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The teleconnections of the two types of the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on autumn rainfall over Southeast Asia and its 20 subregions are investigated during 1979–2019. Under El Niño, autumn rainfall reduces over the West Philippines and most of the Maritime Continent, while Indochina experiences alternating dry and wet conditions. Under El Niño Modoki, more rainfall reduction is evident in Indochina, East Philippines, Malay Peninsula, and North Sumatra. Conversely, El Niño Modoki causes less dry conditions than El Niño in southern Southeast Asia. La Niña and La Niña Modoki tend to increase autumn rainfall, except in Sumatra and some specific areas in Indochina. However, La Niña Modoki causes notably more rainfall than La Niña in northern Southeast Asia, including Indochina (except Myanmar), the Malay Peninsula, and Philippines. Although insignificant, La Niña Modoki also reproduces more rainfall in eastern Indonesia, whereas displaying less rain in the central and western Maritime Continent. These distinctions of ENSO Modoki compared to ENSO are driven by different sea surface temperature anomalies patterns, leading to a more northward Walker circulation and the presence/strength of an anomalous Philippine Sea cyclone. Positive IOD (negative IOD) also generally results in drier (wetter) autumn, with the rainfall anomaly patterns of positive IOD/negative IOD exhibiting similarities to El Niño/La Niña. However, compared to both types of ENSO, IOD impacts are weaker over Pacific-facing subregions while only stronger in specific equatorial Indian Ocean-facing subregions. This stronger impact comes from reduced (intensified) moisture transport from the equatorial Indian Ocean to southwestern Southeast Asia under positive IOD (negative IOD). [ABSTRACT FROM AUTHOR]

The concept of compound hazards refers to the simultaneous occurrence or rapid succession of multiple hazards within a specific area. In this study, we identified compound meteorological hazards based on three types of individual hazards: high temperatures, heavy rainfall, and tropical cyclones (TCs). We computed the number of each type of individual hazards and their compounds at 10 selected stations along the coastline of Vietnam for the period 1980–2018. A temperature hazard (rainfall hazard) is identified if the maximum temperature (rainfall amount) on a given day exceeds the 95th percentile of all daily values recorded over the study period. A location is considered exposed to a TC hazard on a given day if the distance from that location to the TC center is less than 250 km. Subsequently, a compound hazard event is identified if the interval between the ending time of one individual hazard event and the starting time of the following one is less than 10 days. The results indicate that the annual number of compound hazards shows increasing trends at the majority of the stations, mainly due to the increase in temperature hazards. At the seasonal scale, compound hazards typically occur from March to November, primarily associated with temperature hazards in the first half of the year and with TCs and rainfall hazards in the later months. The identification and analysis of compound hazards in this study provide essential information for resilience and adaptation planning in Vietnam. [ABSTRACT FROM AUTHOR]

Remote influences on intraseasonal anomalous rainfall over regions that encompass North and South Vietnam are explored using a 38‐year (1979–2016) global dataset over the extended summer (May–October). Wet and Dry composites in filtered daily data with lags of up to 2 weeks are assembled for various rainfall indices over the two subregions, including atmospheric reanalysis products and in‐situ rainfall data. On the regional scale, the moisture flux convergence correlates well with reanalysed rainfall. The large‐scale dynamics associated with these composites are described. Rainfall composites of opposing signs show asymmetrical large‐scale precursors and different pathways of influence. Wet and Dry anomalies in North Vietnam are seen to originate from Europe and propagate at high latitudes. The exact nature of the precursors is sensitive to the definition of the composite index. There is also a pathway of influence along the Asian jet, which impacts South Vietnam, especially for Wet composites which often coincide with Dry conditions in the North. South Vietnam is also influenced by tropical divergent precursors, which are again asymmetric between Wet and Dry composites. [ABSTRACT FROM AUTHOR]

This paper investigates the process behind climate model biases, especially in rainfall, during the southwest (June to August; JJA) and northeast monsoon (December to February; DJF) seasons over the Philippines using the downscaled Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment-Southeast Asia (SEACLID/CORDEX-SEA). Biases in the surface and near-surface climate are assessed over land in eastern (EastPH) and western (WestPH) Philippines using observation and reanalysis datasets. Results show that models with well-represented sea surface temperature (SST) provided better added values after downscaling. Although the CMIP5-driven models tend to overestimate both surface energy and water partitioning, simulations with well-represented SST have better surface energy partitions. In contrast to the reference simulation driven by ERA-Interim reanalysis, the CMIP5-driven simulations tend to underestimate (overestimate) moisture flux convergence during the DJF (JJA) season, which might have contributed to the overall underestimation (overestimation) of rainfall in this season. Further investigation along a vertical transect shows the differences in governing mechanisms for each season. During DJF (JJA) season, the simulation with underestimated (overestimated) SST, near-surface air temperature, and mixing ratio results in underestimated (overestimated) rainfall over EastPH (WestPH). In addition, the CMIP5-driven models were found to overestimate low-level winds, which may have induced the deep convection regardless of season, suggesting the role of low-level winds in the convection scheme used in RegCM4 aside from topography and land cover. [ABSTRACT FROM AUTHOR]

