Madden–Julian Oscillation Amplitude Research Articles

Responses of the Madden–Julian Oscillation to Global Warming: Impacts from Tropical Sea Surface Temperature Changes

Abstract The impacts of tropical sea surface temperature (SST) changes on the Madden–Julian oscillation (MJO) are investigated using the large-ensemble simulation from the Community Earth System Model version 2 (CESM2-LE) under the shared socioeconomic pathway (SSP370 scenario). Three SST change patterns are featured, distinguished by the zonal gradient of the change in the equatorial Pacific warming. MJO characteristics and its teleconnections responses are composited for the clusters, and their relationships to the zonal SST gradient changes are examined. Results show that the anomalously strong El Niño–like SST change pattern significantly intensifies the MJO amplitude and enhances its eastward extension compared to the anomalously weak El Niño–like SST change pattern. These changes in MJO amplitude are further interpreted through the α framework. We also found no statistically different extratropical geopotential height responses to MJO between the three SST warming patterns, possibly due to strong internal climate variability. Changes in Rossby wave source between clusters also show a weak relationship with the MJO teleconnections. Our results highlight the importance of Indo-Pacific zonal SST gradient changes on the changes of MJO but limited impacts on MJO teleconnections to the midlatitudes.

MJO Influence on Subseasonal-to-Seasonal Prediction in the Northern Hemisphere Extratropics

Abstract The impacts of the Madden–Julian oscillation (MJO) on the subseasonal-to-seasonal (S2S) prediction in the Northern Hemisphere extratropics are examined using the reforecasts from the S2S Project and Subseasonal Experiment Project (SubX). When forecasts are initialized during an active MJO, extratropical prediction skill becomes significantly higher at 3–4-week windows compared to inactive MJO. Such prediction skill improvement is evident in the 500-hPa geopotential height over the Pacific–North America region and the North Atlantic and in surface temperature over North America, especially when the model is initialized during the MJO phases 6–7 and 8–1. However, the extratropical prediction skill is not modulated by the MJO phases 2–3 and 4–5. This phase dependency is likely determined by the arrival time of the MJO at the Maritime Continent (MC) barrier that substantially enhances the MJO amplitude error. This result suggests that only MJO phases whose convection lies east of the MC are a source of wintertime S2S predictability in the extratropics.

The Role of Subtropical Rossby Waves in Amplifying the Divergent Circulation of the Madden–Julian Oscillation

Abstract The composite structure of the Madden–Julian oscillation (MJO) has long been known to feature pronounced Rossby gyres in the subtropical upper troposphere, whose existence can be interpreted as the forced response to convective heating anomalies in the presence of a subtropical westerly jet. The question of interest here is whether these forced gyre circulations have any subsequent effects on divergence patterns in the tropics and the Kelvin-mode component of the MJO. A nonlinear spherical shallow water model is used to investigate how the introduction of different background jet profiles affects the model’s steady-state response to an imposed MJO-like stationary thermal forcing. Results show that a stronger jet leads to a stronger Kelvin-mode response in the tropics up to a critical jet speed, along with stronger divergence anomalies in the vicinity of the forcing. To understand this behavior, additional calculations are performed in which a localized vorticity forcing is imposed in the extratropics, without any thermal forcing in the tropics. The response is once again seen to include pronounced equatorial Kelvin waves, provided the jet is of sufficient amplitude. A detailed analysis of the vorticity budget reveals that the zonal-mean zonal wind shear plays a key role in amplifying the Kelvin-mode divergent winds near the equator, with the effects of nonlinearities being of negligible importance. These results help to explain why the MJO tends to be strongest during boreal winter when the Indo-Pacific jet is typically at its strongest. Significance Statement The MJO is a planetary-scale convectively coupled equatorial disturbance that serves as a primary source of atmospheric predictability on intraseasonal time scales (30–90 days). Due to its dominance and spontaneous recurrence, the MJO has a significant global impact, influencing hurricanes in the tropics, storm tracks, and atmosphere blocking events in the midlatitudes, and even weather systems near the poles. Despite steady improvements in subseasonal-to-seasonal (S2S) forecast models, the MJO prediction skill has still not reached its maximum potential. The root of this challenge is partly due to our lack of understanding of how the MJO interacts with the background mean flow. In this work, we use a simple one-layer atmospheric model with idealized heating and vorticity sources to understand the impact of the subtropical jet on the MJO amplitude and its horizontal structure.

