Structure Of Madden-Julian Oscillation Research Articles

MJO Structure–Propagation Nexus and Impacts of Background Mean States in CMIP6 Models

Abstract Eastward propagation is an essential feature of the Madden–Julian oscillation (MJO). Yet, it remains a challenge to realistically simulate it by global climate system models, and the reasons are not fully understood. This study evaluates the capability of 20 Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating MJO’s eastward propagation and its intrinsic links with the dynamic–thermodynamic structures and the background mean states, aiming at better understanding the sources of the simulation errors. The metrics to evaluate the MJO internal dynamics consists of six parameters: 1) the east–west asymmetry in the low-level circulation, 2) the boundary layer moisture convergence propagation, 3) the vertical tilt of equivalent potential temperature or moist static energy, the vertical structures of 4) diabatic heating and 5) available potential energy generation, and 6) upper-level diabatic heating and divergence. We also gauge the performance of three MJO-related background mean-state fields, including precipitation, sea surface temperature, and low-level moist static energy. It is argued that these parameters are relevant internal and external factors that could affect MJO eastward propagation. We find that the boundary layer moisture convergence is most tightly coupled with the eastward propagation of MJO and controls the premoistening, destabilization, and the leading low-level diabatic heating and available potential energy generation. The CMIP6 models exhibit significant improvements against CMIP5 models in simulating MJO dynamic–thermodynamic structures and the mean states. The diagnostics in this study could help to identify the possible processes related to CMIP6 models’ shortcomings and shed light on how to improve simulation of MJO eastward propagation in the future.

Madden–Julian Oscillation-induced extreme rainfalls constrained by global warming mitigation

The sixth assessment report of the IPCC indicates low-to-high confidence in trends of extreme rainfall with regional inconsistency in the tropics, where a key phenomenon causing intra-seasonal variations in weather is the Madden–Julian Oscillation (MJO). It remains unknown how the MJO-induced extreme rainfall and the societal exposure may change in response to global warming and climate mitigation attempts. Here, using eight CMIP6 models that capture the eastward-propagating MJO structure and amplitude, we detect a nearly 60% increase in extreme rainfall over tropical Asia and Australia by the end of the 21st century under the fossil-fueled warming scenario (SSP5-8.5); 84% of this change is associated with MJO-induced extreme rainfall. Extreme rainfall increases are modulated by the warming-induced asymmetric changes in MJO phase characteristics, occurring mostly over the lands with distinct zonal differences. The region that is most likely to be affected includes Malaysia, Indonesia, and northern Australia where 96.68 million people and 9.72 million km2 of urban areas are exposed to potential danger stemming from extreme rainfall. More than 95% (99%) of the population (urban) exposure can be potentially avoided under the “middle of the road” development (SSP2-4.5) scenario, whereby CO2 emissions hover around current levels before starting to fall mid-century.

Application of MJO dynamics-oriented diagnostics to CMIP5 models

Realistic simulation of Madden–Julian Oscillation (MJO) propagation in coupled global climate models remains a common problem. In this study, the ability of 20 coupled models from Coupled Model Intercomparison Project Phase 5 (CMIP5) in simulating MJO is examined using dynamics-oriented diagnostics. The diagnostics focus on dynamic and thermodynamic structures of MJOs on three dimensions, which help to identify the shortcomings of models and evaluate whether they could reproduce MJO for the right reason. According to the simulation performance of the eastward propagation of MJO, the “good” models and “poor” models are detected. The dynamics-oriented diagnostics are further applied to the good models, poor models, and all models to establish a linkage between MJO simulation skill and their dynamic/thermodynamic structures. Results show that the simulations of good models have the following common features: (1) a horizontal zonal structural asymmetry in low-level zonal wind, upper-level diabatic heating and divergence; (2) a preceding eastward propagation of boundary layer moisture convergence; and (3) a rearward-tilted vertical structure of diabatic heating, equivalent potential temperature and available potential energy generation. The poor models that fail to capture these three-dimensional structures do not reproduce the eastward propagation of MJO. More than half of these 20 CMIP5 models still have difficulties in simulating these dynamic/thermodynamic structures.

