Madden–Julian Oscillation Phases Research Articles

On the MJO Phase Speed among Different Background Moisture and Zonal Wind Base States

Abstract The variability of the phase speed of the Madden–Julian oscillation (MJO) is poorly understood. The authors assess how the phase speed of the convective signal of the MJO associates with the background states over eastern Africa and the Indian Ocean. Relaxation of the coupling between tropical modes and their circulation has been previously linked to faster propagation; for example, the MJO speeds up over the eastern Pacific where its convective signal decouples from the circulation. In contrast, our results show that fast MJO events happen to exist during periods of wetter background states (>90 days) from East Africa across the Indian Ocean, whereas slow MJO is associated with dry background states. We found that fast MJO exhibits strong active and inactive phases with a structure suggesting more hierarchical convection. Results indicate that the association of the phase speed of the MJO as seen in the integrated filtered moist static energy with its tendency is stronger than the association of the phase speed as observed in the dry static energy with its tendency which is consistent with the acceleration of the MJO during wet background states. Also, our results indicate that the MJO may be faster during periods of enhanced low-level moisture because these periods have anomalously weak upper-tropospheric easterly background winds, which reduce the westward advection of the MJO by the background easterly wind, resulting in higher eastward phase speed of the MJO. The acceleration of the MJO by the background zonal wind overwhelms the deceleration associated with the moist-wave dynamics. Significance Statement This study shows that the Madden–Julian oscillation (MJO), which is the dominant subseasonal weather signal in the tropics, moves eastward more quickly across eastern Africa and the Indian Ocean when the region is abnormally moist. The faster propagation does not appear to result from the higher moisture but instead from encountering weaker-than-normal upper-air winds from the east that tend to occur during moist periods.

Possible Mechanisms behind Dominant Modes of Seasonal Cycle of Rainfall over India

Abstract The South Asian summer monsoon that occurs over four months from near summer solstice (June) to near Autumnal equinox (September) evolves within the season due to interaction with other climate components. Since most climate patterns are seasonally phase-locked, their interaction and feedback with monsoon should also be season-dependent. Here, using rain-gauge-based precipitation over India for seventy-four years, we show that the onset phase of the monsoon (June) is uncorrelated with rest of the season (July to September). Propagation of Madden Julian Oscillation (MJO) over the Indian Ocean triggers northward-propagation of precipitable water that plays a key role during the onset phase. MJO appears over the Western equatorial Indian Ocean both during the early and late Central Indian onset years. Normal and slow MJO phase propagation during early and late onset years respectively give rise to interannual variations in June rainfall. After onset, till the end of the monsoon season, interannual variability of the dominant seasonal mode of precipitation is regulated by the phase of the El Nino Southern Oscillation (ENSO) through anomalous moisture convergence. We show that, in June, ENSO doesn’t play key role because of the opposing impact on wind and moisture. However, from July to September, ENSO perturbs wind and moisture in a way that gives rise to positive feedback between monsoon, ENSO, and large-scale circulation. Such feedback of monsoon with ENSO results in a coherent impact of ENSO on monsoon precipitation in July-September. Our results suggest that dominant mechanisms for interannual variations of Indian monsoon rainfall in June and the rest of the season are different.

Vertical structure and occurrence patterns of the cross-equatorial northerly surge under different ENSO and MJO phases

This study investigated the vertical structure of 6 cross-equatorial northerly surge (CENS) events during the Year of Maritime Continent–Cold Surge Observation in 2021 (YMC-CSO2021) campaign. These events, named CENS1 (Jan. 18–20), CENS2 (Jan. 29–30), CENS3 (Feb. 2–5), CENS4 (Feb. 5–9), CENS5 (Feb. 18–20), and CENS6 (Feb. 25–26), occurred under different environmental conditions associated with cold surges (CSs) and the Madden–Julian Oscillation (MJO). Using radiosonde observations, we identified distinct characteristics in the northerly wind layer thickness, westerly wind bursts, and potential temperature anomalies among the events. Notably, CENS6 featured a deep northerly wind layer reaching 400 hPa, influenced by a southward pressure gradient linked to a cyclone in the Southern Hemisphere. Statistical analysis of past CENS events using reanalysis dataset showed that 80% occurred in January and February, with higher frequencies during La Niña years and active MJO phases over the western Pacific, aligning with YMC-CSO2021 observations. Composite analysis showed that CENS events induced significant ascending motion and localized potential temperature gradients, leading to positive precipitation anomalies around the Maritime Continent. These findings enhance our understanding of CENS dynamics and their impact on regional climate variability.

