Madden–Julian Oscillation Activity Research Articles

Evaluating MJO event initiation and decay in the skeleton model using an RMM‐like index

AbstractThe Madden‐Julian oscillation (MJO) skeleton model is a low‐order dynamic model that is capable of simulating many of the observed features of the MJO. This study develops a model‐based “MJO” index that is similar to the well‐known real‐time multivariate MJO (RMM) index to better facilitate comparison between the skeleton model and observational data. Multivariate and univariate empirical orthogonal function (EOF) analyses were performed on the convective heating and zonal wind data taken from the skeleton model for simulations forced with an idealized warm pool and observed sea surface temperatures (SSTs). The leading EOF modes indicated a wave number 1 convectively coupled circulation anomaly with zonal asymmetries that closely resembled the observed RMM EOFs, especially when the model was forced with observed SSTs. The RMM‐like index was used to compute an MJO climatology and document the occurrence of primary, continuing, and terminating MJO events in the skeleton model. The overall amount of MJO activity and event lengths compared reasonably well to observations for such a simple model. Attempts at reconciling the observed geographic distribution of individual MJO initiation and termination events were not successful for the stochastic simulations, though stochasticity is necessary in order to produce composite MJOs that initiate and decay with time scales similar to observations. Finally, analysis indicates that the existence of slow‐moving, eastward traveling waves with higher wave numbers (k ≈ 12) embedded within the large‐scale flow often precedes MJO termination in the skeleton model.

Extended-range forecast of spring rainfall in southern China based on the Madden–Julian Oscillation

Spring (March–May) rainfall after a dry period in winter has a substantial impact on agriculture and water management in populous southern China. The occurrence of low-frequency spring rainfall anomalies has been linked with the tropical Madden–Julian Oscillation (MJO) through its modulation of large-scale circulation and moisture supply over southern China. Using the spatial and temporal information of the MJO as a predictor, an empirical model for extended-range forecasting of spring rainfall in southern China was constructed. We first obtained the coupled patterns between the preceding MJO evolutions (real-time multivariate MJO index) and the succeeding rainfall variability in southern China based on singular value decomposition analysis. Then, a prediction was carried out by projecting the predictor onto the spatiotemporal coupled patterns. Useful skill, in terms of the temporal correlation coefficient (TCC) between the predicted and observed rainfall over southern China, persisted up to a forecast lead-time of six pentads. The forecast amplitude bias in terms of root-mean-square error was around 1.0 standard deviation. Also, the forecast skill was highly dependent on the strength of the MJO signal. During active MJO periods, the TCC skill was around twofold larger than that during weak MJO periods. The current statistical model shows encouraging ability, but additional work is required to improve its forecasting skill.

Global‐scale convective aggregation: Implications for the Madden‐Julian Oscillation

AbstractPrevious work has shown that convection will self‐organize in cloud‐system‐resolving model simulations of radiative‐convective equilibrium, and it has been suggested that the convective envelope of the Madden‐Julian oscillation (MJO) may be organized by similar processes on a much larger scale. Here we present support for that hypothesis based on simulations with SP‐CAM with globally uniform SST. Without rotation, convection self‐organizes into large (∼4000 km) clusters surrounded by dry regions, while with Earth‐like rotation the model produces a robust MJO. The nonrotating aggregation and MJO are found to have similar budgets of moist static energy, both being supported by diabatic feedbacks, particularly cloud‐longwave interaction. Mechanism denial experiments show that longwave heating anomalies associated with high clouds are essential to the nonrotating aggregation, and amplify the MJO. Simulations using the conventional CAM show a weaker MJO and a much weaker tendency for nonrotating aggregation, and both MJO activity and aggregation intensity are found to increase with the entrainment rate in the deep convection parameterization.

