Boreal Summer Intraseasonal Oscillation Research Articles

Multiscale drivers of catastrophic heavy rainfall event in early August 2022 in South Korea

On 8–11 August 2022, South Korea experienced a catastrophic heavy rainfall event (HRE) with 14 fatalities. To elucidate its driving mechanisms, the present study performs a multiscale analysis by hierarchically delineating the synoptic and large-scale characteristics of the HRE. Its synoptic condition was featured by the confrontation of the western North Pacific subtropical high and the continental cyclone in the north of the Korean Peninsula. At their interface, a tremendous amount of moisture was transported in an elongated shape (i.e., atmospheric river) along with strong frontogenetic activity. This provided a favorable environment for potential instability. The continental cyclone was maintained throughout the HRE period, while a transient cyclone was superposed contributing to more intense rainfall in the early stage of the HRE. This persistent cyclone in the north of the Korean Peninsula originated from a far-upstream-originated cutoff low that became a part of the quasi-stationary wave train along the Asian subtropical jet. A linear model experiment suggests that the quasi-stationary wave train was excited by the enhanced tropical convection related to the boreal summer intraseasonal oscillation. The anomalously strong subtropical jet also acted as an effective waveguide. These results suggest that the integration of synoptic and large-scale processes is essential to understand this unprecedented HRE.

Development of Indian summer monsoon precipitation biases in two seasonal forecasting systems and their response to large-scale drivers

Abstract. The Met Office Global Coupled Model (GC) and the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFSv2) are both widely used for predicting and simulating the Indian summer monsoon (ISM), and previous studies have demonstrated similarities in the biases in both systems at a range of timescales from weather forecasting to climate simulation. In this study, ISM biases are studied in seasonal forecasting setups of the two systems in order to provide insight into how they develop across timescales. Similarities are found in the development of the biases between the two systems, with an initial reduction in precipitation followed by a recovery associated with an increasingly cyclonic wind field to the north-east of India. However, this occurs on longer timescales in CFSv2, with a much stronger recovery followed by a second reduction associated with sea surface temperature (SST) biases so that the bias at longer lead times is of a similar magnitude to that in GC. In GC, the precipitation bias is almost fully developed within a lead time of just 8 d, suggesting that carrying out simulations with short time integrations may be sufficient for obtaining substantial insight into the biases in much longer simulations. The relationship between the precipitation and SST biases in GC seems to be more complex than in CFSv2 and differs between the early part of the monsoon season and the later part of the monsoon season. The relationship of the bias with large-scale drivers is also investigated, using the boreal summer intraseasonal oscillation (BSISO) index as a measure of whether the large-scale dynamics favour increasing, active, decreasing or break monsoon conditions. Both models simulate decreasing conditions the best and increasing conditions the worst, in agreement with previous studies and extending these previous results to include CFSv2 and multiple BSISO cycles.

Prediction of Diverse Boreal Summer Intraseasonal Oscillation in the GFDL SPEAR Model

Abstract Boreal summer intraseasonal oscillation (BSISO) is a primary source of predictability for summertime weather and climate on the subseasonal-to-seasonal (S2S) time scale. Using the GFDL SPEAR S2S prediction system, we evaluate the BSISO prediction skills based on 20-yr (2000–19) hindcast experiments with initializations from May to October. It is revealed that the overall BSISO prediction skill using all hindcasts reaches out to 22 days as measured by BSISO indices before the bivariate anomalous correlation coefficient (ACC) drops below 0.5. Results also show that the northeastward-propagating canonical BSISO (CB) event has a higher prediction skill than the northward dipole BSISO (DB) event (28 vs 23 days). This is attributed to CB’s more periodic nature, resulting in its longer persistence, while DB events are more episodic accompanied by a rapid demise after reaching maximum enhanced convection over the equatorial Indian Ocean. From a forecaster’s perspective, a precursory strong Kelvin wave component in the equatorial western Pacific signifies the subsequent development of a CB event, which is likely more predictable. Investigation of individual CB events shows a large interevent spread in terms of their prediction skills. For CB, the events with weaker and fluctuating amplitude during their lifetime have relatively lower prediction skills likely linked to their weaker convection–circulation coupling. Interestingly, the prediction skills of individual CB events tend to be relatively higher and less scattered during late summer (August–October) than those in early summer (May–July), suggestive of the seasonal modulation on the evolution and predictability of BSISO. Significance Statement The advance of subseasonal-to-seasonal (S2S) prediction largely depends on dynamical models’ ability to predict some major intrinsic modes in the climate system, including the boreal summer intraseasonal oscillation (BSISO). Using a newly developed S2S prediction system, we thoroughly evaluated its performance in predicting BSISO, and revealed the skill dependence on the BSISO propagation diversity. Here we provide physical explanations of what influences the BSISO predictions and identify different precursory signals for two types of BSISO, which have important implications for operational forecasts.