Observed Responses of Gravity Wave Momentum Fluxes to the Madden‒Julian Oscillation Around the Extratropical Mesopause Using Mohe Meteor Radar Observations

AbstractThe 12‐year continuous observation of gravity wave momentum fluxes (GWMFs) estimated by the Mohe meteor radar (53.5°N, 122.3°E) revealed prominent intraseasonal variability around the extratropical mesopause (82–94 km) during boreal winters. Composite analysis of the December‒January‒February (DJF) season according to the Madden‒Julian Oscillation (MJO) phases revealed that the zonal GWMFs notably increased in MJO Phase 4 (P4) by ∼2–4 m2/s2, and a Monte Carlo test was designed to examine the statistical significance. The response in zonal winds lags behind the GWMF response by two MJO phases (i.e., 1/2π), indicating a “force‒response” interaction between them. Additionally, time‐lagged composites revealed that strengthened westward GWMFs occurred ∼25–35 days after MJO P4, coincident with the MJO impact on the zonal winds in the stratosphere. The analysis results also suggested that the mechanism of MJO by which the MJO influences the stratospheric circulation might involve poleward propagating effects of stationary planetary waves with zonal wavenumber one. This work emphasizes the importance of GW intraseasonal variability, which impacts tropical sources from the troposphere to the extratropical mesopause.

Madden Julian Oscillation Moves Faster as the Meridional Moisture Gradient Intensifies in a Warming World

AbstractThe eastward‐moving large‐scale convective system associated with the Madden‐Julian Oscillation (MJO) significantly impact global weather and climate. Recent decades have seen notable changes in the MJO's lifecycle due to non‐uniform tropical ocean warming, with the roles of natural climate variability and anthropogenic influence still requiring quantification. This study examines observed and projected long‐term changes in the MJO phase speed using four twentieth‐century reanalyses and CMIP6 simulations. We find a substantial increase in MJO phase speed in three reanalyses during the twentieth century (0.6–1.2 m s⁻1 century⁻1) and further increase in MJO phase speed during the twenty‐first century (0.3–1.5 m s⁻1 century⁻1), with notable multidecadal fluctuations. We attribute the overall acceleration of the MJO to the global warming‐driven increase in the meridional moisture gradient around the warm pool while attributing the multidecadal variability in the MJO phase speed to changes in the zonal moisture gradient associated with the Pacific Decadal Oscillation.

Comparing MJO Intensity over the Tropical Western Pacific during Mega and Equatorial La Niña Winters

Abstract This study investigated variations in the Madden–Julian oscillation (MJO) behavior between two types of La Niña winters: mega and equatorial La Niñas. Results of this work show that in contrast to mega conditions, accompanied by more abundant intraseasonal column-integrated moisture anomalies over the planetary boundary layer, intensities of intraseasonal outgoing longwave radiation anomalies are stronger over the tropical western Pacific (WP) at MJO phases 5–6 under equatorial conditions. The occurrence of the moisture anomaly change averaged over the tropical WP is supported by a distinct moisture tendency difference. Moisture budget and multiscale interaction diagnoses underscore the pivotal effect of an area-averaged vertical gradient change in low-frequency background moisture in driving such tendency change. Considering notable MJO convection variations, the teleconnections associated with MJO phases 5–6 over East Asia (EA) were also explored, including warmer surface air temperature anomalies and stronger positive geopotential height anomalies at 200 hPa on the intraseasonal time scale under equatorial conditions. Results from a linear baroclinic model demonstrate that such teleconnection changes could be effectively explained by the linear response to the MJO’s diabatic heating anomaly. Roles of mean state and MJO itself variations in the linear response change are also discussed. These findings provide novel insights into MJO activity and offer potential improvements for subseasonal forecasting in EA. Significance Statement The relationship between El Niño–Southern Oscillation (ENSO) and MJO has been extensively explored recently. However, variations in the MJO behavior and its climate effects under mega and equatorial La Niña winters have not been thoroughly investigated. In this work, we utilized reanalysis datasets and an idealized linear baroclinic model to address these questions. We observed more robust and abundant intraseasonal convection activity and planetary boundary layer–integrated moisture anomaly averaged over the tropical WP at MJO phases 5–6 under equatorial conditions than mega conditions, which is closely linked with the vertical gradient change in low-frequency background moisture. Additionally, MJO-related teleconnections also exhibit some changes. This work may provide a new perspective on understanding the relationship between the MJO and ENSO and offer potential improvements for the subseasonal forecast.

