Quasi-biweekly Oscillation Research Articles

Generation of the Quasi‐Biweekly Oscillation in the Surface Potential Vorticity Over the Tibetan Plateau During Boreal Summer: A Case Study of 2014

AbstractThe atmospheric circulation around the Tibetan Plateau (TP) exhibits a substantial 10–20‐day quasi–biweekly oscillation (QBWO), profoundly impacting weather and climate locally and remotely. Understanding the factors influencing the generation of QBWO over the TP (QBWOTP) and its physical mechanism is crucial. This study has investigated the influence of multi–timescale and land–atmosphere interactions on the generation of the QBWOTP in surface potential vorticity (SPV), a valuable tool for characterizing the mechanical and thermal variabilities in mountain forcing, based on a 2014 case study. Results indicate that in the free atmosphere, the summer monsoon onset over the Bay of Bengal induces a northward shift in the westerly jet toward the TP, manifested as an increase in low‐frequency zonal winds. This shift facilitates the propagation of wave trains, leading to atmospheric quasi–biweekly potential temperature anomalies (θa) over the TP through a multi‐timescale interaction. Additionally, the TP's surface thermal forcing and arrival of wave trains trigger anomalous upward motion and increase cloud cover. The resultant decrease in net short‐wave radiations and increase in net long‐wave radiations contribute to variations in surface potential temperature (θs) over the TP. As θa and θs evolve, the difference between them enlarges, resulting in the generation of the SPV QBWOTP. Given the relationship between the QBWOTP and downstream rainfall, this study could provide novel insights into understanding and predicting downstream rainfall QBWO.

Moisture dynamic processes of the westward propagation of quasi-biweekly oscillation in Asian tropical summer monsoon rainfall

The quasi-biweekly oscillation (QBWO) is an important component of tropical monsoon variations. In this study, the QBWO rainfall demonstrates a westward propagation across the tropical Asian monsoon region, originating from the western North Pacific, passing through the South China Sea and the Bay of Bengal, and ultimately reaching the northwestern Indian Peninsula. Notably, the rainfall anomalies over the South China Sea precede those over the Bay of Bengal by one quarter of the QBWO life cycle. Enhanced (suppressed) rainfall is closely associated with significant positive (negative) specific humidity anomalies, primarily induced by meridional moisture transport at the lower levels, as revealed by a moisture budget analysis. Specifically, the alternating presence and westward movement of northerly anomalies on the eastern (western) flank of anticyclonic (cyclonic) circulation anomalies are responsible for transporting low-frequency moisture across tropical Asia, thereby instigating the westward propagation of QBWO rainfall. Thus, the multiscale interactions between atmospheric circulation and moisture play a pivotal role in the development of QBWO monsoon rainfall, and understanding the moisture dynamics of this rainfall development would provide useful information for subseasonal rainfall prediction in tropical Asia.

Propagation and Maintenance of the Quasi-Biweekly Oscillation over the Western North Pacific in Boreal Winter

Abstract This study investigates the propagation and maintenance mechanisms of the dominant intraseasonal oscillation over the western North Pacific in boreal winter, the quasi-biweekly oscillation (QBWO). The wintertime QBWO over the western North Pacific is characterized by the westward-northwestward movement from the tropical western Pacific to the western North Pacific and resembles the n = 1 equatorial Rossby wave. Its westward migration is primarily driven by the seasonal-mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. Its northward movement is preconditioned by the vorticity dynamics of the beta effect, the low-level vertical moisture variation, and the local air–sea interaction. The latter involves the atmospheric forcing on the underlying ocean by changing the surface heat flux fluctuations and the sea surface temperature feedback on the low-level atmospheric instability. Its maintenance is primarily through atmospheric external energy sources from diabatic heating, which first generates eddy available potential energy and then converts it to eddy kinetic energy. Significance Statement Atmospheric quasi-biweekly oscillation (QBWO) is an important climate phenomenon over the western North Pacific in boreal winter. It can trigger significant influences both in the tropical and extratropical regions. Given the importance, it is necessary to investigate the propagation and maintenance mechanisms of the QBWO over the western North Pacific in boreal winter. The QBWO usually propagates northwestward from the tropical western Pacific to the western North Pacific. The westward propagation is driven by the seasonal mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. The northward propagation is caused jointly by several processes, including the atmospheric forcing on the ocean via the surface heat flux fluctuations, the feedback of ocean on the low-level atmospheric instability, the vorticity dynamics of the beta effect, and the vertical variation of low-level moisture. In addition, converting from eddy available potential energy to eddy kinetic energy is essential in maintaining the QBWO’s development. The disclosure of these crucial processes deepens the dynamics of the QBWO over the western North Pacific in boreal winter.