Impact of Arctic sea ice on the boreal summer intraseasonal oscillation

AbstractThis study investigates the relationship between sea ice concentration (SIC) in the Arctic Ocean and the Boreal Summer Intraseasonal Oscillation (BSISO) from 1991 to 2020 and its underlying mechanism. A significantly positive (negative) correlation was found between the frequency of phase 7 (3) of BSISO1 (30–60 d) and the preceding winter SIC, which is located the north of the East Siberian-Beaufort Sea (ESBS). Compared with low-SIC years, the conditions including northeasterly vertical wind shear, an enhanced ascending branch of the anomalous Walker circulation, an eastward water vapour transport channel, and an increased humidity gradient induce active convection over the Philippine Sea in high-SIC years, which benefits (hinders) to phase 7 (3) of BSISO1. The positive SIC anomaly during the transition from winter to spring influences local temperature and pressure through anomalous local sensible heat flux. This anomaly induces wave activity flux from the ESBS, which converges over the Bering Sea, enhancing the Aleutian Low (AL). Subsequently, the AL triggers an anomalous subtropical anticyclone through wave-mean flow interaction in the North Pacific. Due to southerly wind stress and increased sea surface heat flux, positive sea surface temperature anomalies near Japan persist in the summer, heating the lower troposphere and increasing baroclinicity. Significant positive geopotential heights and anticyclone anomalies occur over Japan, accompanied by a negative vorticity anomaly. The enhanced ascending motion over the Philippine Sea, facilitated by Ekman pumping, favours convection and influences the frequency of phases 7 and 3.

Marine Heatwaves in the East Asian Marginal Seas Facilitated by Boreal Summer Intraseasonal Oscillations

AbstractDuring 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.

Changes in tropical cyclone response to the Boreal Summer Intraseasonal Oscillation in the western North Pacific under global warming in EC-Earth3P-HR

EC-Earth3P-HR reproduces well the observed Boreal Summer Intraseasonal Oscillation (BSISO) and its impacts on tropical cyclone genesis (TCG) in the western North Pacific (WNP). Hence, the historical simulation (1950–1979) and future projection under the SSP5-8.5 scenario (2020–2049) in EC-Earth3P-HR are adopted to explore possible changes in the BSISO's modification of WNP TCG under global warming to enhance the understanding of TC activities in the WNP. Results show that the BSISO circulation in the WNP shifts northeastward under global warming. This leads to enhanced convection in a northwest–southeast-oriented band crossing the WNP. Along the band, the BSISO-related TCG anomalies are enhanced. Analyses of genesis potential index show that changes in the BSISO-related mid-tropospheric relative humidity play the dominant role in modifying the BSISO's impacts on WNP TCG under global warming. The enhanced BSISO convection in the band moistens the middle troposphere, which helps reduce the entrainment of generally dry mid-tropospheric air in the updrafts and the modification of the boundary layer by the downdraft of generally dry mid-tropospheric air, leading to enhanced TCG.摘要北半球夏季季内振荡 (BSISO) 对西北太平洋 (WNP) 热带气旋形成 (TCG) 有显著的影响, 并且为TC的次季节预报提供重要依据, 因此研究其在全球变暖下的变化有重要意义. EC-Earth3P-HR模式较好地再现了观测到的BSISO及其对TCG的影响. 因此, 采用EC-Earth3P-HR的历史模拟 (1950–1979) 和SSP5-8.5情景 (2020–2049) 下的未来预估, 探讨全球变暖下BSISO对WNP上TCG调制的可能变化. 结果表明, 在全球变暖影响下, WNP上的BSISO环流向东北方向移动, 表现为西北-东南方向分布的增强对流带. 在对流带上, BSISO相关的TCG异常增强. 成因潜力指数分析表明, 全球变暖背景下, 与BSISO相关的对流层中部相对湿度的变化对BSISO对TCG的调制起主导作用. 带内增强的BSISO对流使对流层中部空气变湿润, 这有利于减少上升气流对通常干燥的对流层空气的夹带以及下沉气流对边界层的修正, 从而导致TCG的增强.