Madden Julian Oscillation Influence on Diurnal Variation of Rainfall in Mentawai Islands Indonesia from IMERG Observations

Abstract Mentawai is a small island located in the Indian Ocean, so its rainfall pattern tends to be different from the larger islands in Indonesian region. Various scales of factors influence this, one of which is Madden Julian Oscillation (MJO). The aim of this study is to determine effect of MJO in Mentawai Islands on the diurnal variation of rainfall. Rainfall data was collected from 2000-2022 from Integrated Multi-SatellitE Retrievals for GPM (IMERG). Diurnal variations were observed in the accumulation (PA), frequency (PF), and intensity (PI) of rainfall and MJO was classified into active phase (2, 3, 4, and 5) and inactive phase (1, 6, 7, and 8) during the seasonal period (DJF-MAM-JJA-SON). Diurnal peak times in PA, PF, and PI occur later when MJO is active than when MJO is inactive, especially in JJA and SON periods in Mentawai Islands and Mentawai Strait. When MJO is active, average PA and PF are higher than when MJO is inactive. The average of PI is higher around the ocean during active and inactive phases of MJO. The number of short-duration rainfall events occurred over the land, while long-duration rainfall occurred over the ocean.

Unraveling Subseasonal Drought Dynamics in India: Insights from NCMRWF Extended Range Prediction System

Abstract Drought, a prolonged natural event, profoundly impacts water resources and societies, particularly in agriculturally dependent nations like India. This study focuses on subseasonal droughts during the Indian summer monsoon season using standardized precipitation index (SPI). Analyzing hindcasts from the National Centre for Medium Range Weather Forecasting (NCMRWF) Extended Range Prediction (NERP) system spanning 1993–2015, we assess NERP’s strengths and limitations. NERP replicates climatic patterns well but overestimates rainfall in the Himalayan foothills and the Indo-Gangetic Plain while underestimating it in the core monsoon zone and western coastline. Nonetheless, the NERP system demonstrates its ability to predict subseasonal drought conditions across India. Our research explores the model’s dynamics, emphasizing tropical and extratropical influences. We evaluate the impact of monsoon intraseasonal oscillation (MSIO) and Madden–Julian oscillation (MJO) on drought onset and persistence, noting model performance and discrepancies. While the model consistently identifies MSIO locations, variations in phase propagation affect drought severity in India. Remarkably, NERP excels in predicting MJO phases during droughts. The study underscores the robust response in the near-equatorial Indian Ocean, a crucial factor in subseasonal drought development. Furthermore, we explored upper-level dynamic interactions, demonstrating NERP’s ability to capture subseasonal drought dynamics. For example, unusual westerly winds weaken the tropical easterly jet, and a cyclonic anomaly transports cold air at midlevels and upper levels. These interactions reduce thermal contrast, weakening monsoon flow and favoring drought conditions. Hence, the NERP system demonstrates its skill in assessing prevailing drought conditions and associated teleconnection patterns, enhancing our understanding of subseasonal droughts and their complex triggers.

