Regulation Of Intraseasonal Oscillations Of The Atmosphere By Tropical Indo-Pacific SST Anomalies And Their Effects On Precipitation In The Yangtze River Basin

The Madden-Julian oscillation(MJO)is the most dominant mode of the atmospheric intraseasonal variability over the tropics and has the significant impact on global scale.MJO activity has significant interannual variation and coupled air-sea interaction characteristic,which is significantly related to the sea surface temperature(SST)of the tropical Pacific and the Indian Ocean.However,the relationship between tropical Indo-Pacific SST and MJO activities on interannual timescales has complex characteristics.The mechanism between the MJO activity and tropical Indo-Pacific SST is not entirely clear yet.Based on several observations and reanalysis,the interannual relationship and mechanism between MJO activity anomaly and tropical SST interannual variability over the Indian Ocean and Pacific are explored,and discuss the effect of MJO residence days on precipitation in the middle and lower reaches of the Yangtze River.Main conclusions are summarized as follows:(1)The impact of three super El Ni(?)o events on MJO activity over the western Pacific is inconsistent during the mature phase.1982/83,1997/98,and 2015/16 are the three strongest El Ni(?)o events since the observation data are available.According to the areaaveraged SST anomaly in the Ni(?)o3.4 area,the general evolutions and magnitudes are quite similar for these three super El Ni(?)o events are quite similar.The severely suppressed MJO activities over the western Pacific during mature phases in 1982 /83 and 1997/98 El Ni(?)o events,while the enhanced amplitude and occurrence of the MJO activity over the western Pacific during 2015/16 winter.Further analyses suggest that the enhanced western Pacific MJO is mainly related to its distinctive SSTA structure and associated background thermodynamic conditions.In comparison with the previous super El Ni(?)o events,the warming SSTA center of the 2015/16 El Ni(?)o is located further westward and strong cold SSTA is not detected in the western Pacific.Accordingly,the low-level moisture and atmospheric circulation tend to support the development of MJO in the western Pacific.In contrast,the negative moisture anomalies over the western Pacific during the 1982/83 and 1997/98 super El Ni(?)o events suppressed the development of MJO.It can be seen that the zonal position of the SST anomaly has an important influence on the MJO.(2)The ENSO's combination mode exerts prominent impacts on the western Pacific MJO activity.The traditional SST indices are hard to accurately describe the complex relationship between ENSO and MJO activity.The combination mode is the Interaction of SST annual cycle with ENSO variability,and it can describe the temporal and spatial complexity of ENSO in the western Pacific region.The combination mode has the significant effect on the MJO activities over the western Pacific.Statistical results show that the ENSO combination mode has a significant negative correlation with MJO activity in the western Pacific.The strong combination mode induces the anticyclonic circulation in the northwestern Pacific and suppresses the low-level water vapor convergence,inhibiting the development of MJO.In addition,the correlation coefficient between the combination mode and the MJO activity is increasing gradually as the northward shift of the MJO activity location from winter to spring.Considering the ENSO combination mode,we could accurately grasp the modulation of ENSO on MJO activity in the western Pacific.(3)The Indian Ocean basin warming can increase the MJO residence days on the Indian Ocean.The tropical Indian Ocean warming in summer is largely driven by El Ni(?)o remote forcing in the preceding mature phase.The tropical Indian Ocean warming persists through the summer in El Ni(?)o decaying phase like a discharging capacitor,so the interannual variability of the Indian Ocean SST in summer has a significant impact on the MJO.On the one hand,the warming in the tropical Indian Ocean is beneficial to increasing the water vapor and air temperature in the equatorial lower atmosphere,leading to the stagnant MJO convection staying at the equator.On the other hand,the low-level easterly wind anomaly on the eastern Indian Ocean accompanied by the warming SST inhibits the northeast propagation of MJO.These two processes are not in favor of the northeast propagation of MJO in the region,making the MJO convection stay in the tropical Indian Ocean.This dynamic mechanism is also applicable to the long-term trend while the MJO residence days are increasing in the Indian Ocean since the1980 s.Under the global warming,the rapidly warming in the Indian Ocean and the intensified low-level easterly wind over the eastern Indian Ocean lead to the MJO residence days increasing over the Indian Ocean.(4)The Indian Ocean MJO residence time is significantly correlated with precipitation days over the Yangtze River,and this relationship becomes prominently enhanced after the 2000 s..When the active MJO convection is located over the Indian Ocean,the anticyclonic circulation anomaly in the northwestern Pacific Ocean excited by the MJO is favorable for transporting low-level water vapor to the Yangtze River Basin,resulting in increased precipitation in the middle and lower reaches of the Yangtze River.When the MJO residence days increased in the Indian Ocean,the precipitation days in the middle and lower reaches of the Yangtze River increased,especially for the heavy rainfall days.Further research found that the relationship between active days of MJO over the Indian Ocean and the precipitation days over the middle and lower reaches of the Yangtze River has experienced a decadal change with their relationship being significant since the 2000 s.The decadal change of this relationship might be due to the decreased sea surface temperature anomaly variability in the Tropical Ocean.