Southwest Monsoon Region Research Articles

Comparison of Intra-Event Characteristics of Hydrogen and Oxygen Stable Isotopes between Rainfall and Throughfall and the Effects of Pre-Event Precipitation

The isotopic composition of precipitation provides valuable information about its source and transportation. However, raindrops interact with vegetation before reaching the earth’s surface, leading to isotopic changes in the infiltrating water. Comparing isotopic composition between rainfall and throughfall helps to understand canopy processes and their impact on isotopic variation. Based on observational data collected during the periods of July 2019, July–August 2020, and July–August 2021 in a planted forest located in the southwest monsoon region of China, this study examines hydrogen and oxygen isotopes in rainfall and throughfall at event and intra-event scales, and investigates the effects of pre-event precipitation (PEP) on the isotopic composition. The results indicate that during the initial stage of precipitation, δ18O was enriched in rainfall and it presented a dilution effect gradually, while the d-excess exhibits a low initial value followed by an increasing trend. The difference in δ18O between throughfall and rainfall initially increased and subsequently converged around 0, whereas the difference in d-excess experiences a decreasing phase, followed by an increasing phase, and finally a decreasing phase. Canopy interception led to a lag effect during the early stage of precipitation; the forest exhibited higher water vapor content compared to open land in the intermediate stage, which reduced the degree of non-equilibrium fractionation in throughfall, and the flow pathway enhanced in the later stage. Evaporation processes become more prominent as precipitation intensity weakens. The rainfall and throughfall were influenced by distinct meteorological factors in different precipitation events, and the role of the forest canopy varied across different precipitation periods. PEP was found to augment the intercept and slope of the linear relationship between the H-O isotopic composition of throughfall and rainfall. This pre-event effect also contributes to heightened fluctuations in the δ18O and d-excess values during subsequent precipitation events. The findings contribute to understanding water dynamics, vegetation interception, and mechanisms governing water input in forested areas during precipitation events, which provides valuable insights for analyzing factors influencing water movement in forest ecosystems.

Seasonal variations of cave dripwater hydrogeochemical parameters and δ13CDIC in the subtropical monsoon region and links to regional hydroclimate

Stalagmites are considered natural archives of climate proxies. However, under the combined effects of atmospheric circulation patterns, precipitation, and karst environments, drip hydrogeochemical processes can be coupled and linked to each other to control cave sediment record information. Therefore, the evolution of chemistry and factors controlling the isotopic composition of the dripwater during regional precipitation migration from the surface to caves need to be evaluated. In this study, hydrogeochemical characteristics and the isotopic composition of the dripwater in the Mahuang Cave in Guizhou Province, Southwest China, including stable isotope (δ13CDIC) and trace element ratios, were monitored from August 2018 to December 2020. The results showed seasonal variations in the δ13CDIC, Mg/Ca, and Sr/Ca values of the dripwater in dry and wet seasons under the control of water-gas-rock reactions, such as soil CO2 concentrations and carbonate rock dissolution. In addition, the five monitored dripwater points in the Mahuang Cave showed fast and slow seepage due to the complex cave fractures and stratigraphy, reflecting the effects of precipitation variations to different degrees. Indeed, the δ13CDIC were more sensitive to the recharge changes from extreme precipitation and drought events. Therefore, dripwater δ13CDIC is a reliable indicator of the recorded hydrological signal in the southwest monsoon region.

Correction to: Occurrence and environmental risks of nonsteroidal anti-inflammatory drugs in urban wastewater in the southwest monsoon region of India.

The original version of this article is unfortunately published online with missing acknowledgment section.

Occurrence and environmental risks of nonsteroidal anti-inflammatory drugs in urban wastewater in the southwest monsoon region of India.

Municipal wastewater treatment plants (MWWTPs) are considered to reduce the amount of pollutants that enter water reservoirs as a result of wastewater disposal. An assessment of the occurrence and removal of pharmaceutical compounds, mainly nonsteroidal anti-inflammatory drugs (NSAIDs), in wastewater from the Kavoor MWWTP (southwest monsoon region), India, is presented in this paper. The performance of the MWWTP was monitored in the summer (May) and monsoon (September) periods. The highest inlet concentrations of diclofenac, naproxen, ibuprofen, ketoprofen, and acetylsalicylic acid in the wastewater were observed in May and were 721.37, 2132.48, 2109.875, 2747.29, and 2213.36μg/L, respectively. The ketoprofen content was found to be higher than that of other NSAIDs in the influent in both seasons, whereas the diclofenac content was found to be the lowest. The removal efficiency (RE) of the target NSAIDs in the Kavoor secondary treatment plant varied from 81.82-98.92% during the summer season. During the monsoon season, the influent NSAID concentration level dropped, probably because of infiltration in old sewer pipes. In addition, a 100% RE was achieved for all the target NSAIDs in the wastewater of the MWWTP. The results showed that secondary treatment plants have the potential to remove NSAID compounds from municipal sewage with consistent performance. The environmental hazards caused by the accumulation of such compounds in water reservoirs are due to open discharge. The environmental risk levels of these compounds were also studied by the environmental risk assessment (ERA) using the European Agency for Evaluation of Medicines approach.

