Madden–Julian Oscillation Convection Research Articles

Monitoring the Madden–Julian oscillation with geopotential height

This paper examines the three-dimensional geopotential height structure of the Madden–Julian oscillation (MJO) and proposes that the MJO convection signals can be well reflected by upper-tropospheric zonal anomalous height gradient (ZAHG, $$\nabla_{x} z'$$ ). A case study in 2011 and composite analyses have been done by applying a wavenumber-frequency filtering method on daily variable anomalies derived from different reanalysis datasets. Results show that the upper-level eastward-moving intraseasonal geopotential height anomaly does not exactly display a Rossby–Kelvin wave structure of the Gill model. Instead, it shows a height quadrupole pattern and is well associated with temperature and zonal wind anomalies. The notable tilted upper-tropospheric temperature structure can be derived from the height anomalies by the hydrostatic balance relationship. The first baroclinic mode vertical structure of zonal wind anomalies can be approximated from the height anomalies by the geostrophic equation. A highlight of the paper is the proposal of tracing the MJO convection center by the upper-tropospheric ZAHG ( $$\nabla_{x} z'$$ ). Composite analyses of all MJO events during extended winter (NDJFM) from 1979 to 2013 suggest that positive upper-tropospheric ZAHG is exactly in phase with negative outgoing longwave radiation (OLR) anomalies in the eastern Indian Ocean and Maritime Continent during a MJO event. On the other hand, positive upper-tropospheric ZAHG leads (lags) negative OLR anomalies by 1 (2) days in the western Indian Ocean (western Pacific Ocean), where MJO events generally initiate (dissipate). ZAHG can also be applied to modeling data and observation data before the satellite era. This study provides a new diagnostic parameter option to monitor MJO events, and an alternative for studying MJOs before the satellite era and model evaluation.

The relationship between the Madden–Julian Oscillation and the North Atlantic Oscillation

In this study, the relationship between the Madden–Julian Oscillation (MJO) and the North Atlantic Oscillation (NAO) is investigated by analysing NAO events both when the MJO is active and inactive. The European Centre for Medium‐Range Weather Forecasts Re‐Analysis (ERA‐Interim) dataset is used, for the years 1979–2006 and the months December–February.When the MJO is active, negative (positive) NAO events are preceded by enhanced MJO convection over the tropical western and central Pacific Ocean (tropical Indian Ocean and Maritime Continent). The accompanying sea‐level pressure (SLP) anomaly fields reveal patterns that closely resemble the northern annular mode (NAM). The MJO‐related negative NAO events are preceded by an increase in the vertical propagation of planetary‐scale wave activity into the stratosphere, followed by a marked weakening of the zonally symmetric component of the lower stratospheric zonal wind. The opposite behaviour is found for the MJO‐related positive NAO events. The MJO‐related NAO events are also long‐lived, persisting for approximately 30 days. In contrast, when the MJO is inactive, there is little change to the zonally symmetric component of the lower stratospheric zonal wind, these events are relatively short‐lived, lasting for about 10 days, and the NAO corresponds to a dipole that is confined to the North Atlantic.

Extreme precipitation events and their relationship with ENSO and MJO phases over northern South America

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.

Influence of topography on temperature variations in the tropical tropopause layer

AbstractTemperature variations in the tropical tropopause layer (TTL) play an important role in dehydration in the upper troposphere and lower stratosphere. Equatorial Kelvin waves associated with the Madden‐Julian Oscillation (MJO) are known to induce remarkable temperature variations in the TTL. In this study, the influence of topography on temperature variations in the TTL is investigated by using radiosonde data, satellite data, reanalysis data, and numerically simulated data. When MJO convection passes over the radiosonde sites, temperature variations near mountainous regions are larger than those measured in regions of lower elevation. The difference in temperature amplitude reaches ~1–2 K. Large temperature variations over mountainous regions were also found in other data sets. Numerically simulated data from the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) are also used to investigate the temperature variations in the TTL. The results show that the temperature variations associated with Kelvin waves become large over mountainous regions. A sensitivity test using the stretched version of NICAM gave two important results: (i) the height of mountains affects the magnitude of the temperature variations in the TTL, and (ii) the terrain‐following coordinate system used in the model produces the artificially high‐temperature variation in the TTL. When Kelvin waves pass over mountainous regions, topographic gravity waves are excited and superimposed on the Kelvin waves, thereby producing large temperature variations over these regions. The mountainous region of the Indonesian Maritime Continent is a favorable location for large temperature variations in the TTL.

