Oceanic Moisture Source Research Articles

Large‐scale synoptic atmospheric moisture circulation patterns associated with variability of daily precipitation over East China

AbstractThis study identifies the variability of large‐scale atmospheric patterns (LSMPs) and atmospheric moisture circulation patterns associated with the variability of daily precipitation over East China using the self‐organizing maps (SOM) method. The spatiotemporal characteristics of the occurrence and intensity of daily precipitation were linked with the variabilities of LSMPs. Results show that the synoptic moisture transport over East China in both dry season and rainy season can be classified into eight patterns, and the occurrence frequency and primary occurring months of the eight patterns are varied. In the dry season, the wetter pattern (the drier pattern) exhibits a significant decreasing (increasing) trend with a slope of −0.23 (0.63)% year−1 at the confidence level of 0.95 during 1979–2018. The LSMPs corresponding to each atmospheric moisture circulation pattern, including the composite mean values of vertically integrated water vapour transport fields, 500 hPa geopotential height (Z500) anomalies and sea surface temperature anomalies (SSTA) in surrounding oceans, indicate that the primary oceanic moisture source of the dry season is western North Pacific (WNP), while that of the rainy season is WNP and the Indian Ocean. The location and intensity variabilities of subtropical high affect the transport and amount of water vapour along with its western flank to arrive in East China. SSTA has an influential link with the regional circulations and convection activity, and thus plays an important role in affecting water vapour transport.

Variations of Stable Isotopic Composition in Atmospheric Water Vapor and their Controlling Factors—A 6‐Year Continuous Sampling Study in Nanjing, Eastern China

AbstractLong‐term continuous monitoring of water vapor isotopes has great potential to improve our understanding of mechanisms that control stable isotope variations in hydrological processes, which is essential for accurate interpretation of isotopic proxy records. Here we presented a 6‐year daily data set of stable isotopes in atmospheric water vapor (δ18Ov, δDv, and d‐excessv) and precipitation (δ18Op, δDp, and d‐excessp) at Nanjing in Eastern China, an area under the influence of the East Asian Monsoon. Based on isotopic variations, we divided a year into three seasons, that is, summer monsoon season (June–September), spring (March–May), and fall‐winter (October–February). During summer monsoon season, the low δ18Ov values were mainly caused by the upstream convective rainout process during the large‐scale water vapor transport, and the d‐excessv values were the lowest due to the weak kinetic isotopic fractionation over its oceanic moisture source. Spring had the highest δ18Ov values, which was related to less distillation of local moisture under relatively high air temperatures. The relatively high d‐excessv values resulted from the kinetic isotopic fractionation due to drier condition. The fall‐winter season has the lowest δ18Ov values because the local distillation was the strongest with low air temperature. The d‐excessv values were highest during this season because of the strongest kinetic isotopic fractionation during the continental moisture recycling under dry climate condition. Using the isotopic composition of both precipitation and water vapor, we found that the annual mean subcloud raindrop evaporation is ~11% of precipitation. These findings improve our understanding of hydrological cycle on the East Asian Monsoon region and will potentially improve our interpretation of numerous isotopic proxy records from this region.

Rainwater isotopes in central Vietnam controlled by two oceanic moisture sources and rainout effects

The interpretation of palaeoclimate archives based on oxygen isotopes depends critically on a detailed understanding of processes controlling the isotopic composition of precipitation. In the summer monsoonal realm, like Southeast Asia, seasonally and interannually depleted oxygen isotope ratios in precipitation have been linked to the summer monsoon strength. However, in some regions, such as central Vietnam, the majority of precipitation falls outside the summer monsoon period. We investigate processes controlling stable isotopes in precipitation from central Vietnam by combining moisture uptake calculations with monthly stable isotope data observed over five years. We find that the isotopic seasonal cycle in this region is driven by a shift in moisture source from the Indian Ocean to the South China Sea. This shift is reflected in oxygen isotope ratios with low values (− 8 to − 10‰) during summer and high values during spring/winter (0 to − 3‰), while 70% of the annual rainfall occurs during autumn. Interannual changes in precipitation isotopes in central Vietnam are governed by the timing of the seasonal onset and withdrawal of the Intertropical Convergence Zone, which controls the amount of vapour contributed from each source.

