Enhanced ENSO-unrelated Summer Variability in the Indo-western Pacific Under Global Warming


El Niño-Southern Oscillation (ENSO) is an important but not the only source of interannual variability over the Indo-western Pacific. Non-ENSO forced variability in the region has received recent attention because of the implications for rainy season prediction. Using a 35-member CESM1 Large Ensemble (CESM-LE) and 30 CMIP6 models, this study shows that the ensemble means project intensified interannual variability for precipitation, low-level winds, and sea-level pressure under global warming, associated with the enhanced large-scale anomalous anticyclone (AAC) over the tropical northwestern (NW) Pacific after the ENSO signal is removed. A decomposition based on the column water vapor budget reveals that enhanced precipitation variability is due to the increased background specific humidity. The resultant diabatic heating intensifies the AAC, which further strengthens the precipitation anomalies. Over the tropical NW Pacific, the wind-induced evaporative cooling on the southeastern flank of the AAC is countered by the increased shortwave radiation due to the strengthened precipitation reduction. Tropospheric temperature anomalies in the ensemble means show no significant change, suggesting no apparent change of the inter-basin positive feedback between the AAC and North Indian Ocean SST. Inter-model analysis based on CMIP6 reveals that enhanced background warming in the eastern equatorial Pacific is associated with enhanced ENSO-unrelated precipitation variability over the NW Pacific, due to the modulated Walker and Hadley circulations.