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

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

Precipitation microstructure in different Madden–Julian Oscillation phases over Sumatra

Intraseasonal variations of precipitation and its microstructure are investigated using measurements of the Equatorial Atmospheric Radar (EAR) facilities at Kototabang, west Sumatra, Indonesia (0.20°S, 100.32°E, 864m above sea level). Raindrop size distribution (DSD) observations are obtained from a 2D-Video Disdrometer (2DVD) with a near continuous record of operation over eight consecutive years (2003–2010). Precipitation types are classified using 1.3-GHz wind profiler observation, and are partitioned according to active and inactive convective phases of Madden–Julian Oscillation (MJO). It is found that precipitation systems during the inactive phase are more continental in nature than those during the active phase. Cloud propagation from brightness temperature data indicates that Sumatra receives the rainfall mainly from maritime clouds during the active phase, while it is mainly from the continental clouds (land-based convection) during the inactive phase. Other remarkable differences between active and inactive phase precipitation systems are also observed from the vertical structure of precipitation. The precipitation during the inactive phase has deeper storms, a higher reflectivity aloft, more lightning activity and less stratiform characteristics, as compared to the active phase. Assessment of cloud effective radius of the Moderate Resolution Imaging Spectroradiometer (MODIS) data also shows a slight difference in the cloud droplet between the active and the inactive MJO phases. Different convective storms in different MJO phases lead to different DSD characteristics and Z–R relationships. The DSD during the inactive phase tends to have a higher concentration of medium and large-size drops than the active counterpart, consistent with the previous study during the first campaign of Coupling Processes in the Equatorial Atmosphere project. Although the DSD parameters and coefficient of Z–R relationships fall within the range of tropical maritime precipitation, mass-weighted mean diameter (Dm) for the deep convective rains during the inactive phase are somewhat larger than that for maritime and closer to the continental cluster. Therefore, continental-like DSDs are somewhat dominant during the inactive phase, consistent with the intraseasonal variation of precipitation structure. The causative processes of the observed difference in the DSD for the two phases have also been discussed with the help of satellite and radar data. Evaporation and updraft associated with the intense convection during the inactive phase seem to eliminate the small-sized drops from the spectra. Finally, radar reflectivity during the inactive phase is larger than that during the active MJO phase, at the same rainfall rate. This condition can limit the accuracy of radar-derived rainfall estimates for the tropics when applying a single Z–R relation to the two MJO phases, particularly for deep convective rains.

Diurnal cycle of the South Pacific Convergence Zone in 30 years of satellite images

AbstractA data set of three‐hourly South Pacific Convergence Zone (SPCZ) location and extent, from which IR temperature within the SPCZ may be obtained, for 1980–2012, November–April is used to examine the diurnal cycle of the SPCZ. Maximum SPCZ area occurs at 15:00–18:00 local standard time (LST). Two minima in mean IR temperature are evident, one at 13:00–16:00 LST, nearly coinciding with the maximum in area, and the second in the early morning hours at 05:00–07:00 LST, when area is at a minimum. The morning minimum in mean IR temperature is associated with a peak in deep convection while the afternoon minimum is associated with a peak in midlevel clouds. On average, the morning minimum in IR temperature dominates in the tropical regions of the SPCZ, while the afternoon IR temperature minimum dominates in the subtropical regions of the SPCZ. The relative strength of the two IR temperature minima is affected by the seasonal cycle, intraseasonal variability associated with active Madden Julian Oscillation events, and interannual variability due to the El Niño Southern Oscillation. The morning IR temperature minimum becomes more dominant when the SPCZ is more frequently present or when the SPCZ is shifted toward the equator. In terms of cloud height, midlevel clouds dominate at all times in all regions of the SPCZ and peak in abundance between 15:00–18:00 LST. Low‐level clouds peak near midnight and then transition to high‐level clouds, which peak between 03:00–06:00 LST, just before sunrise.

Evidence for a modulation of the intraseasonal summer temperature in Eastern Patagonia by the Madden‐Julian Oscillation

AbstractWe describe the relationship between the intraseasonal component of surface air temperature (SAT) variability in Eastern Patagonia and the Madden‐Julian Oscillation (MJO) during austral summer based on ~50 years of daily instrumental records, the Twentieth Century Reanalysis, and a century‐long MJO index reconstruction. Our results show that the summer SAT variability in Patagonia is highly driven by the intraseasonal activity (~80%), especially by that associated with the MJO. The active MJO phases modulate the spatial mean intraseasonal temperature signal in Eastern Patagonia with ~1.5°C of amplitude. In most of the region, the warmest (coldest) conditions are found during active phase 8 (4). These opposite states of the temperature perturbations are related to almost inverse midlevel circulation anomalies over southern South America and the southwest Atlantic, which are part of a large‐scale Rossby‐like wave train of alternating circulation anomalies extended along the South Pacific. The corresponding outgoing longwave radiation anomalies suggest that these structures may be triggered by anomalous convection in the tropics. Furthermore, we show that intraseasonal heat waves in southeastern Patagonia tend to occur during active MJO phase 8. These events are also induced by a wave train pattern over the South Pacific, associated with other intraseasonal variability sources. Hence, as shown in a case study, circulation anomalies over the South Pacific triggered, in general, by tropical convection variability and, in particular, by the MJO activity may constructively interact with circulation patterns resulting from the extratropical dynamics, eventually leading to intraseasonal heat waves in southeastern Patagonia.

