Modulations of local rainfall in Northeast Australia associated with the Madden Julian Oscillation

This study investigates the role of the interaction between the Madden–Julian oscillation (MJO) and local‐scale forcings in regulating precipitation and its diurnal variation during the austral summer over coastal areas in northeast (NE) Australia using radar data. The variation of rainfall is influenced by both large‐scale and local‐scale forcings. During the enhanced convection phases of the MJO, widespread increased rainfall signals are generated by large‐scale forcings associated with the MJO, but the environmental factors controlling the type and amount of precipitation during each phase are different. By contrast, the locally enhanced rainfall during suppressed convection phases of the MJO likely results from the interaction of mesoscale land–sea breezes, strong large‐scale background winds and topography. Different responses of mean and heavy precipitation to the MJO occur in some MJO phases. The impact of the MJO on diurnal rainfall characteristics is spatially inhomogeneous and likely regulated by local forcings. Although stratiform rainfall is more common, convective rainfall predominantly contributes to the total precipitation over coastal regions of NE Australia. The MJO's influence on convective rainfall is generally stronger and more statistically significant than its impact on stratiform rainfall. The widespread increased rainfall probability during the enhanced convection phases is likely due to increases in both stratiform and convective rainfall. In contrast, the locally enhanced precipitation signal during some suppressed convection phases mainly results from an increase in convective rainfall.

<p>The Madden-Julian Oscillation (MJO) can create favorable conditions for tropical cyclone (TC) genesis in the Indian Ocean, but past work has not thoroughly investigated how TCs <em>after</em> genesis may influence MJO convection development. This work utilizes long-term composite analysis to broadly establish the relationship between Indian Ocean TCs and each MJO phase over the Indian basin, and then seeks to isolate direct impacts of the TCs on MJO convection coverage and intensity.</p><p>We first examine Indian Ocean TC interactions with MJO convection using daily-mean ERA5 reanalysis and TRMM precipitation products from 1998-2018 for TC and non-TC days per MJO phase, excluding 3 days before and after Best-Track TC lifespans to reduce contamination of non-TC composites. Preliminary analysis suggests that TC periods are associated with stronger MJOs, with an anomalously stronger MJO large-scale circulation and associated subsidence over the equatorial Indian Ocean. We find higher CAPE and increased TRMM rainfall during convectively-active MJO phases over the eastern Indian Ocean when TCs are present, but increased dry-air advection, greater CIN, and decreased TRMM rainfall over the western Indian Ocean during the same phases. These findings allude to suppression of MJO convection development during TC periods in the western MJO convective envelope, with coincident enhancement of MJO convection in the eastern MJO convective envelope. While these broad conclusions are consistent during non-convectively-active MJO phases, changing MJO strength during TC periods for convectively-active MJO phases limit our ability to quantify TC impacts on MJO convection using only composite analysis.</p><p>To better quantify TC influences, we next isolate direct TC impacts on MJO convection using metrics for the TC range of influence, likelihood of interaction with MJO convection, and strength of TC-MJO convection interactions. Since a TC’s influence likely extends beyond the 34-kt wind radii provided by Best-Track, we determine an “outer wind radius” by integrating a radial wind model outward from each TC eye to ~5 m/s. We next quantify the overlapping area between each TC outer wind radius and the coincident MJO convection by using the MJO precipitation boundary determined from a large-scale precipitation tracking dataset, with the size of the overlapping area providing the likelihood of interaction. For time periods when TC outer wind radii and MJO convection overlap, we compare the convection coverage and intensity observed by TRMM between MJO convection sectors with and without TC wind overlap. The strength of a TC-MJO convection interaction is finally quantified by comparing the convection coverage and intensity between these sectors. With climatological statistics on the likelihood and strength of TC-MJO interactions, future MJO prediction and Maritime Continent rainfall forecasts could be adjusted according to the presence or absence of Indian Ocean TCs.</p>

