Madden–Julian Oscillation Cycle 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.

ENSO Modulation of Global MJO and Its Impacts on South America

Abstract 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.

Variability of volume transport in the Taiwan Strait and its response to tropical MJO convection: A numerical approach

Taiwan Strait (TWS) plays a crucial role in material exchange and nutrient budget between the South China Sea and the East China Sea. In this study, we investigate the variability of volume transport in the TWS and its response to tropical Madden–Julian Oscillation (MJO) convection based on the simulated results from a three-dimensional operational numerical model. Validated by the observational data, the model generally reproduced the physical field well. The volume transport in the TWS has strong seasonal cycles as well as higher-frequency variations. Intra-seasonal fluctuations dominate the along-strait currents variability, while secondary and the last signal are seasonal one and inter-annual variability, respectively. Overall, the along-strait wind stress plays a more important role in controlling the variability of volume transport in the TWS than the pressure gradient induced by north-to-south sea level slope. At intra-seasonal time scales, the volume transport in the TWS varies with the movement of the tropical MJO convection from Indian Ocean eastward to the western Pacific Ocean. These oceanic anomalies are related to atmospheric anomalies, with a distinct physical linkage from the tropical atmosphere to the mid-latitude ocean. The tropical MJO deep convection can modify the upper tropospheric heights and generate a wave train pattern that propagates to mid-latitudes. In addition, these anomalous upper tropospheric heights modulate the surface pressure, resulting in a cyclonic anomaly. The upper tropospheric heights anomaly and its corresponding mean sea level pressure anomaly move eastward in the MJO cycle following the migrating MJO heat source in the tropics from phase 2 to 5. Consequently, surface winds change as the cyclonic anomaly moves from central China to the east of Japan, resulting in a northeast volume transport anomaly during MJO phase 2 and 3 and a southwest volume transport anomaly during MJO phase 4 and 5.

The effect of the Madden–Julian Oscillation on the global electric circuit

The Madden–Julian Oscillation (MJO) is the most dominant component of the climate variability in the tropics on the timescale of tens of days. Occurring irregularly, the MJO consists of an eastward propagation of certain large-scale coupled structures in atmospheric circulation and deep convection. Here we investigate the effect of the MJO on the direct current global electric circuit (GEC), using both numerical simulations and the results of electric field measurements. We have reproduced the atmospheric dynamics for 1980–2020 with the help of the Weather Research and Forecasting model and meteorological reanalysis data; this allowed us to simulate 41 years of the GEC variation by parameterising contributions to the ionospheric potential (IP) in terms of precipitation and convection. Besides, we have analysed the results of surface potential gradient (PG) measurements at the Vostok station in Antarctica during 2006–2020. When averaged over many days, both the simulated IP and measured fair-weather PG show sinewave-like variations with the MJO phase, which manifest a statistically significant correlation with the MJO cycle. A more elaborate investigation involving an application of empirical orthogonal functions shows that the dynamics of contributions to the IP follows the patterns of variability of deep convection typical for the MJO. The variation of the IP on the MJO scale can largely be represented as a superposition of two basic oscillating patterns of convection, one of which is located over the Maritime Continent and its surrounding seas and the other is located over the Indian Ocean and part of South-East Asia. These oscillating patterns together describe the eastward progression of perturbations and can be naturally associated with the two components of the Real-time Multivariate MJO index, widely used to describe the MJO. Together with earlier results about links between the GEC and the El Niño—Southern Oscillation, the results concerning the MJO obtained in this study indicate that the GEC is an important part of the Earth system, which reflects climate variability on different timescales.

Cloud Radiative Effects on MJO Development in DYNAMO

Abstract Observed Madden–Julian oscillation (MJO) events are examined with the aid of regional model simulations to understand the role of cloud radiative effects in the MJO development. The importance of this role is demonstrated by the absence of the MJO in the model simulations that contain no cloud radiative effects. Comparisons of model simulations with and without the cloud radiative effects and observation help identify the major processes arising from those effects. Those processes develop essentially from heating in the upper troposphere due to shortwave absorption within anvil clouds in the upper troposphere and the convergence of longwave radiation in the middle to upper troposphere, with a peak at 300 hPa, during deep convection. First, that heating adds extra buoyancy and accelerates the rising motion in the upper troposphere in deep convection. The vertical acceleration in the upper troposphere creates a vacuum effect and demands for more deep convection to develop. Second, in response to that demand and required by mass balance arises the large-scale horizontal and vertical mass, moisture, and energy convergence. It strengthens deep convection and, with the feedback from continuing cloud radiative effect, creates conditions that can perpetuate deep convection and MJO development. That perpetuation does not occur however because those processes arising from the cloud radiative heating in the upper troposphere stabilize the troposphere until it supports no further deep convection. Weakening deep convection reduces cloud radiative effects. The subsequent reduction of the vacuum effect in the upper troposphere diminishes deep convection completing an MJO cycle. These results advance our understanding of the development of the MJO in the radiative–convective system over warm waters in the tropics. They show that while the embryo of intraseasonal oscillation may exist in the system its growth/development is largely dependent on cloud radiative effects and feedbacks.