Comparison of two filter approaches for characterizing boreal summer intraseasonal oscillation

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.

Joint Impacts of Intraseasonal Oscillation and Diurnal Cycle on East Asian Summer Monsoon Rainfall

Abstract Intraseasonal and diurnal variations are two basic periodic oscillations in global/regional climate and weather. To investigate their joint impacts over East Asia, this paper categorizes the boreal summer intraseasonal oscillations (ISOs) in 1998–2019 into two groups with different diurnal cycles. It is shown that the active ISOs with large diurnal cycles feature a northwestward-moving anomalous anticyclone with strong southerlies at the western flank. These ISOs have in-phase patterns of geopotential height anomaly between low and midlatitudes over East Asia, associated with the simultaneous expansions of the western Pacific subtropical high (WPSH) and South Asian high (SAH). They couple with the anomalous ABL heating by daytime solar radiation over East Asia, which acts to enhance monsoon southerlies at midnight. The nocturnally strengthened southerlies facilitate dynamic lifting, moisture transport, and convective instability for producing midnight to morning rainfall at their northern terminus, thereby yielding a remarkable northward propagation of the monsoon rain belt. In contrast, the other ISOs with small diurnal cycles are related to a westward-moving anomalous anticyclone, while the WPSH and SAH have relatively small expansions and the westerly trough is active at middle latitudes. They lead to the dipole patterns of geopotential height anomaly and weak ABL heating over East Asia. The daily-mean southerlies and moisture conditions as well as their nocturnal enhancements are relatively weak, and thus, the northward shift of the monsoon rain belt is less pronounced. These results highlight that the large-scale conditions of ISOs can be distinguished by their different couplings with regional-scale diurnal forcings, which help the understanding and prediction of multiscale rainfall activities.

Contrasting the Impacts of Intraseasonal Oscillations on Yangtze Precipitation during the Summer of 1998 and 2016

In 1998 and 2016, boreal summer intraseasonal oscillation (BSISO) could reach the middle-lower reaches of the Yangtze River basin (YRB), leading to extreme precipitation. Based on multiple daily data, this study reveals the differences in BSISO events and mechanisms between 1998 and 2016. In June–July of 1998 (2016), YRB precipitation was impacted by 30–60-day oscillation, i.e., BSISO1 (10–30-day oscillation, i.e., BSISO2), with two strong (three) precipitation events occurring. In 1998, when BSISO1 was in phases 1–4 (phases 5–8), the YRB experienced a wet (dry) episode. In 2016, when BSISO2 was in phases 1–2 and 7–8 (phases 3–6), the YRB experienced a wet (dry) episode. In 1998, in event 1, the active convection of the YRB first originated in the South China Sea–western Pacific (SCS–WP) and then in the tropical Indian Ocean (IO). In 1998, in event 2, the active convection of the YRB originated in the SCS–WP. In 2016, in events 1 and 3, the active convection of the YRB originated from the SCS–WP. In 2016, in event 2, the active convection of the YRB originated from the tropical IO and the extratropical WP. Different SST and atmospheric circulations explain different BSISO modes that dominate in the YRB. In 1998 (2016), in summer, (no) strong easterly wind anomalies occurred in the SCS–WP, which are favorable (unfavorable) for the enhancement of BSISO1. Accompanying the suppressed BSISO1, BSISO2 was enhanced in 2016.

