Propagation Of Madden-Julian Oscillation Research Articles

Diversity of the Madden-Julian Oscillation.

Madden-Julian Oscillation (MJO) is the dominant mode of atmospheric intraseasonal variability and the cornerstone for subseasonal prediction of extreme weather events. Climate modeling and prediction of MJO remain a big challenge, partially due to lack of understanding the MJO diversity. Here, we delineate observed MJO diversity by cluster analysis of propagation patterns of MJO events, which reveals four archetypes: standing, jumping, slow eastward propagation, and fast eastward propagation. Each type exhibits distinctive east-west asymmetric circulation and thermodynamic structures. Tight coupling between the Kelvin wave response and major convection is unique for the propagating events, while the strength and length of Kelvin wave response distinguish slow and fast propagations. The Pacific sea surface temperature anomalies can affect MJO diversity by modifying the Kelvin wave response and its coupling to MJO convection. The results shed light on the mechanisms responsible for MJO diversity and provide potential precursors for foreseeing MJO propagation.

Changes of MJO propagation characteristics under global warming

The change of Madden–Julian Oscillation (MJO) propagation and periodicity characteristics under global warming is investigated using two selected CMIP5 models. It is found that the MJO period tends to be shorter, while its eastward propagation tends to be accelerated. Meanwhile the main MJO activity center shifts eastward toward the central equatorial Pacific. Two factors are possibly responsible for the increased eastward phase speed of the MJO. The first is attributed to the increase of atmospheric static stability that accelerates equatorial Kelvin wave speed. The second is attributed to the increase of zonal asymmetry of MJO-scale moist static energy (MSE) tendency, which is contributed to the combined effect of anomalous circulation and mean MSE gradient. A theoretical framework is constructed to understand the relative role of anomalous heating and mean static stability changes in determining MJO-scale circulation change. It is found that their effect is phase dependent. During the initial warming phase the circulation change is primarily controlled by the heating change, whereas during the later warming phase it is primarily controlled by the static stability change. The eastward shift of the main MJO activity center is possibly caused by the occurrence of an El Nino-like mean SST change and associated vertical overturning circulation change in the tropical Pacific.

The Role of MJO Propagation, Lifetime, and Intensity on Modulating the Temporal Evolution of the MJO Extratropical Response

AbstractThe Madden‐Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability. Many studies have found that the MJO has significant impacts on extratropical weather. Since the MJO can act as a tropical heat source that excites Rossby waves, midlatitude weather is modulated by the MJO due to the Rossby waves that propagate into the midlatitude and modulate the midlatitude circulation. Heat sources of individual MJO events are different since each event has different eastward propagation speed, lifetime, intensity, and structure. The background flow is also different for each event. These result in different Rossby waves and different extratropical response for each MJO event. In this study, the role of MJO propagation speed, lifetime, and intensity on modulating the structure and temporal evolution of the MJO extratropical response is systematically explored by using an idealized general circulation model. By adding the MJO‐associated heating into the general circulation model as an external forcing, the extratropical response in the Reanalysis is captured reasonably by the model. However, large ensemble model simulations show that the response in the Reanalysis is not robust. Experiments with MJO events of different propagation speed, lifetime, and intensity show that to excite a strong extratropical response, the MJO has to propagate through specific phases (Phases 1–3 and 5–7). The intensity, timing, and duration of the extratropical response strongly depend on when the MJO is initiated and when the MJO decays. The extratropical impacts of slow‐ and fast‐propagating MJO also have significant differences, especially on intensity and duration.

Observational Evidence of Mixed Rossby‐Gravity Waves as a Driving Force for the MJO Convective Initiation and Propagation

AbstractBased on observational data analyses of a Madden‐Julian Oscillation (MJO) event during the field campaign of the Years of the Maritime Continent Project, we propose a new mechanism for MJO convective initiation and propagation to which mixed Rossby‐gravity waves (MRGs) contribute significantly. The intensive observation captured midtropospheric wind variations associated with MRGs, and those signals propagated westward over the Indian Ocean in the MJO‐suppressed phase. The transition to the MJO active phase follows tight MRG‐convection coupling, which is enhanced when the wavelength becomes shorter due to weak midlevel convergence in the western Indian Ocean. Basin‐scale anomalous midtropospheric moistening caused by MRG shallow circulations also contributes. These processes lead to the lower‐tropospheric MRG wave packet formation and successive triggering of MJO convection, with the eastward MRG group velocity corresponding to MJO propagation. Additional analysis for other MJO cases implicates potential for robustness of this MJO‐MRG relationship.

