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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]

Arabian Sea mid‐tropospheric cyclones (MTCs), responsible for extreme rainfall events in Western India, often coincide with monsoon low‐pressure systems (LPSs) over the Bay of Bengal. However, the influence of Bay of Bengal LPSs on the formation of Arabian Sea MTCs remains unclear. This study utilizes the Weather Research and Forecasting Model (WRF) to investigate the atmospheric connection between these two basins. By introducing a balanced bogus vortex over the Bay of Bengal, cyclonic systems are induced over the Arabian Sea in the majority of ensemble members, exhibiting characteristics consistent with observations. In particular, as the Bay of Bengal vortex moves westward, the middle tropospheric trough deepens, horizontal wind shear increases, the low‐level Arabian Sea stable inversion layer weakens, and the middle troposphere moisture content over Western India and the northeast Arabian Sea rises. Subsequently, MTC genesis occurs over the northeast Arabian Sea along the western edge of the trough within 2–4 days of model integration. A vorticity budget analysis highlights the critical role of vorticity advection and tilting during the initial 24 h of MTC genesis, while vortex stretching becomes the dominant vorticity source during rapid intensification. To substantiate these findings further, a mechanism denial experiment is conducted using a real‐world instance of a coexistent Arabian Sea MTC and Bay of Bengal LPS, replicated in the model. In this experiment, conditions unfavorable for LPS genesis are created by cooling and drying the Bay of Bengal. The results demonstrate that the absence or reduced intensity of the Bay of Bengal LPS inhibits formation of the Arabian Sea MTC. In all, this study presents compelling evidence for the significant influence of Bay of Bengal low‐pressure systems on the formation of severe weather‐inducing MTCs over the Arabian Sea and Western India. [ABSTRACT FROM AUTHOR]

Abstract 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.

During the summer of 2016, the northern East China Sea and the southern Yellow Sea (NECS‐SYS) experienced one of the most severe and devastating marine heatwaves (MHWs) on record, with a temperature anomaly exceeding 4°C. This shallow semi‐enclosed continental shelf region is widely recognized as a significant hotspot for MHWs with associated incidences of harmful algae blooms. Previous studies have highlighted the importance of mixed layer shoaling as a crucial factor in the genesis of MHWs in the global ocean. The current study employed the Hybrid Coordinate Ocean Model reanalysis data set during 1994–2015 to delve into the mechanisms driving mixed layer shoaling during NECS‐SYS MHW genesis. Our findings reveal the significant role of the northward propagating boreal summer intraseasonal oscillation in promoting MHW genesis and intensification. Specifically, boreal summer intraseasonal oscillation phases 5, 6, and 7 contribute to the favorable conditions that facilitate MHW formation by inducing mixed layer shoaling and increasing solar influx, with mixed layer shoaling playing a more dominant role. The current study provides insights into the relative influences of wind, salinity, and temperature on mixed layer shoaling. We observe that wind plays the most significant role in mixed layer shoaling, followed by temperature and salinity. The boreal summer intraseasonal oscillation induced wind relaxation, increased shortwave radiation, and freshwater influx lead sea surface temperature by 7, 5, and 4 days, respectively. Importantly, mixed layer shoaling leads SST anomalies by 1–2 days. Therefore, the current study also suggests an intraseasonal predictability source for NECS‐SYS MHWs. Plain Language Summary: The northern East China Sea and southern Yellow Sea (NECS‐SYS) experienced one of the most severe and devastating marine heatwaves (MHWs) during the summer of 2016. The region, situated in the northwest Pacific, is characterized by a shallow semi‐enclosed marginal sea, and receives a significant amount of freshwater influx from the Yangtze River. Previous studies have demonstrated that shoaling of surface mixed layer is a primary factor contributing to the genesis of MHWs in the global ocean. The present study emphasizes the role of the boreal summer intraseasonal oscillation in creating favorable conditions for MHWs genesis in these regions. Our research demonstrates that boreal summer intraseasonal oscillation phases 5, 6, and 7 contribute to the amplification of solar flux and the shoaling of mixed layer by reducing wind speed and introducing freshwater in this region. We provide a comprehensive understanding of the individual influences of wind, salinity, and temperature on mixed layer shoaling, highlighting the significance of mixed layer shoaling as a fundamental driver of MHWs. Moreover, our research also directs to an intraseasonal predictability source of the NECS‐SYS MHWs. Key Points: Boreal Summer Intraseasonal Oscillation phases 5, 6, and 7 facilitate marine heatwaves (MHWs) in the northern East China Sea and southern Yellow SeaMixed layer shoaling is a more critical factor than the enhanced shortwave for MHW genesis and intensification in this regionOccurrences of MHWs in phase with boreal summer intraseasonal oscillation offer subseasonal predictability of MHWs [ABSTRACT FROM AUTHOR]

In this study, based on the ECMWF 20-year (1997 ~ 2016) hindcasts, the subseasonal prediction of the weekly summer rainfall anomaly over the middle and lower reaches of the Yangtze River (YR) were studied. The skill at 2-week lead time exhibits prominent interannual variation, which is significantly correlated with the preceding winter ENSO. That is, rainfall anomaly can be better predicted in El Niño decaying summer than in La Niña decaying summer. Observation analyses show that in El Niño decaying summer the intraseasonal variation of YR summer rainfall is featured by strong low-frequency (> 30 days) intraseasonal oscillation (ISO) partly associated with the first boreal summer ISO mode (BSISO1) activity, in comparison with La Niña decaying summer. This is possibly because El Niño-induced mean state western North Pacific (WNP) anti-cyclone blocks the northward propagation of convection over the WNP, resulting in BSISO1 stagnation in phases 3–4. The phase stagnation could force stable atmospheric teleconnection, which is favorable to sufficient moisture transportation to the YR and persistent rainfall formation. Finally, prediction verification showed that more accurate prediction for the middle-low-level circulation contribute to the better prediction of rainfall anomaly in El Niño decaying summer than in La Niña decaying summer. [ABSTRACT FROM AUTHOR]

