Changes And Mechanisms Of Winter MJO Eastward Propagation Speed Under Global Warmin

Successfully predicting the Madden-Julian Oscillation（MJO）can effectively improve the extended-range prediction.As the structures of atmospheric and the states of underlying surface change with the increase of greenhouse gas emission,there are new challenges in the MJO simulation.One of the main challenges is how the eastward propagation speed of MJO changes under a warmer climate.In this study,we will examine the potential changes of MJO propagation speed during boreal winter under global warming in CMIP6 models through statistical analysis and dynamic diagnosis,and reveal the key factors affecting changes of the MJO phase speed.To select the appropriate models,this study first evaluates the simulation of 29 CMIP6models on the speed of MJO eastward propagation.The results show that most of CMIP6models can well simulate the MJO’s eastward propagation over Indo-Pacific warm pool.It is also shown that the Kelvin-wave response and Rossby-wave response to the MJO heating are the key circulation factors affecting the simulated MJO propagation speed,with stronger（weaker）Kelvin-wave（Rossby-wave）response corresponding to faster propagation.Further study indicates that the variation of the MJO circulation structure among models is attributed to the diverse background sea surface temperature（SST）.According to the model evaluation,we choose 15 models that can well simulate the MJO’s eastward propagation to explore the potential changes of MJO phase speed under global warming.It is found that the propagation speed of canonical MJO（the average state of MJO events）shows 19.6%increase at the end of 21^st century.Due to the similar reasons for the MJO speed variation in different models,the eastward expansion of the Indo-Pacific warm pool caused by the warming in the Central and Eastern Pacific（CEP）is the fundamental reason for the acceleration of MJO in a warmer climate.The warming trend over CEP affects the MJO phase speed in two ways.First,it increases the water vapor content in this area,resulting in the enhancement of the meridional moisture gradient over southern Maritime Continent（MC）,thus enhancing the meridional moisture advection that accelerates the MJO.Second,it increases the moisture in CEP,leading to the enhancement of Kelvin-wave response to the east of MJO convection center and decrease of Rossby-wave response to the west,which promotes the boundary layer moisture convergence to the east of MJO convection,accelerating the eastward propagation of MJO.Only analyzing the changes of canonical MJO may be one-sided,because the MJO had propagation diversity.This study further explores the possible changes of MJO diversity under global warming.The results of K-means cluster analysis show that MJO diversity can be optimally classified into three clusters in CMIP6 models:standing MJO,slow eastward propagating MJO and fast eastward propagating MJO.In terms of propagation speed,the slow cluster and the fast cluster both accelerate at the end of 21^st century,indicating that global warming increases the overall eastward propagation speed of MJO.Due to the same reason for the change of canonical MJO,the changes of background states induce the change of circulation structures,which strengthens the moisture advection over MC,leading to the acceleration of slow MJO and fast MJO.In terms of frequency,the standing MJO decreases,while the slow MJO and the fast MJO increase,indicating that global warming increases the proportion of MJO events that can propagate eastward.These changes are attributed to the El Ni（?）o-like SST changes in the warmer climate.This trend of SST change enhances Kelvin-wave response in general,leading to slow eastward propagation of some MJO events that are supposed to be standing MJO under the influence of La Nina events.The results here not only have implications in understanding the MJO dynamics,which may improve the simulation and prediction of the model,but also advance our knowledge on the global warming’s impact on MJO.