Environmental Controls on the Tropical Island Diurnal Cycle in the Context of Intraseasonal Variability

Abstract The mechanisms regulating the relationship between the tropical island diurnal cycle and large-scale modes of tropical variability such as the boreal summer intraseasonal oscillation (BSISO) are explored in observations and an idealized model. Specifically, the local environmental conditions associated with diurnal cycle variability are explored. Using Luzon Island in the northern Philippines as an observational test case, a novel probabilistic framework is applied to improve the understanding of diurnal cycle variability. High-amplitude diurnal cycle days tend to occur with weak to moderate offshore low-level wind and near to above average column moisture in the local environment. The transition from the BSISO suppressed phase to the active phase is most likely to produce the wind and moisture conditions supportive of a substantial diurnal cycle over western Luzon and the South China Sea (SCS). Thus, the impact of the BSISO on the local diurnal cycle can be understood in terms of the change in the probability of favorable environmental conditions. Idealized high-resolution 3D Cloud Model 1 (CM1) simulations driven by base states derived from BSISO composite profiles are able to reproduce several important features of the observed diurnal cycle variability with BSISO phase, including the strong, land-based diurnal cycle and offshore propagation in the transition phases. Background wind appears to be the primary variable controlling the diurnal cycle response, but ambient moisture distinctly reduces precipitation strength in the suppressed BSISO phase and enhances it in the active phase.

Interplay between Boreal Summer Intraseasonal Oscillation and Southern Hemisphere Stratospheric Polar Vortex Warming

Abstract The boreal summer intraseasonal oscillation (BSISO) features more distinctive and complex propagation characteristics than its wintertime counterpart, the Madden–Julian oscillation (MJO). While the relationship between the MJO and the Arctic stratosphere during boreal winter has been widely documented, the linkage between the BSISO and the Antarctic stratosphere during austral winter has not been extensively discussed. Here, after identifying the Southern Hemisphere (SH) stratospheric polar vortex warming (SPVW) events, we reveal the bidirectional connection between BSISO and SPVW. Before onset of the SPVW events, the occurrence frequency and amplitude of BSISO phase 5 (P5) shows a significant increase. The most significant responses of the SH polar stratospheric temperature to the BSISO are found about 10 days after BSISO P5. Thus, to some extent, BSISO P5 can be regarded as a precursor to the SH SPVW event, which is attributed to the enhanced upward propagation and dissipation of planetary waves in the SH stratosphere induced by the BSISO P5. After onset of the SPVW events, a significant increase in the occurrence and amplitude of BSISO P6 is observed, corresponding to the enhanced convection over the South China Sea and southern Philippine Sea. Tropical upwelling associated with the strengthened Brewer–Dobson circulation (BDC) induced by the SPVW tends to result in unstable circumstances in the tropical upper troposphere. Then the high correlation between static stability at 150 hPa and outgoing longwave radiation (OLR) anomalies over the South China Sea and southern Philippine Sea provides robust evidence that the intensity of convective activity in the tropics can indeed be modulated by the variability in SH stratospheric polar vortex. Significance Statement The relationship between the boreal summer intraseasonal oscillation (BSISO) and the Southern Hemisphere (SH) polar stratosphere remains unclear. After selecting the stratospheric polar vortex warming (SPVW) events, we reveal the two-way connection between BSISO and SPVW. Before the SPVW, the significant increase in occurrence and amplitude of BSISO phase 5 (P5) suggests that BSISO P5 can be regarded as a precursor to the weakened polar vortex. After the SPVW, the significant increase in occurrence and amplitude of BSISO phase 6 (P6) establishes that environmental alteration in tropical upper troposphere induced by the SPVW provides a favorable condition for the growth of ensuing convective activity. The results here help us better understand the potential link between stratospheric polar vortex and tropical intraseasonal oscillation.

