Active Phase Of Madden–Julian Oscillation Research Articles

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).

A Cold Lid on a Warm Ocean: Indian Ocean Surface Rain Layers and Their Feedbacks to the Atmosphere

AbstractOcean surface rain layers (RLs) form when relatively colder, fresher, less dense rain water stably stratifies the upper ocean. RLs cool sea surface temperature (SST) by confining surface evaporative cooling to a thin near‐surface layer, and generate sharp SST gradients between the cool RL and the surrounding ocean. In this study, ocean‐atmosphere coupled simulations of the November 2011 Madden‐Julian Oscillation (MJO) event are conducted with and without RLs to evaluate two pathways for RLs to influence the atmosphere. The first, termed the “SST gradient effect,” arises from the hydrostatic adjustment of the boundary layer to RL‐enhanced SST gradients. The second, termed the “SST effect,” arises from RL‐induced SST reductions impeding the development of deep atmospheric convection. RLs are found to sharpen SST gradients throughout the MJO suppressed and suppressed‐to‐enhanced convection transition phases, but their effect on convection is only detected during the MJO suppressed phase when RL‐induced SST gradients enhance low‐level convergence/divergence and broaden the atmospheric vertical velocity probability distribution below 5 km. The SST effect is more evident than the SST gradient effect during the MJO transition phase, as RLs reduce domain average SST by 0.03 K and narrow vertical velocity distribution, thus delaying onset of deep convection. A delayed SST effect is also identified, wherein frequent RLs during the MJO transition phase isolate accumulated subsurface ocean heat from the atmosphere. The arrival of strong winds at the onset of the MJO active phase erodes RLs and releases subsurface ocean heat to the atmosphere, supporting the development of deep convection.

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.

Observed Anomaly of Temperature and Mixed Layer Depth Associated with the Madden Julian Oscillation (MJO) Active Phase in the Banda Sea, Indonesia

The eastward propagation of atmospheric waves along the equatorial band from the central equatorial Indian Ocean to the western Pacific Ocean passing through the Indonesian Maritime Continent (IMC), known as the intraseasonal Madden Julian Oscillation (MJO) event, plays an important role on modulating both atmospheric and upper ocean dynamics along its path. This study aims to investigate the MJO active phase dynamics and its impact on changes in near-surface seawater temperature and mixed layer depth (MLD) anomaly in the Banda Sea Indonesia, using multi-datasets of atmospheric reanalysis, satellites derived sea surface temperature (SST), and Argo float between 2017 - 2018. This study revealed that the MJO waves propagate eastward along the southern equator-line over the IMC and pass through the Banda Sea, associated with significant decreased on Outgoing Longwave Radiation (OLR) and increased in zonal wind speed at 850hPa. The study result shows anomalous increased on surface wind speed and SST cooling during of MJO active phase. The amplitude peaks of filtered ocean-atmosphere variables range between 30 – 60 days. Argo float datasets in the Banda Sea for the first time captured upper ocean responses to the arrival of the MJO active phase, as characterized by a negative temperature anomaly of ~0.3°C in the surface mixed layer, large temperature anomaly of ~0.8°C in the thermocline layer and the deepening of the MLD of ~25 m. Hence, the MJO active phase impacts significantly on surface and vertical temperature cooling and regulate upper ocean mixing intensity in the Banda Sea.

Rainfall Variation on the West Coast of Sumatra during Each Phase of the Madden-Julian Oscillation from GPM Satellite Observation

The West Coast of Sumatra has a significantly higher annual rainfall than its surrounding areas, making this region crucial in controlling precipitation in Sumatra. The Madden-Julian Oscillation (MJO) heavily influences the West Coast of Sumatra. MJO is an intraseasonal activity that occurs in tropical regions and can be recognized by the eastward movement of convective activity from the Indian Ocean towards the Pacific Ocean for 30-60 days. Using Global Precipitation Measurement (GPM) satellite data from 2014 to 2020, this study investigates the influence of MJO on rainfall on the West Coast of Sumatra. The data used in this study is the attenuation-corrected reflectivity (dBZ) of Ku PR (Z-Ku) and Ka PR (Z-Ka) from GPM DPR Level 2 data. For convective rainfall, the Z-Ku and Z-Ka reflectivity values are higher during the inactive MJO phase than the active MJO phase, consistent with previous research in Sumatra. Thus, during the inactive MJO phase, strong convection occurs, leading to updrafts and increased formation of larger raindrops through coalescence processes. During the active MJO phase, there is a significant decrease in raindrop size, indicating dominant break-up processes.

