Madden–Julian Oscillation Activity Research Articles

Dynamic Characteristics of the Circulation and Diurnal Spatial Cycle of Outgoing Longwave Radiation in the Different Phases of the Madden–Julian Oscillation during the Formation of the South Atlantic Convergence Zone

In this work, we verified the formation of the South Atlantic Convergence Zone (SACZ) during the active, unfavorable, and transition phases of the Madden–Julian Oscillation (MJO), as well as the diurnal spatial variability in the estimated Outgoing Longwave Radiation (OLR) data. The real-time multivariate index (RMM) and the composites of meteorological variables were used, along with the temporal average of the estimated OLR data. All the different patterns for the average period of SACZ showed classic behavior: well-organized and with meteorological variables in phases throughout the troposphere. However, some differences were evident in the organization of each phase of the MJO: at 200 hPa, the Bolivian High (BH) was more flattened during the active phase pattern than in the unfavorable and transition phases, being wider and with a wavier trough embedded in the western flow; at medium levels, the subtropical highs appeared more defined and with a very wide trough; the trough supported the frontal systems on the surface and, together with the subtropical highs, concentrated all the moisture in this layer. In the OLR dataset, the formation of the Coast Squall Line (CSL) occurred during SACZ events in the active phase and MJO transition, whereas in the unfavorable phase, this system was not observed.

The inverse influence of MJO on the cyclogenesis in the north Indian Ocean

We examine the influence of Madden Julian Oscillation (MJO) on tropical cyclones (TCs) in the north Indian Ocean (NIO). To analyse cyclogenesis, we have used the Genesis Potential Index (GPI) and Accumulated Cyclonic Energy (ACE) in the active years (AYs) and the non-active years (NAYs) of MJO during the period 1979–2019. The GPI anomaly shows enhanced cyclogenesis in NIO except for the northern part of the Bay of Bengal (BoB) in AYs, and an exact opposite anomaly of GPI is observed in NAYs in the Arabian Sea (AS) and BoB. Additionally, a strong negative correlation exists in the anomaly of relative humidity, potential intensity and absolute vorticity in AYs and NAYs. The reduction in MJO phase duration by 3–4 days in NIO is in accordance with the declining frequency of cyclones in AYs in BoB, but in contrast with the frequency of cyclones in AS. Similarly, an increasing trend of ACE is observed in AS with a clear indication of shifting the increased frequency of cyclones towards the western NIO (AS) in AYs. An inverse characteristic of cyclonic activity is observed in AS and BoB during AYs, particularly from 2014 onwards. Our analysis exposes apparent changes in MJO activity and its impact on cyclogenesis and cyclone activity in NIO, which may be attributed to the recent warming in NIO.

Seasonality in Prediction Skill of the Madden‐Julian Oscillation and Associated Dynamics in Version 2 of NASA's GEOS‐S2S Forecast System

AbstractThe prediction skill of the Madden‐Julian Oscillation (MJO) in Version 2 of NASA's Global Earth Observing System Subseasonal to Seasonal (GEOS‐S2S) forecast system is investigated for winter and summer focusing on moistening‐related processes crucial for eastward propagating MJO activity. It is found that the annual bivariate correlation of the Real‐time Multivariate MJO time series between prediction and observation is ∼0.70, ∼0.57, and ∼0.50 at 20‐, 25‐, and 30‐day forecast leads. Correlation at long‐leads (>30 days) is noticeably higher for boreal summer initial conditions (June‐September [JJAS]), with correlations remaining above 0.5 at 35–40 days leads. Correlations are lower for boreal winter initial conditions from January through March (JFM), dropping to ∼0.5 at 25‐day lead, still comparable to the skills in the other reliable S2S forecast systems.The predicted eastward MJO propagation across the Indo‐western Pacific sector is well captured in JJAS, but is slower than observed in JFM. Investigations of the moisture field and advection and moisture sink, moist static energy (MSE) budget, and tropical circulation/pressure responses to the MJO convective heating reveal that, in JFM, those responses and moistening processes, especially the vertical MSE advection, are underestimated over and to the east of the Maritime Continent when the MJO anomaly approaches from the west. In contrast, those processes are well represented in JJAS, although moistening is overestimated due to large surface evaporation. This study suggests that improvement of the moistening tendency over that region in the boreal winter could contribute to further increases in MJO prediction skill of the GEOS‐S2S system.