Analyses of Lagrangian model simulations and atmospheric sonde data reveal a key component of the Madden–Julian Oscillation (MJO): circumnavigating equatorial Kelvin waves with dynamics that transform between dry and moist, which initiate and dissipate MJO convection. The same compositing procedure is applied to simulated and observed MJOs, which uses a coordinate system that moves with the precipitation centre, and treats developing, mature and dissipating stages separately. MJO structure and evolution are similar in the simulation and the observations. To the west of the developing convection, there is a broad region of low‐level (upper‐level) perturbation westerlies (easterlies), which is accompanied by a deep negative temperature perturbation. As this feature enters the MJO‐formation region, convection intensifies on its eastern edge, and the zonal wind perturbations decrease in zonal extent and propagation speed. This process is shown to be a dynamical consequence of a largely dry, first baroclinic Kelvin wave entering a region where large‐scale upward motion is mostly balanced by convective heating. As the MJO matures, a Kelvin wave of opposite sign emerges from its eastern edge, and makes the opposite transition (from moist to dry). The resulting wave, which includes low‐level (upper‐level) easterlies (westerlies) and a deep positive temperature perturbation, rapidly propagates around the world and dissipates MJO convection. The Kelvin wave that initiates MJO convection is shown to originate from the inactive phase of a previous MJO, so that the complete MJO cycle is characterized by the two kinds of Kelvin waves emerging from active and suppressed phases of MJO convection, circumnavigating the Tropics, and triggering the opposite phase.

Changes in the Madden–Julian oscillation (MJO) and its impacts on the South American monsoon season during different El Niño–Southern Oscillation (ENSO) states [El Niño (EN), La Niña (LN), neutral (NT)] are analyzed in the global context of the MJO-propagating anomalies of convection and circulation. The background ENSO-related changes influence several aspects of MJO (relative occurrence of phases, propagation, convection, and teleconnections), and therefore modify the MJO impacts on South America (SA), such as precipitation anomalies and frequency of extreme events, as well as their distribution throughout the MJO cycle. Among the changes are the following: 1) a delay in the teleconnection between the central-east Pacific and SA, from MJO phase 8 in LN (MJOLNphase8) to MJO phase1 in EN (MJOENphase1); 2) enhanced MJO convection in the central-east subtropical South Pacific in MJOLNphases7 + 8 and a little farther east in MJOENphases8 + 1, in a region efficient in generating teleconnections that produce rainfall anomalies over central-east SA (CESA), especially the South Atlantic convergence zone (SACZ), strongest one phase earlier in LN (MJOLNphase8) than in EN (MJOENphase1), and a little shifted east in the latter than in the former; 3) enhancement of the extratropical teleconnection and its impacts over the SACZ in both EN and LN (with regard to NT), indicating nonlinear effects on MJO impacts over SA; 4) predominant increase (or reduction) in the frequency of extreme events over SA regions where both ENSO and MJO contribute in the same direction, with the greatest increase over CESA during EN in MJOENphase1 and over Southeast SA (SESA) in MJOENphase3. Significance Statement The changes produced by different El Niño–Southern Oscillation background states (neutral, El Niño, La Niña) in the Madden–Julian oscillation (MJO) and its impacts on precipitation over South America (SA) are disclosed for the austral summer monsoon season, when MJO is strongest. The reliability of the results is enhanced by using observed rainfall data. The background states affect MJO propagation, circulation, and convection, producing significant differences in precipitation and extremes over densely populated regions, besides phase shifts of the strongest impacts. The results are relevant to subseasonal prediction, since MJO is a key predictability source, to validate models and describe realistically the impacts over SA in each MJO phase. Models’ skill in simulating these results is assessed in a follow-up work.

ABSTRACTSeveral droughts and floods in Amazonia and Northeast Brazil have occurred in recent years and projections from Intergovernmental Panel on Climate Change indicate an increase of these extreme events. El Niño Southern Oscillation (ENSO) is one of the phenomena associated with extreme rainfall events in the Amazon. However, recent studies have indicated that the basic response of ENSO is dependent on the Madden–Julian Oscillation (MJO) phase. Hence, this study analyses the MJO influence on precipitation extreme events over northern South America in El Niño and La Niña years. Extreme precipitation events over northern South America for the rainy season (December–May) were obtained through a composite analysis of the combinations of ENSO and MJO phases. Most of the dry extreme events occurred during El Niño periods, while wet extreme events were more recurrent during La Niña or neutral years. However, the results showed that the MJO convection could enhance or weaken the basic response of ENSO on extreme precipitation events. Moreover, dry/wet extreme events over both Amazon and Northeast Brazil are favoured when MJO convection over Indonesia is enhanced (MJO phases 4 and 5)/suppressed (MJO phase 2). Additionally, the interannual variability of the extreme events showed an increasing linear trend for dry extreme events and a decreasing linear trend for wet extreme events. The results presented here contribute to a better understanding of the climate variability and will be helpful for the forecast of ENSO effects on extreme events over northern South America.