Holocene climatic fluctuations and periodic changes in the Asian southwest monsoon region

Climatic changes in the Asian southwest monsoon (ASWM) during the Holocene have become a topic of recent studies. It is important to understand the patterns and causes of Holocene climatic changes and their relationship with global changes. Based on the climate proxies and wavelet analysis of Lugu Lake in the ASWM region, the climatic fluctuations and periodic changes in the ASWM region during the Holocene have been reconstructed with a high-precision chronology. The results indicate the intensification of ASWM began to increase with Northern Hemisphere low-latitude solar insolation (LSI) and solar activity during the early Holocene, and gradually decreased during the late Holocene, exhibiting an apparent synchrony with numerous records of ASWM region. Meanwhile, an apparent 1000-a quasi-periodic signal is present in the environment proxies, and it demonstrates that the environmental change in the ASWM region has been driven mainly by LSI and solar activity.

Potential Value of Expert Elicitation for Determining Differential Credibility of Regional Climate Change Simulations: An Exercise with the NARCCAP co-PIs for the Southwest Monsoon Region of North America

Abstract In this brief article, we report the initial results of an expert elicitation with the co-PIs (regional climate modelers) of the North American Regional Climate Change Assessment Program regarding their evaluation of the relative quality of regional climate model simulations focusing on the subregion dominated by the North American monsoon (NAM). We assumed that an expert elicitation framework might reveal interesting beliefs and understanding that would be different from what would be obtained from calculating quantitative metrics associated with model quality. The simulations considered were of six regional climate models (RCMs) that used NCEP Reanalysis 2 as boundary conditions for the years 1980–2004. The domain covers most of North America and adjacent oceans. The seven participating regional modelers were asked to complete surveys on their background beliefs about model credibility and their judgments regarding the quality of the six models based on a series of plots of variables related to the NAM (e.g., temperature, winds, humidity, moisture flux, precipitation). The specific RCMs were not identified. We also compared the results of the expert elicitation with those obtained from using a series of metrics developed to evaluate a European collection of climate model simulations. The results proved to be quite different in the two cases. The results of this exercise proved very enlightening regarding regional modelers’ perceptions of model quality and their beliefs about how this information should or should not be used. Based on these pilot study results, we believe a more complete study is warranted.

Relationship between sub-cloud secondary evaporation and stable isotope in precipitation in different regions of China

Falling to the ground from the bottom of clouds, rain experiences evaporation process, which has a very important role for accurate analysis of isotope water cycle information and the improvement of isotope hydrology system. As an integral part of the water cycle research, this evaporation process gradually arise people’s attention. Based on the precipitation isotope data obtained from GNIP site or some observation and literature, the existence and influence factors of secondary evaporation effects are studied in different regions of China. The study areas are divided into four regions (northwest arid region, southeast monsoon region, southwest monsoon region, and the Tibetan Plateau region), and the results show that the four regions exist the secondary evaporation effect. The seasonal change is obvious in the northwest arid areas and the north of the southeast monsoon region, and it is strong in summer and weak in winter. In the northwest arid areas and the monsoon region in dry season, it has a strong secondary evaporation effect when rainfall is less, and there is no significant correlation between monsoon rainfall and the secondary evaporation effect during the rainy season. The relationship between temperature and secondary evaporation is not obvious in the north of the southeast monsoon region during rainy season, while the sub-cloud secondary evaporation is more intense in high temperatures in other areas. In addition to the northwest arid region and north of the southeast monsoon region in the dry season, the relationship between the secondary evaporation effect and the relative humidity is significant. In high relative humidity, the secondary evaporation effect gradually weakens with the increase in the relative humidity.

