Madden–Julian Oscillation Events Research Articles

Stratospheric ozone and quasi-biennial oscillation (QBO) interaction with the tropical troposphere on intraseasonal and interannual timescales: a normal-mode perspective

Abstract. The Madden–Julian oscillation (MJO) is the main controller of the weather in the tropics on intraseasonal timescales, and recent research provides evidence that the quasi-biennial oscillation (QBO) influences the MJO interannual variability. However, the physical mechanisms behind this interaction are not completely understood. Recent studies on the normal-mode structure of the MJO indicate the contribution of global-scale Kelvin and Rossby waves. In this study we test whether these MJO-related normal modes are affected by the QBO and stratospheric ozone. The partial directed coherence method was used and enabled us to probe the direction and frequency of the interactions. It was found that equatorial stratospheric ozone and stratospheric zonal winds are connected with the MJO at periods of 1–2 months and 1.5–2.5 years. We explore the role of normal-mode interactions behind the stratosphere–troposphere coupling by performing a linear regression between the MJO–QBO indices and the amplitudes of the normal modes of the atmosphere obtained by projections on a normal-mode basis using ERA-Interim reanalysis data. The MJO is dominated by symmetric Rossby modes but is also influenced by Kelvin and asymmetric Rossby modes. The QBO is mostly explained by westward-propagating inertio-gravity waves and asymmetric Rossby waves. We explore the previous results by identifying interactions between those modes and between the modes and the ozone concentration. In particular, westward inertio-gravity waves, associated with the QBO, influence the MJO on interannual timescales. MJO-related modes, such as Kelvin waves and Rossby waves with a symmetric wind structure with respect to the Equator, are shown to have significantly different dynamics during MJO events depending on the phase of the QBO.

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.

Exploring the long-term changes in the Madden Julian Oscillation using machine learning

The Madden Julian Oscillation (MJO), the dominant subseasonal variability in the tropics, is widely represented using the Real-time Multivariate MJO (RMM) index. The index is limited to the satellite era (post-1974) as its calculation relies on satellite-based observations. Oliver and Thompson (J Clim 25:1996–2019, 2012) extended the RMM index for the twentieth century, employing a multilinear regression on the sea level pressure (SLP) from the NOAA twentieth century reanalysis. They obtained an 82.5% correspondence with the index in the satellite era. In this study, we show that the historical MJO index can be successfully reconstructed using machine learning techniques and improved upon. We obtain a significant improvement of up to 4%, using the support vector regressor (SVR) and convolutional neural network (CNN) methods on the same set of predictors used by Oliver and Thompson. Based on the improved RMM indices, we explore the long-term changes in the intensity, phase occurrences, and frequency of the winter MJO events during 1905–2015. We show an increasing trend in MJO intensity (22–27%) during this period. We also find a multidecadal change in MJO phase occurrence and periodicity corresponding to the Pacific Decadal Oscillation (PDO), while the role of anthropogenic warming cannot be ignored.

Modeling Evidence of QBO‐MJO Connection: A Case Study

AbstractThe boreal winter Madden‐Julian Oscillation (MJO) is modulated by the Quasi‐Biennial Oscillation (QBO). The MJO becomes relatively strong during the easterly QBO (EQBO) winters but weak during the westerly QBO (WQBO) winters. To better understand their relationship, a set of WRF model experiments is conducted with varying lateral boundary conditions. The MJO event in December 2007, during EQBO winter, is chosen as a reference case. The control experiment qualitatively reproduces the observed MJO. When the lateral boundary conditions are switched with those of WQBO or strong WQBO winters, the MJO becomes weak over the Maritime Continent. All eight ensemble members exhibit enhanced outgoing longwave radiation and reduced precipitation from EQBO to WQBO, and to strong WQBO conditions, although the magnitude of changes is smaller than observations. This result, one of the first mesoscale modeling evidences of the QBO‐MJO connection, suggests that the MJO is at least partly modulated by the QBO.