Some Climatological Aspects of the Madden–Julian Oscillation (MJO)

Abstract One of the most commonly used metrics for both locating the Madden–Julian oscillation (MJO) geographically and defining the intensity of MJO convective activity is the real-time multivariate MJO (RMM) index. However, a climatology of the MJO, particularly with respect to the frequency of activity levels or of consecutive days at certain activity thresholds, does not yet exist. Thus, several climatological aspects of the MJO were developed in this study: 1) annual and 2) seasonal variability in MJO intensity, quantified using four defined activity categories (inactive, active, very active, and extremely active); 3) persistence in the above-defined four categories; 4) cycle length; and 5) low-frequency (decadal) variability. On an annual basis, MJO phases 1 and 2 occurred more often, and phase 8 occurred less often, than the other phases throughout the year. Notable seasonality was also found, particularly in the frequency of extremely active MJO in March–May (8% of days) compared with June–August (only 1% of days). The MJO was persistent in time and across intensity categories, and all activity categories the following day had at least an 80% chance of maintaining their amplitudes. Implications of this climatology are discussed, including length of complete MJO cycles (the shortest of which was 17 days) and correlations between MJO amplitude and atmospheric response.

Interannual variability of the Madden‐Julian Oscillation and its impact on the North Atlantic Oscillation in the boreal winter

AbstractInterannual variability of the Madden‐Julian Oscillation (MJO) and its influence on the extratropical teleconnection are analyzed. It is found that there is an interannual shift of the MJO activity between the Indian Ocean and the western central Pacific. This is reflected by the year‐to‐year changes in the occurrence frequency of individual MJO phases. The leading mode of an empirical orthogonal function analysis of occurrence frequency of MJO phases shows an out‐of‐phase relationship between MJO phase 7 and phases 3–5. During winters when the MJO convection occurs more frequently in the Indian Ocean (phases 3–5), a winter mean positive North Atlantic Oscillation (NAO) tends to occur. These results indicate that the seasonal mean convection activity over the Indian Ocean related to the MJO is a possible driving forcing for the seasonal mean NAO variability in the boreal winter.

The Effects of Explicit versus Parameterized Convection on the MJO in a Large-Domain High-Resolution Tropical Case Study. Part II: Processes Leading to Differences in MJO Development

High-resolution simulations over a large tropical domain (~20°S–20°N, 42°E–180°) using both explicit and parameterized convection are analyzed and compared during a 10-day case study of an active Madden–Julian oscillation (MJO) event. In this paper, Part II of this study, the moisture budgets and moist entropy budgets are analyzed. Vertical subgrid diabatic heating profiles and vertical velocity profiles are also compared; these are related to the horizontal and vertical advective components of the moist entropy budget, which contribute to gross moist stability (GMS) and normalized GMS (NGMS). The 4-km model with explicit convection and good MJO performance has a vertical heating structure that increases with height in the lower troposphere in regions of strong convection (like observations), whereas the 12-km model with parameterized convection and a poor MJO does not show this relationship. The 4-km explicit convection model also has a more top-heavy heating profile for the troposphere as a whole near and to the west of the active MJO-related convection, unlike the 12-km parameterized convection model. The dependence of entropy advection components on moisture convergence is fairly weak in all models, and differences between models are not always related to MJO performance, making comparisons to previous work somewhat inconclusive. However, models with relatively good MJO strength and propagation have a slightly larger increase of the vertical advective component with increasing moisture convergence, and their NGMS vertical terms have more variability in time and longitude, with total NGMS that is comparatively larger to the west and smaller to the east.