Accuracy of TRMM precipitation data in the southwest monsoon region of China

Accurate, high-resolution precipitation data is important for hydrological applications and water resource management, particularly within mountainous areas about which data is presently scarce. The goal of the this study was to assess the accuracy of TRMM 3B43 precipitation data from the southwest monsoon region of China between 1998 and 2011 based on the correlation coefficients, regression, and geostatistical methods. We found a strong correlation between TRMM 3B43 data and observational data obtained from meteorological stations, but the TRMM 3B43 precipitation data was consistently lower than that obtained from the weather stations. The TRMM 3B43 data was significantly different from the data obtained by weather stations located in the northwest and northeast regions of the Hengduan Mountains. The spatial distribution of precipitation obtained from TRMM 3B43 was also different from meteorological data, but the deviation was predominantly distributed along the northern longitude and southern latitude. In addition, the TRMM data more accurately reflected the regional precipitation patterns. Our results indicate that the TRMM 3B43 data should be used for hydrological applications and water resource management at meteorological stations that have a sparse and uneven distribution of observation stations in the southwest monsoon region.

Climatic Impacts and Reliability of Large Scale Wind Farms in Tamil Nadu

Objective: The main objective of this paper describes how the large scale windfarms affect the climate of south west monsoon region. Methods/Analysis: Method used for analysing climatic parameters of before and after installation of wind farms is Gaussian mixture model. ArcGIS and QGIS software is used for image and geo-information analysis. Data from the commercial wind turbine of south west monsoon region like temperature, relative humidity, precipitation, wind speed is used to find the climatic variation. Findings: Large scale wind farms significantly affect the various climatic parameters. These impacts depends on the static stability, increase or decrease in the climatic parameters. Conclusion/ Application: Improvements can be made by taking the ground temperature measured by satellite image and identify the warming effect of night and day time warming effect of large scale wind farm area of southwest monsoon regions.

Hydrologic Prediction over the Conterminous United States Using the National Multi-Model Ensemble

Abstract The authors analyzed the skill of monthly and seasonal soil moisture (SM) and runoff (RO) forecasts over the United States performed by driving the Variable Infiltration Capacity (VIC) hydrologic model with forcings derived from the National Multi-Model Ensemble hindcasts (NMME_VIC). The grand ensemble mean NMME_VIC forecasts were compared to ensemble streamflow prediction (ESP) forecasts derived from the VIC model forced by resampling of historical observations during the forecast period (ESP_VIC), using the same initial conditions as NMME_VIC. The forecast period is from 1982 to 2010, with the forecast initialized on 1 January, 1 April, 5 July, and 3 October. Overall, forecast skill is seasonally and regionally dependent. The authors found that 1) the skill of the grand ensemble mean NMME_VIC forecasts is comparable with that of the individual model that has the highest skill; 2) for all forecast initiation dates, the initial conditions play a dominant role in forecast skill at 1-month lead, and at longer lead times, forcings derived from NMME forecasts start to contribute to forecast skill; and 3) the initial conditions dominate contributions to skill for a dry climate regime that covers the western interior states for all seasons and the north-central part of the country for January. In this regime, the forecast skill for both methods is high even at 3-month lead. This regime has low mean precipitation and precipitation variations, and the influence of precipitation on SM and RO is weak. In contrast, a wet regime covers the region from the Gulf states to the Tennessee and Ohio Valleys for forecasts initialized in January and April, the Southwest monsoon region, the Southeast, and the East Coast in summer. In these dynamically active regions, where rainfall depends on the path of the moisture transport and atmospheric forcing, forecast skill is low. For this regime, the climate forecasts contribute to skill. Skillful precipitation forecasts after lead 1 have the potential to improve SM and RO forecast skill, but it was found that this mostly was not the case for the NMME models.

New evidence on deglacial climatic variability from an alpine lacustrine record in northwestern Yunnan Province, southwestern China

New deglacial pollen and conifer stoma records from Tiancai Lake, northwestern Yunnan Province, southwestern China, an alpine lake in the southwest monsoon region, are presented in this study. Based on these records, the lithology of core TCYL1, and PCA analysis of pollen data between ~21 and 11.5kaBP (calibrated 14C years), the deglacial vegetation and climate changes are discussed in detail. The results show that Tiancai Lake was above the upper limit of Picea/Abies forest (a treeline in the study area) between ~21 and 11.5kaBP, and the climate was colder and drier than that of the Holocene. During this period eight significant vegetation changes are recorded that are considered to be responses to changing temperatures and variations in the southwest monsoon in southwestern China. The Heinrich Event 1 (H1), the Bølling/Allerød warm period (BA) and the Younger Dryas cold event (YD) are all clearly detected in this record. In addition, this study finds that the initial late glacial warming in northwestern Yunnan Province was at ~18.7kaBP, which is coincident with the climate records in monsoonal Central Asia, the Indian Ocean, the tropical and subtropical Pacific Ocean, and Antarctica, and is a response to solar insolation changes. A noted temperature increase between 15.8 and 14.4kaBP occurred at the end of the H1 and before the BA, which indicates a strong pre-Bølling warming. Based on the study, we consider that the hypothesis about a slowdown of the ocean's thermohaline circulation is sufficient to explain these late glacial abrupt events.