The Impacts of Horizontal Grid Spacing and Cumulus Parameterization on Subseasonal Prediction in a Global Convection-Permitting Model

AbstractMonthlong simulations targeting four Madden–Julian oscillation events made with several global model configurations are verified against observations to assess the roles of grid spacing and convective parameterization on the representation of tropical convection and midlatitude forecast skill. Specifically, the performance of a global convection-permitting model (CPM) configuration with a uniform 3-km mesh is compared to that of a global 15-km mesh with and without convective parameterization, and of a variable-resolution “channel” simulation using 3-km grid spacing only in the tropics with a scale-aware convection scheme. It is shown that global 3-km simulations produce realistic tropical precipitation statistics, except for an overall wet bias and delayed diurnal cycle. The channel simulation performs similarly, although with an unrealistically higher frequency of heavy rain. The 15-km simulations with and without cumulus schemes produce too much light and heavy tropical precipitation, respectively. Without convection parameterization, the 15-km global model produces unrealistically abundant, short-lived, and intense convection throughout the tropics. Only the global CPM configuration is able to capture eastward-propagating Madden–Julian oscillation events, and the 15-km runs favor stationary or westward-propagating convection organized at the planetary scale. The global 3-km CPM exhibits the highest extratropical forecast skill aloft and at the surface, particularly during week 3 of each hindcast. Although more cases are needed to confirm these results, this study highlights many potential benefits of using global CPMs for subseasonal forecasting. Furthermore, results show that alternatives to global convection-permitting resolution—using coarser or spatially variable resolution—feature compromises that may reduce their predictive performance.

Dynamical Roles of Mixed Rossby–Gravity Waves in Driving Convective Initiation and Propagation of the Madden–Julian Oscillation: General Views

AbstractMotivated by the previous case study, this work shows that dynamical variations of mixed Rossby–gravity waves with tropical depression–type circulations (MRGTDs) are possible drivers of convective initiation and propagation of the Madden–Julian oscillation (MJO) by performing statistical analysis. MJO events initiated in the Indian Ocean (IO) in boreal winter are objectively identified solely using outgoing longwave radiation data. The lagged-composite analysis of detected MJO events demonstrates that MJO convection is initiated in the southwestern IO (SWIO), where strong MRGTD–convection coupling is statistically found. Further classification of MJO cases in terms of intraseasonal convection and MRGTD activities in the SWIO suggests that 26 of 47 cases are related to more enhanced MRGTDs, although they can also be secondarily affected by Kelvin waves. For those MRGTD-enhanced MJO events, MJO convective initiation is primarily triggered by low-level MRGTD circulations that develop via the enhancement of downward energy dispersion in accordance with upper-tropospheric baroclinic conversion. This is supported by the modulation of MRGTD structure associated with zonal wave contraction due to upper-tropospheric zonal convergence, and plentiful moisture advected into the western IO. Following this MRGTD-induced MJO triggering and midtropospheric premoistening in the IO contributed by MRGTD shallow circulations as well as intraseasonal winds during the MJO-suppressed phase, low-level MRGTD winds with eastward group velocity successively trigger convection to the east, which helps MJO convective propagation over the IO. The interannual atmospheric variability may affect whether the presented MRGTD-related processes are effective or not.

The Impact of the Madden–Julian Oscillation on Cyclone Amphan (2020) and Southwest Monsoon Onset

Cyclone Amphan was an exceptionally strong tropical cyclone in the Bay of Bengal that achieved a minimum central pressure of 907 mb during its active period in May 2020. In this study, we analyzed the oceanic and surface atmospheric conditions leading up to cyclogenesis, the impact of this storm on the Bay of Bengal, and how the processes that led to cyclogenesis, such as the Madden–Julian Oscillation (MJO) and Amphan itself, in turn impacted southwest monsoon preconditioning and onset. To accomplish this, we took a multiparameter approach using a combination of near real time satellite observations, ocean model forecasts, and reanalysis to better understand the processes involved. We found that the arrival of a second downwelling Kelvin wave in the equatorial Bay of Bengal, coupled with elevated upper ocean heat content and the positioning of the convective phase of the MJO, helped to create the conditions necessary for cyclogenesis, where the northward-propagating branch of the MJO acted as a trigger for cyclogenesis. This same MJO event, in conjunction with Amphan, heavily contributed atmospheric moisture to the southeastern Arabian Sea and established low-level westerlies that allowed for the southwest monsoon to climatologically onset on June 1.