The transient response to an equatorial heat source and its convergence to steady state: implications for MJO theory

The Matsuno–Gill model for the atmospheric steady-state response to an imposed equatorial heat source is perhaps the most successful set of reduced equations in the theory of tropical dynamics. While it was originally designed to explain some key features of the tropical climatology such as the Walker and Hadley circulations, it is increasingly used for other transient tropical disturbances such as the Madden–Julian oscillation (MJO). Here, we provide a semi-analytic solution to the time dependent Matsuno–Gill equations, with periodic boundary conditions and without the long-wave approximation, based on a meridional expansion using the parabolic cylinder functions combined with the method of characteristics and Fourier series in the zonal direction. Our method of solution leads to an ODE system which is solved numerically. We use this transient solution to investigate the conditions under which the transient response convergences to the steady state and study the effect of a moving heat source on the structure of the response and speculate about its resemblance or not to the MJO structure. In particular, we look at the sensitivity of the long time behavior of the solution with respect to the model parameters, namely, the imposed dissipation rate and the speed of the moving heat source as well as the form of the heating itself.

Seasonality of the Structure and Propagation Characteristics of the MJO

Abstract The seasonality of the Madden–Julian oscillation (MJO) is documented in observational data, and a nonlinear shallow-water model is used to help interpret some of the contrasts in MJO structure between the boreal winter season [November–March (NDJFM)] and the Asian summer monsoon period [June–September (JJAS)]. At upper-tropospheric levels, the flanking Rossby waves remain centered around 28°N/S year-round, but they tend to be stronger in the winter hemisphere, where the climatological-mean jet stream is stronger, rendering the subtropical circulation more sensitive to forcing by a near-equatorial heat source. Amplitudes of the MJO-related deep convection and lower-tropospheric zonal wind are stronger in the summer hemisphere, where the column-integrated water vapor is larger. During NDJFM, the equatorial asymmetry is subtle: as in the annual mean, moisture convergence into swallowtail-shaped regions of enhanced deep convection is an integral part of the equatorial Rossby wave signature, and the eastward propagation is due to moistening of the air to the east of the enhanced convection by poleward moisture advection. During the Asian summer monsoon in JJAS, the convection assumes the form of northward-propagating, west-northwest–east-southeast-oriented rainbands embedded within cyclonic shear lines. These features are maintained by frictional convergence of moisture, and their northward propagation is mainly due to the presence of features in the climatological-mean fields: that is, the west–east moisture gradient over India and the Arabian Sea and the southwesterly low-level monsoon flow over the northwest Pacific.

Effective control parameters in a deep convection scheme for improved simulation of the Madden–Julian oscillation

This work seeks to find the most effective parameters in a deep convection scheme (relaxed Arakawa–Schubert scheme) of the National Centers of Environmental Prediction Climate Forecast System model for improved simulation of the Madden–Julian Oscillation (MJO). A suite of sensitivity experiments are performed by changing physical components such as the relaxation parameter of mass flux for adjustment of the environment, the evaporation rate from large-scale precipitation, the moisture trigger threshold using relative humidity of the boundary layer, and the fraction of re-evaporation of convective (subgrid-scale) rainfall. Among them, the last two parameters are found to produce a significant improvement. Increasing the strength of these two parameters reduces light rainfall that inhibits complete formation of the tropical convective system or supplies more moisture that help increase a potential energy to large-scale environment in the lower troposphere (especially at 700 hPa), leading to moisture preconditioning favorable for further development and eastward propagation of the MJO. In a more humid environment, more organized MJO structure (i.e., space–time spectral signal, eastward propagation, and tilted vertical structure) is produced.