A Unified Moisture Mode Framework for Seasonality of the Madden–Julian Oscillation

The Madden–Julian oscillation (MJO) exhibits pronounced seasonality. While it is largely characterized by equatorially eastward propagation during the boreal winter, MJO convection undergoes marked poleward movement over the Asian monsoon region during summer, producing a significant modulation of monsoon rainfall. In classical MJO theories that seek to interpret the distinct seasonality in MJO propagation features, the role of equatorial wave dynamics has been emphasized for its eastward propagation, whereas coupling between MJO convection and the mean monsoon flow is considered essential for its northward propagation. In this study, a unified physical framework based on the moisture mode theory, is offered to explain the seasonality in MJO propagation. Moistening and drying caused by horizontal advection of the lower-tropospheric mean moisture by MJO winds, which was recently found to be critical for the eastward propagation of the winter MJO, is also shown to play a dominant role in operating the northward propagation of the summer MJO. The seasonal variations in the mean moisture pattern largely shape the distinct MJO propagation in different seasons. The critical role of the seasonally varying climatological distribution of moisture for the MJO propagation is further supported by the close association between model skill in representing the MJO propagation and skill at producing the lower-tropospheric mean moisture pattern. This study thus pinpoints an important direction for climate model development for improved MJO representation during all seasons.

Role of topography on the MJO in the maritime continent: a numerical case study

The role of topography on a Madden–Julian Oscillation (MJO) event in the Maritime Continent (MC) is explored using a regional model. Four simulations are conducted: lower-resolution (12 km) simulations using cumulus parameterization in the presence (LR) and absence (LR-Flat) of topography, and higher-resolution (4 km) simulations without cumulus parameterization in the presence (HR) and absence (HR-Flat) of topography. In the LR simulation, the MJO remains unorganized with no clear eastward propagation, while the LR-Flat simulation captures the MJO and its eastward propagation across the MC. In the absence of cumulus parameterization, both HR and HR-Flat capture the MJO and show several similarities and differences compared to the LR and LR-Flat simulations. To better understand these differences, a moisture budget analysis is conducted during the passage of the MJO. In the LR-Flat simulation, vertical advection of moisture is increased to the east of the islands, leading to continuity in MJO-associated convection, continuity that was not present in the LR simulation. The increase in vertical advection in the absence of topography is due to an increase in the mean moisture advection by the anomalous vertical winds. In the middle of the MC, horizontal advection seems to be the most important for an uninterrupted eastward propagation of the MJO. The increase in the horizontal advection in the absence of topography is primarily due to an increase in the anomalous moisture advection by the mean zonal winds. To what extent the MJO was influenced by the upstream effect from the New Guinea topography was also explored. These results indicate that the important physical processes for MJO-associated convection may be different in different parts of the MC. Further implications of these results in the context of other recent studies on MJO propagation across the MC are discussed.

QBO‐MJO Connection

AbstractActivities of the Madden‐Julian Oscillation (MJO) in boreal winter has recently been found to be stronger in easterly phases of the stratospheric quasi‐biennial oscillation (QBO) than its westerly phases. This QBO‐MJO connection was investigated in this study using a method that identifies individual MJO events by tracking their eastward propagating signals in precipitation. Stronger MJO activities in QBO easterly phases are a consequence of more MJO days, not larger amplitudes of individual MJO events as previously thought. More MJO days come from more MJO events initiated over the Indian Ocean and their longer duration because of a weaker barrier effect of the Maritime Continent on MJO propagation. Zonal heterogeneity exists in the connection between QBO, MJO, and tropical total precipitation in general. This poses a challenge to our current understanding of the MJO dynamics, which has yet to fully include upper‐tropospheric and stratospheric processes.