This study investigates the climatological characteristics of the monsoon breaks during the southwest monsoon season (June–September) of the Philippines from 1979 to 2020. The monsoon break is defined as a period when the average rainfall across the eight western coast stations of the Philippines decreases below 5 mm·day−1 for at least three consecutive days. About 208 monsoon break events were identified, with 131 short‐ (3–4 days; 63% of the total monsoon break events), 71 medium‐ (5–9 days; 34%) and 6 long‐duration (≥10 days; 3%) monsoon break events. The short‐ and medium‐duration monsoon breaks were found to be more frequent during June and less frequent during August. Lag composite analyses of the time series of rainfall, low level zonal wind (UWND850hPa), vertically integrated moisture flux convergence (VIMFC) and mid‐tropospheric geopotential height (HGT500hPa) relative to the starting date of the monsoon break events show that the monsoon break is characterized by decrease in rainfall, weakening of westerly winds, increase in HGT500hPa and suppression of VIMFC over Luzon Island. Further analyses of the large‐scale circulation features show the appearance of an anticyclonic circulation over Luzon Island that suppresses the rainfall in the region and westward extension of the western North Pacific Subtropical High (WNPSH). The occurrence of the monsoon break events appears to be modulated by the 30–60‐day mode of the boreal summer intraseasonal oscillation (BSISO). About 39%, 55% and 82% of the short‐, medium‐ and long‐duration monsoon break events occur between Phases 1 and 4 during significant suppressed phases of the BSISO (i.e., amplitude >1), respectively. Convective activities and tropical cyclone activities are generally suppressed in these phases. The suppression of convective activities near the Philippines may facilitate the westward expansion of WNPSH, which further suppresses the rainfall and weaken the southwest monsoon. [ABSTRACT FROM AUTHOR]

Water isotopes‐climate correlations are used to reconstruct paleoclimate from various natural archives. Intra‐seasonal oscillation is one of the major atmospheric patterns that modulate the ratio of precipitation isotope in low latitudes, yet their spatial and temporal distribution patterns are unclear. Here we presented a detailed analysis of how the intra‐seasonal oscillations (MJO, Madden‐Julian Oscillation, and BSISO, Boreal Summer Intra‐seasonal Oscillation) modulate precipitation δ18O and vapor δ18O over the Asian monsoon region and the equatorial region. This analysis found consistent intra‐seasonal variations between the ISO and rain δ18O. We interpret it as the ISO regulating the active and inactive convective systems on the regional scale, leading to the intra‐seasonal oscillations in precipitation δ18O, with amplitudes from 4‰ up to 15‰. The MJO (BSISO) leads to eastward (northeastward) propagation of δ18O intra‐seasonal oscillations in the Asian equatorial (monsoon) region, coinciding with the spatial patterns of OLR oscillations, reflecting the response of precipitation/vapor δ18O to the oscillation of large‐scale convective activity or accumulated depletion of regional precipitation. We proved that the amplitudes of intra‐seasonal variation of precipitation δ18O are comparable to seasonal variations, and both of them are the main patterns for precipitation δ18O in study regions. Our results are conducive to the accurate interpretation of δ18O in climate proxies such as precipitation, ice cores, tree cellulose, cave stalagmites, et al. on the intra‐seasonal scale in this region. Plain Language Summary: Periodicity is a phenomenon in which similar characteristics or states of things reappear steadily and continuously. The Tropical Intra‐seasonal Oscillation (Andersen et al., 2004, https://doi.org/10.1038/nature02805) is the reappearance of a large‐scale envelope of more abundant convective rain systems moving eastward in the tropical regions with a typical oscillatory period of 30–60 days. The ratios of H218O and HDO vary during the cycle of continuous circulation of water in the atmosphere, and therefore are widely used as proxies for climate records. We used the measured data of precipitation or vapor isotopes to reveal that the propagation of ISO can be recorded by the ratio of H218O covering Asian tropical and monsoon regions. Furthermore, the increased (decreased) ISO intensity leads to the increased (decreased) amplitude of convection intensity, which results in the increased (decreased) amplitude of the H218O ratio in precipitation or vapor. Key Points: The intra‐seasonal oscillations are one of the main patterns of precipitation δ18O covering the Asian monsoon and equatorial regionsThe stronger δ18O intra‐seasonal oscillation corresponds to the season with active convective activities, more rainfall, and low‐level δ18OThe stronger regional convective intensity leads to the cumulative effect in the depletion of rain δ18O during the convective phases of ISO [ABSTRACT FROM AUTHOR]

The present study focuses on the variability of subsurface ocean temperature and associated planetary waves (oceanic Kelvin and Rossby waves) in the Indian Ocean during the boreal summer intraseasonal oscillation (BSISO) phases in the Indian Ocean Dipole (IOD) years. To accomplish this, multi-sensor datasets have been considered for the period 1980–2021. During negative IOD (nIOD) years northward propagation of BSISO convection is consistent with the normal years, whereas incoherent patterns are evident in the positive (pIOD) years. Enhanced meridional gradient of sea surface temperature (SST) anomalies is evident during the nIOD years, which favours the coherent northward propagation of BSISO. Significant intraseasonal variations of subsurface ocean temperature anomalies are observed in the Equatorial Indian Ocean (EIO). Pronounced subsurface (50–150 m) temperature anomalies are evident during the different phases of BSISO during the nIOD years. Symmetric formation of equatorial downwelling Kelvin wave followed by upwelling Kelvin wave packet were observed during the nIOD BSISO phases. This study further suggests that the symmetric patterns of subsurface waves are evident during the nIOD years and led to clear variations of the northward propagation of BSISO. [ABSTRACT FROM AUTHOR]

