Indian Ocean Monsoon Research Articles

Dipole Response of Mesoscale Eddy Formation to Monsoon Transition in the Southeast Tropical Indian Ocean

AbstractThe Southeast Tropical Indian Ocean (SETIO), dominated by the Indian Ocean monsoon, is an important source region for strong mesoscale eddies. To date, the impacts of the Indian Ocean monsoon on mesoscale eddies have not been clarified. Here we report on the dipole response of mesoscale eddy formation to monsoon transition in the SETIO, using satellite and reanalysis data sets. During the summer monsoon season, anticyclonic eddies are mainly concentrated north of 12°S, while cyclonic eddies are south of 12°S. This situation reverses during the winter monsoon season. We attribute this dipole feature to the oceanic perturbations and current shear during the different monsoon periods. A geographical boundary along 12°S aligns with meridional changes in eddy potential energy, which delineates the generation and direction of the newly‐formed eddies. The hot spot region, rich in eddy energy properties, tends to promote eddy formation and endurance during the monsoon periods.

A Cold Breath of Wind: The Storm as Archipelagic Epistemology in the Horn of Africa

The Horn of Africa, located on the western edge of the Indian Ocean, is a zone of extreme weather. Often figuring within discourses of geopolitical fragmentation and environmental disaster, the Horn is also home to rich literary traditions that bear witness to its long history as a site of cultural exchange and imperial ambition. This article presents a close reading of Igabia Scego's Italian-language novel Adua (2015), arguing that a localized iteration of the Indian Ocean monsoon, the rainstorm, organizes the novel's narrative, structure, and epistemology. This “storm form” embedded in the novel is a mode of archipelagic thinking that indexes situated African environmental epistemes and the climatological and embodied effects of empire while dispelling reductive or romanticized representations. As an alternate mode of organizing spatiotemporal relations and a method of inquiry into them, the storm is a situated Horn of Africa contribution to the theorization of the Indian Ocean as monsoonal archipelago.

Multi‐index characterization of compound dry hot events in India

AbstractCompound dry hot events (CDHEs), where hot events and droughts coexist, have received a lot of attention lately due to their catastrophic effects on the economy, environment and human health. In this study, we use two CDHE indices, the Standardized Compound Event Indicator (SCEI) and the Standardized Dry and Hot Index (SDHI), to assess changes in CDHE characteristics (severity, frequency, spatial extent) over the historical past and future CMIP6 simulations across the Indian subcontinent. To understand the role of the drought index selected on CDHE characterization two drought indices namely the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) are employed in the calculation of the CDHE indices. Further, the role of climatic oscillations such as El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Equatorial Indian Ocean Monsoon Oscillation and Indian Ocean Dipole in modulating CDHE characteristics have also been explored. Results show that SPI and SPEI based CDHE indices exhibit contrasting trends in northern India, while similar trends are observed in southern India in the historical past. Also, more frequent, extended, and severe CDHEs are reported by CDHE indices calculated using SPEI than by SPI. Temperature is found to be the dominant factor contributing to increases in CDHEs in the recent past and ENSO phases significantly modulate the severity and frequency of CDHE events in India. CMIP6 simulations generally report an increase in CDHE events for a 3°C global warming scenario. Overall, our findings show that the choice of the drought index has a greater impact on CDHE characterization than the choice of the CDHE index itself. Results from this study provide useful information towards understanding the risk of CDHEs in India under global warming and urge for the development and implementation of adaptation and mitigation measures.