Modulations of Madden–Julian Oscillation and Quasi-Biweekly Oscillation on Early Summer Tropical Cyclone Genesis over the Bay of Bengal and South China Sea

Abstract The Madden–Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO) are prominent components of the intraseasonal oscillations over the tropical Indo-Pacific Ocean. This study examines the tropical cyclone (TC) genesis over the Bay of Bengal (BOB) and the South China Sea (SCS) on an intraseasonal scale in May–June during 1979–2021. Results show that the convection associated with the two types of intraseasonal oscillations simultaneously modulates TC genesis in both ocean basins. As the MJO/QBWO convection propagated, TCs form alternately over the two basins, with a significant increase (decrease) in TC genesis frequency in the convective (nonconvective) MJO/QBWO phase. Based on the anomalous genesis potential index associated with the MJO/QBWO, an assessment of the influence of various factors on TC genesis is further assessed. Middle-level relative humidity and lower-level relative vorticity play key roles in the MJO/QBWO modulation on TC genesis. The MJO primarily enhances large-scale cross-equatorial moisture transport, resulting in significant moisture convergence, while the QBWO generally strengthens the monsoon trough and induces the retreat of the western North Pacific subtropical high, increasing the regional lower-level relative vorticity. The potential intensity and vertical wind shear make small or negative contributions. This study provides insights into the neighboring basin TC relationship at intraseasonal scales, which has a potential to improve the short-term prediction of regional TC activity. Significance Statement The Madden–Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO) are two types of intraseasonal tropical atmospheric oscillations. The development of tropical cyclones (TCs) is often accompanied by intraseasonal convection. This study highlights the distinct roles of MJO and QBWO in TC genesis over the South Asian marginal seas (e.g., Bay of Bengal and South China Sea). The QBWO can co-regulate TC genesis along with the background of the MJO, where the large-scale MJO mainly provides moisture, while the small-scale QBWO mainly contributes to vorticity. These findings provide useful information for subseasonal TCs forecasting. There are many developing countries along the South Asian marginal seacoast; therefore, further research on regional TC climate would help effectively reduce casualties and property damage.

Diversity of the Austral Summer Quasi-Biweekly Oscillation in the Southwestern Indian Ocean

Abstract The 10–20-day quasi-biweekly oscillation (QBWO) is active in the southwestern Indian Ocean (SWIO) during austral summer. Compared with comprehensive analyses of the QBWO in the Asian monsoon regions during boreal summer, studies focusing on the austral summer QBWO in the SWIO are relatively scarce. In this study, the diversity of the austral summer QBWO in the SWIO is examined based on K-means cluster analysis, which objectively classifies two distinct modes: an eastward-propagating mode (EM) and a poleward-propagating mode (PM). For the EM (PM), an active convection center originates from the subtropical ocean (tropical ocean) and exhibits an eastward (poleward) propagation path. Moisture budget analysis reveals that positive moisture time tendency anomalies show a phase-leading relationship relative to both QBWO convection centers. This phase leading in moisture tendency anomalies is mainly due to horizontal moisture advection. Further analysis demonstrates that meridional moisture transport (i.e., the summer mean moisture advected by the meridional quasi-biweekly wind) is fundamentally responsible for moisture phase leading in both QBWO modes in their mature phases. The combined scale interaction among low frequency, quasi-biweekly, and high frequency contributes to the initial movement for both modes in the growing phases. Although the two modes in the SWIO are initiated in different regions and exhibit distinct evolutionary features, they are regulated by similar moisture dynamics: the northerlies (northeasterlies) of the cyclonic wind response bring higher mean moisture levels east (south) of the convective center, which leads to the eastward (southward) movement of the EM (PM). Significance Statement The quasi-biweekly oscillation (QBWO), which can affect extreme weather events, such as extreme precipitation and heat waves, is active in the southwestern Indian Ocean (SWIO) during austral summer. Compared with previous studies of the QBWO in the Asian monsoon regions during boreal summer, studies focusing on the austral summer QBWO in the SWIO are relatively scarce. Specifically, we objectively classify the austral summer QBWO in the SWIO into two distinct modes: an eastward-propagating mode (EM) and a poleward-propagating mode (PM). Through moisture tendency diagnosis, we find that the two QBWO modes are regulated by similar moisture dynamics, although they are initiated in different regions and exhibit distinct evolutionary features. This improved understanding may provide insights into the monitoring and prediction of the QBWO.