Variability of Jakarta Rain-Rate Characteristics Associated with the Madden–Julian Oscillation and Topography

Abstract Research on the interaction between the Madden–Julian oscillation (MJO) and rainfall around Jakarta is limited, although the influence of the MJO on increased rainfall is acknowledged as one of the primary causes of flooding in the region. This paper investigates the local rainfall response around Jakarta to the MJO. We used C-band Doppler radar in October–April during 2009–12 to study rain-rate characteristics at much higher resolution than previous analyses. Results show that the MJO strongly modulates rain rates over the region; however, its effect varies depending on topography. During active phases, MJO induces a high rain rate over the ocean and coast, meanwhile during suppressed phases, it generates a high rain rate mainly over the mountains. In phase 2 of the MJO we find the strongest increase in mean and extreme rain rate, which is earlier in the MJO cycle than most studies reported, based on lower-resolution data. This higher rain rate is likely due to increases in convective and stratiform activities. The MJO promotes more stratiform rain once it resides over Indonesia. In phase 5, over the northwestern coast and western part of the radar domain, the MJO might bring forward the peak of the hourly rain rate that occurs in the early morning. This is likely due to a strong westerly flow arising from MJO superimposed westerly monsoonal flow, blocked by the mountains, inducing a strong wind propagating offshore resulting in convection near the coast in the morning. Our study demonstrates the benefits of using high-resolution radar for capturing local responses to the larger-scale forcing of the MJO in Indonesia. Significance Statement Rainfall in Jakarta and its surroundings is highly variable and often heavy resulting in devastating floods. In this region, in the wet season, rainfall is influenced by large-scale climate variability including the Madden–Julian oscillation (MJO) characterized by eastward propagation of clouds near the equatorial regions on intraseasonal time scales. The MJO has been known to increase the probability of rainfall occurrence and its magnitude, but we show that the impact differs in varying topography. The frequency and intensity of rainfall increase over land areas including mountains even when MJO has not arrived in Indonesia. Meanwhile, once MJO moves through Indonesia, the frequency and magnitude of the rainfall increases over the northern coast and ocean as well as in the west of the radar domain.

Effect of the MJO on East Asian winter rainfall as revealed by an SVD analysis

AbstractThis paper studies the influences of the Madden Julian Oscillation (MJO) on East Asian (EA) winter rainfall using the Singular Value Decomposition (SVD) approach. This method uses two-dimensional instead of the latitudinally-averaged variables in the commonly used Real-time Multivariate MJO (RMM) index. A comparison of the two approaches is made using the same OLR and zonal wind data over 37 boreal winter seasons of December – March. The SVD composite reveals a more conspicuous and coherent variation throughout the MJO cycle, while the RMM composite is more ambiguous. In particular, the SVD analysis identifies the convection anomalies over the Maritime Continent and the subtropical western Pacific (MCWP) as a major cause of enhanced rainfall in EA at RMM phases 8 and 1. This is at least one-eighth cycle earlier than the phases of convection development over Indian Ocean (IO) that were emphasized by previous studies. A linearized global baroclinic model is used to demonstrate the mechanism of MJO forcing on EA rainfall during various phases, with a focus on the MCWP cooling. The result shows that the anomalous MCWP cooling and the resultant low-level anticyclonic flow interact with the East Asian Jet, leading to an overall weakened EA winter monsoon circulation. The associated anomalous overturning circulation, with ascending motion and low-level horizonal moisture convergence in EA, contributes to the enhanced rainfall. This model result supports the interpretation of the SVD analysis, in that the MCWP-cooling induced anomalous meridional circulation is a more direct cause of enhanced EA rainfall than the IO-heating (or the IO-MCWP heating dipole) induced Rossby wave teleconnection.

