Madden–Julian Oscillation Wind Research Articles

Role of Dry Dynamics in the Maritime Continent Barrier Effect in the Madden Julian Oscillation

Abstract Eastward-moving moist deep convection and atmospheric circulation signals associated with the tropical Madden Julian Oscillation (MJO) sometimes break down as they cross the Maritime Continent region, but other times the signal propagates across the region maintaining amplitude or regaining it over the West Pacific Basin. This paper assesses the hypothesis that upper tropospheric zonal diffluence of the background wind over the Maritime Continent causes much of this Maritime Continent barrier effect and its variation over time, through two mechanisms. 1. By slowing down the MJO as stronger than average background upper tropospheric zonal wind over the Indian Ocean advects the MJO circulation signal westward, slowing its eastward advance, and 2. through zonal advection of background wind by subseasonal zonal wind across a region of zonal diffluence of the background wind, which advects background wind of the opposite sign to the MJO wind. Advection of the opposite-signed background wind counteracts the MJO wind and reduces its associated upper tropospheric mass divergence, weakening the mechanisms of the upper tropospheric Kelvin wave component of the MJO circulation. Composites of MJO-associated zonal wind and outgoing longwave radiation signals diminish as they cross the Maritime Continent region when the region’s background zonal winds are diffluent, and composites of data reconstructing the relevant advection terms reveal the direct action of the advection mechanisms.

Seasonal variability of eddy kinetic energy in the Banda Sea revealed by an ocean model: An energy budget perspective

Seasonal eddy kinetic energy (EKE) variability in the Banda Sea during 1993–2014 is studied from an energy budget perspective, based on the outputs of Ocean Forecasting Australian Model version 3. High EKE is confined within the upper 300 m of the western Banda Sea with the largest intensity exceeding 3 × 103 J/m2 in the northwest monsoon (NWM) season. In this strong EKE region during NWM, eddies derive almost two thirds of their kinetic energy from the direct wind power input (WP), with additional contributions from the barotropic (BT) and baroclinic instability (BC) of the background flow. Both WP and BT modulate the EKE seasonality and drive the peak energy during NWM, while BC strengthens in the southeast monsoon (SEM) season because of the intensified baroclinicity of the upper circulation. The westerly wind bursts associated with the Madden-Julian Oscillation (MJO), together with steep topography, facilitate more cyclonic eddy generation events in the western Banda Sea during NWM. During SEM, EKE becomes relatively moderate across the Banda Sea but with a regional peak to the southwest of the Buru Island, which can be attributed to island wake effect. Over the Banda Sea, WP, BT and BC contribute 74% (42%), 14% (19%) and 12% (39%) of total energy to EKE during NWM (SEM), respectively. The majority of EKE generated is diverged horizontally by pressure work and dissipated by turbulent viscosity, with dissipation depleting the most. This study highlights the importance of both monsoon and MJO wind forcing in generating EKE variability along the pathway of the Indonesian Throughflow.

The MJO-driven Indo-Pacific barotropic see-saw

Recent discovery of a see-saw in intraseasonal barotropic sea level driven by boreal winter Madden-Julian Oscillation winds in the tropical Indo-Pacific basin has renewed interest in the barotropic dynamics in the tropics, which was otherwise known to be prominently a region of baroclinic dynamics at intraseasonal timescales. In this study, using a reference model simulation and several sensitivity experiments, we provide comprehensive details of the dynamics associated with this see-saw. It is found that the narrow Indonesian straits are instrumental in relaying barotropic excitations from one basin to the other. The intraseasonal barotropic circulation associated with this see-saw dynamics appears well organized at basin scale, both in the Indian Ocean and in the Pacific Ocean. During the positive phase of the see-saw, the Madden-Julian Oscillation winds over the Maritime Continent drive a basin-wide anti-clockwise circulation of ∼2 Sv around the Australian Continent with the eastern arm of this circulation located along the west African coast. Concurrently, it also drives a clockwise circulation in the southern Pacific Ocean. This circulation pattern reverses during the negative phase of the see-saw. In the Indonesian Throughflow region, the relatively narrow Ombai Strait and Lombok Strait facilitate this circulation in comparison to the wider Timor Passage. We discuss and point out the significant differences in this flow pattern with the mean flow across the Indonesian seas. We also address how some regions in the northern Pacific Ocean are decoupled from this large-scale dynamics. We show that it takes about a week for both the basins to adjust and to establish the see-saw, once the Madden-Julian Oscillation winds reach the Maritime Continent.