Quasi-Biweekly Oscillation of Surface Sensible Heating over the Central-Eastern Tibetan Plateau and Its Relationship with Spring Rainfall in China

Abstract This study examines the characteristics and phase evolution of the quasi-biweekly oscillation of surface sensible heating (SH) over the central-eastern Tibetan Plateau (CETP) during spring. The mechanism connecting CETP SH to spring rainfall in China on the quasi-biweekly time scale is further investigated. Results show that the dominant mode of quasi-biweekly CETP SH presents a monopole pattern, in which the peak leads the maximum of the quasi-biweekly rainfall in the middle and lower reaches of the Yangtze River (MLYR) and South China by approximately 5 and 7 days, respectively. As an upper-level Rossby wave train propagates eastward, an anomalous center of convergence moves to the CETP, which leads to a strong downdraft and reduced cloud cover. The resultant elevated shortwave radiation input and drier soil conditions are favorable for the CETP SH quasi-biweekly oscillation to enter a positive phase. When reaching its peak, the CETP SH efficiently heats the lower atmosphere, resulting in a local updraft. Due to the “SH-driven air pump” effect, abundant water vapor is transported from the oceans to China. A lower-layer southerly anomaly on the east side of the TP develops into an anomalous cyclonic circulation via the effect of topographic friction, which leads to the expansion of the positive potential vorticity anomaly and the maximum of the quasi-biweekly rainfall in the MLYR. Further southeastward propagation of the wave train leads to a shift in the rainfall anomaly center to South China. These findings suggest that the CETP monopole SH warming could be a good indicator for predicting intraseasonal variations in spring rainfall over China.

Comparison of changes in quasi-biweekly oscillation intensity over the western North Pacific during the developing late-summer of super El Niño events

This study investigated the differences in the changes in the quasi-biweekly oscillation (QBWO) intensity over the western North Pacific during the developing late-summer (1982, 1997 and 2015) of three super El Niño events and the possible reasons. The late-summer QBWO intensity was enhanced in these three years and the enhanced QBWO intensity in 2015, which was the strongest during 1980–2017, was remarkably stronger than that in 1982 and 1997. This mainly resulted from the differences in the anomalous late-summer background atmospheric conditions over the northwestern tropical Pacific, which were further modulated by the differences in the sea surface temperature anomalies in the Pacific. While strong warming appeared in the central and eastern equatorial Pacific (CEEP) in these three years, the warming and its center extended further west in 2015. More importantly, the warming in the central and eastern North Pacific (CENP) in 2015 was the strongest during 1980–2017, whereas there was cooling in 1982 and moderate warming in 1997. In 2015, the strong and westward-extended warming in the CEEP and the strongest warming in the CENP led to the strongest increased lower-level moisture and anomalous easterly vertical shear over the northwestern tropical Pacific during 1980–2017, favoring the strongest QBWO intensity. Numerical experiments confirmed the role of warming in the CENP in 2015. Besides, the frequency of extreme precipitation events over southern China during the late-summer of 2015 was the maximum of 1980–2017 and was closely related to the enhanced QBWO intensity over the western North Pacific.