This paper investigates the impact of the Madden-Julian Oscillation (MJO) on the diurnal cycle of rainfall over the western Maritime Continent during the austral summer. For this purpose, cyclostationary empirical orthogonal function analysis is applied to the tropical rainfall measuring mission rain rate and the Japanese Reanalysis-25 data for the period 1998–2008. The real-time multivariate MJO index by Wheeler and Hendon (Mon Wea Rev 132:1917–1932, 2004) is adopted to define the intensity and the phase of MJO. It is demonstrated that the hourly maximum rain rate over the domain tends to increase when convectively active phase of MJO approaches the Maritime Continent. In contrast, the hourly maximum rain rate tends to decrease when convectively suppressed phase of MJO resides over the region. The changes in the rain rate due to MJO differ over the ocean and the land. This difference is the greatest when the MJO is in the mature stage. Throughout the day during this stage, terrestrial rain rates show minimum values while diurnally varying oceanic rain rates record maximum values. Thus, precipitation becomes more intense in the morning over the Java Sea and is weakened in the evening over Borneo and Sumatra during the mature stage of MJO. During the decaying stage of MJO over the Maritime Continent, the diurnal cycle of precipitation weakens significantly over the ocean but only weakly over land. Analyses suggest that the anomalous lower level winds accompanied by MJO interact with the monsoonal flow over the Maritime Continent. Westerlies induced by MJO convection in the mature stage are superimposed on the monsoonal westerlies over the equator and increase wind speed mainly over the Java Sea due to the blocking effect of orography. Mountainous islands induce flow bifurcation, causing near-surface winds to converge mainly over the oceanic channels between two islands. As a result, heat flux release from the ocean to the atmosphere is enhanced by the increased surface wind resulting in instability as described in the wind-induced surface heat exchange mechanism. This may contribute to heavy rainfall over the Java Sea in the morning during the mature stage. On the other hand, convergence and vertical velocity over the islands, which play important roles in inducing nighttime rainfall, tend to be weak in the evening during the mature stage of MJO. Strong westerlies arising from MJO and the seasonal flow during the mature stage tend to interrupt convergence over islands. This interruption of convergence by MJO gives rise to decreased rain rates over the land regions.

Using the specified-dynamics (SD) Whole Atmosphere Community Climate Model (SD-WACCM), the effects of the Madden–Julian oscillation (MJO) on the midwinter stratosphere and mesosphere in the Southern Hemisphere (SH) are investigated. The most significant responses of the SH polar cap temperature to the MJO are found about 30 days after MJO phase 1 (P1) and about 10 days after MJO phase 5 (P5) in both the ERA-Interim data and the SD-WACCM simulation. The 200- and 500-hPa geopotential height anomalies in the SH reveal that wave trains emanate from the Indian and Pacific Oceans when the MJO convection is enhanced in the eastern Indian Ocean and the western Pacific. As a result, the upward propagation and dissipation of planetary waves (PWs) in the middle and high latitudes of the SH stratosphere is significantly enhanced, the Brewer–Dobson (BD) circulation in the SH stratosphere strengthens, and temperatures in the SH polar stratosphere increase. Wavenumber 1 in the stratosphere is the dominant component of the PW perturbation induced by the MJO convection. In the SH mesosphere, the MJO leads to enhancement of the dissipation and breaking of gravity waves (GWs) propagating as a result of wind-filtering change in the SH extratropics and causes anomalous downwelling in the middle and high latitudes of the mesosphere. The circulation thus changes significantly, resulting in anomalous cooling in the mesosphere in response to MJO P1 and P5 at lags of 10 and 30 days, respectively.