The impact of the diurnal cycle on the propagation of Madden‐Julian Oscillation convection across the Maritime Continent

AbstractInfluences of the diurnal cycle on the propagation of the Madden‐Julian Oscillation (MJO) convection across the Maritime Continent (MC) are examined using cloud‐permitting regional model simulations and observations. A pair of ensembles of control (CONTROL) and no‐diurnal cycle (NODC) simulations of the November 2011 MJO episode are performed. In the CONTROL simulations, the MJO signal is weakened as it propagates across the MC, with much of the convection stalling over the large islands of Sumatra and Borneo. In the NODC simulations, where the incoming shortwave radiation at the top of the atmosphere is maintained at its daily mean value, the MJO convection signal propagating across the MC is enhanced. Examination of the surface energy fluxes in the simulations indicates that the surface downwelling shortwave radiation is larger in the presence of the diurnal cycle (CONTROL simulations) primarily because clouds preferentially form in the afternoon and are smaller during day time in comparison to nighttime. Furthermore, the diurnal covariability of surface wind speed and skin temperature results in a larger sensible heat flux and a cooler land surface in the CONTROL runs compared to NODC runs. An analysis of observations indicates that ahead of and behind the MJO active phase, the diurnal cycle of cloudiness enhances downwelling shortwave radiation and hence convection over the MC islands. This enhanced stationary convection competes with and disrupts the convective signal of MJO events that propagate over the waters surrounding the islands.

Gross moist stability and the Madden–Julian Oscillation in reanalysis data

This study uses the ERA‐Interim and Climate Forecast System reanalysis datasets to examine the relationship between the convective activity of the Madden–Julian Oscillation (MJO) and the normalized gross moist stability (NGMS) on various time‐scales. Previous studies based on proposed theories for the MJO and the assessments of general circulation models suggest that MJO convection tends to be more active where the NGMS is smaller or effectively negative. This relationship only appears using reanalysis data on some time‐scales and limited geographical regions. On seasonal time‐scales, MJO convective activity shifts with the regions of positive NGMS and net moisture import, indicating positive correlation between MJO convective activity and the NGMS. On interannual to decadal time‐scales, stronger MJO convective events tend to occur with a background state of smaller NGMS exclusively over the central Pacific basin, indicating negative correlation. When the environment switches between net moisture export to import, MJO convective activity tends to follow the region of net moisture import, leading to its positive correlation with NGMS. However, when the environment already has net moisture import and positive NGMS, MJO convective activity is negatively correlated with the NGMS and stronger MJO convective events tend to occur with smaller positive NGMS. Therefore, the background NGMS is not a globally applicable metric for MJO convective activity and it needs to be used with caution over such environments where the NGMS can oscillate around zero. On intraseasonal time‐scales, as shown by previous modelling and observational studies, the NGMS becomes anomalously negative prior to the onset of MJO enhanced convection and becomes positive as the convection peaks over the Indo‐Pacific basin. The results of this study demonstrate that the relationship between MJO convective activity and the NGMS in reanalysis data largely depends on time‐scales and geographical region.