Impact of the Madden–Julian Oscillation on Western North Pacific Tropical Cyclogenesis Associated with Large-Scale Patterns

AbstractThe intraseasonal variability of tropical cyclogenesis in the western North Pacific (WNP) basin is explored in this study. The relation of cyclogenesis in each of the five large-scale patterns identified in recent work by Yoshida and Ishikawa is associated with the Madden–Julian oscillation (MJO). Confirming previous results, more events of cyclogenesis are found during the active MJO phase in the WNP. Furthermore, results indicate that most of the tropical cyclogenesis is associated with the monsoon shear line large-scale pattern during the active phase. The genesis potential index (GPI) and its individual components are used to evaluate the environmental factors that most contribute toward cyclogenesis under the different phases of the MJO. GPI exhibits a large positive anomaly during the active phase of the MJO, and such an anomaly is spatially correlated with the events of cyclogenesis. The analysis of each factor indicates that low-level relative vorticity and midlevel relative humidity are the two dominant contributors to the MJO-composited GPI anomalies. The positive GPI anomalies during the active phase are partially offset by the negative contributions from vertical wind shear and potential intensity. This is valid for all five large-scale patterns. It is noteworthy that the easterly wave (EW) large-scale pattern, while exhibiting the same influence of relative vorticity and midlevel humidity contributing toward positive GPI anomalies, presents slightly more cyclogenesis events under the inactive phase of the MJO. This unexpected result suggests that other factors not included in the definition of the GPI and/or changes in environmental flows on other time scales contribute to the tropical cyclogenesis associated with the EW large-scale pattern.

Correlating MJO Activity with Argentina Rainfall and Atlantic Hurricanes Using ICI-RAFT

Knowledge of how the frequency and intensity of extreme events are affected by climate variability will contribute to answering questions related to drought and water availability that are important in water resources management. A tool titled the “International Center for Integrated Water Resources Management–Regional Analysis of Frequency Tool” (ICI-RAFT), which includes a database of several climate indices, was developed to accomplish this using rainfall observations on the ground. The method used was applied to rainfall obtained from 20 rain gauges in western central Argentina and hurricanes in the Atlantic Ocean during the respective active seasons. The global climate index that was found to have the greatest impact in both cases was the Madden-Julian Oscillation (MJO), which is the focus of the current study. Specifically, MJO activity over central Africa (MJO9) using a lag of 4 months and the western Indian Ocean (MJO10) using zero lag had the greatest influences, respectively. It was found that knowledge of the value of the MJO at these locations allows the prediction of below- or above-average rainfall or hurricane conditions for the current season and even allows a more precise quantitative estimate of this activity to be made.

Extraction and predictability of coherent intraseasonal signals in infrared brightness temperature data

This work studies the spatiotemporal structure and regime predictability of large-scale intraseasonal oscillations (ISOs) of tropical convection in satellite observations of infrared brightness temperature (\(T_b\)). Using nonlinear Laplacian spectral analysis (NLSA), a data analysis technique designed to extract intrinsic timescales of dynamical systems, the \(T_b\) field over the tropical belt \(15^{\circ }\hbox {S}-15^{\circ }\hbox {N}\) and the years 1983–2006 (sampled every 3 h at \(0.5^{\circ }\) resolution) is decomposed into spatiotemporal modes spanning interannual to diurnal timescales. A key advantage of NLSA is that it requires no preprocessing such as bandpass filtering or seasonal partitioning of the input data, enabling simultaneous recovery of the dominant ISOs and other patterns influenced by or influencing ISOs. In particular, the eastward-propagating Madden–Julian oscillation (MJO) and the poleward-propagating boreal summer intraseasonal oscillation (BSISO) naturally emerge as distinct families of modes exhibiting non-Gaussian statistics and strong intermittency. A bimodal ISO index constructed via NLSA is found to have significantly higher discriminating power than what is possible via linear methods. Besides MJO and BSISO, the NLSA spectrum contains a multiscale hierarchy of modes, including the annual cycle and its harmonics, ENSO, and modulated diurnal modes. These modes are used as predictors to quantify regime predictability of the MJO amplitude in \(T_b\) data through a cluster-based framework. It is found that the most predictable MJO regimes occur before the active-MJO season (November–December), when ENSO has a strong influence on the future statistical behavior of MJO activity. In forecasts initialized during the active-MJO period (February), both ENSO and the current state of MJO are significant predictors, but the predictive information provided by the large-scale convective regimes in \(T_b\) is found to be smaller than in the early-season forecasts.

