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.
To investigate the mechanism related to the influence of ENSO, regression analysis was applied [26]. The PC1s-res, which represent the variability of northeasterly wind over the IDP without the influence of the thermal contrast in the temperate region, were analyzed to separate them into the ENSO-related part (PC1s-res-EN) and the ENSO-unrelated part (PC1s-res-uEN) by the regression technique, as described in a previous study [26]. The contributions of ENSO to PC1pri-res and PC1sec-res are 17% and 25% of their total variances, respectively, whereas the contributions of the ENSO unrelated part are 83% and 75% for PC1pri-res and PC1sec-res, respectively. The ENSO-related patterns show the positive anomalous SLP over the temperate mainland and the north Eastern Pacific Ocean (Figure 8a,c). They also present the anomalous low pressure in the west Pacific Ocean with the cyclonic circulation around the maritime continent. These indicate that the variability of northeasterly wind over the IDP is influenced by wind blowing from the mid-latitude to the IDP, which is induced by the cyclonic
circulation. These reveal the role of ENSO. On the other hand, the ENSO-unrelated part of PC1pri-res (Figure 8b) presents wider positive anomalous SLP over the eastern Pacific Ocean, which implies the high pressure over this area influencing the wind variability over the IDP in a parallel direction to the directions of wind vectors given by the eigenvector. For the PC1sec-res, which is related to the variation of winds in the perpendicular direction, the pattern (Figure 8d) shows two poles of the positive anomalous SLP over the areas around Siberia and the Gulf of Alaska without the obvious SLP forcing in the tropical area. These indicate that the two poles in the mid-latitude influence the variability of northeasterly wind over the IDP without the ENSO forcing by driving winds in the high latitude to the tropical region. Therefore, the variability of northeasterly wind over the IDP is influenced by the modulation of the tropical and temperate forcing, which are the ENSO and the SLP forcing in the mid-latitude.