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Extended-range forecast has long maintained a difficult point for the seamless forecast system due to the lack of predictability, with intraseasonal oscillation (ISO), an important signal in many high-impact weather events, being an important source of that. To improve the accuracy of ISO extended-range forecast and make up the gaps in previous researches in this regard, a data-driven model ISOX is proposed for the intraseasonal components of atmospheric fields. Compared with the subseasonal forecast results from climate forecast system (CFS), and the climatological forecast, ISOX achieves higher accuracy for lead times longer than 13 days, with few spatial or temporal weak points. It also performed better in predicting the positive 2 m temperature ISO and lower tropospheric conditions in a heatwave event, surpassing CFS for lead times longer than 13 days. Finally, through gradient evaluation, the model is proved to be able to study the ISO signal movements of atmospheric systems. Thus, the success of this model may shed light on improving extended-range forecast skills and assist the timely detection and prevention of possible meteorological disasters. [ABSTRACT FROM AUTHOR]

Atmosphere-ocean interactions are understood to significantly modulate climate variability and ocean circulation patterns. In this study, the influence of climate oscillations, particularly the North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO), on the circulation dynamics of the Gulf of Mexico (GoM) is investigated. Empirical Orthogonal Function (EOF) analysis was used to identify the principal modes of variability in the GoM circulation, and cross-spectral analysis was conducted to examine the coherence between the GoM circulation, NAO, and ENSO indices. The results reveal that Gulf of Mexico circulation patterns share significant frequencies with both NAO and ENSO. These shared frequencies suggest synchronization phenomena between NAO, ENSO, and the Atlantic Meridional Overturning Circulation (AMOC), indicating a strong influence of these climate oscillations on the GoM's circulation. Key frequencies observed include a near 7-year period aligning with ENSO's natural variability and semiannual periods linked to NAO and the Madden-Julian Oscillation (MJO). These climate oscillations are found to modulate heat transfer intensity in the GoM, influencing large-scale ocean-atmosphere interactions. The findings highlight the critical role of NAO-ENSO teleconnections in shaping GoM circulation variability and their broader implications for global oceanic heat transport mechanisms. [ABSTRACT FROM AUTHOR]

This study identifies 86 active events and 66 break events of the western North Pacific summer monsoon (WNPSM) from 1979 to 2020. These active and break events exhibit sharp contrast in large-scale convection and circulation fields. During the active period, deep convection almost covers the entire WNP domain (0°–25°N, 120°–170°E), accompanied by the strengthening of a local monsoon trough and monsoon westerlies extending from the Arabian Sea to the WNP. In contrast, during the break period, along with weakened monsoon westerlies, the subtropical high replaces the monsoon trough to dominate the WNP domain, leading to the disappearance of deep convection. Results about the convection evolution at multiple time scales indicate that active/break events are primarily contributed to by the wet/dry phases of intraseasonal oscillations (ISOs). The phase transitions of ISOs are nearly symmetric between active and break events. However, these two types of events present notable asymmetric features in convection anomalies relative to climatology, which can be attributable to the modulation of the seasonal mean component. Specifically, both active and break events correspond to anomalously strong seasonal mean convection, which amplifies (weakens) the convection enhancement (suppression) during the active (break) period and results in normal (enhanced) convection during adjacent periods. The preferred occurrence of active and break events in strong summer monsoons is because related mean backgrounds, including increased low-level positive vorticity, specific humidity, and easterly vertical shear, facilitate the intensification of ISOs. These mean backgrounds are likely induced by SST cooling in the north Indian Ocean and warming in the equatorial central Pacific during the simultaneous summer. Significance Statement: The subseasonal variation in monsoon rainfall typically manifests as fluctuations between active events with abundant rainfall and break events with deficit rainfall, which can induce converse extremes such as floods and droughts, significantly affecting water management, agriculture planning, and economic activities. This study aims to reveal the characteristics, formation, and occurrence regularity of active and break events during the western North Pacific summer monsoon (WNPSM). We note that the occurrence of these active and break events is determined not only by intraseasonal oscillations but also by seasonal mean backgrounds, which result in certain asymmetrical features between the two types of events. These results will enhance our understanding of the monsoon's intraseasonal variability and the physical association between multiple time scales over the WNP domain (0°–25°N, 120°–170°E). [ABSTRACT FROM AUTHOR]

An atmospheric duct is an anomalous atmospheric structural phenomenon that causes trap refraction in the troposphere to change the propagation paths and ranges of electromagnetic waves, thereby increasing the detection distances of radar and other communications. Atmospheric ducts include evaporation, surface, and elevated ducts. Evaporation ducts are commonly found near the sea surface and can impact electromagnetic wave propagation, radar communication, and livelihood applications at low altitudes. The meteorological environment of the Indian Ocean is complex, and the characteristics of its evaporation duct are still poorly understood. We validate four select atmospheric duct models using the observed evaporation duct height (EDH) in the Indian Ocean and present their spatial as well as temporal characterizations in spring using in situ meteorological variables. The results show that the EDH derived using the Babin model has the lowest bias with the observed value; thus, the Babin model can be used by researchers in the future to study changes to the EDHs in sea areas with similar seasonal conditions. The EDH in the Indian Ocean has diurnal variations, with higher (lower) values during the daytime (nighttime). The EDH has a negative correlation with the relative humidity, and the standard deviation of the EDH is largest when the relative humidity is 90%. The EDH has the maximum value when the wind speed is in the range of 8–14 m/s and when the air–sea temperature difference is zero, i.e., ∆ T = T a i r − T s e a = 0. This study validates four different EDH models in the Indian Ocean, thus broadening our knowledge on the tropical atmospheric boundary layer. [ABSTRACT FROM AUTHOR]

As a typical hotspot of heatwaves, southern China experienced a marked shift from dry to humid heatwaves in the recent decade of 2013–2022. Significant 10–30‐day intraseasonal oscillation (ISO) in temperature and humidity is a major contributor to the humid heatwaves. Configuration of the 10–30‐day quasi‐barotropic wave train at mid–high latitudes with the dipolar pattern of tropical convection leads to the concurrence of strong moisture convergence and anomalous descents over southern China. The increased downward longwave radiation in association with moisture enhancement, shortwave radiation and adiabatic heating in association with descents bring about the persisting hot and humid conditions in humid heatwaves. Due to stronger barotropic energy conversion from mean flow to 10–30‐day circulation at mid–high latitudes, and stronger tropical convection activities in the recent decade, the occurrence of 10–30‐day circulation configuration induced humid heatwaves has increased by 13.3%, thus leading to the predominance of humid heatwaves. Plain Language Summary: Southern China is a typical hotspot of heatwaves. In the recent decade of 2013–2022, the southern China experienced a marked transition from dry heatwaves to humid ones, with the occurrence frequency of humid heatwaves becoming more than 4 times that of dry counterpart. As the humid heatwaves usually lasting for 3–5 days are more directly associated with the intraseasonal (10–90‐day) variability, we have identified significant 10–30‐day intraseasonal oscillation (ISO) in temperature and humidity in southern China, and found their critical role in inducing the lethal humid heatwaves. This finding provides the scientific clues for extending the forecast lead time of humid heatwaves. The circulation configuration of the 10–30‐day atmospheric ISOs at mid–high latitudes and tropics leads to the concurrence of strong moisture convergence and anomalous descents over southern China. The increased downward longwave radiation in association with moisture enhancement, shortwave radiation and adiabatic heating in association with descents bring about the persisting hot and wet conditions in humid heatwaves. As more barotropic energy are transferred from mean flow to 10–30‐day circulation at mid–high latitudes, and tropical 10–30‐day convection activities get stronger, the occurrence of 10–30‐day circulation configuration induced humid heatwaves have increased from 39% in the period of 2003–2012 to 52.3% in the period of 2013–2022. Therefore, humid heatwaves become the predominant type. Key Points: Significant 10–30‐day ISO is a major contributor to the concurrence of hot and wet conditions in humid heatwaves of southern ChinaSimultaneous increases in temperature and humidity are related to the synergistic effect of 10–30‐day tropical and mid–high‐latitude atmospheric ISOsStronger configuration of 10–30‐day atmospheric ISOs contributes to rapid increases in humid heatwaves in recent decade [ABSTRACT FROM AUTHOR]

Monsoonal precipitation is dominated by intraseasonal variabilities, whose skillful prediction lead time is currently less than 5 days and remains a grand challenge. Here we show that an intrinsic variability in the Indian Ocean, the Central Indian Ocean (CIO) mode, when combined with a machine learning (ML) algorithm, can produce skillful predictions of intraseasonal precipitation over the monsoon region with a lead time of over 15 days, which is close to the theoretical predictability limit. This remarkable skill improvement stems from the fact that the CIO mode is dynamically related to the intraseasonal monsoon rainfall, while the data‐driven ML algorithm suppresses unwanted high‐frequency noise. Using the CIO mode and the ML algorithm, the forecast system hybridizes physical fundamentals and versatility of data‐driven algorithms. The identification of CIO mode and the verification of its significant contribution to intraseasonal predictions advance our understanding of the coupled monsoon system and also underscores the great potential of ML techniques in weather forecasts and climate predictions. Plain Language Summary: Rainfall during the Indian summer monsoon is dominated by variations with a period of tens of days, which are referred to as intraseasonal variabilities. Current prediction skill of intraseasonal monsoonal rainfall is less than 5 days and it remains a grand challenge in terms of increasing the current prediction skill. Here we show that an intrinsic mode of variability in the Indian Ocean, called the Central Indian Ocean (CIO) mode, when combined with a machine learning (ML) algorithm, can produce skillful predictions of intraseasonal precipitation over the monsoon region with a lead time of over 15 days. This remarkable skill improvement stems from the fact that the CIO mode is dynamically related to intraseasonal monsoon rainfall, while data‐driven ML algorithm suppresses disruptive noise with a period shorter than 10 days. Using the CIO mode and an ML algorithm, the forecast system synergizes physical fundamentals and versatility of data‐driven algorithm. The identification of CIO mode and the verification of its significant contribution to intraseasonal prediction advance our understanding of the coupled monsoon system and also demonstrate the great potential of ML techniques in weather forecasts and climate predictions. Key Points: The Central Indian Ocean (CIO) mode provides a dynamical basis for the prediction of monsoon intraseasonal rainfallThe machine learning (ML) algorithm suppresses high‐frequency noise while capturing the real‐time CIO mode indexThe dynamics and ML hybrid forecast system can skillfully predict monsoon intraseasonal rainfall with a lead time of ∼15 days [ABSTRACT FROM AUTHOR]