Impact of Madden Julian Oscillation on the diurnal cycle of precipitation over the west coast of India

Madden Julian Oscillation (MJO) is an eastward propagating convective system that passes over the Indian and Pacific Oceans with a periodicity of 30–90 days and influences most of the tropical weather systems. This study investigates and quantifies the impact of MJO on the diurnal cycle of precipitation over the west coast of India. The work uses TRMM precipitation, profiles of wind and humidity from ERA5 re-analysis and satellite Infrared window channel measurements from GridSat-B1 for the period 2000–2019. Real-time Multivariate MJO index has been used for the identification of active and suppressed MJO phases. Harmonic analysis is used to derive the diurnal and semi-diurnal amplitudes of precipitation. The active MJO phases enhance the diurnal amplitude of precipitation over the west coast with respect to the climatological diurnal cycle, whereas the suppressed phases reduce it. Active phases help in the formation of mesoscale convective cloud systems with much more spatial extension and temporal duration than that in suppressed phase. This kind of convective organization is observed especially during the active MJO phases of monsoon and post-monsoon seasons. The vertical structure of wind and humidity in the troposphere are modified by MJO where the active (suppressed) phase enhances (decreases) them. The overall modification of tropospheric dynamics, thermodynamics and deep convection by MJO can cause anomalous rainfall activity over the west coast.

Transition of Large‐Scale Environmental Conditions and Characteristics of Four Rainfall Types Observed by S‐PolKa During the MJO‐1 Active Phase of DYNAMO/CINDY/AMIE

AbstractAnalyses of National Center for Atmospheric Research (NCAR) S‐PolKa dual‐polarization radar data and ERA5 reanalysis fields indicate gradual changes in convection characteristics and large‐scale environmental conditions during a central Indian Ocean Madden‐Julian Oscillation (MJO) event observed by the DYNAMO/CINDY/AMIE field campaigns in late October 2011 (MJO‐1). Examination of four rainfall types (isolated convective cores, convective, mixed, and stratiform) reveals a transition of convection characteristics (i.e., areal coverage and depth) between distinct 5‐day environmental periods at the beginning and end of this ∼2‐week MJO active phase. A shift toward less frequent rainfall covering less of the radar domain for all four rainfall types occurs when large‐scale lower‐tropospheric dry air advects into the region with the westerly wind burst (WWB). Drier and warmer lower‐free‐tropospheric conditions associated with the WWB contribute to increased large‐scale surface‐based convective inhibition (CIN), surface‐based convective available potential energy (CAPE), and cloud base heights. A thermodynamic budget analysis indicates reduced surface heat fluxes contribute to the increased surface‐based CAPE. Greater CAPE at the end of MJO‐1 coincides with deeper 50‐dBZ convective echoes, while decreased 10‐dBZ depth for all rainfall types corresponds in time with WWB‐related dry‐air advection. Increased (decreased) reflectivity values in the lower‐level vertical reflectivity distribution of convective (stratiform) precipitation indicate increased (decreased) convective (stratiform) intensities when the WWB is present. The opposite depth changes for convective echoes and opposite shifts in convective and stratiform precipitation intensities underscore how the WWB can have differing impacts at different reflectivity thresholds and stages of the deep convection lifecycle.

Assimilating C-Band Radar Data for High-Resolution Simulations of Precipitation: Case Studies over Western Sumatra