Large‐scale precipitation tracking and the MJO over the Maritime Continent and Indo‐Pacific warm pool

AbstractA large‐scale precipitation tracking (LPT) method is developed to track convection and precipitation associated with the Madden‐Julian oscillation (MJO) using the Tropical Rainfall Measurement Mission 3B42 rainfall data from October to March 1998–2015. LPT uses spatially smoothed 3 day rainfall accumulation to identify and track precipitation features in time with a minimum size of 300,000 km2 and time continuity at least 10 days. While not all LPT systems (LPTs) are attributable to the MJO, among the 199 LPTs, there were 42 with a mean eastward propagation of at least 2 m s−1, which are considered to be MJO convective initiation events. These LPTs capture the diversity of the MJO convection, which is not well depicted by the Real‐time Multivariate MJO (RMM) index or the outgoing longwave radiation MJO index. During the 17 years, there were 17 instances out of 45 with a MJO signature in the RMM without eastward propagating LPTs. Among the 42 eastward propagating LPTs, 24 propagated across the Maritime Continent (MC), which confirms the MC barrier effect. Among the cases that crossed the MC from the Indian Ocean to the western Pacific (MC crossing), 18 (75%) had a significant MJO signature in the RMM index. In contrast, only six (33%) of the non‐MC‐crossing cases occurred with a RMM MJO signal. There is a significant seasonal and interannual variability with MC‐crossing LPTs occurring in December more commonly than other months. More MC‐crossing events were observed during La Niña than El Niño, which is consistent with the observations of stronger and more frequent MJO events identified by RMM during La Niña years.

Diagnosing ocean feedbacks to the MJO: SST‐modulated surface fluxes and the moist static energy budget

AbstractThe composite effect of intraseasonal sea surface temperature (SST) variability on the Madden‐Julian Oscillation (MJO) is studied in the context of the column‐integrated moist static energy (〈m〉) budget using data from the European Centre for Medium‐Range Weather Forecasts Interim Reanalysis (ERA‐I). SST fluctuations influence the Δq and ΔT parts of the bulk surface latent and sensible heat flux calculations, respectively, each of which influence column 〈m〉. Reynolds decomposition of latent and sensible heat fluxes (LH and SH) reveal that the thermodynamic perturbations (e.g., for LH) modestly offset the equatorial wind‐driven perturbations ( ) and 〈m〉, but strongly offset the subtropical and 〈m〉. Column moistening east of MJO convection is opposed by and supported by . Impacts of intraseasonal SST fluctuations are analyzed by recomputing surface flux component terms using 61 day running‐mean SST. Differences between “full SST” and “smoothed SST” projections onto 〈m〉 and its tendency (∂〈m〉/∂t) yield the “SST effect” on the MJO 〈m〉 budget. Particularly in the Indian Ocean, intraseasonal SST fluctuations maintain equatorial 〈m〉 anomalies at a rate of 1%–2% d−1 and damp subtropical 〈m〉 anomalies at a similar rate. Vertical advection (−〈ω∂m/∂p〉) exports 10%–20% of 〈m〉 per day, implying that the SST modulation of surface fluxes offsets roughly 10% of equatorial 〈m〉 export and amplifies by 10% the subtropical 〈m〉 export by −〈ω∂m/∂p〉. SST fluctuations support MJO propagation by encouraging on‐equator convection and the circulation anomalies that drive MJO propagation, and by contributing up to 10% of ∂〈m〉/∂t across the Warm Pool.

MJO prediction skill, predictability, and teleconnection impacts in the Beijing Climate Center Atmospheric General Circulation Model

This study evaluates performance of Madden–Julian oscillation (MJO) prediction in the Beijing Climate Center Atmospheric General Circulation Model (BCC_AGCM2.2). By using the real-time multivariate MJO (RMM) indices, it is shown that the MJO prediction skill of BCC_AGCM2.2 extends to about 16–17days before the bivariate anomaly correlation coefficient drops to 0.5 and the root-mean-square error increases to the level of the climatological prediction. The prediction skill showed a seasonal dependence, with the highest skill occurring in boreal autumn, and a phase dependence with higher skill for predictions initiated from phases 2–4. The results of the MJO predictability analysis showed that the upper bounds of the prediction skill can be extended to 26days by using a single-member estimate, and to 42days by using the ensemble-mean estimate, which also exhibited an initial amplitude and phase dependence. The observed relationship between the MJO and the North Atlantic Oscillation was accurately reproduced by BCC_AGCM2.2 for most initial phases of the MJO, accompanied with the Rossby wave trains in the Northern Hemisphere extratropics driven by MJO convection forcing. Overall, BCC_AGCM2.2 displayed a significant ability to predict the MJO and its teleconnections without interacting with the ocean, which provided a useful tool for fully extracting the predictability source of subseasonal prediction.