The growing importance of oceanic moisture sources for continental precipitation

The precipitation that falls on the continents defines the extent and nature of terrestrial ecosystems and human activity in them, all of which are adapted to and maintained by present-day precipitation. In essence, precipitation is supplied by moisture that either comes directly from the ocean, or is subsequently recycled from the continents themselves. Both the processes that control evaporation and the main mechanisms of moisture transport clearly differ between the ocean and the continent, thus within the context of a changing climate, it may be expected that the relationship between precipitation of oceanic and terrestrial origin varies globally and regionally, as will the influence of these two basic components of total precipitation on global and regional precipitation trends, especially in tropical regions. We describe an approach based on a Lagrangian technique for estimating the precipitation in a target region given the proportions of moisture transported from the two sources (ocean and continent) to reveal that the percentage of precipitation of oceanic origin has increased globally in the current climate (1980–2016). The greatest observed rate of increase is in the tropical regions; furthermore, the trends of precipitation in these regions are controlled by trends in precipitation for which the source of moisture is the ocean.

Near-Surface Salinity Reveals the Oceanic Sources of Moisture for Australian Precipitation through Atmospheric Moisture Transport

AbstractThe long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on interseasonal to interannual time scales, and to locate the source of moisture. Seasonal composites during El Niño–Southern Oscillation/Indian Ocean dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies toward Australia. During co-occurring La Niña and negative IOD events, salty anomalies around the Maritime Continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, a moisture transport divergence anomaly over Australia results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean–atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g., the 2010/11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall.

Moisture Sources for Wintertime Intense Precipitation Events Over the Three Snowy Subregions of the Tibetan Plateau

AbstractWintertime intense precipitation events often lead to severe snow disasters. In this study, a Lagrangian approach is employed to examine the evaporative moisture sources for wintertime intense precipitation events over the three snowy subregions of the Tibetan Plateau (TP) during 1979–2016, including the western TP (WTP), south central TP (SCTP), and southeastern TP (SETP). More than 80.0% of the moisture for intense precipitation over each subregion originates from terrestrial areas. Although prevailing westerly winds dominate above the TP and its surrounding areas during winter, half of the precipitation over the three subregions is supplied by evaporation from the south (i.e., the Indian Peninsula). Specifically, evaporation from the Indian Peninsula contributes 68.0%, 65.0%, and 45.0% of the moisture for intense precipitation over the WTP, SCTP, and SETP, respectively. The two primary oceanic moisture source regions for intense precipitation are the Arabian Sea and the Bay of Bengal, playing complementary roles in supplying moisture. The relative contributions of the Arabian Sea to intense precipitation over the WTP, SCTP, and SETP are 9.2%, 6.9%, and 1.1%, while those of the Bay of Bengal are 1.1%, 12.1%, and 8.6%. Southerly winds downstream of a cyclonic anomaly over the Indian Peninsula are crucial for the low‐level moisture transport from the south to the Himalayan foothills. Under the combined effects of orographic lifting and favorable large‐scale circulation patterns, moisture ascends further into the three subregions. Changes in the position and intensity of the cyclonic anomaly are particularly crucial to facilitating moisture contributions from the key source regions.