Accurate high-resolution precipitation forecasts are critical yet challenging for weather prediction under complex topography or severe synoptic forcing. Data fusion and assimilation aimed at improving model forecasts, as one possible approach, has gained increasing attention in past decades. This study investigates the influence of the observations from a C-band Doppler radar over the west coast of Sumatra on high-resolution numerical simulations of precipitation around its vicinity under the Madden–Julian oscillation (MJO) in January and February 2018. Cases during various MJO phases were selected for simulations with an advanced research version of the weather research and forecasting (WRF) model at a cloud-permitting scale (~3 km). A 3-dimensional variational (3DVAR) data assimilation method and a hybrid three-dimensional ensemble–variational data assimilation (3DEnVAR) method, based on the NCEP Gridpoint Statistical Interpolation (GSI) assimilation system, were used to assimilate the radar reflectivity and the radial velocity data. The WRF-simulated precipitation was validated with the Integrated Multi-satellitE Retrievals for GPM (IMERG) precipitation data, and the fractions skill score (FSS) was calculated in order to evaluate the radar data impacts objectively. The results show improvements in the simulated precipitation with hourly radar data assimilation 6 h prior to the simulations. The modifications with assimilation were validated through the observation departure and moist convection. It was found that forecast improvements are relatively significant when precipitation is more related to local-scale convection but rather small when the background westerly wind is strong under the MJO active phase. The additional simulation experiments, under a 1- or 2-day assimilation cycle, indicate better improvements in the precipitation simulation with 3DEnVAR radar assimilation than those with the 3DVAR method.

Intraseasonal variability of the Indonesian throughflow associated with the Madden-Julian Oscillation

Intraseasonal variability (20–90 days) of the Indonesian Throughflow (ITF), which is primarily forced by the Madden-Julian Oscillation (MJO), is investigated using a high-resolution global ocean reanalysis and satellite altimeter data. Previous studies show that during the MJO active phase, downwelling Kelvin waves generated in the central equatorial Indian Ocean propagate along the coast of Sumatra and Java islands, affecting the ITF transport at exit passages in the Indonesian Seas and Makassar Strait. However, the intraseasonal variation of the ITF transport through these straits over the MJO life cycle, especially during the suppressed phase, has not been quantified. To quantify the ITF transport associated with the MJO, composites of ITF transport through major straits in the Indonesian Seas are constructed using a 0.08° global HYbrid Coordinate Ocean Model (HYCOM) reanalysis. A prominent reduction of ITF transport through major straits is found during the MJO active phase, and a transport enhancement comparable to the reduction is evident during the suppressed phase. As a result, the net effect of the MJO on the mean ITF transport is very small due to the cancellation of the enhancement and reduction. The magnitude of the MJO-associated ITF transport variation through all ITF exit passages is about 6 Sv, which is about 50% of the total transport. While the propagation of coastal Kelvin waves during the MJO active phase and their impact on ITF transport is clearly evident in the composite, such Kelvin wave influence on ITF transport is not clearly detected during the suppressed phase. This suggests that anomalous winds over the Maritime Continent (MC) area are mostly responsible for the ITF variation during the suppressed phase in many MJO events. Yet, during some MJO events, remotely-forced upwelling Kelvin waves and their significant impact on the ITF transport are evident during the MJO suppressed phase.

Deterministic and Probabilistic Evaluation of Sub-Seasonal Precipitation Forecasts at Various Spatiotemporal Scales over China during the Boreal Summer Monsoon

Skillful sub-seasonal precipitation forecasts can provide valuable information for both flood and drought disaster mitigations. This study evaluates both deterministic and probabilistic sub-seasonal precipitation forecasts of ECMWF, ECCC, and UKMO models derived from the Sub-seasonal to Seasonal (S2S) Database at various spatiotemporal scales over China during the boreal summer monsoon. The Multi-Source Weighted-Ensemble Precipitation, version 2 (MSWEP V2), is used as the reference dataset to evaluate the forecast skills of the models. The results suggest that skillful deterministic sub-seasonal precipitation forecasts are found when the lead time is within 2 weeks. The deterministic forecast skills reduce quickly when the lead time is beyond 2 weeks. Positive ranked probability skill scores (RPSS) are only found when the lead time is within 2 weeks for probabilistic forecasts as well. Multimodel ensembling helps to improve forecast skills by removing large negative skill scores in northwestern China. The forecast skills are also improved at larger spatial scales or longer temporal scales. However, the improvement is only observed for certain regions where the predictable low frequency signals remain at longer lead times. The composite analysis suggests that both the El Niño–Southern Oscillation (ENSO) and Madden–Julian Oscillation (MJO) have an impact on weekly precipitation variability over China. The forecast skills are found to be enhanced during active ENSO and MJO phases. In particular, the forecast skills are found to be enhanced during active MJO phases.

