Barrier Effect of the Indo-Pacific Maritime Continent on the MJO: Perspectives from Tracking MJO Precipitation

<p>The Madden-Julian Oscillation (MJO) is one of the leading sources of tropical and extra-tropical predictability on subseasonal-to-seasonal timescales, but numerical models often suffer from systematic errors in capturing the MJO dynamics. Large-scale convection associated with the MJO is initiated over the Indian Ocean and propagates eastward across the Maritime Continent (MC) and into the western Pacific. As an MJO event enters the MC, it often weakens or completely dissipates due to complex interactions between the large-scale MJO and the MC landmass and its topography. This MC barrier effect is responsible for the dissipation of 40-50% of observed MJO events, though the exact nature of the barrier effect is unclear. Common mechanisms include the physical barrier of the islands of the MC, and the dynamical barrier of strong diurnally driven circulations that exist around those islands. The MC barrier effect is often exaggerated in when it comes to MJO prediction.</p><p>In this study, we examine convection-permitting, atmosphere-ocean coupled model simulations of an MJO event to determine how the MJO responds to physical and dynamical changes implemented over the MC region. In addition to the control simulation with real topography, we introduce two idealized simulations – (1) where we flatten the topography of the MC to sea level, but leave the land-sea distribution as is, and (2) where we entirely remove the MC islands and replace them with a 50-m deep ocean. How the MJO responds to the implemented changes can help us determine whether some physical processes that occur over the MC are more detrimental to MJO propagation than others. The differences between the control simulation and the first scenario can tell us about the physical barrier effect of the MC on MJO propagation. The complete removal of land in the second scenario also removes the diurnal changes associated with air-sea boundaries (e.g., land-sea breezes and convergence zones between islands), exploring whether the barrier effect of the MC on the MJO is more dynamically driven.</p><p>Results show that flattening the MC terrain only has a small impact on large-scale MJO characteristics. However, as expected, removing the land, and diurnal cycle associated with it, drastically smooths the MJO’s propagation and the produced MJO shows no sign of dissipation over the MC region. We examine the model simulations to gain insight on what physical processes are behind the changes among model simulations and to expose some modeling difficulties that could contribute to numerical models’ exaggerating the effects of the MC barrier effect.</p>

Deforestation is a major issue affecting both regional and global hydroclimates. This study investigated the effect of deforestation in the Maritime Continent (MC) on tropical intraseasonal climate variability. Using a global climate model with credible Madden–Julian oscillation (MJO) simulations, we examined the effect of deforestation over the MC region by replacing the forest canopy with grassland. The results revealed that under constant orographic and land–sea contrast forcing, the modification of the canopy over the MC altered the characteristics of the MJO. We noted the amplification of the MJO and increases in wet–dry fluctuation and the zonal extent. We analyzed more than 100 MJO cases by performing K-means clustering and determined that the continuous propagation of the MJO over the MC increased from 35% in the control experiment to 61% in the deforestation experiment. This phenomenon of less blocked MJO over the MC in the deforestation run was associated with more substantial precipitation, increased soil moisture, and a suppressed diurnal cycle in land convection. Furthermore, when the MJO convection was over the Indian Ocean (IO), we observed the enhancement of low-level moisture over the MC region in the deforestation experiment. Grassland surface forcing provides a thermodynamic source for triggering instability in the atmosphere, resulting in low-level moisture convergence. The MJO exhibited a stronger energy recharge–discharge cycle in the deforestation experiment than in the control experiment, and this difference between the experiments enlarged as the MJO progressed from the IO to MC.

This study explores possible mechanisms for the barrier effect of the Indo-Pacific Maritime Continent (MC) on MJO propagation. In particular, this study examines whether similar mechanisms can be found in both observations and CMIP5 simulations. All models simulate individual MJO events but underestimate the percentage of MJO events propagating into the MC. The simulations are grouped into the top and bottom 50% based on their capability of reproducing the MJO spectral signal. When compared with the observations, the bottom 50% of the simulations significantly underestimate the MJO strength and exaggerate the barrier effect intensity, whereas these discrepancies are not significant in the top 50% of the simulations. From the top 50% of the simulations, the MJO strength, moisture processes, and surface evaporation in the MC all play important roles in constituting the barrier effect. No such evidence is found in observations. The discrepancies may come from small observed sample size and/or misrepresentations of key physical processes in the models. A consistent result is found in the observations and simulations: Whether MJO events can cross the MC depends on the degree to which dominant precipitation over land shifts to over water in the MC as MJO convection centers approach the MC and cross it. This result emphasizes the critical role of precipitation over water in carrying convective signals of the MJO through the MC. The results suggest that diagnosing the model alone on mechanisms for the barrier effect could be misleading; further investigations using a combination of observations, global gridded data, and high-resolution models are needed.