Why Australia was not wet during spring 2020 despite La Niña

The austral spring climate of 2020 was characterised by the occurrence of La Niña, which is the most predictable climate driver of Australian springtime rainfall. Consistent with this La Niña, the Bureau of Meteorology’s dynamical sub-seasonal to seasonal forecast system, ACCESS-S1, made highly confident predictions of wetter-than-normal conditions over central and eastern Australia for spring when initialised in July 2020 and thereafter. However, many areas of Australia received near average to severely below average rainfall, particularly during November. Possible causes of the deviation of rainfall from its historical response to La Niña and causes of the forecast error are explored with observational and reanalysis data for the period 1979–2020 and real-time forecasts of ACCESS-S1 initialised in July to November 2020. Several compounding factors were identified as key contributors to the drier-than-anticipated spring conditions. Although the ocean surface to the north of Australia was warmer than normal, which would have acted to promote rainfall over northern Australia, it was not as warm as expected from its historical relationship with La Niña and its long-term warming trend. Moreover, a negative phase of the Indian Ocean Dipole mode, which typically acts to increase spring rainfall in southern Australia, decayed earlier than normal in October. Finally, the Madden–Julian Oscillation activity over the equatorial Indian Ocean acted to suppress rainfall across northern and eastern Australia during November. While ACCESS-S1 accurately predicted the strength of La Niña over the Niño3.4 region, it over-predicted the ocean warming to the north of Australia and under-predicted the strength of the November MJO event, leading to an over-prediction of the Australian spring rainfall and especially the November-mean rainfall.

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

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

Record‐Low WNP Tropical Cyclone Activity in Early Summer 2020 due to Indian Ocean Warming and Madden‐Julian Oscillation Activity

AbstractDuring early boreal summer 2020, the western North Pacific (WNP) was characterized by record‐low tropical cyclone (TC) activity, reaching the lowest historical level since 1979. Only one short‐lived TC was observed in June and the whole of July underwent an unprecedented quiescent TC period. Concurrent Indian Ocean warming provided unfavorable conditions for cyclogenesis over the WNP, which partly (∼40%) explains the extremely low TC number from a statistical standpoint. Accompanied with the Indian Ocean warming, an unprecedentedly long‐lasting Madden‐Julian Oscillation (MJO) event stalled over the Indian Ocean sector, persisting almost through the entire June and July. This simultaneous quasi‐stationary MJO activity further caused unfavorable background conditions for cyclogenesis in the WNP. The Indian Ocean warming combined with the anomalous MJO conditions together largely (>80%) explain the record‐low TC number over the WNP during early summer 2020.

Impact of the Madden–Julian Oscillation on extreme precipitation over the western Maritime Continent and Southeast Asia

AbstractThe western Maritime Continent (MC) and Southeast Asia lie at the heart of the largest area of high precipitation on Earth. Extreme precipitation is one of the major high‐impact weather events to affect the population of over 500 million in this region. The deep convection associated with this extreme precipitation is difficult to forecast, even with modern high‐resolution numerical weather prediction with explicit convection. However, larger‐scale organised convective systems, such as the Madden–Julian Oscillation (MJO), can be skilfully predicted to 3–5 weeks lead time. The MJO has a well‐known precipitation signal, and it is likely that it also modulates extreme precipitation. Here, the extreme precipitation signal of the MJO is analysed in detail for the western MC and Southeast Asia using 19 years of high‐resolution Integrated Multi‐satellitE Retrievals for Global Precipitation Measurement (GPM IMERG) data. The probability of experiencing extreme precipitation increases robustly by a factor of two, and decreases by a factor of half, dependent on location and the phase of the MJO. The spatial pattern of these changes in extreme precipitation does not describe a smooth eastward propagation, but shows rapid variation over short distances, tied to the complex distribution of land and sea within the archipelago. There is also a seasonal dependence of this MJO modulation in some locations. A more detailed analysis of the effect of the MJO on extreme precipitation is presented for the major cities in the region. Extreme precipitation days over the MC are generally linked with an amplification of the diurnal cycle. However, although an active MJO increases the frequency of extreme precipitation days and therefore an amplified diurnal cycle, there was no further amplification of the diurnal cycle in the active MJO, compared with extreme precipitation days during non‐active MJO periods.