Chemical composition of monsoon bulk precipitation in the Salalah area, Oman

Precipitation chemistry data provide important information for environmental studies on large-scale element cycling and anthropogenic impacts on our atmosphere, but also for hydrochemical models and groundwater recharge estimations via the Chloride Mass Balance method. Such recharge data play a crucial role in groundwater management, particularly in (semi-)arid areas. Unfortunately, precipitation analyses are often scarce in such regions. This also applies to the Arabian Peninsula, including southern Oman. To overcome this lack of rain chemistry data, we developed a strategy for automatic weekly bulk precipitation sampling, using recently designed automatic rainwater samplers. The integral samples were gathered along an elevation gradient from the Salalah coast to the Dhofar mountains during the Indian Ocean Monsoon seasons 2017 and 2018.Our major ion analyses of the rainwater samples revealed considerable temporal and spatial heterogeneity, in terms of ion proportions and absolute concentrations. Samples from the coast were relatively salty (EC mostly >3000 μS cm−1) and rich in Na+ and Cl−, reflecting small rain amounts and a sea spray effect. Further inland, solute concentrations were lower, partly due to more precipitation, and ions such as Ca2+ and SO42− gained importance, probably due to calcite and gypsum dust. This pattern reflects the interplay between solute availability (influenced by regional geology, wind direction at different altitudes, and wind speed) and precipitation amounts. Cl−/Br− ratios were fairly uniform and scattered around the seawater value. Combining ion concentrations and rain amounts yielded bulk depositions that showed an erratic pattern along the elevation gradient, i.e., depositions did not decrease steadily in inland direction, as one may assume. This suggests that the occasionally reported approach of collecting a few opportunistic grab samples at a single site is unlikely to yield data that are representative for a larger coastal study area.

Feasibility study on utilizing oscillating wave surge converters (OWSCs) in nearshore regions, case study: Along the southeastern coast of Iran in Oman sea

Considering the need for energy basket diversification and reducing fossil fuel consumption to control global warming, making renewable energies an appropriate solution. Wave energy is one of the most notable solutions offering the highest energy density among renewable energies. However, this energy source is not yet extensively developed. Finding an appropriate location to install the interested Wave Energy Converters (WECs) requires exquisite consideration. This study introduced a methodology through a case study to assess the use of OWSCs in nearshore regions. The southeast coasts of Iran along the Oman Sea were investigated as the case study, is the country's only oceanic coast and offers a higher potential for wave energy extraction. Therefore, to achieve this aim, the wave characteristics data were obtained from the Iranian National Institute for Oceanography and Atmospheric Science (INIO). The data was validated based on two buoys implemented by the Iran Meteorological Organization. By overlapping four key variables (average wave power, wave direction, distance from shoreline, and bathymetry) in annual and seasonal intervals via the geographic information system (GIS) program, five points were identified as optimal. Next, the distribution of wave height and period as well as angle distribution of wave power were analyzed for five considered points. Therefore, two points with appropriate distributions were selected with the following specifications an average annual wave power of 2–3.5 kW/m, wave direction arriving from the south-southwest, a depth of below 40 m, and coastal distance less than 5 km. In summer, due to the Indian Ocean monsoon, the potential to harness wave energy significantly increases.

Seasonal Variation in Microphysical Characteristics of Precipitation at the Entrance of Water Vapor Channel in Yarlung Zangbo Grand Canyon

Mêdog is located at the entrance of the water vapor channel in the Yarlung Zangbo Grand Canyon (YGC). This area has the largest annual accumulated rainfall totals and precipitation frequency on the Tibetan Plateau (TP). This paper investigates the seasonal variation in raindrop size distribution (DSD) characteristics in Mêdog based on disdrometer observations from 1 July 2019 to 30 June 2020. The DSD characteristics are examined under six rain rate classes and two rainfall types (stratiform and convective) in the winter, premonsoon, monsoon and postmonsoon periods. The highest (lowest) concentration of small raindrops is observed in monsoon (winter) precipitation, whereas large raindrops predominate in premonsoon precipitation. For stratiform rainfall, the mean mass-weighted mean diameter (Dm) exhibits overlooked differences in the four periods, while the mean normalized intercept parameter (Nw) is significantly higher in the monsoon period than in the other three periods. The convective rainfall in the monsoon and postmonsoon periods is characterized by a high concentration of limited-size drops and can be classified as maritime-like. This is probably attributed to abundant warm and humid airflow transported by the Indian Ocean monsoon into Mêdog. The westerly winds prevail over the TP during the premonsoon period, and thereby the premonsoon convective rainfall in Mêdog has a larger mean Dm and a lower mean Nw. In addition, the relationships of radar reflectivity Z and rain rate R for different precipitation types in different periods are also derived. A better understanding of the seasonal variation in the microphysical characteristics of precipitation in Mêdog is important for improving the microphysical parameterization scheme and the precipitation forecast of models on the TP.