Deforestation is a major issue affecting both regional and global hydroclimates. This study investigated the effect of deforestation in the Maritime Continent (MC) on tropical intraseasonal climate variability. Using a global climate model with Madden–Julian Oscillation (MJO) simulations, we examined the effect of deforestation over the MC region by replacing the forest canopy with grassland. The results revealed that under constant orographic and land–sea contrast forcing, the modification of the canopy over the MC altered the characteristics of the MJO. We noted the amplification of the MJO and increases in wet–dry fluctuation and the zonal extent. We analyzed more than 100 MJO cases by performing K-means clustering and determined that the continuous propagation of the MJO over the MC increased in 35% and 61% of the total 110 cases in the control and deforestation experiments, respectively. This phenomenon was associated with more substantial vanguard precipitation, increased soil moisture, and a suppressed diurnal cycle in land convection. Furthermore, when the MJO convection was over the Indian Ocean (IO), we observed the enhancement of low-level moisture over the MC region in the deforestation experiment. Grassland surface forcing provides a thermodynamic source for triggering instability in the atmosphere, resulting in low-level moisture convergence. The MJO exhibited a stronger energy recharge–discharge cycle in the deforestation experiment than in the control experiment, and this difference between the experiments enlarged from the IO to MC. 

While the Madden–Julian Oscillation (MJO) has been shown to affect tropical cyclones (TCs) worldwide through its modulation of large-scale circulation in the atmosphere, little or no role for the ocean has been identified to date in this influence of MJO on TCs. Using observations and numerical model simulations, we demonstrate that MJO events substantially impact TCs over the Maritime Continent (MC) region through an oceanic pathway. While propagating across the MC region, MJO events cause significant sea surface cooling with an area-averaged value of about 0.35 ± 0.12 °C. Hence, TCs over the MC region immediately following the passage of MJO events encounter considerably cooler sea surface temperatures. Consequently, the enthalpy fluxes under the storms are reduced and the intensification rates decrease by more than 50% on average. These results highlight an important role played by the ocean in facilitating MJO-induced sub-seasonal variability in TC activity over the MC region.

Possible effects of the diurnal cycle in land convection on propagation of the Madden–Julian Oscillation over the Indo‐Pacific Maritime Continent (MC) were investigated using satellite observations. Four features distinguishable from their respective climatology are uniquely associated with MJO events that cross the MC: strong precipitation over land as their convection centers approach the MC, subsequent increased soil moisture, reduced diurnal amplitude of land convection, and the dominance of precipitation over water by nondiurnal convection as their convection centers move over the MC. These results provide observational evidence for a proposed MAritime Continent Convective diurnal Cycle mechanism in which the diurnal cycle in land convection acts as an intrinsic barrier effect on MJO propagation over the MC.

The Maritime Continent (MC) region is known as a “barrier” in the life cycle of the Madden–Julian oscillation (MJO). During boreal winter, the MJO detours the equatorial MC land region southward and propagates through the oceanic region. Also, about half of the MJO events that initiate over the Indian Ocean cease around the MC. The mechanism through which the MC affects MJO propagation, however, has remained unanswered. The current study investigates the MJO–MC interaction with a particular focus on the role of MC land convection. Using a global climate model that simulates both mean climate and MJO realistically, we performed two sensitivity experiments in which updraft plume radius is set to its maximum and minimum value only in the MC land grid points, making convective top deeper and shallower, respectively. Our results show that MC land convection plays a key role in shaping the 3D climatological moisture distribution around the MC through its local and nonlocal effects. Shallower and weaker MC land convection results in a steepening of the vertical and meridional mean moisture gradient over the MC region. The opposite is the case when MC land convection becomes deeper and stronger. The MJO’s eastward propagation is enhanced (suppressed) with the steeper (lower) mean moisture gradient. The moist static energy (MSE) budget of the MJO reveals the vertical and meridional advection of the mean MSE by MJO wind anomalies as the key processes that are responsible for the changes in MJO propagation characteristics. Our results pinpoint the critical role of the background moisture gradient on MJO propagation.

Using the outgoing long-wave radiation (OLR) data from NOAA, the ERA-Interim reanalysis products from ECMWF, and daily observations at 756 stations provided by National Meteorological Information Center of China Meteorology Administration, we have examined the relationships of interannual variations of summer precipitation in Southwest China with the convective activity in the Maritime Continent (MC) region by employing the singular value decomposition (SVD) method and the regional climate model RegCM4.4. The first SVD mode (here after SVD1) indicates that high precipitation anomalies in Southwest China correspond to abnormally weak convective activity in northeastern MC and strong convective activity in Southwestern MC if the time-series of coefficients of SVD1 is in positive phase. When convective activity are anomalously strong in Southern MC, the anomalous divergence at 700 hPa and convergence at 200 hPa are observed over the tropical western Pacific and South China Sea. The propagation of wave energy at 700 hPa from Western Europe and the tropics provide a favorable condition for inducing more precipitation in most of Southwest China. The atmospheric water vapor transport from northern Indochina and the South China Sea to Southwest China intensifies while the western Pacific subtropical high is stronger than normal and extends more westward. All these results along with the simulations demonstrate that the summer precipitation in Southwest China is significantly affected by the convective activity over the MC region. These results above are helpful for our better understanding the role of the MC in regulating the summer climate in Southwest China.