The Madden‐Julian Oscillation Modulates the Air Quality in the Maritime Continent

AbstractAir quality in the Maritime Continent, which worsens particularly during peatland fire periods, causes climate and environmental concerns. The occurrence of peatland fire haze has been related to the interannual variations of the El Niño‐Southern Oscillation, Indian Ocean Dipole, and inter‐seasonal monsoon cycle. However, the factors affecting the region's air quality on subseasonal timescales have not been clearly identified. We investigated the role of the Madden‐Julian Oscillation (MJO), which has a 30–90‐day cycle, in air quality using a long‐term observation data set from Malaysia. The results show that in MJO Phase 3, during which the region is covered by convective clouds, only accounted for 4 ± 2% of high PM10 concentration days (daily average PM10 concentration at site X > average + standard deviation of daily average PM10 concentration at site X) among all active MJO periods. In contrast, the corresponding values were considerably higher in MJO Phases 1 and 5–8. The MJO also modulated CO and ozone concentrations. This study shows that the MJO influences air quality during both fire and non‐fire seasons, demonstrating that variations in wildfire activity are not the only cause of air quality changes. Other factors, such as precipitation, also need to be considered to better understand the relationship between the MJO and air quality.

Interannual variability of the frequency of MJO phases and its association with two types of ENSO

In this study, we reexamine the effect of two types of El Niño Southern Oscillation (ENSO) modes on Madden Julian Oscillation (MJO) activity in terms of the frequency of MJO phases. Evaluating all-season data, we identify two dominant zonal patterns of MJO frequency exhibiting prominent interannual variability. These patterns are structurally similar to the Wheeler and Hendon (Mon. Weather Rev. 132:1917–1932, 2004) RMM1 and RMM2 spatial patterns. The first pattern explains a higher frequency of MJO activity over the Maritime Continent and a lower frequency over the central Pacific Ocean and the western Indian Ocean, or vice versa. The second pattern is associated with a higher frequency of MJO active days over the eastern Indian Ocean and a lower frequency over the western Pacific, or vice versa. We find that these two types of MJO frequency patterns are related to the central Pacific and eastern Pacific ENSO modes. From the positive to the negative ENSO (central Pacific or eastern Pacific) phases, the respective MJO frequency patterns change their sign. The MJO frequency patterns are the lag response of the underlying ocean state. The coupling between ocean and atmosphere is exceedingly complex. The first MJO frequency pattern is most prominent during the negative central-Pacific (CP-type) ENSO phases (specifically during September–November and December-February seasons). The second MJO frequency pattern is most evident during the positive eastern-Pacific (EP-type) ENSO phases (specifically during March–May, June–August and September–November). Different zonal circulation patterns during CP-type and EP-type ENSO phases alter the mean moisture distribution throughout the tropics. The horizontal convergence of mean background moisture through intraseasonal winds are responsible for the MJO frequency anomalies during the two types of ENSO phases. The results here show how the MJO activity gets modulated on a regional scale in the presence of two types of ENSO events and can be useful in anticipating the seasonal MJO conditions from a predicted ENSO state.

The Role of Background Meridional Moisture Gradient on the Propagation of the MJO over the Maritime Continent

AbstractThis study investigates the role of the background meridional moisture gradient (MMG) on the propagation of the Madden–Julian Oscillation (MJO) across the Maritime Continent (MC) region. It is found that the interannual variability of the seasonal mean MMG over the southern MC area is associated with the meridional expansion and contraction of the moist area in the vicinity of the MC. Sea surface temperature anomalies associated with relatively high and low seasonal mean MMG exhibit patterns that resemble those of the El Niño–Southern Oscillation. By contrasting the years with anomalously low and high MMG, we show that MJO propagation through the MC is enhanced (suppressed) in years with higher (lower) seasonal mean MMG, though the effect is less robust when MMG anomalies are weak. Column-integrated moisture budget analysis further shows that sufficiently large MMG anomalies affects MJO activity by modulating the meridional advection of the mean moisture via MJO wind anomalies. Our results suggest that the background moisture distribution has a strong control over the propagation characteristics of the MJO in the MC region.