This study investigates the impact of El Niño Southern Oscillation (ENSO) and ENSO Modoki on rainfall variability over Southeast Asia (SEA) with a focus on its twenty sub-regions. For the period 1979–2019, seven El Niño (EN), six La Niña (LN), five El Niño Modoki (EM), and five La Niña Modoki (LM) events were identified. In the boreal summer (JJA) and winter (DJF) of EN events, rainfall reduction occurred over the Maritime Continent and the Philippines except for West Philippines in JJA and West Kalimantan in DJF. Similar patterns but with enhanced drier/wetter conditions were detected during EM. During LN, rainfall increased in most sub-regions except for West Philippines and parts of mainland Indochina in JJA, and in DJF in some southern areas such as South Sumatra, West Kalimantan, and Papua. Compared to LN, LM generally exhibited less wet/drier conditions in JJA over most sub-regions and wetter conditions in DJF over a major part of the Maritime Continent. The decrease (increase) in rainfall over SEA during Modoki events compared to the canonical ENSO events was explained by a reduced (enhanced) moisture transport into the region and a weakening (strengthening) of the ascending branch of the Walker circulation. [ABSTRACT FROM AUTHOR]

This paper examines the projected changes in rainfall in Southeast Asia (SEA) in the twenty-first century based on the multi-model simulations of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment–Southeast Asia (SEACLID/CORDEX–SEA). A total of 11 General Circulation Models (GCMs) have been downscaled using 7 Regional Climate Models (RCMs) to a resolution of 25 km × 25 km over the SEA domain (89.5° E–146.5° E, 14.8° S–27.0° N) for two different representative concentration pathways (RCP) scenarios, RCP4.5 and RCP8.5. The 1976–2005 period is considered as the historical period for evaluating the changes in seasonal precipitation of December–January–February (DJF) and June–July–August (JJA) over future periods of the early (2011–2040), mid (2041–2070) and late twenty-first century (2071–2099). The ensemble mean shows a good reproduction of the SEA climatological mean spatial precipitation pattern with systematic wet biases, which originated largely from simulations using the RegCM4 model. Increases in mean rainfall (10–20%) are projected throughout the twenty-first century over Indochina and eastern Philippines during DJF while a drying tendency prevails over the Maritime Continent. For JJA, projections of both RCPs indicate reductions in mean rainfall (10–30%) over the Maritime Continent, particularly over the Indonesian region by mid and late twenty-first century. However, examination of individual member responses shows prominent inter-model variations, reflecting uncertainty in the projections. [ABSTRACT FROM AUTHOR]

This paper highlights detailed projected changes in rainfall over Thailand for the early (2011–2040), middle (2041–2070) and late (2071–2099) periods of the 21st century under the representative concentration pathways (RCP) 4.5 and RCP 8.5 using the high‐resolution multi‐model simulations of the Coordinated Regional Climate Downscaling Experiment (CORDEX) Southeast Asia. The ensemble mean is calculated based on seven members consisting of six general circulation models (GCMs) and three regional climate models (RCMs). Generally, the ensemble mean precipitation agrees reasonably well with observations, best represented by the Global Precipitation Climatology Center (GPCC) data, over Thailand during the historical period (1976–2005). However, inter‐model variations can be large among ensemble members especially during dry months (December to March) for northern‐central‐eastern parts, and throughout the year for the southern parts of Thailand. Similarly for future projection periods, inter‐model variations in the sign and magnitude of changes exist. The ensemble means of projected changes in rainfall for both RCPs during dry months show distinct contrast between the northern‐central‐eastern parts and the southern parts of Thailand with generally wetter and drier conditions, respectively. The magnitude of change can be as high as 15% of the historical period, which varies depending on the sub‐region, season, projection period, and RCP scenario. In contrast, generally drier conditions are projected during the wet season (June to September) throughout the country for both RCPs where the rainfall reduction can be as high as 10% in some areas. However, the magnitude of projected rainfall changes of some individual models can be much larger than the ensemble means, exceeding 40% in some cases. These projected changes are related to the changes in regional circulations associated with the winter and summer monsoons, which are projected to weaken. The drier (wetter) condition is associated with the enhanced subsidence (rising motion). [ABSTRACT FROM AUTHOR]