Diversity in the quasi‐biweekly oscillation sources of the daily maximum temperature over South China during summer

AbstractThe summer daily maximum temperature (Tmax) over South China features obvious quasi‐biweekly oscillation (QBWO) and its QBWO sources mainly include three types. The first type is influenced by tropical and mid–high‐latitude antiphase coupling QBWO. Successive suppressed and enhanced convection, accompanying with anticyclonic and cyclonic anomalies, propagate northward from the northwest of the Philippines to South China. At the peak warm phase, an anomalous high dominates over South China and causes significant warming. Meanwhile, an anomalous low associated with a southeastward‐propagating wave train occurs to the north and leading to abnormal cooling in situ. The second type originates from the mid–high latitudes. Before the warm phase, an anomalous high around the Ural Mountains triggers two wave trains propagating southeastward along the great circle route and eastward along the westerly jet, respectively, favouring an anomalous high over North–Northeast China. At the peak warm phase, the anomalous high extends southward, causing significant warming over South–Central China. The third type is influenced by tropical and mid–high‐latitude in‐phase coupling QBWO. The tropical anomalies are similar to the first type but the convection anomalies are centred to the northeast of the Philippines and propagate northwestward. The mid–high‐latitude wave trains are similar to the second type but with relatively weaker southeastward‐propagating wave train. At the peak warm phase, the anomalous highs from the Tropics and mid–high latitudes together dominate a large meridional scope and induce wide‐range warming over eastern China. Intrinsically, the QBWO of the convection north of the Philippines and mid–high‐latitude wave trains are important signals of the atmospheric variability and might be applied to the forecast of South China Tmax.

Sea Surface Energy Fluxes' Response to the Quasi‐Biweekly Oscillation: A Case Study in the South China Sea

AbstractThe South China Sea (SCS) owns the world's strongest quasi‐biweekly oscillation (QBWO) in boreal summer, but the mechanism is still unclear. This case study summarizes two modes of QBWO over the summer SCS in 2019 by using empirical orthogonal function on the 10–20‐day bandpass‐filtered outgoing longwave radiation fields. The maximum positive irradiance anomalies for the two modes are 90 W m−2. The upward solar and downward longwave radiation anomalies own about 4%–8% of the irradiance magnitude, and the surface upward longwave radiation shows a weak response. Sea surface turbulent heat fluxes' responses to QBWO display different spatial patterns compared to radiation fluxes. Their changes are mainly ascribed to the surface wind in Mode1 and the air‐sea thermal contrast in Mode2. We also discuss the cause and impact of sea surface turbulent heat fluxes on QBWO.

Quasi-Biweekly Oscillation of PM2.5 in Winter over North China and Its Leading Circulation Patterns

Persistent pollution often occurs in North China in winter. The study of the sub-seasonal evolution characteristics of fine particles (PM2.5) can provide a theoretical basis for the prediction and prevention of persistent pollution. Based on the high-resolution gridded data of PM2.5 and NCEP/NCAR reanalysis, the sub-seasonal variation in PM2.5 in North China in winter and its dominant circulation patterns from 1960/61 to 2019/20 were analyzed. The results show that, in winter, PM2.5 in North China shows a dominant period of 10–20 days, and persistent heavy pollution occurs at the active phase of oscillation. Based on the PM2.5 quasi-biweekly oscillation (QBWO) events, the 850 hPa wave train can be classified into four categories. It was found that, during the active phase of PM2.5 QBWO, the wind speed is weak and humidity is high in the low-troposphere for all of the four event types, while the quasi-biweekly 850 hPa wave train and the track of geopotential height anomaly are significantly different. Based on the characteristics of circulation evolution, these four types of events can be named as eastward, split southward, southeastward, and merged event. The energy conversion between the basic flow and the quasi-biweekly disturbance, and the mean flow difference are responsible for the circulation diversity for different PM2.5 QBWO events. The above research results can provide a theoretical basis for pollutant prediction.