Two‐way feedback between the Madden–Julian Oscillation and diurnal warm layers in a coupled ocean–atmosphere model

AbstractDiurnal warm layers develop in the upper ocean on sunny days with low surface wind speeds. They rectify intraseasonal sea‐surface temperatures (SSTs), potentially impacting intraseasonal weather patterns such as the Madden–Julian Oscillation (MJO). Here we analyse 15‐lead‐day forecast composites of coupled ocean–atmosphere and atmosphere‐only numerical weather prediction (NWP) models of the UK Met Office to reveal that the presence of diurnal warming of SST (dSST) leads to a faster MJO propagation in the coupled model compared with the atmosphere‐only model. To test the feedback between the MJO and the dSST, we designed a set of experiments with instantaneous vertical mixing over the top 5 or of the ocean component of the coupled model. Weaker dSST in the mixing experiments leads to a slower MJO over 15 lead days. The dSST produces a increase in the MJO phase speed between the coupled and the atmosphere‐only model. An additional increase is found for other coupling effects, unrelated to the dSST. A two‐way feedback manifests in the coupled model over the 15 lead days of the forecast between the MJO and the dSST. The MJO regime dictates the strength of the dSST and the dSST rectifies the intraseasonal anomalies of SST in the coupled model. Stronger dSST in the coupled model leads to stronger intraseasonal anomalies of SST. The MJO convection responds to these SSTs on a seven‐lead‐day timescale, and feeds back into the SST anomalies within the next three lead days. Overall, this study demonstrates the importance of high vertical resolution in the upper ocean for predicting the eastward propagation of the MJO in an NWP setting, which is potentially impactful for seasonal predictions and climate projections, should this feedback be unrepresented in the models.

Impact of a fresh-core mesoscale eddy in modulating oceanic response to a Madden-Julian Oscillation

Theories of ocean–atmosphere interaction during a Madden–Julian Oscillation (MJO) are generally based on a thermodynamic model with surface fluxes dictating changes in sea surface temperature. Evidence from a two month ocean glider deployment in early 2019 in the southeast Indian Ocean suggests the impact of mesoscale dynamics on upper-ocean stratification likely affects ocean–atmosphere interaction at MJO scales. Until mid-February, local surface fluxes consistent with a convectively suppressed MJO phase drove near-surface ocean evolution. With the advection of a fresh-core eddy to the glider location in late February, ocean dynamics then becomes an additional driver of this evolution by modulating local stratification and generating a barrier layer of ≈12 m thickness for 10 days. One-dimensional modelling experiments based on the ocean and atmospheric conditions experienced during our sampling period show that the ocean subsurface structure within the eddy induce changes in SST of physical significance for ocean-atmosphere interaction. Moreover, results also suggest that the presence of a thick eddy-induced barrier layer during the MJO suppressed phase modulates the magnitude of temperature anomalies forced by surface fluxes during the following enhanced MJO phase. As eddies are abundant in this area, their dynamics must be considered to correctly represent SST variability for MJO modelling.

Impact of MJO Propagation Speed on Active Atlantic Tropical Cyclone Activity Periods

AbstractThe Madden‐Julian Oscillation (MJO) is often used for subseasonal forecasting of tropical cyclone (TC) activity. However, TC activity still has considerable variability even given the state of the MJO. This study evaluates the connection between MJO propagation speed with Atlantic TC activity and possible physical mechanisms guiding this relation. We find the Atlantic sees the highest accumulated cyclone energy (ACE) during MJO phase 2. However, the odds of above average ACE in the Atlantic is greatest during slow MJO propagation. We find that slow propagation of the MJO results in lower vertical wind shear anomalies over the Caribbean and main development region compared with typical MJO propagation. Typical MJO propagation produces an amplified height pattern and lower height anomalies along the region of the tropical upper tropospheric trough which is known to impede Atlantic TC activity. Slow MJO propagation sees weaker height anomalies over the Atlantic.