Diatom response to climate forcing of a deep, alpine lake (Lugu Hu, Yunnan, SW China) during the Last Glacial Maximum and its implications for understanding regional monsoon variability

The Yunnan Plateau of southwest China is strongly influenced by the Asian monsoon and is a critical area in terms of determining its development and variability since the Last Glacial Maximum. High-resolution diatom analysis of a 14C-dated sediment core retrieved from Lugu Lake, Yunnan provides a detailed history of palaeoenvironment changes from 30,000 to 10,000 cal. yr BP. A model of climate–diatom interactions (linking thermal stratification, ice cover and catchment-derived nutrients) is proposed to explain the observed changes. Prior to 24,500 cal. yr BP the dominance of the planktonic Cyclotella rhomboideo-elliptica indicates a period of stable stratification and low nutrient levels, with a moderate change of temperature and precipitation in the southwest monsoon region. During the LGM (24,500–14,500 cal. yr BP), the loss of the planktonic diatoms and a switch to small Fragilaria spp. suggest an altered thermal regime in the lake (i.e. weaker stratification and possibly ice cover). From ∼18,000 cal. yr BP, the expansion of planktonic taxa (Stephanodiscus and the Asterionella formosa) and increased %TOC are indicative of increased nutrient availability reflecting altered vegetation and soil processes due to rising regional temperatures. After 14,500 cal. yr BP, a rapid reversal to dominance by planktonic diatoms indicates a further increase in temperature and continued development of catchment vegetation and soils. The abrupt changes in diatom species composition around 24,500 and 14,500 cal. yr BP suggest that the ecological status of the lake exceeded limnological thresholds. Interestingly, however, the Younger Dryas event is not clearly recognizable in either the diatom or pollen records in the Lugu Lake sediment record. In a partial redundancy analysis, 44.2% of the variability in the total diatom assemblages in the period 30,000–10,000 cal. yr BP was accounted for by changes in summer solar radiation. The relationships between the diatom assemblages and the C/N and pollen data are further evidence of climate–catchment–vegetation interactions and their impact on lake functioning. The composition of diatom assemblages in the Lugu Lake sediment core reflects direct and indirect (i.e. via catchment processes) climate forcing and in particular, the strength of the southwest monsoon, which in turn corresponds to orbitally-induced variability in Northern Hemisphere summer solar insolation from 30,000 cal. yr BP to 10,000 cal. yr BP.

Do Climate Forecast System (CFSv2) forecasts improve seasonal soil moisture prediction?

We investigated whether seasonal soil moisture forecasts derived from a land surface model forced by seasonal climate forecast model outputs are more skillful than benchmark forecasts derived from the same land surface model but with forcings taken from resampled climatological precipitation, temperature and low level winds. For most forecast leads and over the western United States, soil moisture forecasts based on seasonal climate forecasts are no more skillful than the benchmark. For relatively short (one month) leads, the climate model‐based forecasts are more skillful than the benchmark along a swath from the Gulf States to the Tennessee and Ohio Valleys and the Southwest monsoon region, where the climate model has skillful precipitation forecasts.

Seasonal variations of stable isotope in precipitation and moisture transport at Yushu, eastern Tibetan Plateau

Precipitation δ 18O at Yushu, eastern Tibetan Plateau, shows strong fluctuation and lack of clear seasonality. The seasonal pattern of precipitation stable isotope at Yushu is apparently different from either that of the southwest monsoon region to the south or that of the inland region to the north. This different seasonal pattern probably reflects the shift of different moisture sources. In this paper, we present the spatial comparison of the seasonal patterns of precipitation δ 18O, and calculate the moisture transport flux by using the NCAR/NCEP reanalysis data. This allows us to discuss the relation between moisture transport flux and precipitation δ 18O. This study shows that both the southwest monsoon from south and inland air mass transport from north affected the seasonal precipitation δ 18O at Yushu, eastern Tibetan Plateau. Southwest monsoon brings the main part of the moisture, but southwest transport flux is weaker than in the southern part of the Tibetan Plateau. However, contribution of the inland moisture from north or local evaporation moisture is enhanced. The combined effect is the strong fluctuation of summer precipitation δ 18O at Yushu and comparatively poor seasonality.