Hydroclimatic and geochemical palaeoenvironmental records within tufa: A cool-water fluvio-lacustrine tufa system in the Wadi Dabsa volcanic setting, western Saudi Arabia

A series of uncharacterised tufa deposits within the lowland volcanic provinces of western Saudi Arabia are investigated here in order to account for their presence and to examine their utility as a palaeoenvironmental archive. The Wadi Dabsa basin, in the Harrat Al Birk, is part of the Saharo-Arabian region, where there is increasing evidence for multiple hominin dispersals during windows of reduced aridity during the last ~400 ka of the Late Quaternary. This first and detailed petrological and geochemical characterisation of these tufa carbonates is particularly important for palaeoenvironmental and climatological reconstruction at a site that contains a major concentration of Early and Middle Stone Age artefacts. We aim to determine the nature of deposition, to answer whether these are cool, freshwater tufa or hydrothermal travertine, and to establish what the tufa settings, stratigraphies and facies reveal about the hydroclimatic regime in this semi-arid environment.A widespread lacustrine-paludal and fluvial tufa system is preserved, as well as topographically-controlled tufa barrages and cascades. Observable sections, exposed by incision, reveal a thickness of tufa deposits of 3 to 6 m in places, which contain two packages of tufa-cemented conglomerate facies. This allows us to provide a conceptual model for the deposition of semi-arid tufa, based on existing type sites in the Naukluft Mountains of Namibia and the Ebro Basin of Spain. The varied topography within this volcanic harrat provides a setting at Wadi Dabsa that combines the low-lying lacustrine-paludal model from Spain with the steeper and higher-energy model from Namibia. The two tufa-cemented bedload units and the evidence for post-depositional erosion of other tufa facies indicates a shifting hydroclimatic regime, with episodes during which the basin contained water and there was steady flow in the fluvial channels, alternating with episodes of high-energy flashy flows.Geochemical analysis using 87Sr/86Sr isotopic ratios (0.7049 to 0.7056 with one sample at 0.7062) and stable carbon isotopes (−12.9 to −6.3‰) reveal that these are cool-water tufas were deposited from meteoric water that has interacted with both metamorphosed limestone in the Asir Escarpment and the mafic metavolcanic bedrock of the Wadi Dabsa Basin, without a geothermal influence. These tufa are low-concentration‑magnesium calcite, with traces of quartz (to a maximum of 3%) and other silicates. Some samples have ≤7.5% palygorskite, which along with petrological evidence for microbial filaments suggests the importance of biomineralisation by cyanobacteria within some of these tufa. Petrological analysis shows the fabrics are dominated by micrite cements with some microspar, and very little post-depositional alteration. Where samples are banded, this reflects variation in calcite crystal size as opposed to any striking variations in mineralogical composition. Oxygen stable isotopic composition (−14.6 to −1.9‰) indicates deposition from an isotopically more depleted rainfall source than the modern day, noting additional corrections for unknown past temperature and any other fractionation effects are needed. Depleted rainfall sources in this region may relate to either a far-travelled Atlantic moisture source or a higher-intensity Indian Ocean monsoon source, or some combination.

