Indian Monsoon Activity Research Articles

Influences of Spring Land Surface Thermal Anomalies over West Asia on Indian Early Summer Monsoon Activity and Its Pathway

Abstract Exploring the premonsoonal land thermal predictor of the Indian summer monsoon is a hot topic under the background of global warming, and West Asia is one of the regions with the most significant warming in spring. In this study, we investigated the impact of anomalous spring land surface warming over West Asia on early summer (June) Indian monsoon precipitation as well as its possible mechanisms based on statistical analysis and numerical simulations. It has been found that spring land surface anomalous warming over West Asia corresponds to the enhancement of the leading mode of early summer precipitation in the Indian subcontinent, especially in its northern part. Further analysis indicates that an anomalously warm land surface over West Asia can advance the transition of atmospheric conditions toward the warm season by heating the atmosphere above. The increased land–sea meridional thermal contrast favors the intensification of the low-level jet and monsoon trough, further inducing anomalous moisture convergence and ascending motion over northern India. Additionally, the heat-driven anomalous upper-tropospheric anticyclone over West Asia favors the intensification of the tropical easterly jet and the northwestward development of the South Asian high (SAH). The enhanced SAH dynamically couples with the lower- to middle-level cyclonic circulation over northern India, resulting in a stronger monsoon and increased precipitation. These findings are helpful for better understanding and prediction of Indian early summer monsoon. Significance Statement The land surface thermal condition is critical to the monsoon activity and exploring the premonsoonal land thermal predictor of Indian summer monsoon remains a hot topic. The purpose of this study is to explore how spring land surface thermal anomalies over West Asia impact Indian monsoon activity in early summer (June). The anomalous land surface warming over West Asia can lead to a stronger Indian monsoon in early summer by heating and driving the atmosphere, which benefits the precipitation increase over northern India. Our results provide a further scientific basis for the prediction of early summer Indian precipitation.

A climatological-dynamical analysis of tropopause folds over Southwest Asia in the period of 1989–2018

Tropopause folding is believed to be connected to the development of cyclonic systems in midlatitudes as well as the larger-scale, lower-frequency processes in the lower latitudes. To investigate such connections, a statistical and dynamical analysis of tropopause folds occurred in Southwest Asia is carried out for the period of 1989–2018. Tropopause folds are identified using the Era-Interim reanalysis dataset from the European Centre for Medium-Range Weather Forecasts. The spatial–temporal distributions of tropopause folding show that the frequency of folding is higher in the winter hemisphere than in the summer hemisphere and greater values of the frequency are found mostly within an elongated zonal band which is situated between 20 and 40 degrees latitude. Southwest Asia also has strong positive anomalous values of tropopause folding frequency and achieves the values more than five times the average of the Northern Hemisphere in the summer, especially in the northern latitudes of this region including Afghanistan, Iran and the eastern Mediterranean. This can be attributed mainly to the formation of summer monsoon in the subtropical latitudes of the Indian Ocean. In this regard, the examination of the effects of El Niño and La Niña on the spatial–temporal distributions of tropopause folding reveals that the values of 3-month running average of tropopause folding frequency coincident with La Niña tend to be larger than those with El Nino in both the hemispheres and Southwest Asia. This result is consistent with the previous findings on the tendency for higher occurrence of anticyclonic (cyclonic) baroclinic wave life cycles during La Niña (El Niño). Dynamically, the folds occurred in the study area are associated with the formation of waves at 500 hPa level and subsequently the creation of surface cyclones. In areas of high frequency of folding events, strong meridional shear of the zonal winds and jet stream amplification are observed at 200 hPa level. The analysis of wave activity flux and its divergence for the selected month of June 2015 shows two regions of wave propagation, one with a wave-packet like signature of flux divergence/convergence centers in the upper troposphere and one over the lower troposphere. The upper-tropospheric wave packet ends with an area of flux convergence related with the receipt and decay of Rossby waves, while in the lower troposphere, a flux divergence region as a source of stationary waves associated with the Indian monsoon activity is identified.