Systematic decomposition of the Madden‐Julian Oscillation into balanced and inertio‐gravity components

AbstractWe present a new method for the three‐dimensional multivariate decomposition of the Madden‐Julian Oscillation (MJO) into balanced and inertio‐gravity (IG) components. The method analyzes global fields with no filtering involved, and it provides a quantitative comparison between the contribution of the Rossby, Kelvin, and other balanced and IG modes to the MJO circulation and its teleconnections. Results based on the ERA Interim reanalysis data and the multivariate MJO index show that the Rossby mode with the lowest meridional index is the largest contributor to the MJO circulation over the Pacific. A smaller role of the Kelvin mode is diagnosed over the Indian Ocean and the maritime continent. The MJO teleconnections in the polar stratosphere appear associated with the leading balanced vertical modes. The presented method shows new ways of evaluating the MJO structure and its global impacts in weather and climate models.

Convective Momentum Transport Associated with the Madden–Julian Oscillation Based on a Reanalysis Dataset

Abstract A better understanding of multiscale interactions within the Madden–Julian oscillation (MJO), including momentum exchanges, is critical for improved MJO prediction skill. In this study, convective momentum transport (CMT) associated with the MJO is analyzed based on the NOAA Climate Forecast System Reanalysis (CFSR). A three-layer vertical structure associated with the MJO, as previously suggested in the mesoscale momentum tendency profile based on global cloud-resolving model simulations, is evident in the subgrid-scale momentum tendency from the CFSR. Positive (negative) subgrid-scale momentum tendency anomalies are found near the surface, negative (positive) anomalies are found in the low to midtroposphere, and positive (negative) anomalies in the upper troposphere are found within and to the west (east) of the MJO convection. This tends to damp the MJO circulation in the free atmosphere, while enhancing MJO winds near the surface. In addition, it could also reduce the MJO eastward propagation speed and lead to the backward tilt with height in the observed MJO structure through a secondary circulation near the MJO center. Further analyses illustrate that this three-layer vertical structure in subgrid-scale momentum tendency largely balances the grid-scale momentum transport of the zonal wind component u, mainly through the transport of seasonal mean u by the MJO-scale vertical motion. Synoptic-scale systems, which were previously proposed to be essential for the u-momentum transport of the MJO, however, are found to play a minor role for the total grid-scale momentum tendency. The above momentum tendency structure is also confirmed with the ECMWF analysis for the Year of Tropical Convection (YOTC) that lends confidence to these above results based on the CFSR.

The influence of El Niño on MJO over the equatorial pacific

In this paper, the influence of El Nino event on the Madden-Julian Oscillation (MJO) over the equatorial Pacific is studied by using reanalysis data and relevant numerical simulation results. It is clearly shown that El Nino can reduce the intensity of MJO. The kinetic energy of MJO over the equatorial Pacific is stronger before the occurrence of the El Nino event, but it is reduced rapidly after El Nino event outbreak, and the weakened MJO even can continue to the next summer. The convection over the central-western Pacific is weakened in El Nino winter. The positive anomalous OLR over the central-western Pacific has opposite variation in El Nino winter comparing to the non-ENSO cases. The vertical structure of MJO also affected by El Nino event, so the opposite direction features of the geopotential height and the zonal wind in upper and lower level troposphere for the MJO are not remarkable in the El Nino winter and tend to be barotropic features. El Nino event also has an influence on the eastward propagation of the MJO too. During El Nino winter, the eastward propagation of the MJO is not so regular and unanimous and there exists some eastward propagation, which is faster than that in non-ENSO case. Dynamic analyses suggest that positive SSTA (El Nino case) affects the atmospheric thickness over the equatorial Pacific and then the excited atmospheric wave-CISK mode is weakened, so that the intensity of MJO is reduced; the combining of the barotropic unstable mode in the atmosphere excited by external forcing (SSTA) and the original MJO may be an important reason for the MJO vertical structure tending to be barotropic during the El Nino.

