Madden–Julian Oscillation Phases Research Articles

Impact of Madden Julian Oscillation on the diurnal cycle of precipitation over the west coast of India

Madden Julian Oscillation (MJO) is an eastward propagating convective system that passes over the Indian and Pacific Oceans with a periodicity of 30–90 days and influences most of the tropical weather systems. This study investigates and quantifies the impact of MJO on the diurnal cycle of precipitation over the west coast of India. The work uses TRMM precipitation, profiles of wind and humidity from ERA5 re-analysis and satellite Infrared window channel measurements from GridSat-B1 for the period 2000–2019. Real-time Multivariate MJO index has been used for the identification of active and suppressed MJO phases. Harmonic analysis is used to derive the diurnal and semi-diurnal amplitudes of precipitation. The active MJO phases enhance the diurnal amplitude of precipitation over the west coast with respect to the climatological diurnal cycle, whereas the suppressed phases reduce it. Active phases help in the formation of mesoscale convective cloud systems with much more spatial extension and temporal duration than that in suppressed phase. This kind of convective organization is observed especially during the active MJO phases of monsoon and post-monsoon seasons. The vertical structure of wind and humidity in the troposphere are modified by MJO where the active (suppressed) phase enhances (decreases) them. The overall modification of tropospheric dynamics, thermodynamics and deep convection by MJO can cause anomalous rainfall activity over the west coast.

Improving Madden–Julian oscillation simulation in atmospheric general circulation models by coupling with a one-dimensional snow–ice–thermocline ocean model

Abstract. A one-column, turbulent, and kinetic-energy-type ocean mixed-layer model (snow–ice–thermocline, SIT), when coupled with three atmospheric general circulation models (AGCMs), yields superior Madden–Julian oscillation (MJO) simulations. SIT is designed to have fine layers similar to those observed near the ocean surface; therefore, it can realistically simulate the diurnal warm layer and cool skin. This refined discretization of the near-surface ocean in SIT provides accurate sea surface temperature (SST) simulation, and thus facilitates realistic air–sea interaction. Coupling SIT with the European Centre/Hamburg Model version 5, the Community Atmosphere Model version 5, and the High-Resolution Atmospheric Model significantly improved MJO simulation in three coupled AGCMs compared to the AGCM driven by a prescribed SST. This study suggests two major improvements to the coupling process. First, during the preconditioning phase of MJO over the Maritime Continent (MC), the often underestimated surface latent heat bias in AGCMs can be corrected. Second, during the phase of strongest convection over the MC, the change in intraseasonal circulation in the meridional circulation enhancing near-surface moisture convergence is the dominant factor in the coupled simulations relative to the uncoupled experiments. The study results show that a fine vertical resolution near the surface, which better captures temperature variations in the upper few meters of the ocean, considerably improves different models with different configurations and physical parameterization schemes; this could be an essential factor for accurate MJO simulation.

Effect of Advection by Upper-Tropospheric Background Zonal Wind on MJO Phase Speed

Abstract A robust linear regression algorithm is applied to estimate 95% confidence intervals on the background wind associated with Madden–Julian oscillation (MJO) upper-tropospheric atmospheric circulation signals characterized by different phase speeds. Data reconstructed from the ERA5 to represent advection by the upper-tropospheric background flow and MJO-associated zonal wind anomalies, together with satellite outgoing longwave radiation anomalies, all in the equatorial plane, are regressed against advection data filtered for zonal wavenumber 2 and phase speeds of 3, 4, 5, and 7 m s−1. The regressed advection by the background flow is then divided by the negative of the zonal gradient of regressed zonal wind across the central Indian Ocean base longitude at 80°E to estimate the associated background wind that leads to the given advection. The median estimates of background wind associated with these phase speeds are 13.4, 11.2, 10.5, and 10.3 m s−1 easterly. The differences between estimated values at neighboring speeds suggests that advection acts most strongly in slow MJO events, indicating that the slowest events happen to be slow because they experience stronger easterly advection by the upper-tropospheric background wind. Significance Statement The Madden–Julian oscillation (MJO) is the dominant subseasonal rainfall signal of the tropical atmosphere. This project shows that the background wind of the tropical atmosphere most especially slows down the slowest MJO events. Understanding what controls its speed might help scientists better predict events.