Prediction of Coral Sea tropical cyclone power and latitude of maximum intensity using climate indices

Tropical cyclone (TC) wind power, often highly destructive, can be quantified using the power dissipation index (PDI) and in this study, the PDIs for Coral Sea TC tracks, as well as the latitude of maximum intensity (LMI) were investigated for correlation with climatological indices. Multiple linear regression with collinearity removed and an overall Pearson correlation of 0.7 or above was used for this. The results for all tracks showed that three indices dominated for PDI: Niño 4 Sea Surface Temperature (SST), the Dipole Mode Index (DMI) and the Madden Julian Oscillation (MJO). Coral Sea TC tracks clustered by maximum windspeed-weighted locations were then examined: For cluster 1 (located more south-east), the additional influence of the Southern Annular Mode (SAM) was apparent, whilst for cluster 2 (located more north-west), the same indices dominated as for the all-tracks model. For LMI, four indices were identified: the Indian Ocean East SST Anomaly (DMI E), the MJO, the Southern Oscillation Index (SOI) and the SAM. Only TCs clustered in the northeast of Australia had a valid model for LMI, with correlation 0.8, using three indices: DMI E, DMI and the SOI. Overall, raised Niño 4 SST combined with a negative DMI and low MJO amplitude were shown to predict large increases in TC power, whilst a combination of increasing DMI E temperature anomaly with a positive SOI moves LMI equator-wards. The models compiled in this study identified the most significant climatic indices and successfully predicted TC power dissipation and LMI.

Captured QBO‐MJO Connection in a Subseasonal Prediction System

AbstractThe quasi‐biennial oscillation (QBO) impacts the Madden‐Julian Oscillation (MJO) activity with a stronger MJO in QBO easterly (QBOE) than QBO westerly (QBOW) winters. However, this relationship is poorly represented in the current generation climate models. For the first time, this paper applies a stratospheric zonal‐mean nudging in a subseasonal prediction system to capture it. Two strong MJO cases in a QBO‐neutral winter are investigated. The QBO temperature and zonal wind anomalies are added separately as well as together to the stratosphere using nudging in MJO case hindcast. Only by nudging the QBO temperature anomalies while leaving the zonal wind free, can the prediction system capture the observed QBO‐MJO connection. The tropopause instability is found positively correlated to the MJO amplitude, but it cannot fully explain the captured connection. The free‐evolving zonal wind anomalies in the stratosphere due to the nudged QBO temperature are crucial for the captured connection.

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.

Ensemble forecast of an index of the Madden–Julian Oscillation using a stochastic weather generator based on circulation analogs

Abstract. The Madden–Julian Oscillation (MJO) is one of the main sources of sub-seasonal atmospheric predictability in the tropical region. The MJO affects precipitation over highly populated areas, especially around southern India. Therefore, predicting its phase and intensity is important as it has a high societal impact. Indices of the MJO can be derived from the first principal components of zonal wind and outgoing longwave radiation (OLR) in the tropics (RMM1 and RMM2 indices). The amplitude and phase of the MJO are derived from those indices. Our goal is to forecast these two indices on a sub-seasonal timescale. This study aims to provide an ensemble forecast of MJO indices from analogs of the atmospheric circulation, computed from the geopotential at 500 hPa (Z500) by using a stochastic weather generator (SWG). We generate an ensemble of 100 members for the MJO amplitude for sub-seasonal lead times (from 2 to 4 weeks). Then we evaluate the skill of the ensemble forecast and the ensemble mean using probabilistic scores and deterministic skill scores. According to score-based criteria, we find that a reasonable forecast of the MJO index could be achieved within 40 d lead times for the different seasons. We compare our SWG forecast with other forecasts of the MJO. The comparison shows that the SWG forecast has skill compared to ECMWF forecasts for lead times above 20 d and better skill compared to machine learning forecasts for small lead times.