The two‐way relationship between the Madden–Julian oscillation and anticyclonic wave breaking

The Madden– Julian oscillation (MJO) is the primary mode of intraseasonal variability in the Tropics. The MJO often couples with convection as it moves over the Indian Ocean and decouples as it leaves the west Pacific Ocean. The association between convection coupled to the MJO and the midlatitude flow pattern has been the primary focus of numerous studies. In particular, it has been shown that there is a statistical relationship between the position and strength of the MJO and anticyclonic wave breaking (AWB). This study expands on this statistical relationship by examining the two‐way relationship between the MJO and AWB.Using a climatology of AWB during an extended boreal cold season, we demonstrate that there is enhanced AWB over the central North Pacific when active MJO convection is over the eastern Indian Ocean and Maritime Continent. Convection associated with the MJO can induce AWB when active MJO convection is over the Indian Ocean. Once the wave breaks over the central North Pacific, the residual cut‐off low phases with the MJO circulation to enhance it as it propagates eastward, resulting in a higher amplitude MJO than when no AWB occurs.

The Relationship between the Madden–Julian Oscillation (MJO) and Southeastern New England Snowfall

AbstractThe winter of 2014/15 brought record snow totals to portions of southeastern New England. Additionally, over 90% of Boston Logan Airport snowfall during the winter fell during phases 7 and 8 of the Madden–Julian oscillation (MJO) index. This motivated the authors to investigate potential connections between intense southeastern New England snowstorms and the MJO in the historical record. It was found that southeastern New England snowfall, measured since the 1930s at several stations in the region, recorded higher than average winter snowfalls when enhanced MJO convection was located over the western Pacific and the Western Hemisphere (phases 7–8). Similarly, snowfall was suppressed when enhanced MJO convection was located over the Maritime Continent (phases 4–5). The MJO also modulates the frequency of nor’easters, which contribute the majority of New England’s snowfall, as measured by reanalysis-derived cyclone tracks. These tracks were more numerous during the same MJO phases that lead to enhanced snowfall, and they were less common during phases with less snowfall.

The intraseasonal atmospheric angular momentum associated with MJO convective initiations

The first part of this study examines the driving mechanisms of the equatorial intraseasonal relative atmospheric angular momentum (AAM) and its dynamical relationship to the upper‐tropospheric zonal wind over the Western Hemisphere (WH) during the convective initiation of the Madden–Julian Oscillation (MJO) over the Indian Ocean. The budget analysis shows that the main driver of the equatorial intraseasonal AAM anomaly is the meridional transport of momentum induced by the modulation of the background subtropical eddies by the intraseasonal eddies. While the subtropical eddies over the central Pacific basin partly drive the equatorial AAM by meridionally transporting the momentum, the equatorial zonal wind associated with the same subtropical eddies is zonally advected and locally amplified over the east Pacific and Atlantic basins. The common source phenomena that transport momentum result in simultaneous evolution of the WH upper‐tropospheric zonal wind and the AAM on intraseasonal time‐scales, but their main driving mechanisms are different.The second part of the study investigates the influence of the equatorial intraseasonal AAM state on the subsequent development of initiating MJO convection over the Indian Ocean. In the presence of the WH upper‐tropospheric easterly wind, MJO convection tends to develop a stronger enhanced convective envelope when the initiation occurs during the negative intraseasonal AAM state, which strengthens and extends the upper‐tropospheric easterly wind in the WH. When the AAM anomaly is positive, it tends to induce stronger mid‐tropospheric convergence above the region of convective initiation, thereby suppressing the lower‐tropospheric updraught and suppressing the further growth of convection. The results show that the combined effects of the WH circumnavigating circulation and the AAM can influence the subsequent development of MJO convection over the Indian Ocean.