The relationship between boreal summer intraseasonal oscillation (BSISO) and precipitation extremes over Indonesia is investigated using observational datasets from 30 years (1987–2016) of rain gauge measurements and the gridded Asian Precipitation‐Highly Resolved Observational Data Integration Towards Evaluation of Water Resources from 1998 to 2015. The results indicate that the frequency of daily extreme precipitation events in Indonesia (defined as total precipitation above the 95th percentile) during extended boreal summer (May–August) is significantly modulated by BSISO, especially over the western and northern regions. Under the influences of BSISO1, the probability of the precipitation extremes over Sumatra and Borneo increases by 20–120% (relative to the seasonal probability) during phases 1–3 and approximately 50–80% over the eastern part of Borneo and Sulawesi during phase 4. Under BSISO2, the probability of the extremes increases up to 40% over Sumatra during phases 1 and 2 and up to 140% over Borneo and Sulawesi during phases 2 and 3. The increase in the probability of extreme summer precipitation is associated with enhanced large‐scale moisture flux convergence and upward moisture transport induced by the active phases of BSISO. These results provide potential information for medium‐ to extended‐range predictions of summer precipitation extremes in Indonesia. [ABSTRACT FROM AUTHOR]

This study investigates the roles of the boreal summer intraseasonal oscillation (BSISO) in the diurnal rainfall cycle over Hainan Island during the warm season (April-September) using 20-year satellite-based precipitation, ERA5 and the outgoing longwave radiation data with the phase composite analysis method. Results show that the spatial distributions of the hourly rainfall anomaly significantly change under the BSISO phases 1-8 while no clear variations are found on the daily and anomaly daily area-averaged rainfall over the island. During the BSISO phase 1, the rainfall anomaly distinctly increases in the morning over the southwest and late afternoon over the northeast of the island, while suppressed convection occurs in the early afternoon over the southwest area. Under this circumstance, strong low-level westerly winds bring abundant moisture into the island, which helps initiate the nocturnal-morning convection over the south coastal area, and drives the convergence region of sea breeze fronts to concentrate into the northwest. Opposite to Phase 1, an almost completely reversed diurnal cycle of rainfall anomaly is found in Phase 5, whereas a positive anomalous rainfall peak is observed in the early afternoon over the center while negative peaks are found in the morning and late afternoon over the southwest and northeast, owing to a strong low-level northeasterly anomaly flow, which causes relatively low moisture and enlarges a sea-breeze convergence area over the island. During Phase 8, strongest moisture is found over the island all through the day, which tends to produce highest rainfall in the afternoon with enhanced anomalous northerly. These results further indicate that multiscale interactions between the large-scale circulations and local land-sea breeze circulations play important roles in modulating diurnal precipitation cycles over the tropical island. [ABSTRACT FROM AUTHOR]

The interannual El Niño-Southern Oscillation (ENSO) and the Boreal Summer Intraseasonal Oscillation (BSISO) strongly modulate sub-seasonal to seasonal rainfall variability, leading to dry extreme rainfall events (DEREs) over Northeast (NE) Thailand. In this study, the ability of climate models to simulate the ENSO-BSISO-induced DEREs and associated synoptic features are evaluated using self-organizing maps. Observed DEREs occur most frequently during ENSO Neutral and La Niña conditions, when enhanced convection is located over central India and the Bay of Bengal. The intensity of observed DEREs are strengthened during El Niño when enhanced convection is observed over the western Pacific region. The climate models exhibit a diverse frequency of DEREs during ENSO phases, with some models showing better skill than others. On intraseasonal time scales, observed DEREs are favored when enhanced BSISO convection in phases 3–5 is located over the Bay of Bengal and the western Pacific region. Five models out of the 19 examined capture the observed pattern during BSISO phases 4 and 5, while only three models capture the BSISO phase 3 behavior. Composite maps of observed DEREs during the combined BSISO and ENSO conditions indicate that BSISO convection increases (decreases) DERE frequency during El Niño and Neutral (La Niña) phase(s). Climate models can simulate the occurrence of dry events associated with the combined BSISO and ENSO phases, but they do so in different BSISO phases.

This study investigates the impact of the northward/northwestward propagating 30–60‐day mode of the boreal summer intraseasonal oscillation (BSISO) on the extreme rainfall events in the Philippines during the June–September (JJAS) season from 1979 to 2018. The Philippines domain is divided into the three latitudinal regions: Luzon region (13°–22°N), Visayas region (10°–13°N), and Mindanao region (5°–10°N) to account for the regional differences in the timing of extreme rainfall events. The probability density functions of JJAS rainfall are skewed towards higher values relative to the non‐BSISO days in BSISO Phases 6–8, Phases 5–7, and Phases 4–6 over the Luzon, Visayas, and Mindanao regions, respectively, during which the probability of extreme rainfall events at the 95th percentile increases by as much as 80% in some stations in these regions. Further analyses of the large‐scale circulation features show that the increase (decrease) in the probability of extreme rainfall events is associated with enhanced moisture convergence (divergence) induced by the cyclonic (anticyclonic) circulation anomalies of the BSISO and appearance of multiple tropical cyclones. About 36% of the total extreme rainfall events over the Luzon region are associated with TCs during Phases 7–8. On the other hand, TCs contribute by no more than 24% in all phases over the Visayas and Mindanao regions, indicating less TC influence in these regions. This study is the first attempt to clarify the impact of the BSISO on the extreme rainfall events in the Philippines. [ABSTRACT FROM AUTHOR]

In this study, subseasonal precipitation forecast skills over Maritime Continent in boreal summer are investigated for the ECMWF and CMA models involved in the S2S Project. Results indicate that the ECMWF model shows generally superior forecast performances than CMA, which is characterized by lower errors and higher correlations compared with the observations. Meanwhile, ECMWF tends to produce wet biases with increasing lead times, while the mean errors of CMA are revealed to be approximately constant throughout lead times of 2–4 weeks over most areas. Besides, the temporal correlations between model outputs and observations obviously decrease with growing lead times, with a high-low distribution presented from north to south. In addition, the roles of large-scale drivers like ENSO and BSISO in modulating subseasonal precipitation forecast skills are also assessed in the models. Both ECMWF and CMA can reasonably capture the ENSO related precipitation anomalies for all lead times, while their capabilities of capturing BSISO related precipitation anomalies decrease with growing lead times, which is more obvious in CMA. The enhanced subseasonal precipitation forecast skills mainly respond to the BSISO associated precipitation variability. For most MC areas such as southern Indochina, western Indonesia, Philippines and the eastern ocean, the forecast skills of both ECMWF and CMA can be improved to a great extent by enhancing the capture of BSISO related precipitation anomalies, with the temporal correlations for both ECMWF and CMA increased by about 0.15 for lead times of 3–4 weeks. It provides an opportunity window for the models to improve precipitation forecasts on the subseasonal timescale.