Impacts of the Western Pacific and Indian Ocean Warm Pools on Wildfires in Yunnan, Southwest China: Spatial Patterns With Interannual and Intraannual Variations

AbstractThe Indo‐Pacific warm pool affects the regional climatic conditions in Southeast Asia mediated by the Western Pacific (WP) and Indian Ocean (IO) monsoons, which are recognized as critical regulators of wildfires in this region. Our study explored the spatiotemporal effects of the warm pools on wildfire records in Yunnan, Southwest China. We showed the interannual records had a negative relationship with the simultaneous WP warm pool area index. The Ailao Mountains in the central Yunnan could be a geographical barrier that blocked the influences of the WP warm pool from the east side. For the intraannual changes, the wildfire number variations had a stronger connection to the IO warm pool dynamics than the WP. Our study provided the novel evidence on the teleconnection between the Indo‐Pacific warm pool dynamics and regional environmental processes in Yunnan, also indicating this area as a part of the transitional zone between the two warm pools.

A RIC<sub>p</sub> criterion for periodicity analysis of hydrological time series

<p indent="0mm">Identification of periodicity in hydrological time series is important for understanding the complex variability of hydrological processes. Previous studies mainly focused on the comparison, improvement and development of different methods for the identification of periodicity. However, these methods have big subjectivity in the selection of significance levels, which may lead to pseudo-periodic components in the identification results. In this article, a new criterion called RIC_p<italic> </italic>(R for correlation coefficient, IC for information criterion, and subscript p for periodicity) is proposed to accurately identify periodic components in hydrological time series and evaluate their statistical significance. The RIC_p is based on the idea of Akaike information criterion, and it is established by consisting of a fitting accuracy index and an uncertainty measurement. The fitting accuracy index is based on the correlation coefficient, and the uncertainty measurement is based on the form of information entropy. Different synthetic time series, which are composed of multiple periodic components and Pearson-III noise data, are used to verify the superiority of the RIC_p method. The performance of RIC_p is compared with the moving correlation coefficient-based method for periodicity identification (MCCP), harmonic analysis method (HAM), and maximum entropy spectral analysis (MESA), by checking the accuracy and reliability of the identification results. The results suggest that in HAM and MCCP methods, more pseudo-periodic components are identified compared to other methods. Differently, the number of periodic components identified by MESA is less than the true value, as some significant periodic components can be missed. By comparison, the RIC_p performs the best among these four methods. The advantage of RIC_p is that it can describe the periodicity and also evaluate its statistical significance level. The RIC_p method is further applied to analyze the surface water resource time series in six secondary water resource regions in Southwest Rivers basin, by comparing with HAM and MESA. Results further verified the advantages of the RIC_p method developed. It also indicates that the runoff in the basin generally has a 9-year periodic pattern; the precipitation has the same periodicity as the runoff except Lancang River region, which can be attributed to the 11-year periodic cycle of the Indian Ocean monsoon.

Mid-Late Holocene Sub-Millennial Scale Inverse Trends of South Asian Summer and Winter Monsoons in Sri Lanka

The South Asian Monsoon (SAM) brings precipitation crucial for agriculture across the densely populated region of South Asia. Identifying the key long-term drivers of the SAM is essential to improve the predictability of future monsoonal trends in the context of current global climate scenarios and increasingly frequent drought and flooding events in this part of the world. Here, we reconstruct ∼6000 years of climatic and environmental history of the South Asian summer monsoon-fed Bolgoda South Lake and the Horton Plains, and the winter monsoon-fed Panama lagoon, in Sri Lanka to better understand monsoonal operation over this island and its connection to broader climate systems. Multiple proxies (diagnostic biomarkers, hydrogen and carbon isotopes of individual n-alkane, grain size, and Zr/Rb elemental ratio) indicate a sub-millennial scale decreasing trend of summer monsoon rainfall in the wet zone of Sri Lanka alongside an increasing trend of winter monsoon rainfall in the dry zone during the last ∼6000 years. We also observed multi-centennial scale arid events in the Bolgoda South Lake and Horton Plains records at ∼3,500 and ∼1,000 cal years BP. Inverse monsoonal behavior during the mid- and late Holocene seems to be led by the southward migration of the mean latitudinal position of ITCZ, induced by varying solar energy distribution between the Northern and Southern hemispheres due to Earth’s processional cycle. Our observations are broadly supported by existing paleoclimatic records from the Indian sub-continent, but abrupt arid phases are asynchronous in the regional records. In addition, these short-term arid conditions do not show systematic correlations with the different modes of climate variables known to have teleconnections with the Indian Ocean monsoon.

