Role Of Nonlinear Moisture-convection And Oceanic Feedbacks In Shaping MJO Events

As the strongest intraseasonal variability?ISV?in the tropical atmosphere,the Madden-Julian Oscillation?MJO?plays an important role in modulating global weather and climate.Theoretical and predictability studies related to MJO can provide possibility for seamless weather and climate prediction.In this paper,the shaping of MJO events is systematically studied for the first time from the perspectives of nonlinear moisture-convection and oceanic feedbacks.The research results mainly reveal the following conclusions with important scientific significance and potential application value:nonlinearly convective heating and thermodynamically air-sea interaction play an important role in controlling how MJO develops from small-scale disturbances to a planetary-scale,slowly eastward-propagating,circulation-convection coupled system.Nonlinear moisture-convection and oceanic feedbacks play a key role in the onset of primary MJO events.The conditional nonlinear optimal perturbation?CNOP?method is firstly applied here to investigate the primary MJO initiation,and the results not only reveal the importance of assimilating moisture in the predictability study of primary MJO events,but also highlight the necessity of considering nonlinearity in subseasonal-scale?two weeks^one month?prediction.This paper has constructed air-sea coupled dynamical moisture?ASDM?model to study one of the most fundamental scientific questions related to the MJO,that is,the planetary scale selection.The nonlinear ASDM model can well simulate some basic characteristics of MJO in observation no matther in the zonally uniform or warm-pool-like basic state,including:1)zonally planetary scale,equatorially trapped,circulation-convection coupled,mixed Rossby-Kelvin wave horizontal structure;2)unstable and slowly eastward propagation(4.2 m s^-1)in the warm pool area and the decaying while fast propagation(9.2 m s^-1)in the cold tongue area;3)easterly wind and positive sea surface temperature?SST?anomalies leading the MJO convetion activcity center;4)zonal asymmetry in the circulation-precipitation.The planetary scale selectivity characteristic of MJO can only be reproduced in the ASDM model with nonlinear convective precipitation parameterization,while the linear and uncoupled models cannot simulate this phenomenon.This shows that the planetary scale selection of MJO is essentially determined by nonlinear heating,and the sea-air interaction,especially the wind-evaporation-entrainment feedback,will provide additional unstable energy to support the planetary scale selection of MJO.In addition,the adoption of warm pool background state will accelerate the planetary scale selection process of MJO.The process of planetary scale selection for MJO is as follows:starting from the initial small disturbance,one convection branch will randomly grow dramatically and lead the others.The warm pool background state will accelerate the process of random selection.The dry Kelvin wave?east wind anomaly?excited by the strong convection branch would catch up with other slowly eastward wet convection branch systems and partially offset its west wind anomaly after 5 days.The decrease of SST gradient and water vapor convergence will further weaken convective precipitation.Finally,the structure of the planetary-scale,zonal wavenumber-1 MJO is selected.In this paper,the coupled general circulation model of the University of Hawaii,which shows a good fidelity in simulating the Indo-Pacific climatic variabilities,especially the ISV/MJO,combined with the CNOP method has bee chosen to develop a new nonlinear methodology framework for the study of the predictability of primary MJO events.Starting from the non-MJO reference states,this methodological framework aims to solve the corresponding nonlinear optimization problem to explore what kind of initial moisture perturbations can lead to the onset of the strongest primary MJO event.Results have shown that,compared with random perturbations,CNOPs of moisture with specific spatial structure can be regarded as an optimal precursor in triggering the primary MJO initiation,and is manifested as uniform humidification mode along the equatorial Indian Ocean,while it is manifested as dry anomaly in the direction away from the equator.The authenticity of moisture CNOPs is confirmed by both observation data and long-term simulation data.Through the moisture budget analysis,it is revealed that horizontal moisture advection,especially the nonlinear moisture advection associated with high-frequency meridional wind,is the key nonlinear physical process dominating the onset of the primary MJO events.The wavelet analysis further shows the importance of high-frequency equatorial waves associated with quasi-3–4 and quasi-6–8 day perturbations in modulating the nonlinear moisture advection.Vertical column processes,including subgrid-scale evaporation and entrainment,also play a role in the humidification of boundary layer.These results are valuable in that they highlight the importance of assimilating humidity fields and nonlinear moisture-convection feedback in the lower troposphere in the onset of primary MJO events.Oceanic feedback plays an important role in preconditioning primary MJO events in two aspects:triggering the deep convective cloud and maintaining its slow and unstable eastward propagation.The oceanic mixed layer heat budget diagnosis indicated that the main factors determining the SST warming in the pre-onset stage of primary MJO event include the entrainment beneath the mixed layer and net surface heat flux feedback.The air-sea interaction plays an important role in explaining the asymmetry of positive and negative phase amplitudes of MJO and the"detour"of MJO in boreal winter via the southern Maritime Continent.