Madden-Julian oscillation influences United States springtime tornado and hail frequency

The Madden–Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics and has a documented influence on extratropical extreme weather through modulation of synoptic atmospheric conditions. MJO phase has been correlated with anomalous tornado and severe hail frequency in the United States (US). However, the robustness of this relationship is unsettled, and the variability of physical pathways to modulation is poorly understood, despite the socioeconomic impacts that tornadoes and hail evoke. We approached this problem using pentad MJO indices and practically perfect severe weather hindcasts. MJO lifecycles were cataloged and clustered to document variability and potential pathways to enhanced subseasonal tornado and hail predictability. Statistically significant increases in US tornado and hail probabilities were documented 3–4 weeks following the period of the strongest upper-level divergence for the 53 active MJO events that propagated past the Maritime continent, contrasting with the 47 MJO events that experienced the barrier effect, during boreal spring 1979–2019. The 53 MJO events that propagated past the Maritime continent revealed three prevailing MJO evolutions—each containing unique pathways and modulation of US tornado and hail frequency—advancing our knowledge and capability to anticipate these hazards at extended lead times.

How Long Can the MJO be Predicted During the Combined Phases of ENSO and QBO?

AbstractThe predictability limit of the Madden‐Julian Oscillation (MJO) in the El Niño and La Niña boreal winters (November‐February) is investigated using observational data and historical run outputs of CMIP models. The MJO predictability is computed by the nonlinear local Lyapunov exponents approach for various MJO indices obtained from bandpass‐filtered (30–80 days) outgoing longwave radiation, 850‐hPa zonal wind, and 200‐hPa zonal wind data. The result shows that the MJO predictability during La Niña winters is enhanced by up to 8 days compared to El Niño winters. Enhanced convection of the MJO phases 2–4 with persistence propagation during La Niña winters leads to higher predictability, as compared with those for El Niño winters. The highest (lowest) predictability of the MJO shows 39 (31) days during La Niña/EQBO (El Niño/EQBO) winters. The longer persistence of the MJO during La Niña/EQBO winters leads to higher predictability.

Importance of integration of subseasonal predictions to improve climate services in Sri Lanka case study: Southwest monsoon 2019

The climate outlook for the 2019 southwest monsoon (SWM) season was prepared through an expert assessment of the prevailing global climate conditions and forecasts from different climate models from around the world during the fourteenth session of the South Asian Climate Outlook Forum (SASCOF14). Above-normal rainfall was predicted over Sri Lanka for SWM 2019, and information was shared at the monsoon forum. Even though SWM 2019 seasonal rainfall wasslightly above average, highly uneven rainfall distribution with a deficit of rainfall at the beginning and a surplus of rainfall during the latter part of the season was observed. Unusual dry conditioned prevailed during the month of May 2019, which led to delay the onset of SWM by 2 weeks. Due to the delay of onset and rainfall deficit during the early part of2019 SWM, late cultivation of paddy was observed.Hydro-catchment areas recorded large shortfalls in early part Southwest Monsoon rainfall reducing hydropower generation to 15–18%, from May to July.The suppressed phase of Madden Julian Oscillation (MJO) (phase 6 to 8) with anomalous easterly winds over Sri Lanka was evident from 04th May to 25th May. As MJO being a major predictive source in subseasonal timescale and Sri Lanka being a country located in the heart of the MJO envelope, integration of subseasonal information into seasonal outlook provide much greater value to decision-makers in Agriculture and Energy sector.

Effect of the Quasi‐Biennial Oscillation on the Madden Julian Oscillation Teleconnections in the Southern Hemisphere

AbstractThe Madden Julian Oscillation (MJO) is the main source of intraseasonal variability in the tropics. MJO modulates rainfall in remote areas in the Southern Hemisphere by exciting tropical and extratropical wave trains. We use newly released reanalysis data to analyze how the Quasi‐Biennial Oscillation (QBO) can influence the MJO's effects over the Southern Hemisphere, focusing on precipitation anomalies over South America. The anomalies in the intensity of the South Atlantic Convergence Zone (SACZ) and precipitation over Southeastern South America during MJO phases 1 and 4 are intensified during the easterly QBO. The extratropical wave train excited by the anomalous convection over the maritime continent during MJO phase 4 is affected by the QBO, with a stronger, better‐defined pattern during westerly QBO. The conclusions are supported by a perturbation experiment of a case study, conducted using a high‐top atmospheric global climate model where the QBO and MJO are controlled.

