Origin of Seasonal Predictability for Summer Climate over the Northwestern Pacific

Summer climate in the Northwestern Pacific (NWP) displays large year-to-year variability, affecting densely populated Southeast and East Asia by impacting precipitation, temperature and tropical cyclones. El Niño-Southern Oscillation (ENSO) is the leading predictor for summer East Asian climate through its correlation with a meridional teleconnection called the Pacific-Japan (PJ) pattern over the NWP. Using coupled climate model experiments, we show that the PJ pattern is the atmospheric manifestation of an air-sea coupled mode spanning the Indo-NWP warm pool. The PJ pattern forces the Indian Ocean (IO) via a westward propagating atmospheric Rossby wave. In response, IO sea surface temperature feeds back and reinforces the PJ pattern via a tropospheric Kelvin wave. Ocean coupling increases both the amplitude and temporal persistence of the PJ pattern. Cross-correlation of ocean-atmospheric anomalies confirms the coupled nature of this mode. This coupled PJIO mode can exist without ENSO, but in nature, ENSO excites this coupled mode, explaining why the last echoes of ENSO are found in the summer IO-NWP. We demonstrate that the PJIO mode is indeed highly predictable, giving hopes for skillful seasonal forecast over the densely populated region.