Simulated Madden–Julian Oscillation Research Articles

Quantifying the impact of SST feedback frequency on Madden–Julian oscillation simulations

Abstract. This study uses the Community Atmosphere Model 5.3 coupled to a 1-D ocean model to investigate the effects of intraseasonal sea surface temperature (SST) feedback frequency on Madden–Julian oscillation (MJO) simulations with intervals at 30 min and 1, 3, 6, 12, 18, 24, and 30 d. The large-scale nature of the MJO in simulations remains intact with decreasing feedback frequency, although it becomes increasingly unrealistic in both structure and amplitude, until 1 per 30 d when the intraseasonal fluctuations are overwhelmingly dominated by unorganized small-scale perturbations in both atmosphere and ocean, as well as at the atmosphere–ocean interface where heat and energy are rigorously exchanged. The main conclusion is that the less frequent the SST feedback, the more unrealistic the simulations. Our results suggest that more spontaneous atmosphere–ocean interaction (e.g., ocean response once every time step to every 3 d in this study) with high vertical resolution in the ocean model is a key to the realistic simulation of the MJO and should be properly implemented in climate models.

Effects of Surface Turbulence Flux Parameterizations on the MJO: The Role of Ocean Surface Waves

Abstract This study investigates the sensitivity of the Madden–Julian oscillation (MJO) to changes to the bulk flux parameterization and the role of ocean surface waves in air–sea coupling using a fully coupled ocean–atmosphere–wave model. The atmospheric and ocean model components of the Energy Exascale Earth System Model (E3SM) are coupled to a spectral wave model, WAVEWATCH III (WW3). Two experiments with wind speed–dependent bulk algorithms (NCAR and COARE3.0a) and one experiment with wave-state-dependent flux (COR3.0a-WAV) were conducted. We modify COARE3.0a to include surface roughness calculated within WW3 and also account for the buffering effect of waves on the relative difference between air-side and ocean-side momentum flux. Differences in surface fluxes, primarily caused by discrepancies in drag coefficients, result in significant differences in MJO’s properties. While COARE3.0a has better convection–circulation coupling than NCAR, it exhibits anomalous MJO convection east of the date line. The wave-state-dependent flux (COR3.0-WAV) improves the MJO representation over the default COARE3.0 algorithm. Strong easterlies over the Pacific Ocean in COARE3.0a enhance the latent heat flux (LHFLX). This is responsible for the anomalous MJO propagation after the date line. In COR3.0a-WAV, waves reduce the anomalous easterlies, leading to a decrease in LHFLX and MJO dissipation after the date line. These findings highlight the role of surface fluxes in MJO simulation fidelity. Most importantly, we show that the proper treatment of wave-induced effects in bulk flux parameterization improves the simulation of coupled climate variability.

MJO Seasonality in Its Scale Selection: Perspectives From Space‐Time Spectral Analysis of Moisture Budget

AbstractThe Madden–Julian Oscillation (MJO) displays an evident seasonality in its spatiotemporal scale selection. In boreal winter, the MJO is best behaved at wavenumber 2 and oscillates on a narrow time scale centering on a ∼55‐day period. In contrast, a wavenumber‐1 selection and a broad oscillation frequency centering on a ∼33‐day period are observed in boreal summer. Using the space‐time cross‐spectral analysis between convection and moisture budget, we reveal that column processes determine seasonal variations in the MJO spatial organization, while the MJO is strongly damped by the horizontal moisture advection mainly due to its meridional component (Vadv). A timescale decomposition analysis suggests that the damping effect of Vadv results primarily from high‐frequency synoptic‐scale disturbances, and the Vadv component related to the seasonal‐mean moisture generally supports the MJO growth at large wavenumbers while inhibits the growth of small wavenumbers, which is the most evident in boreal summer. The advection of seasonal‐mean moisture by anomalous zonal winds supports the MJO growth at all wavenumbers in boreal winter, but in boreal summer the growth becomes weak and even turns negative for small wavenumbers. Furthermore, different MJO timescale selections in winter versus summer are rooted in the Vadv, as both the column processes and zonal moisture advection mainly support the propagation of high‐frequency modes. Observational results related to the horizontal seasonal‐mean moisture advection are reasonably validated in a dynamical moisture model. These findings advance our understanding of the MJO seasonality and offer alternative diagnoses for evaluating the MJO simulation fidelity in contemporary climate models.