MJO Phase Swings Modulate the Recurring Latitudinal Shifts of the 2020 Extreme Summer‐Monsoon Rainfall Around Yangtse

AbstractConsidering the unprecedented extremity of the Meiyu episodes in early summer of 2020, here the authors perform a comprehensive study of the intraseasonal evolution characteristics of the observed rainfall round Yangtse as well as the potential modulating effect of Madden‐Julian Oscillation (MJO). Results suggest that the Meiyu rainfall manifests as recurring latitudinal shifts around Yangtse during June–July of 2020, corresponding well to the continued swings of MJO circulation and convection between Phase 1 and Phase 2. The MJO affects the meridional migration of the 2020 Meiyu rainfall mainly through regulating the westward extended ridge line and southwesterly of the anomalous anticyclone over the western North Pacific (WNP). Specifically, the northeastward propagation of boreal summer MJO from Phase 1 to 2 shifts the ridge line of the anomalous WNP anticyclone northeastward. Consequently, the associated southwesterly anomaly over the southeastern China penetrates to the north of Yangtse and enhances the lower‐tropospheric moisture flux convergence, supporting heavy precipitation there. However, when the MJO swings back to Phase 1, the anomalous southwesterly and moisture flux convergence retreat southward, so does the Meiyu rain band. The present results advanced our understanding of the interactions between the latitudinal shifts of extreme Meiyu rainfall episodes and MJO Phase swings.

Modulation of East African Boreal Fall Rainfall: Combined Effects of the Madden–Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO)

Abstract Climate variabilities can have significant impacts on rainfall in East Africa, leading to disruption in natural and human systems and affecting the lives of tens of millions of people. Subseasonal and interannual variabilities are critical components of total rainfall variability in the region. The goal of this study is to examine the combined effects of the Madden–Julian oscillation (MJO), operating at a subseasonal time scale, and El Niño–Southern Oscillation (ENSO), operating at an interannual scale, on the modulation of East African boreal fall [October–December (OND)] rainfall, commonly called the short rains. Composite analysis shows that daily rainfall responses depend on MJO phase and its interaction with ENSO state. In particular, MJO modulation of rainfall is generally stronger under El Niño conditions relative to ENSO neutral and La Niña conditions, leading to increased potential for daily precipitation excesses during wet MJO phases under El Niño. There is evidence for both dynamic and thermodynamic mechanisms associated with these impacts, including an increase in westerly moisture transport and easterly advection of temperature and moist static energy. Seasonal analysis shows that the frequency and intensity of wet MJO phases during an El Niño contribute notably to the seasonal total precipitation anomaly. This suggests that the MJO can mediate El Niño’s impact on OND rainfall in East Africa.

Role of the Tropics in State‐Dependent Improvements of US West Coast NOAA Unified Forecast System Precipitation Forecasts

AbstractBoreal‐wintertime hindcasts in the Unified Forecast System with the tropics nudged toward reanalysis improve United States (US) West Coast precipitation forecasts at Weeks 3–4 lead times when compared to those without nudging. To diagnose the origin of these improvements, a multivariate k‐means clustering method is used to group hindcasts into subsets by their initial conditions. One cluster characterized by an initially strong Aleutian Low demonstrates larger improvements at Weeks 3–4 with nudging compared to the others. The greater improvements with nudging for this cluster are related to model errors in simulating the interaction between the Aleutian Low and the teleconnection patterns associated with the Madden‐Julian oscillation (MJO) and El Niño‐Southern Oscillation (ENSO). Improving forecasts of tropical intraseasonal precipitation, especially during early MJO phases under non‐cold ENSO, may be important for producing better Weeks 3–4 precipitation forecasts for the US West Coast.