A connection between the quasi‐biennial oscillation (QBO), solar variability, and the short‐term convective climate oscillation, the Madden‐Julian oscillation (MJO), in boreal winter has been found in observational data, yet it is generally lacking in current global climate models (GCMs). A proposed mechanism is changes in tropical lower stratospheric upwelling rates and static stability caused by QBO and solar UV effects on extratropical wave forcing of the stratospheric residual meridional circulation (the Brewer‐Dobson circulation). The extent to which this mechanism, which operates only in boreal winter and enhances similar effects of the QBO‐induced meridional circulation, is simulated in a series of GCMs participating in the Coupled Model Intercomparison Project 6 (CMIP6) is investigated. The models are found to be often lacking complete representation of several elements of the mechanism, with particular issues being QBOs that are westerly biased and weak in the lower stratosphere, insufficient solar or QBO modulation of extratropical wave activity (the Holton‐Tan effect), too weak reductions in equatorial tropopause static stability in response to extratropical wave forcing, and MJOs that in some cases do not respond to these reductions. Through bypassing many of these deficiencies via data selection, it is demonstrated that effects on the MJO that resemble those found in observations (strengthening of the MJO following early winter sudden stratospheric warmings and during easterly QBO winters) can be simulated by a subset of the models. This supports operation of the proposed mechanism, and points to needed model improvements, although caveats exist and further work is needed. Plain Language Summary: The 28–29‐month quasi‐biennial oscillation (QBO) and solar ultraviolet variations on the 11‐year and 27‐day timescales have signatures that have been linked to the 30–60‐day convective climate oscillation, the Madden‐Julian Oscillation (MJO). However, this connection is lacking in the current generation of global climate models (GCMs). A proposed mechanism is changes in tropical lower stratospheric upwelling rates and vertical temperature gradient caused by QBO and solar UV effects on the stratospheric residual meridional circulation. The extent to which both components of this modulation is simulated via this mechanism in a selection of models participating in the Coupled Model Intercomparison Project 6 (CMIP6) is examined. Key issues with multiple models are found to be weak QBO lower stratospheric wind and temperature signals, a lack of solar/QBO modulation of wave propagation into the stratosphere, weak reductions in equatorial lower stratospheric vertical temperature stratification in response to increased wave propagation, and MJOs that are sometimes unresponsive to such reductions. Through bypassing many of these deficiencies via data selection, we show that, if all of the elements of the proposed mechanism were properly represented, a QBO/solar‐MJO connection could be simulated in at least some of the models. Key Points: A mechanism for modulating the Madden‐Julian oscillation via QBO/solar effects on extratropical wave forcing is evaluated in 6 modelsSeveral necessary elements of this mechanism (e.g., the Holton‐Tan effect) are found to be incompletely represented by the modelsArtificial imposition of these elements via data selection produces effects on the MJO that resemble those seen in observations [ABSTRACT FROM AUTHOR]

The Madden-Julian Oscillation (MJO), having far-reaching impact on Earth's climate and human society, is an important tropical phenomenon characterized by a typical 30–90-day period in phase evolution. The MJO was hardly being connected to chaos, which is featured with aperiodicity. However, unlike the quasi-periodic phase evolution, the event-to-event changes of MJO, to some extent represented by its amplitude evolution, is irregular. By presenting multiple evidence, we demonstrate that the MJO's amplitude evolution is deterministically chaotic. Combining eigen-time-delay embedding and Koopman operator into a regression model, we further reveal the cause of chaos through a data-drive approach. We show that the dynamics of MJO amplitude can be decomposed into multiple periodic obits and nonlinear interaction between them. The aperiodic motion resulted from these nonlinear interactions disturbs the periodic obits asynchronously, leading to chaotic evolution of MJO amplitude. The findings here unveil the strong nonlinear nature of the MJO, explaining the difficulties in the long-term MJO prediction and providing new insight into MJO's complexity. [ABSTRACT FROM AUTHOR]

The boreal summer intraseasonal oscillation (BSISO) is a major mode of sub‐seasonal variability that regulates the summer climate in East Asia. This study investigates the four possible effects of two different time‐scale BSISOs on temperature and precipitation variations in South Korea. When active BSISO convection is positioned over the subtropical western Pacific, it induces anomalous anticyclonic circulation accompanied by subsidence, leading to significant positive temperature anomalies. Conversely, the anomalous cyclonic circulation near the Korean Peninsula, resulting from suppressed convection in the subtropical western Pacific, along with low‐level cold advection anomalies, contributes to a decrease in temperature. The spatial distribution of BSISO convection, which drives precipitation variation, shows a distinctive pattern of three meridionally narrow cells extending from the Philippines to the Korean Peninsula. Suppressed (enhanced) convection to the north of 20°N in the western North Pacific (WNP) promotes the northwestward expansion (eastward contraction) of the WNP Subtropical High in conjunction with a southwesterly (northeasterly) moisture flux anomaly. Furthermore, enhanced (reduced) moisture flux convergence and intensified ascending (descending) motion create favorable conditions for positive (negative) precipitation anomalies in South Korea. The combined effect of BSISOs not only amplifies the mean temperature and precipitation anomalies compared to individual modes but also increases the frequency of warmer, wetter, and drier events. Therefore, monitoring both BSISO modes together is crucial for comprehending and predicting the anomalous summer climate in South Korea. [ABSTRACT FROM AUTHOR]

Eastward-moving moist deep convection and atmospheric circulation signals associated with the tropical Madden–Julian oscillation (MJO) sometimes break down as they cross the Maritime Continent region, but other times, the signal propagates across the region maintaining amplitude or regaining it over the west Pacific basin. This paper assesses the hypothesis that upper-tropospheric zonal diffluence of the background wind over the Maritime Continent causes much of this Maritime Continent barrier effect and its variation over time, through two mechanisms: 1) by slowing down the MJO as stronger-than-average background upper-tropospheric zonal wind over the Indian Ocean advects the MJO circulation signal westward, slowing its eastward advance, and 2) through the zonal advection of the background wind by subseasonal zonal wind across a region of zonal diffluence of the background wind, which advects the background wind of the opposite sign to the MJO wind. Advection of the opposite-signed background wind counteracts the MJO wind and reduces its associated upper-tropospheric mass divergence, weakening the mechanisms of the upper-tropospheric Kelvin wave component of the MJO circulation. Composites of MJO-associated zonal wind and outgoing longwave radiation signals diminish as they cross the Maritime Continent region when the region's background zonal winds are diffluent, and composites of data reconstructing the relevant advection terms reveal the direct action of the advection mechanisms. Significance Statement: The Madden–Julian oscillation (MJO) is the leading subseasonal variation of the tropical atmosphere. This project addresses how diffluence of the upper-tropospheric background zonal wind can break down MJO events through advection of and by the background wind. [ABSTRACT FROM AUTHOR]

Analysis of the daily rainfall records from 43 synoptic stations of the Philippine Atmospheric Geophysical and Astronomical Services Administration (PAGASA) from 1979 to 2019 reveals that the wettest station in the Philippines is in Hinatuan, Surigao del Sur, in eastern Mindanao Island in terms of the mean annual total rainfall. Despite being located at a low elevation (∼ 3 m above sea level), the mean annual total rainfall in this station is about 4554 mm, which is approximately 700 mm more than the mean annual total rainfall in Baguio City station, the station with the highest elevation (∼ 1500 m above sea level) in the country. Further analysis of the statistical characteristics of rainfall and comparison with other stations in terms of intensity, frequency, duration (i.e., short (1 − 2 days), medium (3 − 7 days), long (8 − 14 days), and very long (> 14 days) events), and 95th percentile extremes show that this station ranks first in terms of the frequency of wet months (200–500 mm month− 1) and heavy rainfall months (> 500 mm month− 1), mean monthly rainfall amounts from January to April, and the mean rainfall amount in the short duration category. The contributions of multiscale factors such as Tropical Cyclones (TCs), Low Pressure Systems (LPSs), and the Madden-Julian Oscillation (MJO) to the rainfall extremes over Hinatuan station are also quantified. The results show that TCs, LPSs, and MJO contribute about 0–5%, 0–38%, 3–38% to the monthly extremes over Hinatuan station, respectively. Cases when TCs or LPSs are located within 1100 km radius centered at Hinatuan station while MJO is active were also found and their contributions to the monthly extremes are 0–4% and 0–12%, respectively. The largest portion of the extremes are associated with other unaccounted factors, which contribute about 49–71%. The results of this study may serve as a basis for future characterization of the spatial variation of rainfall including the variations in extremes and their potential causes over the Philippines [ABSTRACT FROM AUTHOR]

This study reported the unprecedented tropical cyclone (TC) activity in the western North Pacific (WNP) and North Atlantic (NA) during the developing year of the 2023/2024 El Niño. The possible causes behind these unusual features were addressed. In contrast to previous El Niño events, an unusual low/high TC genesis number in the WNP/NA was identified during the typhoon season (June–November) in 2023. Meanwhile, the mean TC genesis location in the WNP exhibited a La Niña‐like northwestward shift, rarely observed in an El Niño developing year. An observational diagnosis on TC‐genesis‐related large‐scale dynamics and thermodynamics revealed that the lower/higher TC numbers in the WNP/NA were primarily attributed to an anticyclonic/cyclonic anomaly linked to trans‐basin sea surface temperature anomalies in the tropics and extratropics. Additionally, weaker intraseasonal oscillation activity compared to previous El Niños also partially contributed to fewer TCs in the WNP. Plain Language Summary: During the 2023 El Niño, tropical cyclone (TC) activity in the western North Pacific (WNP) and North Atlantic (NA) was unusual. Extremely few TCs formed in the WNP during the typhoon season (June–November), while more TCs were observed in the NA compared to the climatology, opposite to the feature of TC genesis in previous El Niño events. Additionally, the mean genesis location of WNP TCs shifted northwestward, contrary to the typical southeastward shift seen in past El Niño events. These anomalies were linked to an anticyclonic anomaly in the WNP and a cyclonic anomaly in the NA, which respectively hindered and facilitated favorable large‐scale environments related to TC genesis. Diagnostic analysis revealed that these circulation anomalies were associated with trans‐basin sea surface temperature anomalies (SSTAs). Extremely warm SSTAs in the northern Indian Ocean, tropical NA (TNA), and extratropical regions, along with a negative Pacific meridional mode, favored the anticyclonic anomaly in the WNP. In contrast, the extremely warm SSTA in the TNA facilitated the cyclonic anomaly in the NA. Additionally, weak intraseasonal oscillation activity also contributed to the low number of WNP TCs. Key Points: Unlike previous El Niño events, unusually fewer TCs, with a northwestward shift in genesis location, were observed in the WNP in 2023By contrast, the number of TCs in the North Atlantic (NA) was approximately twice that of the composite mean of previous El NiñosIn addition to high SST in the tropical NA and negative PMM, high extratropical SST along with inactive ISO contributed to the features [ABSTRACT FROM AUTHOR]