Projected Changes in the Seasonal Cycle of Madden‐Julian Oscillation Precipitation and Wind Amplitude

AbstractAlthough recent studies have examined the responses of annual‐ and seasonal‐mean Madden‐Julian oscillation (MJO) activity to global warming, little is known about the seasonal cycle changes that determine the timing of the peak MJO influence on local weather and climate. By analyzing the Community Earth System Model version 2 large ensemble at the end of the 21st century, we report changes in both the phase and amplitude of the seasonal cycle of MJO precipitation over the tropical Indian Ocean and southwestern Pacific. While the seasonal delay in phase of the MJO is related to the seasonal delay of total precipitation, with contributions from surface latent heat flux and circulation changes, the amplification in seasonal amplitude is consistent with the increase in MJO amplitude due to mean temperature increases. The shifts in phase of the seasonal cycle of the MJO under global warming may influence the predictability of extreme events.

Excitation of the Madden‐Julian Oscillation in Response to Transient Ocean Warming in SPCAM

AbstractRecent studies have suggested that the Madden‐Julian oscillation (MJO) could be generated as part of atmospheric adjustment to localized equatorial heating in models properly resolving moist convection. Here we use the Super‐parameterized Community Atmosphere Model (SPCAM) to study atmospheric response to transient sea surface warming in the equatorial Indian Ocean, which leads to the excitation of an MJO event, robust across different experiments, in addition to the expected equatorial Rossby and Kelvin waves. A moist static energy (MSE) budget analysis suggests that longwave and surface turbulent latent heat flux anomalies are predominantly responsible for the MJO excitation; however, these terms oppose its eastward propagation. In contrast, advection of MSE generally weakens the MJO amplitude, but contributes to its eastward propagation. These results highlight the role of the MJO in the moist atmospheric adjustment to localized tropical heating, with implications for the MJO mechanisms and prediction.

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.

Modulation of the Convectively Coupled Kelvin Waves by the MJO over different domains

Abstract In this study we have examined the modulation of convectively coupled Kelvin waves (CCKWs) by different Madden-Julian oscillation (MJO) states over the Indian, Pacific and Atlantic Ocean domains. Convectively active CCKW events associated with active MJO convection, suppressed MJO convection, and quiescent MJO states were derived using wavenumber-frequency filtered outgoing long-wave radiation (OLR) indices over the three domains. Composite analysis of CCKW events during different MJO states indicate that the amplitude and phase speed of CCKW are modulated by the MJO state. CCKW amplitude is stronger (weaker), and it propagates relatively slower (faster) and more (less) eastward when the MJO amplitude is strong (weak). The phase speed of CCKW is much slower over the Indian Ocean domain, while it propagates relatively faster over the Atlantic Ocean domain. It is hypothesized that the observed difference in CCKW phase speeds is related to the Gross Moist Stability (GMS). The clear linear relationship observed between GMS and CCKW phase speeds over the different domains, during different MJO states and the observed differences in CCKW vertical structures support this hypothesis. It is found that the CCKW exhibits a baroclinic vertical structure over the Indian and Pacific Ocean domains and a barotropic vertical structure over the Atlantic Ocean. Planetary-scale convection associated with the MJO reduces the static stability allowing for baroclinic modes to prevail, which in turn reduces the GMS and the effective equivalent depth, eventually slowing down the CCKW phase propagation. The results suggest that CCKW may be treated as a mixed-moisture mode.

