Diagnosis Of The Role Of Water Vapor In MJO Propagation And Its Diversity

As the “moisture mode” theory is one of the most popular theories to explain MJO's eastward propagation,many studies have diagnosed the column-integrated moist static energy(MSE)and water vapor(CWV)budget,mostly using reanalysis(RA)data.However,there are large uncertainties between RAs,and few studies pay attention to the differences of MJO-CWV budget in different RAs.This study analyzes and compares the MJO-MSE and CWV budget in six modern RAs.The relative contributions of each budget term to MJO propagation and maintenance and their differences in RAs are examined.The reasons for terms have large uncertainty in RAs and non-negligible CWV budget residuals are analyzed.One of the complexities of MJO lies in its propagation diversity.In this study,we propose a new method to distinguish the different propagation characteristics of MJO events.The role of water vapor in the propagation diversity of MJO is illustrated by diagnosing the MJO-CWV budget.A noticeable inter-RA spread is found in the mean state moisture,especially its vertical structure in all RAs.Horizontal advection dominates the propagation of the MJO while column-integrated longwave radiative heating and vertical advection are found to be the key processes for MJO maintenance.The budget terms directly affected by the model parameterization schemes exhibit high uncertainty.The differences in anomalous vertical velocity mainly contribute to the large differences in vertical advection among the RAs.The budget residuals of CWV show large inter-RA differences and have non-negligible contributions to MJO propagation in most RAs.RAs that underestimate(overestimate)the strength of cloud-radiation feedbacks and the convective moisture adjustment timescale tend to have positive(negative)MJO CWV budget residuals.Our results emphasize that a correct representation of the interactions among moisture,convection,cloud,and radiation is the key to an accurate depiction of the MJO MSE and CWV budget in RAs.The strong correlations between the budget residual and various process-oriented metrics provide an emergent constraint on the metrics and thereby offer the target against which the models used in the construction of RAs can be tuned.ERA5 provides the most practical value of these metrics,thus is more suitable for MJO related theoretical research.Given the propagation diversity of MJO.This study identifies two key areas to represent MJO propagation diversity using ERA5 data: The tropical Maritime Continent(MC)and Western Pacific are the keys to distinguish between propagating and non-propagating,slow and fast MJO events,respectively.Two propagation indices are defined based on the abovementioned two areas and verified that they have good abilities to separate the four types of MJO events.As horizontal advection plays a dominant role in MJO eastward propagation,the differences of horizontal moisture advection in different MJO events are compared.The results show that whether the MJO convection passes through the MC depends on the preceding dry anomaly,which can trigger anomalous meridional wind to transport background water vapor,thereby promoting the eastward propagation of MJO.The zonal scale of MJO and its related dry anomaly can affect the zonal and meridional moisture advection in the Western Pacific,which regulates the propagation speed of MJO there.Our study emphasizes the importance of background water vapor on the propagation speed of MJO.On the one hand,the enhanced meridional moisture gradient in MC can accelerate the propagation speed of MJO there.On the other hand,the wetter(or warmer)environment in the central and eastern Pacific can reduce the background westerly wind in the tropical Indian Ocean through teleconnection,thereby accelerating the eastward propagation of MJO there.