Extratropical intraseasonal oscillations (EISO), which is akin to the tropical ISO, significantly influence mid‐high‐latitude intraseasonal variability. Previous studies have shown that biases in the dynamical prediction of EISOs reduce the accuracy of subseasonal forecasts for extreme events, particularly in extratropical regions. However, the current dynamical prediction skills of EISO and associated dependence remain unclarified. Based on the recently proposed EISO metrics, which represent three dominant zonally propagating EISO modes traveling along the westerly jets, this study conducts a comprehensive evaluation of the EISOs' prediction skill in 13 state‐of‐the‐art subseasonal‐to‐seasonal (S2S) prediction project models. Firstly, the S2S models display skill to predict the EISOs out to a range of 9–20 days, which demonstrates higher predictability compared to other conventional circulation modes in the extratropics. In the subseasonal prediction, EISOs' amplitudes are underestimated, and their propagating speeds feature large random errors in all S2S models. Secondly, the EISOs' prediction skill depends on their initial/target amplitude, particularly for strong target amplitudes, which showcase higher skill than weak‐amplitude EISOs. Thirdly, although the EISOs' prediction skill is sensitive to the EISOs' phase, it is not locked to any fixed phase, indicating that EISOs do not have an inherent prediction barrier like the tropical ISO. This study also discusses how the humidity initialization and ensemble sizes influence the EISOs' prediction skill, suggesting that increasing the ensemble members in the saturation range is an effective way of improving the prediction for extratropical subseasonal variation and related extreme events. Plain Language Summary: Extratropical intraseasonal oscillations (EISO) significantly influence mid‐to‐high‐latitude subseasonal variability and trigger extreme events, making their accurate prediction crucial for effective disaster prevention and reduction. However, the current dynamical prediction skills of EISO and associated dependence remain the gap of knowledge. This research highlights the higher prediction of EISO, with skill extending up to 20 days, compared to other conventional circulation modes in the extratropics. The findings emphasize the dependence of prediction skill on the initial and target amplitudes of EISO, with strong target amplitude EISO extending up to 24 days. This study also identifies that humidity initialization and increasing ensemble sizes in the saturation range can effectively enhance prediction accuracy for extratropical subseasonal variation, thus providing valuable insights for improving subseasonal prediction for related extreme events. Key Points: The dynamical models have high skills in predicting the extratropical intraseasonal oscillations (EISOs) out to a range of 9–20 daysEISOs' prediction depends on their initial/target amplitude and phasesHumidity initialization and increasing ensemble sizes can effectively enhance the EISOs' prediction skill in dynamical models [ABSTRACT FROM AUTHOR]

Capabilities of 43 models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) to simulate the Madden‐Julian Oscillation (MJO) were compared in this study. Models with higher MJO simulation skills reproduce organized large‐scale convection more frequently over the equatorial western Pacific (EWP), suggesting that MJO simulations are tightly connected to organization of large‐scale precipitation over the EWP. MJO simulation skills are not significantly correlated with the mean sea surface temperature (SST) over the EWP, but tightly connected to SST threshold for convection there. The top simulations exhibit lower SST threshold and more realistic SST‐precipitation relationship over the EWP, suggesting that convection can be organized more efficiently and intensified more rapidly with the increasing SST in these simulations. Our results emphasize that reproducing the relationship between SST and large‐scale precipitation over the EWP is critical to MJO simulations. Plain Language Summary: The Madden‐Julian Oscillation (MJO) is one of the most important tropical phenomena that modulate weather and climate systems at the intraseasonal time scales. However, accurately reproducing the MJO in global climate models remains a challenge. The eastward propagation of the MJO convection is usually not well‐reproduced in models. Comparing to observations, they tend to underestimate the occurrence frequency of individual MJO events with lower amplitude and shorter propagation range. This study explore whether the MJO simulations in models are linked to the relationship between sea surface temperature (SST) and large‐scale precipitation they reproduced. It was found that MJO simulation skills are greatly dependent on the organized large‐scale precipitation over the equatorial western Pacific. We further revealed that realistically reproducing the SST‐precipitation relationship over this region is critical for simulating the MJO in models. Key Points: Madden‐Julian Oscillation (MJO) simulation skills depend on organized large‐scale precipitation over the equatorial western Pacific (EWP) reproduced in modelsModels with higher MJO simulation skills exhibit lower sea surface temperature (SST) threshold for convection over the EWPRealistically reproducing the relationship between the SST and organized large‐scale precipitation is critical to MJO simulations [ABSTRACT FROM AUTHOR]

Westerly wind bursts (WWBs) in the western–central equatorial Pacific are critical in El Niño–Southern Oscillation (ENSO) dynamics. Understanding how they may change with global warming has important implications for future projections of El Niño. In this study, we investigate how the enhanced eastern equatorial Pacific warming pattern, emerging in future climate projections, can influence WWB characteristics in an atmospheric general circulation model, CAM6. Changes in three main factors affecting WWBs—El Niño-conditions mean westerly wind stress anomalies, the Madden–Julian oscillation (MJO), and tropical cyclones (TCs)—are analyzed. We find that during El Niño onset (December–April), the WWB wind stress intensity remains largely unchanged but WWBs shift westward by about 20° of longitude. In contrast, during El Niño development (May–November), the WWB intensity increases by 41% or 79% in two warming scenarios considered [shared socioeconomic pathways (SSP5-8.5) and SSP2-4.5, respectively], which is mainly caused by a higher frequency of TC occurrence in the central tropical Pacific within 15°N/S. Further, we find that westerly wind speed anomalies associated with El Niño and the MJO also increase during El Niño development. However, as trade winds weaken in the central-eastern equatorial Pacific, the mean strength of the resulting westerly wind stress anomalies does not change much, and no significant eastward shift of WWBs is observed. Thus, our atmospheric model simulations suggest a strong TC-driven increase in WWB wind stress anomalies during El Niño development and hence a more important role of TCs in WWB generation, which in a coupled system could lead to stronger ENSO events, enhanced TC–ENSO coupling, and even greater intensification of WWBs. [ABSTRACT FROM AUTHOR]

The impact of the Madden-Julian Oscillation (MJO) on India's intraseasonal and interannual variability of rainfall has been investigated in this study. Anomalous MJO amplitude points from 1979 to 2021 are classified into post-, pre- and normal onset years of the Indian Summer Monsoon (ISM). High MJO amplitudes, particularly those exceeding three standard deviations from the mean, correlate with significant rainfall anomalies, affecting both the timing and intensity of the monsoon. Specific MJO phases contribute differently to rainfall patterns, with phases 3, 4, and 5 generally suppressing rainfall, while phases 1 and 8 enhance it. Pre-onset years show significant rainfall anomalies driven by strong MJO activity, particularly during phases 3, 4, and 5. Normal onset years exhibit notable suppression and enhancement patterns, with significant fluctuations observed in 1996 and 2020. Post-onset years have the highest frequency of anomalies, with phases 3 and 4 prominently influencing rainfall variability. Overall, this research underscores the critical role of MJO dynamics in modulating rainfall variability in India. The findings suggest that understanding MJO phases can improve rainfall predictability and monsoon onset forecasting, aiding in better climate prediction and water resource management. This study contributes to the broader climatological discourse by elucidating the interplay between MJO activity and regional precipitation patterns. [ABSTRACT FROM AUTHOR]

The response of agricultural productivity anomalies to drought stress plays a crucial role in the carbon cycle within terrestrial ecosystems and in ensuring food security. However, detailed analysis of how global agricultural productivity anomalies response to drought stress, particularly within irrigated and rainfed agricultural systems, remains insufficient. In this study, the impact of drought stress on agricultural productivity anomalies during the growing season (zcNDVIS), across both irrigated and rainfed agriculture, were analyzed using a suite of hydro-climatic variables. Specifically, the investigation utilized the multi-scalar Standardized Precipitation Evapotranspiration Index (SPEI), the Multivariate ENSO Index (MEI), and the Madden-Julian Oscillation (MJO). Meanwhile, the relationships between hydroclimatic variables and zcNDVIS were analyzed at one, two, three and four months before the ending of growing season (EOS). Results showed that (1) the percentages of significant (p<0.1) drying trends varied across the globe from 8.30% to 13.42%, 6.50% to 14.63%, 6.52% to 14.23%, and 6.47% to 14.95% at one-, two-, three-, and four-month lead times before EOS, respectively, during 2001–2020, which represented by the multiscalar SPEI. This observation highlights that most regions across the globe tend to be arid, which could significantly impact agricultural productivity; (2) the global mean correlation coefficients (rmax) for SPEI-1, SPEI-3, SPEI-6, SPEI-12 (indicating SPEI at 1-, 3-, 6-, and 12-month lags), MEI, and MJO with zcNDVIS ranged between 0.24–0.25, 0.27–0.28, 0.25–0.26, 0.21–0.22, −0.02–0.01 and 0.06–0.11, respectively, across both irrigated and rainfed agriculture system from 2001 to 2020. Agricultural productivity anomalies demonstrated a significant correlation with drought stress. The strongest correlations were noted for SPEI-3 and SPEI-6, suggesting a delayed response of crops to drought conditions. This indicates that agriculture ecosystem experiences prolonged disturbances due to abiotic drought stress; and (3) the percentages of regions that showed significant correlations (p<0.1) between zcNDVIS and drought indices (SPEI-1, SPEI-3, SPEI-6, and SPEI-12), as well as climate indices (MEI and MJO) ranged as follows: 14.77%–20.27%, 21.51%–32.55%, 22.60%–35.68%, 21.89%–35.16%, 7.93%–11.20% and 9.44%–17.94%. Quantitatively identifying how zcNDVIS spatially responds to hydro-climatic variables can help us better understand the impact of drought on agricultural productivity anomalies worldwide. [ABSTRACT FROM AUTHOR]