Changes in MJO Characteristics and Impacts in the Past Century

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

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

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

Deep learning for bias correction of MJO prediction

Producing accurate weather prediction beyond two weeks is an urgent challenge due to its ever-increasing socioeconomic value. The Madden-Julian Oscillation (MJO), a planetary-scale tropical convective system, serves as a primary source of global subseasonal (i.e., targeting three to four weeks) predictability. During the past decades, operational forecasting systems have improved substantially, while the MJO prediction skill has not yet reached its potential predictability, partly due to the systematic errors caused by imperfect numerical models. Here, to improve the MJO prediction skill, we blend the state-of-the-art dynamical forecasts and observations with a Deep Learning bias correction method. With Deep Learning bias correction, multi-model forecast errors in MJO amplitude and phase averaged over four weeks are significantly reduced by about 90% and 77%, respectively. Most models show the greatest improvement for MJO events starting from the Indian Ocean and crossing the Maritime Continent.

Improving the MJO Forecast of S2S Operation Models by Correcting Their Biases in Linear Dynamics

AbstractThe operational dynamic subseasonal to seasonal (S2S) models for Madden‐Julian oscillation (MJO) forecasting mostly still suffer from systematic errors in capturing the MJO's key dynamic features, such as its growth rate and propagation speed. By deriving the linear dynamic operators using the linear inverse modeling (LIM) approach, we propose a method to partly correct the errors in MJO linear dynamic operators to improve the MJO predictions of three operational dynamic S2S models. Correcting the deficiencies of the too‐fast decay rates and the unrealistic propagating phase speeds lead to MJO prediction skills being extended by approximately 2–4 days. The improvements are more significant for the models with larger biases in MJO amplitude and propagation. This approach in principle may be extendable to predictions of other types of climate variability such as ENSO on one hand, and possible inclusions of nonlinear dynamics effects on the other hand.

Common Mechanism for Interannual and Decadal Variability in the East African Long Rains

AbstractThe East African long rains constitute the main crop‐growing season in the region. Interannual predictability of this season is low in comparison to the short rains, and recent decadal drying contrasts with climate projections of a wetter future (the “East African climate paradox”). Here, we show that long rains rainfall totals are strongly correlated with 700 hPa zonal winds across the Congo basin and Gulf of Guinea ( ). Westerly anomalies align with more rainfall, with the same mechanism controlling covariability on interannual and decadal time scales. On both time scales wind anomalies are linked to geopotential anomalies over the Sahel and Sahara, and warming there. Rainfall and wind are significantly correlated with the Madden‐Julian Oscillation (MJO) amplitude, and around 18% of the decadal drying can be explained by MJO amplitude variability. This work shows that predictions of East African rainfall across time scales require robust prediction of both zonal winds and MJO activity.

Factors Regulating the Multidecadal Changes in MJO Amplitude over the Twentieth Century

AbstractThis study examined multidecadal changes in the amplitude of the boreal-winter Madden–Julian oscillation (MJO) over the twentieth century using two century-long reanalysis datasets (20CR and ERA-20C). Both revealed reasonable MJO variability compared to other state-of-the-art reanalysis datasets. We detected pronounced multidecadal variations along with an increasing trend in MJO amplitude during the period 1900–2009 in both datasets, although this linear trend was less significant in the reconstructed MJO index proposed by Oliver and Thompson. The two twentieth-century reanalysis datasets and the Oliver–Thompson MJO index consistently showed the intensified amplitude of MJO precipitation and circulation in the later decades (1970–99) compared to the earlier decades (1920–49). The most significant enhancement of MJO precipitation in the later decades appeared over the western Pacific warm pool. To understand the mechanisms controlling the changes in western Pacific MJO precipitation amplitude over the twentieth century, we diagnosed the moisture budget equation. The enhanced MJO precipitation variability in the later decades mainly came from increased moisture associated with a strengthened low-level convergence anomaly working on background mean moisture []. Further diagnosis showed that the effect of anomalous circulation (∇ ⋅ V′) change on the MJO precipitation amplitude change over the twentieth century was about an order larger than that of mean moisture () change, different from the mechanisms (i.e., increased gradient of ) responsible for the intensified MJO precipitation amplitude under future warmer climate. The enhanced MJO circulation anomalies during 1970–99 may be caused by an enhanced diabatic heating anomaly, offset partly by the increased mean static stability.