Evaluating and Improving the Models' Prediction Skills for the Relationship Between the Summer NWPSH in Different Months

AbstractCompared to interannual and interdecadal variations, inadequate attention has been given to intraseasonal changes in the summer northwest Pacific subtropical high (NWPSH). During 1981–2005, the north Indian Ocean (NIO) and NWPSH exhibit significant relationships from June to August, while the influences of the Boreal Summer Intraseasonal Oscillation 1 (BSISO1) in June and August differ from its impacts in July, resulting in a significant correlation between the NWPSH in June and August. The models capture NIO's roles but underestimate BSISO1's influences, leading to an underestimation (overestimation) of the relationship between NWPSH in June and August (adjacent two months). A dynamical‐statistical approach based on the close relationship between June and August NWPSH is proposed to improve models' prediction capabilities. Improved results show the mean correlation coefficients (root mean square errors) between prediction and observation for August precipitation and geopotential height increase by 57% and 21% (decrease by 0.27 mm/day and 1.61 m).

Propagation pathways of Indo-Pacific rainfall extremes are modulated by Pacific sea surface temperatures

Intraseasonal variation of rainfall extremes within boreal summer in the Indo-Pacific region is driven by the Boreal Summer Intraseasonal Oscillation (BSISO), a quasi-periodic north-eastward movement of convective precipitation from the Indian Ocean to the Western Pacific. Predicting the spatiotemporal location of the BSISO is essential for subseasonal prediction of rainfall extremes but still remains a major challenge due to insufficient understanding of its propagation pathway. Here, using unsupervised machine learning, we characterize how rainfall extremes travel within the region and reveal three distinct propagation modes: north-eastward, eastward-blocked, and quasi-stationary. We show that Pacific sea surface temperatures modulate BSISO propagation — with El Niño-like (La Niña-like) conditions favoring quasi-stationary (eastward-blocked) modes—by changing the background moist static energy via local overturning circulations. Finally, we demonstrate the potential for early warning of rainfall extremes in the region up to four weeks in advance.

Distinct changes in boreal summer intraseasonal oscillation over the western North Pacific under mega and equatorial La Niña conditions

Distinct changes in boreal summer intraseasonal oscillation over the western North Pacific under mega and equatorial La Niña conditions

The influence of 10–30-day boreal summer intraseasonal oscillation on the extended-range forecast skill of extreme rainfall over southern China

The influence of 10–30-day boreal summer intraseasonal oscillation on the extended-range forecast skill of extreme rainfall over southern China

Interannual Variability in the Boreal Summer Intraseasonal Oscillation Modulates the Meridional Migration of Western North Pacific Tropical Cyclone Genesis

Abstract Prior studies have emphasized the influence of the boreal summer intraseasonal oscillation (BSISO) on basin-scale and global-scale tropical cyclones (TCs). An improved understanding of the BSISO’s impact on TCs at various climate time scales will likely lead to improved subseasonal-to-seasonal prediction. This study explores the impact of BSISO interannual variability on western North Pacific (WNP) TCs. We find that interannual meridional variability in the BSISO modulates the meridional migration of WNP TC genesis and is related to changes in BSISO phase structure. These structural changes are characterized by occurrence frequency changes for individual BSISO phases between the equatorial region over the eastern Indian Ocean (EIO)–Maritime Continent (MC) and the subtropical region over the South China Sea (SCS)–western Pacific. This interannual north–south change of the BSISO appears to be associated with changes in sea surface temperature over the MC region and WNP mid-to-low-tropospheric moisture advection, mainly via remote forcing of a Gill-type Rossby response to BSISO convection and a large meridional asymmetry of the low-level background moisture distribution. During years with increased BSISO residence time over the subtropical SCS–western Pacific, more TCs occur over the northern WNP due to TC-favorable BSISO-associated convection and circulation. In contrast, more TCs occur over the southern WNP in years with increased BSISO convection residence time over the equatorial EIO–MC. We find that the increase in frequency of BSISO phase occurrence over the subtropical SCS–western Pacific since the late 1990s has potentially contributed to a recent poleward shift of WNP TC genesis.