Propagation Diversity of Quasi-Biweekly Oscillation of Summer Precipitation in the Middle and Lower Reaches of the Yangtze River

Abstract Summer precipitation in the middle and lower reaches of the Yangtze River (MLYR) has obvious characteristics of intraseasonal oscillation, but there are different inferences about its propagation. Based on observed precipitation and ERA5 data, the propagation diversity of summer quasi-biweekly precipitation in the MLYR is analyzed. The results show that summer precipitation in the MLYR has obvious quasi-biweekly oscillation (QBWO) characteristics, and precipitation QBWO events can be classified into three categories: southward, northward, and local oscillation events. The frequency of southward events is significantly higher than that of northward or local oscillation events. For southward events, a wave train is observed in the mid–high latitudes and propagates southward in East Asia. The northward and local oscillation events are closely related to the QBWO of the northwest Pacific in low latitudes. The meridional asymmetry of quasi-biweekly divergence and background wind is responsible for the diversity of QBWO propagation. Quasi-biweekly convergence is usually located on the south of the quasi-biweekly cyclone, so the convergence interacts with geostrophic vorticity to guide the quasi-biweekly cyclone to move southward; in the southerly basic flow, north of the quasi-biweekly cyclone is positive relative vorticity advection, which leads the quasi-biweekly cyclone to move northward. Therefore, quasi-biweekly disturbances originating from north of the MLYR, with weak basic wind, mainly move southward under the influence of convergence. However, quasi-biweekly disturbances at low latitudes move northward under the guidance of relative vorticity advection due to the strong basic flow. A disturbance originating from the MLYR presents local oscillation under the coaction of quasi-biweekly relative vorticity advection and divergence.

Role of the Low-Latitude Quasi-Biweekly Oscillation in the Extreme Persistent Heavy Rainfall in the Mei-Yu Season over the Middle and Lower Reaches of the Yangtze River

Abstract Persistent heavy rainfall events (PHREs), as a result of the interaction among multiscale systems, are prone to continuously affect the middle and lower reaches of the Yangtze River valley (MLYR). Based on the observation and reanalysis data, a total of 41 PHREs during the mei-yu seasons of 1979–2020 are first identified over the MLYR and divided into prolonged (over 5 days) and normal (3–5 days) groups. The contributions of quasi-biweekly-scale and synoptic-scale components to the abovementioned two types of PHREs are analyzed. Prolonged PHREs are dominated by the quasi-biweekly component of precipitation (QBW_Pr), while normal PHREs depend on synoptic-scale components (SS_Pr). The quantitative moisture budget indicates that the favorable environment for the QBW_Pr of prolonged PHREs is associated with the interaction between background moisture and quasi-biweekly circulation disturbances. Approximately 80% of prolonged PHREs occurred in phases 6–8 of the boreal summer QBWO (BSISO2) life cycle. The large-scale meridional vertical circulation along the South China Sea (SCS)–MLYR, and the westward-propagating suppressed convection at low latitudes, accompanied by the substantial northward and upward moisture supply, significantly promotes and maintains the QBW_Pr for prolonged PHREs. In contrast, the QBWO signals are relatively insignificant in the evolution of normal PHREs. The impact of the low-latitude QBWO on the prolonged PHREs is further confirmed by sensitivity experiments with RegCM4.9. As the QBWO are weakened on the southeast lateral boundary near the SCS, the vertical circulation and moisture transport clearly diminish along the SCS–MLYR. Consequentially, precipitation reduces visibly during prolonged PHREs. Significance Statement Persistent heavy rainfall events (PHREs), as the reflection of the interaction among multiscale circulation systems, characterized by high intensity and wide coverage, are prone to cause severe flooding and death in the middle and lower reaches of the Yangtze River valley (MLYR). This study aims to investigate the connection between PHREs and different time scales of atmospheric variability, thus elucidating the crucial factors and the influencing mechanisms for the prolonged PHREs during the mei-yu season. Our results reveal that the prolonged PHREs are dominated by the quasi-biweekly component of precipitation, while normal PHREs depend on the synoptic-scale component. The quasi-biweekly precipitation of prolonged PHREs is promoted and maintained by the meridional vertical circulation along the South China Sea (SCS)–MLYR, which is accompanied by the westward-propagating SCS QBWO-related suppressed convection at low latitudes. This study highlights the specific role of the low-latitude QBWO to the formation of prolonged PHREs relative to normal PHREs.