Contrasting vegetation response to climate change between two monsoon regions in Southwest China: The roles of climate condition and vegetation height

Southwest China is an important biodiversity hotspot in the world and is controlled by the Pacific and Indian Ocean monsoon in the east and west part respectively. However, how abiotic and biotic factors affect the response of vegetation to climate change in different monsoon regions is still not clear. Here we used the annual change rate of growing-season normalized difference vegetation index (NDVI trend) during 1982–2015 to explore the vulnerability of vegetation (forests and shrubs) activity to climate change in southwest China. We examined NDVI trend in relation to: 1) climate change trends, i.e. annual change rate of water and energy availability, indicated by the Palmer Drought Index (PDSI) and potential evapotranspiration (PET), respectively; 2) climatic condition, i.e. mean PDSI and PET during 1982–2015; 3) vegetation height; 4) biome type; 5) monsoon region. The results showed that NDVI generally increased in the Pacific monsoon region, especially in the southern areas, probably because the vegetation under more productive climate were more resistant to climate change, and also because decreased temperature lead to lower evapotranspiration which alleviated the slight drought trend in this region. In contrast, NDVI generally decreased in the Indian Ocean monsoon region which showed more pronounced drought trend, especially in the tall subalpine and tropical forests of Southeast Tibetan Mountains, which supports the “hydraulic limitation hypothesis” that vegetation height interacted with climate change in affecting vegetation vulnerability. Our analysis highlighted the critical roles of different monsoon systems, climate condition and vegetation height in affecting ecosystem vulnerability. We suggest that the (sub)tropical forests in the Pacific monsoon region may have act as an important carbon sink during the past decades, while the tall forests in Southeast Tibetan mountains (a biodiversity center with high carbon stock) are highly vulnerable to climate change and should have priority in ecosystem protection.

Meridional migration of Indian Ocean Monsoon precipitation during the early Holocene: Evidence from the Andaman Sea

The Holocene evolution of the Indian Ocean Monsoon (IOM), especially the spatiotemporal nature of its maximum precipitation period, remains controversial. Here we reconstruct high-resolution local δ18Osw and sea surface temperature (SST) values by pairing Mg/Ca values and the δ18O data from planktonic foraminifera Globigerinoides ruber (white) in the southern Andaman Sea in order to understand IOM rainfall variability during the Holocene. Local δ18Osw values show that local maximum rainfall occurred between 10.6 ka and 8.6 ka in the southern Andaman Sea, before declining markedly during the 8.6–5.0 ka period, and then decreasing slightly after 5.0 ka. The Mg/Ca-SST was relatively low during the 10.3–8.6 ka period, before increasing slightly between 8.6 ka and 4.0 ka; it declined thereafter. But the sedimentation rate at the site of Core ADM-C1 relating to monsoonal precipitation of the source region of sediments exhibited its highest value during the 8.6–5.5 ka period. After making further comparisons with other published records for this region, we would propose that maximum IOM precipitation during the early Holocene has exhibited a clearly meridional migration: it was located in the tropics around 7°N during the 10.6–8.6 ka period, migrated northward to the subtropics during the 8.6–5.5 ka period, before migrating southward again after 5.0 ka. We would suggest that the northward shift of maximum precipitation during the Early Holocene in the IOM region was in response to the progressively northward migration of the Intertropical Convergence Zone (ITCZ) and the change of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) activities.