Identifying key driving mechanisms of heat waves in central Chile

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

Intensification of Westerly Wind Bursts Caused by the Coupling of the Madden‐Julian Oscillation to SST During El Niño Onset and Development

AbstractThe Madden‐Julian Oscillation (MJO) plays an important role in El Niño dynamics in connection with westerly wind bursts (WWBs) that occur in the equatorial Pacific; however, our understanding of the relationship between these climate phenomena remains incomplete. Here, we use a Lagrangian Atmospheric Model (LAM) to examine the response of the MJO and resulting WWBs to sea surface temperature (SST) anomalies similar to those observed during the onset and development of El Niño. We find that when a localized SST anomaly (≤1°C) is imposed in the western‐central equatorial Pacific between December and April (mimicking El Niño onset), the MJO convective envelope shifts from the southern hemisphere toward the equator. When an SST anomaly (≤1.5°C) is imposed in the central‐eastern equatorial Pacific between May and November (mimicking El Niño development), the MJO weakens in the northwestern Pacific, but strengthens in the central equatorial Pacific. In both experiments, the ensuing changes enhance MJO activity along the equator, facilitating WWBs.

Distinctive MJO Activity during the Boreal Winter of the 2015/16 Super El Niño in Comparison with Other Super El Niño Events

Many previous studies have demonstrated that the boreal winters of super El Nino events are usually accompanied by severely suppressed Madden-Julian oscillation (MJO) activity over the western Pacific due to strong descending motion associated with a weakened Walker Circulation. However, the boreal winter of the 2015/16 super El Nino event is concurrent with enhanced MJO activity over the western Pacific despite its sea surface temperature anomaly (SSTA) magnitude over the Nino 3.4 region being comparable to the SSTA magnitudes of the two former super El Nino events (i.e., 1982/83 and 1997/98). This study suggests that the MJO enhanced over western Pacific during the 2015/16 super El Nino event is mainly related to its distinctive SSTA structure and associated background thermodynamic conditions. In comparison with the previous super El Nino events, the warming SSTA center of the 2015/16 super El Nino is located further westward, and a strong cold SSTA is not detected in the western Pacific. Accordingly, the low-level moisture and air temperature (as well as the moist static energy, MSE) tend to increase in the central-western Pacific. In contrast, the low-level moisture and MSE show negative anomalies over the western Pacific during the previous super El Nino events. As the MJO-related horizontal wind anomalies contribute to the further westward warm SST-induced positive moisture and MSE anomalies over the western tropical Pacific in the boreal winter of 2015/16, stronger moisture convergence and MSE advection are generated over the western Pacific and lead to the enhancement of MJO convection.

Indispensable Role of the Madden‐Julian Oscillation in the 2019 Extreme Autumn Drought Over Eastern China

AbstractEastern China suffered an extensive drought during autumn (September–October 2019) with precipitation deficits reaching as high as 70% of the climatology, the worst on record since the 1980s. Concurrent with this extreme drought, the tropical Indo–Pacific oceans experienced a central–Pacific (CP) El Niño event with the westernmost location, and the third–ranking positive Indian Ocean Dipole (IOD) event. However, the CP El Niño associated convection anomalies are unexpectedly absent over the western equatorial Pacific, leading to the decoupling of traditional atmospheric–oceanic connections between the tropical Pacific and Indian Oceans. A strong Madden–Julian oscillation (MJO) event stayed in the western Indian Ocean for 32 days during September–October 2019, nearly triple the frequency of climatology. This extraordinary long–lasting MJO activity suppressed the western Pacific convection associated with the CP El Niño and simultaneously enhanced the local IOD–related sea surface temperature anomalies via the atmospheric forcing. The MJO associated convection over the Indian Ocean can excite an anomalous cyclone in the western North Pacific, producing persistently extensive precipitation deficiencies over eastern China. It highlights that the abnormal MJO activity plays a crucial role on the Indo–Pacific atmospheric–oceanic features and the extreme precipitation deficits over eastern China.