<p>The impacts of the Madden-Julian Oscillation (MJO) on the South American monsoon season (December, January, and February – DJF) and their possible changes during positive (El Niño – EN) and negative (La Niña – LN) phases of the El Niño-Southern Oscillation (ENSO) are analyzed in the UK Met Office Unified Model Global Ocean Mixed Layer configuration (MetUM-GOML3). Experiments sixty years long, with and without ENSO cycle, considering lower (200 km) and higher (90 km) spatial resolution, are performed to assess if the ENSO influences MJO characteristics such as the phase distribution, propagation, convection, and teleconnections to South America (SA). The analyzes use daily continental precipitation data, daily global outgoing longwave radiation (OLR), and zonally asymmetric streamfunction computed with daily wind data. Composites of daily filtered anomalies in the 20-90 day band are assessed. Simulations without ENSO show (1) an established MJO extratropical teleconnection triggered by enhanced convection in the central-east subtropical South Pacific (SP) (source region), and its strongest impact on precipitation over SA in phase 8, earlier than in observations (phase 1); (2) an extratropical teleconnection via Rossby wave train, triggered by suppressed convection over the same region, with strongest impact on precipitation over SA in phase 4, with opposite sign; (3) increased horizontal resolution enhances the MJO convection and the anomalous circulation-precipitation dipole over SA, mainly over subtropical SA. However, the extratropical teleconnections via Rossby wave train at upper levels are slightly shifted east at higher resolution due to an enhanced SA westerly jet with respect to the lower resolution. The ENSO affects the basic state and the MJO convective anomalies, which modulate the MJO teleconnections and their impacts on SA in simulations with ENSO cycles. The EN (LN) basic state improves (worsens) MJO eastward propagation and its convection. However, both EN and LN states produce enhanced convection over the source region in phases 8+1, while suppressed convection over the same region in phase 4 is simulated only in EN. The extratropical teleconnections via Rossby wave train (phases 8+1, 4) and their impacts are stronger under ENSO with respect to those in simulations without ENSO. Hence, both ENSO states in the model generate forcing in the central-east subtropical SP that more efficiently triggers teleconnections than simulations without ENSO, indicating nonlinear ENSO effects on MJO anomalies over SA. As the MJO and its teleconnections improve during ENSO, other coupled global climate models (CGCMs) may reproduce these features, and subseasonal to seasonal (S2S) predictions to SA may be better forecast when ENSO and MJO peak in DJF, though the MJO impacts in phase 1 remain challenging.</p><p>Keywords: Coupled global models; ENSO-MJO Interaction; South American monsoon; Teleconnections.</p>

The present study reveals that the Madden–Julian oscillation (MJO)-related temperature anomalies over East Asia have notable differences among positive, neutral, and negative Arctic Oscillation (AO) phases. In MJO phases 2–3, cold anomalies over eastern China occur mainly during positive AO. In MJO phase 7, warm anomalies over eastern China are observed mostly during neutral AO, and in MJO phase 8 warm anomalies appear in positive and neutral AO. Regional mean temperature anomalies over northeastern East Asia tend to be negative during negative AO but positive during positive AO in six of eight MJO phases. In MJO phases 2–3, the AO-related mid- to high-latitude wave train over Eurasia and the MJO convection-triggered poleward wave train work together in contributing to negative height anomalies over eastern China and leading to cold anomalies there. The mid- to high-latitude wave train is stronger when the AO is negative than positive, which is associated with stronger zonal winds. In MJO phases 7–8, the positive AO-related mid- to high-latitude wave train over Eurasia and the MJO-induced poleward wave train cooperate in inducing positive height anomalies and leading to warm anomalies over eastern China. The mid- to high-latitude wave train is the main contributor to negative height anomalies over eastern China when the AO is negative during MJO phases 7–8. Meanwhile, the intensity of the South Asian wave source associated with the MJO convection is subjected to the modulation of southeastward dispersion of wave energy from western Europe during negative AO.