Emergence of Madden-Julian oscillation precipitation and wind amplitude changes in a warming climate

The Madden-Julian oscillation (MJO) has profound impacts on weather and climate phenomena, and thus changes in its activity have important implications under human-induced global climate change. Here, the time at which the MJO change signal emerges from natural variability under anthropogenic warming is investigated. Using simulations of the Community Earth System Model version 2 large ensemble forced by the shared socioeconomic pathways SSP370 scenario, an increase in ensemble mean MJO precipitation amplitude and a smaller increase in MJO circulation amplitude occur by the end of the 21st century, consistent with previous studies. Notably, the MJO precipitation amplitude change signal generally emerges more than a decade earlier than that of MJO wind amplitude. MJO amplitude changes also emerge earlier over the eastern Pacific than other parts of the tropics. Our findings provide valuable information on the potential changes of MJO variability with the aim of improving predictions of the MJO and its associated extreme events.

Diurnal Cycle and Dipolar Pattern of Precipitation Over Borneo During the MJO: Linear Theory and Nonlinear Sensitivity Experiments

AbstractThe precipitation anomaly over Borneo features a dipolar pattern under the influence of the Madden‐Julian Oscillation (MJO). To understand the formation mechanisms of this pattern, a linear theory is developed to study the factors controlling its diurnal cycles and the dipolar pattern. The theory predicts that the prevailing wind is primarily responsible for the asymmetry over an idealized island, while the topography plays a critical role in the leeward convergence and convection asymmetry. The results are largely consistent with the observed composites of the dipole at Borneo. Nonlinear cloud‐permitting simulations are further conducted to test the effects of island topography and solar radiative heating in different MJO phases. The results show that the island topography can cause the mesoscale flow to split around the mountain due to the topographic blocking, and favor the development of the lee side sea breeze. These processes strengthen the low‐level convergence and convection at the leeside of the island during the late afternoon and night, which is very important to the formation of island dipolar precipitation anomaly. In contrast, the inland convergence is weakened and the dipole disappears when the terrain is flattened. The diurnal cycle of solar insolation is the dominant factor driving the land‐sea breeze circulation, which intensifies the island convection at the leeside. These results indicate that the MJO wind anomaly, island topography and solar insolation play distinct roles in the formation of the dipolar pattern of Borneo precipitation.

Topographic Influences on Diurnally Driven MJO Rainfall Over the Maritime Continent

AbstractThis work investigates impacts of the Madden‐Julian Oscillation (MJO) on the daily and diurnal rainfall over the Maritime Continent, with emphasis on the influences of topography over Sumatra, Borneo and New Guinea. Eighty‐nine MJO events were identified during 2001–2019 using IMERG satellite data. Daily and diurnal rainfall maxima on the east side of topography lag the west side as the MJO passes. In addition, the island vanguard (pre‐MJO) rain is more convective, while the island rearguard (behind the main MJO body) rain is more stratiform. The timing and magnitude of diurnal rainfall is defined using the maximum hourly rain rate instead of the first diurnal harmonic to avoid smoothing. While a single sharp peak is observed over the mountains around 19 LT, a much broader delayed peak occurs over land to the west of topography and two peaks are observed over land to the east of topography at 15 and 2 LT. Rain peaks offshore from 3 to 7 LT. Cluster analysis shows that the afternoon and nighttime rainfall peaks are highest before the MJO arrives, then gradually decrease and become earlier on the west side of the topography, whereas the afternoon (nighttime) peak east of the topography is enhanced before (after) the MJO arrives. The contrasting east‐west features can be attributed to topographic influences on the moisture flux convergence of the mean column moisture by MJO‐induced winds. The MJO wind modulation of diurnal rainfall over most of the open ocean areas is insignificant.

The Role of Background Meridional Moisture Gradient on the Propagation of the MJO over the Maritime Continent

AbstractThis 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.