Simulation of MJO with improved deep convection scheme in different resolutions of BCC-CSM2 models

AbstractThis study investigates the impacts of modifying the deep convection scheme on the ability to simulate the Madden–Julian Oscillation (MJO) in the Beijing Climate Center Climate System Model version 2 with a medium resolution (BCC-CSM2-T159) and a high resolution (BCC-CSM2-T382). On the basis of the original deep convection scheme, a modified scheme is suggested, which involves the transport processes of deep convective cloud water. The liquid cloud water that is detrained is transferred horizontally to its neighboring grids, and a portion of the cloud water that is horizontally transported is allowed to be transported downward into the lower troposphere. Both BCC-CSM2-T159 and BCC-CSM2-T382 with the modified deep convection scheme perform better than that used the original deep convection scheme in reproducing the major features of the MJO, such as its spectrum, period, intensity, eastward propagation and life cycle. Further analysis shows that those pronounced improvements in the MJO features in both BCC-CSM2-T159 and BCC-CSM2-T382 with the modified scheme are caused by transport processes of deep convective cloud water. The modified deep convection scheme enhances moisture and energy exchange from the lower troposphere to the upper troposphere around convective cloud, and promotes the convergence of moisture in the lower troposphere to the east of the MJO convection center, and then induces eastward propagation of the MJO. The comparisons between the coupled experiments and their corresponding experiments following Atmospheric Model Intercomparison Project (AMIP) simulations indicated that atmosphere–ocean interactions are also important to improve MJO simulations in the models.

Global Simulation of the Madden–Julian Oscillation With Stochastic Unified Convection Scheme

AbstractA new spectral convection scheme, the stochastic unified convection scheme (stochastic UNICON), is implemented in a general circulation model. The global climate simulation using stochastic UNICON is evaluated and compared with UNICON, focusing on the simulation of the Madden–Julian oscillation (MJO). Stochastic UNICON extends the original UNICON by randomly sampling convective updrafts from the joint probability density function constructed at the near‐surface, generating a spectrum of convective updrafts in a physically based manner. The performances of UNICON and stochastic UNICON on simulating observed mean climates are comparable, while stochastic UNICON slightly reduces the mean bias of climate variables. For the simulation of intraseasonal variabilities, stochastic UNICON outperforms UNICON in many aspects. Stochastic UNICON improves the simulation of the intensity and propagation patterns of boreal winter MJO, which are too weakly simulated in UNICON. The coherency between MJO‐related convection and large‐scale circulation is also enhanced, which many climate models underestimate. The improvement of MJO simulation by stochastic UNICON is related to a better representation of the relationship between moisture and convection in the model. The increased frequency of shallow convection in stochastic UNICON leads to stronger moisture convergence that precedes convection activity peak and results in the more robust development of organized convection and more frequent intense precipitation. A precipitation budget analysis reveals that the moisture tendencies due to horizontal advection and convective process are consistently enhanced during MJO developing periods by stochastic UNICON.

MJO in Different Orbital Regimes: Role of the Mean State in the MJO’s Amplitude during Boreal Winter

