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Atmospheric mercury (Hg) cycling is sensitive to climate-driven changes, but links with various teleconnections remain unestablished. Here, we revealed the El Niño-Southern Oscillation (ENSO) influence on gaseous elemental mercury (GEM) concentrations recorded at a background station in East Asia using the Hilbert-Huang transform (HHT). The timing and magnitude of GEM intrinsic variations were clearly distinguished by ensemble empirical mode decomposition (EEMD), revealing the amplitude of the GEM concentration interannual variability (IAV) is greater than that for diurnal and seasonal variability. We show that changes in the annual cycle of GEM were modulated by significant IAVs at time scales of 2-7 years, highlighted by a robust GEM IAV-ENSO relationship of the associated intrinsic mode functions. With confirmation that ENSO modulates the GEM annual cycle, we then found that weaker GEM annual cycles may have resulted from El Niño-accelerated Hg evasion from the ocean. Furthermore, the relationship between ENSO and GEM is sensitive to extreme events (i.e., 2015-2016 El Niño), resulting in perturbation of the long-term trend and atmospheric Hg cycling. Future climate change will likely increase the number of extreme El Niño events and, hence, could alter atmospheric Hg cycling and influence the effectiveness evaluation of the Minamata Convention on Mercury.

In this study, we investigated the influences of stratospheric intrusion (SI) on the tropospheric ozone (O3) at Lulin Atmospheric Background Station (LABS, 2862 m MSL), a representative high-altitude site in East Asia. Thresholds for surface O3, carbon monoxide (CO), and relative humidity (RH) were set to identify SI events at LABS. Accordingly, 3.0% of the observation period was impacted by SI leading to a net O3 enhancement of 14.6 ± 9.6 ppb at LABS. Factors influencing the two most common SI event types affecting LABS, i.e. tropopause folds (major) and tropical cyclones (minor), were discussed. We utilized ERA5 reanalysis products (meteorology and O3) to trace and analyze the pathway and mechanism of these SI types over sub-tropical East Asia. With a conceptual model, we illustrate two driving circulations for SI air at ∼200 hPa that affect subtropical East Asia. SI air generated due to Rossby wave breaks are transported equatorward and downward into the troposphere. The southern westerlies passing south of the Tibetan Plateau can further push the SI air eastward. A SI event in 2007 with a significant O3 enhancement of 43.4 ppb caused by typhoon Nari was detected and discussed to depict the second SI type.

The outflow of East Asian haze (EAH) has gathered much attention in recent years. For downstream areas, it is meaningful to understand the impact of crucial upstream sources and the process analysis during transport. This study evaluated the impact of PM2.5 from the three biggest industrial regions in Asian continent: Bohai Rim industrial region (BRIR), Yangtze River Delta industrial region (YRDIR), and Pearl River Delta industrial region (PRDIR) on Taiwan and discussed the processes during transport with the help of air quality modeling. The simulation results revealed the contributions of monthly average PM2.5 from BRIR and YRDIR were 0.7∼1.1 µg m−3 and 1.2∼1.9 µg m−3 (∼5 % and 7.5 % of total concentration) on Taiwan, respectively in January 2017. When the Asian anticyclone moved from Asian continent to the West Pacific, e.g. on Jan 9th 2017, the contributions from BRIR and YRDIR to northern Taiwan could reach 6∼8 and 9∼12 µg m−3. The transport of EAH from BRIR and YRDIR to low latitude regions was horizontal advection (HADV), vertical advection (ZADV), and vertical diffusion (VDIF) over Bohai Sea and East China Sea. Over Taiwan Strait and northern South China Sea, cloud processes (CLDS) was the major production process of PM2.5 due to high relative humidity environment. Along the transport from high latitude regions to low latitude regions, Aerosol chemistry (AERO) and Dry deposition (DDEP) were the major removal processes. When the EAH intruded northern Taiwan, the major production processes of PM2.5 at northen Taiwan were HADV and AERO. The stronger the EAH was the easier the EAH could influence central and southern Taiwan. Although PRDIR was located at the downstream of Taiwan under northeast wind, the PM2.5 from PRDIR could transport upward above boundary layer and moved eastwards. When the PM2.5 plume moved overhead Taiwan, PM2.5 could transport downward via boundary layer mixing (VDIF) and further enhanced by the passing cold surge. In contrast, for the simulation of July 2017, the influence from three industrial regions was almost negligible unless there was special weather system like thermal lows, which may carried pollutants from PRDIR to Taiwan, but the occurrence was rare.

The transport of aerosols relies primarily upon air flow for conveyance; however, the air flow pattern is dominated by large-scale circulation conditions. One mission of the 2013 7-SEAS/BASELInE (Seven SouthEast Asian Studies/Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment) was to capture/confirm the downwind effect on the surface air quality due to the long-range transport of Southeast Asia biomass-burning (SEA BB) pollutants. This phenomenon was first discovered during the 2010 Dongsha experiment and directly observed by a lidar system at Hengchun in southern Taiwan during 7-SEAS/BASELInE. Through three-dimensional structural analysis, it was found that the sinking motion behind the upper-level active short wave trough is the major mechanism that enhances subsidence along the cold surge leading edge. In turn, the enhanced subsidence could bring the long-range transport of the SEA BB pollutants down to the surface. Furthermore, the HYSPLIT backward air trajectories helped identify the SEA BB pollutants in the mid-troposphere, while the fine-resolution WRF model simulation combined with dual-polarization lidar observations demonstrated the evolution of the brought-down aerosols process. An additional significant finding of this study is that the upper-level ridge-trough short wave within 20°–35°N was very active during spring 2013, highlighting the inter-annual variability of the long-range transport of SEA BB pollutants.