Influences of MJO on the Diurnal Variation and Associated Offshore Propagation of Rainfall near Western Coast of Sumatra

Madden-Julian Oscillation (MJO) plays an important role in modulating precipitation at Maritime Continent (MC) not only on a larger scale, but also in the diurnal cycle. Diurnal rainfall offshore propagation is one of the most evident features near coasts. This study investigates the impacts of MJO on diurnal rainfall and its offshore propagation at the western coast of Sumatra during boreal winters using ERA5 reanalysis. The real-time multivariate MJO (RMM) index was applied to locate the active MJO convection through eight different phases, in the western hemisphere and Africa in P8–P1, at the Indian Ocean in P2–P3, at MC in P4–P5, and the western Pacific Ocean in P6–P7. The rainfall characteristics, including the daily rate, the absolute and normalized diurnal variation amplitudes, and the strengths of diurnal offshore propagation, not only depend on active/inactive MJO stages but also vary under different MJO phases, through the combined modulations of large-scale backgrounds and local-scale land–sea circulations. The offshore rainfall propagation is associated with meso-large-scale gravity waves generated from land–sea thermal contrast and thus is affected by the radiation effect of cloud under different MJO phases. The stronger wave signals in P8–P1 and P6–P7 enhance the diurnal rainfall variation amplitudes away from the coast, while the strong coupling of moist convection with gravity waves contributes greatly to the diurnal rainfall cycle in P2–P3.

Characteristics of Precipitation Diurnal Cycle over a Mountainous Area of Sumatra Island including MJO and Seasonal Signatures Based on the 15-Year Optical Rain Gauge Data, WRF Model and IMERG

In this study we investigate the characteristics of the diurnal precipitation cycle including the Madden–Julian oscillation (MJO) and seasonal influences over a mountainous area in Sumatra Island based on the in situ measurement of precipitation using the optical rain gauge (ORG). For comparison with ORG data, the characteristics based on the Global Precipitation Measurement (GPM) mission (IMERG) and Weather Research and Forecasting (WRF) simulations were also investigated. Fifteen years of ORG data over a mountainous area of Sumatra, namely, at Kototabang (100.32° E, 0.20° S), were analyzed to obtain the characteristics of the diurnal cycle of precipitation in this region. The diurnal cycle of precipitation presented a single peak in the late afternoon, and the peak time difference was closely related to the rain event duration. The MJO acts to modulate the diurnal amplitude but not the diurnal phase. A high precipitation amount (PA) and frequency (PF) were observed during phases 2, 3, and 4, along with an increase in the number of longer-duration rain events, but the diurnal phase was similar in all MJO phases. In terms of season, the highest PA and PF values were observed during pre-southwest and pre-northeast monsoon seasons. WRF simulation reproduced the diurnal phase correctly and more realistically than the IMERG products. However, it largely overestimated the amplitude of the diurnal cycle in comparison with ORG. These disagreements could be related to the resolution and quality of IMERG and WRF data.

Assimilating C-Band Radar Data for High-Resolution Simulations of Precipitation: Case Studies over Western Sumatra

Accurate high-resolution precipitation forecasts are critical yet challenging for weather prediction under complex topography or severe synoptic forcing. Data fusion and assimilation aimed at improving model forecasts, as one possible approach, has gained increasing attention in past decades. This study investigates the influence of the observations from a C-band Doppler radar over the west coast of Sumatra on high-resolution numerical simulations of precipitation around its vicinity under the Madden–Julian oscillation (MJO) in January and February 2018. Cases during various MJO phases were selected for simulations with an advanced research version of the weather research and forecasting (WRF) model at a cloud-permitting scale (~3 km). A 3-dimensional variational (3DVAR) data assimilation method and a hybrid three-dimensional ensemble–variational data assimilation (3DEnVAR) method, based on the NCEP Gridpoint Statistical Interpolation (GSI) assimilation system, were used to assimilate the radar reflectivity and the radial velocity data. The WRF-simulated precipitation was validated with the Integrated Multi-satellitE Retrievals for GPM (IMERG) precipitation data, and the fractions skill score (FSS) was calculated in order to evaluate the radar data impacts objectively. The results show improvements in the simulated precipitation with hourly radar data assimilation 6 h prior to the simulations. The modifications with assimilation were validated through the observation departure and moist convection. It was found that forecast improvements are relatively significant when precipitation is more related to local-scale convection but rather small when the background westerly wind is strong under the MJO active phase. The additional simulation experiments, under a 1- or 2-day assimilation cycle, indicate better improvements in the precipitation simulation with 3DEnVAR radar assimilation than those with the 3DVAR method.