Active and break phases of the South American summer monsoon: MJO influence and subseasonal prediction

The role of the Madden–Julian Oscillation (MJO) in producing active and break periods of the South American (SA) monsoon and the performance of the ECMWF and NCEP models in predicting these periods at multiweek lead times are assessed. Two monsoon indices, based on precipitation and wind, are proposed to characterize these periods. The models represent well the observed association of active and break monsoon days with large scale convection and circulation anomalies. Although reproducing approximately the distribution of active and break days proportions in each phase of the MJO cycle, models produce a phase shift between observed and simulated distributions because they establish the teleconnection between Central Pacific and South America, as well as its impacts, sooner than in observations. The predictive skill of both rainfall and wind anomalies is limited to about 2 weeks, with the monsoon wind index displaying higher correlation score till week 3. The forecast performance is apparently not affected by initialization on active or break monsoon days. However, it is higher for prediction of lower precipitation in break days than heavier rainfall in active days. Wind is much better predicted than rainfall for active days, which could be used for extreme rainfall events forecast. Although relatively small at shorter lead times, the MJO contribution is the major source of rainfall predictability after week 3. To improve the multiweek prediction of SA monsoon, models need not only to predict correctly the MJO phase, but also to reproduce in the right phase the MJO-related SA rainfall anomalies.

Madden–Julian Oscillation impacts on South American summer monsoon season: precipitation anomalies, extreme events, teleconnections, and role in the MJO cycle

The impacts of the Madden Julian Oscillation (MJO) on the South American monsoon season are analyzed in the global context of the MJO propagating anomalies of convection and circulation. Unexplored aspects, such as the continental-scale daily precipitation anomalies in the MJO frequency band and changes in the frequency of extreme rainfall events, are disclosed throughout its cycle. Among other effects, the MJO increases the average daily precipitation by more than 30% of the climatological value and doubles the frequency of extreme events over central-east South America (SA), including the South Atlantic Convergence Zone (SACZ). The evolution of the most intense precipitation anomalies depends on the interplay between tropics–tropics and tropics–extratropics teleconnections, and the topography over central-east SA seems to play a role in enhancing low-level convergence. The maximum anomalies are produced by a tropics-extratropics wave train. It not only favors precipitation anomalies over the SACZ and subtropical SA, but also strengthens the anomalies over tropical SA when the system propagates northeastward. Influence function analysis and simulations of the responses to different components of upper-level anomalous divergence associated with the MJO anomalous convection indicate the probable origin of the anomalous circulation leading to the main precipitation anomalies over SA. It is triggered by secondary anomalous convection, while the main tropical anomalous circulation is produced by the strongest equatorial convection anomalies. There are indications that MJO-related anomalies over SA contribute to the impacts on other regions and to the initiation of the MJO in the Indian Ocean.

Impact of Convective Gravity Waves on the Tropical Middle Atmosphere During the Madden‐Julian Oscillation

AbstractThe Madden‐Julian oscillation (MJO) prominently influences weather systems in the tropical Indian Ocean region. Although understanding of the MJO has improved over the last decade, insufficient simulation of the MJO still considerably degrades forecasting skill of atmospheric models. In this study we investigate the interaction of gravity waves (GWs) from convection with the background atmosphere during MJO cycles between 1979 and 2010. For this, we utilize a parameterization of convective GW (CGW) forcing and propagation to evaluate the entire life cycle of GWs from their convective excitation to their dissipation in the tropical upper stratosphere region. CGW forcing at source level was calculated using the Song and Chun (2005, https://doi.org/10.1175/JAS-3363.1) parameterization. We performed simulations for all respective MJO phases and MJO cycles during a period of 32 years using Climate Forecast System Reanalysis data for the full spectrum of CGWs. Our results show a clear connection between 850 hPa winds and zonal momentum flux during all MJO phases. This prominent momentum flux feature changes with the progress of each MJO cycle. Zonal means of eastward directed momentum flux also show this correlation. Furthermore, we estimated the spectral characteristics of launched GWs and found significant GW momentum flux for horizontal wavelengths as short as 80 km at launch altitude. Eastward GW drag (GWD) consistently shows a maximum at altitudes below 40 km and prominent westward GWD at altitudes higher than 40 km. Additionally, we investigated the impact of the phase of the quasi‐biennial oscillation (QBO) on our findings with GWD found stronger during the QBO easterly phase compared to the QBO westerly phase.