During March and April, widespread burning occurs across farmlands in Indochina in preparation for planting at the monsoon onset. The resultant aerosols impact the air quality downwind. In this study, we investigate the climatic aspect of the interannual variation of springtime biomass burning in Indochina and its correlation with air quality at Mt. Lulin in Taiwan using long‐term (2005–2015) satellite and global reanalysis data. Based on empirical orthogonal function (EOF) analysis, we find that the biomass burning activities vary with two geographical regions: northern Indochina (the primary EOF mode) and southern Indochina (the secondary EOF mode). We determine that the variation of biomass burning over northern Indochina is significantly related with the change in aerosol concentrations at Mt. Lulin. This occurs following the change in the so‐called India‐Burma Trough in the lower and middle troposphere. When the India‐Burma Trough is intensified, a stronger northwesterly wind (to the west of the trough) transports the dryer air from higher latitude into northern Indochina, and this promotes local biomass burning activities. The increase in upward motion to the east of the intensified India‐Burma Trough lifts the aerosols, which are transported toward Taiwan by the increased low‐level westerly jet. Further diagnoses revealed the connection between the India‐Burma Trough and the South Asian jet's wave train pattern as well as the previous winter's El Niño–Southern Oscillation phase. This information highlights the role of the India‐Burma Trough in modulating northern Indochina biomass burning and possibly predicting aerosol transport to East Asia on the interannual time scale., Key Points The springtime biomass burning activities in Indochina vary with two geographical regions: northern Indochina and southern IndochinaThe interannual variation of biomass burning over northern Indochina is closely related with the changes of PM10 at Mt. Lulin in TaiwanThe changes of the India‐Burma Trough are important to the modulation of the aerosol transport pattern from northern Indochina to Taiwan

Haze over east China has been extensively studied, but its impacts on downwind areas remain unclear. In this study, we applied long-term (9 yr) air quality, meteorological, and ground-based remote sensing data for investigating the transported haze events over Northern Taiwan in winter. Thirty-six haze events were identified using a statistical method and information pertaining to wind direction and episode duration. In contrast to haze events over China, the transported haze exhibits low relative humidity (approximately 70%) and high wind speed (approximately 5 m s−1) and is associated with a migrating high-pressure system. The mass concentration of fine particulate matter (PM2.5) for the haze events was 57.1 ± 13.6 μg m−3, nearly four times higher than that of the background (13.7 ± 7.4 μg m−3). Such high PM levels persisted for 120 h in winter. Back trajectory analysis suggests that the haze particles were transported from the east coast of China, particularly from the Yangtze River Delta (YRD) area with a traveling time of nearly 28 h. Lidar observations clarify that the Chinese haze was transported to Northern Taiwan and accompanied by low clouds and a temperature inversion layer, which confined the particles to a height of less than 1 km. Moreover, a relatively high PM2.5 concentration (65.0 μg m−3) was observed when air mass stagnated over YRD before advancing to Taiwan. This scenario, most frequently seen in the winter of 2013, resulted in the highest historical PM2.5 concentration (approximately 77.2 μg m−3). We propose that the heavy haze year is primarily attributable to synoptic weather variability and less to climate.

The Community Multi-scale Air Quality Model (CMAQ) is used to simulate the long-range transport of biomass burning (BB) pollutants from Southeast Asia (SEA) towards the Taiwan Central Mountain Range (CMR) in March and April 2010. The results show that a proportion of the BB plume was blocked and compressed at the windward side of CMR. High-altitude BB plume is shown to influence air quality on the ground via three mechanisms: (1) the subsidence in the anticyclone, (2) the downward motion in the cold surge, and (3) the vertical mixing of the boundary layer over land. Two case studies are further investigated to probe the chemical evolution of the air parcel approaching Mt. Lulin. The first case shows that the third mechanism also explained the increase in the concentrations of peroxyacyl nitrate (PAN), higher peroxyacyl nitrate (PANX), NH3, SO2, and volatile organic compounds in the BB plume when entering the land over western Taiwan. Meanwhile, the percentage of NO3– in the plume is also significantly increased. The second case reveals that valley wind transported air pollutants from the ground to the mountains. The air parcel, accompanied with considerable concentrations of PAN, PANX, SULF, and anthropogenic secondary organic aerosol, moved up Mt. Lulin. The pollutant concentrations, except for elemental carbon, in the air parcel decreased on approach to Mt. Lulin because the air parcel was mixed with a clean air.

This study aimed to simulate the transport of biomass burning (BB) aerosol originating from Southeast Asia (SEA) during the Dongsha Experiment conducted from March 2010 to April 2010. Transport pathways were reanalyzed and steering flow in the mid-latitude areas and anticyclones in low-latitude areas were found to control the transport of BB plume after it was injected to a high atmosphere. For the 12 simulated and observed events at Mt. Lulin (2862 m MSL; 23°28′07″ N, 120°52′25″ E), the 72 h backward trajectories were all tracked back to southern China and northern Indochina, which were the locations of the largest BB fire activities in SEA. Chemical evolutions of BB pollutants along the moving trajectories showed that organic matter was always the dominant component in PM2.5, consistent with the observations at both near-source regions and Mt. Lulin. For nitrogen species, nearly all NOx molecules oxidized into HNO3, NO3−, PAN, and PANX in fires or near fires. The synchronic consumption of NOx, SO2, and NH3 explained the production of the major components of inorganic salts. In the moving BB plume, sulfate concentration increased with decreased nitrate concentration. Ratios of ammonium to PM2.5 and elemental carbon to PM2.5 remained nearly constant because additional sources were lacking.

The objective of this study is to understand the influence of regional climate change on local meteorological conditions and their subsequent effect on local ozone (O3) dispersion in Taiwan. The 33-year NCEP-DOE Reanalysis 2 (NNR2) data set (1979–2011) was analyzed to understand the variations in regional-scale atmospheric conditions in East Asia and the western North Pacific. To save computational processing time, two scenarios representative of past (1979–86) and current (2004–11) atmospheric conditions were selected but only targeting the autumn season (September, October and November) when the O3 concentrations were at high levels. Numerical simulations were performed using weather research and forecasting (WRF) model and Community Multiscale Air Quality (CMAQ) model for the past and current scenarios individually but only for the month of October because of limited computational resources. Analysis of NNR2 data exhibited increased air temperature, weakened Asian continental anticyclone, enhanced northeasterly monsoonal flow, and a deepened low-pressure system forming near Taiwan. With enhanced evaporation from oceans along with a deepened low-pressure system, precipitation amounts increased in Taiwan in the current scenario. As demonstrated in the WRF simulation, the land surface physical process responded to the enhanced precipitation resulting in damper soil conditions, and reduced ground temperatures that in turn restricted the development of boundary layer height. The weakened land–sea breeze flow was simulated in the current scenario. With reduced dispersion capability, air pollutants would tend to accumulate near the emission source leading to a degradation of air quality in this region. The conditions would be even worse in southwestern Taiwan due to the fact that stagnant wind fields would occur more frequently in the current scenario. On the other hand, in northern Taiwan, the simulated O3 concentrations are lower during the day in the current scenario due to the enhanced cloud conditions and reduced solar radiation.