The dominant intraseasonal variability mode of tropical convection in the 2 weeks before a Northern Hemisphere extreme stratospheric polar vortex

AbstractThe Madden–Julian Oscillation (MJO) phase 7 has been recognized as a driver for Northern Hemisphere (NH) sudden stratospheric warmings (SSWs) in the literature recently. However, uncertainties remain on the exact relationship between MJO phase 3 and the stratospheric polar vortex. Here, we re‐examined and compared the observed evolution of the MJO before the two types of NH extreme stratospheric polar vortex events (i.e. strong vortex (SV) events and weak vortex (WV) events) in this study. Our analysis reveals that MJO phase 3 (7) is the dominant intraseasonal variability mode in about 2 weeks before onset of the SV (WV) events, suggesting these two phases are closely linked to variation in the intensity of the stratospheric polar vortex. MJO phase 3 (7) leads to a strengthened (weakened) polar vortex due to the suppressed (enhanced) wave‐number 1 wave activity in the extratropical stratosphere resulting from destructive (constructive) interference between the negative (positive) Pacific–North American (PNA)‐like pattern induced by MJO phase 3 (7) and the background field. Note that the time‐scales for the peak responses of wave activity in the extratropical stratosphere to MJO phases 3 and 7 are around 2 weeks and 1 week, respectively. Thus, within 2 weeks, MJO phase 3 (7) can be treated as the precursor to the strengthened (weakened) polar vortex.

Influence of the Madden–Julian Oscillation on the Arctic Oscillation Prediction in S2S Operational Models

The connections between the Madden–Julian Oscillation (MJO) and the Arctic Oscillation (AO) are examined in both observations and model forecasts. In the observations, the time-lag composites are carried out for AO indices and anomalies of 1,000-hPa geopotential height after an active or inactive initial MJO. The results show that when the AO is in its positive (negative) phase at the initial time, the AO activity is generally enhanced (weakened) after an active MJO. Reforecast data of the 11 operational global circulation models from the Sub-seasonal to Seasonal (S2S) Prediction Project are further used to examine the relationship between MJO activity and AO prediction. When the AO is in its positive phase on the initial day of the S2S prediction, an initial active MJO can generally improve the AO prediction skill in most of the models. This is consistent with results found in the observations that a leading MJO can enhance the AO activity. However, when the AO is in its negative phase, the relationship between the MJO and AO prediction is not consistent among the 11 models. Only a few S2S models provide results that agree with the observations. Furthermore, the S2S prediction skill of the AO is examined in different MJO phases. There is a significantly positive relationship between the MJO-related AO activity and the AO prediction skill. When the AO activity is strong (weak) in an MJO phase, including the inactive MJO, the models tend to have a high (low) AO prediction skill. For example, no matter what phase the initial AO is in, the AO prediction skill is generally high in MJO phase 7, in which the AO activity is generally strong. Thus, the MJO is an important predictability source for the AO forecast in the S2S models.

Evidence of a chlorophyll “tongue” in the Malacca Strait from satellite observations

Remotely sensed surface chlorophyll-a concentration from the Moderate Resolution Imaging Spectroradiometer (MODIS) unveils signatures of a “tongue” like feature extending into the Malacca Strait from the west of Malaysia. A threefold increase of chlorophyll-a amplitudes is observed in the strait during the northeast monsoon (December–February) compared to the lowest values during the southwest monsoon (May–September). This seasonal variability of chlorophyll-a is linked with the seasonal variabilities of the meridional component of Ekman mass transport (M y ), sea surface temperature (SST), wind stress curl, and the sea surface height anomaly (SSHA). A multiple regression analysis indicates that the monsoonal winds and SST are the dominant contributors to the chlorophyll-a variability in the strait. The role of SSHA is limited to the presence of a seasonal cyclonic eddy in the strait during December–February and its associated upwelling. The monsoonal winds associated with the active phase of Madden Julian Oscillation (MJO) events during November induced strong coastal upwelling along the west coast of Malaysia. Later, strong upwelling associated with both wind-induced coastal upwelling and open ocean upwelling due to wind stress curl is observed in the strait during December–February, which is also well supported by the entrainment of cold and saline waters from the subsurface and advection of colder SST within the strait. The advection of higher chlorophyll-a in the north-westward extension of the “tongue” off the west coast of Malaysia during December – February is attributed to the Ekman mass transport due to higher wind stress. On the other hand, lower chlorophyll-a during May–September is due to the negative wind stress curl and downwelling. Rectified wavelet analysis shows higher correlations of chlorophyll-a variability with the active phases of the MJO events than with the ENSO and IOD events, indicating substantial impacts of the MJO events in the Malacca Strait. • First study to link Chl-a seasonality in Malacca Strait with winds, SST and SSH. • Highest Chl-a values during December–February manifest as a “tongue.” • Winds drive open ocean and coastal upwelling processes to generate the “tongue”. • MJO events influences chlorophyll-a variability more than ENSO and IOD events.