The effects of intraseasonal oscillations on landfalling tropical cyclones in the Philippines during the boreal summer

As a kind of weather phenomenon with destructive wind and heavy rainfall, tropical cyclones (TCs), especially the landing TCs, can cause severer economic damage and losses of life. Philippines is one of the countries mostly affected by Tropical cyclones (TCs). Based on the best-track TC data and global atmospheric and oceanic reanalysis data, the present paper investigates the isolated and combined effects of two intraseasonal oscillations, the Madden-Julian oscillation (MJO) and the quasi-biweekly oscillation (QBWO), on landfall of TCs in the Philippines during boreal summer (May-September) in 1979-2019. The results show that both the MJO and the QBWO can significantly affect the frequency, landfall intensity, location, and translation speed of TCs that make landfall in the Philippines. During the convective (non-convective) phases of the MJO and the QBWO, more (less) frequent and stronger (weaker) TCs make landfall in the Philippines. This is due to the increased (decreased) frequency of TCs formation in the NWP and environmental factors in the region east of the Philippines that are favorable (unfavorable) for the development of TCs. With the northward propagation of the convective signals of the MJO and QBWO, the Western Pacific Subtropical High (WPSH) shifts eastward, and the steering flow is unfavorable for westward movement of NWP TCs. This, in turn, causes a northward shift in the landfall locations and a decrease in the translation speed of TCs. These results are helpful for the prediction of the TCs affecting the Philippines.

Large-scale background and role of quasi-biweekly moisture transport in the extreme Yangtze River rainfall in summer 2020

A severe flooding hit southern China along the Yangtze River in summer 2020. The floods were induced by extreme rains, and the associated dynamic and thermodynamic conditions are investigated using daily gridded rainfall data of China and NCEP-NCAR reanalysis. It is found that the June–July rainfall over the Yangtze River Basin (YRB) experienced pronounced subseasonal variation in 2020, dominated by a quasi-biweekly oscillation (QBWO) mode. The southwestward-moving anomalous QBWO circulation was essentially the fluctuation of cold air mass related to the tropospheric polar vortex or trough-ridge activities over the mid-high latitude Eurasian in boreal summer. The southwestward-transport of cold air mass from mid-high latitudes and the northeastward-transport of warm and moist air by the strong anomalous anticyclone over the western North Pacific provided important large-scale circulation support for the extreme rainfall in the YRB. The analysis of streamfunction of water vapor flux demonstrates that a large amount of water vapor eastward zonal transport from the Bay of Bengal and Indo-China and northward transport from the South China Sea provided the background moisture supply for the rainfall. The quasi-biweekly anomalies of potential and divergent component of vertically integrated water vapor flux played an important role in maintaining the subseasonal variability of rainfall in June–July of 2020. The diagnosis of moisture tendency budget shows that the enhanced moisture closely related to the quasi-biweekly fluctuated rainfall was primarily attributed to the moisture convergence. Further analysis of time-scale decomposition in the moisture convergence indicates that the convergence of background mean specific humidity by the QBWO flow and convergence of QBWO specific humidity by the mean flow played dominant roles in contributing to the positive moisture tendency. In combination with adiabatic ascent over the YRB induced by the warm temperature advection, the boundary layer moisture convergence strengthened the upward transport of water vapor to moisten the middle troposphere, favoring the persistence of rainfall during June–July. The vertical moisture transport associated with boundary layer convergence was of critical importance in causing low-level tropospheric moistening. By comparison, the horizontal moisture advection played a secondary important role in the quasi-biweekly oscillation of rainfall in June–July 2020.