The daily rainfall data at 13 stations over the Central Highlands (CH) Vietnam were collected for the period 1981-2014. Two different sets of criteria using daily observed rainfall and 850 hPa daily reanalysis wind data were applied to determine the onset (retreat) dates of the summer rainy season (RS) and summer monsoon (SM) season, respectively. Over the study period, the mean RS and SM onset dates were April 20 and May 13 with standard deviations of 17.4 and 17.8 days, respectively. The mean RS and SM retreat dates were November 1 and September 30 with standard deviations of 17.9 and 10.2 days, respectively. The year-to-year variations of the onset dates and the rainfall amount within the RS and SM season were closely linked with the preceding winter and spring sea surface temperature in the central-eastern and western Pacific. It was also found that the onset dates were significantly correlated with the RS and SM rainfall amount. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, simulations over Southeast Asia (15°S-40°N, 80°-145°E) at 36 km resolution were conducted for the period 1989-2007 using the Regional Climate Model version 4.3 ( RegCM4.3) under the framework of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment - Southeast Asia (or SEACLID/ CORDEX- SEA) project. Forced by the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim), 18 experiments were carried out using different combinations of cumulus parameterization and ocean flux schemes. Twelve extreme indices for both rainfall and temperature were estimated from the model output. A statistical omega index was used to measure the degree of similarity among the 18 experiments in phase and shape. The results showed relatively high similarities among the experiments over mainland Asia compared to those over the Maritime Continent for both seasonal and inter-annual variability. The extreme rainfall indices had a lower omega compared to that of temperature. Observed daily rainfall and temperature data at 52 meteorological stations over the SEA region were used to validate the simulated extreme indices. The results showed that extreme temperature indices were generally underestimated across the region. Systematic biases for each simulated rainfall index were also identified. A score ranking system was established to compare the relative performance of the 18 experiments over the 52 selected stations objectively. It was shown that the experiments with the Massachusetts Institute of Technology ( MIT)-Emanuel scheme performed relatively better than the other convective schemes. The combination of the MIT-Emanuel convective scheme with the Biosphere-Atmosphere Transfer scheme ( BATS1e) ocean flux scheme produced the best performance. [ABSTRACT FROM AUTHOR]

The onset dates of rainy season over the eastern Indochina Peninsula (8.5°-23.5°N, 100°-110°E) are objectively determined for individual years from 1958 to 2007 using the empirical orthogonal function (EOF) analysis. On average, the onset of the summer rainy season (SRS) determined by EOF1 is 6 May, with a standard deviation of 13 days. The autumn rainy season (ARS) indicated by EOF2 has a mean onset and standard deviation of 16 September and 12 days, respectively. The SRS onset is characterized by the evolution of summer monsoon westerlies and the northward propagation of strong convection from the equatorial region. Conversely, the withdrawal of the summer monsoon over northeastern Indochina in late summer-early autumn favors the ARS onset. Both onsets are strongly associated with intraseasonal oscillation on 30-60- and 10-20-day time scales. Examination of the precursory signals associated with the early/late onsets of both SRS and ARS implies that ENSO has a significant impact on their year-to-year variations. In La Niña years, the subsequent SRS tends to have early onsets. Simultaneously, the western Pacific subtropical high (WPSH) weakens and retreats eastward earlier. In contrast, advanced ARS onset generally occurs during an El Niño-developing autumn with weakened equatorial easterlies and suppressed convection over the central Indian Ocean from the preceding summer, as evident in weakened Walker circulation. However, robust precursory signals in SST are observed only from midsummer (July-August). An earlier ARS onset is also associated with the development of an anomalous Philippine Sea anticyclone and a westward-extended WPSH from midsummer. However, no coherent correlation is found between the late onset and La Niña. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, the climatological summer monsoon onset in Vietnam was investigated by using the 25-year in situ observations and reanalysis gridded dataset recorded from 1979 to 2003. Results suggested that the onset date varies considerably among the sub-regions. Rainfall criteria indicate that the earliest onset generally occurs in the northwestern mountainous area around April 25. That in the northern and southern plains is in mid-May, along with the full establishment of monsoon circulation over the Indochina Peninsula. In the central coastal plain, however, the Foehn wind, an effect of the Asian summer monsoon, causes a dry summer rather than a rainy season. This peculiar characteristic differs substantially from the rainy season in the adjacent regions. Therefore, a new specific criterion for this region is proposed: The summer dry season onset date, which is subjectively indicated in late May, is defined as the appearance time of the dry, downslope Foehn wind. Monsoon onset in Vietnam and surrounding areas is preceded by the development of convective activity and circulation features over the equatorial Indian Ocean and the southern Bay of Bengal. It also coincides with the northward retreat of mid-latitude westerlies and the eastward retreat of the easterly trade wind associated with the western Pacific sub-tropical high. A relationship between the occurrence of the Meiyu front and the summer monsoon onset over the South China Sea ( SCS) is identified. Particularly, cyclonic development concurrent with monsoon onset appears over the SCS and Taiwan area, and a large amount of moisture is observed to converge along the southern side of the front. Simultaneously, the northward extension of the tropical easterly jet ( TEJ), which is driven by the reversed poleward temperature and pressure gradient as a consequence of warming over inland India relative to the surrounding ocean, also plays an important role in summer monsoon circulation. [ABSTRACT FROM AUTHOR]

