Inter-decadal Change In Potential Predictability Of The East Asian Summer Monsoon And Its Possible Causes

The East Asian summer monsoon system includes the East Asian subtropical summer monsoon(EASSM)and the South China Sea summer monsoon(SCSSM).The EASSM and the SCSSM are different.The inter-decadal change in potential predictability of the EASSM and the SCSSM has been investigated using the signal to noise ratio method based on the National Centers for Environmental Prediction(NCEP/NCAR)reanalysis data,the National Oceanic and Atmospheric Administration 20th Century Reanalysis version 2(20CRv2)dataset and the pacemaker experiment of the National Center for Atmospheric Research Community Earth System Model version 1.1.1(CESM 1.1.1):(1)The potential predictability of the EASSM underwent a significant decadal change from a low phase to a high phase during the late 1970s,followed by a sharp decline at the end of the 1990s.(2)The decadal change in potential predictability of the EASSM can be attributed mainly to variations in the external signal of the EASSM.The latter is found to include contributions from variations in the phase and intensity of the El Nifio-Southern Oscillation(ENSO)decadal variability.As a major external signal of the EASSM,the decadal variability of ENSO has changed from a negative phase to a positive phase in the late 1970s,and back to a negative phase in the late 1990s.This phase shift of the ENSO decadal variability is associated with a decadal change in the intensity of the ENSO decadal variability.Thus ENSO is generally strong(weak)during a positive(negative)phase of the ENSO decadal variability.The strong ENSO is expected to have a greater influence on the EASSM,and vice versa.As a result,the potential predictability of the EASSM tends to be high(low)during a positive(negative)phase of the ENSO decadal variability.Furthermore,a suite of Pacific Pacemaker experiments suggests that the ENSO decadal variability may be a key pacemaker for the decadal change in potential predictability of the EASSM.(3)The potential predictability of the SCSSM underwent a significant decadal change from a low to high phase in the 1990s,followed by a decreased trend in the early 2000s.The interdecadal change in potential predictability of the SCSSM differs from the interdecadal variation in potential predictability of the EASSM.(4)The inter-decadal change in potential predictability of the SCSSM has a significant positive correlation with the sea surface temperature(SST)of the East Indian Ocean and the Western Pacific(EIOWP).The EIOWP area averaged SST with a high(low)phase of its inter-annual variability would have strong(weak)influence on the SCSSM and hence enhance(weaken)the SCSSM signal.As a result,the potential predictability of the SCSSM tends to be high(low).