An Index Intercomparison for MJO Events and Termination

AbstractThe Madden‐Julian oscillation (MJO) is a significant contributor to climate variability in the tropics. While many studies have examined the MJO, few have considered its termination. Multiple indices exist to track and forecast the MJO although the influence of each index on MJO decay has not been documented. This study presents an intercomparison of several MJO indices for a 34‐year period to examine the evolution of MJO events and comprises an index‐specific climatology for MJO termination. We present an analysis of common indices including the OLR‐based MJO index (OMI) and filtered OMI (FMO) in addition to the real‐time multivariate MJO (RMM) and velocity potential MJO (VPM) index. Normalized empirical orthogonal function comparisons reveal only minor shifts in longitude and time in the corresponding convection and circulation anomalies between indices. The daily rate‐of‐change (ROC) for each index is derived, and we find that the RMM and VPM indices experience twice as large of daily amplitude changes as the univariate indices. Results from the ROC analysis are implemented in an MJO event identification algorithm to study the sensitivity of MJO termination to changes in the temporary minimum amplitude and threshold used to define events. The corresponding counts of primary, continuing, and circumnavigating events are recorded with primary events being robust to changes in the temporary minimum amplitude. Finally, an MJO climatology is created that presents MJO termination as a function of phase for each index. Physical reasons behind differences in the termination climatology are proposed in addition to applications for future work.

Can geostrophic adjustment of baroclinic disturbances in the tropical atmosphere explain MJO events?

AbstractUsing the two‐layer moist‐convective rotating shallow‐water model, we study the process of relaxation (adjustment) of localized large‐scale pressure anomalies in the lower equatorial troposphere, and show that it engenders coherent structures strongly resembling Madden–Julian Oscillation (MJO) events, as seen in vorticity, pressure, and moisture fields. We demonstrate that baroclinicity and moist convection substantially change the scenario of the quasi‐barotropic “dry” adjustment, which was established in the framework of the one‐layer shallow‐water model and consists, in the long‐wave sector, of the emission of equatorial Rossby waves, with dipolar meridional structure, to the west, and of equatorial Kelvin waves to the east. If moist convection is strong enough, a dipolar cyclonic structure, which appears in the process of adjustment as a Rossby‐wave response to the perturbation, transforms into a coherent modon‐like structure in the lower layer, which couples with a baroclinic Kelvin wave through a zone of enhanced convection and produces, at initial stages of the process, a self‐sustained slowly eastward‐propagating zonally dissymmetrical quadrupolar vorticity pattern. At the same time, a weaker quadrupolar structure of opposite sign arises in the upper layer, the whole picture similar to the active phase of MJO events. The baroclinic Kelvin wave then detaches from the dipole, which keeps slow eastward motion, and circumnavigates the Equator, catching up and interacting with the dipole.

Analysis of Coupled Oceanic and Atmospheric Preconditioning for Primary Madden‐Julian Oscillation Events Across ENSO Phases

AbstractThe Madden‐Julian Oscillation (MJO) is the dominant mode of air‐sea interaction over intraseasonal timescales. The effects of the MJO are well understood, but the initiation of the MJO remains less conclusive, particularly under El Niño Southern Oscillation (ENSO) conditions. Primary MJO events are those not immediately preceded by existing MJO activity of sufficient strength. As they are rare by definition, primary MJOs remain difficult to study, especially so when observations of events are scarce and of low spatiotemporal resolution. The advent of satellites allows for more expansive observations to be made more frequently than in situ methods, thus improving the observational capabilities of pre‐primary MJO conditions in the ocean and atmosphere. We examined oceanic and atmospheric intraseasonal signals preceding two primary MJO events during contrasting ENSO events in an attempt to bridge the connection between oceanic and atmospheric observations as potentially coupled trigger mechanisms. Satellite observations and model simulations of the central and western Indian Ocean show that intraseasonal peaks in absolute dynamic topography (ADT) and sea surface temperature (SST) upward of 1 to 2 weeks prior to the observed outgoing longwave radiation (OLR) minimum. Surface ocean warming moistens the near surface through anomalous surface fluxes, which destabilizes the lower atmosphere to deep convection. Low‐level moisture flux convergence (MFC) moistens the lower atmosphere prior to convective initiation, thus forcing an increase of total column moist static energy (MSE). Coupled midtropospheric cooling is observed that further destabilizes the atmosphere. Zonal shifts in contributing initiating parameters are observed during ENSO phases.

