Modulation of the Predictability of the East Asian Summer Monsoon by the Interdecadal Pacific Oscillation

We identified an interdecadal modulation of the potential predictability (measured by the signal-to-noise ratio) of the East Asian summer monsoon (EASM) by the interdecadal Pacific oscillation (IPO). The EASM potential predictability is relatively high during the warm phase of the IPO, but relatively low during the cold phase. The IPO has been in a cold phase since the late 1990s and this has coincided with a decrease in the potential predictability of the EASM, which has posed a severe challenge to the seasonal forecasting of the EASM in recent decades. The El Nino-Southern Oscillation (ENSO)-EASM relationship is stronger in the warm phase of the IPO than in the cold phase, resulting in an enhanced influence of the ENSO on the EASM and a stronger external signal of predictability for the EASM in the warm phase of the IPO. Changes in the ENSO-EASM relationship can be attributed to a significant difference in the amplitude of the variability of the ENSO during the IPO cold and warm phase. The latter was found to have contributions from the modulation of the IPO on the El Nino and La Nina frequency asymmetry. These results imply that the phase of the IPO should be considered when assessing the prediction skill of the EASM.