Impact of the Madden–Julian oscillation and equatorial waves on tracked mesoscale convective systems over southeast Asia

AbstractSoutheast Asia is a region dominated by high‐impact weather, but numerical weather prediction here is a challenge owing to the complex orography and interactions between small‐ and large‐scale phenomena. Localised mesoscale convective systems (MCSs) can produce intense precipitation. Here, we track MCSs over a 5‐year period in Himawari satellite data, characterise the distribution of MCSs in the region, and investigate how they are modulated by the Madden–Julian oscillation (MJO) and equatorial waves. Between 10°S and 10°N in southeast Asia, MCSs account for 45–70% of the precipitation during boreal extended winter (November–April). Over most of the region, the fractional MCS contribution to rainfall is higher than average on days with extreme rainfall (>55%). Long‐lived (>12 hr) MCSs contribute disproportionately, providing 85% of the rainfall despite comprising only 34% of all MCSs. Variability in MCS rainfall accounts for >50% of the total rainfall variability during an MJO cycle, mostly due to larger numbers of MCSs in convectively active MJO phases. Variations in MCS size and mean rain rate due to shifts in the stratiform proportion provide compensating effects. In the west of the region, a shift to faster moving MCSs in active MJO phases and slower moving MCSs in inactive phases resulted in fast‐moving MCSs having the greatest impact on the MJO‐associated variability. Variability is larger in the west than in the east. Equatorial Kelvin waves modulate MCS rainfall, with MCSs accounting for 20–50% of local rainfall anomalies. This variability is again enhanced in the west. By contrast, rainfall anomalies due to westward‐propagating mixed Rossby–gravity waves and Rossby‐1 waves are dominated by tropical‐cyclone‐related rainfall. Skill at local scales may be extracted from forecasts of subseasonal drivers such as the MJO and Kelvin waves, by understanding how these modulate the number and characteristics of MCSs.

The impact of the Madden-Julian oscillation on spring and autumn afternoon diurnal convection in Sri Lanka

This study examines the impact of strong Madden-Julian Oscillation (MJO) phases (P1–P8) on diurnal rainfall patterns focusing on Afternoon Diurnal Convection (ADC) events in Sri Lanka during 2001–2020 spring and autumn. Daily mean rainfall increases (decreases) during the P2-to-P3 (P6-to-P7) MJO phases in both seasons, while the diurnal rainfall amplitude peaks during the P2-to-P3 (P8-to-P1) MJO phases in spring (autumn). ADC events also occur more frequently and intensely during MJO P2-to-P3 (P8-to-P1) in spring (autumn). The MJO’s modulation of diurnal rainfall amplitude and ADC events is more apparent in autumn than in spring. Active MJO phases enhance the westward propagation of diurnal rainfall associated with ADC events, sustained by moisture flux convergence and enhanced upward motion. The prevailing mid-to-upper level easterly wind, combined with deep convection over Sri Lanka, contributes to a more pronounced westward propagation during the P2-to-P3 (P8-to-P1) phases for ADC events in spring (autumn).

Propagation Characteristics of Madden Julian Oscillation in the Indonesian Maritime Continent: Case Studies for 2020-2022

Madden-Julian Oscillation (MJO) can affect weather and climate variability in the Indonesian Maritime Continent. MJO propagation is not always the same, previous research has classified MJO into 4 categories: slow, fast, stand, and jump. The objective of this study is to investigate the differences in MJO propagation and the factors that impact it. Daily data for variables such as Outgoing Longwave Radiation (OLR), zonal wind, and sea surface temperature are utilized in this research. The collected data is processed using composite methods based on the 8 MJO phases, with a specific focus on the years 2020, 2021, and 2022. The research findings suggest that warm sea surface temperatures in the Pacific Ocean and zonal winds dominated by Kelvin waves are favorable for MJO propagation. Conversely, cooling sea surface temperatures in the Pacific Ocean and zonal winds dominated by equatorial Rossby waves can hinder MJO propagation. Future researchers are expected to examine the impact of MJO propagation during extreme rainfall occurrences in several regions of Indonesia, as well as the application of machine learning and deep learning methods to predict MJO propagation in the future.