Diagnosing Moisture Sources for Flash Floods in the United States. Part II: Terrestrial and Oceanic Sources of Moisture

Abstract Backward trajectories were derived from North American Regional Reanalysis data for 19 253 flash flood reports published by the National Weather Service to determine the along-path contribution of the land surface to the moisture budget for flash flood events in the conterminous United States. The impact of land surface interactions was evaluated seasonally and for six regions: the West Coast, Arizona, the Front Range, Flash Flood Alley, the Missouri Valley, and the Appalachians. Parcels were released from locations that were impacted by flash floods and traced backward in time for 120 h. The boundary layer height was used to determine whether moisture increases occurred within the boundary layer or above it. Moisture increases occurring within the boundary layer were attributed to evapotranspiration from the land surface, and surface properties were recorded from an offline run of the Noah land surface model. In general, moisture increases attributed to the land surface were associated with anomalously high surface latent heat fluxes and anomalously low sensible heat fluxes (resulting in a positive anomaly of evaporative fraction) as well as positive anomalies in top-layer soil moisture. Over the ocean, uptakes were associated with positive anomalies in sea surface temperatures, the magnitude of which varies both regionally and seasonally. Major oceanic surface-based source regions of moisture for flash floods in the United States include the Gulf of Mexico and the Gulf of California, while boundary layer moisture increases in the southern plains are attributable in part to interactions between the land surface and the atmosphere.

Dominant influence of the humidity in the moisture source region on the 17O-excess in precipitation on a subtropical island

A high-precision analysis of the triple oxygen isotope composition of water provides a new tracer denoted as 17O-excess. Early theoretical and experimental studies suggest that changes in the 17O-excess in precipitation can be interpreted as variations in the relative humidity in moisture source regions for precipitation. However, subsequent studies on 17O-excess in the precipitation in polar and dry regions suggest the importance of additional fractionation, such as snow formation and raindrop re-evaporation processes, which diminish the relative humidity information in oceanic source regions. To date, whether humidity in the moisture source region can be quantified based on observations of 17O-excess in precipitation has not been proven. Here, we show a two-year record of the 17O-excess in precipitation on a maritime island in the East Asian monsoon region. The normalized relative humidity in the source region was reconstructed from 17O-excess data using a simple evaporation model. We demonstrate that the reconstructed relative humidity is quantitatively consistent with observations in the oceanic moisture source region. This result suggests that the 17O-excess in precipitation on the subtropical island is determined largely by diffusional fractionation during evaporation in the ocean. We also test the impact of a possibly different 17O–18O slope suggested by a recent study on the 17O-excess definition. This different definition results in a minor impact on the data from tropical and subtropical regions but a significant change in the data from polar regions. These results suggest that the 17O-excess in precipitation in tropical and subtropical regions is a unique quantitative tracer for the relative humidity in oceanic moisture source regions and thus will be a useful tracer and proxy for hydrological and paleoclimate studies.

Rainfall isotope variations over the Australian continent – Implications for hydrology and isoscape applications

This paper presents a continental scale interpretation of δ2H and δ18O in Australian precipitation, incorporating historical GNIP data at seven sites (1962–2002) and 8–12 years of new monthly data from 15 sites from 2003 to 2014. The more than doubling of stations and the significant time series duration allow for an improved analysis of Australian precipitation isotopes. Local meteoric water lines were developed for each site, and for the Australian continent. When the annual precipitation weighted values were used, the Australian meteoric water line was δ2H = 8.3 δ18O + 14.1‰.Precipitation amount was found to be a stronger driver of precipitation isotopes than temperature at most sites, particularly those affected by tropical cyclones and the monsoon. Latitude, elevation and distance from the coast were found to be stronger drivers of spatial variability than temperature or rainfall amount.Annual isoscapes of δ2H, δ18O and deuterium excess were developed, providing an improved tool to estimate precipitation isotope inputs to hydrological systems. Because of the complex climate, weather and oceanic moisture sources affecting Australia, regional groupings were used instead of the climate zone approach and additional data was included to improve the coverage in data poor regions. Regression equations for the isoscape were derived using latitude, altitude and distance from the coast as predictor variables.We demonstrate how this isoscape can be used as a tool for interpreting groundwater recharge processes using examples from across Queensland and New South Wales, including the Murray Darling Basin. Groundwater isotopes at sites where direct local recharge occurs are similar to rainfall, but for inland sites, which are often arid or semi-arid, a disconnect between shallow groundwater and local rainfall is observed; the departure in deuterium excess for these sites increases with aridity and distance from the headwaters where flooding originates.

