Modulation of PDO and the Victoria mode on the ENSO–East Asian summer monsoon relationship

This study investigates how the relationship between the preceding winter ENSO and the East Asian summer monsoon (EASM) varies with the phases of Pacific Decadal Oscillation (PDO) and the Victoria mode (VM), considering whether ENSO was El Niño or La Niña. Results show that the strongest El Niño–EASM relationship occurs during positive PDO and negative VM. This combination features a dipolar SST anomalies pattern characterized by tropical Western North Pacific (TWNP) cooling and North Indian Ocean (NIO) warming, which sustains an anomalous Western North Pacific anticyclone (WNPAAC) through the Indo-Western Pacific Ocean Capacitor (IPOC) effect. In spring, El Niño-induced northeasterly anomalies intensify the prevailing trades, enhancing TWNP cooling via wind–evaporation–SST (WES) feedback. This cooling suppresses convection and triggers a westward-propagating Rossby wave, reinforcing the WNPAAC. The TWNP cooling persists into summer and couples with NIO warming under the westerly monsoon regime, further intensifying the WNPAAC via Matsuno-Gill type response. During positive PDO and positive VM, however, significant TWNP cooling is lacking and only NIO warming is prominent. The WNPAAC still forms but is significant mostly in low latitudes, resulting in a weaker El Niño–EASM relationship. This relationship further weakens during negative PDO and negative VM, where NIO warming is significant only in spring and TWNP cooling is absent. The weakest El Niño–EASM relationship occurs during negative PDO and positive VM, where both NIO warming and TWNP cooling are lacking. Meanwhile, the La Niña–EASM relationship is generally insignificant across all PDO–VM combinations, as the relatively weaker and slower-decaying La Niña induces less pronounced anomalous SST and atmospheric responses than El Niño.