Measurement report: Altitudinal variation of cloud condensation nuclei activation across the Indo-Gangetic Plain prior to monsoon onset and during peak monsoon periods: results from the SWAAMI field campaign

Abstract. Vertical distributions (altitude profiles) of condensation nuclei (CN) and cloud condensation nuclei (CCN) and their spatial variations across the Indo-Gangetic Plain (IGP) have been investigated based on airborne measurements carried out during the SWAAMI field campaign (June to July 2016) capturing the contrasting phases of the Indian monsoon activity in 2016 just prior to its onset and during its active phase. Prior to the monsoon onset, high concentrations of CN and CCN prevailed across the IGP, and the profiles revealed frequent occurrence of elevated layers (in the altitude range 1–3 km). Highest concentrations and elevated peaks with high values occurred over the central IGP. The scenario changed dramatically during the active phase of the monsoon, when the CN and CCN concentrations dropped (CN by 20 % to 30 % and CCN by 6 % to 25 %) throughout the IGP with more pronounced changes at altitudes higher than 3 km where decreases as high as > 80 % were observed. These reductions have an east-to-west decreasing gradient, being most remarkable in the eastern IGP and very weak over the western IGP where the CN concentrations above 3 km increased during the monsoon. The activation ratios (ARs) showed contrasting features, increasing with increase in altitude, prior to the onset of monsoon, reversing the trend to decrease with increase in altitude during the active phase of the monsoon. The supersaturation spectrum became flatter during the active phase of the monsoon, indicating an increase in the hygroscopicity of aerosols following the mixing of surface-based emissions with the advected marine air mass.

Provenance tracing of dust using rare earth elements in recent snow deposited during the pre-monsoon season from mountain glaciers in the central to northern Tibetan Plateau.

To provide insight into dust sources in snow deposited during the non-monsoon period on the Tibetan Plateau, detailed post-Archean Australian shale (PAAS)-normalized rare earth element (REE) distribution patterns and variations in REE tracers, such as La/Yb, La/Er, and Gd/Yb with depth, as well as the distribution of samples in the plots of REE ratio pairs, such as La/Yb vs. Y/ΣREE, La/Er vs. Gd/Er, and Y/La vs. Nd/Er, were compared in seven potential dust source areas in Asia. Snow samples from five glaciers, i.e., Qiumianleike (QMLK), Meikuang (MK), Yuzhufeng (YZF), Xiaodongkemadi (XDKMD), and Gurenhekou (GRHK), were collected from April 26 to May 13 before the onset of monsoon activity. The results show that dust trapped in snow pits from the studied glaciers, i.e., QMLK, MK, XDKMD, and GRHK, has varying degrees of contribution from the Taklimakan Desert, Qaidam Basin, and the surface soil of the Tibetan Plateau. There are signals of the Tengger and Badain Jaran Desert and Chinese Loess in the MK and XDKMD snow pit samples or in the QMLK and GRHK surface samples from the REE tracers; however, from the point of view of the land location, the three dust sources should not be the major contributors. Signals of dust from the Indian Thar Desert were occasionally detected from the MK, YZF, and XDKMD snow pit samples and GRHK surface snow samples, implying the intrusion of early Indian monsoon activities to the sites. The dust signal from the Thar Desert in India from the YZF glacier is significantly greater than that from the XDKMD and MK glaciers. These findings were also supported by the tracer of dust transmitted to the three snow pits through the air mass backward trajectories. The new finding of this study is that dust from the Indian Desert can even reach the Kunlun Mountains in the northern region of the Tibetan Plateau. The conclusions are helpful in interpreting the sources of dust and the pollutants absorbed by dust particles, as well as the extent of the impact of Indian monsoon activities at the end of the non-monsoon season on the plateau.