This study examines the multiscale modulation of mean and extreme rainfall in Northeast (NE) Australia under different background modes of variability, which is a new aspect given the high-resolution and long-term observational datasets. Daily rainfall probability is significantly modified by the Madden–Julian oscillation (MJO), and its influence varies with the seasons and is associated with atmospheric circulation anomalies. Rainfall generally decreases during El Niño and increases during La Niña years; however, there is a notable spatial nuance to El Niño–Southern Oscillation (ENSO)-associated extreme rainfall, with some locations showing the opposite precipitation response to the typical ENSO–rainfall relationship. Despite more precipitation overall in La Niña years, the mean and extreme precipitation responses to the MJO appear to be stronger and more often statistically significant during El Niño compared to La Niña periods. The impact of ENSO on the MJO–rainfall relationship is stronger than the variation of the MJO itself with ENSO, and likely reflects a change in the MJO modulation of rain-bearing atmospheric processes. During El Niño periods, diurnal rainfall amplitude is generally stronger in the central and southern subtropical parts of the study area than during La Niña periods, while the opposite tendency occurs in the northern tropical part. The diurnal cycle of both mean and extreme precipitation is amplified during suppressed convection phases compared to enhanced convection phases of the MJO. In general, the peak time of diurnal cycle does not change with MJO regimes, but there are some notable differences in rainfall propagation between enhanced and suppressed MJO phases. Significance Statement This study presents a new perspective on the relationship between precipitation in Northeast (NE) Australia and two important climate modes, the Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO). Rainfall in NE Australia is strongly influenced by the MJO, ENSO, and their interaction, suggesting that climate models need to capture both individual climate modes and their interactions for reliable rainfall projections. These findings have societal climate risk implications, as NE Australia is an area prone to catastrophic flooding due to extreme rainfall.

The responses of the Madden–Julian oscillation (MJO) to greenhouse warming have been widely studied, given its important role in subseasonal to seasonal prediction. However, due to the short satellite observation records, the long-term variability of MJO convective activity in the past century remains underexplored. Thus, this study reconstructed the MJO convection index [outgoing longwave radiation (OLR)-based MJO index (OMI)] since 1900, based on two twentieth century reanalyses. Based on the constructed century-long OMI, we further investigated dominant patterns of long-term changes in MJO convective activity from 1900 to 2009. Results show an overall significant increasing trend in MJO convective activity, while the MJO circulation feature exhibits an insignificant trend. Furthermore, results reveal that the changes in MJO convective activity over the past century are phase dependent, despite the overall enhancement in MJO convection. Empirical orthogonal function (EOF) analyses indicate that the first leading mode of MJO convective days is characterized by an increase across phase 2–3 and 6–7, while that of MJO convection intensity exhibits significant rising trends across all phases. These changes are attributed to the Indo-Pacific warm pool warming and the resulting increase in atmospheric low-level water vapor, which act as the heat and moisture sources for enhancing the moist static energy (MSE) and thus strengthen MJO convection. Meanwhile, the second leading mode of MJO convective days exhibits interdecadal oscillations, primarily modulated by the Pacific decadal oscillation–like sea surface temperature and vertical motion patterns. Finally, the contributions of each EOF mode to the variability of different MJO phases were also quantified.

This study evaluates the relationship between the Madden–Julian oscillation (MJO) and the occurrence of equatorial Pacific westerly wind bursts (WWBs). During the convective MJO phase, anomalous surface westerlies prevail in and west of the convective MJO center, providing favorable conditions for WWBs. Compared with the probability of WWBs expected under a null hypothesis that WWBs occur randomly, the convective MJO phase almost doubles the probability of a WWB occurring. Nevertheless, only 34.46% of WWBs co-occur with the convective MJO, which is much less than that reported in previous studies. We show that when the MJO and WWBs are defined using the same field with overlapping frequencies, the percentage of WWBs co-occurring with the convective MJO shows a significant increase. However, the higher percentage is simply caused by the fact that the strong WWBs during a convective MJO are more likely to be identified than those during the suppressed and neutral MJO phases. A total of 45.80% of WWBs are found to occur in the full MJO phase (both the convective and suppressed MJO phases), which is slightly higher than that expected based on randomness. Although the full MJO has statistically significant impact on the likelihood of WWBs, the influence from the full MJO on the tropical Pacific sea surface temperature anomaly is much weaker as compared to that from the WWBs. The relationships between the MJO and WWBs simulated in CMIP5 models are also assessed, and the percentage of WWBs that co-occur with the MJO simulated in models is in general less than that in observations.