Origin of summer monsoon rainfall identified by δ18O in precipitation

A negative correlation between δ18O in monsoon precipitation andf, the ratio of precipitable water in monsoon region to that in water source area, is hypothesized. Using the Rayleigh model, a new method for identifying origin of summer monsoon rainfall is developed based on the hypothesis. In order to validate the method, the isotopic data at New Delhi, a typical station in the southwest monsoon region, and Hong Kong, a typical station in the southeast monsoon region, were collected and analyzed for case studies. The case studies indicate that the water source areas of the monsoon rainfall at the two stations identified by the method are accordant with the general atmosphere circulation patterns. The method developed in this paper is significantly important for tracing the origin of summer monsoon precipitation.

Satellite estimated water vapour in relation to Asian monsoon Circulation

Water vapour plays a crucial role in various exchange and transport processes in the atmosphere and its knowledge in the tropics is extremely important for input to various global circulation models. The vast oceans of earth's surface provide a large source of moisture and continuously modify the thermodynamics of the atmosphere through latent heat flux and condensational heating. In the tropics, especially in the Indian ocean the water vapour is highly heterogeneousin nature, and is one of the parameters which is responsible for cloud formation, associated with tropical systems like monsoon flows, depression, cyclones etc. The present paper reviews the various information’s available from deferent geostationary and polar orbiting satellites about water vapour affecting the southwest monsoon region around the Indian Ocean and Indian subcontinent.
 
 The temperature and moisture data from TIROS operational vertical sounder (TOVS) and INSAT-2E water vapour channel are examined to study water vapour distribution. Their usefulness in characterizing the Asian south-west (SW) monsoon circulation is focused. The Western Indian ocean showed an increase in mid-tropospheric moisture (700-500hPa) over about 8 to 10 days prior to the onset over Kerala coast. NOAA/TOVS layer tmperature and humidity is used to extrapolate the humidity profile at standard pressure levels. It is also used to compute latent and sensible heat flux. Total integrated water vapour from SSM/I is also used for estimating latent heat fluxes and for the diagnostics of NWP models. Recently, INSAT-2E water vapour channel was used to monitor the monsoon circiulation features. The new WV channel brought out clearly the feeding of various air masses, especially water vapour associated with monsoon onset and monsoon lows.

Prospects for harnessing wind-power economically in the sultanate of Oman

Wind power as a source of harnessible energy is assessed. Average data over the 10 year period, 1985–1994, have been compiled for 12 different widely spaced locations in Oman. It is concluded that the three most promising sites for the economic harnessing of wind-power are Thumrait (latitude 17.67°N and altitude 466.86 m above sea level), Masirah (latitude 20.67°N and altitude 18.80 m above sea level) and Sur (latitude 22.54°N, and altitude 13.7 m above sea level). These locations are situated in the Southwest monsoon region of influence. The largest wind speeds occur during the summer from June to August. The highest (i.e. ∼10 m s −1) long-term (i.e. over 10 years) monthly mean wind speed is that for Thumrait and ensues during the month of July.

Controls on precipitation distributions in Sri Lanka

The rainfall patterns and regions of the monsoonal island of Sri Lanka have been investigated. On the basis of quality-controlled monthly rainfall data from 110 gauges for the period 1931–60, three major homogeneous rainfall regions were derived based on a variety of rainfall parameters using cluster and discriminant analysis. For each of these regions, identified as the South-west Monsoon (SWM), the Northeast Monsoon (NEM) and the Inter-monsoon II (IMII) regions, multiple regression methods were used to determine the geographical factors influencing the spatial patterns of rainfall. For the NEM and IMII regions, the level of explanation as indicated by the coefficients of multiple determination were reasonable with values generally above 0.60. For the SWM region, the level of explanation was much lower at about 0.20. Examination of the residuals of the regression equations demonstrated the significance of local factors in influencing the patterns of rainfall distribution in Sri Lanka.

Satellite-determined cloudiness in the tropics in relation to large scale flow patterns Pt. II. Case-studies of developments outside the Indian southwest monsoon region

The paper contains the results of 4 case studies of satellite observations of clouds especially over se-areas in the tropics, in relation to monthly mean large-scale flow-patterns during the period June to September 1965. Three of the case-studies relate to the southern hemisphere in the region of westerlies and one to the northern hemisphere in the region of easterlies. These studies have shown that there are close associations between satellite-determined cloudiness and certain specifie types of flow patterns at the 700, 500 and 300-mb levels. The observed quasi-empirical relationships would be useful to the forecaster in his day-to-day predictions, especially for sea areas wherefrom he may not have any cloud or rainfall observations at the time of forecasting.