The Madden–Julian oscillation (MJO) is often observed to weaken or sometimes completely decay as its convective anomaly moves from the Indian Ocean over to the Maritime Continent (MC), which is known as the MC barrier effect on the MJO. This barrier effect is often exaggerated in numerical models. Using 23 years of the retrospective intraseasonal forecast from two coupled model systems with useful MJO prediction skills, we show that the predictive skill of the real-time multivariate MJO (RMM) index for the continuously propagating MJO events across the MC region is higher than for the blocked MJO events. The greater prediction skill is not related to the higher initial RMM amplitude for the continuous MJO events. Rather the higher skill arises from the more persistent behavior of the propagating MJO events as the convective anomaly moves through the MC region into the western Pacific. The potential predictability is similar for both types of MJO events, suggesting the forecast models hardly differentiate the two types of MJO events in prediction; they only maintain higher RMM magnitudes of the continuously propagating events. The global reanalysis dataset indicates that the blocked events are often associated with persistent higher surface pressures over colder sea surface temperatures in the central Pacific, suggesting the large-scale environment plays a role in promoting or inhibiting the MJO propagation across the MC region. Caveats in the models to reproduce the observed MJO events are also discussed.

This study investigates the role of the background meridional moisture gradient (MMG) on the propagation of the Madden–Julian Oscillation (MJO) across the Maritime Continent (MC) region. It is found that the interannual variability of the seasonal mean MMG over the southern MC area is associated with the meridional expansion and contraction of the moist area in the vicinity of the MC. Sea surface temperature anomalies associated with relatively high and low seasonal mean MMG exhibit patterns that resemble those of the El Niño–Southern Oscillation. By contrasting the years with anomalously low and high MMG, we show that MJO propagation through the MC is enhanced (suppressed) in years with higher (lower) seasonal mean MMG, though the effect is less robust when MMG anomalies are weak. Column-integrated moisture budget analysis further shows that sufficiently large MMG anomalies affects MJO activity by modulating the meridional advection of the mean moisture via MJO wind anomalies. Our results suggest that the background moisture distribution has a strong control over the propagation characteristics of the MJO in the MC region.

Eastward propagation of the Madden‐Julian Oscillation (MJO) detours the Maritime Continent (MC) region southward during austral summer, exhibiting enhanced convective activity preferentially in the southern part of the MC area with much weaker anomalies in the central MC area. Column‐integrated moist static energy budget of the MJO is analyzed to understand the processes responsible for the MJO detouring. Results show that zonal and meridional moisture advection is the essential process to the MJO detouring, causing the difference between the southern and central MC regions in the moisture recharge before and during the MJO onset. Further analysis reveals that moisture advection by MJO perturbation winds acting upon the background moisture gradient has the dominant contribution to the regional contrast between the central and southern MC areas. The zonal moisture advection is greater in the southern MC region because the zonal gradient of the background moisture field is much steeper in the southern MC area than in the central MC area. The onset of the Australian monsoon in austral summer contributes to the establishment of the sharp zonal moisture gradient in the southern MC region. The meridional moisture advection is weaker in the central MC area because meridional wind anomalies associated with the MJO vary regionally, presumably through interactions with the topography and land‐sea contrast.

The “barrier effect” of the Maritime Continent (MC) is a known hurdle in understanding the propagation of the Madden–Julian oscillation (MJO). To understand the differing dynamics of MJO events that propagate versus stall over the MC, a new tracking algorithm utilizing 30–96-day-filtered NOAA Interpolated OLR anomalies is presented. Using this algorithm, MJO events can be identified, tracked, and described in terms of their propagation characteristics. Latent heat flux from OAFlux and CYGNSS surface winds and fluxes are compared for MJO events that do and do not propagate through the MC. Events that successfully propagate through the MC demonstrate regional surface flux anomalies that are stronger, more spatially coherent, and have a larger fetch. The spatial scale of convective anomalies for events that successfully propagate through the MC region is also larger than for terminating events. Large-scale enhancement of latent heat fluxes near and to the east of the date line, equally driven by dynamic and thermodynamic effects, also accompanies MJO events that successfully propagate through the MC. These findings are placed in the context of recent theoretical models of the MJO in which latent heat fluxes are important for propagation and destabilization.