Eastward-moving equatorial modons in moist-convective shallow-water models

It is shown that steady large-scale slowly eastward-moving twin-cyclone coherent structures, the equatorial modons, exist in both one- and two layer versions of the rotating shallow water model on the equatorial beta plane. They arise via the process of “ageostrophic adjustment” from the analytic asymptotic modon solutions of the vorticity equation obtained in the limit of small pressure perturbations. Evolution of these structures in adiabatic and moist-convective environments, and also in the presence of topography is analysed, showing their robustness in the one-layer model. It is demonstrated that moist convection enhances and helps maintain the modons. In the two-layer model the barotropic and quasi-barotropic modons display similar to one-layer modon features, while increasing baroclinicity leads to eventual loss of coherence and arrest of the eastward propagation. Some features of equatorial modons resemble those observed in the Madden-Julian Oscillation events in tropical atmosphere, which hints at their possible relevance to the dynamics of this phenomenon.

Tracking Air–Sea Exchange and Upper-Ocean Variability in the Indonesian–Australian Basin during the Onset of the 2018/19 Australian Summer Monsoon

AbstractSea surface temperatures (SSTs) north of Australia in the Indonesian–Australian Basin are significantly influenced by Madden–Julian oscillation (MJO), an eastward-moving atmospheric disturbance that traverses the globe in the tropics. The region also has large-amplitude diurnal SST variations, which may influence the air–sea heat and moisture fluxes, that provide feedback to the MJO evolution. During the 2018/19 austral summer, a field campaign aiming to better understand the influences of air–sea coupling on the MJO was conducted north of Australia in the Indonesian–Australian Basin. Surface meteorology from buoy observations and upper-ocean data from autonomous fast-profiling float observations were collected. Two MJO convective phases propagated eastward across the region in mid-December 2018 and late January 2019 and the second MJO was in conjunction with a tropical cyclone development. Observations showed that SST in the region was rather sensitive to the MJO forcing. Air–sea heat fluxes warmed the SST throughout the 2018/19 austral summer, punctuated by the MJO activities, with a 2°–3°C drop in SST during the two MJO events. Substantial diurnal SST variations during the suppressed phases of the MJOs were observed, and the near-surface thermal stratifications provided positive feedback for the peak diurnal SST amplitude, which may be a mechanism to influence the MJO evolution. Compared to traditionally vessel-based observation programs, we have relied on fast-profiling floats as the main vehicle in measuring the upper-ocean variability from diurnal to the MJO time scales, which may pave the way for using cost-effective technology in similar process studies.

Predicting Daily Mean Wind Speed in Europe Weeks ahead from MJO Status

ABSTRACTThe Madden–Julian oscillation (MJO), a prominent feature of the tropical atmospheric circulation at subseasonal time scales, is known to modulate atmospheric variability in the Euro-Atlantic region. However, current subseasonal prediction systems fail to accurately reproduce the physical processes involved in these teleconnection mechanisms. This paper explores the observed impact of strong MJO events on surface wind speed over Europe. It is found that some MJO phases are accompanied by strong wind anomalies in Europe. After showing that this teleconnective mechanism is not present in the predictions of the ECMWF monthly forecasting system, a methodology to reconstruct forecasts of daily mean wind speed in the continent weeks ahead is proposed. This method combines MJO forecasts from the S2S project database and the observed teleconnection impacts in the historical records. Although it is found that strong MJO events cannot be skillfully predicted more than 10 days ahead with current prediction systems, a theoretical experiment shows that this method can effectively transform a dynamical MJO forecast into a probabilistic wind speed prediction in Europe.

Impacts of Insolation and Soil Moisture on the Seasonality of Interactions Between the Madden‐Julian Oscillation and Maritime Continent

AbstractThis study investigates the seasonality of the interaction of the Madden‐Julian Oscillation (MJO) with the Maritime Continent (MC). Beside their seasonal east‐west migration with the monsoons, observations show that the MJO amplitude and precipitation over the MC islands exhibit semiannual variability, with apparent strengthening of MJO signal during March and September, when solar insolation is strongest over the MC region. Furthermore, during the high‐insolation months, soil moisture shows wetter conditions during the early phases of the MJO. Motivated by these results, a series of regional convection permitting simulations are performed for the November 2014 MJO event as a case study under the following conditions: (i) increased solar insolation to mimic the local summer, (ii) reduced initial soil moisture, and (iii) the two conditions combined to isolate their local effects from their impacts on the large‐scale circulation. Results show that increased insolation increases precipitation including that associated with MJO moisture convergence over the MC region. On the other hand, decreased initial soil moisture slightly increases precipitation over the MC islands and reduces it over the surrounding waters, but such effect is short lived as the increased precipitation gradually restores the soil moisture to wetter conditions. Using a moisture budget analysis that isolates the MJO and non‐MJO signals and additional idealized simulations, the MJO response to high insolation is demonstrated to be related to an increase in the basic state moisture over the MC region rather than a direct response of the MJO to the insolation.