Identifying key driving mechanisms of heat waves in central Chile

This study explores the main drivers of heat wave (HW) events in central Chile using state-of-the-art reanalysis data (ERA5) and observations during the extended austral summer season (November to March) for the period 1979–2018. Frequency and intensity aspects of the HW events are considered using the total number of the HW events per season and the amplitude. We first contrast ERA5 with several surface meteorological stations in central Chile to evaluate its ability to capture daily maximum temperature variability and the HW events. We then use synoptic- and large-scale fields and teleconnection patterns to address the most favorable conditions of the HW events from a climatological perspective as well as from the extreme January 2017 HW event that swept central Chile with temperature records and wildfires. ERA5 tends to capture temperature extremes and the HW events at the inland stations; on the contrary, it has difficulties in capturing the maximum temperature variability at the coastal stations, which is plausible given the complex terrain features and confined coastal climate zone (only $$\sim $$ 7% of all grid boxes within central Chile). The composite HW days based on ERA5 reveals a mid-level trough-ridge dipole pattern exhibiting a blocking anticyclone on the surface over a large part of southwest South America. Relatively dry and warm easterly flow appears to accompany the anomalous warming in a large part of central Chile. The temporal evolution of the HW events yields a wave-like propagation pattern and enhancement of trough-ridge pattern along the South Pacific. This meridional dipole pattern is found to be largely associated with the Pacific South American pattern. In addition, the Madden–Julian Oscillation (MJO) appears to be a key component of the HW events in central Chile. In particular, while active MJO phases 2 and 7 promote sub-seasonal patterns that favor the South Pacific dipole mode, synoptic anomalies can superimpose on them and favor the formation of a migrating anticyclone over central-southern Chile and coastal lows over central Chile. Agreeing with the climatological findings, the extreme January 2017 HW analysis suggests that an eastward migratory mid-latitude trough-ridge pattern associated with MJO phase 2 was at work. We highlight that in addition to large- and synoptic-scale features, sub-synoptic processes such as coastal lows can have an important role in shaping the HW events and can lead to amplification of temperature extremes during the HW events.

Relationship between extreme rainfall based on GSMaP data with Madden Julian Oscillation (MJO) in Bangka Island

The effect of MJO events on extreme rainfall events on Bangka Island needs to be spatially studied in more detail. The low quality of rain station data encourages the use of alternative data such as Global Satellite Mapping of Precipitation (GSMaP). This study aimed to analyze the influence of active MJO on extreme rainfall events using GSMaP data on Bangka Island. The methods used are extreme threshold of 95th, 98th and 99th percentile, the Spearman Rank correlation between extreme rainfall events and active MJO and its correlation significance test. The results showed that northern coast has the highest range of extreme threshold. The correlation significance test showed GSMaP grids that have a significant correlation with the MJO are in the north of the Island, especially coast area. MJO phases 3 and 5 have an impact on decreasing of extreme rainfall events which are known through the negative correlation values. From all the three extreme thresholds, it was only MJO phase 4 that increasing extreme rainfall events. Active MJO phase 4 causes Bangka Island as a convective area, while MJO phases 3 and 5 affect the formation of suppressed areas on Bangka Island.

Comparison of the Madden–Julian Oscillation-Related Tropical Cyclone Genesis over the South China Sea and Western North Pacific under Different El Niño-Southern Oscillation Conditions

This paper explores the Madden–Julian Oscillation (MJO) modulation of tropical cyclone (TC; hereafter, MJO-TC) genesis over the South China Sea (SCS) and Western North Pacific (WNP) under different El Niño-Southern Oscillation (ENSO) conditions. Analyses used Joint Typhoon Warning Center (JTWC) best-track data, the Real-Time Multivariate MJO (RMM) index, and European Center for Medium-Range Weather Forecasts (ECMWF) Interim (ERA-Interim) reanalysis data. The results showed that the MJO has significant modulation on both the SCS and WNP TC genesis in neutral years, with more (fewer) TCs forming during the active (inactive) MJO phases. However, during the El Niño and La Niña years, the MJO-TC genesis modulation over the two regions differs from each other. Over the SCS, the MJO modulation of TC genesis is stronger in the La Niña years, while it becomes weaker in the El Niño years. Over the WNP, the MJO has a stronger influence on TC genesis in the El Niño years compared to that in the La Niña years. Related Genesis Potential Index (GPI) analysis suggests that midlevel moisture is the primary factor and vorticity is the secondary factor, for the MJO-TC genesis modulation over the SCS in the La Niña years. Over the WNP, midlevel moisture is the dominant factor for the MJO-TC genesis modulation during the El Niño years. These results can be explained by increased water vapor transport from the Bay of Bengal, associated with enhanced westerly wind anomalies, during the active phases relative to the inactive MJO phases; these conditions prevail over the SCS during the La Niña years, and over the WNP during the El Niño years.