The Atmospheric Branch of the Hydrological Cycle over the Negro and Madeira River Basins in the Amazon Region

The Amazon region, in South America, contains the largest rainforest and biodiversity in the world, and plays an important role in the regional and global hydrological cycle. In the present study, we identified the main sources of moisture of two subbasins of the Amazon River Basin, the Negro and Madeira River Basins respectively. The source-sink relationships of atmospheric moisture are investigated. The analysis is performed for the period from 1980–2016. The results confirm two main oceanic moisture sources for both basins, i.e., oceanic regions in the Tropical North and South Atlantic oceans. On the continents are, the Negro River Basin itself, and nearby regions to the northeast. For the Madeira River Basin, the most important continental sources are itself, and surrounding regions of the South American continent. Forward-trajectory analysis of air masses over the source regions is used to compute the moisture contribution to precipitation over basins. Oceanic (continental) sources play the most important role in the Negro River Basin (Madeira River Basin). The moisture contribution from the Tropical North Atlantic region modulates the onset and demise of the rainy season in the Negro River Basin; while the moisture contribution from the rest of the Amazon River Basin, the Madeira Basin itself, and Tropical South America leads to the onset of the rainy season in the Madeira River Basin. These regions also played the most important role in decreasing the moisture supply during most severe dry episodes in both basins. During ‘’El Niño’’, generally occurs a reduction (increase) of the moisture contribution to the Negro River Basin (Madeira River Basin; mainly from April to August) from almost all the sources, causing a decrease in the precipitation. Generally, the contrary occurs during ‘’La Niña’’.

Asynchrony between Antarctic temperature and CO2 associated with obliquity over the past 720,000 years

The δD temperature proxy in Antarctic ice cores varies in parallel with CO2 through glacial cycles. However, these variables display a puzzling asynchrony. Well-dated records of Southern Ocean temperature will provide crucial information because the Southern Ocean is likely key in regulating CO2 variations. Here, we perform multiple isotopic analyses on an Antarctic ice core and estimate temperature variations at this site and in the oceanic moisture source over the past 720,000 years, which extend the longest records by 300,000 years. Antarctic temperature is affected by large variations in local insolation that are induced by obliquity. At the obliquity periodicity, the Antarctic and ocean temperatures lag annual mean insolation. Further, the magnitude of the phase lag is minimal during low eccentricity periods, suggesting that secular changes in the global carbon cycle and the ocean circulation modulate the phase relationship among temperatures, CO2 and insolation in the obliquity frequency band.

Sources of moisture for different intensities of summer rainfall over the Chinese Loess Plateau during 1979–2009

ABSTRACTIn this study, moisture sources for different intensities of summer (June–July–August) rainfall over the Chinese Loess Plateau (CLP) were investigated by performing a transient simulation for the period 1979–2009 using the Lagrangian particle dispersion model FLEXPART. The results showed that continental areas surrounding the CLP, namely northwestern China–eastern central Asia and central eastern China, were major moisture source regions for summer rainfall over the CLP and that oceanic moisture originated from the East China Sea, the South China Sea, the Bay of Bengal, and the Arabian Sea played a secondary role. The moisture contribution of northwestern China–eastern central Asia to the CLP significantly decreased with increasing intensity of rainfall, whereas the contribution of central eastern China to the CLP slightly increased. Moreover, the moisture contribution of oceanic source regions to the CLP increased with the increased rainfall intensity. The East China Sea and the Bay of Bengal were the two most substantial oceanic moisture source regions for the 0–5th percentile (0–P5th) to 50th–75th percentile (P50th–P75th) rainfall classes, whereas the South China Sea and the Bay of Bengal were the two most substantial oceanic moisture source regions for the 75th–95th percentile (P75th–P95th) and the 95th–100th percentile (P95th–P100th) rainfall classes.