Impacts of the Madden‐Julian Oscillation on the intensity and spatial extent of heavy precipitation events in northern Northeast Brazil

AbstractThis paper investigates the spatiotemporal variability of Contiguous Extreme Precipitation Events (CEPE) related to Madden‐Julian Oscillation (MJO) activity during the pre‐wet season and wet season in northern Northeast Brazil for 30 years (1979–2010). Inherent differences in rainy season regimes are associated with northern Northeast Brazil precipitation systems; these are isolated during pre‐wet and wet season and analyse the intensity and the spatial area covered by the CEPE. A daily index of MJO activity and its life cycle were used to identify activity phases during heavy rainfall events. Composite analysis was performed for days with CEPE during the most frequent MJO phases in the pre‐wet and wet seasons. The analysis shows the influence of the MJO on the occurrence of CEPE in northern Northeast Brazil. In the pre‐wet season, the MJO phases 8‐1‐2 affect the occurrence of heavy rainfall events, modifying the atmospheric circulation and water vapour transport over northern South America. Indications of Rossby wave propagation over the South Pacific associated with MJO were found, suggesting tropic‐extratropic teleconnection during occurrences of CEPE in the pre‐wet season. In the wet season, CEPE events are more frequent during MJO phases 8‐2‐3. Conversely, no wave propagation over the South Pacific is observed in the wet season, changes in wind anomalies at high and low atmospheric levels only point to the influence of the MJO via tropical teleconnection. This study may help monitoring and forecasting of very heavy rainfall events in northern Northeast Brazil in pre‐wet season and wet season.

On the Relationship Between the Madden‐Julian Oscillation and the Hadley and Walker Circulations

AbstractThis study investigates: (i) how the local meridional (Hadley) and zonal (Walker) circulations change in each phase of the Madden‐Julian Oscillation (MJO); and (ii) the effect of enhanced and suppressed MJO‐related convection on the poleward extent of the local Hadley circulations and, thus, the strengths and positions of the subtropical jets. We examine these effects in ERA‐Interim reanalysis by decomposing the vertical mass flux into zonal and meridional components. We show for the first time, that as the envelope of enhanced convection moves eastwards from Africa to the Central Pacific the local Hadley circulation is enhanced. The regional Walker circulation in the Pacific is strengthened when the envelope of active MJO convection is located over the Maritime Continent and weakened when the region of suppressed convection is located there. In regions of anomalous upper‐level divergence the subtropical jet is enhanced. The core of the subtropical jet over Asia shifts eastwards with the progression of the MJO and shifts farther poleward in regions of anomalous upper‐level divergence linked with enhanced convection. The region of either enhanced or suppressed convection over the Maritime Continent strengthens or weakens the local Hadley circulation, producing disturbances in the subtropical jet. These disturbances then force midlatitude Rossby waves that propagate across the Pacific Ocean in both hemispheres.

Interdecadal variability of intensity of the Madden–Julian oscillation

AbstractThe interdecadal variability of intensity of the Madden–Julian oscillation (MJO) during boreal winter was investigated based on two reanalysis datasets (NOAA 20CR and ERA‐20C). Both the reanalysis datasets reveal statistically significant power spectrum peaks at 12–20‐year periods. A composite analysis shows that during the active interdecadal phase, eastward propagating MJO activity was strengthened over the equatorial warm pool expanded from the Indian Ocean (IO) to the western Pacific (WP). The cause of the enhanced MJO variability was attributed to the interdecadal change of background mean precipitation in situ. A further diagnosis shows that the increase of the background precipitation over the Maritime Continent and the WP was attributed to interdecadal warm sea surface temperature (SST) anomalies in situ, whereas the increase of the precipitation over the IO resulted from the local convective instability due to the increase of wind induced surface evaporation and moisture. The strengthened background precipitation impacted the MJO strength through the increase of the interdecadal background moisture in the lower and middle troposphere induced by anomalous vertical moisture advection.