This study evaluates the simulated modulation of western North Pacific (WNP) tropical cyclone (TC) genesis by the boreal summer intraseasonal oscillation (BSISO) in five high-resolution models from the Coupled Model Intercomparison Project phase 6 (CMIP6) High Resolution Model Intercomparison Project (HighResMIP). By comparing reanalysis data with historical simulations from these five HighResMIP models during 1979–2014, we find that these models can realistically capture observed BSISO features including northward propagation, propagation speed and life cycle. These models can also simulate the modulation of WNP TC genesis by the BSISO in observations. Similar to observations, there is a significant difference in simulated TC genesis frequency between active and inactive BSISO phases, which in most models can be largely explained by changes in large-scale environmental factors and synoptic-scale wave trains. Compared with observations, however, there is a poleward shift of intraseasonal variability of TC genesis in all model simulations. This shift is likely due to a northward shift of the simulated WNP monsoon trough and an associated northward shift of simulated BSISO activity, as well as changes in synoptic-scale eddy kinetic energy in these models. This study documents the ability of HighResMIP models to reproduce the BSISO and its modulation on TCs, and also highlights the importance of the large-scale monsoon circulation in TC genesis. [ABSTRACT FROM AUTHOR]

This study evaluates the observed non-linearity in the wind-precipitation relationship over India's southwest coast and maps this non-linearity to monsoon intraseasonal oscillations. The wind-precipitation data is subjected to clustering, and five distinct regimes are identified. The large-scale dynamical and thermodynamical condition that leads to wind-precipitation clustering is explored. It uses novel observations from the Stratosphere–Troposphere wind profiler radar placed over Cochin (10.04° N; 76.33° E) for the period 2019–2021 to derive characteristics of monsoon low-level jets. Out of five wind-precipitation clusters, low wind low precipitation (LWLP) and low wind high precipitation clusters (LWHP) bear signatures of monsoon break conditions over central India. The occurrence of Madden Julian events or monsoon lows in LWHP generates heavy rainfall over the southwest coast. The high wind low precipitation (HWLP) cluster occurs during the weak phase of northward propagating monsoon intraseasonal oscillation (MISO) over the southwest coast, while the high wind high precipitation (HWHP) cluster lies within the strong phase of MISO. The fifth cluster, the high wind extreme precipitation cluster (HWEP), although technically lying within the strong phase of MISO, however has more apparent signatures of northwestward propagating monsoon depressions. The Lagrangian trajectories launched from 500 hPa level for each cluster reveal some intriguing aspects. The dry air intrusion from adjacent regions near the west coast makes the mid-troposphere dry over the southwest coast in LWLP and HWLP clusters. In HWHP and HWEP, strong moisture sources confined over the west coast create mid-troposphere moistening owing to the moisture supply from below 500 hPa. This study highlights that wind-precipitation clustering effectively defines sub-seasonal variability during the monsoon. [ABSTRACT FROM AUTHOR]

In August 2022, Pakistan experienced unprecedented monsoon rains, leading to devastating floods and landslides affecting millions. While previous research has mainly focused on the contributions of seasonal and synoptic anomalies, this study elucidates the dominant influences of tropical and extratropical intraseasonal oscillations on both the occurrence and subseasonal prediction of this extreme rainfall event. Our scale-decomposed moisture budget analysis revealed that intense rainfall in Pakistan was triggered and sustained by enhanced vertical moisture transport anomalies, primarily driven by interactions between intraseasonal circulation anomalies and the prevailing background moisture field when tropical and mid-latitude systems coincided over Pakistan. Evaluation of subseasonal-to-seasonal prediction models further highlighted the critical role of tropical intraseasonal modes in causing this extreme rainfall event in Pakistan. Models that accurately predicted northward-propagating intraseasonal convection with a forecast lead time of 8–22 days demonstrated good skill in predicting the extreme event over Pakistan. [ABSTRACT FROM AUTHOR]

Monsoon intraseasonal transitions are intrinsically chaotic; hence, their predictability is limited. In the past the majority of studies were conducted solely keeping in account of dynamic prospective like boreal summer intraseasonal oscillation or Madden–Julian oscillation; however its thermodynamic aspect got limited attention. The current study looks at the internal thermodynamic mechanisms pertaining to monsoon intraseasonal transition using two independent sets of reanalysis data (European Centre for Medium‐range Weather Forecasts Reanalysis version 5 and Indian Monsoon Data Assimilation and Analysis), along with the observed cloud type from Met Office Integrated Data Archive System Land and Marine surface station data. A lead–lag composite analysis shows that the midtropospheric warming by diabatic heating is a thermal control to restrict the deepening of convective cloud (cumulonimbus) as the wet spell progresses. This midtroposhperic heating cuts off the formation of deep convective clouds and results in the formation of relatively shallow clouds, such as stratocumulus and stratus. On the other hand, surface forcing during dry spells humidifies the troposphere by detrainment from the shallow convection with the formation of cumulus and cumulus congestus clouds. This facilitates the preconditioning of the atmosphere for deep convection and results in the onset of wet spells by the formation of cumulonimbus clouds. The comprehensive approach used in this work may further help in identifying the limitation of the predictability in monsoon transition period from the numerical weather prediction models. [ABSTRACT FROM AUTHOR]

Despite its substantial impacts on human society, winter weather is difficult to predict on subseasonal‐to‐seasonal (S2S) timescales. To improve its predictability at longer timescales, this study examines whether the Madden–Julian Oscillation (MJO) has a significant impact on the frequency of extreme winter weather (i.e., heavy accumulations of snow, ice pellets, and freezing rain) over the contiguous United States. We find an increased frequency of extreme winter weather in the Ohio Valley and Mid‐Atlantic regions when MJO enhanced convection is over Africa and the western Indian Ocean, due to imposed lower 850‐hPa temperatures associated with MJO teleconnections. More frequent subfreezing temperatures lead to an increased likelihood of frozen precipitation compared with liquid, increasing the frequency of extreme winter weather. The MJO also increases the frequency of winter weather in the Central Great Plains region of the United States when MJO enhanced convection is over the eastern Indian Ocean and the Maritime Continent. Rather than through effects on temperature, the likely mechanism of increased winter weather over this region is enhanced synoptic forcing that increases overall precipitation. These effects can be seen up to 15 days in advance, suggesting the utility of using the MJO in S2S forecasting of extreme winter weather. [ABSTRACT FROM AUTHOR]

The prediction skill for individual Madden–Julian oscillation (MJO) events is highly variable, but the key factors behind this remain unclear. Using the latest hindcast results from the subseasonal-to-seasonal (S2S) phase II models, this study attempts to understand the diverse prediction skill for the MJO events with an enhanced convective anomaly over the eastern Indian Ocean (IO) at the forecast start date, by investigating the preference of the prediction skill to the MJO-associated convective anomalies and low-frequency background states (LFBS). Compared to the low-skill MJO events, the high-skill events are characterized by a stronger intraseasonal convection–circulation couplet over the IO before the forecast start date, which could result in a longer zonal propagation range during the forecast period, thereby leading to a higher score for assessing the prediction skill. The difference in intraseasonal fields can further be attributed to the LFBS of IO sea surface temperature (SST) and quasi-biannual oscillation (QBO), with the high-skill (low-skill) events corresponding to a warmer (colder) IO and easterly (westerly) QBO phase. The physical link is that a warm IO could increase the low-level convective instability and thus amplify MJO convection over the IO, whereas an easterly QBO phase could weaken the Maritime Continent barrier effect by weakening the static stability near the tropopause, thus favoring eastward propagation of the MJO. It is also found that the combined effects of IO SST and QBO phases are more effective in influencing MJO prediction skill than individual LFBS. Significance Statement: Given the importance of the Madden–Julian oscillation (MJO) in the subseasonal predictability of global weather and climate, how well the MJO itself can be predicted is a matter of concern. This study reveals the critical observational factors that determine the variation in MJO prediction skill across events. Without making any presumptions about what the factors would be, the observed MJO events are separated according to their individual prediction skill, and the difference between the MJO events with higher and lower skill is then investigated. The results show that the low-frequency background states of the Indian Ocean sea surface temperature and the quasi-biannual oscillation are good indicators for MJO prediction skill, for their modulatory role in the MJO propagation range. [ABSTRACT FROM AUTHOR]

This study investigated variations in the Madden–Julian oscillation (MJO) behavior between two types of La Niña winters: mega and equatorial La Niñas. Results of this work show that in contrast to mega conditions, accompanied by more abundant intraseasonal column-integrated moisture anomalies over the planetary boundary layer, intensities of intraseasonal outgoing longwave radiation anomalies are stronger over the tropical western Pacific (WP) at MJO phases 5–6 under equatorial conditions. The occurrence of the moisture anomaly change averaged over the tropical WP is supported by a distinct moisture tendency difference. Moisture budget and multiscale interaction diagnoses underscore the pivotal effect of an area-averaged vertical gradient change in low-frequency background moisture in driving such tendency change. Considering notable MJO convection variations, the teleconnections associated with MJO phases 5–6 over East Asia (EA) were also explored, including warmer surface air temperature anomalies and stronger positive geopotential height anomalies at 200 hPa on the intraseasonal time scale under equatorial conditions. Results from a linear baroclinic model demonstrate that such teleconnection changes could be effectively explained by the linear response to the MJO's diabatic heating anomaly. Roles of mean state and MJO itself variations in the linear response change are also discussed. These findings provide novel insights into MJO activity and offer potential improvements for subseasonal forecasting in EA. Significance Statement: The relationship between El Niño–Southern Oscillation (ENSO) and MJO has been extensively explored recently. However, variations in the MJO behavior and its climate effects under mega and equatorial La Niña winters have not been thoroughly investigated. In this work, we utilized reanalysis datasets and an idealized linear baroclinic model to address these questions. We observed more robust and abundant intraseasonal convection activity and planetary boundary layer–integrated moisture anomaly averaged over the tropical WP at MJO phases 5–6 under equatorial conditions than mega conditions, which is closely linked with the vertical gradient change in low-frequency background moisture. Additionally, MJO-related teleconnections also exhibit some changes. This work may provide a new perspective on understanding the relationship between the MJO and ENSO and offer potential improvements for the subseasonal forecast. [ABSTRACT FROM AUTHOR]