Using stable isotopes to understand seasonal and interannual dynamics in moisture sources and atmospheric circulation in precipitation

AbstractStable isotopes in precipitation are useful tracers to strengthen understanding of climate change and hydrological processes. In this study, the moisture sources of 190 precipitation events in Beijing were analysed using the Hybrid Single‐particle Lagrangian Integrated Trajectory model, based on which we studied the relation between variations in precipitation δ18O and dynamics in moisture sources and atmospheric circulation in seasonal and interannual timescales. Categorization of 7 groups of moisture sources was performed, among which oceanic moisture sources presented lower δ18O in precipitation than continental moisture sources. The results show that seasonal variations of precipitation δ18O were caused by changes of moisture sources. In summer, moisture from proximal oceans dominated vapour transport to Beijing due to increasing monsoon strength and resulted in a relatively small variation of precipitation δ18O. At the interannual timescale, the variations of δ18O in summer precipitation were related to dynamics in oceanic moistures, showing depleted values when the contribution of oceanic moistures, especially the proportion of long‐distance oceanic moisture, was high. Further analysis indicated that changes of oceanic moisture sources were controlled by the strength of summer monsoons. These findings address the complexity of moisture sources in midlatitude monsoon areas and suggest that isotopic signals in precipitation have the potential to deduce changes in moisture sources and atmospheric circulation and can therefore serve for palaeoclimate reconstruction.

Role of Oceanic and Terrestrial Atmospheric Moisture Sources in Intraseasonal Variability of Indian Summer Monsoon Rainfall

Summer Monsoon Rainfall over the Indian subcontinent displays a prominent variability at intraseasonal timescales with 10–60 day periods of high and low rainfall, known as active and break periods, respectively. Here, we study moisture transport from the oceanic and terrestrial sources to the Indian landmass at intraseasonal timescales using a dynamic recycling model, based on a Lagrangian trajectory approach applied to the ECMWF–ERA–interim reanalysis data. Intraseasonal variation of monsoon rainfall is associated with both a north-south pattern from the Indian landmass to the Indian Ocean and an east-west pattern from the Core Monsoon Zone (CMZ) to eastern India. We find that the oceanic sources of moisture, namely western and central Indian Oceans (WIO and CIO) contribute to the former, while the major terrestrial source, Ganga basin (GB) contributes to the latter. The formation of the monsoon trough over Indo-Gangetic plain during the active periods results in a high moisture transport from the Bay of Bengal and GB into the CMZ in addition to the existing southwesterly jet from WIO and CIO. Our results indicate the need for the correct representation of both oceanic and terrestrial sources of moisture in models for simulating the intraseasonal variability of the monsoon.

Spatial and temporal variation in the stable isotope composition (δ18O and δ2H) of rain across the tropical island of Sri Lanka

ABSTRACTSeasonal and spatial variation in δ18O and δ2H in rainwater was determined in three selected transects across Sri Lanka, the tropical island in the Indian Ocean. Local meteoric water lines (LMWLs) for three distinguished climatic zones; wet, dry and intermediate were constructed. LMWLs show slight variations in their gradients and respective d-excess values, depending on the air moisture origin, circulation and environmental conditions of each climatic zone. The elevation effect and amount effect could be identified but the continental effect is not significantly seen in the isotope composition of rain in the concerned areas. The results reasonably revealed that the distinct rainfall regimes; two monsoonal rains and two convectional (inter-monsoon) rains have characteristic isotopic signatures. Also the impact of (i) terrestrial and oceanic moisture sources, (ii) depression and cyclonic conditions of the Bay of Bengal, and (iii) topography of the country on the variation of the isotopic composition of rain in Sri Lanka could be satisfactorily identified.