The spatial structure and propagation characteristics of tropical intraseasonal oscillations vary substantially by season. In this study, these seasonal variations are identified using a multivariate sliding‐window Empirical Orthogonal Function (EOF) analysis. The two modes comprising the leading EOF pair have equal variances and depict the propagation of intraseasonal oscillations in convection and low‐level circulation over the Indian Ocean, the Maritime Continent, and the western Pacific region in the equatorial summer hemisphere. In contrast, the upper tropospheric circulation shows more structure in the winter hemisphere. It is suggested that this variation in seasonality with height is an inherent feature of intraseasonal oscillations. A new multivariate index for tropical intraseasonal oscillations (MII) is developed based on the leading EOFs and represents the three‐dimensional structure of intraseasonal variability in all seasons. The MII is computed by projecting intraseasonal anomalies onto the leading EOFs pair, and it exhibits clearly delineated but smooth seasonal transitions and rich meridional structure. The real‐time version of this new index, rMII, is shown to be similar to MII, with a correlation of 0.9. Compared to the widely used Real‐time Multivariate MJO (RMM) index, the power spectrum of rMII represents substantially greater intraseasonal variance, and the application of rMII in dynamical forecast models indicates rMII is skillfully predicted for an additional week compared to RMM. Plain Language Summary: The Madden‐Julian Oscillation and boreal summer intraseasonal oscillation are the most important oscillations in the tropical atmosphere on the intraseasonal time scale (a few weeks to a couple of months). While they are similar in a number of respects, there are important differences between the two: they are active in different seasons, often display different shapes in weather maps, and move in different directions. An important research question is how to track these oscillations in real‐time across all the seasons. Toward this goal, we develop a new machine learning algorithm to automatically identify spatial and temporal patterns of the oscillations in this research. We apply the new algorithm to identify the oscillations in output from state‐of‐the‐art subseasonal weather forecast models, and find that doing so allows skillful prediction of these oscillations up to 5 weeks, a longer time horizon than if we use the algorithms currently in common use. Key Points: A sliding window, multi‐variate EOF analysis identifies spatial structures of tropical intraseasonal oscillations in all the seasonsIntraseasonal lower‐level zonal winds and convection maximize in the winter hemisphere; upper‐level winds in summer hemisphereA Real‐time Multivariate Intraseasonal Index derived from the sliding‐window EOFs is skillfully predicted by subseasonal forecast models [ABSTRACT FROM AUTHOR]

The study finds out Climate Forecast System (CFS) version 2 (CFSv2T126 and CFSv2T382) models’ fidelity in capturing mean flow–eddy interaction, circulation–heating feedback and, the energy conversion processes during organization and intensification of Boreal Summer Intra-Seasonal Oscillation (BSISO) in the backdrop of the mechanism put forward by earlier observation-based study. Ten years of free-run is used to evaluate the models. CFSv2-T126 over-estimates the BSISO intensity from lag –24 and the over-estimation is even more for CFSv2-T382 compared to observation-based study. T126 model underestimates the mean kinetic energy (MKE) related to upper-level easterly wind and the under-estimation is notably more prominent for the strong events. The under-estimation is more for T382 model and the model has difficulty in capturing the upper-level MKE. However, both the model can capture the lower level MKE structure with a slight over-estimation for both the events. At the upper level, MKE to eddy kinetic energy (EKE) conversion for T126 model is significantly weak, and contrary to the observation-based study which shows a decrease in conversion in the subsequent lags. Nevertheless, T382 model shows an increase in MKE to EKE conversion for strong events which agrees with ERA analysis. CFSv2-T126 shows decreasing mean available potential energy (MAPE) to MKE conversion (CA process) for the strong events at the upper level in complete contrast to observation-based study. CFSv2-T382 model can capture the increasing MAPE to MKE conversion as BSISO approaches towards the organized and intense phase. For weak events, CA process is very weak in both models. Both models show a very weak vertical eddy momentum-vertical wind shear interaction (CK3 conversion) for strong events at the upper level. T126 model can capture the conversion for strong events at the lower level, but the magnitude is under-estimated. The process-based analyses of CFSv2 simulation bring out that the model has a significant deficiency in capturing the energy conversions and circulation–heating feedback processes. All these eventually led to lesser fidelity of the model in capturing the organization and intensification of BSISO. [ABSTRACT FROM AUTHOR]