Dynamics-oriented diagnostics for the Madden-Julian Oscillation

AbstractRealistic simulations of the Madden–Julian oscillation (MJO) by global climate models (GCMs) remain a great challenge. To evaluate GCM simulations of the MJO, the U.S. CLIVAR MJO Working Group developed a standardized set of diagnostics, providing a comprehensive assessment of statistical properties of the MJO. Here, a suite of complementary diagnostics has been developed that provides discrimination and assessment of MJO simulations based on the perception that the MJO propagation has characteristic dynamic and thermodynamic structures. The new dynamics-oriented diagnostics help to evaluate whether a model produces eastward-propagating MJOs for the right reasons. The diagnostics include 1) the horizontal structure of boundary layer moisture convergence (BLMC) that moistens the lower troposphere to the east of a convection center, 2) the preluding eastward propagation of BLMC that leads the propagation of MJO precipitation by about 5 days, 3) the horizontal structure of 850-hPa zonal wind and its ...

Characterization of Moist Processes Associated With Changes in the Propagation of the MJO With Increasing CO2.

The processes that lead to changes in the propagation and maintenance of the Madden‐Julian Oscillation (MJO) as a response to increasing CO2 are examined by analyzing moist static energy budget of the MJO in a series of NASA GISS model simulations. It is found changes in MJO propagation is dominated by several key processes. Horizontal moisture advection, a key process for MJO propagation, is found to enhance predominantly due to an increase in the mean horizontal moisture gradients. The terms that determine the strength of the advecting wind anomalies, the MJO horizontal scale and the dry static stability, are found to exhibit opposing trends that largely cancel out. Furthermore, reduced sensitivity of precipitation to changes in column moisture, i.e., a lengthening in the convective moisture adjustment time scale, also opposes enhanced propagation. The dispersion relationship of Adames and Kim, which accounts for all these processes, predicts an acceleration of the MJO at a rate of ∼3.5% K−1, which is consistent with the actual phase speed changes in the simulation. For the processes that contribute to MJO maintenance, it is found that damping by vertical MSE advection is reduced due to the increasing vertical moisture gradient. This weaker damping is nearly canceled by weaker maintenance by cloud‐radiative feedbacks, yielding the growth rate from the linear moisture mode theory nearly unchanged with the warming. Furthermore, the estimated growth rates are found to be a small, negative values, suggesting that the MJO in the simulation is a weakly damped mode.

MJO Propagation Shaped by Zonal Asymmetric Structures: Results from 24 GCM Simulations

Abstract Eastward propagation is an essential characteristic of the Madden–Julian oscillation (MJO). Yet, simulation of MJO propagation in general circulation models (GCMs) remains a major challenge and understanding the causes of propagation remains controversial. The present study explores why the GCMs have diverse performances in MJO simulation by diagnosis of 24 GCM simulations. An intrinsic linkage is found between MJO propagation and the zonal structural asymmetry with respect to the MJO convective center. The observed and realistically simulated MJO eastward propagations are characterized by stronger Kelvin easterly waves than Rossby westerly waves in the lower troposphere, which is opposite to the Gill pattern where Rossby westerly waves are 2 times stronger than Kelvin easterly waves. The GCMs simulating stronger Rossby westerly waves tend to show a stationary MJO. MJO propagation performances are robustly correlated with the quality of simulated zonal asymmetries in the 850-hPa equatorial zonal winds, 700-hPa diabatic heating, 1000–700-hPa equivalent potential temperature, and convective instability. The models simulating realistic MJO propagation are exemplified by an eastward propagation of boundary layer moisture convergence (BLMC) that leads precipitation propagation by about 5 days. The BLMC stimulates MJO eastward propagation by preconditioning and predestabilizing the atmosphere, and by generating lower-tropospheric heating and available potential energy to the east of precipitation center. The MJO structural asymmetry is generated by the three-way interaction among convective heating, moisture, and equatorial wave and boundary layer dynamics. In GCMs, differing convective heating representation could produce different MJO structural asymmetry, and thus different propagations. Diagnosis of structural asymmetry may help revealing the models’ deficiency in representing the complex three-way interaction processes, which involves various parameterized processes.