Abstract The Madden–Julian oscillation (MJO) exhibits pronounced seasonality, with one of the key unanswered questions being the following: what controls the maximum in MJO precipitation variance in the Southern Hemisphere during boreal winter? In this study, we examine a set of global climate model simulations in which the eccentricity and precession of Earth’s orbit are altered to change the boreal winter mean state in an attempt to reveal the processes that are responsible for the MJO’s amplitude in the boreal winter. In response to the forced insolation changes, the north–south asymmetry in sea surface temperature is amplified in boreal fall, which intensifies the Hadley circulation in boreal winter. The stronger Hadley circulation yields higher mean precipitation and stronger mean lower-tropospheric westerlies in the southern part of the Indo-Pacific warm pool. The MJO precipitation variability increases significantly where the mean precipitation and lower-tropospheric westerlies strengthen. In the column-integrated moisture budget of the simulated MJO, only surface latent heat flux feedback shows a trend that is consistent with the MJO’s amplitude, suggesting an important role for the surface latent heat flux feedback in the MJO’s amplitude during the boreal winter. An analysis of the moisture–precipitation relationship in the simulations shows that the increase in the mean precipitation lowers the convective moisture adjustment time scale, leading to the increase in precipitation variance. Our results suggest that the mean-state precipitation plays a critical role in the maintenance mechanism of the MJO.

Response of the Madden–Julian Oscillation and Tropical Atmosphere to Changes in Oceanic Mixed Layer Depth over the Indian Ocean

Abstract The Madden–Julian oscillation (MJO) is a component of tropical variability that influences high-impact events such as hurricane activity and Asian monsoons on intraseasonal (2–8 weeks) time scales. However, the atmosphere–ocean dynamics responsible for the MJO are highly debated. To gain insight into MJO–Indian Ocean dynamics, we conduct climate model experiments where the ocean is replaced by a motionless slab whose thickness, called the mixed layer depth (MLD), varies in space but not in time. Changes in the MLD and ocean heat convergence over the Indian Ocean have no discernible impact on MJO propagation, predictability, or variability within the Community Earth System Model (CESM) version 1.2.1. This suggests that ocean dynamics may not be critical to the MJO over the Indian Ocean in this dynamical model (CAM5 coupled to motionless slab). To diagnose changes in intraseasonal variability beyond the MJO, a discriminant analysis technique is used to optimize differences in variability between experiments. This analysis reveals that differences caused by changing Indian Ocean MLD were restricted mostly to local surface fluxes and could be explained by simple energy balance physics. Despite modeling adjustments intended to preserve the climate, the control slab has a warmer climate than the fully coupled model. The resulting changes in the mean climate are consistent with changes theoretically expected from global warming, particularly the “wet-gets-wetter” mechanism. Significance Statement The Madden–Julian oscillation (MJO) is a tropical eastward-moving pulse of convection that can influence a wide variety of global phenomena. Currently, models do not capture many key aspects of the MJO such as speed and spatial structure. The uppermost ocean layer that communicates with the atmosphere is widely believed to play a large role in the evolution of the MJO. To test the importance of ocean dynamics on simulated MJO, a slab-model configuration is used to allow the atmosphere and ocean to communicate through surface fluxes of heat and moisture while suppressing interactive ocean currents. Changes to the upper ocean did not impact the MJO in our model setup.

Tropical Intraseasonal Variability Response to Zonally Asymmetric Forcing in an Idealized Moist GCM

Abstract The tropical intraseasonal variability in an idealized moist general circulation model (GCM) coupled to a slab ocean is investigated. The model has a simple moist convection scheme and realistic radiative transfer, but no parameterization of cloud processes. In a zonally symmetric aquaplanet state, variability is dominated by westward-propagating Rossby waves. Enforcing zonal asymmetry through the application of a prescribed ocean heat flux in the bottom boundary leads to the development of a slow, eastward propagating mode that bears some of the characteristics of the observed Madden–Julian oscillation (MJO). When the ocean heat flux is made stronger, high-frequency Kelvin waves exist alongside the MJO mode. The strength of the disturbances and the spatial distribution of their precipitation anomalies are sensitive to the strength of intraseasonal sea surface temperature (SST) anomalies. The greatest resemblance to the MJO is observed when shallow slab ocean depths (1 m) are used, but the mode still exists at deeper slabs. Sensitivity experiments to the parameters of the convection scheme suggest that the simulated MJO mode couples to convection in a way that is distinct from both Kelvin and Rossby waves generated by the model. Analysis of the column moist static energy (CMSE) budget of the MJO mode suggests that column radiative heating plays only a weak role in destabilizing the mode relative to the stabilizing contribution of vertical advection. The CMSE budget analysis highlights the importance of the life cycle of horizontal advection for the destabilization and propagation of the MJO. Synergies between the generated MJO mode and linear theories of the MJO are discussed as well.