Nonstationarity in timing of extreme precipitation across China and impact of tropical cyclones

This study examines the seasonality and nonstationarity in the timing of extreme precipitation obtained by annual maximum (AM) sampling and peak-over-threshold (POT) sampling techniques using circular statistics. Daily precipitation data from 728 stations with record length of at least 55years across China were analyzed. In general, the average seasonality is subject mainly to summer season (June–July−August), which is potentially related to East Asian monsoon and Indian monsoon activities. The strength of precipitation seasonality varied across China with the highest strength being in northeast, north, and central-north China; whereas the weakest seasonality was found in southeast China. There are three seasonality types: circular uniform, reflective symmetric, and asymmetric. However, the circular uniform seasonality of extreme precipitation was not detected at stations across China. The asymmetric distribution was observed mainly in southeast China, and the reflective distribution of precipitation extremes was also identified the other regions besides the above-mentioned regions. Furthermore, a strong signal of nonstationarity in the seasonality was detected at half of the weather stations considered in the study, exhibiting a significant shift in the timing of extreme precipitation, and also significant trends in the average and strength of seasonality. Seasonal vapor flux and related delivery pathways and also tropical cyclones (TCs) are most probably the driving factors for the shifts or changes in the seasonality of extreme precipitation across China. Timing of precipitation extremes is closely related to seasonal shifts of floods and droughts and which means much for management of agricultural irrigation and water resources management. This study sheds new light on nonstationarity in timing of precipitation extremes which differs from existing ones which focused on precipitation extremes from perspective of magnitude and intensity.

Role of distinct flavours of IOD events on Indian summer monsoon

The summer monsoon contributes to about 75 % of mean annual rainfall over the various meteorological subdivisions of India. The role of ocean–atmosphere phenomena such as Indian Ocean Dipole (IOD) and El Nino–Southern Oscillation (ENSO) on the Indian monsoon activity is intriguing. The impacts of ENSO, IOD and Equatorial Indian Ocean Oscillation on monsoon are distinct. The ENSO (IOD) in general affects the monsoon negatively (positively). The present study aims to understand the role of different types of IOD such as early IOD (EIOD), normal IOD and prolonged IOD (PIOD) on Indian Summer Monsoon Rainfall (ISMR). We find that an EIOD, which peaks in the mid-monsoon months (July and August), plays a significant role like PIOD in enhancing ISMR even though it has a medium Dipole Mode Index amplitude value compared to other IODs. During an EIOD, the combined effect of excess evaporation from Arabian Sea and the stronger cross-equatorial flow leads to the enhanced monsoon activity. In addition, there is a substantial decrease in the number of break spells during EIOD years.

Influences of relative humidity and Indian monsoon precipitation on leaf water stable isotopes from the southeastern Tibetan Plateau

AbstractThis project report results of the first leaf water δ18O and δD time series of the woody plant Quercus aquifolioides undertaken on the southeastern Tibetan Plateau (STP). The data show that δD of leaf water underwent less enrichment than δ18O relative to precipitation and soil water, and could “inherit” the δD value characteristics of precipitation far better than δ18O. The leaf water δD values were strongly modified by evaporative enrichment in response to lower and more drastically changing relative humidity observed outside of the rainy season. In comparison, leaf water δD values more closely tracked those observed in the monsoon precipitation (acted as source water) in the rainy season. Our findings suggest that leaf water δD values from the STP were sensitive to the Indian monsoon activities, and the effects of relative humidity and the Indian monsoon precipitation should be considered for stable isotopes in tree ring cellulose or other biomarkers studies on the STP.

Stable isotopic compositions of precipitation events from Kathmandu, southern slope of the Himalayas

Investigation of temporal variations in the stable δ18O and δD isotopes from Kathmandu’s precipitation events shows that the relatively enriched δ18O and δD values in the winter (the dry season, dominated by the westerlies) were positively correlated with temperature, indicating a temperature effect controlling the changes of δ18O and δD. However, the δ18O and δD values were depleted in the summer (the wet season, dominated by the Indian monsoon), which were negatively correlated with precipitation amount, indicating an amount effect. In addition, the comparison of stable isotopes in precipitation from Kathmandu and Mawlong (near the Bay of Bengal) shows that the overall trends of δ18O and δD values at Kathmandu generally approximate those at Mawlong. However, there remain many differences between the details of the isotopic changes at Kathmandu versus those at Mawlong. Compared with those at Mawlong, the further rainout effect and the more intense lift effect of the oceanic moisture by the high mountains resulted in the more depleted δ18O and δD values in summer precipitation at Kathmandu. A deuterium excess and the local meteoric water lines reveal that evaporation at Kathmandu exceeds that at Mawlong. The data also show that the Indian monsoon activities at Mawlong are more intense than those at Kathmandu.