Evolution of the Madden–Julian Oscillation in Two Types of El Niño

Abstract Evolution characteristics of the Madden–Julian oscillation (MJO) during the eastern Pacific (EP) and central Pacific (CP) types of El Niño have been investigated. MJO activities are strengthened over the western Pacific during the predeveloping and developing phases of EP El Niño, but suppressed during the mature and decaying phases. In contrast, MJO activities do not show a clear relationship with CP El Niño before their occurrence over the western Pacific, but they increase over the central Pacific during the mature and decaying phases of CP El Niño. Lag correlation analyses further confirm that MJO activities over the western Pacific in boreal spring and early summer are closely related to EP El Niño up to 2–11 months later, but not for CP El Niño. EP El Niño tends to weaken the MJO and lead to a much shorter range of its eastward propagation. Anomalous descending motions over the Maritime Continent and western Pacific related to El Niño can suppress convection and moisture flux convergence there and weaken MJO activities over these regions during the mature phase of both types of El Niño. MJO activities over the western Pacific are much weaker in EP El Niño due to the stronger anomalous descending motions. Furthermore, the MJO propagates more continuously and farther eastward during CP El Niño because of robust moisture convergence over the central Pacific, which provides adequate moisture for the development of MJO convection.

The development of upper‐tropospheric geopotential height anomaly in the Western Hemisphere during MJO convective initiations

This study uses geopotential height budget and composite analysis of reanalysis data to examine the driving dynamics and structural evolution of intraseasonal upper‐tropospheric circulation over the Western Hemisphere (WH) prior to and during the convective initiation of the Madden–Julian Oscillation (MJO) over the Indian basin. Previous studies by the authors showed that, during most of MJO convective initiation events over the Indian basin, upper‐tropospheric equatorial easterly wind anomaly initially develops in association with subtropical and midlatitude Rossby ridges over the east Pacific basin. As the easterly wind continues to amplify and propagate farther eastward, a negative upper‐tropospheric geopotential height anomaly develops over tropical South America and couples with the easterly wind anomaly, forming a structure consistent with theoretical Kelvin waves. This study shows that the development of the negative geopotential height anomaly over South America is driven by net tropospheric adiabatic cooling induced by updraughts. The convection associated with the updraughts is triggered by the intrusion of a midlatitude wave train from the Southern Hemisphere, suggesting that the interaction with midlatitude circulation is a part of the dynamics driving the formation of upper‐tropospheric Kelvin wave structure over the WH.During some MJO convective initiation events, the upper‐tropospheric zonal wind anomaly is positive over the east Pacific basin and the formation of Kelvin wave structure is weak or absent over South America. Those events subsequently develop weaker MJO convective envelopes over the Indian basin, apparently due to the absence of upper‐tropospheric divergence induced by the incoming Kelvin wave circulation that enhances convective updraughts over the region of initiating convection. The results suggest that the formation of the upper‐tropospheric Kelvin wave over the WH partly determines the subsequent development of MJO convection over the Indian basin.

Assessments of surface latent heat flux associated with the Madden–Julian Oscillation in reanalyses

To understand the accuracy and uncertainty of surface latent heat flux (LHF) associated with the Madden–Julian Oscillation (MJO), the LHF from each of the six global reanalysis datasets is compared with LHF based on in situ data and the objectively analyzed air–sea flux (OAFlux), in terms of tropical intraseasonal variability. The reanalysis products used in this study include the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-I), the Modern-Era Retrospective Analysis for Research and Applications (MERRA), three generations of reanalysis from the National Center for Environmental Prediction (NCEP R1, R2 and CFSR), and the twentieth century reanalysis (20CR). We find that the intraseasonal LHF of the reanalysis products agrees well with the OAFlux over the tropical oceans in terms of patterns, but there is a significant spread in amplitude among the reanalysis products. Both ERA-I and MERRA show smaller biases in the power spectral analysis, while the other reanalysis products (NCEP R1, NCEP R2, CFSR, and 20CR) tend to overestimate the intraseasonal LHF when compared with the TAO buoy products and OAFlux. The role of anomalous LHF in supporting the MJO convection identified by previous TAO buoy data studies is confirmed by the long-term global reanalyses. The feature of increasing LHF accompanied by growing MJO observed in the recent MJO field campaign in the central Indian Ocean (DYNAMO/CINDY2011) is also well captured by the reanalysis products. Among the reanalysis datasets, MERRA has the smallest bias in temporal variability of LHF during the DYNAMO/CINDY2011 period.