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.

Modulation of mid-Atlantic tropical cyclone landfalls by the Madden-Julian Oscillation

Tropical cyclone (TC) landfalls over the U.S. mid-Atlantic region are very infrequent. However, when they do occur, the resulting human and material losses can be severe, as was the case with Hurricane Sandy in 2012. Therefore, it is important to predict these land-falling events as far in advance as possible. In this study, we investigate the relationship between mid-Atlantic TC landfalls and the Madden-Julian Oscillation (MJO), which is the dominant source of climate variability in the tropics on intraseasonal time scales. This is largely accomplished by using a high-atmospheric-resolution ensemble prediction system based on the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model (Project Minerva) to compile the statistics of these rare events, and the velocity potential MJO (VPM) index to define the phase and amplitude of the MJO. We find that at longer lead times (between 14 and 7 days prior to landfall), statistically significant peak landfall probabilities are present during MJO phases 1 and 7 and, to some extent, phase 8. This result is largely supported by observational data. At shorter lead times (between 6-day lead and landfall), phase 1 is strongly favored with some contribution from phases 2 and 3. These findings suggest a potential for extended-range predictions of the mid-Atlantic TC landfall risk based on the phase of the MJO.

Year‐round sub‐seasonal forecast skill for Atlantic–European weather regimes

AbstractWeather regime forecasts are a prominent use case of sub‐seasonal prediction in the midlatitudes. A systematic evaluation and understanding of year‐round sub‐seasonal regime forecast performance is still missing, however. Here we evaluate the representation of and forecast skill for seven year‐round Atlantic–European weather regimes in sub‐seasonal reforecasts from the European Centre for Medium‐Range Weather Forecasts. Forecast calibration improves regime frequency biases and forecast skill most strongly in summer, but scarcely in winter, due to considerable large‐scale flow biases in summer. The average regime skill horizon in winter is about 5 days longer than in summer and spring, and 3 days longer than in autumn. The Zonal Regime and Greenland Blocking tend to have the longest year‐round skill horizon, which is driven by their high persistence in winter. The year‐round skill is lowest for the European Blocking, which is common for all seasons but most pronounced in winter and spring. For the related, more northern Scandinavian Blocking, the skill is similarly low in winter and spring but higher in summer and autumn. We further show that the winter average regime skill horizon tends to be enhanced following a strong stratospheric polar vortex (SPV), but reduced following a weak SPV. Likewise, the year‐round average regime skill horizon tends to be enhanced following phases 4 and 7 of the Madden–Julian Oscillation (MJO) but reduced following phase 2, driven by winter but also autumn and spring. Our study thus reveals promising potential for year‐round sub‐seasonal regime predictions. Further model improvements can be achieved by reduction of the considerable large‐scale flow biases in summer, better understanding and modeling of blocking in the European region, and better exploitation of the potential predictability provided by weak SPV states and specific MJO phases in winter and the transition seasons.

NAO Influence on the MJO and its Prediction Skill in the Subseasonal-to-Seasonal Prediction Models

AbstractBased on the database of the Subseasonal to Seasonal (S2S) Prediction project of the World Weather Research Programme (WWRP) / World Climate Research Programme (WCRP), the influence of the North Atlantic Oscillation (NAO) on the Madden- Julian Oscillation (MJO) and its forecast skill is investigated. It is found that most models can capture the MJO phase changes following positive and negative NAO events. About 20 days after initialized with a positive (negative) NAO, the forecast MJO appears more frequently in phase 7 (3), which corresponds to reduced (enhanced) convection in the tropical Indian Ocean and enhanced (suppressed) convection in the western Pacific. In most S2S models the MJO prediction skill is dependent on the NAO amplitude and phase in the initial condition. A strong NAO leads to a better MJO forecast skill than a weak NAO. The MJO skill tends to be higher when the forecast starts from a negative NAO than a positive NAO. These results indicates that there is a strong Northern extratropical influence on the MJO and its forecast skill. It is important for numerical models to better represent the NAO influence to improve the simulation and prediction of the MJO.