Role of Oceanic and Land Moisture Sources and Transport in the Seasonal and Interannual Variability of Summer Monsoon in India

Abstract Three key issues of moisture supply and Indian summer monsoon rainfall (ISMR) variability are discussed in the present work: identification of the oceanic and terrestrial sources of moisture; the extent to which each source affects the ISMR; and their individual contributions to the interannual variability of ISMR. The modified Dynamic Recycling Model, based on a Lagrangian trajectory approach, is used to estimate the relative contributions from 27 terrestrial and oceanic moisture source regions to the monsoon during 1979–2013. ERA-Interim data are used for the study. The results show that the ISMR is strongly influenced by the land–ocean–atmosphere interactions, and a significant fraction of atmospheric moisture to the ISMR comes from five main moisture sources: the western Indian Ocean (WIO), central Indian Ocean (CIO), upper Indian Ocean (UIO), Ganges basin (GB), and Red Sea and the neighboring gulf (RDG). The moisture flux from WIO is very high during the initial period of monsoon seasons. From the mid-monsoon season, the contribution from this moisture source decays and land sources through evapotranspiration (ET) become more active. Early decay of moisture contributions from the WIO and the GB is observed during weak monsoon years. El Niño years are associated with low contributions of moisture from all sources, whereas warm Indian Ocean years are associated with low moisture flux from the major sources except WIO. ISMR is characterized by the prolonged and increasing moisture supply from WIO during the first half of the monsoon along with contributions from GB during the end of season. The results are consistent across several reanalyses (CFSR, ERA-Interim, and MERRA).

Source apportionment of atmospheric water over East Asia – a source tracer study in CAM5.1

Abstract. The atmospheric water tracer (AWT) method is implemented in the Community Atmosphere Model version 5.1 (CAM5.1) to quantitatively identify the contributions of various source regions to precipitation and water vapour over East Asia. Compared to other source apportionment methods, the AWT method was developed based on detailed physical parameterisations, and can therefore trace the behaviour of atmospheric water substances directly and exactly. According to the simulation, the northern Indian Ocean (NIO) is the dominant oceanic moisture source region for precipitation over the Yangtze River valley (YRV) and southern China (SCN) in summer, while the north-western Pacific (NWP) dominates during other seasons. Evaporation over the South China Sea (SCS) is responsible for only 2.7–3.7 % of summer precipitation over the YRV and SCN. In addition, the Indo-China Peninsula is an important terrestrial moisture source region (annual contribution of ∼ 10 %). The overall relative contribution of each source region to the water vapour amount is similar to the corresponding contribution to precipitation over the YRV and SCN. A case study for the SCS shows that only a small part ( ≤ 5.5 %) of water vapour originates from local evaporation, whereas much more water vapour is supplied by the NWP and NIO. In addition, because evaporation from the SCS represents only a small contribution to the water vapour over the YRV and SCN in summer, the SCS mainly acts as a water vapour transport pathway where moisture from the NIO and NWP meet.

Response of water vapour D-excess to land–atmosphere interactions in a semi-arid environment

Abstract. The stable isotopic composition of water vapour provides information about moisture sources and processes difficult to obtain with traditional measurement techniques. Recently, it has been proposed that the D-excess of water vapour (dv = δ2H − 8 × δ18O) can provide a diagnostic tracer of continental moisture recycling. However, D-excess exhibits a diurnal cycle that has been observed across a variety of ecosystems and may be influenced by a range of processes beyond regional-scale moisture recycling, including local evaporation (ET) fluxes. There is a lack of measurements of D-excess in evaporation (ET) fluxes, which has made it difficult to assess how ET fluxes modify the D-excess in water vapour (dv). With this in mind, we employed a chamber-based approach to directly measure D-excess in ET (dET) fluxes. We show that ET fluxes imposed a negative forcing on the ambient vapour and could not explain the higher daytime dv values. The low dET observed here was sourced from a soil water pool that had undergone an extended drying period, leading to low D-excess in the soil moisture pool. A strong correlation between daytime dv and locally measured relative humidity was consistent with an oceanic moisture source, suggesting that remote hydrological processes were the major contributor to daytime dv variability. During the early evening, ET fluxes into a shallow nocturnal inversion layer caused a lowering of dv values near the surface. In addition, transient mixing of vapour with a higher D-excess from above the nocturnal inversion modified these values, causing large variability during the night. These results indicate d