Sensitivity of MJO propagation to a robust positive Indian Ocean dipole event in the superparameterized CAM

AbstractThe superparameterized Community Atmosphere Model (SPCAM) is used to investigate the impact and geographic sensitivity of positive Indian Ocean Dipole (+IOD) sea‐surface temperatures (SSTs) on Madden‐Julian oscillation (MJO) propagation. The goal is to clarify potentially appreciable +IOD effects on MJO dynamics detected in prior studies by using a global model with explicit convection representation. Prescribed climatological October SSTs and variants of the SST distribution from October 2006, a +IOD event, force the model. Modest MJO convection weakening over the Maritime Continent occurs when either climatological SSTs, or +IOD SST anomalies restricted to the Indian Ocean, are applied. However, severe MJO weakening occurs when either +IOD SST anomalies are applied globally or restricted to the equatorial Pacific. MJO disruption is associated with time‐mean changes in the zonal wind profile and lower moist static energy (MSE) in subsiding air masses imported from the Subtropics by Rossby‐like gyres. On intraseasonal scales, MJO disruption arises from significantly smaller MSE accumulation, weaker meridional advective moistening, and overactive submonthly eddies that mix drier subtropical air into the path of MJO convection. These results (1) demonstrate that SPCAM reproduces observed time‐mean and intraseasonal changes during +IOD episodes, (2) reaffirm the role that submonthly eddies play in MJO propagation and show that such multiscale interactions are sensitive to interannual SST states, and (3) suggest that boreal fall +IOD SSTs local to the Indian Ocean have a significantly smaller impact on Maritime Continent MJO propagation compared to contemporaneous Pacific SST anomalies which, for October 2006, resemble El Niño‐like conditions.

The Madden–Julian Oscillation’s Influence on Spring Rainy Season Precipitation over Equatorial West Africa*

Abstract This paper characterizes the influence of the Madden–Julian oscillation (MJO) on spring rainy season (March–June) convection variability over equatorial West Africa (EWA) and investigates mechanisms of association. It is found that the MJO has a significant impact on convection and precipitation anomalies over the region. Over large portions of EWA, MJO impacts on rainfall constitute a difference on the order of 20%–50% from average daily rain rates for the season. This impact is primarily due to the direct influence of the eastward movement of the MJO convective core into EWA, which is associated with westerly low-level wind anomalies that advect moisture from the Atlantic Ocean to the region. In addition, equatorial Rossby and Kelvin waves triggered by MJO convection anomalies over the Indian Ocean have a significant and systematic influence on EWA spring rainy season precipitation. The Kelvin wave contribution and the relative strength of the direct MJO convective influence compared to that of equatorial wave activity differs from findings of studies that have examined MJO influence on EWA during boreal summer. In addition, MJO is found to influence precipitation extremes during spring rains in a manner that is not observed in summer. Importantly, in this analysis the influences of MJO convection and each of the MJO-associated convectively coupled equatorial waves frequently coincide, reaching EWA approximately 20 days after MJO convection initiates in the Indian Ocean. This coincident timing enhances the total MJO impact on the region, and it also implies that MJO events have potential for prediction of regional-scale convection and rainfall anomalies over EWA in this season.

