Pivotal Factors Controlling Seasonal And Interannual Variations Of The Madden-Julian Oscillation

Although the seasonal variation of the Madden-Julian Oscillation(MJO)was described in the previous studies,the key factors controlling the seasonal changes in MJO have not been fully understood.In this study,we defined the MJO events of 1979-2014 and analyzed their features in different seasons by using the Real-time Multivariate MJO(RMM)index and the newly proposed RMM-r(RMM-revised)index.The results indicate that the MJO events show a higher frequency of occurrence,a stronger intensity and a longer duration during the boreal winter(Dec.-Feb.)and spring(Mar.-May).However,the frequency,strength and lifespan of MJO events are all reduced during the boreal summer(Jun.-Aug.)and autumn(Sep.-Nov.).The enhanced MJO events in winter—spring also show a large ratio of variance for eastward to westward components.To elucidate how large-scale background fields affect seasonal MJO variation,a series of sensitivity experiments was conducted by using a 2.5-layer model that can simulate MJO-like features.It is found that the variation in low-level moisture(vertical wind shear)is the key large-scale factor affecting the seasonal variation in MJO strength(in propagation).In comparison with the summer-autumn seasons when the MJO is relatively weakened,the relatively abundant low-level moisture near the equator during boreal winter-spring may strengthen the development of MJO convection and circulation,whereas the relatively weak easterly shear(or the westerly shear anomaly)is conducive to the enhancement of an eastward-propagating MJO component.In addition to the seasonal variations,the MJO also reveals a prominent variation at the interannual timescale.Based on the composite analysis,we compare the environment fields during boreal winter-spring between strong MJO activity years and weak years.Results show that the interannual changes on MJO activitiy over the tropical India Ocean to the westem Pacific are modulated by the water vapors,vertical shear anomaly and low-level zonal wind anomaly.During the strong MJO years,the abundant moisture,vertical easterly wind shear and low-level westerly are observed,all of which are favorable for the MJO strength and propagation.To further explore the relative roles of large-scale background fields in MJO interannual variation,a series of sensitivity experiments was conducted by using a 2.5-layer model.We find that the relatively abundant low-level moisture in the regions from Maritime Continent to the western Pacific is conducive to maintaining the development of MJO,whereas the vertical westerly shear in the equatorial eastern Pacific favors the eastward propagation component.To understand the key processes that determine the seasonal and interannual variations of MJO in the state-of-the-art general circulation models(GCMs),we assessed the 24 GCMs participated the MJOTF/GASS multimodel comparison project.We calculated the correlation between seasonal difference in MJO eastward-propagating component and the related large-scale environment fields,and found that low-level zonal wind fields have obviously positive correlation with MJO eastward-propagating signals in the GCMs.This suggests that the the model capability in simulating the seasonal MJO variation is highly linked with the zonal wind similation.The investigation on interannual variations of MJO in the general circulation models shows that the relatively abundant water vapors in the western Pacific are conducive to the development of MJO strength,whereas the vertical westerly shear anomaly in the equatorial eastern Pacific favors the propagation.