Environmental conditions for the roughly three million people living in the Taipei basin of Taiwan are greatly affected by the land–sea breeze and afternoon thunderstorm activities. A new perspective on the land–sea breeze life cycle and how it is affected by afternoon thunderstorm activity in the Taipei basin during the dry season is provided. During the summer monsoon break–revival phase, about 75% of rainfall in the Taipei basin is produced by afternoon thunderstorms triggered by sea-breeze interactions with the mountains to the south of this basin. Because the basic characteristics of the land–sea breeze and the changes it undergoes through the influence of afternoon thunderstorms have not been comprehensively analyzed/documented, a mini–field experiment was conducted during the summers of 2004 and 2005 to explore these aspects of the land–sea breeze in this basin. Thunderstorm rainfall is found to change not only the basin’s land–sea-breeze life cycle, but also its ventilation mechanism. On the nonthunderstorm day, the sea breeze supplies the open-sea fresh air for about 8 h during the daytime, but the land breeze persists on the thunderstorm day from afternoon to the next morning, acting to sweep polluted urban air out of the basin.

Climatologically, the East Asian high merges with the westward expansion of the northwestern Pacific subtropical high in March. The local East–West cell/circulation is enhanced by a well-organized convergent center that forms over Indochina at 925 h Pa in March. This allows the emitted air pollutants from the biomass burning to be uplifted to the free troposphere and immediately transported downwind to the east. Using the data collected from the 2010 7-SEAS/Dongsha Experiment, six cases of long-range transport events involving air pollutants during March–April were identified at the Hengchun air quality monitoring station in southern Taiwan. These events were related to active burning phases over Indochina Peninsula. The air pollutants produced by these events were transported to Taiwan after a 2–3 day journey. A composite analysis for these identified six cases showed that the boundary layer of the southwesterly flow confluence coupled with a well-organized convergent center located over a thermal low under clear skies over the Indochina Peninsula may induce a distinct ascending motion to form the upward branch of the transient local East–West cell/circulation during the burning phase. This inferred upward motion together with the strong thermal buoyancy created by the active biomass burnings could carry the air pollutants to the lower free troposphere where they would effectively be conveyed downwind along the westerly flow above 850 h Pa level. The air pollutants were brought down to the surface by downward branch of the transient local East–West cell/circulation, which was induced by the subsidence of a cold surge anticyclone. Using continuous point sources, the six identified cases were simulated with the Hybrid Single-Particle Lagrangian-Integrated Trajectory (HYSPLIT) particle dispersion forward model to confirm our supposition regarding the mechanism for the long-range transport of Southeast Asian biomass burning pollutants that has a significant impact on the surface air quality of the downstream areas, particularly in Taiwan.

By modulating the Earth-atmosphere energy, hydrological and biogeochemical cycles, and affecting regional-to-global weather and climate, biomass burning is recognized as one of the major factors affecting the global carbon cycle. However, few comprehensive and wide-ranging experiments have been conducted to characterize biomass-burning pollutants in Southeast Asia (SEA) or assess their regional impact on meteorology, the hydrological cycle, the radiative budget, or climate change. Recently, BASE-ASIA (Biomass-burning Aerosols in South-East Asia: Smoke Impact Assessment) and the 7-SEAS (7-South-East Asian Studies)/Dongsha Experiment were conducted during the spring seasons of 2006 and 2010 in northern SEA, respectively, to characterize the chemical, physical, and radiative properties of biomass-burning emissions near the source regions, and assess their effects. This paper provides an overview of results from these two campaigns and related studies collected in this special issue, entitled “Observation, modeling and impact studies of biomass burning and pollution in the SE Asian Environment”. This volume includes 28 papers, which provide a synopsis of the experiments, regional weather/climate, chemical characterization of biomass-burning aerosols and related pollutants in source and sink regions, the spatial distribution of air toxics (atmospheric mercury and dioxins) in source and remote areas, a characterization of aerosol physical, optical, and radiative properties, as well as modeling and impact studies. These studies, taken together, provide the first relatively complete dataset of aerosol chemistry and physical observations conducted in the source/sink region in the northern SEA, with particular emphasis on the marine boundary layer and lower free troposphere (LFT). The data, analysis and modeling included in these papers advance our present knowledge of source characterization of biomass-burning pollutants near the source regions as well as the physical and chemical processes along transport pathways. In addition, we raise key questions to be addressed by a coming deployment during springtime 2013 in northern SEA, named 7-SEAS/BASELInE (Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment). This campaign will include a synergistic approach for further exploring many key atmospheric processes (e.g., complex aerosol–cloud interactions) and impacts of biomass burning on the surface–atmosphere energy budgets during the lifecycles of biomass-burning emissions.