Two commonly used filter approaches for boreal summer intraseasonal oscillation (BSISO) are investigated with the observations during 1979–2021. One is the real-time BSISO index and the other is based on 20–80-day Lanczos bandpass filter. Both two filter approaches are able to isolate spatial structure and life cycle of BSISO realistically in the Indo-NWP region. But the BSISO phase pattern match does not guarantee consistent propagation characteristics and phase occurrence frequency. The real-time monitoring methods such as real-time BSISO index does not use future information beyond the current date, so it is influenced by seasonal mean anomalies induced by El Niño on the interannual timescale over the Indo-NWP region. During post-El Niño summers, the real-time BSISO index shows a weaker northward propagation than bandpass filter approach. Phases 2–4 of BSISO show relatively higher occurrence frequency than other phases during post-El Niño summers in real-time BSISO index, resulting in the seasonal mean of active eight BSISO phases largely resembling the El Niño induced mean state anomaly pattern with depressed convection and low-level anomalous anticyclone in the tropical northwest Pacific region. Bandpass filter approach exhibits a more uniform phase occurrence frequency for both normal and post-El Niño summers, which is consistent with the random nature of BSISO as one of atmospheric internal variability. Bandpass filter approach is recommended for studies of BSISO northward propagation and its relationship with low-frequency variability such as ENSO. • Both two commonly used filter approaches are able to isolate spatial structure and life cycle of BSISO realistically in the Indo-NWP region. • The real-time monitoring method is influenced by low frequency variability such as ENSO. • During post-El Niño summers, real-time method shows reduced northward propagation and increased frequency of phases 2–4 than bandpass method. [ABSTRACT FROM AUTHOR]

The tropical intraseasonal oscillation (ISO) strongly influences tropical cyclone (TC) genesis. However, little is known concerning its impact on TC tracks. Here, we examine how TC tracks in the western North Pacific are modulated by the boreal summer ISO (BSISO) during each month of the TC season (June–October) using the best track data and how well the modulation are reproduced in the European Centre for Medium‐Range Weather Forecasts (ECMWF) model forecasts. The results reveal that the impact of the BSISO on the typhoon tracks varies month to month. The ECMWF model successfully reproduces this impact up to a month in advance. A simple advection model shows that the reproduction of the modulations in the TC genesis location leads to accurate TC track forecasts. These results suggest that the BSISO is one of the major sources of TC track predictability at the subseasonal time scale (2 weeks to 2 months). Plain Language Summary: A 30–90‐day oscillation of convective activity in the tropical atmosphere affects the formation of tropical cyclones (TCs) in the western North Pacific (WNP) in summer. However, how this oscillation modulates the TC tracks is not yet well documented. In this study, we analyze the observational data and how well the modulation is simulated by a state‐of‐the‐art operational numerical weather prediction model to discuss further improvements in TC predictions beyond 2 weeks that are necessary to mitigate the risk of TCs. The results show that the modulation of the TC track by the oscillation varies month to month. For example, TCs formed in the enhanced convection phases in the WNP have a lower chance of hitting Japan than those formed in the suppressed phases in September. In October, however, TCs formed in the enhanced phases have a higher chance of hitting Japan. The model can simulate the TC track modulation well in June, July, and September but not in August and October. Further detailed analyses suggest that model error in the modulation of the TC formation distribution caused by the oscillation causes errors in the TC tracks. These findings will help further improvement of the TC predictions beyond 2 weeks. Key Points: The impact of the boreal summer intraseasonal oscillation on the typhoon tracks varies month to monthThe European Centre for Medium‐Range Weather Forecasts model successfully reproduces the impact up to a month in advanceReproduction of modulations in the typhoon genesis location leads to accurate tropical cyclone track forecasts [ABSTRACT FROM AUTHOR]

Boreal summer intraseasonal oscillation (BSISO) profoundly impacts Northern Hemisphere monsoon onsets and breaks, tropical cyclones, and many climate extremes. BSISO exhibits more complex propagation patterns than the dominant eastward propagation of the Madden‐Julian Oscillation. Previous studies have extensively examined the dominant northeastward propagating BSISO mode and its northward component, but this mode only accounts for about half of the total cases. We conducted an objective cluster analysis of the two‐dimensional BSISO propagation and revealed two new forms of BSISO propagation besides the northeastward propagation: the dipolar northward propagation and the eastward expansion. We investigate processes governing the different propagation forms using moisture tendency analysis. We show that the propagation diversity is related to BSISO's circulation structural asymmetries and the associated moistening processes. The Rossby‐wave component in the background zonal wind shear favors northward propagation while the Kelvin‐wave component favors eastward propagation. The circulation differences are affected by the variation of background states, especially those season‐dependent variations. The results provide insights into the BSISO diversity and potential precursors for foreseeing BSISO propagation. Key Points: Three types of boreal summer intraseasonal oscillation (BSISO) have been identified, which exhibit various 2D propagation featuresThe diverse propagation features are attributed to the BSISO circulation structural differencesThe variations in the season‐dependent background can cause differences in the BSISO circulation structures [ABSTRACT FROM AUTHOR]

This study aims to extend the bimodal intraseasonal oscillation (ISO) index, developed by Kikuchi et al. (Clim Dyn 38:1989–2000, 2012), using JRA-55 reanalysis back in time to 1958. The bimodal ISO index is composed of two distinct ISO indices: the Madden–Julian oscillation (MJO) index and the boreal summer ISO (BSISO) index, each aiming to capture the ISO behavior during boreal winter and summer, respectively. These indices are derived by means of extended empirical orthogonal function (EEOF) analysis applied to outgoing longwave radiation (OLR) during boreal winter and summer, respectively. By combining the MJO and BSISO indices, the state of the ISO can be reasonably represented over the course of the year. First, the original index is updated using observed OLR until recently (–Jan. 2017) with a modification of the use of extended winter and summer months (Dec.–Apr./Jun.–Oct.) instead of normal winter and summer months (Dec.–Feb./Jun.–Aug.) in EEOF analysis. The updated version is quite consistent with the original counterpart, whereas it is arguably able to represent the ISO more faithfully throughout the year. In the same manner, a JRA-55-based index is constructed. Although JRA-55 OLR has systematic biases, the resulting JRA-55-based bimodal ISO index is in excellent agreement with the observation-based index. A comparison between JRA-55 and its subset, JRA-55C (satellite observations are not assimilated at all) suggests that the JRA-55-based index is reliable even in the pre-satellite era. Interannual variability and long-term trend of the ISO activity are also discussed. The newly developed, long-term JRA-55-based index will be useful for a number of applications. [ABSTRACT FROM AUTHOR]