Understanding the impacts of the Indonesian Throughflow (ITF) on the eastward propagation of the Madden-Julian Oscillation (MJO) is crucial for accurately simulating the MJO and achieving high-skill sub-seasonal predictions. Our analyses demonstrate a significant enhancement of MJO eastward propagation due to the strong ITF. Blocking the ITF decreases the eastward sea surface temperature (SST) gradient over the tropical Indian Ocean, hindering MJO propagation across the Maritime Continent (MC). Removing the MJO circulation-induced intraseasonal variability of the ITF transport also weakens the eastward propagation of the MJO, as the MJO easterly winds enhance the ITF transport and warm the eastern tropical Indian Ocean. These experiments reveal that mean and intraseasonal variability of the ITF transport contribute to 73% and 42% of the eastward propagation of the MJO over the MC, respectively. The findings presented in this study highlight the significant role of the ITF in shaping the propagation of the MJO. [ABSTRACT FROM AUTHOR]

El Niño typically induces cooling in the Southwest Pacific Ocean during austral summers, usually leading to decreased marine heatwave frequency and severity. However, the 2016 extreme El Niño unexpectedly coincided with the longest and most extensive marine heatwave ever recorded in the region. This heatwave, spanning over 1.7 million square kilometers, persisting for 24 days with a peak intensity of 1.5°C, resulted in massive coral bleaching and fish mortality. This exceptional warming resulted from anomalously strong shortwave radiation and reduced heat loss via latent heat fluxes, owing to low wind speed and increased air humidity. These anomalies are attributed to a rare combined event "Madden-Julian Oscillation and extreme El Niño." Following 10 February, the rapid dissipation of this marine heatwave results from the most intense cyclone ever recorded in the South Pacific. The hazardous ecological impacts of this extreme event highlight the needs for improving our understanding of marine heatwave-driving mechanisms that may result in better seasonal predictions. [ABSTRACT FROM AUTHOR]

Deep convection in the Indo-Pacific warm pool is vital in driving global atmospheric overturning circulations. Year-to-year variations in the strength and location of warm pool precipitation can lead to significant local and downstream hydroclimatic impacts, including floods and droughts. While the El Niño-Southern Oscillation (ENSO) is recognized as a key factor in modulating interannual precipitation variations in this region, atmospheric internal variability is often as important. Here, through targeted atmospheric model experiments, we identify an intrinsic low-frequency atmospheric mode in the warm pool region during the austral summer, and show that its impact on seasonal rainfall is comparable to ENSO. This mode resembles the horizontal structure of the Madden-Julian Oscillation (MJO), and may play a role in initiating ENSO as stochastic forcing. We show that this mode is not merely an episodic manifestation of MJO events but primarily arises from barotropic energy conversion aided by positive feedback between convection and circulation. [ABSTRACT FROM AUTHOR]

The occurrence of extreme hot and dry summer conditions in the Pacific Northwest region of North America (PNW) has been known to be influenced by climate modes of variability such as the El Niño-Southern Oscillation and other variations in tropospheric circulation such as stationary waves and blocking. However, the extent to which the subseasonal remote tropical driver influences summer heat extremes and fire weather conditions across the PNW remains elusive. Our investigation reveals that the occurrence of heat extremes and associated fire-conducive weather conditions in the PNW is significantly heightened during the boreal summer intraseasonal oscillation (BSISO) phases 6-7, by ~50–120% relative to the seasonal probability. The promotion of these heat extremes is primarily attributed to the enhanced diabatic heating over the tropical central-to-eastern North Pacific, which generates a wave train traveling downstream toward North America, resulting in a prominent high-pressure system over the PNW. The ridge, subsequently, promotes surface warming over the region primarily through increased surface radiative heating and enhanced adiabatic warming. The results suggest a potential pathway to improving subseasonal-to-seasonal predictions of heatwaves and wildfire risks in the PNW by improving the representation of BSISO heating over the tropical-to-eastern North Pacific. [ABSTRACT FROM AUTHOR]

Diurnal warm layers develop in the upper ocean on sunny days with low surface wind speeds. They rectify intraseasonal sea‐surface temperatures (SSTs), potentially impacting intraseasonal weather patterns such as the Madden–Julian Oscillation (MJO). Here we analyse 15‐lead‐day forecast composites of coupled ocean–atmosphere and atmosphere‐only numerical weather prediction (NWP) models of the UK Met Office to reveal that the presence of diurnal warming of SST (dSST) leads to a faster MJO propagation in the coupled model compared with the atmosphere‐only model. To test the feedback between the MJO and the dSST, we designed a set of experiments with instantaneous vertical mixing over the top 5 or 10 m$$ 10\;\mathrm{m} $$ of the ocean component of the coupled model. Weaker dSST in the mixing experiments leads to a slower MJO over 15 lead days. The dSST produces a 3%$$ 3\% $$ increase in the MJO phase speed between the coupled and the atmosphere‐only model. An additional 5%$$ 5\% $$ increase is found for other coupling effects, unrelated to the dSST. A two‐way feedback manifests in the coupled model over the 15 lead days of the forecast between the MJO and the dSST. The MJO regime dictates the strength of the dSST and the dSST rectifies the intraseasonal anomalies of SST in the coupled model. Stronger dSST in the coupled model leads to stronger intraseasonal anomalies of SST. The MJO convection responds to these SSTs on a seven‐lead‐day timescale, and feeds back into the SST anomalies within the next three lead days. Overall, this study demonstrates the importance of high vertical resolution in the upper ocean for predicting the eastward propagation of the MJO in an NWP setting, which is potentially impactful for seasonal predictions and climate projections, should this feedback be unrepresented in the models. [ABSTRACT FROM AUTHOR]

Deserts and semi-desert countries have become increasingly affected by dust storms in recent years. In this research, the effect of the real-time multivariate of the Madden-Julian Oscillation (MJO) index on the frequency of dust storms in the west of Iran (Sarpol-E-Zahab, Islamabad Gharb, Kermanshah, and Kangavar stations located in Kermanshah province) during February-July for the period 1987-2022 and the frequency percentage of this index for Phases 7-2 (7, 8, 2 and 1) and Phases 3-6 (3, 4, 5 and 6) were investigated. The results of the analysis of the relationship between the MJO and dust storms showed that between 57 and 75% of dust storms occurred in phases 7-2 of the MJO and 25 to 42% in phases 3-6. Phases 7-2 of the MJO have a higher percentage of dust storms than phases 3-6. According to the results of the Mann-Whitney test, the displacement of phases 7-2 and phases 3-6 of the MJO have significantly led to dust storms at the Kermanshah and Kangavar stations. Due to the suitability of the HYSPLIT model for tracking dust storms, the paths of dust entering Kermanshah province were investigated with this model. Tracking the paths of dust entering Kermanshah province with the HYSPLIT model and analyzing the wind speed and direction maps of 850 hPa level indicate an increase in the speed of westerly winds. The increase in the speed of westerly winds and the decrease in soil moisture in phases 7-2 and 3-6 have caused the movement of dust particles from Iraq, eastern parts of Syria, and Khuzestan province to the studied area. [ABSTRACT FROM AUTHOR]

While decadal changes in Madden–Julian oscillation (MJO) have received considerable attention, the corresponding changes in Boreal Summer Intraseasonal Oscillation (BSISO) have yet to be well understood. In this study, we show the enhanced northward propagation of BSISO in the Western North Pacific (WNP) during the 2000s compared to the 1980s–1990s. Observational analyses and model experiments suggest this enhancement is partially attributed to the tropical Indian Ocean (TIO) warming. The TIO warming tends to increase the air-sea interaction, enhancing moisture anomalies in the free atmosphere. Consequently, this increase in moisture anomalies strengthens BSISO-scale convection through increased convective anomalies at the north of the BSISO center, thereby enhancing the northward propagation of BSISO. Additionally, vorticity changes resulting from mean-state zonal vertical shear also contribute to the BSISO decadal change. Our findings underscore the importance of considering the interaction between BSISO and changes in the ocean mean state in future assessments. [ABSTRACT FROM AUTHOR]

As the primary interannual signal of variability in the tropical ocean-atmosphere interaction, the El Niño-Southern Oscillation has a considerable impact on tropical cyclone (TC) activity over the western North Pacific (WNP). Both 2018 and 2021 were La Niña decay years, but TC activity over the WNP during the two summers (June–August) showed notable differences. In 2018, summer TC activity was unusually high with a total of 18 TCs, and the region of TC genesis was mainly in the central and eastern WNP. In contrast, only 9 TCs were generated in summer 2021, and the region of TC genesis was primarily in the western WNP. By comparing the characteristics of the large-scale environmental conditions over the regions of TC genesis, the thermal factors of the tropical oceans, and the activity of the Madden-Julian Oscillation (MJO), this study revealed the possible causes for the marked differences in TC genesis over the WNP during the two summers, which both had a similar background of La Niña decay. The Indian Ocean Basin Mode (IOBM) transitioned of a cold anomaly in the winter of 2017/2018 and persisted until summer 2018. At the same time, the Pacific Meridional Mode (PMM) maintained a positive phase, leading to eastward and northward displacement of the Western Pacific Subtropical High in summer, and eastward extension of the tropical monsoon trough, which presented conditions conducive to TC genesis over the Northwest Pacific. Moreover, the days when the MJO stagnated in phases 5 and 6 in the summer of 2018 increased by approximately 150% relative to climatological state, providing dynamic conditions favorable for TC formation. In 2021, the IOBM quickly turned to a warm anomaly in March and persisted until summer, whereas the PMM became a negative phase in January and remained so until summer. At the same time, the MJO stagnated in phases 2 and 3 for up to 47 days, with the center of convection located over the western Maritime Continent, producing conditions unconducive to TC genesis over the Northwest Pacific. Thus, despite being under a similar background of La Niña decaying year, the distinct evolutions of the IOBM, PMM, and MJO in spring and summer of 2018 and 2021 were the main causes of the notable differences in TC activity over the WNP during these two summers, and the anomalies in IOBM and MJO contributed more significantly than those of the PMM. [ABSTRACT FROM AUTHOR]