The impact of the mean moisture bias on the key physics of MJO propagation in the ECMWF reforecast

AbstractThe impact of the seasonal mean background state on the physical processes responsible for better Madden‐Julian Oscillation (MJO) propagation and prediction was investigated using 20 year European Centre for Medium‐Range Weather Forecasts (ECMWF) reforecast data sets. While the reforecast captures the MJO propagating signal in the circulation field, the outgoing longwave radiation and column‐integrated moist static energy (MSE) anomalies show a weaker signal than the reanalysis when the MJO detours and propagates in the southern Maritime Continent. By comparing individual MSE budget terms, it is found that the predicted positive MSE tendency to the east of the MJO convection center is weaker than the reanalysis due to the weaker horizontal MSE advection. The dry bias in the seasonal mean lower tropospheric moisture is a key factor that weakens the horizontal MSE advection, and thus deteriorates the MJO propagation in the reforecasts. Therefore, improvements in the mean state could further extend the MJO prediction and associated subseasonal weather phenomena over the globe.

Madden—Julian Oscillation prediction and teleconnections in the S2S database

The Madden—Julian Oscillation (MJO) has been diagnosed in the World Weather Research Program (WWRP)/World Climate Research Program (WCRP) Sub‐seasonal to Seasonal prediction project (S2S) database using the Wheeler and Hendon index over the common hindcast period 1999—2010. The S2S models display skill to predict the MJO over a period between two and four weeks. The majority of S2S models tend to produce a weaker MJO than in ERA‐Interim, with a phase speed decreasing with lead time.All the S2S models produce realistic patterns of MJO teleconnections at 500 hPa, with an increased probability of positive North Atlantic Oscillation (NAO) following an active MJO over the Indian Ocean and of negative NAO following an active MJO over the West Pacific. However, the amplitude of the MJO teleconnection patterns are significantly weaker than in ERA‐Interim over the Euro‐Atlantic sector and are often too strong over the western North Pacific. Models with lower horizontal resolution tend to produce weaker teleconnections. In the lower stratosphere, several S2S models produce teleconnections which are too strong compared to ERA‐Interim. These results suggest that, although the S2S models display significant skill in predicting the MJO propagation beyond two weeks, all the S2S models do not fully exploit the predictability associated with the MJO in the Northern Extratropics, particularly over Europe.

The role of global circumnavigating mode in the MJO initiation and propagation

AbstractA series of convection‐permitting regional‐model simulations are used to study the influences of intraseasonal forcings from the periphery of the tropical Indian Ocean on the October 2011 Madden‐Julian Oscillation (MJO) event well observed during the Dynamics of the Madden‐Julian Oscillation field campaign. These sensitivity experiments demonstrate the predominant role of a global circumnavigating mode in the initiation and propagation of this MJO event, although the regional framework would not completely rule out influences from the extratropics through the western boundary of the model domain. It is found that filtering out intraseasonal forcing leads to the absence of Kelvin waves, which may reduce moisture transport and weakened lower level convergence at the leading edge of the westerlies and further leads to diminishing MJO signatures in these sensitivity experiments. Meanwhile, for the same regional domain initiated 2 weeks before the onset of the MJO convection, there is little impact of whether or not the global MJO signals retained in the initial conditions. On the other hand, the removal of the global MJO signals from the lateral boundary conditions will not affect the propagation of the MJO after it has been initiated in the regional model domain.

Successive MJO propagation in MERRA‐2 reanalysis

AbstractComposite circumnavigating Madden‐Julian oscillation (MJO) events in Version 2 of the NASA Modern Era Reanalysis for Research and Applications (MERRA‐2) reanalysis propagate as convectively coupled Kelvin waves over the Western Hemisphere and moisture waves like that described by Adames and Kim (2016) over the warm pool. Estimated zonally variable phase speeds of coupled Kelvin waves in the tropics are calculated by determining the “effective static stability” experienced by the wave. The wave is structured similarly to a classically derived deep tropospheric Kelvin wave, and its phase speed is up to 33 m s−1 or 40 m s−1 over the central/eastern Pacific or Atlantic/equatorial Africa, respectively, during boreal winter. Theoretically, estimated phase speeds of convectively coupled Kelvin waves over the tropical warm pool are greater than 15 m s−1, much faster than the propagation of the reanalyzed MJO. A complete theory for MJO propagation around the globe must allow both coupled Kelvin waves and moisture waves.

