Interdecadal Variations of ENSO Impacts over the Indo–Northwest Pacific Region and the Related Mechanisms

Owing to limited observations, it remains unknown whether the impact of El Niño–Southern Oscillation (ENSO) on the Indian Ocean–Northwest Pacific (IO–NWP) climate showed decadal changes in the early 20th century. Using multi-source reanalysis and hindcast datasets from the ECMWF and NOAA extending back to 1901, this study investigates interdecadal variations of the impact of ENSO on the IO–NWP climate from 1901 to 2009. It is found that the influence of ENSO on the IO-NWP climate shows “strong–weak–strong” interdecadal change during 1901–2009. This is characterized by much weaker Indian Ocean sea surface temperature (SST) warming and a weaker NWP subtropical anticyclone (NWPSA) in the following summer of El Niño during 1946–1967, compared with those in the other two periods (1901–1945 and 1968–2009). Analyses of the datasets indicate that the interdecadal variation is mainly associated with the change in ENSO amplitude. In contrast to the period of 1946–1967, a greater SST variance occurred in the central–eastern equatorial Pacific during 1901–1945 and 1968–2009. A stronger El Niño tends to generate more significant anticyclonic anomalies over the southeast Indian Ocean through teleconnection. The northwesterly anomalies to the south of the anticyclone weaken the southeast trade winds and warm the south Indian Ocean SST via wind–evaporation–SST feedback, and the positive south Indian Ocean SST anomalies trigger westward-propagating oceanic Rossby waves to induce stronger warming of the southwest Indian Ocean, leading to a significant asymmetric wind pattern across the equator in spring. The profound northeastward winds on the north side weaken the southwest monsoon, leading to a “second warming” over the north Indian Ocean in summer, which anchors the eastward-propagating warm Kelvin waves and results in a stronger NWPSA by inducing surface divergence and suppressing deep convection.