ANALISIS PENGARUH MADDEN JULIAN OSCILLATION (MJO) TERHADAP ANOMALI CURAH HUJAN DI WILAYAH NGURAH RAI

Madden Julian Oscillation (MJO) merupakan suatu gelombang atau osilasi non seasonal yang terjadi di lapisan troposfer yang bergerak dari barat ke timur dengan periode osilasi kurang lebih 30-60 hari. Fenomena ini sangat berdampak terhadap kondisi anomali curah hujan pada suatu wilayah yang dilaluinya. Dalam penelitian ini delapan fase MJO dikelompokan menjadi 4 bagian sesuai dengan pergerakannya yaitu fase 1 dan 8 (Western and Africa), fase 2 dan 3 (Indian Ocean), fase 4 dan 5 (Maritime Continent), fase 6 dan 7 (Western Pacific). Data yang digunakan dalam penelitian ini yaitu data curah hujan periode 1996-2015 dan data MJO periode yang sama. Lokasi penelitian yaitu wilayah Ngurah Rai. Data curah hujan dihitung anomalinya lalu dipisahkan antara anomali positif dan negatif lalu disandingkan dengan fase MJO aktif. Penelitian ini bertujuan untuk menganalisis porsentase kelompok fase MJO mana yang mendominasi anomali curah hujan pada saat anomali positif maupun negatif. Hasil kajian menunjukan persentase kejadian anomali curah hujan positif dominan terjadi saat fase MJO berada di Maritime Continent sebesar 36%. Kemudian persentase kejadian anomali curah hujan negatif dominan terjadi saat MJO berada pada fase Indian Ocean dengan porsentasi sebesar 33%.

The Diurnal and Microphysical Characteristics of MJO Rain Events during DYNAMO

Abstract The Madden–Julian oscillation (MJO) dominates the intraseasonal variability of cloud populations of the tropical Indian and Pacific Oceans. Suppressed MJO periods consist primarily of shallow and isolated deep convection. During the transition to an active MJO, the shallow and isolated deep clouds grow upscale into the overnight hours. During active MJO periods, mesoscale convective systems occur mostly during 2–4-day bursts of rainfall activity with a statistically significant early morning peak. Yet when these rain events are separated into individual active periods, some periods do not follow the mean pattern, with the November events in particular exhibiting an afternoon peak. The radar-observed microphysical processes producing the precipitation during the major rain events of active MJO periods evolve in connection with synoptic-scale wave passages with varying influences of diurnal forcing. MJO studies that do not account for the intermittency of rainfall during active MJO phases through averaging over multiple events can lead to the misimpression that the primary rain-producing clouds of the MJO are modulated solely by the diurnal cycle.

Modulation of bay of bengal tropical cyclone activity by the madden-julian oscillation

The influence of the Madden-Julian oscillation (MJO) on global tropical cyclone (TC) activity has been well documented in many earlier studies. However, no prior studies have focused specifically on the MJO's impacts on TCs in the Bay of Bengal (BoB). Therefore, the present study examines the impact of the MJO on BoB TC activity during the two peak TC periods i.e. April–June (AMJ) and October–December (OND) from 1974 to 2015. The MJO considerably modulates various measures of TC activity in the BoB, including the number of TCs, the number of TC days, accumulated cyclone energy, the power dissipation index, TC genesis location and TC tracks. TC activity is significantly enhanced (suppressed) over the BoB when the convectively active MJO phase is positioned over the eastern Indian Ocean and the Maritime Continent (the Western Hemisphere and Africa). Alterations in the TC genesis locations and their tracks are more pronounced in OND than in AMJ. These changes in TC characteristics are mainly attributed to MJO-driven modulations in large-scale environmental conditions such as deep convection, mid-tropospheric relative humidity, sea surface temperature, sea level pressure, lower- and upper-level winds and vertical wind shear. A comprehensive knowledge of the MJO-TC relationship may be beneficial for short-term predictions of TC activity in the BoB.