MJO-Induced Intraseasonal Mixed Layer Depth Variability in the Equatorial Indian Ocean and Impacts on Subsurface Water Obduction

AbstractChange of oceanic surface mixed layer depth (MLD) is critical for vertical exchanges between the surface and subsurface oceans and modulates surface temperature variabilities on various time scales. In situ observations have documented prominent intraseasonal variability (ISV) of MLD with 30–105-day periods in the equatorial Indian Ocean (EIO) where the Madden–Julian oscillation (MJO) initiates. Simulation of Hybrid Coordinate Ocean Model (HYCOM) reveals a regional maximum of intraseasonal MLD variability in the EIO (70°–95°E, 3°S–3°N) with a standard deviation of ~14 m. Sensitivity experiments of HYCOM demonstrate that, among all of the MJO-related forcing effects, the wind-driven downwelling and mixing are primary causes for intraseasonal MLD deepening and explain 83.7% of the total ISV. The ISV of MLD gives rise to high-frequency entrainments of subsurface water, leading to an enhancement of the annual entrainment rate by 34%. However, only a small fraction of these entrainment events (<20%) can effectively contribute to the annual obduction rate of 1.36 Sv, a quantification for the amount of resurfacing thermocline water throughout a year that mainly (84.6%) occurs in the summer monsoon season (May–October). The ISV of MLD achieves the maximal intensity in April–May and greatly affects the subsequent obduction. Estimation based on our HYCOM simulations suggests that MJOs overall reduce the obduction rate in the summer monsoon season by as much as 53%. A conceptual schematic is proposed to demonstrate how springtime intraseasonal MLD deepening events caused by MJO winds narrow down the time window for effective entrainment and thereby suppress the obduction of thermocline water.

Atmospheric River Lifecycle Responses to the Madden‐Julian Oscillation

AbstractWe investigate how the Madden‐Julian Oscillation (MJO), the dominant mode of tropical subseasonal variability, modulates the lifecycle of cool‐season North Pacific atmospheric rivers (ARs). When the enhanced (suppressed) convection center is located over the Indian Ocean (western Pacific), more AR events originate over eastern Asia and with fewer over the subtropical northern Pacific. When the enhanced (suppressed) convection is over the western Pacific (Indian Ocean), the opposite changes occur, with more AR events originate over the subtropical northern Pacific and fewer over eastern Asia. Dynamical processes involving anomalous MJO wind and seasonal mean moisture are found to be the dominant factors impacting these variations in AR origins. The MJO‐related anomalous geopotential height patterns are also shown to modulate the propagation of the AR events. These MJO–AR lifecycle relationships are further supported by model simulations.

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.

Investigating Recent Changes in MJO Precipitation and Circulation in Multiple Reanalyses

AbstractRecent work using CMIP5 models under RCP8.5 suggests that individual multimodel mean changes in precipitation and wind variability associated with the Madden‐Julian oscillation (MJO) are not detectable until the end of the 21st century. However, a decrease in the ratio of MJO circulation to precipitation anomaly amplitude is detectable as early as 2021–2040, consistent with an increase in dry static stability as predicted by weak temperature gradient balance. Here, we examine MJO activity in multiple reanalyses (ERA5, MERRA‐2, and ERA‐20C) and find that MJO wind and precipitation anomaly amplitudes have a complicated time evolution over the record. However, a decrease in the ratio of MJO circulation to precipitation anomaly amplitude is detected over the observational period, consistent with the change in dry static stability. These results suggest that weak temperature gradient theory may be able to help explain changes in MJO activity in recent decades.

MJO Wind Energy and Prediction of El Niño

AbstractThis study applies the energetic framework developed to quantify the interaction between the Madden‐Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO) to El Niño forecasting. The MJO wind power, measured by the covariability of MJO‐related wind stress and oceanic Kelvin wave activity, is combined with the sea surface temperature (SST) anomalies, and two new indices are proposed. The MJO‐Kelvin wave‐ENSO (MaKE) index is proposed as an ENSO predictor and is shown to slightly outperform Niño 3.4 when applied to observational data sets and to greatly outperform Niño 3.4 when applied to CFSv2 reforecasts of the years 1980–2014. The MJO‐Kelvin wave Influence (MaKI) index is proposed to predict MJO influence on developing El Niño events. This index performs reasonably well when applied to observations. The forecast skill of MaKI in the CFSv2 reforecasts suggests that this model does not predict the observed MJO‐ENSO relationship as measured by this index.

Prospects for Erratic and Intensifying Madden-Julian Oscillations

The Madden–Julian Oscillation (MJO) is a planetary-scale convective disturbance that typically forms in the equatorial Indian Ocean, propagates slowly eastward, and dissipates near the date line. This study examines how the MJO changes in response to a changing radiative forcing in a fully-Lagrangian coupled model (LCM) that is shown to simulate robust and realistic MJOs. After the LCM is spun up for 160 years to reproduce the late 20th century climate, non-water-vapor longwave optical depth is increased over 70 years to model the effects of increasing concentrations of greenhouse gases. The model is then run for another 30 years without additional changes to the radiative forcing. After the radiative forcing is modified, the MJO generally becomes more frequent and intense, but it is also more variable from one year to the next. Not only do composite MJO rainfall perturbations increase, but wind, temperature, and moisture perturbations also become stronger. The aspect of the MJO’s structure that changes the most is the largely dry equatorial Kelvin wave circulation that circumnavigates the globe between moist phases of the MJO. Potential impacts of these changes included alterations to the way in which the MJO modulates tropical cyclones, monsoon disturbances, and El Niño.