The Relationship between Northern Hemisphere Winter Blocking and Tropical Modes of Variability

In the present study, the influence of some major tropical modes of variability on Northern Hemisphere regional blocking frequency variability during boreal winter is investigated. Reanalysis data and an ensemble experiment with the ECMWF model using relaxation toward the ERA-Interim data inside the tropics are used. The tropical modes under investigation are El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), and the upper-tropospheric equatorial zonal-mean zonal wind [[Formula: see text]] E. An early (late) MJO phase refers to the part of the MJO cycle when enhanced (suppressed) precipitation occurs over the western Indian Ocean and suppressed (enhanced) precipitation occurs over the Maritime Continent and the western tropical Pacific. Over the North Pacific sector, it is found that enhanced (suppressed) high-latitude blocking occurs in association with El Niño (La Niña) events, late (early) MJO phases, and westerly (easterly) [[Formula: see text]] E. Over central to southern Europe and the east Atlantic, it is found that late MJO phases, as well as a suppressed MJO, are leading to enhanced blocking frequency. Furthermore, early (late) MJO phases are followed by blocking anomalies over the western North Atlantic region, similar to those associated with a positive (negative) North Atlantic Oscillation. Over northern Europe, the easterly (westerly) phase of [[Formula: see text]] E is associated with enhanced (suppressed) blocking. These results are largely confirmed by both the reanalysis and the model experiment.

Impact of atmospheric convectively coupled equatorial Kelvin waves on upper ocean variability

AbstractConvectively coupled Kelvin waves (CCKWs) are atmospheric weather systems that propagate eastward along the equatorial wave guide with phase speeds between 11 and 14 m s−1. They are an important constituent of the convective envelope of the Madden‐Julian oscillation (MJO), for which ocean‐atmosphere interactions play a vital role. Hence, ocean‐atmosphere interactions within CCKWs may be important for MJO development and prediction and for tropical climate, in general. Although the atmospheric structure of CCKWs has been well studied, their impact on the underlying ocean is unknown. In this paper, the ocean‐atmosphere interactions in CCKWs are investigated by a case study from November 2011 during the CINDY/DYNAMO field experiment, using in situ oceanographic measurements from an ocean glider. The analysis is then extended to a 15 year period using precipitation data from the Tropical Rainfall Measuring Mission and surface fluxes from the TropFlux analysis. A methodology is developed to calculate trajectories of CCKWs. CCKW events are strongly controlled by the MJO, with twice as many CCKWs observed during the convectively active phase of the MJO compared to the suppressed phase. Coherent ocean‐atmosphere interaction is observed during the passage of a CCKW, which lasts approximately 4 days at any given longitude. Surface wind speed and latent heat flux are enhanced, leading to a transient suppression of the diurnal cycle of sea surface temperature (SST) and a sustained decrease in bulk SST of 0.1°C. Given that a typical composite mean MJO SST anomaly is of the order of 0.3°C, and more than one CCKW can occur during the active phase of a single MJO event, the oceanographic impact of CCKWs is of major importance to the MJO cycle.

Atmosphere‐ocean coupled processes in the Madden‐Julian oscillation

AbstractThe Madden‐Julian oscillation (MJO) is a convectively coupled 30–70 day (intraseasonal) tropical atmospheric mode that drives variations in global weather but which is poorly simulated in most atmospheric general circulation models. Over the past two decades, field campaigns and modeling experiments have suggested that tropical atmosphere‐ocean interactions may sustain or amplify the pattern of enhanced and suppressed atmospheric convection that defines the MJO and encourage its eastward propagation through the Indian and Pacific Oceans. New observations collected during the past decade have advanced our understanding of the ocean response to atmospheric MJO forcing and the resulting intraseasonal sea surface temperature fluctuations. Numerous modeling studies have revealed a considerable impact of the mean state on MJO ocean‐atmosphere coupled processes, as well as the importance of resolving the diurnal cycle of atmosphere‐upper ocean interactions. New diagnostic methods provide insight to atmospheric variability and physical processes associated with the MJO but offer limited insight on the role of ocean feedbacks. Consequently, uncertainty remains concerning the role of the ocean in MJO theory. Our understanding of how atmosphere‐ocean coupled processes affect the MJO can be improved by collecting observations in poorly sampled regions of MJO activity, assessing oceanic and atmospheric drivers of surface fluxes, improving the representation of upper ocean mixing in coupled model simulations, designing model experiments that minimize mean state differences, and developing diagnostic tools to evaluate the nature and role of coupled ocean‐atmosphere processes over the MJO cycle.