Impact of tibetan plateau snow cover on tropical cyclogenesis via the Madden–Julian oscillation during the following boreal summer

This study investigates the role of the interannual variation of boreal winter (December–February) snow cover over the Tibetan Plateau (TPSC) in modulating the relationship between the Madden Julian Oscillation (MJO) and tropical cyclogenesis over the western North Pacific (WNP) during the following boreal summer (June–October). During the boreal summer following a high snow cover anomaly (SCA), MJO-associated convection tends to be confined to the west of 140 °E, coinciding with more frequent tropical cyclone (TC) activity in this region. By contrast, there tends to be stronger MJO-associated convection extending farther east during years following low SCA. The MJO-associated convection extends to ~ 160 °E over the WNP basin with a peak around 140 °E. Thus, more TCs form farther east in the WNP in these years. We use composite analyses of large-scale environmental factors to demonstrate that low-level relative vorticity is one of the most important factors in controlling TCs in response to the change of MJO-associated convection between years with high SCA and low SCA. Meanwhile, eddy kinetic energy variations associated with the MJO are consistent with changes in tropical cyclogenesis over the WNP basin between years with high and low SCA. The Asian summer monsoon plays an important role in linking prior winter TPSC to MJO activity in the following summer. During high-SCA years, increased snow cover results in positive soil moisture anomalies and cooling over the Tibetan Plateau, thus weakening the following summer Asian monsoon and weakening associated water vapor advection, while the opposite chain of events occurs in low-SCA years. These changes in high-SCA years are favorable for MJO propagation and are unfavorable for MJO propagation in low-SCA years. Meanwhile, the interannual variation of SCA exerts a significant impact on the upper-tropospheric circulation and thus changes in vertical wind shear. During high-SCA years, there are anomalous upper-level easterlies in the Indian Ocean and upper-level westerlies in the Pacific Ocean, which result in anomalous easterly wind shear in the Indian Ocean and anomalous westerly shear in the Pacific Ocean. The opposite shear pattern arises in low-SCA years. In addition to changes in wind shear, the background vertical motion associated with changes in TPSC may also be partly responsible for changes in MJO activity.

MJO induced diurnal sea surface temperature variations off the northwest shelf of Australia observed from Himawari geostationary satellite

High frequency coupling between the tropical upper ocean and atmosphere at diurnal time scales is being recognised to be important for the behaviour of large scale climate modes such as the Madden Julian Oscillation (MJO). Sea surface temperature (SST) variability in the Eastern Indian Ocean warm pool and on the Northwest Shelf of Australia is strongly influenced by the MJO activity. In this study, we use the hourly Himawari-8 geostationary satellite SST data to understand the modulation of the amplitude of SST diurnal variations in the region by the MJO during two austral summer seasons of 2015–16 and 2016–17. The high temporal and spatial resolutions of the Himawari-8 SST data are able to capture the temporal and spatial variability of diurnal SST amplitude. Our results show that the MJO onset (active phase) is preceded by strong diurnal SST variations during the MJO suppressed phase. The amplitudes of the SST diurnal variations are mainly modulated by surface wind variability in the region, as denoted by their spatial and temporal correlations. The very strong El Niño event in 2015–16 may have significantly weakened the MJO activities in the region, resulting in prolonged MJO suppressed phase, high diurnal SST variation and mean SST. This study provides background knowledge for future research on how the diurnal SST variation in the region may influence the progression of the MJO events.

The Role of the Mean State on MJO Simulation in CESM2 Ensemble Simulation

AbstractThis study examines the role of the mean state in the propagation of the Madden‐Julian oscillation (MJO) over the Maritime Continent (MC). We use an ensemble of simulations made with a single model—the Community Earth System Model version 2—to assess the effect of the mean state that is unaffected by that of model components such as parameterization schemes. Results show that the background meridional moisture gradient is much steeper over the MC region in the periods with a stronger MJO propagation. Column water vapor budget of the MJO strongly suggests that the simulated mean state affects MJO via its impacts on moisture dynamics—a greater advection of mean moisture by MJO wind in the MC region is responsible for the anomalous MJO activity.