Changes in the activities of the Boreal Summer Intraseasonal Oscillation (BSISO) at the end of 21st century under the SSP5-8.5 scenario are assessed by adopting 17 CMIP6 models and the weak-temperature-gradient assumption. Results show that the intraseasonal variations become more structured. The BSISO-related precipitation anomaly shows a larger zonal scale and propagates further northward. However, there is no broad agreement among models on the changes in the eastward and northward propagation speeds and the frequency of individual phases. In the western North Pacific (WNP), the BSISO precipitation variance is significantly increased, at 4.62% K−1, due to the significantly increased efficiency of vertical moisture transport per unit of BSISO apparent heating. The vertical velocity variance is significantly decreased, at −3.51% K−1, in the middle troposphere, due to the significantly increased mean-state static stability. Changes in the lower-level zonal wind variance are relatively complex, with a significant increase stretching from the northwestern to southeastern WNP, but the opposite in other regions. This is probably due to the combined impacts of the northeastward shift of the BSISO signals and the reduced BSISO vertical velocity variance under global warming. Changes in strong and normal BSISO events in the WNP are also compared. They show same-signed changes in precipitation and large-scale circulation anomalies but opposite changes in the vertical velocity anomalies. This is probably because the precipitation anomaly of strong (normal) events changes at a rate much larger (smaller) than that of the mean-state static stability, causing enhanced (reduced) vertical motion. [ABSTRACT FROM AUTHOR]

The interannual variation of the South China Sea (SCS) summer monsoon onset (SMO) may bring extreme weather and climate disasters in East Asia. However, its skillful forecast still remains challenging. This study investigates the intraseasonal ocean‐atmosphere interaction that affects the SCSSMO through diagnostic analysis and numerical experiments. It reveals that the cold sea surface temperature in the Southern SCS during winter (referred as cold tongue, CT) is the key pathway controlling the propagation of the 30–60 days intraseasonal oscillation (ISO) convective system from the Bay of Bengal (BOB) to the SCS. The CT variations affect the interannual variation of the SCSSMO. Specifically, the strong (weak) CT after the peak of La Niña (El Niño) years suppresses (enhances) the propagating ISO from the BOB to the SCS, resulting in a delayed (advanced) SCSSMO. This finding offers the new scientific insights for improving the forecasting of the SCSSMO. Plain Language Summary: The occurrence of a strong cold tongue (CT) in winter, commonly detected in the southern South China Sea (SCS) during La Niña decay years, plays a significant role in delaying the SCS summer monsoon onset (SCSSMO). It adversely affects the northeastward progression of the 30‐60‐day convective Intraseasonal Oscillation (ISO) from the equatorial Indian Ocean to the southern SCS. This interference leads to the delayed onset of the SCSSMO. It highlights the crucial influence of the CT sea surface temperatures (SST) on the linkage between Bay of Bengal and SCS at the intraseasonal scale, thus significantly impacting the timing of the SCSSMO. Key Points: The winter cold tongue in South China Sea (SCS) is a crucial oceanic precursor for the interannual variation of the SCS summer monsoon onset (SCSSMO)Strong cold tongue in the SCS can suppress the propagation of the 30‐60‐day intraseasonal oscillation into the SCSStrong SCS cold tongue during La Niña years tends to delay the SCSSMO [ABSTRACT FROM AUTHOR]

In this study, the mechanisms for the long‐lived and medium‐lived regional consecutive cloudy‐rainy events (RCCREs) in the Yangtze‐Huaihe River valley (YHRV) are investigated by comparing the influences of 10–30‐day and 30–90‐day intraseasonal oscillations (ISOs). The evolution of medium‐lived RCCREs is generally dominated by the 10–30‐day ISO, while the 10–30‐day and 30–90‐day ISOs synergistically contribute to the occurrence of long‐lived RCCREs. Further analysis reveals that, when the East Asian westerly jet (EAWJ) shifts northward to the north of YHRV, a RCCRE in the YHRV occurs in 2 days later. In the medium‐lived RCCREs, the EAWJ retreats southward rapidly; while in the long‐lived RCCREs, the stronger 30–90‐day ISO prevents the southward retreat of EAWJ, which favors the persistence of the RCCREs in the YHRV. The northward shifted EAWJ facilitates the RCCREs by modulating the secondary atmospheric circulation, consequently enhancing the moisture convergence and ascending motion anomalies over the YHRV. The RCCREs‐related precursor wave train originates from northern Europe with at least a 4‐day lead. The wave train propagates southeastward to excite an anomalous high over East Asia, which contributes to the northward shift of EAWJ and subsequently the occurrence of RCCREs. Different from the rapid weakness of the precursor wave train associated with the medium‐lived RCCREs, the precursor wave train associated with the long‐lived RCCREs can be maintained by the 30–90‐day ISO, which consequently leads to a persistence of the northward located EAWJ and long‐lived RCCREs in the YHRV. Plain Language Summary: Regional consecutive cloudy‐rainy events (RCCREs) have a disastrous impact on economy, agriculture, and natural environment over the Yangtze‐Huaihe River valley (YHRV). This study investigates the mechanisms contributing to the long‐ and medium‐lived RCCREs in the YHRV by comparing the role of 10–30‐day and 30–90‐day intraseasonal oscillations (ISOs). The medium‐lived RCCREs are dominated by the 10–30‐day ISO, while the long‐lived RCCREs are caused by a combination of the 10–30‐day and 30–90‐day ISOs. The East Asian westerly jet (EAWJ) plays a crucial role in RCCRE occurrence. When the EAWJ shifts northward, it triggers RCCREs by enhancing moisture convergence and ascending motion over the YHRV. The EAWJ retreats southward rapidly in the medium‐lived RCCREs. Conversely, the stronger 30–90‐day ISO prevents its southward retreat in the long‐lived RCCREs, favoring the persistence of RCCRE in the YHRV. Additionally, the precursor wave train originating from northern Europe contributes to the northward shift of the EAWJ and the occurrence of the RCCREs over the YHRV. Although both the long‐ and medium‐lived RCCREs exhibit similar precursor wave train, the influence of different ISOs varies, with the 30–90‐day ISO showing a larger impact on the precursor wave train and northward shift of the EAWJ in the long‐lived RCCREs. Key Points: Medium (long)‐lived RCCREs are mainly influenced by 10–30‐day intraseasonal oscillation (ISO) (both 10–30‐day and 30–90‐day ISOs)In the long (medium)‐lived RCCREs, the East Asian westerly jet (EAWJ) remains stable (retreats southward rapidly) after shifting northwardThe precursor wave train of the medium‐lived RCCREs weakens rapidly, while that of the long‐lived RCCREs is maintained by 30–90‐day ISO [ABSTRACT FROM AUTHOR]

This study investigates the influences of intraseasonal oscillation on two types of regional extreme consecutive dry days events (RECDD) in spring over Southern China (SC). For type‐1 RECDD occurring in entire SC, the 7–25 days and 25–90 days high over Lake Balkhash‐Baikal are important. The 7–25 days high first causes lower‐tropospheric northerlies and moisture deficit in SC, then propagates to Yangtze River Basin and causes lower‐tropospheric warming and decaying moisture deficit in SC. These processes favor dry condition in SC for 1 week. In comparison, the 25–90 days high and associated lower‐tropospheric warming and moisture deficit in SC favor dry condition there for more than one week. For type‐2 RECDD occurring in the southern part of SC, the 7–25 days and 25–90 days high over Northeast Asia are important. The 7–25 days high contributes for one week. It first causes lower‐tropospheric anticyclone around the East China Sea and south‐negative–north‐positive moisture pattern in SC, then propagates to the Sea of Japan and causes lower‐tropospheric south‐warming–north‐cooling pattern and decaying south‐negative–north‐positive moisture pattern in SC. In comparison, the 25–90 days high and associated lower‐tropospheric south‐warming–north‐cooling pattern and south‐negative–north‐positive moisture pattern in SC favor south‐dry–north‐wet pattern there for more than one week. Moreover, the 7–25 days low and decaying 25–90 days high over Lake Balkhash‐Baikal, concurring with 7–25 days low over Bay of Bengal, favor the end of type‐1 RECDD; the 7–25 days low and decaying 25–90 days high over Northeast Asia favor the end of type‐2 RECDD. Plain Language Summary: The extreme consecutive dry days with long duration could increase the risk of drought, even if the mean precipitation does not show significant change. In this study, we identify two types of regional extreme consecutive dry days events in spring over Southern China (type‐1 occurs in entire Southern China, type‐2 occurs in southern part of Southern China), with the aim of understanding the direct physical processes responsible for the persistence of extreme consecutive dry days. The results show that anomalous high over Lake Balkhash‐Baikal/Northeast Asia on two time‐scales (7–25 days and 25–90 days) are important for the persistence of type‐1/type‐2 events. In comparison, the 7–25 days high favors the persistence of regional events for approximately one week, whereas the 25–90 days high favors longer. Moreover, the 7–25 days low and decaying 25–90 days high over Lake Balkhash‐Baikal, combined with 7–25 days low over Bay of Bengal, provide a favorable condition for the end of type‐1 events. The 7–25 days low and decaying 25–90 days high over Northeast Asia favor the end of type‐2 events. Key Points: Two types of regional extreme consecutive dry days events in Southern China (SC) are identified in this study7–25 days high over Lake Balkhash‐Baikal/Northeast Asia favors type‐1/type‐2 events for one week, and 25–90 days high over there favors longer7–25 days low over Lake Balkhash‐Baikal/Northeast Asia, combined with decaying 25–90 days high there, favors the end of type‐1/type‐2 events [ABSTRACT FROM AUTHOR]

