely to be the factor driving the EAWM [8], but our result shows little difference in the time period, showing early significant correlation. Although this is different, it is interesting to study further, including the direct and indirect influences or the modulation of SST anomalies in TIO with ENSO. For the next analysis, we only focus on ENSO during the boreal winter