Transforming circumnavigating Kelvin waves that initiate and dissipate the Madden–Julian Oscillation

Analyses of Lagrangian model simulations and atmospheric sonde data reveal a key component of the Madden–Julian Oscillation (MJO): circumnavigating equatorial Kelvin waves with dynamics that transform between dry and moist, which initiate and dissipate MJO convection. The same compositing procedure is applied to simulated and observed MJOs, which uses a coordinate system that moves with the precipitation centre, and treats developing, mature and dissipating stages separately. MJO structure and evolution are similar in the simulation and the observations. To the west of the developing convection, there is a broad region of low‐level (upper‐level) perturbation westerlies (easterlies), which is accompanied by a deep negative temperature perturbation. As this feature enters the MJO‐formation region, convection intensifies on its eastern edge, and the zonal wind perturbations decrease in zonal extent and propagation speed. This process is shown to be a dynamical consequence of a largely dry, first baroclinic Kelvin wave entering a region where large‐scale upward motion is mostly balanced by convective heating. As the MJO matures, a Kelvin wave of opposite sign emerges from its eastern edge, and makes the opposite transition (from moist to dry). The resulting wave, which includes low‐level (upper‐level) easterlies (westerlies) and a deep positive temperature perturbation, rapidly propagates around the world and dissipates MJO convection. The Kelvin wave that initiates MJO convection is shown to originate from the inactive phase of a previous MJO, so that the complete MJO cycle is characterized by the two kinds of Kelvin waves emerging from active and suppressed phases of MJO convection, circumnavigating the Tropics, and triggering the opposite phase.

Structural evolution of the Madden‐Julian Oscillation from COSMIC radio occultation data

The atmospheric temperature and specific humidity profiles derived from 4 years of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements during the boreal winter (November thru April in 2006–2010) were employed to study the large‐scale vertical structure of the Madden‐Julian Oscillation (MJO). In a composited MJO cycle, both RO temperature and moisture anomalies propagate eastward along with the enhanced MJO convection over the Indo‐Pacific region. In the same region, the RO temperature anomaly is positively correlated with the convection (Tropical Rainfall Measuring Mission (TRMM) rainfall) anomaly in the middle and upper troposphere (800 hPa–200 hPa) and negatively correlated in the lower troposphere and at the tropopause. A positive RO moisture anomaly has a westward tilt below 300 hPa and is well associated with the convection anomaly. RO bending angle and refractivity anomalies show vertical structure similar to that of temperature MJO anomaly above ∼300 hPa and to that of the moisture anomaly below that. These salient MJO features agree well with those revealed by global reanalysis data and meteorological satellite observations, such as the Atmospheric Infrared Sounder (AIRS), even though RO anomalies exhibit sharper structure at the tropopause region due to the higher vertical resolution of the RO. Investigation of the cold‐point tropopause anomaly indicates that the enhanced convection is preceded by cooling of the tropopause and followed by lowering of the tropopause height. The MJO moisture structures during two boreal winters, November 2007–April 2008 (La Niña) and November 2009–April 2010 (El Niño), are individually presented to demonstrate the El Niño‐Southern Oscillation (ENSO) effect on the MJO structure.

Critical roles of convective momentum transfer in sustaining the multi-scale Madden–Julian oscillation

To understand the nature and role of multi-scale interaction involved in the Madden–Julian oscillation (MJO), a dynamical model is built based on two essential processes: the convective complex of the MJO modulates the strength and location of synoptic-scale motions, which in turn feed back to the MJO through the convective momentum transfer (CMT). Our results exhibit that: (1) The lower tropospheric easterly CMT coming from the 2-day waves slows down the MJO dramatically; (2) although the lower tropospheric westerly CMT coming from the superclusters can produce the horizontal quadrupole vortex and vertical westerly wind-burst structures of the MJO, it drives the large-scale motions to propagate eastward too fast; (3) the planetary boundary layer provides an instability source for the MJO and pulls the MJO to propagate eastward at a speed of 0∼10 ms−1; and (4) the optimal structure of the multi-scale MJO should be: the stronger superclusters/2-day waves prevail in the rear/front part of the MJO and produce lower tropospheric westerly/easterly CMT there. These theoretical results emphasize the role of CMT and encourage further observations in the multi-scale MJO.