Relative humidity and agricultural activities dominate wildfire ignitions in Yunnan, Southwest China: Patterns, thresholds, and implications

Wildfires are land-surface processes and ecological disturbances occurring around the world. The wildfire regime in Yunnan Province of Southwest China is recognized as similar to that of the adjoining Indo-China Peninsula, a global hotspot for wildfires. However, the ignition mechanisms in this region remain unclear, with interactions among ecosystem features, local agricultural activities, and fire weather controlled by the West Pacific and Indian Ocean monsoons. Based on the ground records of 5145 confirmed wildfire events in Yunnan during the period of 2003–2015, we used a logistic regression model to estimate the local environmental controls on wildfire ignitions. Results highlighted the primary role of meteorology-driven characters, especially the prevalent importance of daily minimum relative humidity across the region. The threshold of the relative humidity was 37.48% ± 15.60% for the 50% ignition probability. Relative humidity also dominated ignition over years, with fuel conditions relatively stable and playing a minor role in contrast to the inter-annual climate changes. Moreover, agriculture-related ignition comprised most wildfire records, and human activities deeply shaped the spatiotemporal patterns of ignition. The distance to the nearest village was the primary factor during the beginning of the agricultural season, with a farming radius of 1.2 km as a key threshold for ignition. The complementary roles among influential factors were prominent at county scale. Among counties, the variation of ignition mechanisms corresponded to the influence paths of the two monsoons. This study highlights the importance of ground wildfire records in deriving critical wildfire ignition information such as environmental thresholds and change rates, which can provide important insights for sustainable forest management in this region, including wildfire monitoring, ignition control, fuel structure adjusting, and implementing differentiated strategies for fire prevention with regard to the environmental contexts.

Quantitative analysis of the profiles of twelve major compounds in Gentiana straminea Maxim. Roots by LC-MS/MS in an extensive germplasm survey in the Qinghai-Tibetan plateau

Ethnobotanical relevanceGentiana straminea Maxim. is a well-known Tibetan traditional herb, which has been used to treat rheumatic arthritis, iceteric hepatitis, and other diseases for thousands years. However, there is still lack of comprehensive active constituents profiling of this species throughout the Qinghai-Tibet Plateau (QTP). Aim of studyThis study was designed to provide a comprehensive quality map of G.straminea germplasm based on twelve active constituents (loganic acid, gentiopicroside, swertiamarin, sweroside, 6-O-β-D-glucosylgentiopicroside, oleanic acid, morroniside, trilobatin, isoorientin, isovite, Shanzhisidemethylester and quercetin) on the QTP. Materials and methodsG.straminea root samples collected throughout QTP in the flowering period were analyzed by the LC-MS/MS. Statistics analysis methods PCA, clustering and ecological regions analysis for G.straminea constituents differentiation was demonstrated. ResultsThe active constituents varied greatly across the QTP; the majority of constituents were secoiridoid derivatives, with gentiopicroside being the most abundant compound. Most constituents were significantly affected by the latitudes and altitudes other than longitudes. PCA and hierarchical clustering analysis showed that all samples could be separated into six distinct groups, and 15 populations showed the highest constituent abundances. Further, geographical region analysis showed that the highest quality populations mainly located near the source region of Yellow River, especially in the Qinghai and Sichuan areas. Additionally, correlation analysis showed that there were relationships among genetiopicroside, loganic acid, and other compounds, which might be related to the enzymatic pathways involved in the metabolism of these constituents. Conclusion: LC-MS/MS method allowed separation of quality profiling of G.straminea on the QTP, 15 populations showed the highest constituent abundances. In six geographical groups, the highest quality populations mainly located near the source region of Yellow River, especially in the Qinghai and Sichuan areas, which may be due to the climate caused by the westerlies and Indian Ocean monsoons in the QTP.

A shrubby resprouting pine with serotinous cones endemic to southwest China.

The Yunnan Province of China is wet and warm enough to support magnificent forests. However, many mountains in the region are covered by a shrubland formation dominated by an unusual pine, the shrubby and multi-stemmed Pinus yunnanensis (Yunnan pine; Fig. 1). This formation is natural, and not the product of forest degradation; and fire appears to be a major factor explaining the features of this pine. This region has a subtropical monsoon climate influenced by both the Indian Ocean Monsoon from the west, and the Pacific Ocean Monsoon from the east.