Different region climate regimes and topography affect the changes in area and mass balance of glaciers on the north and south slopes of the same glacierized massif (the West Nyainqentanglha Range, Tibetan Plateau)

This project launched a comparative study to investigate the areas of the Zhadang glacier (on the leeward slope/north slope of the West Nyainqentanglha Range, Tibetan Plateau) and the Gurenhekou glacier (on the windward slope/south slope) for nearly 40 years (1970–2007) and measure the mass balance of the two glaciers for three (2005–2008) and four (2004–2008) mass balance years, respectively. Results show that, in 1970–2007, overall annual precipitation decreased slightly caused by the weakening Indian monsoon and strengthened westerlies and annual mean air temperatures increased gradually in the areas to the north and south of the West Nyainqentanglha Range, respectively, resulting in the areas of the two glaciers reduced over the last nearly four decades. The rate of air temperature increase in the north is higher than that in the south. Moreover, mean annual precipitation in the south exceeds that in the north, due to the weakening of the Indian Monsoon activities across the West Nyainqentanglha Range. As a result, the area decrease of Zhadang glacier in the north slope of the West Nyainqentanglha Range is more intensive than that of Gurenhekou glacier in the south slope. Results also show that these two small glaciers experienced gradual reduction of the mass loss during the observation period of 2005–2008 and 2004–2008, and exhibited positive mass balances in 2008, possibly resulting from increases of annual precipitation and decreases in mean annual air temperatures, especially the lowering of air temperatures and the notably increasing of precipitation during the ablation period in 2006–2008. However, the mass balances of the two glaciers differed considerably. The differences in mass balance between the glaciers appear closely related to local climatic factors (different local moisture recycling and different seasonal distributions of precipitation) and glacier topography (the leeward/windward slopes and different elevations of the accumulation and ablations zones). Interestingly, the mass balances in the final year of our study period (2007/2008) for these two glaciers are much more similar, although they differed during the first year (2005/2006), resulting from differences between their annual precipitation trends. Observations indicate that changes in either the air temperature or precipitation will drive mass balance changes for the two glaciers, and the changes in the mass balance will directly affect the local water resource. These findings will provide important data for hydrological modelling of glacierized catchments and local water resource management strategies.

Potential of Stable Carbon and Oxygen Isotope Variations of Speleothems from Andaman Islands, India, for Paleomonsoon Reconstruction

The Indian monsoon activity, coinciding with the Inter-Tropical Convective Zone (ITCZ), progresses from the southern Indian Ocean during the boreal summer and withdraws towards the south in winter. Islands situated to the south of India receive, therefore, the first and last showers of the monsoon; speleothems in such islands have not yet been explored for their potential to reconstruct past monsoon rainfall. Here, we present the first measurements of stable carbon and oxygen isotopic compositions (δ13C andδ18O) of a stalagmite collected from the Baratang Island of Andamans, along with new data onδ18O of modern monsoon precipitation (May to July 2010). The aim was to detect (i) whether these samples are amenable to dating using14C, (ii) whether their oxygen isotopes indicate precipitation under isotopic equilibrium, and (iii) if (i) and (ii) above are true, can we reconstruct monsoon activity during the past few millennia? Our results indicate that whileδ18O of speleothem does show evidence for precipitation under isotopic equilibrium; dating by14C shows inversions due to varying contributions from dead carbon. The present work highlights the problems and prospects of speleothem paleomonsoon research in these islands.

