Interannual Variability Of The Semiannual Oscillation Over The Western Pacific Warm Pool And Its Possible Mechanism

Based on the monthly mean datasets of the NOAA’s optimum interpolation SST,the air-sea interface heat flux,the OLR, the reanalysis precipitation and the atmospheric circulation fields for 1982 to 2014, the semiannual oscillations and the interannual variability over the western pacific warm pool were analyzed by using statistical methods,such as REOF, Morlet wavelet analysis, power spectrum analysis and butterworth band-pass filter, etc.Furthermore,through the air-sea interaction and ENSO cycle to explore the factors affecting its formation and the mechanism of interannual variability of the semiannual oscillation.Finally, analyze the impact of interannual variability of the semiannual oscillation for the atmospheric circulation anomalies in east Asia and the precipitation anomalies in eastern China between P-SAO and N-SAO patterns.The main conclusions are as follows:(1) Set the analysis domain of this study as 20°S-20°N,120°E-160°E over the western pacific warm pool and choose the first mode as the leading spatial pattern by REOF method. Using the methods of power spectrum analysis and Morlet wavelet analysis,we can proved that the time series of the first mode exist a significant semiannual oscillations by period of 5-7 months.In addition,we identify the following three categories:positive SAO (P-SAO), negative SAO (N-SAO)and neutral cases, based on the different phase evolution of the time series of each year by the 5-7 months bandpass filtering process,thus we can confirm that there is a significant interannual variability over the western pacific warm pool. It is worth mentioning that the P-SAO and N-SAO evolve in opposite ways and both exhibit a remarkable seasonal dependence.(2) The semiannual oscillation over the western pacific warm pool closely related to the sea surface heat balance,the abnormal changes in sea surface temperature can be basically determined by the net heat flux or by OLR combine with windspeed.The semiannual oscillation over the western pacific warm pool closely related to the ENSO cycle,where P-SAO patterns corresponding to the clod event of ENSO,and N-SAO patterns corresponding to the warm event of ENSO.The causes of interannual variability of the semiannual oscillation can be explained by the trade winds anomalies and a negative feedback of process convection-cloud-solar radiation.(3) In summer, P-SAO patterns corresponds to the precipitation patterns in eastern China as"Southern droughts- North floods",on the contrary, N-SAO patterns corresponds to the "North floods-South droughts"; In winter, the flood occurred more frequently in most parts of eastern China for P-SAO patterns and N-SAO patterns shows the opposite situation. In general,interannual variability of the semiannual oscillation over the western pacific warm pool closely related to the atmospheric circulation anomalies in east Asia and the precipitation anomalies in eastern China.