The impact of the quasi‐biennial oscillation (QBO) on the prediction of tropical intraseasonal convection, including the Madden Julian Oscillation (MJO) and Boreal Summer Intraseasonal Oscillation (BSISO), is assessed in the WMO Subseasonal to Seasonal (S2S) forecast database using the real‐time OLR based MJO (ROMI) index. It is shown that the ROMI prediction skill for the boreal winter MJO, measured by the maximum time at which the anomaly correlation coefficient exceeds 0.6, is higher by 5 to 10 days in the QBO easterly phase than its westerly phase. This difference occurs even in models with low tops and poorly resolved stratospheres. MJO predictability, as measured by signal to noise ratio in the S2S ensemble, also shows a similar difference between the two QBO phases, and results from a simple linear regression model show consistent behavior as well. Analysis of the ROMI index derived from observations indicates that the MJO is more coherent and stronger in the QBO easterly phase than in the westerly phase. These results suggest that the skill dependence on QBO phase results from the initial state of the MJO, the regularity of its propagation in the verifying observations, or most likely a combination of the two, but not on an actual stratospheric influence on the MJO within the model simulations. In contrast to the robust QBO‐MJO connection in boreal winter, the BSISO prediction skill exhibited by the S2S models in boreal summer is greater in the QBO westerly phase than in the easterly phase during the 1999 to 2010 period. This is consistent with the observation that BSISO OLR anomalies are stronger in the QBO westerly phase during that period. However, this relationship between the QBO and BSISO in boreal summer changes in recent decades: BSISO is weaker in QBO westerly than easterly during 1979–2000. Correspondingly, the QBO impact on BSISO prediction in boreal summer also reverses in that period as well in a statistical model, whereas this statistical model shows a consistent QBO impact on MJO prediction in boreal winter over the past four decades. Key Points: Prediction skill of boreal winter MJO convection is higher by 5–10 days in QBO easterly than QBO westerly in the WMO/S2S modelsModel forecasted stratosphere has little influence on MJO predictionBSISO prediction skill in QBO westerly is higher than easterly in 2000–2010 but reverses before 1999 [ABSTRACT FROM AUTHOR]

The tropical intraseasonal (30–90 days) oscillation (ISO) displays distinctive behaviors in boreal summer and winter. How well each mode is simulated in climate models has been investigated; however, very few studies have examined whether these modes are simulated in appropriate season. Here we developed diagnostics to assess this aspect and applied these diagnostics to numerous atmosphere‐only and atmosphere–ocean‐coupled models. We found out that all models share serious biases and that they sometimes incorrectly simulate the boreal summer ISO mode even in boreal winter and underestimate the appearance frequency of the boreal summer ISO in boreal summer. Nearly all atmosphere‐ocean‐coupled models show some improvements in the ISO seasonality representation compared to their atmosphere‐only counterparts. It is suggested that good models for simulating the ISO seasonality have good life cycles for each ISO mode and that an accurate reproduction of the seasonal mean low‐level zonal wind is crucial. Plain Language Summary: The leading intraseasonal (30–90 days) oscillation (ISO) in the tropical atmosphere has different characteristics in the boreal summer and winter. Accordingly, it has different impacts on extreme weather such as heavy precipitation and tropical cyclone activity. Many studies have examined how well the boreal summer and winter modes are simulated in climate models; however, very few studies have examined whether climate models can simulate the boreal summer and winter ISOs appropriately in the boreal summer and winter, respectively. In this study, we developed a new method to examine this aspect and applied it to numerous present‐day climate simulations using both atmosphere‐only and atmosphere–ocean‐coupled models. We found out that all models share serious biases and that they inadequately simulate the boreal summer mode even in boreal winter and underestimate the appearance frequency of the boreal summer mode in boreal summer. Nearly all atmosphere–ocean‐coupled models show some improvements in the ISO seasonality representation compared to their atmosphere‐only counterparts. It is suggested that good models for simulating the seasonality of the ISO show good life cycles for each ISO mode and a seasonal mean zonal wind in the lower troposphere. These results are helpful for further improving climate models. Key Points: The seasonality of two major modes of ISO (MJO/BSISO) in global climate models was examinedGlobal climate models have larger deficits in their performances when representing the BSISO than the MJOThe presented method helps in examining the model performance in detail and suggesting further improvements [ABSTRACT FROM AUTHOR]

The 2013 subseasonal asymmetry in tropical cyclone (TC) genesis over the western North Pacific (WNP) was investigated by using the 1979-2013 RSMC best track dataset. The genesis frequency of the 2013 WNP TCs between June-August (summer) and September-November (fall) manifested an abnormal temporal asymmetry: fewer typhoons (more tropical storms) in summer and more typhoons (normal tropical storms) in fall. The 2013 active summer-tropical storm genesis arose from both a failure of eastward extension of monsoon confluence region, especially in August and a lack of moisture supply for TC genesis over the eastern part of WNP, and consequently from fewer probability to reach typhoon intensity due to the westward movement of favorable location for genesis. Thereafter, the eastward extension of monsoon shear line in September and the establishment of monsoon gyre in October induced the eastward movement of favorable location for genesis which increased probability to reach typhoon intensity. The relative contribution of mid-level relative humidity to the positive GPI change played a major role in favorable condition for typhoon genesis in September (45.2%) and October (50.9%). The monsoon gyre pattern played a leading role in the most active fall-typhoon in 2013 contributing to the highest number of October-typhoon. The eastward-migration of convection mainly contributed to the subseasonal shift of TC genesis location following eastward movement of local SST warming from summer to fall under the La Nina-like neutral state. The enhanced active boreal summer intraseasonal oscillation (BSISO) in fall provided more favorable conditions for TC genesis showing about twice as many TCs occurred regarding BSISO in fall than those in summer. This spatiotemporal asymmetry in the large-scale circulations and moisture conditions between summer and fall accounted for the subseasonal shift of genesis location of TCs, and consequently for the active summer-tropical storm genesis and the active fall-typhoon genesis in 2013. [ABSTRACT FROM AUTHOR]