Recent studies have shown that individual Madden–Julian oscillation (MJO) events can be categorized into four types based on their propagation characteristics: standing, jumping, slow-propagating, and fast-propagating MJOs. While their structures and impacts are well documented in observations, their representation in state-of-the-art climate models has not been investigated. This study evaluates the four types of MJOs in phase 6 of Coupled Model Intercomparison Project (CMIP6) models. While most models show reasonable MJO propagation characteristics, their performance varies among MJO types, with better performance for the propagating MJO types. To identify the factors that drive each MJO type, background conditions at the surface, troposphere, and stratosphere are examined. We found that the standing and fast MJOs are closely associated with sea surface temperature (SST) and precipitable water (PW) distributions. Specifically, negative and positive SST and PW anomalies are observed over the equatorial Pacific during the standing and fast-propagating MJOs, respectively. In contrast, the jumping and slow-propagating MJOs do not show noticeable SST and PW distributions, suggesting that they may be more strongly influenced by the internal dynamics than by the background conditions. It is further found that the models with more frequent standing MJO than fast MJO generally have stronger negative biases in both SST and PW over the equatorial western Pacific. Although the standing MJO is also preferred when the stratospheric wind at 50 hPa is westerly, such a relationship is absent in the models. These results suggest that MJO diversity in climate models could be improved by better simulation of the mean state, especially for the standing and fast-propagating MJOs. [ABSTRACT FROM AUTHOR]

The RMM (Real-time Multivariate MJO) index is used to evaluate the skill of simulating the amplitude and phase of the Madden–Julian Oscillation (MJO) for various versions of the INM RAS (Institute of Numerical Mathematics of the Russian Academy of Sciences) climate model. The model results are compared to the other CMIP6 climate models and ERA-Interim reanalysis. It has been shown that all the versions of the INM RAS model correctly reproduce the MJO amplification in the Northern Hemisphere winter, but significantly underestimate the amplitude of the oscillations as compared to the reanalysis data. The models correctly reproduce the eastward propagation of the MJO, but the INM-CM5-0 and INM-CM6-L are unable to simulate the crossing of Indonesia by the convective center of MJO. In the INM-CM6-L*, the MJO trajectory is better simulated as compared to previous versions, and the amplitude for the individual MJO events is consistent with the reanalysis data. However, the MJO events in the INM-CM6-L* make up only about 30% of their total number according to the reanalysis data over the period 1980–2014. [ABSTRACT FROM AUTHOR]

Sea ice drift in the Arctic Ocean impacts ice mass balance, ocean currents, ice deformation, and freshwater output into lower latitudes. Satellite observation reveals that the Arctic sea ice drift has accelerated under global warming. Meanwhile, previous studies also found that local atmospheric intraseasonal oscillation modulates the Arctic sea ice drift. However, the mechanisms linking the Arctic sea ice drift change to the general warming and intraseasonal oscillation in the local atmosphere are not clearly addressed. Based on a sea ice‒ocean coupled model, this study finds that: (1) The atmospheric intraseasonal oscillation leads to a higher climatological sea ice drift speed despite it produces thicker ice in the Arctic marginal seas, since the elevating effect of increased wind speed yields the suppressing effect of increased ice thickness on sea ice drift speed. (2) The warming of local atmosphere results in substantial elevation of the Arctic sea ice drift speed through generating basin-scale reduction of sea ice thickness. Developing a more sophisticated sea ice dynamical equation may be an essential way to reduce the wide-existing positive bias in sea ice drift modeling. [ABSTRACT FROM AUTHOR]

The onset of the rainy season in Vietnam displays distinct characteristics compared to other Asian summer monsoon (ASM) regions due to its elongated shape running from south to north, coupled with its complex terrain and the influences of multiple weather systems. Determining the onset is challenging, although numerous empirical indices exist. Here, we proposed an alternative approach to determine the onset by examining the large-scale weather patterns associated with the abrupt increase in rainfall in the country. In this new approach, the evolutions of the ASM system were interpreted as consecutive occurrences of multiple weather patterns, detected by a clustering algorithm Self Organizing Maps (SOMs). Specifically, SOMs were applied on outgoing longwave radiation, 850 hPa wind, and geopotential height over the ASM regions from April to July. The primary features of these atmospheric variables were generalized to 16 patterns. A high-resolution Vietnam Gridded Precipitation (VnGP) dataset was employed to investigate the linkage between these large-scale patterns and local-scale rainfall. The large-scale patterns associated with the onset were distinguished, which are characterized by the development of westerly winds and large-scale convections. Then, the onset of summer rainfall in Vietnam is identified based on the initial appearance of wet-condition weather patterns. This approach offers several advantages: it is objective and does not require additional thresholds of rain or wind to determine the monsoon onset. It also more effectively captures the characteristics of large-scale circulation and deep convection compared to relying on predefined monsoon indices alone. Additionally, this approach helped to elucidate two types of weather patterns associated with the onset: those associated with the northeastward development of deep convection and westerly wind from the Indian Ocean to the Indochina Peninsula and activities of the Boreal Summer Intraseasonal Oscillation. [ABSTRACT FROM AUTHOR]

A better understanding and simulation of the tropical intraseasonal oscillation (ISO) is the cornerstone of subseasonal-to-seasonal predictions. Here, we have revealed crucial roles of interference effects between Madden–Julian Oscillation (MJO) and low-frequency equatorial Rossby (ER) waves on the intensity, structure and initiation of the ISO. Over where ER waves are sufficiently strong, the ISO convection usually manifests localized strengthening or weakening due to constructive or destructive MJO-ER interferences. For the ISO interannual variability, though the strength is determined by the MJO, the area is largely controlled by ER waves. The Maritime Continent MJO (ER) varies synchronously (asynchronously) with El Niño-Southern Oscillation likely controlled by the meridional mean moisture gradient. Additionally, separating MJO and ER components helps explain the northwest-southeast tilted structure of boreal summer ISOs, which dramatically arises from decreased phase speeds of the MJO away from the equator. Moreover, the considerable damping of ISO deep convection over the Maritime Continent partly arises from the further weakened MJO aloft due to the dry intrusion of ER waves triggered from the central-eastern Pacific. Finally, a primary ISO event with jumping-like propagation behaviors is likely initiated over the Indian Ocean with the strengthening of preceding ER waves, implying a novel "initiation-propagation" linkage of the ISO. These ER signals may be detected approximately 20 days ahead from the western Pacific and are thus potentially useful for monitoring and predicting ISO initiation early. Taking together, the findings here highlight the importance of ER waves in understanding the dynamics and predictability of the ISO/MJO. [ABSTRACT FROM AUTHOR]

The climatological connection between tropical cyclones (TCs) and boreal summer intraseasonal oscillation (BSISO) over the western North Pacific (WNP) is known, but here we additionally find that the influences of May–October BSISO on TC genesis and intensification at the interannual timescale reveal distinctive regional features. The strengthened BSISO variability over the subtropical WNP (7.5°–27.5°N, 145°E–165°W) contributes positively to the basin-total TC genesis count, while the BSISO amplitude over the central-western equatorial Pacific (5°S–5°N, 140°E–160°W) is significantly correlated with the average intensity that TCs can reach. The physical processes causing the differences in TC behavior associated with interannual changes in BSISO are quantified from the viewpoint of eddy energetics. During summers with enhanced BSISO over the subtropical WNP, more TCs form there. Composite diagnostic analysis results for TC genesis days show that local synoptic eddies (or TC embryos) gain more kinetic energy to grow into TCs from both the seasonal-mean and BSISO anomalies via barotropic energy conversion. Eddy self-augmentation processes related to baroclinic conversion and eddy geopotential flux also aid the generation of TCs. During summers with enhanced central-western equatorial BSISO, TCs stay longer over the open ocean before they reach maximum intensity. Further diagnosis along the TC tracks shows that these eddies may grow faster (with a larger kinetic energy tendency) and reach a higher intensity with the help of enhanced barotropic and baroclinic energy conversion processes. [ABSTRACT FROM AUTHOR]

The seasonal prediction of the Indian summer monsoon (ISM) and Monsoon Intraseasonal Oscillations (MISO), as well as the Madden Julian Oscillations (MJO) that strongly modulate MISO, is important to the country for water and crop management. We have analyzed the precipitation, convection, and cloud in the selected models from the sixth generation Coupled Model Intercomparison Projects (CMIP6). This study highlights the significant differences in simulating MISO and MJO features between the selected CMIP6 models and their possible reasons. The mean and intraseasonal features of MISO and MJO varied significantly in CMIP6 models, which may impact better depiction of convection and total cloud fraction. The probability distributions of rainfall and OLR in CMIP6 models also indicate significant variations in simulating the MISO and mean ISM. The results demonstrate the importance of cloud and convection in CMIP6 models to depict realistic MISO and MJO and provide a road map for improving ISM climate prediction and projections. [ABSTRACT FROM AUTHOR]

Seasonal reversal of winds and equatorial remote forcing influences the circulation of the Arabian Sea (AS) and Bay of Bengal (BoB) basins in the northern Indian Ocean. In this study, we numerically modelled the physical characteristics of the AS and BoB, using the Massachusetts Institute of Technology general circulation model (MITgcm) at a high spatial resolution of 1/20 ° forced with climatological initial and boundary conditions. The simulated temperature, salinity, and flow fields were validated with satellite and in situ datasets. We then studied the exchange of coastal waters by evaluating transports computed from the model simulations. The alongshore volume transport on the eastern coast is stronger with high seasonal variability due to the poleward-flowing western boundary current and equatorward-flowing East Indian Coastal Current. West coast transport is influenced by large intraseasonal oscillations. The alongshore freshwater transport is an order less than the alongshore volume transport. Out of the net volume transport, freshwater accounts for a maximum of 6.03 % during the southwestern monsoon season, followed by 4.85 % in the post-monsoon season. We observe an inverse relationship between alongshore freshwater and volume transport on the western coast and a direct relationship on the eastern coast. The contribution of eddy-induced heat and freshwater transport was also examined. The relation between net heat transport and net heat flux illustrates the role of coastal currents and equatorial forcing in dissipating heat within the coastal waters. We observed that meridional heat transport over the AS is stronger than over the BoB. Both basins act as a heat source during the summer monsoon and heat sink during the winter. This high-resolution model setup simulates all the important physical climatological patterns, leading to a better understanding of the state of the northern Indian Ocean. [ABSTRACT FROM AUTHOR]