The Role of the North Atlantic Blocking High during Large-Scale Heavy Rainfall Events over Central India

Abstract Large-scale extreme rainfall events (LEREs) over central India are produced by monsoon low pressure systems (LPSs) when assisted by a secondary cyclonic vortex (SCV). Both the LPS and the SCV are embedded in a monsoon trough and form mainly during the positive phase of the boreal summer intraseasonal oscillation. Here, we observe that tropical–extratropical interactions exist during LEREs. Using ray tracing, we show that extratropical Rossby waves propagate to the Indian subcontinent during the summer monsoon season. Stationary Rossby wave rays originating over the North Atlantic Ocean reach India following approximately a great circle path at midtropospheric levels. This pathway appears to play an important role in tropical–extratropical interactions during LEREs. Seventy-seven percent of LEREs are preceded by a North Atlantic blocking high and 90% by a quasi-stationary central Asian high. The Atlantic blocking high triggers a quasi-stationary Rossby wave response and strengthens the downstream central Asian high. In turn, the quasi-stationary central Asian high facilitates Rossby wave breaking, transporting high-PV streamers and cutoffs equatorward. The central Asian high is in close proximity to the monsoon trough in the mid- and lower troposphere. It interacts with the monsoon trough over the northwest Indian subcontinent. The equatorial monsoon trough is strengthened due to the supply of dynamic forcing and static instabilities from the extratropics. This additional forcing from the extratropics creates an environment that is conducive for LEREs.

Influence of three types of boreal summer intraseasonal oscillation on summer precipitation over the Yangtze River Valley

Previous studies have demonstrated that boreal summer intraseasonal oscillation (BSISO) active in the Indian Ocean can affect precipitation over the Yangtze River Valley (YRV). In this study, the impacts of three types of BSISO modes on precipitation over the YRV were investigated and found to vary significantly. First, moisture budget analysis indicated that the process responsible for the precipitation anomalies in all BSISO modes is moisture convergence, while moisture advection is insignificant. In the canonical mode, precipitation anomalies are negative (positive) at pentad −2 (−1 to 3) owing to a BSISO-related cyclone over the northwestern Pacific (anticyclone moving from the subtropical western to northwestern Pacific) accompanied by enhanced (suppressed) convection propagating northeastward. Comparatively, in the eastward expansion mode, precipitation anomalies are positive (negative) at pentad −2 to 0 (1 to 3) via a BSISO-related anticyclonic (cyclonic) circulation over the northwestern Pacific induced by the teleconnection effect of enhanced (suppressed) convection over the Indian Ocean. Different from the previous two BSISO modes, in the northward dipole mode, precipitation anomalies are negative (positive) at pentad −2 to 0 (1 to 3) owing to a BSISO-related cyclonic (anticyclonic) circulation, which moves from the South China Sea to the northwestern Pacific because of the propagation of enhanced (suppressed) convection.

Climatological characteristics of the monsoon breaks during the southwest monsoon season of the Philippines

AbstractThis 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.

Influence of boreal summer intraseasonal oscillation on summer precipitation in North China

AbstractTo understand the impact of the boreal summer intraseasonal oscillation (BSISO) propagation on summer precipitation in North China, this study analysed the quantitative relationship between the BSISO and summer precipitation in North China and its influence mechanism using summer precipitation data from 1981 to 2020, the NCEP/NCAR reanalysis data and the NOAA outgoing longwave radiation (OLR). The results show that there is a strong correlation between summer precipitation in North China and the BSISO. Over the past 40 years, the variance in summer precipitation anomalies in North China caused by BSISO1 accounted for 16.8%, whereas the impact of BSISO2 accounted for 13.1%. Their combined impact accounted for 30% of the variance in summer precipitation in North China. The BSISO influences summer precipitation process in North China primarily through upper tropospheric circulation and lower‐level water vapour transport. At 500 and 200 hPa, the BSISO excited some disturbance centres of a deep barotropic structure propagating from west to east along the upper‐tropospheric westerly jet. North China is controlled by the blocking circulation pattern of “high in the east and low in the west” while there is a negative (positive) anomaly near Lake Baikal and a positive (negative) anomaly near the Korean Peninsula. At 850 hPa, the BSISO stimulates an anticyclonic circulation anomaly on the north side of its convective area, and the southerly winds on the west side of the anticyclone enhance water vapour transport to North China. This high‐ and low‐level circulation configuration is favourable for precipitation processes in North China. The results suggest that the influence of both low‐frequency signals in the middle and high latitudes and the BSISO signal in the tropics should be considered to make extended‐range forecasts of precipitation processes in North China.