A Madden–Jillian oscillation (MJO) and boreal summer intraseasonal oscillation (BSISO) are important climate variabilities, which affect a forecast of weather and climate. In this study, the MJO and the BSISO hindcasts from the Global Seasonal Forecast System, version 5 (GS5) were diagnosed using dynamic-oriented theories. We additionally analyzed the GS5 climatological run to identify whether the weakness of the GS5 hindcast results from the model physics or initialization processes. The GS5 hindcast captures three-dimensional dynamics and thermodynamics structure of MJO eastward propagation well in the Indian Ocean. The model produces the boundary layer (BL) moisture convergence anomalies to the east of the MJO deep precipitation with easterly anomalies associated with the Kelvin wave. The enhanced BL moisture convergence increases upward transport of moisture from the surface to the lower troposphere, inducing the moist lower troposphere and the positive convective instability by destabilization of the lower atmosphere and, thus, generating the next convection to the east of MJO deep convection and promoting MJO eastward propagation. However, the signal for eastward propagation is relatively weak in the Maritime Continent (MC) and the Western Pacific (WP). To improve the MJO eastward propagation in the MC and WP, improved heating induced by shallow (or congestus) clouds interacting with enhanced BL dynamics may be required. On the other hand, the GS5 hindcast reproduces the BSISO northward propagation reasonably well in the Indian Ocean, which is attributed to positive vorticity anomalies induced by strong vertical shear.

The study investigates the Climate Forecast System version 2 (CFSv2) model fidelity in capturing large‐scale dynamical field and the energy conversion processes during organization and intensification of Boreal Summer Intra‐Seasonal Oscillation (BSISO). Experiment is carried out to make 10‐year free run to evaluate the model. Based on rainfall analyses, two types of events are defined one strong and other with weaker intensity. The result reveals that model over estimates the BSISO intensity starting from the initial phase. Strong events based on MERRA data show a persistent increasing lower‐level moisture convergence with respect to convection centre (CC) and a collocated vertical velocity with CC. However, model moisture convergence and vertical velocity are slightly stronger than observation at the initial phase but it is not persisting in the subsequent lags. The analysis of eddy kinetic energy (EKE) budget equation shows that the model intra‐seasonal EKE is very weak at 850 hPa level. The vertical structure of EKE has shown that a positive EKE at the lower to middle troposphere increases in the subsequent lags but it is very weak for model. The analysis of each term of EKE budget has shown that the barotropic energy conversion (ΔCK) and convergence of eddy geopotential fluxes (ΔFG) are the dominant contributors for positive EKE tendency in the ERA analysis and both the terms increase as the BSISO approaches towards its organized and intense phase. However for model, the above terms are very weak. The EKE advection through large‐scale background flow (ΔAM) and synoptic‐scale eddy (ΔAE) adds negatively to the EKE tendency for strong events. However for model, the magnitudes of the above terms are higher than that of the observation for both strong and weak events. Thus, the model has deficiency in capturing the dynamical processes and the energy conversion processes properly. Composite structures of 850 hPa intra‐seasonal EKE (m2/s2) for strong and weak events based on (a) ERA (b) CFSv2T126. [ABSTRACT FROM AUTHOR]

The cloud water content (CW) and radiative heating rate (QR) structures related to northward propagating boreal summer intraseasonal oscillations (BSISOs) are analyzed using data from A-train satellites in conjunction with the ERA-Interim reanalysis. It is found that the northward movement of CW- and QR anomalies are closely synchronized with the northward movement of BSISO precipitation maxima. Commensurate with the northward propagating BSISO precipitation maxima, the CW anomalies exhibit positive ice (liquid) CW maxima in the upper (middle/low) troposphere with a prominent tilting structure in which the low-tropospheric (upper-tropospheric) liquid (ice) CW maximum leads (lags) the BSISO precipitation maximum. The BSISO-related shortwave heating (QSW) heats (cools) the upper (low) troposphere; the longwave heating (QLW) cools (heats) the upper (middle/low) troposphere. The resulting net radiative heating (QRN), being dominated by QLW, cools (heats) the atmosphere most prominently above the 200 hPa level (below the 600 hPa level). Enhanced clouds in the upper and middle troposphere appears to play a critical role in increasing low-level QLW and QRN. The vertically-integrated QSW, QLW and QRN are positive in the region of enhanced CW with the maximum QRN near the latitude of the BSISO precipitation maximum. The bottom-heavy radiative heating anomaly resulting from the cloud-radiation interaction may act to strengthen convection. [ABSTRACT FROM AUTHOR]

Several monsoon indices have been applied to multiple models from the sub-seasonal to seasonal ( S2S) prediction project database during the period May to October 1999-2010 to assess their ability to simulate the Asian monsoon. The bivariate anomaly correlation ( BAC) of the Boreal Summer IntraSeasonal Oscillation ( BSISO) index suggests that the operational models can predict the BSISO1 and BSISO2 events up to 6-24.5 and 6.5-14 days in advance respectively, although the models tend to underestimate the amplitude of BSISO as the lead time increases. For the strong BSISO events, BSISO1 ( BSISO2) display lower skill mostly in phases 3-5 for all the models, suggesting that the BSISO1 ( BSISO2) is not easy to predict when it is located over India and the Maritime Continent (South China Sea and Bay of Bengal). On the other hand, the higher skills appear in different phases for different models. For instance, the limit of predictive skill of strong BSISO1 and BSISO2 events in phases 6-7 for the ECMWF ensemble forecast could exceed 30 and 28 days, respectively. The comparisons of the BSISO life cycle among the ECMWF, NCEP and CMA models also indicate that the ECMWF model can better predict the evolution of strong BSISO events. Predictions of additional monsoon circulation indices including the Webster-Yang index ( WY), Indian Summer Monsoon index ( ISM), South Asian Monsoon index ( SAM), and SouthEast Asian Monsoon index ( SEAM) in the S2S models have statistically significant skill over the corresponding monsoon regions up to 9-31, 3-17, 7-13 and 7-14 days, respectively. However, the significant skill of summer monsoon precipitation over the SAM and SEAM regions varies significantly among the models, with the skill ranging from 2 days to 2 weeks lead time. [ABSTRACT FROM AUTHOR]