Embedding a one-column ocean model in the Community Atmosphere Model 5.3 to improve Madden–Julian Oscillation simulation in boreal winter

Abstract. The effect of the air–sea interaction on the Madden–Julian Oscillation (MJO) was investigated using the one-column ocean model Snow–Ice–Thermocline (SIT 1.06) embedded in the Community Atmosphere Model 5.3 (CAM5.3; hereafter CAM5–SIT v1.0). The SIT model with 41 vertical layers was developed to simulate sea surface temperature (SST) and upper-ocean temperature variations with a high vertical resolution that resolves the cool skin and diurnal warm layer and the upper oceanic temperature gradient. A series of 30-year sensitivity experiments were conducted in which various model configurations (e.g., coupled versus uncoupled, vertical resolution and depth of the SIT model, coupling domains, and absence of the diurnal cycle) were considered to evaluate the effect of air–sea coupling on MJO simulation. Most of the CAM5–SIT experiments exhibit higher fidelity than the CAM5-alone experiment in characterizing the basic features of the MJO such as spatiotemporal variability and the eastward propagation in boreal winter. The overall MJO simulation performance of CAM5–SIT benefits from (1) better resolving the fine vertical structure of upper-ocean temperature and therefore the air–sea interaction that results in more realistic intraseasonal variability in both SST and atmospheric circulation and (2) the adequate thickness of a vertically gridded ocean layer. The sensitivity experiments demonstrate the necessity of coupling the tropical eastern Pacific in addition to the tropical Indian Ocean and the tropical western Pacific. Coupling is more essential in the south than north of the Equator in the tropical western Pacific. Enhanced MJO could be obtained without considering the diurnal cycle in coupling.

Improving Madden–Julian oscillation simulation in atmospheric general circulation models by coupling with a one-dimensional snow–ice–thermocline ocean model

Abstract. A one-column, turbulent, and kinetic-energy-type ocean mixed-layer model (snow–ice–thermocline, SIT), when coupled with three atmospheric general circulation models (AGCMs), yields superior Madden–Julian oscillation (MJO) simulations. SIT is designed to have fine layers similar to those observed near the ocean surface; therefore, it can realistically simulate the diurnal warm layer and cool skin. This refined discretization of the near-surface ocean in SIT provides accurate sea surface temperature (SST) simulation, and thus facilitates realistic air–sea interaction. Coupling SIT with the European Centre/Hamburg Model version 5, the Community Atmosphere Model version 5, and the High-Resolution Atmospheric Model significantly improved MJO simulation in three coupled AGCMs compared to the AGCM driven by a prescribed SST. This study suggests two major improvements to the coupling process. First, during the preconditioning phase of MJO over the Maritime Continent (MC), the often underestimated surface latent heat bias in AGCMs can be corrected. Second, during the phase of strongest convection over the MC, the change in intraseasonal circulation in the meridional circulation enhancing near-surface moisture convergence is the dominant factor in the coupled simulations relative to the uncoupled experiments. The study results show that a fine vertical resolution near the surface, which better captures temperature variations in the upper few meters of the ocean, considerably improves different models with different configurations and physical parameterization schemes; this could be an essential factor for accurate MJO simulation.

Deceleration of Madden–Julian Oscillation Speed in NICAM AMIP‐Type Simulation Associated With Biases in the Walker Circulation Strength

AbstractThe eastward movement speed of Madden–Julian Oscillation (MJO) events simulated in a 30‐year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model, following the atmospheric model intercomparison project protocol with a slab ocean was analyzed and compared with the observation. The simulation reproduced the observed tendency of the MJO to decelerate when it is embedded within the stronger Walker circulation, intensified by background sea surface temperature states with larger zonal gradients between the warmer western Pacific and the cooler Indian Ocean and eastern Pacific. However, the simulated MJO events occurred disproportionately during El Niño events and displayed a slow bias. These biases were associated with an overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of accurately reproducing the mean atmospheric circulation for the realistic reproduction of the MJO in long term simulations.