Mechanism of persistent heavy PM2.5 pollution over the Beijing-Tianjin-Hebei region of China: A combination of aerosol radiative effect and atmospheric quasi-biweekly oscillation

The persistent heavy PM2.5 pollution (PHP) occurs frequently over the Beijing-Tianjin-Hebei (BTH) region in winter, but the formation mechanism is not fully understood. Using a regional climate-chemistry-ecology model RegCM-Chem-YIBs, we investigated the synergetic impacts of aerosol radiative effect (ARE) and atmospheric quasi-biweekly oscillation (QBWO) on a PHP event in December 2013. The event consists of two peak stages and a persistence stage between them. Process analysis shows that chemical reactions and advection are the most important processes in the accumulation of PM2.5. Chemical reactions contribute 30–40 μg m−3 hr−1 to the rapid development of PHP, which is far more than that in clean days. Through sensitivity experiments, the influences of ARE and QBWO on PM2.5 variation via changing meteorological conditions were quantified respectively. During the event, ARE leads to a reduction of 24.0% in solar radiation and −33.8% in surface air temperature. The planetary boundary layer height is lowered by 20.8%. The feedback of ARE to PM2.5 ranges from 20 to 170 μg m−3, with the largest changes at PM2.5 peak stages. In comparison, QBWO exerts dominant influence at the persistence stage, during which the East Asia trough and northerlies are weakened constantly. Besides, QBWO causes a decrease of −39.5% in wind speed and an increase of 14.3% in relative humidity near the surface. The rates of chemical process and advection are increased considerably at the first peak stage and the persistence stage. QBWO also slows down the dissipation of PHP, prolonging the pollution duration. Finally, a range of 60–110 μg m−3 increase in PM2.5 is attributed to QBWO. In all, ARE and QBWO take different roles in the formation of PHP. ARE mainly affects the pollution intensity by accelerating chemical reactions, while QBWO determines the persistence of heavy pollution by changing both advection and chemical processes.

The Combined Effects of the Tropical and Extratropical Quasi-biweekly Oscillations on the Record-setting Mei-yu Rainfall in the Summer of 2020

The Combined Effects of the Tropical and Extratropical Quasi-biweekly Oscillations on the Record-setting Mei-yu Rainfall in the Summer of 2020

The intensification of consecutive three tropical cyclones passing over a warm ocean eddy in the South China Sea

An extremely strong and long-lasting (more than 8 months) oceanic warm core eddy existed in the South China Sea (SCS) from February–October 2010. From July–August 2010, three tropical cyclones (TCs; TC Conson, Chanthu, and Mindulle) consecutively passed over this eddy and sustained at least 21 h. The intensity change of all three TCs reached 20 kt within 24 h when they encountered this eddy. In mid-late July, tropical cyclone heat potential (TCHP) is overall stronger in the eddy region than in its surrounding region, thus TCHP plays an important role in the intensification of TC Conson and Chanthu. It is also found that the intraseasonal oscillation (ISO) and the quasi-biweekly oscillation (QBWO) can be important in favor of the further enhancement of TCs. The TCHP is too low to favor the intensity increase of TC Mindulle in late August, 2010, but weak vertical wind shear, ISO and QBWO act as key roles in the intensification of TC Mindulle.

Record-breaking rainfall over the middle reaches of the Yangtze River in August 2021: Sub-seasonal perspective and its predictability