Moisture sources of the Chinese Loess Plateau during 1979–2009

The Chinese Loess Plateau (CLP) is one of the regions in China with the most serious water scarcity, and precipitation is a key issue in the CLP climate. To investigate the main sources of moisture for the CLP, we apply a Lagrangian particle dispersion model known as the flexible particle dispersion model (FLEXPART) to perform a 31–year transient simulation (1979–2009) with backtracking particles residing over the CLP for 10days in this study. Further, a moisture source attribution method, the areal source–receptor attribution method, is employed to quantify the contributions of the moisture source regions to the CLP precipitation. The results show that in summer (June–July–August), the moisture transported into the CLP originates mainly from central–eastern China, northwestern China–eastern Central Asia, the South China Sea, the East China Sea, the Bay of Bengal, and the Arabian Sea. In contrast, in winter (December–January–February) the main moisture source regions are northwestern China–eastern Central Asia, central–eastern China, the sea around the Arabian Peninsula, the southern Tibetan Plateau–northwestern Indo-China Peninsula, and the Mediterranean Sea. Because of significant losses en route, such as those caused by terrain obstruction and the long path to the continental interior, the eventual moisture release to the CLP is much less than the original uptake. Of the above-mentioned sources, the continental moisture sources near the CLP are the main contributors to precipitation over the CLP in both summer and winter; of these, a high proportion is contributed by central–eastern China and northwestern China–eastern Central Asia, while the oceanic moisture sources make a very small contribution. The reasons for this phenomenon may include the great distance between the related oceans and the CLP; thus, the direct oceanic water vapor transport into the CLP encounters obstacles on conventional backtracking days. Furthermore, the indirect oceanic moisture sources of the CLP are also preliminarily discussed herein.

Hydro-chemical evolution of groundwater in a sequence of Tertiary Formations in Northwest Oman

Tertiary Formations, which are widely spread in the Arab Peninsula, are fundamental to the oil industry in the world’s largest oil producing area. These formations host major aquifers that satisfy water demands for oil production such as steam injection and reservoir pressure maintenance . The current study investigates the hydro-chemical evolution of groundwater in a sequence of Tertiary Formations in Northwest Oman in the Kingdom of Saudi Arabia and the United Arab Emirates borders. Stable isotopes (18O and 2H) show three different groundwater types: (1) groundwater formed from precipitation originating from the Mediterranean Sea, (2) groundwater formed from mixing of northern (Mediterranean) and southern (Indian Ocean) moisture sources, and (3) groundwater affected by evaporation. Radiogenic isotopes 14C and 3H indicate younger groundwater close to the recharge source in North Oman Mountains and older age in the plain desert area that constitutes the discharge zone. Groundwater chemistry notably evolves from Ca–Mg–HCO3 dominant in the recharge area to Na–Cl–SO4 dominant type in the discharge zone. Water–rock interaction is the main process controlling groundwater chemistry in the recharge zone while evaporation becomes progressively effective down the gradient. The geochemistry and stable and radiogenic isotopes of the Tertiary Formations indicate that the study area has witnessed wet periods during the Early Holocene with surface water bodies formed by groundwater discharge at distal parts. The sustainability of such old groundwater that is being exploited for oil production should be thoroughly investigated to secure future oil production.