Evaluating the Eastward Propagation of the MJO in CMIP5 and CMIP6 Models Based on a Variety of Diagnostics

Abstract Given the climatic importance of the Madden–Julian oscillation (MJO), this study evaluates the capability of CMIP6 models in simulating MJO eastward propagation in comparison with their CMIP5 counterparts. To understand the representation of MJO simulation in models, a set of diagnostics with respect to MJO-associated dynamic and thermodynamic structures is applied, including large-scale zonal circulation, vertical structures of diabatic heating and equivalent potential temperature, moisture convergence at the planetary boundary layer (PBL), and the east–west asymmetry of moisture tendency relative to the MJO convection. The simulated propagation of the MJO in CMIP6 models shows an overall improvement in realistic speed and longer distance, which displays a robust linear regression relationship against the above-mentioned dynamic and thermodynamic structures. The improved MJO propagation in CMIP6 largely benefits from better representation of premoistening processes that is primarily contributed by improved PBL moisture convergence. In addition, the convergence of moisture and meridional advection of moisture prior to the MJO convection are enhanced in CMIP6, while the zonal advection of moisture is as weak as that in CMIP5. The increased convergence of moisture is a result of enhanced lower-tropospheric moisture and divergence, and the enhanced meridional advection of moisture can be caused by sharpened meridional gradient of mean lower-tropospheric moisture in the western Pacific. Further examination of the lower-tropospheric moisture budget reveals that the enhanced zonal asymmetry of the moisture tendency in CMIP6 is driven by the drying process to the west of the MJO convection, which is attributed to the negative vertical and zonal advections of moisture.

The Madden–Julian Oscillation in the Energy Exascale Earth System Model Version 1

AbstractThe present study examines the characteristics of the Madden–Julian Oscillation (MJO) events represented in the Energy Exascale Earth System Model version 1 (E3SMv1), DOE’s new Earth system model. The coupled E3SMv1 realistically simulates the eastward propagation of precipitation and moist static energy (MSE) anomalies associated with the MJO. As in observations, horizontal moisture advection and longwave radiative feedback are found to be the dominant processes in E3SMv1 that lead to the eastward movement and maintenance of the MJO MSE anomalies, respectively. Modulation of the diurnal cycle of precipitation in the Maritime Continent region by the MJO is also well represented in the model despite systematic biases in the magnitude and phase of the precipitation diurnal cycle. On the MJO impact over the midlatitude, E3SMv1 reasonably captures the pattern of the MJO teleconnections across the North Pacific and North America, with improvement in the performance in a high‐resolution version, despite the magnitude being a bit weaker than the observed. Regarding the interannual variability of the MJO, the El Niño‐Southern Oscillation (ENSO) modulation of the zonal extent of MJO’s eastward propagation, as well as associated changes in the mean state moisture gradient in the tropical west Pacific, are well reproduced in the model. However, MJO in E3SMv1 exhibits no sensitivity to the Quasi‐Biennial Oscillation (QBO), with the MJO propagation characteristics being almost identical between easterly QBO and westerly QBO years. Processes that have been suggested as critical to MJO simulation are also examined by utilizing recently developed process‐oriented diagnostics.