Heavy rainfall/flood (HRF) cyclones contribute close to two-thirds of the total rainfall PT in both parts of Malaysia [peninsular Malaysia (M) and west Borneo (B)]. Judging by the rainfall variance produced by these cyclones and its correlation (~0.9) with the interannual PT variation, this variation is caused primarily by HRF cyclones through two factors: 1) their westward propagation properties and 2) their rain-producing efficiency. The former is regulated by the change of the cyclonic shear flow around the near-equator trough, while the latter is determined by the change of the convergence of water vapor flux toward tropical Southeast Asia. During November–December of cold (warm) ENSO phases, the westward propagation of the cyclone's parent cold surge vortices (CSVs) from the Philippine (P) vicinity (Borneo) to peninsular Malaysia CSVPMs (CSVBMs) and intensified (weakened) convergence of water vapor flux toward tropical South/Southeast Asia act to enhance (reduce) the rain-producing efficiency of HRFPM (HRFBM) cyclones. During winter cold (warm) phases, the deepening (filling) of the near-equator trough crossing west Borneo allows some CSVs formed/trapped in Borneo CSVBBs to develop into HRFBB (HRFBBM) cyclones (to propagate westward to peninsular Malaysia). The rain-producing efficiency of HRFBB and HRFBBM cyclones is also increased (reduced) by the intensified (weakened) convergence of water flux toward tropical South/Southeast Asia. Interannual variations of both PT(M) and PT(B) caused by the impacts of the circulation pattern changes on occurrences of HRFPM and HRFBB/HRFBBM cyclones, respectively, and their rain-producing efficiency may pose a new challenge to simulate the weather–climate relationship in climate modeling.

Malaysia is geographically separated into Peninsular Malaysia and west Borneo. The rainfall maximum in the former region occurs during November–December, whereas that in the latter region occurs during December–February. This difference of maximum rainfall period indicates that the formation mechanism is different for the rainfall centers in these two parts of Malaysia. Since rainfall is primarily produced by severe weather systems, the formation of a climatological rainfall center is explored through synoptic activity and the rainfall amount of this center is estimated through contributions by rain-producing disturbances. The major cause of the rainfall maximum of Peninsular Malaysia is cold surge vortices (CSVs) and heavy rainfall/flood (HRF) events propagating from the Philippine area and Borneo. In contrast, the major cause of the rainfall maximum of Borneo is these rain-producing disturbances trapped in Borneo. Disturbances of the former group are formed by the cold surge flows of the Philippine Sea type, whereas disturbances of the latter group are formed by cold surge flows of the South China Sea (SCS) type. The population of HRF events is about one-fourth of the rain-producing disturbances in both Peninsular Malaysia and Borneo, but they produce less than ~60% rainfall for these two regions. It is revealed from the synoptic and dynamic analyses that the major Borneo rain-producing disturbances propagate westward before December by strong tropical easterlies, but they are trapped after December by strong northeasterlies of the SCS-type cold surge flow.

The formations of heavy rainfall/flood (HRF) events in Vietnam are studied from diagnostic analyses of 31 events during the period 1979–2009. HRF events develop from the cold surge vortices formed around the Philippines. These vortices’ speed, size, and rainfall, which evolve into HRF events, are enhanced distinguishably from non-HRF vortices, as they reach Vietnam. The HRF cyclone, the North Pacific anticyclone, and the northwestern Pacific explosive cyclone simultaneously reach their maximum intensities when the HRF event occurs. An HRF cyclone attains its maximum intensity by the in-phase constructive interference of three monsoon (30–60, 12–24, and 5 days) modes identified by the spectral analysis of zonal winds. The rainfall center of an HRF event is formed and maintained by the in-phase constructive interference of rainfall and convergence of water vapor flux anomalies, respectively, from three monsoon modes. Forecast times of regional models are dependent and constrained on the scale of the limited domain. For 5-day forecasts, a global or at least a hemispheric model is necessary. Using the salient features described above, a 5-day forecast advisory is introduced to supplement forecasts of HRF events made by the global model. Non-HRF vortices are filtered by threshold values for the deepening rate of explosive cyclones and basic characteristics of the HRF events predicted by the global model. A necessary condition for an HRF event is the in-phase superposition of the three monsoon modes. One-week forecasts for 12 HRF events issued by the NCEP Global Forecast System are tested. Results demonstrate the feasibility of the forecast advisory to predict the occurrence dates of HRF events.

The heavy rainfall/flood (HRF) event in central Vietnam usually occurs in October–November, the maximum rainfall season. This rainfall maximum undergoes a distinct interannual variation, opposite the interannual variation of sea surface temperature (SST) anomalies averaged over the NOAA Niño-3.4 area—ΔSST(Niño-3.4)—but coincident with the intensification (weakening) of the low-level easterlies at 15°N and westerlies at 5°N. The changes of low-level zonal winds reflect the strengthening (weakening) of the tropical cyclonic shear flow in tropical South/Southeast Asia in response to the tropical Pacific SST anomalies. Because the rainfall maximum in central Vietnam is primarily produced by the HRF cyclone, the interannual rainfall variation in this region should be attributed to the HRF cyclone activity—a new perspective of the climate change in precipitation. On average, one HRF cyclone occurs in each cold late fall. The population of the HRF cyclone may not be an important factor causing the interannual rainfall variation in central Vietnam. During the cold late fall, the rain-producing efficiency of the individual HRF cyclone is statistically almost twice those during warm and normal late falls and the most crucial factor leading to the interannual rainfall variation in central Vietnam. It is shown by further hydrological analysis that the increase (decrease) of the HRF cyclone’s rain-producing efficiency is determined by the large-scale environmental flow through the enhancement (weakening) of the regional convergence of water vapor flux.

Major rainfall (≥60%) in the northern part of the South China Sea (between North Vietnam and Taiwan) during May–June (the mei-yu season—the first phase of the Southeast–East Asian monsoon) is produced by rainstorms originating over the northern Vietnam–southwestern China region and the northern part of the South China Sea. As observed in this study, the occurrence frequency of rainstorms and rainfall contribution by these rainstorms undergoes a distinct interannual variation, in-phase with those of monsoon westerlies in northern Indochina and sea surface temperature (SST) anomalies over the NOAA Niño-3.4 region ΔSST (Niño-3.4). This in-phase relationship between monsoon westerlies and the ΔSST (Niño-3.4) anomalies is a result of the filling (deepening) of the subtropical Asian continental thermal low in response to the ΔSST (Niño-3.4) warm (cold) anomalies. Accompanied with this response is a slight southward (northward) shift of the North Pacific convergence zone (NPCZ), which extends from southern China to the North Pacific east of Japan. Thus, a favorable environment that meets the Charney–Stern instability criterion in initiating rainstorm genesis is enhanced (suppressed) by the intensification (weakening) of the monsoon shear flow formed by the midtropospheric northwesterly flow around the northeast periphery of the Tibetan Plateau and the monsoon westerlies. The meridional shift of the NPCZ established an elongated anomalous convergence (divergence) zone of water vapor flux along rainstorm tracks to increase (reduce) the rain-producing efficiency of rainstorms. Consequently, this interannual rainfall variation between northern Vietnam and Taiwan is primarily caused by rainstorm genesis and rain-producing efficiency.