The Relationship between the Madden–Julian Oscillation and Tropical Cyclone Rapid Intensification

AbstractThe relationship between the Madden–Julian oscillation (MJO) and tropical cyclone rapid intensification in the northern basins of the Western Hemisphere is examined. All rapid intensification events in the part of the Western Hemisphere north of the equator and the MJO phase and amplitude are compiled from 1974 to 2015. Rapid intensification events and the MJO tend to move in tandem with each other from west to east across the hemisphere, though rapid intensification appears most likely during a neutral MJO phase. The addition of this information to an operational statistical rapid intensification forecasting scheme does not significantly improve forecasts.

Impacts of Insolation and Soil Moisture on the Seasonality of Interactions Between the Madden‐Julian Oscillation and Maritime Continent

AbstractThis study investigates the seasonality of the interaction of the Madden‐Julian Oscillation (MJO) with the Maritime Continent (MC). Beside their seasonal east‐west migration with the monsoons, observations show that the MJO amplitude and precipitation over the MC islands exhibit semiannual variability, with apparent strengthening of MJO signal during March and September, when solar insolation is strongest over the MC region. Furthermore, during the high‐insolation months, soil moisture shows wetter conditions during the early phases of the MJO. Motivated by these results, a series of regional convection permitting simulations are performed for the November 2014 MJO event as a case study under the following conditions: (i) increased solar insolation to mimic the local summer, (ii) reduced initial soil moisture, and (iii) the two conditions combined to isolate their local effects from their impacts on the large‐scale circulation. Results show that increased insolation increases precipitation including that associated with MJO moisture convergence over the MC region. On the other hand, decreased initial soil moisture slightly increases precipitation over the MC islands and reduces it over the surrounding waters, but such effect is short lived as the increased precipitation gradually restores the soil moisture to wetter conditions. Using a moisture budget analysis that isolates the MJO and non‐MJO signals and additional idealized simulations, the MJO response to high insolation is demonstrated to be related to an increase in the basic state moisture over the MC region rather than a direct response of the MJO to the insolation.

Seasonal Dependence of Cold Surges and their Interaction with the Madden–Julian Oscillation over Southeast Asia

AbstractNortheasterly cold surges strongly influence the rainfall patterns over the Malay Peninsula during the northeast monsoon season. This study looks at the changes in the cold surges and Madden–Julian oscillation (MJO) characteristics through the northeast monsoon season and their interaction. Nearly 75% of the cold surge events tend to cross the equator around the Java Sea area (100°–110°E) in February–March with drier conditions prevailing over the Malay Peninsula and increased rainfall over Java. Both the cold surges and the MJO undergo seasonal variations with well-defined regional features. Wavelet analysis shows that MJO amplitude and high-frequency rainfall variations over Southeast Asia peak in November–December. MJO amplitude is suppressed during February and March. This is linked to the high-frequency surges of meridional winds that are prominent during the early part of the season, but February–March is dominated by low-frequency (~20–90 days) cross-equatorial monsoon flow. These prolonged periods of strong meridional flow at the equator interact with the MJO both dynamically and thermodynamically and act as a barrier for convection from propagating from the Indian Ocean to the Maritime Continent (MC). These interactions may have implications for weather and seasonal forecasting over the region. An evaluation of the properties of cold surges and their interaction with the seasonal cycle in the Met Office Unified Model is performed. The atmosphere–ocean coupled model performs better in representing the pattern of influence of the cold surges despite the biases in intensity and spatial distribution of rainfall extremes. These diagnostics are presented with the aim of developing a set of model evaluation metrics for global and regional models.