The performance of the Global Satellite Mapping of Precipitation data Microwave-Infrared Combined Reanalysis Product (GSMaP RNL), version 6, was evaluated, using northern Vietnam as the test area. The Vietnam Gridded Precipitation (VnGP) Dataset was used for comparison purposes. Particular emphasis was placed on the investigation of heavy-rain days (precipitation over 50 mm day −1). Wind data from operational radiosonde observations at Hanoi were also used to examine the effect of interaction between wind and topography, on the GSMaP performance, and the basic relationship between the VnGP precipitation and lower tropospheric wind. Results showed that heavy-rain days generally occurred during May–August. Regions with higher and lower precipitation in the GSMaP, compared to the VnGP, were found to be distributed in a complex manner, at a scale similar to that of tributary basins. The GSMaP and the VnGP showed similar precipitation values when associated with westerly wind in the lower troposphere at Hanoi. Positive influences of westerly wind on estimated precipitation were observed along the northeastern foot of the mountain ranges, whereas negative influences were found along the southeastern foot. The results implied that the GSMaP rainfall estimation algorithm, taking account of orographic effects, may improve the accuracy of rainfall reproducibility, by using a more appropriate wind dataset to evaluate orographic convection. [ABSTRACT FROM AUTHOR]

This study examines the projected precipitation extremes for the end of 21st century (2081–2100) over Southeast Asia (SEA) using the output of the Southeast Asia Regional Climate Downscaling/Coordinated Regional Climate Downscaling Experiment – Southeast Asia (SEACLID/CORDEX-SEA). Eight ensemble members, representing a subset of archived CORDEX-SEA simulations at 25 km spatial resolution, were examined for emission scenarios of RCP4.5 and RCP8.5. The study utilised four different indicators of rainfall extreme, i.e. the annual/seasonal rainfall total (PRCPTOT), consecutive dry days (CDD), frequency of extremely heavy rainfall (R50mm) and annual/seasonal maximum of daily rainfall (RX1day). In general, changes in extreme indices are more pronounced and covering wider area under RCP8.5 than RCP4.5. The decrease in annual PRCPTOT is projected over most of SEA region, except for Myanmar and Northern Thailand, with magnitude as much as 20% (30%) under RCP4.5 (RCP8.5) scenario. The most significant and robust changes were noted in CDD, which is projected to increase by as much as 30% under RCP4.5 and 60% under RCP8.5, particularly over Maritime Continent (MC). The projected decrease in PRCPTOT over MC is significant and robust during June to August (JJA) and September to November (SON). During March to May (MAM) under RCP8.5, significant and robust PRCPTOT decreases are also projected over Indochina. The CDD changes during JJA and SON over MC are even higher, more robust and significant compared to the annual changes. At the same time, a wetting tendency is also projected over Indochina. The R50mm and RX1day are projected to increase, during all seasons with significant and robust signal of RX1day during JJA and SON. • Future changes of Southeast Asia precipitation extremes were examined. • The result was derived from the CORDEX-SEA simulations experiments. • The dry tendency is projected over the maritime continent. • Increasing extreme rainfall frequency and intensity over the mainland Indo-China. • Significant and robust change signals during JJA and SON. [ABSTRACT FROM AUTHOR]