Effect of dry large‐scale vertical motions on initial MJO convective onset

AbstractAnomalies of eastward propagating large‐scale vertical motion with ~30 day variability at Addu City, Maldives, move into the Indian Ocean from the west and are implicated in Madden‐Julian Oscillation (MJO) convective onset. Using ground‐based radar and large‐scale forcing data derived from a sounding array, typical profiles of environmental heating, moisture sink, vertical motion, moisture advection, and Eulerian moisture tendency are computed for periods prior to those during which deep convection is prevalent and those during which moderately deep cumulonimbi do not form into deep clouds. Convection with 3–7 km tops is ubiquitous but present in greater numbers when tropospheric moistening occurs below 600 hPa. Vertical eddy convergence of moisture in shallow to moderately deep clouds is likely responsible for moistening during a 3–7 day long transition period between suppressed and active MJO conditions, although moistening via evaporation of cloud condensate detrained into the environment of such clouds may also be important. Reduction in large‐scale subsidence, associated with a vertical velocity structure that travels with a dry eastward propagating zonal wavenumbers 1–1.5 structure in zonal wind, drives a steepening of the lapse rate below 700 hPa, which supports an increase in moderately deep moist convection. As the moderately deep cumulonimbi moisten the lower troposphere, more deep convection develops, which itself moistens the upper troposphere. Reduction in large‐scale subsidence associated with the eastward propagating feature reinforces the upper tropospheric moistening, helping to then rapidly make the environment conducive to formation of large stratiform precipitation regions, whose heating is critical for MJO maintenance.

The Global Influence of the Madden–Julian Oscillation on Extreme Temperature Events

AbstractThe authors investigated the influence of the Madden–Julian oscillation (MJO) on extreme warm and cold events, which may have large social and economic impacts. The frequencies of extreme temperature events were analyzed and compared between active and inactive MJO periods by using the 7-day running average of the 850-hPa temperature during the extended boreal winter (November–April). The results show that the frequency of extreme events is significantly modulated (i.e., increased by a factor of more than 2) by the MJO with a time lag over some areas in the extratropics as well as in the tropics. In the extratropics, the modulation of the frequency of the extreme events is roughly associated with midlatitude wave responses to tropical forcing and anomalous lower-level circulation due to the MJO.The relationship between the MJO and forecast skill of extreme temperature events was also investigated by using a suite of hindcasts made with the operational one-month ensemble prediction system of the Japan Meteorological Agency. Forecast skill of extreme events occurring after active MJO periods tend to be better over some areas, compared with after inactive MJO periods. These results suggest that a realistic representation of the MJO and of the atmospheric response to the MJO in forecast models is important for providing reliable early warning information about extreme events.

The MJO skeleton model with observation‐based background state and forcing

The Madden–Julian Oscillation (MJO) skeleton model, a nonlinear oscillator model, has previously been shown to have solutions which exhibit several features in common with observed MJOs. In prior studies, model solutions have been found using mathematically simple (constant or sinusoidal) identical moistening and radiative forcing functions. Here we investigate whether this model can also produce realistic regional variability. To do this, observation‐based forcing functions are prescribed for latent heat flux and radiative cooling, and model solutions –both linear and nonlinear –are studied. In the stochastic nonlinear model, solutions reproduce the climatological mean convective activity well and the climatological variance reasonably well for such a simple model. In the linearized model, the solutions are found to contain additional structure including a realistic wave envelope of convective activity centred over the warm pool. These linear solutions are then used to identify and compare significant MJO activity in both reanalysis data and stochastic model solutions. Additional potential uses for these analytical and numerical solutions are discussed.

The Impact of the Initial Flow on the Extratropical Response to Madden–Julian Oscillation Convective Heating

Abstract The response to the Madden–Julian oscillation (MJO) over the Pacific–North American (PNA) region is investigated. In addition, the sensitivity of this response to the interaction between Madden–Julian oscillation forcing and the extratropical initial flow is explored. First, a simple dynamical model is run with an ensemble of 100 randomly selected initial conditions from ERA-Interim data, with no heating, MJO phase-1-like heating, and MJO phase-5-like heating. The 300-hPa geopotential height field is separated into a part that would evolve without an active MJO present, and a part that is a consequence of the MJO heating. A negative 300-hPa geopotential height anomaly centered over northeastern China bounded by positive anomalies on its equatorward and poleward flanks is found to be followed by a large amplitude negative PNA-like response for MJO phase 1 and a positive PNA-like response for phase 5. A similar study is carried out using observational data. An analog approach—using projections to determine the analogs—is used to approximate the part of the flow that results from the MJO heating. The composite initial flow that corresponds to a large MJO response in observational data somewhat matches that in the model, although there is more variability between phases. Finally, the analog method is used to examine questions related to predictability and the MJO. It is found that predictability is improved by taking into account the presence of the MJO and by choosing analogs with high projections. The presence of an active MJO also increases predictability.