Warm conveyor belt activity over the Pacific: modulation by the Madden–Julian Oscillation and impact on tropical–extratropical teleconnections

Abstract. Research in the last few decades has revealed that rapidly ascending airstreams in extratropical cyclones – so-called warm conveyor belts (WCBs) – play an important role in extratropical atmospheric dynamics. However on the subseasonal timescale, the modulation of their occurrence frequency, henceforth referred to as WCB activity, has so far received little attention. Also, it is not yet clear whether WCB activity may affect tropospheric teleconnection patterns, which constitute a source of predictability on this subseasonal timescale. Using reanalysis data, this study analyzes the modulation of WCB activity by the Madden–Julian Oscillation (MJO). A key finding is that WCB activity increases significantly over the western North Pacific when the convection of the MJO is located over the Indian Ocean. This increased WCB activity, which is stronger during La Niña conditions, is related to enhanced poleward moisture fluxes driven by the circulation of subtropical Rossby gyres associated with the MJO. In contrast, when the convection of the MJO is located over the western North Pacific, WCB activity increases significantly over the eastern North Pacific. This increase stems from a southward shift and eastward extension of the North Pacific jet stream. However, while these mean increases are significant, individual MJO events exhibit substantial variability, with some events even exhibiting anomalously low WCB activity. Individual events of the same MJO phase with anomalously low WCB activity over the North Pacific tend to be followed by the known canonical teleconnection patterns in the Atlantic–European region; i.e., the occurrence frequency of the positive phase of the North Atlantic Oscillation (NAO) is enhanced when convection of the MJO is located over the Indian Ocean and similarly for the negative phase of the NAO when MJO convection is over the western North Pacific. However, the canonical teleconnection patterns are modified when individual events of the same MJO phase are accompanied by anomalously high WCB activity over the North Pacific. In particular, the link between MJO and the negative phase of the NAO weakens considerably. Reanalysis data and experiments with an idealized general circulation model reveal that this is related to anomalous ridge building over western North America favored by enhanced WCB activity. Overall, our study highlights the potential role of WCBs in shaping tropical–extratropical teleconnection patterns and underlines the importance of representing them adequately in numerical weather prediction models in order to fully exploit the sources of predictability emerging from the tropics.

Intensity duration and frequency of Heat wave in different phases of MJO over India

The Madden-Julian Oscillation (MJO) is an important tropical atmospheric phenomenon that plays a significant role in modulating weather patterns across India. Heat Waves (HW) are extreme weather events that have severe impacts on human health, agriculture, and various socio-economic sectors. This research paper aims to investigate the dynamical relationship between MJO phases, as measured by the Real-time Multivariate MJO (RMM) index, and the occurrence of HW in India, as defined by the Indian Meteorological Department. This paper demonstrated the relationship among three aspects, i.e., intensity, duration, frequency of HW during the different MJO phases. The differential dynamics associated for about two-fold rise in the HW occurrence in the South East, Eastern and North West India in the MJO active phase. Inactive phase witnessed enhancement (suppression) of HW during wet (dry) over the regions. The novel finding of this paper is that, the weaker the MJO, the higher probability of HW occurrence in respective regions depending on the position of the MJO convection center. The maximum duration is observed to be significantly decreased by about 2 days in the active MJO phase over NW and north central parts of the country. It is also inferred from this study that there is a significantly decrease of 0.5 °C in HW intensity during the active wet phase of MJO over the NW India. To explain the linkage of HW with MJO, geopotential height (GPH), wind circulation, velocity potential (VP), sea surface temperature (SST) and outgoing longwave radiation (OLR) composite anomalies have been computed and used in the analysis. It is observed that all these parameters have a direct impact on the evolution, strength and spread of the HW in India. This dynamical study mechanism can be prescribed in the numerical weather prediction models for the improved prediction of such extreme weather events during summer for proactive disaster management.

The tropical influence on sub‐seasonal predictability of wintertime stratosphere and stratosphere–troposphere coupling