The twenty nine years (1979-2007) rain-gauge based gridded precipitation data generated by the Asian Precipitation−Highly Resolved Observational Data Integration Towards Evaluation of water resources (APHRODITE) are used to depict the rainfall climatology in Vietnam. The rain gauge observations of 163 stations in Vietnam for year 2007 are employed to validate the analysis results of the APHRODITE precipitation and to verify two distinct rainfall regimes: the October-November regime in central Vietnam and the May- October regime in the northern and southern part of this country identified with APHRODITE data. It appears that the Truong Son Range along the western border of Vietnam with Laos and Cambodia provides a natural separation of the October-November rainfall regime in central Vietnam from others. The interannual variation of the October-November rainfall regime can be well depicted by a principal mode obtained from the empirical orthogonal function analysis on the 29-year-APHRODITE precipitation. Time variation of this interannnual mode is out-of-phase with the SST(NINO3.4) index. It is inferred from this negative correlated relationship that central Vietnam is drier (wetter) when the SSTs over the NINO3.4 region is warmer (colder). It is found from the water vapor transport analysis that an anomalous cyclonic (anticyclonic) circulation over south Asia is paired with an anomalous anticylonic (cyclonic) circulation over the western north Pacific during cold (warm) episodes. Water vapor is converged (diverged) by these two anomalous circulations toward (out of ) the South China Sea and Philippine Sea west of 150°E during cold (warm) years. In turn, the anomalous cyclonic (anticyclonic) circulation in South Asia enhances (reduces) the water vapor supply to Indochina, particularly to Vietnam. Coupling with this pair of anomalous circulations, water vapor is converged (diverged) by the anomalous divergent circulation, coupling with the aforementioned pair of anomalous circulation, toward (out of ) Southeast Asia to maintain excessive (deficient) rainfall during cold (warm) episodes. Evidently, the response of the divergent circulation to the tropical Pacific SST anomalies contributes to the interannual variation of the October-November rainfall in central Vietnam.

Intense southwest monsoon (SWM) rainfall events causing massive landslides and flash floods along the western sections of the Philippines were studied. These rainfall events, are not directly coming from the tropical cyclones (TCs) for they are situated far north to northeast of Luzon Island. The heavy rainfall is hypothesized as caused by the interaction of strong westerlies with the mountain ranges along the west coast of Luzon that produces strong vertical motion and consequently generates heavy rainfall. Four of heavy SWM rainfall cases were examined to determine how the presence and position of tropical cyclones in the Philippine vicinity affect these SWM rainfall events; three cases with TC of varying positions within the Philippine area of responsibility (PAR) and the fourth case without TC. Using a spatial Fourier decomposition approach, the total streamfunction is decomposed into two flow regimes: monsoon basic flow (Waves 0-1) and tropical cyclone perturbation flow (Waves 2-23) over a domain of (20°E-140°W, 5°S-35°N). The purpose of this flow decomposition is to determine the latter’s effect on or contribution to the monsoon activity. The analysis utilized the NCEP Final (FNL) data with 1° long. × 1° lat. resolution. Results show that the tropical cyclones over the Pacific Ocean located northeast of Luzon generate strong southwesterly winds over the west coast of Luzon. These in addition to the southwesterlies from the basic flow strengthened the southwest winds that interact with the high Cordillera Mountain ranges

Stretched from Indochina, across the South China Sea, to the Philippine Sea, a monsoon cyclonic shear flow was formed by easterlies of the cold surge-like flow in the north and monsoon westerlies in the south before the onset of the tropical Southeast Asian monsoon on 12 May 2008. On this date, two named tropical cyclones (Halong and Matmo) evolved with a 12-h lag from a closed vortex adjacent to the coast of central Vietnam and another closed vortex near Palawan Island (Philippines) within this shear flow. These two cyclones, named the twin Philippine tropical cyclones, moved almost on the same track, along the anomalous shear line (departure from the climatological one) across the Philippines, and turned northeastward to the ocean south of Japan. It was revealed from synoptic analysis that the cold surge-like flow was coupled with the midlatitude eastward-propagating short wave in northeast Asia, and part of the monsoon westerlies were fed by the cross-equatorial flow, the downstream flow of easterlies around the northern rim of the Southern Hemisphere subtropical high. The environment favorable for the formation of the twin cyclones was developed from the tropics–midlatitude interaction between synoptic systems in these two latitudinal zones. Formations of these cyclones were a result of drastic spinups of the two closed vortices (within the monsoon shear flow) following the surge of monsoon westerlies, which coincided with those of easterlies of the cold surge-like flow, and the cross-equatorial flow originating from easterlies between the Southern Hemisphere subtropical high and the Southern Hemisphere shear flow.

Typhoon passages across Taiwan can generate sudden surface warming in downslope regions. Special characteristics and mechanisms for 54 such warming events that were identified during the 1961–2007 period are examined. Preferred warming regions were identified in northwest Taiwan, where warming is generated by downslope flow from east or northeast winds in westward-moving typhoons, and in southeast Taiwan, where it is generated by downslope flow from west or northwest winds in northwestward-moving typhoons. In addition to the orographic effect, warmings occurred exclusively within nonprecipitation zones of typhoons. Most northwest (southeast) warmings occur during the day (night) with an average lifetime of 4 (5) h, which roughly corresponds to the average time a nonprecipitation zone remains over a station. During the period examined, three typhoons generated warming events in both northwest and southeast Taiwan, and only Typhoon Haitang (2005) generated warmings with comparable magnitudes (∼12-K increase) in both regions. For Typhoon Haitang as an example, diagnostic analyses with two different approaches reveal that the majority of the warming is contributed by downslope adiabatic warming, but the warming associated with the passage of a nonprecipitation zone is not negligible. Similar results were found when these two diagnostic approaches were applied to the other warming events. The diurnal mode of the atmospheric divergent circulation over East Asia–western North Pacific undergoes a clockwise rotation. The vorticity tendency generated by this diurnal divergent circulation through vortex stretching may modulate the arrival time of typhoons to cause daily (nighttime) warming in the northwest (southeast).