The impact of the boreal summer intraseasonal oscillation (BSISO) on rainfall anomalies in the Yangtze–Huaihe River Basin (YHRB) was found to be modulated by the stratospheric quasi‐biennial oscillation (QBO), which is reasonable for the close associations between the QBO and summer extreme precipitation in the YHRB. It was found that the extreme precipitation preferentially occurred in the YHRB during the easterly phase of the QBO (EQBO) compared to the westerly phase of the QBO (WQBO). This was primarily because the BSISO was more prone to cause rainfall extremes in the YHRB during the EQBO summers than during the WQBO summers. The EQBO can induce the background stratospheric easterly wind to arch downward into the troposphere, which enhanced the BSISO‐associated moisture transport and moisture convergence and thus resulted in stronger rainfall extremes in the YHRB. Plain Language Summary: Since previous studies have demonstrated that the stratospheric quasi‐biennial oscillation (QBO) plays a critical role in rainfall variability and extremes in East Asia, in this study, we focused on the associations between the QBO and summer extreme precipitation in the Yangtze–Huaihe River Basin (YHRB). Due to QBO, the long‐standing interannual background cannot fully explain the phenomenon that rainfall extremes can hardly occur at all times under the same background. The boreal summer intraseasonal oscillation (BSISO) was considered in this study because it is one of the most critical factors that directly affects the rainfall variability and extremes in the Asian monsoon region. The impact of the BSISO on summer rainfall extremes in the YHRB is evidently modulated by the QBO. Enhanced rainfall extremes preferentially occurred in the YHRB during the easterly phase of the QBO (EQBO) compared to the westerly phase of the QBO (WQBO). Due to the EQBO backgrounds in the stratosphere, the easterly wind can arch downward into the troposphere and enhance the BSISO‐associated moisture transport and moisture convergence, contributing to the much stronger rainfall extremes in the YHRB. Key Points: QBO is closely associated with summer rainfall extremes in the Yangtze–Huaihe River Basin (YHRB)BSISO during the EQBO summers is more prone to cause rainfall extremes in the YHRB than those during WQBO summersEQBO enhances BSISO‐associated moisture transport and moisture convergence, resulting in rainfall extremes in the YHRB [ABSTRACT FROM AUTHOR]

We simulate the Madden‐Julian oscillation (MJO) over an aquaplanet with uniform surface temperature using the multiscale modeling framework (MMF) configuration of the Energy Exascale Earth System Model (E3SM‐MMF). The model produces MJO‐like features that have a similar spatial structure and propagation behavior to the observed MJO. To explore the processes involved in the propagation and maintenance of these MJO‐like features, we perform a vertically resolved moist static energy (MSE) analysis for the MJO (Yao et al., 2022, https://doi.org/10.1175/jas‐d‐20‐0254.1). Unlike the column‐integrated MSE analysis, our method emphasizes the local production of MSE variance and quantifies how individual physical processes amplify and propagate the MJO's characteristic vertical structure. We find that radiation, convection, and boundary layer (BL) processes all contribute to maintaining the MJO, balanced by the large‐scale MSE transport. Furthermore, large‐scale dynamics, convection, and BL processes all contribute to the propagation of the MJO, while radiation slows the propagation. Additionally, we perform mechanism‐denial experiments to examine the role of radiation and associated feedbacks in simulating the MJO. We find that the MJO can still self‐emerge and maintain its characteristic structures without radiative feedbacks. This study highlights the role of convective MSE transport in the MJO dynamics, which was overlooked in the column‐integrated MSE analysis. Plain Language Summary: We conduct simulations of the Madden‐Julian oscillation (MJO) using a computer model that can explicitly simulate deep convective clouds. The simulated MJO behaves similarly to what has been observed in the real world in terms of its spatial structure and propagation. We then delve into the detailed mechanisms behind the MJO, using a method that analyzes how energy and moisture move vertically through the atmosphere, rather than just averaging these properties across the whole atmosphere. This novel analysis shows that radiation, convection, turbulence in the atmospheric boundary layer, and large‐scale atmospheric flows all play roles in sustaining the MJO and affect its eastward propagation. Interestingly, the MJO can still develop and maintain its unique features without the influence of radiation, indicating other processes are also key. This research underscores the importance of understanding the vertical transport of energy and moisture by convective storms in studying the MJO, an aspect previously underappreciated in some simpler models and diagnoses. Key Points: We have successfully simulated the Madden‐Julian Oscillation (MJO) using the Energy Exascale Earth System Model‐multiscale modeling framework over an aquaplanet with uniform surface temperatureVertically resolved analyses of moist static energy highlight the role of convection in the maintenance and propagation of the MJOMechanism‐denial experiments show that radiative feedbacks are not essential to simulate the MJO [ABSTRACT FROM AUTHOR]

Over the East Africa region forecasts of the onset of the rainy seasons have the potential to support decision‐making, especially in the largely rain‐fed agricultural sector. However, the understanding of key features of onset remains limited. Here, we analyse the variability of onset and associated drivers at interannual and subseasonal timescales, using several onset definitions. Results show that the onset date is especially variable from year to year in some of the high‐potential agricultural areas (standard deviation >20 days), which has implications for agricultural risk management. The choice of onset definition metric matters; agronomic definitions have limited applicability at the regional scale and are also highly sensitive to the spatial scale of analysis and to the choice of rainfall data. Onset information provided at coarse scales should be used with caution for decision‐making at the local scale; the "hit rate" of coarse‐scale tercile onset information at the local scale is less than 40% on average. To varying degrees, onset is related to total seasonal rainfall and thus to dominant interannual drivers of rainfall, including the Indian Ocean Dipole and ENSO modes in October–December and the western Pacific "V‐gradient" pattern in March–May. However, by analysing the dominant proportion of onset variance unrelated to total rainfall during the climatological season we show a substantial influence of subseasonal drivers, notably the Madden–Julian Oscillation. As such, there is an opportunity for rainfall onset information to be provided across seasonal and subseasonal timescales. Our work reinforces the need for enhanced co‐production of such onset information with stakeholders, especially regarding the choice of metric, alignment of forecasts with livelihood calendars, interpretation of the credibility of information content for local‐level decision‐making, as well as appropriate strategies for staggered risk management interventions informed by forecasts over "seamless" lead times. [ABSTRACT FROM AUTHOR]

Northeast India (NEI) receives most of its rainfall in the southwest monsoon (SWM) season. The region is known to have frequent and persistent rainfall events of high intensities and the region is vulnerable to potential high‐impact meteorological events. This study analyses observed daily rainfall data for the SWM months (JJAS) during 1991–2020 to better understand the climatology of high‐intensity rainfall (HIR) occurrences and their persistence. The agglomerative hierarchical cluster analysis has delineated four distinct clusters and Cramer's t test indicates no significant fluctuations at 5% significant level in the HIR events in these clusters. ERA‐5 reanalysis data have been used to find the moisture transport for synoptic‐scale (3–7 days), quasi‐bi‐weekly oscillations (10–20 days) and intraseasonal oscillations (30–60 days) for each cluster. Under positive synoptic‐scale phases, HIR in the clusters on southern latitudes of NEI occurs due to moisture incursion from the Bay of Bengal from southwesterlies at 850 hPa, and in the clusters on the northern latitudes, it is primarily due to westerlies. For, quasi‐bi‐weekly oscillations, westerlies at 850 hPa are favourable for moisture transport in most of the clusters during its positive phase. On the other hand, for positive phase of intraseasonal oscillations, westerlies at 850 hPa dominate the moisture transport in all the clusters. Also, most of the HIR events occur whenever Madden–Julian Oscillation (MJO) is in phase 1 and 2 with higher amplitude (RMM ≥1). [ABSTRACT FROM AUTHOR]

El Niño Southern Oscillations (ENSO), being the largest global air-sea interaction phenomena, has a profound effect on global monsoon. In this study, for the first time we have examined the ENSO – Monsoon relation through nonlinear kinetic energy (KE) exchanges outlook across scales. Indian summer monsoon (ISM) and Western North Pacific Summer Monsoon (WNPSM) are considered to be the two major components of global monsoon on which ENSO has a huge impact. The ENSO – Monsoon nonlinearity can be explored by unraveling the unstable (stable) relation between ENSO and Monsoon during El Niño (La Niña) period through the mechanism of scale interactions among El Niño / La Niña, dominant high (HFO) and low frequency oscillation (LFO) considering ENSO itself as a LFO of time period 12–18 months. It is found from the analysis that more (less) KE is transferred to synoptic scale and 10-20 day LFO from El Niño 1997-98 compared to that from 2015 - 16 episode over ISM (WNPSM) region. Moreover, the opposite energy exchange between El Niño and 30-60 day wave is observed over these regions, attributing to contrasting rainfall variation over ISM and WNPSM zone. Sharp (comparatively less) variability in energy exchange among El Niño (La Niña), HFO and LFO over ISM and WNPSM regions make El Niño – Monsoon relation highly unstable whereas La Niña – Monsoon relation a stable one. Unveiling El Niño-ISM, El Niño-WNPSM, La Niña-ISM and La Niña-WNPSM relations in terms of inter-scale KE exchanges for the two El Niño and two La Niña events may lead to enhance the understanding of ENSO-monsoon nonlinearity. This work will make an avenue for further research in the space of nonlinearity between the two large scale systems that may be expected to be more complex due to the global warming and climate change as a futuristic study. [ABSTRACT FROM AUTHOR]

Streamflow in source region of Yellow River (SRYR) matters with regard to the adjacent and downstream water resources. Intraseasonal oscillation (ISO) in the streamflow in SRYR is of great significance to the sub-seasonal prediction of streamflow in SRYR, but is unknown. Here, we first report a 30–60-day ISO in the streamflow in SRYR, which is regulated by the atmospheric 30–60-day ISO at mid-high latitude over North Eurasia. The 30–60-day ISO in atmosphere is featured by a Rossby wavetrain, and the wave energy propagates southward onto the TP, which causes anomalous wind response over TP. The leading anomalous high (low) with anti-cyclonic (cyclonic) wind anomalies over the TP favors dry (wet) air in lower troposphere in SRYR, via enhancing the water vapor divergence (convergence). Dry (wet) air always results in strong (weak) evaporation from the Yellow River, which causes the later streamflow valley (peak) and thereby the 30–60-day ISO in the streamflow in SRYR. [ABSTRACT FROM AUTHOR]