Impact of Cloud Longwave Scattering on Radiative Fluxes Associated With the Madden‐Julian Oscillation in the Indian Ocean and Maritime Continent

AbstractPrevious studies suggested that cloud longwave radiation contributes to the development and maintenance of the Madden‐Julian Oscillation (MJO) and model‐based convection is highly sensitive to the radiation scheme. However, currently used radiation schemes do not take cloud longwave scattering into account, resulting in an overestimation of the outgoing longwave radiation (OLR) and an underestimation of the downward longwave flux at the surface. We use combined active and passive satellite cloud property retrievals to quantify the one‐layer cloud OLR and heating rate (HR) biases introduced by neglecting cloud longwave scattering in the Indian Ocean and Maritime Continent in the context of MJO, with a focus on its phases 3, 5, and 6. The results show that the satellite‐detected one‐layer cloud area consists primarily of ice clouds, particularly during the boreal winter in the 4‐year study period. An increased ice cloud area fraction of one‐layer cloud groups is present up to 5 days before the onset of MJO events. If longwave scattering is neglected, the composite mean OLR overestimation over the one‐layer ice cloud area from 5 days before to 4 days after the MJO passage is approximately 3.5 to 5.0 W m−2. Neglecting longwave scattering also leads to a HR underestimation at cloud base and an overestimation at cloud top, making the base‐to‐top heating gradient less sharp at the cloud‐resolving scale.

The extratropical response to a developing MJO: forecast and climate simulations with the DREAM model

The Madden Julian oscillation (MJO) has a systematic influence on the state of the extratropical atmosphere. The physical mechanism involves teleconnections from tropical deep convective heat sources through wave propagation. MJO heating resembles a propagating dipole that cycles through phases on a timescale similar to the response timescale. This complicates the attribution of the response to a well identified phase in the tropical heat source. In this paper the Dynamical Research Empirical Atmospheric Model (DREAM) is used to examine the extratropical response to a moving cyclic MJO-like tropical heat source based on outgoing long-wave radiation data. A set of ensemble forecast experiments and consistent stationary wave solutions are analysed, together with longer equilibrium integrations, to investigate how the extratropical response relates to the phase of the MJO heating. We find that the response is modified by the propagation of the source in a way that depends on the initial phase. It also displays both stationary and transient nonlinearity which can account for an asymmetric response to opposite-phase dipole heat sources. The solution for Indian Ocean heating/West Pacific cooling is more zonally oriented and weaker than the solution for the opposite phase. If the MJO heat source is specified as a continuous cycle, it modifies the time-mean extratropical circulation and generates remote teleconnections. The timing and starting phase of MJO events can greatly modify the relationship between MJO phase and extratropical response, especially when an unforced “resting” period is inserted between simulated MJO cycles.

How MJO Teleconnections and ENSO Interference Impacts U.S. Precipitation

AbstractA composite analysis reveals how the Madden–Julian oscillation (MJO) impacts North American rainfall through perturbations in both the upper-tropospheric flow and regional low-level moisture availability. Upper-level divergence associated with the MJO tropical convection drives a quasi-stationary Rossby wave response to the midlatitudes. This forces a midlatitude upper-level dipolar geopotential height anomaly that is accompanied by a westward retraction of the jet stream and reduced rainfall over the central-eastern North Pacific. A reverse effect is found as the MJO propagates eastward across the Maritime Continent. These large differences in the extratropical upper-level flow, combined with anomalies in the regional supply of water vapor, have a profound impact on southeastern U.S. rainfall. The low-frequency variability, including that associated with ENSO, can modify the seasonal background flow (e.g., El Niño and La Niña basic states) affecting the distribution, strength, and propagation of the intraseasonal oscillation and the extratropical teleconnection patterns. The combined effects of the ENSO and the MJO signals result in both spatial and temporal patterns of interference and modulation of North American rainfall. The results from this study show that during a particular phase of an active MJO, the extratropical response can considerably enhance or mask the interannual ENSO signal in the United States, potentially resulting in anomalies of the opposite sign than that expected during a specific ENSO phase. Analyses of specific MJO events during an El Niño or La Niña episode reveal significant contributions to extreme events via constructive and destructive interference of the MJO and ENSO signals.