An afternoon heavy rainfall event in northern Taiwan, observed on June 20, 2000 during the monsoon break, is investigated using surface observation and Doppler radar data and a nonhydrostatic model with a horizontal grid spacing of 1.33 km. Heavy rainfall was brought majorly by two precipitation systems, namely A and B. System A was initiated and developed in Taipei Basin, associated with a local wind convergence line. System B was formed on the western slopes south of Taipei Basin, extended northward of the Taipei Basin, and lasted for 4 h. The formation and maintenance mechanisms of the two systems are examined using sensitivity experiments in which the terrain of Central Mountain Range (CMR) or Yangmin Mountain (YM) located north of the Taipei Basin is removed or reduced. In addition, the effect of cold outflow from System A on System B is examined.The wind convergence line responsible for forming and maintaining System A is brought from the interaction between the prevailing southwesterly winds and the easterly/northeasterly flow caused by the terrain effect of CMR. Furthermore, the northeasterly onshore flow and the low-level northwesterly/northerly flow resulting from the prevailing southwesterly winds turning southward over southern YM, enhance System A inside Taipei Basin. System B is produced over relatively high sloped areas (elevations exceeding 200 m) in southwestern Taipei Basin by the prevailing southwesterly winds. Over relatively low lands (elevations less than 200 m), the cold outflow from System A inside Taipei Basin converges with the prevailing southwesterly winds and helps the maintenance of System B.

The life cycle of the Southeast Asian–western North Pacific monsoon circulation is established by the northward migrations of the monsoon trough and the western Pacific subtropical anticyclone, and is reflected by the intraseasonal variations of monsoon westerlies and trade easterlies in the form of an east–west seesaw oscillation. In this paper, an effort is made to disclose the influence of this monsoon circulation on tropical cyclone tracks during its different phases using composite charts of large-scale circulation for certain types of tracks. A majority of straight-moving (recurving) tropical cyclones appear during weak (strong) monsoon westerlies and strong (weak) trade easterlies. The monsoon conditions associated with straight-moving tropical cyclones are linked to the intensified subtropical anticyclone, while that associated with recurving tropical cyclones is coupled with the deepened monsoon trough. The relationship between genesis locations and track characteristics is evolved from the intraseasonal variation of the monsoon circulation reflected by the east–west oscillation of monsoon westerlies and trade easterlies. Composite circulation differences between the flows associated with the two types of tropical cyclone tracks show a vertically uniform short wave train along the North Pacific rim, as portrayed by the Pacific–Japan oscillation. During the extreme phases of the monsoon life cycle, the anomalous circulation pattern east of Taiwan resembles this anomalous short wave train. A vorticity budget analysis of the strong monsoon condition reveals a vorticity tendency dipole with a positive zone to the north and a negative zone to the south of the deepened monsoon trough. This meridional juxtaposition of vorticity tendency propagates the monsoon trough northward. The interaction of a tropical cyclone with the monsoon trough intensifies the north–south juxtaposition of the vorticity tendency and deflects the tropical cyclone northward. In contrast, during weak monsoon conditions, the interaction between a tropical cyclone and the subtropical high results in a northwestward motion steered by the intensified trade easterlies. The accurate prediction of the monsoon trough and the subtropical anticyclone variations coupled with the monsoon life cycle may help to improve the forecasting of tropical cyclone tracks.

It has been observed that the percentage of tropical cyclones originating from easterly waves is much higher in the North Atlantic (∼60%) than in the western North Pacific (10%–20%). This disparity between the two ocean basins exists because the majority (71%) of tropical cyclogeneses in the western North Pacific occur in the favorable synoptic environments evolved from monsoon gyres. Because the North Atlantic does not have a monsoon trough similar to the western North Pacific that stimulates monsoon gyre formation, a much larger portion of tropical cyclogeneses than in the western North Pacific are caused directly by easterly waves. This study also analyzed the percentage of easterly waves that form tropical cyclones in the western North Pacific. By carefully separating easterly waves from the lower-tropospheric disturbances generated by upper-level vortices that originate from the tropical upper-tropospheric trough (TUTT), it is observed that 25% of easterly waves form tropical cyclones in this region. Because TUTT-induced lower-tropospheric disturbances often become embedded in the trade easterlies and resemble easterly waves, they have likely been mistakenly identified as easterly waves. Inclusion of these “false” easterly waves in the “true” easterly wave population would result in an underestimation of the percentage of easterly waves that form tropical cyclones, because the TUTT-induced disturbances rarely stimulate tropical cyclogenesis. However, an analysis of monsoon gyre formation mechanisms over the western North Pacific reveals that 82% of monsoon gyres develop through a monsoon trough–easterly wave interaction. Thus, it can be inferred that 58% (i.e., 82% × 71%) of tropical cyclones in this region are an indirect result of easterly waves. Including the percentage of tropical cyclones that form directly from easterly waves (∼25%), it is found that tropical cyclones formed directly and indirectly from easterly waves account for over 80% of tropical cyclogeneses in the western North Pacific. This is more than the percentage that has been documented by previous studies in the North Atlantic.