Role of Longwave Cloud–Radiation Feedback in the Simulation of the Madden–Julian Oscillation

Abstract The role of the cloud–radiation interaction in the simulation of the Madden–Julian oscillation (MJO) is investigated. A special focus is on the enhancement of column-integrated diabatic heating due to the greenhouse effects of clouds and moisture in the region of anomalous convection. The degree of this enhancement, the greenhouse enhancement factor (GEF), is measured at different precipitation anomaly regimes as the negative ratio of anomalous outgoing longwave radiation to anomalous precipitation. Observations show that the GEF varies significantly with precipitation anomaly and with the MJO cycle. The greenhouse enhancement is greater in weak precipitation anomaly regimes and its effectiveness decreases monotonically with increasing precipitation anomaly. The GEF also amplifies locally when convection is strengthened in association with the MJO, especially in the weak precipitation anomaly regime (<5 mm day−1). A robust statistical relationship is found among CMIP5 climate model simulations between the GEF and the MJO simulation fidelity. Models that simulate a stronger MJO also simulate a greater GEF, especially in the weak precipitation anomaly regime (<5 mm day−1). Models with a greater GEF in the strong precipitation anomaly regime (>30 mm day−1) represent a slightly slower MJO propagation speed. Many models that lack the MJO underestimate the GEF in general and in particular in the weak precipitation anomaly regime. The results herein highlight that the cloud–radiation interaction is a crucial process for climate models to correctly represent the MJO.

The MJO Cycle Forcing of the North Atlantic Circulation: Intervention Experiments with the Community Earth System Model

Abstract A three-dimensional evolution of Madden–Julian oscillation (MJO) diabatic heating for October–March from satellite data is constructed: the heating propagates eastward for three cycles, modulated by the likelihood for a given MJO phase to occur on a given calendar day. This heating is added to the temperature tendencies of each member of an ensemble of 48 (1 October–31 March) simulations with the Community Earth System Model. The leading two most predictable modes of the planetary wave vertically integrated total (added plus model generated) heating capture 81% of the ensemble-mean variance and form an eastward-propagating oscillation with very high signal-to-noise ratio. The two most predictable modes of the extratropical Northern Hemisphere 200-hPa height form an oscillation, as do those of the 300-hPa height tendency due to synoptic vorticity flux convergence, the 200-hPa Rossby wave source, and the envelope transient kinetic energy. The North Atlantic Oscillation (NAO+) occurs 15–25 days after the MJO convection crosses the 90°E meridian, supported by synoptic vorticity flux convergence and a distinct pattern of Rossby wave source. The daily North Atlantic circulation anomalies are categorized into four circulation regimes with a cluster analysis. The NAO+ and NAO− are equally likely in the control model runs, but the NAO+ is 10% more likely in the model runs with heating, compared to a difference of 14% in reanalyses. The daily occurrence of the NAO+ regime in the heating ensemble shows maxima at times when the leading two optimal modes of height also indicate NAO+ but also shows maxima at other times.

MJO Modulation of Station Rainfall in the Semiarid Seridó, Northeast Brazil

The influence of the Madden Julian Oscillation (MJO) on station rainfall over the Seridó region of Rio Grande do Norte state, Northeast Brazil is examined based on 17 raingauge daily data over 30-year period (1 January-31 December, 1981-2010). The Seridó is one of the driest regions in Northeast Brazil and is recognized as particularly vulnerable to desertification by the United Nations Convention to Combat Desertification. Firstly, daily anomalies were calculated by removing the 30-year daily climatology. Then, to distinguish the MJO signal from other patterns of climate variability, the daily anomalies were band pass filtered in the frequency domain (20 - 90 days) by applying Fast Fourier Transform (FFT). Composites of rainfall anomalies were computed for each of the eight phases of the MJO during February-May (FMAM) rainy season, based on the Jones-Carvalho MJO index. Only days classified as MJO events were considered in the composites. For each phase composite, statistical significance tests were computed independently at each individual station by applying a two-tailed Student’s t-test at 5% significance level. Preliminary results showed that the rainfall anomalies have a spatial coherence throughout the MJO cycle. Extreme positive (negative) anomalies occurred in phase 2 (phase 5), where 13 (12) out of the 17 stations showed statistically significant anomalies in the range of 0.9 - 1.9 mm/day (0.8 - 1.7 mm/day). The typical difference between the wet MJO phase 2 and dry phase 5 represented at least 50% modulation of the daily mean rainfall.

