Search

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]

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]

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]

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]

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]

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 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]