It is the first time for the non-hydrostatic icosahedral atmospheric model (NICAM), at a horizontal mesh size of approximately 14-km, to conduct a continuous long-term Atmospheric Model Intercomparison Project-type simulation. This study examines the performance of NICAM in simulating the tropical intraseasonal oscillation (ISO) from a statistical point of view using 30-year data (1979-2008) in the context of the bimodal ISO representation concept proposed by Kikuchi et al., which allows us to examine the seasonally varying behavior of the ISO in great detail, in addition to the MJO working group level 2 diagnostics. It is found that many of the fundamental features of the ISO are well captured by NICAM. The evolution of the ISO convection as well as large-scale circulation over the course of its life cycle is reasonably well reproduced throughout the year. As in the observation, the Madden-Julian oscillation (MJO) mode, characterized by prominent eastward propagation of convection, is predominant during boreal winter, whereas the boreal summer ISO (BSISO) mode, by a combination of pronounced eastward and northward propagation, during summer. The overall shape of the seasonal cycle as measured by the numbers of significant MJO and BSISO days in a month is relatively well captured. Two major biases, however, are also identified. The amplitude of the simulated ISO is weaker by a factor of ~2. Significant BSISO events sometimes appear even during winter (December-April), amounting to ~30 % of the total significant ISO days as opposed to ~2 % in the observation. The results here warrant further studies using the simulation dataset to understand not only many aspects of the dynamics and physics of the ISO but also its role in weather and climate. It is also demonstrated that the concept of the bimodal ISO representation provides a useful framework for assessing model's capability to simulate, and illuminating model's deficiencies in reproducing, the ISO. The nature and causes of the two major biases are also discussed. [ABSTRACT FROM AUTHOR]

Historical runs of 30 Coupled Model Intercomparison Project Phase-6 (CMIP6) General circulation models (GCMs) are evaluated for the representation of Boreal Summer Intraseasonal Oscillations (BSISO). Several statistical metrics were developed to evaluate the characteristic features of BSISO, such as propagation, phase speed, and exchange of air-sea fluxes at the air-sea interface over the major regions of the Indian Summer Monsoon (ISM). The mean state of the monsoon precipitation in the CMIP6 models is evaluated by using seasonal mean bias, pattern correlation, and root mean square error. The majority of CMIP6 models underestimate the precipitation over central India and overestimate the precipitation over the eastern equatorial region. Multi-model mean (MME) of the models shows good agreement of precipitation pattern with the observations. In the observations, the precipitation anomalies propagate northward from the equatorial latitudes to the northern latitudes over the ISM region (60°E-100°E longitudes). However, the initiation of northward propagating convection shows a significant variation with time in the CMIP6 models. Most of the models well simulated the BSISO propagation over the Bay of Bengal (BoB) and the Indian subcontinent. The majority of the models underestimate the phase speed of BSISO over the Arabian Sea (AS), and easterlies from the western north Pacific, which led to the failure of models in representing the northwest-southeast tilt of convection. Surface turbulent fluxes and zonal winds lag the deep convection over the North Indian Ocean on intraseasonal timescales. However, misrepresentation of air-sea fluxes in the CMIP6 models leads to the significant biases of intraseasonal variances. This study examines the simulation characteristic features of BSISO by CMIP6 models and is mainly attributes them to the atmospheric internal dynamics and air-sea interactions. The present study further suggests that improving atmospheric-oceanic feedback mechanisms, specific humidity, and low-level winds in the CMIP6 models is necessary to accurately predict the ISM intraseasonal variability. • The majority of the CMIP6 models well capture the northward propagation of precipitation anomalies over North Indian Ocean. • In the observations, the phase-speed of BSISO over Bay of Bengal is less than that of the Arabian Sea. • The majority of the models underestimate the phase-speed of BSISO over Arabian Sea, leads to the failure of models in represent the northwest-southeast tilt of convection. • Maximum number of models capture the ocean to atmosphere response with SST lead of precipitation by ~10 (~7) days over Arabian Sea (Bay of Bengal). [ABSTRACT FROM AUTHOR]

A Madden–Jillian oscillation (MJO) and boreal summer intraseasonal oscillation (BSISO) are important climate variabilities, which affect a forecast of weather and climate. In this study, the MJO and the BSISO hindcasts from the Global Seasonal Forecast System, version 5 (GS5) were diagnosed using dynamic-oriented theories. We additionally analyzed the GS5 climatological run to identify whether the weakness of the GS5 hindcast results from the model physics or initialization processes. The GS5 hindcast captures three-dimensional dynamics and thermodynamics structure of MJO eastward propagation well in the Indian Ocean. The model produces the boundary layer (BL) moisture convergence anomalies to the east of the MJO deep precipitation with easterly anomalies associated with the Kelvin wave. The enhanced BL moisture convergence increases upward transport of moisture from the surface to the lower troposphere, inducing the moist lower troposphere and the positive convective instability by destabilization of the lower atmosphere and, thus, generating the next convection to the east of MJO deep convection and promoting MJO eastward propagation. However, the signal for eastward propagation is relatively weak in the Maritime Continent (MC) and the Western Pacific (WP). To improve the MJO eastward propagation in the MC and WP, improved heating induced by shallow (or congestus) clouds interacting with enhanced BL dynamics may be required. On the other hand, the GS5 hindcast reproduces the BSISO northward propagation reasonably well in the Indian Ocean, which is attributed to positive vorticity anomalies induced by strong vertical shear. [ABSTRACT FROM AUTHOR]