Role of Maritime Continent Land Convection on the Mean State and MJO Propagation

AbstractThe 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.

A Damping Effect of the Maritime Continent for the Madden‐Julian Oscillation

AbstractThe damping effect of the Maritime Continent (MC) on propagation of the Madden‐Julian Oscillation (MJO) has been widely recognized; however, its underlying physics remains largely elusive. Capitalizing on the latest high‐resolution reanalysis data (ERA‐5) from the ECMWF, analyses in this study suggest that the interruption of lower‐tropospheric moistening over the MC land during MJO eastward propagation is mainly due to a drying effect induced by zonal advection of the low‐level winter mean moisture by MJO winds. The low‐level mean moisture pattern over the MC tends to closely follow local topography, with moisture maxima collocated with local mountain peaks. Given this mean moisture distribution, the moisture advection by anomalous easterly MJO winds corresponding to the active MJO convection over the eastern Indian Ocean will lead to a drying (moistening) effect to the east (west) of the mountain peaks. Diagnosis based on hourly output from ERA‐5 further illustrates that the diurnal cycle plays a crucial role for the formation of the topographically locked mean moisture pattern over the MC. This study, therefore, indicates that high‐resolution models capable of resolving detailed topography and associated diurnal cycle over the MC could be necessary for realistic depiction of the interaction between the MC and MJO.

Influence of Subseasonal Variability on the Diurnal Cycle of Precipitation on a Mountainous Island: The Case of New Caledonia

Abstract The relationship between the large-scale intraseasonal variability, synoptic wind regimes, and the local daily variability of precipitation over the main island of New Caledonia (southwest tropical Pacific) is investigated with a focus on the austral summer wet season (November–April). The average diurnal cycle of precipitation over the island is characterized by a sharp afternoon maximum around 1600 local time, with significant differences between the windward east coast, the leeward west coast, and the mountain range. The afternoon peak is related to the afternoon sea-breeze circulation and to the diurnal cycle of convection over land. In general, its magnitude follows the same evolution as the daily mean. In agreement with past studies, a clear modulation of the Madden–Julian oscillation (MJO) on both the diurnal cycle of precipitation and the probability of occurrence of four robust wind regimes can be identified in the New Caledonia region during the wet season. From the evidence that there is a qualitative correspondence between the effects of both the MJO phases and the wind regimes on features in the diurnal cycle of precipitation, a simple model is proposed to inspect the MJO forcing mediated by wind regimes on the diurnal variability of rain. The complete decomposition of the MJO impact shows that the modulation of diurnal cycle by the MJO relies on complex interactions between the MJO and synoptic winds that involve both large-scale MJO convective anomalies and MJO-induced modification of wind patterns.

Transient Response of MJO Precipitation and Circulation to Greenhouse Gas Forcing

AbstractRecent studies have shown that Madden‐Julian oscillation (MJO) precipitation anomaly amplitude tends to increase while associated circulations weaken at the end of 21st century in Coupled Model Intercomparison Project phase 5 models under Representative Concentration Pathway 8.5. Transient changes of MJO characteristics earlier in the 21st century have received less attention. In this study, changes of MJO precipitation and circulation amplitude during these interim time periods under Representative Concentration Pathway 8.5 are examined in Coupled Model Intercomparison Project phase 5 models. Multimodel mean changes in MJO precipitation and circulation amplitude are not individually detectable in the early and middle 21st century relative to the historical period (1986–2005). However, robust multimodel mean decreases in the ratio of MJO wind to precipitation anomalies occur even early in the 21st century. This decreased ratio is explained by increasingly large tropical static stability as the climate warms, which under weak temperature gradient balance mandates that a diabatic heating anomaly is balanced by an increasingly weaker circulation anomaly. These results suggest the robustness of weak temperature gradient theory for explaining MJO dynamics, not only in an equilibrium climate but also in the transient response.