AbstractA unique set of relaxation experiments with a forecast model initialized during December–January 1999–2019 is used to explore tropical influence on the Northern Hemisphere polar stratosphere and stratosphere–troposphere coupling, to quantify predictability benefits due to the perfect knowledge of the tropical variability. On average, predictability of the polar stratosphere, as represented by the 50 hPa geopotential height anomalies north of 50°N (Z50), increases from 17 days in freely running (control) forecasts to 21 days in the tropical relaxation experiments. At sub‐seasonal time‐scales, a statistically significant improvement in weekly mean skill scores can be demonstrated in 14%–20% of individual forecast ensembles, mostly in cases when the skill of the corresponding control forecast is worse than average. In these forecasts, root‐mean‐square errors and forecast spread of Z50 during forecast weeks 3–5 are decreased by 10%–15%. Stratospheric improvements are detected during periods of both vortex strengthening and vortex weakening, including most major Sudden Stratospheric Warmings that occurred during the study period, via modulation of the upward wave activity fluxes. An active Madden–Julian Oscillation (MJO) is found in most of these events with MJO phase 5–7 preceding vortex weakening and MJO phase 3–4 preceding vortex strengthening. Forecasts with improved stratospheric circulation also have improved tropospheric circulation during and after the periods when improvements are detected in the stratosphere. We attribute these improvements to both stratosphere–troposphere coupling and tropospheric tropical teleconnections.

QBO modulation of MJO teleconnections in the North Pacific: impact of preceding MJO phases

This study examines the influence of the Quasi-Biennial Oscillation (QBO) on the Madden-Julian Oscillation (MJO) teleconnections in the North Pacific using ERA5 data. It is found that the Rossby wave trains induced by MJO phase 6–7 exhibit greater strength and robustness during the westerly QBO winter (WQBO) than during the easterly QBO winter (EQBO), although the MJO itself is weaker during the former. This counter-intuitive dependency of MJO teleconnections on the QBO is attributed to the preexisting MJO teleconnections prior to the MJO phase 6–7. The MJO phase 6–7 is more frequently preceded by stronger MJO phase 3–4 during the EQBO than during the WQBO. The preceding MJO phase 3–4 teleconnections, which have opposed signs to the MJO phase 6–7 teleconnections, result in a considerable attenuation of the MJO phase 6–7 teleconnections by destructive interference. This result is supported by linear model experiments. The subseasonal-to-seasonal prediction models also indicate improved prediction skills of MJO phase 6–7 teleconnections during the WQBO compared to the EQBO. These results suggest that enhanced MJO activities during the EQBO do not necessarily result in stronger and more robust MJO teleconnections in the North Pacific.

Role of Madden–Julian Oscillation in predicting the 2020 East Asian summer precipitation in subseasonal-to-seasonal models

The 2020 summer monsoon season in East Asia was unusually long and intense, and the Madden–Julian Oscillation (MJO) has been proposed as an underlying reason. This study analyzes the role of the MJO in the 2020 East Asian precipitation forecasts of the subseasonal-to-seasonal (S2S) model. The S2S models underestimated the cumulative precipitation over East Asia, and the models with good forecast performance yielded a distinct precipitation band over East Asia and a western pacific subtropical high (WPSH) during the analysis period. East Asian precipitation forecast performance was more closely related to the location of the center than the strength of the WPSH, with precipitation increasing with a decrease in the latitude at the center. MJO Phases 1–3 activation intensified the WPSH and shifted the center of WPSH to lower latitudes. Our results confirm that the strong East Asian precipitation in summer 2020 was partly due to changes in the characteristics of the MJO and indicate the importance of accurately estimating the MJO-WPSH coupling for reliable East Asian precipitation forecasts.

The Role of the Stratosphere in Teleconnections Arising From Fast and Slow MJO Episodes

AbstractThe Madden‐Julian Oscillation (MJO) can influence the extratropical circulation on timescales up to several weeks, with a dependence on the MJO characteristics: MJO episodes that propagate slowly across the Maritime Continent have a stronger impact on Euro‐Atlantic weather than fast MJO episodes. While the tropospheric pathway for MJO teleconnections with varying phase speeds is well understood, in this study, we investigate the contribution of the Northern Hemisphere stratospheric pathway for fast versus slow MJO episodes. During slow MJO episodes, Phases 5–6 lead to increased upward wave propagation in the North Pacific sector, and subsequently enhanced heat flux at 100 hPa, leading to the weakening of the polar vortex. The results suggest a clear role of stratosphere‐troposphere coupling for slow MJO episodes, which is proposed as a mechanism for anomalously strong positive polar cap height anomalies in MJO Phases 7–8.