Interaction between the MJO and High-Frequency Waves over the Maritime Continent in Boreal Winter

AbstractThe two-way interaction between Madden–Julian oscillation (MJO) and higher-frequency waves (HFW) over the Maritime Continent (MC) during boreal winter of 1984–2005 is investigated. It is noted from observational analysis that strengthened (weakened) HFW activity appears to the west (east) of and under MJO convection during the MJO active phase and the opposite is seen during the MJO suppressed phase. Sensitivity model experiments indicate that the control of HFW activity by MJO is through change of the background vertical wind shear and specific humidity. The upscale feedbacks from HFW to MJO through nonlinear rectification of condensational heating and eddy momentum transport are also investigated with observational data. A significantly large amount (25%–40%) of positive heating anomaly () at low levels to the east of MJO convection is contributed by nonlinear rectification of HFW. This nonlinear rectification is primarily attributed to eddy meridional moisture advection. A momentum budget diagnosis reveals that 60% of MJO zonal wind tendency at 850 hPa is attributed to the nonlinear interaction of HFW with other scale flows. Among them, the largest contribution arises from eddy zonal momentum flux divergence . Easterly (westerly) vertical shear to the west (east) of MJO convection during the MJO active phase causes the strengthening (weakening) of the HFW zonal wind anomaly. This leads to the increase (decrease) of eddy momentum flux activity to the east (west) of the MJO convection, which causes a positive (negative) eddy zonal momentum flux divergence in the zonal wind transitional region during the MJO active (suppressed) phase, favoring the eastward propagation of the MJO.

The Madden‐Julian Oscillation's Impact on the Makassar Strait Surface Layer Transport

AbstractThe Indonesian Throughflow (ITF) surface layer dynamics within the Makassar Strait, responsible for ~77% of the ITF, are investigated using in situ and satellite‐derived observations from January 2004 to August 2011 and August 2013 to December 2016. Surface layer southward transport attains its minima during boreal winter in response to atmospheric and oceanic processes attributed to Australian‐Indonesian monsoon as well as the Madden‐Julian Oscillation (MJO). While the monsoon's impact on seasonal variability of the ITF transport has been well documented, the MJO's role to modulate the variability of the ITF transport is less studied. Eleven MJO events traversed from Indian Ocean to Western Pacific during boreal winter months over the course of the Makassar Strait time series. Intensified along‐strait wind stress, reduced outgoing longwave radiation, increased sea surface height in the southern Makassar Strait, and a reduction in the surface layer ITF transport by up to 4 Sv marked the impact of the active phase of the MJO. Analysis of the momentum budget in the surface layer indicates that the excess of northward momentum due to the along‐channel pressure gradient over northward momentum attributed to the vertical divergence of Reynold stress governs the northward acceleration of the surface layer during the MJO active phase.

Observational Evidence of Mixed Rossby‐Gravity Waves as a Driving Force for the MJO Convective Initiation and Propagation

AbstractBased on observational data analyses of a Madden‐Julian Oscillation (MJO) event during the field campaign of the Years of the Maritime Continent Project, we propose a new mechanism for MJO convective initiation and propagation to which mixed Rossby‐gravity waves (MRGs) contribute significantly. The intensive observation captured midtropospheric wind variations associated with MRGs, and those signals propagated westward over the Indian Ocean in the MJO‐suppressed phase. The transition to the MJO active phase follows tight MRG‐convection coupling, which is enhanced when the wavelength becomes shorter due to weak midlevel convergence in the western Indian Ocean. Basin‐scale anomalous midtropospheric moistening caused by MRG shallow circulations also contributes. These processes lead to the lower‐tropospheric MRG wave packet formation and successive triggering of MJO convection, with the eastward MRG group velocity corresponding to MJO propagation. Additional analysis for other MJO cases implicates potential for robustness of this MJO‐MRG relationship.