Observations and Modeling of Ocean Circulation in the Seychelles Plateau Region

AbstractThe ocean circulation around and over the Seychelles Plateau (SP) is characterized using 35 months of temperature and velocity measurements along with a numerical model. The results here provide the first documented description of the ocean circulation atop the SP. The SP is an unusually broad (∼200 km), shallow (∼50 m) plateau, dropping off steeply to the abyss. It is situated in a dynamic location (3.5–5.5°S, 54‐57°E) in the south‐western tropical Indian Ocean where northwesterly winds are present during austral summer and become southeasterly in austral winter, following the reversal of the western Indian ocean monsoon winds. Measurements around the Inner Islands, on the SP, have been carried out since 2015. Velocity measurements show that most of the depth‐averaged current variance on the SP arises from near‐inertial oscillations and lower‐frequency variability. Lower‐frequency variability encompasses seasonal and intraseasonal variability, the latter of which includes the effects of mixed Rossby‐gravity waves and mesoscale eddies. A global 0.1° numerical ocean simulation is used in conjunction with these observations to describe the regional circulation around the SP. Atop the SP, circulation is dominated by ageostrophic processes consistent with Ekman dynamics, while around the SP, both geostrophic and ageostrophic processes are important and vary seasonally. Stratification responds to the sea surface height semiannual signal which is due to Ekman pumping‐driven upwelling (related to the Seychelles‐Chagos Thermocline Ridge) and the arrival of an annual downwelling Rossby wave.

Spatial distribution of extreme precipitation in the Tibetan Plateau and effects of external forcing factors based on Generalized Pareto Distribution

Abstract The daily precipitation data of the years 1955–2017 from May to September were retrieved; then a Generalized Pareto Distribution (GPD) and maximum likelihood methods were adopted to understand trends and calculate the reappearance period of heavy precipitation in the Tibetan Plateau (TP). The daily precipitation values at 22 stations in the TP were found to conform to the model, and theoretical and measured frequencies were consistent. According to the spatial distribution of the maximum precipitation value, the extreme values of Shigatse and Lhasa showed large fluctuations, and the probability of record-breaking precipitation events was low. In the western part of Nagqu, the probability of extreme precipitation was relatively low, and that of record-breaking precipitation was relatively high. The peak values of extreme precipitation in the flood season in the TP generally exhibited a decreasing trend from southeast to northwest, and the extreme value of the flood season that reappeared in the southeast region was approximately twice that of the northwest region. The maximum rainfall in most areas will exceed 20 mm in the next 5–10 years, and the maximum rainfall in Shigatse will reach 52.7 mm. After 15 years of recurrence in various regions, the peak rainfall in the flood season has become low. Most of the regions in the model have different responses to ENSO and Indian Ocean monsoon indices with external forcing factors.

Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)

Northern Africa’s past climate is characterized by a prolonged humid period known as the African Humid Period (AHP), giving origin to the “Green Sahara” and supporting human settlements into areas that are now desert. The spatial and temporal extent of climate change associated with the AHP is, however, subject to ongoing debate. Uncertainties arise from the complex nature of African climate, which is controlled by the strength and interactions of different monsoonal systems, resulting in meridional shifts in rainfall belts and zonal movements of the Congo Air Boundary. Here, we examine a ∼12,500-years record of hydroclimate variability from Lake Dendi located in the Ethiopian highlands based on a combination of plant-wax-specific hydrogen (δD) and carbon (δ13C) isotopes. In addition, pollen data from the same sediment core are used to investigate the response of the regional vegetation to changing climate. Our δD record indicates high precipitation during peak AHP (ca. 10 to 8 ka BP) followed by a gradual transition toward a drier late Holocene climate. Likewise, vegetation cover changed from predominant grassland toward an arid montane forest dominated by Juniperus and Podocarpus accompanied by a general reduction of understory grasses. This trend is corroborated by δ13C values pointing to an increased contribution of C3 plants during the mid-to late Holocene. Peak aridity occurred around 2 ka BP, followed by a return to a generally wetter climate possibly linked to enhanced Indian Ocean Monsoon strength. During the last millennium, increased anthropogenic activity, i.e., deforestation and agriculture is indicated by the pollen data, in agreement with intensified human impact recorded for the region. The magnitude of δD change (40‰) between peak wet conditions and late Holocene aridity is in line with other regional δD records of East Africa influenced by the CAB. The timing and pace of aridification parallels those of African and Indian monsoon records indicating a gradual response to local insolation change. Our new record combining plant-wax δD and δ13C values with pollen highlights the sensitive responses of the regional vegetation to precipitation changes in the Ethiopian highlands.