Impacts of Idealized Air–Sea Coupling on Madden–Julian Oscillation Structure in the Superparameterized CAM

Abstract Air–sea interactions and their impact on intraseasonal convective organization are investigated by comparing two 5-yr simulations from the superparameterized Community Atmosphere Model version 3.0 (SP-CAM). The first is forced using prescribed sea surface temperatures (SSTs). The second is identical except that a simplified oceanic mixed-layer model is used to predict tropical SST anomalies that are coupled to the atmosphere. This partially coupled simulation allows SSTs to respond to anomalous surface fluxes. Implementation of the idealized slab ocean model in the SP-CAM results in significant changes to intraseasonal convective variability and organization. The more realistic treatment of air–sea interactions in the coupled simulation improves many aspects of tropical convection on intraseasonal scales, from the relationships between precipitation and SSTs to the space–time structure and propagation of the Madden–Julian oscillation (MJO). This improvement is associated with a more realistic convergence structure and longitudinal gradient of SST relative to MJO deep convection. In the uncoupled SP-CAM, SST is roughly in phase with the MJO convective center and the development of the Kelvin wave response and boundary layer convergence east of the convective center is relatively weak. In the coupled SP-CAM, maxima in SST lead maxima in MJO convection by cycle. Coupling produces warmer SSTs, a stronger Kelvin wave response, enhanced low-level convergence, and increased convective heating ahead (east) of the MJO convective center. Convective development east of the MJO precipitation center is more favorable in the coupled versus the uncoupled version, resulting in more realistic organization and clearer eastward propagation of the MJO in the coupled SP-CAM.

The Madden–Julian oscillation wind-convection coupling and the role of moisture processes in the MM5 model

The Madden–Julian oscillation (MJO) produced by a mesoscale model is investigated using standardized statistical diagnostics. Results show that upper- and lower-level zonal winds display the correct MJO structure, phase speed (8 m s−1) and space–time power spectrum. However, the simulated free atmosphere moisture, outgoing longwave radiation and precipitation do not exhibit any clear MJO signal. Yet, the boundary layer moisture, moist static energy and atmospheric instability, measured using a moist static energy instability index, have clear MJO signals. A significant finding is the ability of the model to simulate a realistic MJO phase speed in the winds without reproducing the MJO wind-convection coupling or a realistic propagation in the free atmosphere water vapor. This study suggests that the convergence of boundary layer moisture and the discharge and recharge of the moist static energy and atmospheric instability may be responsible for controlling the speed of propagation of the MJO circulation.

Sensitivity of Intraseasonal Perturbations in SST to the Structure of the MJO

Abstract This study demonstrates that intraseasonal perturbations in sea surface temperature (SST′) in the western Pacific warm pool is very sensitive to the structure, as well as other characteristics, of the Madden–Julian oscillation (MJO). SST′ is simulated using a one-dimensional ocean mixed-layer model and idealized MJO surface forcing that mimics observations. The amplitude and phase of simulated SST′ and its sensitivity to precipitation all depend on the structure of the MJO. It is concluded that a realistic structure of simulated MJO in a coupled model is as vital as realistic phase speed and zonal scale to correctly interpreting the effect of air–sea interaction on the MJO.

Interpreting Low-Frequency Modes of Southern Hemisphere Atmospheric Variability as the Rotational Response to Divergent Forcing

Abstract The principal modes of Southern Hemisphere low-frequency variability have recently been calculated using a 39-yr record of 300-hPa streamfunction fields from the NCEP–NCAR reanalysis dataset. The authors attempt to interpret these modes as the rotational response to some divergent forcing. For a range of mean states the linearized barotropic vorticity equation (BVE) is used to solve for the divergent wind that would generate (or at least be consistent with) the observed vorticity modes. Several of these low-frequency modes can be generated by forcing the BVE with fairly simple divergent wind fields that could easily be interpreted as resulting from anomalous tropical convection. In particular this is found to be true for streamfunction anomalies with El Nino–Southern Oscillation (ENSO), high-latitude mode, South Pacific wave, and Madden–Julian oscillation structure. The authors speculate that it may be possible to relate these calculated divergent wind fields to recently observed OLR fields and h...