Revisiting MJO, Kelvin waves, and El Niño relationships using a simple ocean model

Mechanisms governing interactions between the Madden–Julian Oscillation (MJO), Kelvin wave activity, and El Nino development are reexamined using the oceanic component of the Zebiak–Cane (ZCocn) model of the Pacific basin. Prescribed wind stress from a free run of the super-parameterized Community Climate System Model version 4 (SP-CCSM4) is used to force ZCocn and the simulated El Nino events are analyzed with respect to their relationship with the MJO wind forcing. Composites of El Nino events strongly influenced by the MJO show the earlier onset of a flattened, El Nino-like state of the thermocline. In contrast, the composites of El Nino events not influenced by the MJO winds show a later onset, and are dominated by a transient-like thermocline along with periods of upwelling Kelvin wave activity. Sensitivity experiments performed to identify whether modifying MJO wind stress and oceanic Kelvin wave activity influences these features show that although MJO contributes to the development of these features, it is not necessarily the primary driver. The relative phasing between MJO and oceanic Kelvin wave activity seems to be the most important factor governing the influence of MJO on El Nino. When in phase and collocated with Kelvin wave activity, MJO westerly wind stress contributes to the amplification of preexisting downwelling Kelvin waves, leading to earlier onset and greater strength of the resulting El Nino events. The out-of-phase interactions between MJO and oceanic Kelvin waves explain the observed lack of influence of MJO onto some El Nino events.

Representation of the Madden–Julian Oscillation in CAMS-CSM

The Madden–Julian Oscillation (MJO) has a significant impact on global weather and climate and can be used as a predictability resource in extended-term forecasting. We evaluate the ability of the Chinese Academy of Meteorological Sciences Climate System Model (CAMS-CSM) to represent the MJO by using the diagnostic method proposed by the US Climate Variability and Predictability Program (CLIVAR) MJO Working Group (MJOWG). In general, the model simulates some major characteristics of MJO well, such as the seasonality characteristics and geographical dependence, the intensity of intraseasonal variability (ISV), dominant periodicity, propagation characteristics, coherence between outgoing longwave radiation (OLR) and wind, and life cycle of MJO signals. However, there are a few biases in the model when compared with observational/reanalyzed data. These include an overestimate of precipitation in the convergence zone of the North and South Pacific, a slightly weaker eastward propagation, and a shift in the dominant periodicity toward lower frequencies with slower speeds of eastward propagation. The model gives a poor simulation of the northward propagation of MJO in summer and shows less coherence between the MJO convection and wind. The role of moistening in the planetary boundary layer (PBL) in the eastward/northward propagation of MJO was also explored. An accurate representation of the vertical titling structure of moisture anomalies in CAMS-CSM leads to moistening of the PBL ahead of convection, which accounts for the eastward/northward propagation of MJO. Poor simulation of the vertical structure of the wind and moisture anomalies in the western Pacific leads to a poor simulation of the northward propagation of MJO in this area. Budget analysis of the PBL integral moisture anomalies shows that the model gives a good simulation of the moisture charging process ahead of MJO convection and that the zonal advection of moisture convergence term has a primary role in the detour of MJO over the Maritime Continent.

Changes in Madden‐Julian Oscillation Precipitation and Wind Variance Under Global Warming

AbstractThe Madden‐Julian oscillation (MJO) is the leading mode of tropical intraseasonal variability, having profound impacts on many weather and climate phenomena across the tropics and extratropics. Previous studies using a limited number of models have suggested complex changes in MJO activity in a warmer climate. While most studies have argued that MJO precipitation amplitude will increase in a future warmer climate, others note that this is not necessarily the case for MJO wind variability. This distinction is important since MJO wind fluctuations are responsible for producing remote impacts on extreme weather through teleconnections. In this study, we examine projected changes of MJO precipitation and wind variance at the end of the 21st century in Representative Concentration Pathway 8.5 using the multimodel Coupled Model Intercomparison Project phase 5 data set. Under global warming, most models show an increase in MJO band precipitation variance, while wind variability decreases. The discrepancy between MJO precipitation and wind variance changes under global warming is shown to be due to increases in tropical static stability in a warmer climate. The multimodel mean shows a 20% increase in both the 500‐hPa vertical tropical dry static energy gradient and the ratio of intraseasonal precipitation to 500 hPa omega fluctuations, consistent with scaling by weak temperature gradient theory. These results imply that tropical static stability increases may weaken the MJO's ability to influence extreme events in future warmer climate by weakening wind teleconnections, even though MJO precipitation amplitude may increase.