Changes in MJO Characteristics and Impacts in the Past Century

Abstract The changes of the MJO behavior and its impact on global precipitation, precipitation extremes, and high-frequency variability during the past century (1901–2010) were investigated using the NOAA-20CR dataset. It was found that the MJO amplitude was significantly strengthened, while its eastward phase speed hardly changed. The impacts of the MJO on precipitation in South China (SC), northern Australia (AU), and California (CA) were investigated. The anomalous rainfall in the regions was strengthened from the early to late twentieth century, with the percentage increase ranging from 67% in AU to 14% in CA. A moisture budget analysis indicated that the enhanced precipitation was primarily attributed to the effect of anomalous wind while the effect of the mean moisture change was small. The impact of the local meridional wind anomaly was critical in SC, while in AU and CA the zonal wind component dominated. The precipitation extremes had a significant increase from the early to late twentieth century. The cumulative extreme precipitation amount increased by 140%–150% in SC and AU and by 100% in CA, and the number of the extreme days increased by 110% in SC and AU and 70% in CA. Such increases are consistent with the strengthening of the high-frequency (HF; with a period less than 10 days) variability across the globe. The pattern of percentage increase of the HF variability resembles the pattern of percentage increase of MJO-scale precipitation. This implies that the MJO teleconnection pattern could exert a large-scale control on the HF variability and weather extremes. Significance Statement The Madden–Julian oscillation (MJO), a dominant mode of tropical intraseasonal oscillations, plays an important role in affecting global weather and climate. To understand to what extent the MJO and associated teleconnections change with global warming, this study adopts an alternative approach by analyzing a century-long reanalysis dataset rather than the outputs of climate models that consist of large uncertainties due to poor MJO simulations. We found that MJO-induced tropical and midlatitude precipitation and associated extremes and high-frequency variabilities were strengthened during the past century while the global mean surface temperature increases approximately by 1°C. The results derived from the present study provide a basis for assessing the future MJO behavior and associated climate impacts and for improving the extended-range prediction of severe weather and climate extremes.

The MJO From CMIP5 to CMIP6: Perspectives From Tracking MJO Precipitation

AbstractReproducing the Madden‐Julian Oscillation (MJO) remains a challenge in many climate models. This study compares the MJO simulation skills in 17 pairs of Coupled Model Intercomparison Project Phase 5 (CMIP5) and Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations. Generally, the MJO characteristics are better reproduced in CMIP6, their inter‐model spreads are obviously reduced in CMIP6. However, most CMIP6 models still underestimate the frequency of MJO initiation over the Indian Ocean and the MJO amplitude, exaggerate the barrier effect of the Maritime Continent (MC) on MJO propagation. Models with lower MJO simulation skills in their CMIP5 versions show improvements in CMIP6, while other models tend to show degraded MJO simulation skills in CMIP6. The possible reasons for the apparent improvements of MJO simulations from CMIP5 to CMIP6 in six models was further explored. It was found that the enhanced land precipitation over the MC prior to the MJO convection entering this region is more obvious in CMIP6.

The Amplification of Madden–Julian Oscillation Boosted by Temperature Feedback

Abstract Due to a small Coriolis force in tropics, the theoretical study of Madden–Julian oscillation (MJO) often assumes weak temperature gradient balance, which neglects the temperature feedback (manifested in the temperature tendency term). In this study, the effect of the temperature feedback on the MJO is investigated by using the MJO trio-interaction model, which can capture the essential large-scale features of the MJO. The scale analysis indicates that the rotation effect is strong for the MJO scales, so that the temperature feedback is as important as the moisture feedback (manifested in the moisture tendency term); the latter is often considered to be critical for MJO. The experiments with the theoretical model show that the temperature feedback has significant impact on the MJO’s maintenance. When the temperature feedback is turned off, the simulated MJO cannot be maintained over the warm pool. This is because the temperature feedback could boost the energy generation. Without the temperature feedback, only the latent heat can be generated. With the temperature feedback, not only the latent heat but also the enthalpy (and therefore the available potential energy) can be generated. Therefore, the total energy generation is more efficient with the temperature feedback, favoring the self-maintenance of the MJO. Further investigation shows that this effect of the temperature feedback on MJO amplification can be inferred from observations. The findings here indicate that the temperature feedback could have nonnegligible impacts on the MJO and have implications in the simulation of MJO.