Located in northern Taiwan, Taipei is a metropolis surrounded by hills and mountains that form a basin in which two river valleys funnel the surface airflow of this basin to the open sea. Because of the southwest monsoon, summer is a dry season in northern Taiwan but is the season of maximum rainfall in the Taipei basin. This unusual summer rainfall maximum in Taipei is largely produced by afternoon/evening thunderstorms—in particular, on the downwind side and slopes of mountains south of the city. The population in the city of Taipei and the county in which this city is located has more than tripled during the past four decades while land use for building and surface construction increased by a factor of 3. This urbanization may contribute to an increase of 1.5°C in daily mean temperature, a decrease of 1°C in daily temperature range, an increase of more than 67% in the frequency of afternoon/evening thunderstorms, and an increase of 77% in rainfall generated by thunderstorms. These findings may explain the reduction in the water supply deficit to the Taipei metropolitan area and the ground subsidence of the Taipei basin caused by the excessive use of groundwater. Results of this study also provide important information for urban planning and pollution control and for management of the increasing traffic hazards caused by the enhanced thunderstorm activity and rainfall.

An effort was made to search for relationships between interannual variations of population, lifetime, genesis locations, and intensity of named typhoons and numbered tropical depressions in the western North Pacific during the 1979–2002 period. To support this research task, climatological relationships of tropical cyclone characteristics were also investigated for these cyclones. Major findings of this study are summarized as follows:Climatology: Measured by the intensity scale of the Japan Meteorological Agency, three groups of tropical cyclones were identified in terms of population versus intensity: Group 1 [tropical depression (TD) + typhoon (TY)], Group 2 (strong + very strong TY), and Group 3 (catastrophic TY). This group division coincides with that formed in terms of lifetime of tropical cyclones versus intensity. Weak cyclones (Group 1) have a larger population than strong cyclones (Group 3), while the former group has shorter lifetime than the latter group. For genesis locations, the monsoon trough is established as a favorable region of tropical cyclone genesis because it provides an environment of large vorticity. Therefore, the northward latitudinal displacement of the maximum genesis frequency in the three groups of tropical cyclones follows that of the monsoon trough.Interannual variation: Any mechanism that can modulate the location and intensity of the monsoon trough affects the genesis location and frequency of tropical cyclones. In response to tropical Pacific sea surface temperature anomalies, a short wave train consisting of east–west oriented cells emanates from the Tropics and progresses along the western North Pacific rim. Population of the Group-1 tropical cyclones varies interannually in phase with the oscillation of the anomalous circulation cell northeast of Taiwan and south of Japan in this short wave train, while that of Group 3 fluctuates coherently with the tropical cell of this short wave train. Because these two anomalous circulation cells exhibit opposite polarity, the out-of-phase interannual oscillation between these two cells results in the opposite interannual variation of genesis frequency between tropical cyclones of Groups 1 and 3.

As revealed from the interannual variation of outgoing longwave radiation in the western Pacific, deep cumulus convection along the Meiyu-Baiu front and ITCZ is modulated by the anomalous summer circulation in the following manner: when the sea surface temperatures on the eastern tropical Pacific are anomalously warm (cold), cumulus convection is enhanced (suppressed) along the equator east of 150° E and along the Meiyu-Baiu front, but is suppressed (enhanced) along the equator west of 150° E and along a longitudinal zone (10° N–30° N) extending from the northern section of the South China Sea to the International Dateline. Since tropical deep cumulus convection exhibits a pronounced diurnal variation, the diurnal convection cycle in the western Pacific may undergo an interannual variation coherent with that of deep tropical cumulus convection. This inference is substantiated by our analysis of the diurnal convection cycle for 1980–1993 with 3-hour equivalent black-body temperature observed by the Japanese Geostationary Meteorological Satellite (GMS). As expected, the diurnal convection cycle in the western Pacific is subjected to an interannual variation in accordance with deep cumulus convection along the Meiyu-Baiu front and ITCZ. Except along the equator east of 150° E, the diurnal convection cycle does not exhibit a drastic interannual change in phase.

The south-southeast periphery of a monsoon gyre in the western North Pacific (WNP) is a favorable region for tropical cyclone/tropical depression (TC/TD) genesis. The TC genesis frequency is interannually modulated by the WNP monsoon circulation in response to a change in tropical Pacific sea surface temperature (SST). These findings from previous studies lead to the hypothesis that the effect of tropical Pacific SST changes on the WNP TC/TD genesis frequency is accomplished through a modulation of the monsoon gyre activity by WNP monsoon circulation variations. The 6-h TC/TD track records and NCEP–NCAR reanalysis data for the period of 1979–2002 were analyzed to test this hypothesis. Results show that roughly 70% of WNP TC/TD geneses are linked to monsoon gyres. The interannual variation of these geneses is highly correlated (with a correlation coefficient of 0.89) with that of monsoon gyre activity, which is out-of-phase with the interannual variation of SST over the NOAA Nino-3 region.

Twelve years (1985–96) of monthly house fire reports for 22 districts in Taiwan, a maritime subtropical island of east Asia, were analyzed to characterize its fire nature. The major effort focused on the identification of temporal variation signals and their possible links with meteorological variables. Two significant modes of house fires were identified: annual and diurnal. As revealed from the power spectral analyses of fire time series in every fire district, a pronounced annual cycle peak emerges, with a peak phase in December and a minimum phase in June. In contrast to the warm and dry summer fire season of three continental landmasses (i.e., the U.S. West, the Northwest Territories of Canada, and the large wildland of Australia), an active fire season appears during the cool, dry winter in Taiwan. The fires on this island are highly correlated with several hydrometeorological variables; a decrease (increase) in rainfall in the dry (wet) cool (warm) environment with strong (weak) winds faci...

Rainfall variation of the East Asian summer monsoon has long been believed to be caused by the transition of weather regimes in company with the evolution of monsoon circulation. However, this claim was neither comprehensively analyzed nor convincingly demonstrated. Four datasets [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall, NCEP–NCAR reanalysis data, Japan Meteorological Agency (JMA) 6-h surface analysis maps, and Joint Typhoon Warning Center (JTWC) 6-h tropical cyclone tracks] were used to depict the climatology of the monsoon rainfall variation and to explore the cause of this variation, particularly through the relationship between the seasonal evolution of the monsoon circulation and the associated synoptic disturbance activity. The monsoon life cycle in the southern part of East Asia is basically developed by the sequential passages of the mei-yu rainband in early summer, the western Pacific subtropical high in midsummer, and the tropical cyclone activit...