This study investigates the evolution of cloud and rainfall structures associated with Madden–Julian oscillation (MJO) using Tropical Rainfall Measuring Mission (TRMM) data. Two complementary indices are used to define MJO phases. Joint probability distribution functions (PDFs) of cloud-top temperature and radar echo-top height are constructed for each of the eight MJO phases. The genesis stage of MJO convection over the western Pacific (phases 1 and 2) features a bottom-heavy PDF, characterized by abundant warm rain, low clouds, suppressed deep convection, and higher sea surface temperature (SST). As MJO convection develops (phases 3 and 4), a transition from the bottom-heavy to top-heavy PDF occurs. The latter is associated with the development of mixed-phase rain and middle-to-high clouds, coupled with rapid SST cooling. At the MJO convection peak (phase 5), a top-heavy PDF contributed by deep convection with mixed-phase and ice-phase rain and high echo-top heights (>5 km) dominates. The decaying stage (phases 6 and 7) is characterized by suppressed SST, reduced total rain, increased contribution from stratiform rain, and increased nonraining high clouds. Phase 7, in particular, signals the beginning of a return to higher SST and increased warm rain. Phase 8 completes the MJO cycle, returning to a bottom-heavy PDF and SST conditions similar to phase 1. The structural changes in rain and clouds at different phases of MJO are consistent with corresponding changes in derived latent heating profiles, suggesting the importance of a diverse mix of warm, mixed-phase, and ice-phase rain associated with low-level, congestus, and high clouds in constituting the life cycle and the time scales of MJO.

The summer monsoon rainfall over India exhibits strong intraseasonal variability. Earlier studies have identified Madden Julian Oscillation (MJO) as one of the most influencing factors of the intraseasonal variability of the monsoon rainfall. In this study, using India Meteorological Department (IMD) high resolution daily gridded rainfall data and Wheeler–Hendon MJO indices, the intra-seasonal variation of daily rainfall distribution over India associated with various Phases of eastward propagating MJO life cycle was examined to understand the mechanism linking the MJO to the intraseasonal variability. During MJO Phases of 1 and 2, formation of MJO associated positive convective anomaly over the equatorial Indian Ocean activated the oceanic tropical convergence zone (OTCZ) and the resultant changes in the monsoon circulation caused break monsoon type rainfall distribution. Associated with this, negative convective anomalies over monsoon trough zone region extended eastwards to date line indicating weaker than normal northern hemisphere inter tropical convergence zone (ITCZ). The positive convective anomalies over OTCZ and negative convective anomalies over ITCZ formed a dipole like pattern. Subsequently, as the MJO propagated eastwards to west equatorial Pacific through the maritime continent, a gradual northward shift of the OTCZ was observed and negative convective anomalies started appearing over equatorial Indian Ocean. During Phase 4, while the eastwards propagating MJO linked positive convective anomalies activated the eastern part of the ITCZ, the northward propagating OTCZ merged with monsoon trough (western part of the ITCZ) and induced positive convective anomalies over the region. During Phases 5 and 6, the dipole pattern in convective anomalies was reversed compared to that during Phases 1 and 2. This resulted active monsoon type rainfall distribution over India. During the subsequent Phases (7 and 8), the convective and lower tropospheric anomaly patterns were very similar to that during Phase 1 and 2 except for above normal convective anomalies over equatorial Indian Ocean. A general decrease in the rainfall was also observed over most parts of the country. The associated dry conditions extended up to northwest Pacific. Thus the impact of the MJO on the monsoon was not limited to the Indian region. The impact was rather felt over larger spatial scale extending up to Pacific. This study also revealed that the onset of break and active events over India and the duration of these events are strongly related to the Phase and strength of the MJO. The break events were relatively better associated with the strong MJO Phases than the active events. About 83% of the break events were found to be set in during the Phases 7, 8, 1 and 2 of MJO with maximum during Phase 1 (40%). On the other hand, about 70% of the active events were set in during the MJO Phases of 3 to 6 with maximum during Phase 4 (21%). The results of this study indicate an opportunity for using the real time information and skillful prediction of MJO Phases for the prediction of break and active conditions which are very crucial for agriculture decisions.

<strong class="journal-contentHeaderColor">Abstract.</strong> Research in the last decades revealed that rapidly ascending airstreams in extratropical cyclones &ndash; so-called warm conveyor belts (WCBs) &ndash; play an important role in extratropical atmospheric dynamics. However on the subseasonal time scale, 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 time scale. 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 particularly pronounced during La Ni&ntilde;a 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 favoured 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.