Paleoenvironmental significance of compound-specific δ 13C variations in n-alkanes in the Hongyuan peat sequence from southwest China over the last 13 ka

Compound-specific 13C/ 12C ratios of the C 23, C 25, C 27, C 29 and C 31 n-alkanes in the Hongyuan peat sequence from southwest China were measured to decipher paleoenvironmental information recorded in the δ 13C variations over the last 13 ka. The δ 13C values of the n-alkanes range between −35.4‰ and −30.5‰, falling within the range of those from modern C 3 peat-forming vegetation. However, the vertical trends do not match with those of the δ 13C value for the C 3 peat-forming plant cellulose. Such a discrepancy between the δ 13C profiles implies that the n-alkane δ 13C values are unlikely to reflect signals from emerged aquatic plants in the bog. Because submerged/floating aquatic plants are major contributors of mid-chain (C 23 and C 25) n-alkanes in the sequence, the decoupling between the C 23 and C 25 n-alkanes and the peat cellulose likely reflects the situation that these mid-chain homologues primarily record the isotopic signals of submerged/floating aquatic plants. The stratigraphic profile of δ 13C values of submerged/floating aquatic plant n-alkanes (C 23 and C 25) reveals two prominent positive excursions (0.8–2.4‰) during the early to middle Holocene. The excursions coincide with peat accumulation maxima and stronger Indian monsoon activity in southwest China, indicating that the δ 13C variations in submerged/floating aquatic plants are closely related to changes in bog primary productivity controlled by the Asian monsoon activity.

Paleovegetation and paleoclimate in the Kathmandu Valley and Lake Baikal during the Late Quaternary

The Lake Baikal sediment is one of the best archives which records the Late Quaternary paleoclimatic changes at high-latitude inland of Asian continent. On the other hand, basin-fill sediment of the Kathmandu Valley is ideal for studying the history of Indian monsoon activities. We tried to reconstruct paleovegetation and paleoclimate changes in both regions during Late Quaternary based on palynological studies.

Possible teleconnections of winter rainfall in southern Brazil with Indian monsoon activity

Three homogeneous subregions of rainfall anomaly are identified in southern Brazil from the precipitation data for the months of June to September for the period 1960–1993. The area average monthly rainfall of these regions is correlated with the Indian monsoon rainfall index (MRI). The correlations are weak; however some significant negative correlation coefficients of the order of 0.3 or higher are found, indicating that more than normal monsoon activity in July has an effect of reducing the rainfall in southern Brazil in austral winter. The relevance of this result lies in the fact that any rainfall shortage in the midwinter and spring seasons can increase ambiental hazards such as forest fires.

The interannual variability of mid-latitude meridional circulation and its teleconnection with Indian monsoon activity

The statistical relationship between the summer monsoon rainfall over all India, northwest India and peninsular India, onset dates of monsoon and the index of mid latitude, (35° to 70°N) meridional circulation at 500 hPa level over different sectors and hemisphere based on 19 years (1971–1989) data, have been examined. The results indicate that (i) the summer monsoon rainfalls over all India, northwest India and peninsular India show a significant inverse relationship with the strength of meridional index during previous January over sector 45°W to 90°E. (ii) The summer monsoon rainfalls over all India and peninsular India show a significant inverse relationship with the strength of meridional index during previous December over sector 90°E to 160°E, (iii) The summer monsoon rainfall over northwest India shows a significant direct relationship with the meridional index during previous May over sector 160°E to 45°W.

Five-day oscillation in East African low-level jet

EASTERN edges of the continents are favourable for the deflection of tropical easterlies from the winter hemisphere to the summer hemisphere. Low-level southerly jet stream along the East African coast is the most pronounced manifestation of this deflected air current which feeds into the Indian monsoon current. We have subjected the jet stream data to a smoothing process to filter out noise and then found that this jet stream has a well marked 5-d periodicity. We discuss here whether this 5-d period is related to the well-known 5-d pulsations of the Indian monsoon activity.