The Maritime Continent (MC) is characterized by a seasonal evolution of rainfall distinct from other regions, due to its unique land–sea distribution and topography. In this study, the roles of surface properties and terrains in controlling the regional climatological rainfall were investigated, based on general circulation model experiments. Results show that the existence of terrain can increase the MC land (MCL) rainfall mainly through its dynamical lifting effect, but otherwise has only moderate influence on rainfall over the MC ocean (MCO). On the other hand, the impact of MC land–sea distribution on the regional rainfall is more seasonally dependent. When replacing the MC flat-land with ocean, rainfall is significantly increased over both MCL and MCO during boreal summer-to-fall, but not in the winter-to-spring season. Further inspection showed that by eliminating the MC flat-land, there is enhanced atmospheric water vapor and convective instability in the summer-to-fall period, contributing to a dramatic increase in precipitation. On the other hand, changes in convective instability and atmospheric water vapor over the MCL act to counteract each other, leading to an only moderate change in rainfall during boreal winter-to-spring. The model-based results suggest that this seasonally dependent influence of the MC flat-land on regional climate mean rainfall is not determined by its modulation on the diurnal cycle. Our results also suggest a larger sensitivity of model bias in representing land–sea distribution/fraction over the MC region during the dry season (i.e. boreal summer) than in the wet season (i.e. boreal winter).

Based on the observation and reanalysis data, the relationship between the Madden–Julian Oscillation (MJO) over the Maritime Continent (MC) and the tropical Pacific–Indian Ocean associated mode was analyzed. The results showed that the MJO over the MC region (95°–150° E, 10° S–10° N) (referred to as the MC–MJO) possesses prominent interannual and interdecadal variations and seasonally “phase-locked” features. MC–MJO is strongest in the boreal winter and weakest in the boreal summer. Winter MC–MJO kinetic energy variation has significant relationships with the El Niño–Southern Oscillation (ENSO) in winter and the Indian Ocean Dipole (IOD) in autumn, but it correlates better with the tropical Pacific–Indian Ocean associated mode (PIOAM). The correlation coefficient between the winter MC–MJO kinetic energy index and the autumn PIOAM index is as high as −0.5. This means that when the positive (negative) autumn PIOAM anomaly strengthens, the MJO kinetic energy over the winter MC region weakens (strengthens). However, the correlation between the MC–MJO convection and PIOAM in winter is significantly weaker. The propagation of MJO over the Maritime Continent differs significantly in the contrast phases of PIOAM. During the positive phase of the PIOAM, the eastward propagation of the winter MJO kinetic energy always fails to move across the MC region and cannot enter the western Pacific. However, during the negative phase of the PIOAM, the anomalies of MJO kinetic energy over the MC is not significantly weakened, and MJO can propagate farther eastward and enter the western Pacific. It should be noted that MJO convection is more likely to extend to the western Pacific in the positive phases of PIOAM than in the negative phases. This is significant different with the propagation of the MJO kinetic energy.

<p>The environment of the Maritime Continent (MC) is known to be affected by numerous interactions between local and large-scale weather modes. Here, the primary mode of variability in tropospheric winds – the diurnal cycle – is investigated based on Equatorial Atmosphere Radar (EAR) which provides powerful information about tropospheric dynamics over a wide range of altitudes, with high temporal frequency and over a long period.</p><p>The study focuses on mean profiles for the three wind components, as well as their decomposition into diurnal and semi-diurnal cycles. The mean diurnal evolution of winds during boreal winter as well as variability associated with assorted weather phenomena has been investigated. Interannual modes such as Quasi-Biennial Oscillation (QBO), ENSO and Indian Ocean Dipole (IOD) were analyzed. On subseasonal time scale, the effects of Madden-Julian Oscillations (MJO) and convectively coupled Kelvin waves (CCKW) on diurnal wind evolution were studied. All of the above-mentioned weather phenomena are known to affect precipitation patterns across the MC region. This analysis contributes to understanding of physical processes responsible for such interactions. Obtained results were compared against the ERA-5 reanalysis.</p><p>The results show a large discrepancy between the vertical wind profiles between EAR and reanalysis. The observed variability in the vertical profiles of wind components was related to the temperature profile and the occurrence of cumulus congestus clouds in the MC area. Furthermore, a substantial effect of ENSO phase, as well as MJO and CCKW on the magnitude of diurnal and semi-diurnal cycle amplitudes, was observed at all altitudes. Meanwhile, it is found that the influence of IOD is imperceptible, while QBO effects are limited to levels above 200mb. It is noteworthy that the described impacts are larger in the EAR observations than in the reanalysis data.</p>