Further inquiry into characteristics of MJO in boreal winter

ABSTRACTThe characteristics of Madden–Julian Oscillation (MJO), especially over the equatorial western Pacific, during the boreal winter (November to March) have got a further investigation based on the reanalysis data. The MJO activity prevails over the equatorial Indian Ocean and western Pacific and its main active centres are in the Southern Hemisphere. The MJO also has the highest percentage in the atmospheric variability and strong interannual variability here. The MJO over the equatorial western Pacific shows a prominent interannual variability with a period of 2–8 years and an interdecadal variability with a period of 12–16 years. These variabilities of MJO are closely related to the intensity and duration of strong discrete MJO events over there. During the equatorial western Pacific MJO activity strong winter, the MJO is intensified throughout the tropics and its eastward propagation is more continuous, more coherent, and further east. Anomalous ascending motion and moisture convergence over the equatorial western Pacific could enhance the Walker circulation over the equatorial Pacific and favour the activity of MJO over the equatorial western Pacific. The further eastward propagation of MJO in strong MJO activity winter is mainly due to the strong moistening at the lower troposphere to the east of the MJO convection centre. This moistening is dominated by the vertical advection of moisture. In weak MJO activity winter, the above characteristics are reversed. Furthermore, the MJO activity over the Indian Ocean and Pacific has undergone several interdecadal changes in both of its intensity and propagation during the past 150 years.

Fundamental Causes of Propagating and Nonpropagating MJOs in MJOTF/GASS Models

Abstract This study investigates the fundamental causes of differences in the Madden–Julian oscillation (MJO) eastward propagation among models that participated in a recent model intercomparison project. These models are categorized into good and poor groups characterized by prominent eastward propagation and nonpropagation, respectively. Column-integrated moist static energy (MSE) budgets are diagnosed for the good and the poor models. It is found that a zonal asymmetry in the MSE tendency, characteristic of eastward MJO propagation, occurs in the good group, whereas such an asymmetry does not exist in the poor group. The difference arises mainly from anomalous vertical and horizontal MSE advection. The former is attributed to the zonal asymmetry of upper-midtropospheric vertical velocity anomalies acting on background MSE vertical gradient; the latter is mainly attributed to the asymmetric zonal distribution of low-tropospheric meridional wind anomalies advecting background MSE and moisture fields. Based on the diagnosis above, a new mechanism for MJO eastward propagation that emphasizes the second-baroclinic-mode vertical velocity is proposed. A set of atmospheric general circulation model experiments with prescribed diabatic heating profiles was conducted to investigate the causes of different anomalous circulations between the good and the poor models. The numerical experiments reveal that the presence of a stratiform heating at the rear of MJO convection is responsible for the zonal asymmetry of vertical velocity anomaly and is important to strengthening lower-tropospheric poleward flows to the east of MJO convection. Thus, a key to improving the poor models is to correctly reproduce the stratiform heating. The roles of Rossby and Kelvin wave components in MJO propagation are particularly discussed.

Seasonality and mechanisms of tropical intraseasonal oscillations

This study has compared the monsoon intraseasonal oscillation (MISO) during the boreal summer and Madden Julian Oscillation (MJO) during the boreal winter. Based on MISO and MJO in high-resolution three-dimensional diabatic heating, the possible mechanisms are discussed through observational analyses of dynamical and thermodynamical variables. The MISO and MJO are extracted as nonlinear oscillations during boreal summer and winter, respectively, by applying multi-channel singular spectrum analysis on daily anomalies of diabatic heating over the Indo-Pacific region. Lead and lag relations among moisture, temperature and surface fields relative to diabatic heating are analyzed to compare the mechanisms of MISO and MJO. While both the oscillations show eastward propagation, MISO has a strong northward propagation and MJO has a weak southward propagation as well. The analysis shows that MJO and MISO are essentially driven by the same mechanisms but with some difference in the meridional propagation. The westerly shear leads the diabatic heating, while the vorticity has weak correlation. Large-scale circulation creates positive moisture preconditioning before convection and negative moisture preconditioning before suppressed conditions. A positive lower level horizontal advection of temperature and upper level temperature tendencies lead the convective state while a negative lower level horizontal advection of temperature and upper level temperature tendencies lead the suppressed state. There is positive feedback from the SST to atmosphere. The difference in the meridional propagation of MISO and MJO is hypothesized to be because of the different differential heating meridionally during the two seasons.