Transforming circumnavigating Kelvin waves that initiate and dissipate the Madden–Julian Oscillation

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.

Three-Dimensional Structure and Evolution of the MJO and Its Relation to the Mean Flow

Abstract The two leading principal components of the daily 850- minus 150-hPa global velocity potential in the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) (1979–2011) data are used as time-varying Madden–Julian oscillation (MJO) indices. Regression maps and meridional cross sections based on these indices are used to document the structure and evolution of the zonal wind (u) and geopotential height (Z) anomalies in the MJO cycle. The data are daily, and they are not separated by season. At upper-tropospheric levels the MJO signature is dominated by eastward-propagating planetary wave packets consisting of equatorial Kelvin waves flanked by Rossby waves centered along 28°N/S, for which the westerly jet streams serve as waveguides. At lower-tropospheric levels the pattern more closely resembles the response to a pulsating heat source over the Maritime Continent, where the Andes block the eastward-propagating Kelvin wave pulse. The contrasting upper- and lower-tropospheric patterns are made up of the same building blocks: a deep, baroclinic modal structure with a node at the 400-hPa level, which dominates the tropical signature, and a barotropic residual field consisting mainly of extratropical wave trains oriented along great circles. The extratropical wave trains emanate from the flanking Rossby waves in the baroclinic modal structure. The strongest of them, which resembles the Pacific–North America (PNA) pattern, extracts kinetic energy from the climatological-mean flow in the jet exit region. At other longitudes the jet stream seems to act as a barrier to the poleward propagation of MJO-related wave activity.

Aquarius surface salinity and the Madden‐Julian Oscillation: The role of salinity in surface layer density and potential energy

AbstractSea surface salinity (SSS) data from the Aquarius satellite are analyzed along with auxiliary data to investigate the SSS signature of the Madden‐Julian Oscillation (MJO) in the equatorial Indian and Pacific Oceans, the effect of evaporation‐minus‐precipitation (E‐P), the implication for the role of ocean dynamics, and the SSS influence on surface density and potential energy. MJO‐related SSS changes are consistent with E‐P forcing in the western Indian Ocean throughout the MJO cycle and in the central Indian Ocean during the wet phase of the MJO cycle. However, SSS changes cannot be explained by E‐P in the central Indian Ocean during the dry phase and in the eastern Indian and western Pacific Oceans throughout the MJO cycle, implying the importance of ocean dynamics. SSS has an overall larger contribution to MJO‐related surface density and potential energy anomalies than SST. It partially offsets the SST effect in the western‐to‐central Indian Ocean and reinforces the SST effect in the eastern Indian and western Pacific Oceans. Ocean modeling and assimilation need to properly account for salinity effects in order to correctly represent mixed layer variability associated with the MJO. Our results also clarify some discrepancy in previous studies about the E‐P effect on MJO‐related SSS variations.

Modulation of the Convectively Coupled Kelvin Waves over South America and the Tropical Atlantic Ocean in Association with the Madden–Julian Oscillation

Abstract In this study, evidence of the strong modulation of the convectively coupled Kelvin wave (CCKW) activity by the Madden–Julian oscillation (MJO) is presented, with a particular focus over the South America and tropical Atlantic region. The MJO impacts on CCKWs over this region, as noted in anomalous fields of rainfall as well as vertical profiles of wind, moisture, and temperature, are primarily through the modulation of Kelvin wave amplitude, with secondary effects on vertical structure, and little impact on wavenumber. CCKW activity is enhanced during MJO phases 8, 1, and 2 and damped during MJO phases 4, 5, and 6. Further analyses reveal that the strong modulation of the MJO on the CCKW activity could be largely through two factors: namely, the vertical zonal wind shear and the lower- to middle-tropospheric specific humidity. The CCKW activity tends to be enhanced during MJO phases when the easterly vertical wind shear and positive low- to midtroposphere moisture anomalies are present and vice versa. These two physical processes associated with the MJO are found to have positively (negatively) reinforcing influences on the CCKW activity in phase 1 (4 and 5), while counteracting influences in phases 2, 3, 6, 7, and 8, leading to the observed MJO cycle of the CCKW activity anomalies in the study region. The results presented in this study may have important implications for extended-range prediction of tropical wave activity and might suggest possible roles of the upstream CCKWs in the initiation of the MJO in the western Indian Ocean.