An extremely heavy rainfall cluster was observed over the middle reaches of the Yangtze River (MYR) in August 2021, breaking the historical record since 1981 and causing severe floods. The controlling circulation regime and possible predictability sources of the un-expected rainfall are still unclear, especially from a sub-seasonal perspective. Our results show that the successive heavy rainfall events in August 2021 had significant intraseasonal oscillations for 10–24 d and 30–60 d, and they were synergistically influenced by tropical and extra-tropical circulation regime. Above all, the East Asia–Pacific teleconnection pattern (EAP) turned into a negative phase in late July and maintained throughout August, providing favorable background conditions for the northward transport of tropical water vapor and the southward intrusion of cold air from mid-high latitudes. The 30–60-d oscillation of precipitation was dominated by the first mode of boreal summer intraseasonal oscillation (BSISO1, with 30–60-d period), which strengthened the tropical moisture transport, while the quasi-biweekly oscillation of the western Pacific subtropical high and the anomalous mid-latitude cyclone of EAP intensified the precipitation on the scale of 10–24 d. In addition, the second mode of BSISO (BSISO2, with 10–30-d period) may also intensify the 10–24-d precipitation in late August. The forecast leading time of the S2S models for the persistent heavy rainfall in August 2021 was basically 1–2 weeks, while the EAP and BSISO1 were revealed as the main predictable sources of the abnormal rainfall events. These results highlight the importance of superposition of different intraseasonal oscillations in forming the extreme rainfall event and demonstrate a potential chance to enhance the prediction skill of extreme event if the ensemble members in S2S models could be reasonably selected according to their performances on key predictability sources.

Impact of Quasi-Biweekly Oscillation on Southeast Asian Cold Surge Rainfall Monitored by TRMM Satellite Observation

Based on the Tropical Rainfall Measuring Mission (TRMM) satellite observation and ERA5 re-analysis dataset, this paper studies the influence of the northwestward-propagating quasi-biweekly oscillation (QBWO) over the western North Pacific on cold surge rainfall (CSR) over Southeast Asia. Cold surges are the most important driver affecting Southeast Asian rainfall on a synoptic scale. The presence of the QBWO during phases 6–8, in which the associated active convection coupling with a cyclonic circulation reaches Southeast Asia, provides a favorable environment for the increase of CSR. The increase in CSR primarily occurs east of the Philippines, leading to a high likelihood of triggering extreme rainfall. The effects from the QBWO are independent of those from the active MJO phases over Southeast Asia. Additionally, cold surge activity could also be influenced by the QBWO. An examination of the QBWO and MJO indicates that the most preferred phases for the occurrence of cold surges are the time when phase 1 of the QBWO co-exists with phase 7 of the MJO or the time when phase 7 of the QBWO couples with phase 5 of the MJO. Accordingly, about 40% of the total cold surge days would fall in either combination.

The atmospheric quasi-biweekly oscillation during the Jiangnan Meiyu onset period

Meiyu is a typical rainy season during the East Asian Summer Monsoon whose early or late onset is closely related to an abnormal amount of rainfall. Based on the ERA-interim daily reanalysis data from 1979 to 2019 and the Meiyu index dataset provided by the CMA (the China Meteorological Administration), the characteristics of the intraseasonal oscillation (ISO) in different latitudes during the Jiangnan Meiyu onset period were examined. During the Meiyu onset period, the analysis of 500 hPa geopotential height shows that the key regions of circulation anomaly include the Ural Mountains in mid-high latitudes and the Northwest Pacific in low latitudes. Moreover, the geopotential height anomaly of the two regions shows significant quasi-biweekly scale (10–30 days) characteristics. The diagnosis of quasi-biweekly geopotential tendency shows that height variation in the mid-high latitude key region mainly depends on the influence of temperature advection, while the variation in the low-latitude key region relies on vorticity advection. Over the mid-high latitudes, the height anomaly of the Ural Mountains gradually increases before the day of Meiyu onset, and the contribution of quasi-biweekly oscillation (QBWO) to the total anomaly is approximately 55.9%. The fluctuations from Europe and the Aleutians spread toward the key region, and the abnormal warm advection lies over the key region, contributing to formation of the Ural-blocking pattern. Meanwhile, in the key region located over the Western Pacific, the contribution of the quasi-biweekly component reaches 51.2%. The oscillations over the Western Pacific propagate southwestward along the East Asian coast, while fluctuations over the East Asian continent migrate southward. Throughout this period, the negative vorticity advection occupies the key region, which is conducive to both the positive geopotential height variation and maintenance of the Western Pacific Subtropical High. Thus, the migration of QBWO in different latitudes could be an extended-range signal of the Jiangnan Meiyu prediction.