Since a cold surge is a hazardous weather phenomenon in east Asia, the rapid population increase and economic growth over the past two decades require improvement in forecasting cold surges and their related weather events over this region. However, without a better understanding of these events, this task cannot be accomplished. A cold surge with a well-defined cold front passing through Taiwan was selected to illustrate its impact on the east Asian weather system. This case is typical of a large portion of surges occurring in the region. Major findings of this study are as follows. Coupling with the upper ridge–trough structures of the wave train straddling the eastern seaboard of northeast Asia, cold surges occur sequentially. A cold front with a prefront high pressure zone is formed by the new surge outflow interacting with the anticyclone of the aging surge. The warm moist air advected northeastward along the cold front assists the development of the new surge's low center, while the prefron...

The midocean trough in the North Pacific may form a favorable environment for the genesis of some synoptic disturbances. In contrast, the North Pacific anticyclone may hinder the downward penetration of these disturbances into the lower troposphere and prevent the moisture supply to these disturbances from the lower troposphere. Because no thick clouds, rainfall, and destructive surface winds are associated with these disturbances to attract attention, they have not been analyzed or documented. Actually, the upper-level wind speed within these disturbances is sometimes as strong as tropical cyclones and has the possibility of causing air traffic hazards in the western subtropic Pacific. With infrared images of the Japanese Geostationary Meteorological Satellite and the NCEP–NCAR reanalysis data, 25 North Pacific disturbances were identified over six summers (1993–98). Two aspects of these disturbances were explored: spatial structure and basic dynamics. For their structure, the disturbances possess a well-organized vortex in the middle to upper troposphere with a descending dry/cold core encircled by the moist ascending air around the vortex periphery; the secondary circulation of the vortex is opposite to other types of synoptic disturbances. Since vorticity reaches maximum values along the midocean trough line, barotrophic instability is suggested as a likely genesis mechanism of the vortex. After the vortex is formed, the horizontal advection of total vorticity results in its westward propagation, while the secondary circulation hinders this movement. Along its westward moving course, close to East Asia, there is a reduction in vortex size and a tangential speed increase inversely proportional to the vortex size. Diminishing its horizontal convergence/descending motion by the upper-tropospheric East Asian high and the lower-tropospheric monsoon low, the vortex eventually dissipates along the East Asian coast.

The midocean trough in the North Pacific may form a favorable environment for the genesis of some synoptic disturbances. In contrast, the North Pacific anticyclone may hinder the downward penetration of these disturbances into the lower troposphere and prevent the moisture supply to these disturbances from the lower troposphere. Because no thick clouds, rainfall, and destructive surface winds are associated with these disturbances to attract attention, they have not been analyzed or documented. Actually, the upper–level wind speed within these disturbances is sometimes as strong as tropical cyclones and has the possibility of causing air traffic hazards in the western subtropic Pacific. With infrared images of the Japanese Geostationary Meteorological Satellite and the NCEP–NCAR reanalysis data, 25 North Pacific disturbances were identified over six summers (1993–98). Two aspects of these disturbances were explored: spatial structure and basic dynamics. For their structure, the disturbances posse...

A vertical phase shift of the diurnal harmonic of geopotential height S1(Z) in the lower troposphere of low latitudes is shown by a comparison of diurnal harmonic of surface pressure at two surface stations that have an elevation difference close to 4 km. It is inferred from longitude–height cross sections of S1(Z) that this vertical phase shift is caused by the differential response of the lower and mid–upper troposphere to the diurnal variation of radiative heating/cooling.

The summer monsoons in East and Southeast Asia are characterized, respectively, by the Mei-yu (in eastern China)–Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. As indicated by the 850-mb zonal wind and cumulus convection over some key areas, a distinct opposite-phase intraseasonal variation exists between the two monsoon elements. Two approaches are adopted to explore the cause of this opposite-phase variation (which reflects the coupling between the two monsoon components): 1) the correlation coefficient patterns between the 850-mb zonal-wind monsoon index and the 850-mb streamfunction field and 2) the composite 850-mb streamline charts and the 120°E zonal-wind cross sections. It is shown that the opposite-phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward-migrating 30–60-day monsoon tr...

Two different approaches [empirical orthogonal function (EOF) analysis and rainfall regime identification] were applied to analyse the seasonal variation in Taiwan rainfall using 15 coastal stations administrated by the Central Weather Bureau of Taiwan, along the coastal zone of this island during the period 1981–1997. The major findings are as follows: Using the EOF analysis, the major seasonal variations in Taiwan rainfall may be shown as a seasonal seesaw in rainfall between the southern and northern Taiwan modes following the alternation of the two East Asian monsoon components. The former mode is associated with the summer southwest monsoon, while the latter with the winter northeast monsoon. Four (spring, summer, summer–fall and fall–winter) rainfall regimes are identified by the second approach: the four regimes cover northwest, southwest, east, and northeast Taiwan, respectively. The southern Taiwan mode is primarily formed by the second rainfall regime, while the northern Taiwan mode by the fourth. The seasonal variation in Taiwan rainfall is formed by the transition of these four rainfall regimes; this is reflected by a counter-clockwise rotation in these rainfall regimes around the island. Copyright © 2000 Royal Meteorological Society

Under a weather hazard prevention program in Taiwan, the Vaisala Meteorological Instrument, Inc., of Finland and the Environmental Science and Engineering Corporation of Taiwan jointly developed and installed more than 300 stations of the Automatic Rainfall and Meteorological Telemetry System (ARMTS) over this subtropical island with an area of approximately 30 000 km2. The Taiwan ARMTS network can provide the hourly rainfall data of not only a densely instrumented, but also very uniquely observed, geographically complex island embedded in the east Asian monsoon environment. The hourly ARMTS rainfall for the period 1993–96 supplemented with the hourly surface winds and pressure observed by 25 conventional surface stations around this island are analyzed to understand three aspects of the diurnal and seasonal variations of Taiwan rainfall. The preliminary results of the analysis are presented in this paper. 1) The maximum daily rainfall occurs at about 1600 ∼1700 LST, which is about 2–3 h behind t...