Climate Model State Estimation Using Variants of EnKF Coupled Data Assimilation

Abstract Data assimilation (DA) experiments are performed to assess impacts of observations in climate model state estimation through the cross-domain ocean–atmosphere forecast error covariances (cross covariances). Specifically, we explore strongly and weakly coupled DA variants using the Climate Analysis Forecast Ensemble (CAFE) system. This comprises 96 ensemble members of the Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 climate model assimilating observational data from the ocean, atmosphere, and sea ice realms with the ensemble Kalman filter (EnKF). Sequences of atmospheric synoptic time-scale coupled forecasts (7 days) are carried out with model consistent initialization. Unassimilated forward-independent observations are used to quantify forecast innovation error-growth rates. The results show benefit for the slow components of the atmosphere and ocean subsurface when strongly coupling ocean observations to the atmosphere. In the present system, projecting fast atmospheric observations onto the ocean subsurface through the cross covariances benefits the oceanic and atmospheric near-surface layers; however, this leads to deterioration in the ocean subsurface. Particular variants of coupled DA are able to constrain the ocean and atmosphere. The forecasts initialized with these variants have predictability at intraseasonal time scales. Errors associated with the dominant intraseasonal mode of variability, the Madden–Julian oscillation (MJO), are decomposed into normal mode functions. Consistent with recent studies showing large MJO events are concurrent with rapid error growth associated with nonlinear interactions, we find a clear relationship between the strength of a given MJO event and the related forecast innovations. Our results demonstrate consistent system behavior in relation to capturing real-world disturbances that affect climate predictability.

Subseasonal midlatitude prediction skill following Quasi-Biennial Oscillation and Madden–Julian Oscillation activity

Abstract. The Madden–Julian Oscillation (MJO) is known to force extratropical weather days to weeks following an MJO event through excitation of stationary Rossby waves, also referred to as tropical–extratropical teleconnections. Prior research has demonstrated that this tropically forced midlatitude response leads to increased prediction skill on subseasonal to seasonal (S2S) timescales. Furthermore, the Quasi-Biennial Oscillation (QBO) has been shown to possibly alter these teleconnections through modulation of the MJO itself and the atmospheric basic state upon which the Rossby waves propagate. This implies that the MJO–QBO relationship may affect midlatitude circulation prediction skill on S2S timescales. In this study, we quantify midlatitude circulation sensitivity and prediction skill following active MJOs and QBOs across the Northern Hemisphere on S2S timescales through an examination of the 500 hPa geopotential height field. First, a comparison of the spatial distribution of Northern Hemisphere sensitivity to the MJO during different QBO phases is performed for European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis and ECMWF and the National Centers for Environmental Prediction (NCEP) hindcasts. Secondly, differences in prediction skill in ECMWF and NCEP hindcasts are quantified following MJO–QBO activity. In both hindcast systems, we find that regions across the Pacific, North America, and the Atlantic demonstrate an enhanced MJO impact on prediction skill during strong QBO periods with lead times of 1–4 weeks compared to MJO events during neutral QBO periods.

Optimal error analysis of MJO prediction associated with uncertainties in sea surface temperature over Indian Ocean

In this study, the predictability of the Madden–Julian Oscillation (MJO) is investigated using the coupled Community Earth System Model (CESM) and the climatically relevant singular vector (CSV) method. The CSV method is an ensemble-based strategy to calculate the optimal growth of the initial error on the climate scale. We focus on the CSV analysis of MJO initialized at phase II, facilitating the investigation of the effect of the initial errors of the sea surface temperature (SST) in the Indian Ocean on it. Six different MJO events are chosen as the study cases to ensure the robustness of the results. The results indicate that for all the study cases, the optimal perturbation structure of the SST, denoted by the leading mode of the singular vectors (SVs), is a meridional dipole-like pattern between the Bay of Bengal and the southern central Indian Ocean. The MJO signal tends to be more converged and significant in the Eastern Hemisphere while the model is perturbed by leading SV. The moist static energy analysis results indicate that the eastward propagation is much more evident in the terms of vertical advection and radiation flux than others. Therefore, the SV perturbation can strengthen and converge the MJO signal mostly by increasing the vertical advection of the moist static energy. Further, the sensitivity studies indicate that the structure of the leading SV is not sensitive to the initial states, which suggests that we might not need to calculate SVs for each initial time in constructing the ensemble prediction, significantly saving computational time in the operational forecast systems.