Evolution, Properties, and Spatial Variability of MJO Convection near and off the Equator during DYNAMO

Abstract This study investigates the evolution, structure, and spatial variability of Madden–Julian oscillation (MJO) convection observed during the 2011/12 Dynamics of the MJO (DYNAMO) field campaign. Generally, the C-band radars located in the near-equatorial Indian Ocean—Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) on Addu Atoll (Gan) and NASA TOGA on the R/V Roger Revelle (Revelle)—observed similar trends in echo-top heights, stratiform rain fraction, and precipitation feature size across the MJO life cycle. These trends are closely related to changes in mid- to upper-tropospheric moisture, sea surface temperature (SST), zonal wind, and diagnosed vertical air motions. However, the evolution of convection, moisture, and vertical air motion at the R/V Mirai (Mirai), located in the intertropical convergence zone (ITCZ) at 8°S, exhibited a pattern nearly opposite to Gan and Revelle. When the MJO was active over the equator, convection was suppressed around Mirai owing to induced subsidence by the strong upward motion to the north. SST and zonal winds near Mirai were nearly invariant across the MJO life cycle, indicating little influence from the MJO in these fields. Compared to Gan and Revelle, Mirai had a significant amount of precipitation that fell from shallow and isolated convection. There were subtle differences in the evolution and properties of the convection observed between Gan and Revelle. Deep convection occurred slightly earlier at Gan compared to Revelle, consistent with the west-to-east progression of the MJO in the central Indian Ocean. Furthermore, convective deepening was more gradual over Revelle compared to Gan, especially during the October MJO event.

Impact of East Asian winter monsoon on MJO over the equatorial western Pacific

This paper investigates the processes and mechanisms by which the East Asian winter monsoon (EAWM) affects the Madden-Julian oscillation (MJO) over the equatorial western Pacific in boreal winter (November–April). The results show that both the EAWM and MJO over the equatorial western Pacific have prominent interannual and interdecadal variabilities, and they are closely related, especially on the interannual timescales. The EAWM influences MJO via the feedback effect of convective heating, because the strong northerlies of EAWM can enhance the ascending motion and lead the convection to be strengthened over the equatorial western Pacific by reinforcing the convergence in the lower troposphere. Daily composite analysis in the phase 4 of MJO (i.e., strong MJO convection over the Maritime Continent and equatorial western Pacific) shows that the kinetic energy, outgoing longwave radiation (OLR), moisture flux, vertical velocity, zonal wind, moist static energy, and atmospheric stability differ greatly between strong and weak EAWM processes over the western Pacific. The strong EAWM causes the intensity of MJO to increase, and the eastward propagation of MJO to become more persistent. MJO activities over the equatorial western Pacific have different modes. Furthermore, these modes have differing relationships with the EAWM, and other factors can also affect the activities of MJO; consequently, the relationship between the MJO and EAWM shows both interannual and interdecadal variabilities.

The Development of Upper-Tropospheric Wind over the Western Hemisphere in Association with MJO Convective Initiation

Abstract This study examines the structure and driving mechanisms of upper-tropospheric intraseasonal zonal wind anomalies over the Western Hemisphere (WH) during the convective initiation of the Madden–Julian oscillation (MJO) over the Indian Ocean using composite and budget analyses. The initiating MJO convection is more often associated with WH upper-tropospheric intraseasonal easterly wind anomalies, and when it is, it tends to develop a stronger and zonally broader envelope of enhanced convection than events associated with westerly wind anomalies. The WH upper-tropospheric zonal wind anomaly associated with the MJO is often described as a dry Kelvin wave radiated from MJO convection, but the results show that both the structure and driving mechanisms are different from the ones of theoretical Kelvin waves. Unlike the theoretical Kelvin wave, the zonal wind anomaly is not driven mainly by the zonal pressure gradient force and it is strongly coupled with rotational wind associated with subtropical and equatorward-propagating midlatitude Rossby waves. The intraseasonal zonal wind anomaly amplifies over the eastern Pacific and Atlantic basins because of advection of the background wind by intraseasonal wind in the presence of background zonal wind convergence, which allows acceleration in the same sign of the present intraseasonal zonal wind anomaly. A part of the WH intraseasonal easterly wind initiates in the lower stratosphere and is advected downward as it merges with eastward-propagating easterly wind in the upper troposphere. The initial sources of the lower-stratospheric intraseasonal easterly wind include equatorward intrusion of midlatitude waves and an equatorial Rossby wave.