Winter mean lower tropospheric moisture over the Maritime Continent as a climate model diagnostic metric for the propagation of the Madden‐Julian oscillation

AbstractDespite its widespread influences on the atmosphere, the Madden‐Julian oscillation (MJO) remains poorly represented in state‐of‐the‐art general circulation models (GCMs). Motivated by recent findings that the horizontal advection of the mean low‐tropospheric moist static energy or moisture by the MJO winds plays a crucial role in the eastward propagation of the MJO, we investigate the relationship between lower tropospheric moisture patterns over the Indo‐Pacific and MJO eastward propagation in a suite of 23 GCM simulations. Model capability of reproducing the observed November–April mean lower tropospheric moisture pattern over the Indo‐Pacific, especially near the Maritime Continent (MC), is highly correlated with model skill in simulating MJO eastward propagation. In GCMs with difficulty capturing realistic MJO propagation, the amplitude of the mean low‐level moisture over the MC is greatly underestimated, leading to weak horizontal moisture gradients and thus discrepancies in moisture advection, significantly affecting MJO propagation. This study suggests that the mean lower tropospheric moisture pattern over the MC can serve as an important diagnostic metric for MJO propagation in climate models.

Simulations of MJO Propagation across the Maritime Continent: Impacts of SST Feedback

Abstract The observed Madden–Julian oscillation (MJO) tends to propagate eastward across the Maritime Continent from the eastern equatorial Indian Ocean to the western Pacific. However, numerical simulations present different levels of fidelity in representing the propagation, especially for the tropical convection associated with the MJO. This study conducts a series of coupled simulations using the NCEP CFSv2 to explore the impacts of SST feedback and convection parameterization on the propagation simulations. First, two simulations differing in the model horizontal resolutions are conducted. The MJO propagation in these two simulations is found generally insensitive to the resolution change. Further, based on the CFSv2 with a lower resolution, two additional experiments are performed with model SSTs nudged to climatologies with different time scales representing different air–sea coupling strength. It is demonstrated that weakening the air–sea coupling strength significantly degrades the MJO propagation simulation, suggesting the critical role of SST feedback in maintaining MJO propagation. Last, the sensitivity to convection parameterization is explored by comparing two simulations with different convection parameterization schemes. Analyses of these simulations indicate that including air–sea coupling alone in a dynamical model does not result in realistic maintenance of the MJO eastward propagation without the development of favorable SST conditions in the western Pacific. In both observations and one simulation with realistic MJO propagations, the preconditioning of SSTs is strongly affected by surface latent heat fluxes that are modulated by surface wind anomalies in both zonal and meridional directions. The diagnostics highlight the critical contribution from meridional winds in wind speed variations, which has been neglected in most MJO studies.

Key processes for the eastward propagation of the Madden‐Julian Oscillation based on multimodel simulations

AbstractAs a prominent climate variability mode with widespread influences on global weather extremes, the Madden‐Julian Oscillation (MJO) remains poorly represented in the latest generation of general circulation models (GCMs), with a particular challenge in simulating its eastward propagating convective signals. In this study, by analyzing multimodel simulations from a recent global MJO model evaluation project, an effort is made to identify key processes for the eastward propagation of the MJO through analyses of moisture entropy (ME) processes under a “moisture mode” framework for the MJO. The column‐integrated horizontal ME advection is found to play a critical role for the eastward propagation of the MJO in both observations and good MJO models, with a primary contribution through advection of the lower tropospheric seasonal mean ME by the MJO anomalous circulations. By contrast, the horizontal ME advection effect for the eastward propagation is greatly underestimated in poor MJO GCMs, due to model deficiencies in simulating both the seasonal mean ME pattern and MJO circulations, leading to a largely stationary MJO mode in these GCMs. These results thus pinpoint an important guidance toward improved representation of the MJO in climate and weather forecast models. While this study mainly focuses on fundamental physics for the MJO propagation over the Indian Ocean, complex influences by the Maritime Continent on the MJO and also ME processes associated with the MJO over the western Pacific warrant further investigations.