Millennial-scale variability of Indian summer monsoon during the last 42 kyr: Evidence based on foraminiferal Mg/Ca and oxygen isotope records from the central Bay of Bengal

Studying marine sedimentary archives from the Indian Ocean have provided great opportunities to assess the forcing/response relationship between the Indian monsoons, millennial-scale climate variability as revealed in Greenland ice cores, and possible roles of the Atlantic meridional overturning circulation (AMOC) changes that are most likely responsible for driving both. Here we present new evidence of sea surface temperature (SST) estimates and δ18O of seawater (δ18Osw, presumably salinity-driven) from the paired data of Mg/Ca and δ18O measured from planktonic foraminifer Globigerinoides ruber (white) in core BoB-24 collected from the central Bay of Bengal (BoB), tropical Indian Ocean. Eleven accelerator mass spectrometry (AMS) radiocarbon dates on planktonic foraminifera Neogloboquedrina dutertrei from the core provide a reliable age model. Our results indicate a gradual surface warming with several short-lived fluctuations since ~42 kyr ago, and a ~3 °C cooling during the Last Glacial Maximum (LGM) and a ~3 °C warming from the last glacial to the Holocene. Our δ18Osw revealed an increased salinity during the Marine Isotope Stage (MIS) 2, which is likely attributable to a weakening of Indian Summer Monsoon (ISM) precipitation and associated river discharge in the BoB. Our record also indicates clearly a phase of strengthening ISM during the mid-MIS 3 (37.5–32 ka) as evidenced by low foraminiferal δ18Osw values of ~0.2‰. Lower than present δ18Osw values during the Bølling/Allerød (B/A) event (14.5–12.6 ka) and the early Holocene (10–5 ka) in core BoB-24 indicate lower salinity that in turn suggest enhanced ISM precipitation and increased freshwater output from the Ganges-Meghna-Brahmaputra-Irrawaddy (GMBI) rivers together with peninsular rivers and Irrawaddy-Salween rivers from Myanmar. This study brings data evidence that implies weakened ISM was linked with Northern Hemisphere cooling events, especially during the MIS 2. Furthermore, the millennial-scale hydroclimate changes in the BoB presented in this study support the dynamic role of AMOC in driving a teleconnection between the North Atlantic and Indian Ocean monsoon regions.

Global atmospheric changes versus the Indian rainfall variation

Apropos the asymmetry in temperature at the surface as well as in the vertical atmospheric layers above the Earth’s surface is quite germane to the vantage point of societal and human impacts. They are also generally regarded as an important indicator of human-induced climate change. Even the cause of a global climate change is regarded due to the increase in temperature of the Earth’s atmosphere. This in turn also affects the Indian rainfall. Furthermore, it negatively influences the Indian climate, by increasing floods, hurricanes, and storms. So, analyzing the variation in global temperature change has become an integral part to further study the Indian rainfall variation. According to Sir Gilbert Walker, the Indian rainfall (summer monsoon) is a global phenomenon. There are many factors like jet streams, Tibetan Plateau, El Nino, Indian Ocean Dipole, monsoon trough, cyclonic storms, and mid troposphere cyclones which influence the monsoon. Despite an increase in temperature, the monsoon rainfall has shown a decreasing trend. Even, in recent years, extreme rain events (flood, drought) have increased. Thus, to enhance the Indian rainfall variation (especially monsoon rain), it is important to explore the global atmospheric changes and observe its relationship with the Indian rainfall.