Large‐Scale Precipitation Systems: Essential Elements of the Madden‐Julian Oscillation

AbstractPossible roles of large‐scale precipitation on model simulations of the Madden‐Julian Oscillation (MJO) were investigated using the Large‐scale Precipitation Tracking (LPT) method. Individual LPT systems and eastward propagating MJO LPT systems were identified in both observations and 25 general circulation model (GCM) simulations. LPT systems show dominant eastward propagation over the Indian Ocean in the observation, whereas LPT systems are too stationary in GCM simulations. The MJO statistical signals in the model simulations are represented by the occurence frequencies of LPT systems with a size of ∼4–9 × 106 km2 (∼20–30° in longitude and latitude) and a strength of ∼15–18 mm d−1. Growth rate in size of LPT systems in early stage determines the size of LPT systems, which differentiates the MJO statistical signals among the GCM simulations. Comprehensive diagnostics of MJO simulations in GCMs should include both individual LPT systems as essential elements and the MJO statistical signals.

Reevaluating the impacts of oceanic vertical resolution on the simulation of Madden–Julian Oscillation eastward propagation in a climate system model

The upper ocean plays a critical role in determining the Madden–Julian Oscillation (MJO) characteristics through modulating the tropical atmosphere–ocean interaction. By increasing the oceanic vertical resolution, its impacts on the MJO eastward propagation are discussed in this study by using a climate system model. With a refined vertical resolution in the upper ocean, warmer surface ocean and shallower mixed layer depth are produced in the tropics, which induces associated atmospheric changes as the response to the ocean feedbacks. Enhanced November–April-mean vertically-integrated specific humidity is found around the equatorial region with the increased vertical resolution, which strengthens the zonal and meridional moisture gradients. The lead-lag correlation of MJO precipitation demonstrates that the simulated MJO eastward propagation is improved with increased oceanic vertical resolution by improving the simulations of convective instability at the east of the MJO convective center, the boundary layer moisture convergence, the low- and upper-level circulation, and the vertical structure of equivalent potential temperature and diabatic heating. Moreover, the zonal asymmetry of the tendency of specific humidity is also improved by increasing the oceanic vertical resolution. The vertically-integrated moisture budget analysis is applied to further investigate the dominance of the moistening and drying processes. Results reveal that the drying processes are successfully reproduced over the central Indian Ocean in the case of increased oceanic vertical resolution, whilst the moistening processes are not well captured over the Maritime Continent and the MJO “detour” region. It suggests that additional modifications are needed to further improve the MJO simulation.

The Role of the Mean State on MJO Simulation in CESM2 Ensemble Simulation

AbstractThis study examines the role of the mean state in the propagation of the Madden‐Julian oscillation (MJO) over the Maritime Continent (MC). We use an ensemble of simulations made with a single model—the Community Earth System Model version 2—to assess the effect of the mean state that is unaffected by that of model components such as parameterization schemes. Results show that the background meridional moisture gradient is much steeper over the MC region in the periods with a stronger MJO propagation. Column water vapor budget of the MJO strongly suggests that the simulated mean state affects MJO via its impacts on moisture dynamics—a greater advection of mean moisture by MJO wind in the MC region is responsible for the anomalous MJO activity.

Intraseasonal Surface Salinity Variability and the MJO in a Climate Model

AbstractDue to the sparsity of ocean observations, understanding inconsistencies exist even in qualitative respect about the mechanisms of intraseasonal sea surface salinity (SSS) over the Indo‐Pacific basin. In this study, a quantitative diagnostic of salinity/freshwater budgets related to the Madden‐Julian oscillation (MJO) was conducted based on model simulations. The model applied is a modified version of the NCEP Climate Forecast System, version 2. The application of a coupled model other than ocean‐only models is one of the unique aspects of this study. The model modifications resulted in a more realistic MJO simulation and a better simulation of intraseasonal SSS variability as validated against the Aquarius estimates. The mechanisms of the MJO‐related SSS variability are further explored by conducting a salinity budget analysis over the mixed layer. The analysis indicates a strong spatial variation of dominant physical processes controlling the local SSS variability. For example, while the role of surface fluxes dominates in the western Indian basin, oceanic advection plays an important role in the eastern Indian and western Pacific oceans.