MJO Intensification with Warming in the Superparameterized CESM

Abstract The Madden–Julian oscillation (MJO) is the dominant mode of tropical intraseasonal variability, characterized by an eastward-propagating envelope of convective anomalies with a 30–70-day time scale. Here, the authors report changes in MJO activity across coupled simulations with a superparameterized version of the NCAR Community Earth System Model. They find that intraseasonal OLR variance nearly doubles between a preindustrial control run and a run with 4×CO2. Intraseasonal precipitation increases at a rate of roughly 10% per 1 K of warming, and MJO events become 20%–30% more frequent. Moist static energy (MSE) budgets of composite MJO events are calculated for each scenario, and changes in budget terms are used to diagnose the physical processes responsible for changes in the MJO with warming. An increasingly positive contribution from vertical advection is identified as the most likely cause of the enhanced MJO activity. A decomposition links the changes in vertical advection to a steepening of the mean MSE profile, which is a robust thermodynamic consequence of warming. Surface latent heat flux anomalies are a significant sink of MJO MSE at 1×CO2, but this damping effect is reduced in the 4×CO2 case. This work has implications for organized tropical variability in past warm climates as well as future global warming scenarios.

Latent heating characteristics of the MJO computed from TRMM Observations

AbstractThe Tropical Rainfall Measurement Mission's (TRMM) Spectral Latent Heating algorithm shows the contributions of different forms of convection to the latent heating profiles of the Madden‐Julian Oscillation over the central Indian and West Pacific Oceans. In both oceanic regions, storms containing broad stratiform regions produce increased upper level heating during active Madden‐Julian Oscillation (MJO) phases. The largest differences between the central Indian and West Pacific Ocean heating are associated with heating produced by convective elements. Examination of the most extreme forms of convection shows that mesoscale organized convection often produces at least as much latent heat as young vigorous deep convection. Heating from nonextreme (often midlevel‐topped) convection is an important component of the MJO heating in both regions in all stages of the MJO. Over the central Indian Ocean the heating profile changes from having a maximum at 2 km due to nonextreme convection to a profile during the active stage that has two maxima: one at 3 km due to nonextreme convection and 6 km owing to numerous mature mesoscale storms with broad stratiform precipitation components. Over the West Pacific, the maxima at 3 and 6 km are present in all MJO stages, but the magnitude of the 6 km maximum sharply increases in the active MJO stage due to an increase in the number of storms with broad stratiform precipitation areas.

Eastern Pacific Intraseasonal Variability: A Predictability Perspective

Abstract The eastern Pacific (EPAC) warm pool is a region of strong intraseasonal variability (ISV) during boreal summer. While the EPAC ISV is known to have large-scale impacts that shape the weather and climate in the region (e.g., tropical cyclones and local monsoon), simulating the EPAC ISV is still a great challenge for present-day global weather and climate models. In the present study, the predictive skill and predictability of the EPAC ISV are explored in eight coupled model hindcasts from the Intraseasonal Variability Hindcast Experiment (ISVHE). Relative to the prediction skill for the boreal winter Madden–Julian oscillation (MJO) in the ISVHE (~15–25 days), the skill for the EPAC ISV is considerably lower in most models, with an average skill around 10 days. On the other hand, while the MJO exhibits a predictability of 35–45 days, the predictability estimate for the EPAC ISV is 20–30 days. The prediction skill was found to be higher when the hindcasts were initialized from the convective phase of the EPAC ISV as opposed to the subsidence phase. Higher prediction skill was also found to be associated with active MJO initial conditions over the western Pacific (evident in four out of eight models), signaling the importance of exploring the dynamic link between the MJO and the EPAC ISV. The results illustrate the possibility and need for improving dynamical prediction systems to facilitate more accurate and longer-lead predictions of the EPAC ISV and associated weather and short-term climate variability.