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We assess the spatial structure of moisture flux divergence, regional moisture sources and transport processes over Colombia, in northern South America. Using three independent methods the dynamic recycling model (DRM), FLEXPART and the Quasi-isentropic back-trajectory (QIBT) models we quantify the moisture sources that contribute to precipitation over the region. We find that moisture from the Atlantic Ocean and terrestrial recycling are the most important sources of moisture for Colombia, highlighting the importance of the Orinoco and Amazon basins as regional providers of atmospheric moisture. The results show the influence of long-range cross-equatorial flow from the Atlantic Ocean into the target region and the role of the study area as a passage of moisture into South America. We also describe the seasonal moisture transport mechanisms of the well-known low-level westerly and Caribbean jets that originate in the Pacific Ocean and Caribbean Sea, respectively. We find that these dynamical systems play an important role in the convergence of moisture over western Colombia.

From a global perspective, the stable isotope altitude effect is crucial for understanding climate information. However, the intensity of this effect can be influenced by the source of moisture, particularly in inland mountainous regions where the moisture sources are complex. Different combinations of moisture sources might affect the altitude effect. Focusing on the upper Shiyang River in the northern part of the Qilian Mountains in China, this study calculated the proportion of recycled moisture in precipitation and utilized the HYSPLIT model to determine the source of advective moisture. It explored the characteristics and mechanisms by which moisture sources affect the spatiotemporal variations in precipitation isotope effects within the study area. The findings indicated that: (a) The altitude effect follows a seasonal pattern: winter < autumn < spring < summer, with a reverse effect in winter. (b) As the contribution of recycled moisture to precipitation increases, the altitude effect of stable isotopes weakens, primarily due to the disruptive influence of recycled moisture on this effect. (c) The altitude effect of stable isotopes in precipitation is determined by the direction of the moisture source and its attributes. When the primary source of advective moisture runs perpendicular to the mountain range and the moisture migration speed is slow, the altitude effect is pronounced. Thus, although temperature directly causes the altitude effect, water vapor sources significantly influence it in inland mountainous regions. Key Points The altitude effect has significant seasonal variation, being strong in summer and weakest in winter The contribution of recirculating water vapor to precipitation is large, weakening the altitude effect The source of water vapor and the nature of the air masses contribute to the differences in elevation effects

In this study, we investigated the changes in the origin of moisture for the precipitation associated with tropical cyclones (TCs) after extratropical transition (ET) over the North Atlantic Ocean basin from 1980 to 2018. We analyzed the 24 hr before and after the occurrence of ET events. By applying a TC‐centric methodology we found that the moisture uptake (MU) occurred predominantly in the south and southwest sectors within ∼2,000 km of TC center before ET and from the southwest and west sectors after ET. In addition, the development of the cold front and the warm conveyor belt after ET induces changes in the moisture transport pattern. Overall, the secondary circulation of TCs favored the moisture flux inward for TCs precipitation, while the large‐scale baroclinic environment controlled the MU after ET. Plain Language Summary Tropical cyclones (TCs) are one of the weather systems that often cause fatalities and strong damage to socioeconomic infrastructures in tropical and subtropical latitudes. During the poleward movement, they can undergo an extratropical transition (ET), experiencing notable changes in their structure, such as the replacement of the warm core with a cold core and an increase in the area of the precipitation pattern. In this study, we investigate the changes in the moisture sources' contribution to the precipitation within the TCs' outer radius before and after ET over the North Atlantic Ocean basin. Our results show that the moisture sources were mainly located in the south and southwest sectors within ∼2,000 km of the TC center before ET and in the southwest and west sector after ET. Additionally, the large‐scale baroclinic environment modulated the moisture transport pattern after ET. These findings contribute to improving our knowledge about the changes in a TC during the ET event. Key Points The west North Atlantic Ocean was identified as the main moisture source, accounting for ∼44%–69% of total moisture uptake The moisture was predominately originated from the south and southwest sectors during PRE‐extratropical transition (ET) and from the southwest‐west during POST‐ET The large‐scale baroclinic environment controlled the moisture transport for tropical cyclones precipitation after ET

Sources of moisture-feeding for the incipient tropical cyclones (TCs) over the Bay of Bengal (BoB) are least explored. Using a three-dimensional Lagrangian model running with NCEP-FNL data, the present work identified various source locations and major transport routes that contributed moisture to pre- and post-monsoon TCs genesis (TCG) over the BoB. The clustered mean trajectories identified using the k -means method demonstrated that moisture transport towards the TCG region during pre-monsoon is dominated by intense low-level westerly/south-westerly trajectories propagating across the North Indian Ocean covering the Somalia coast and the Arabian Sea. Meanwhile, during post-monsoon, north-easterly/easterly trajectories are more prevalent and bring moisture to the BoB TCG region from the South China Sea and North-West Pacific. Moreover, the study highlights the spatiotemporal variations of moisture uptake (MU) pattern and its contribution and demonstrates that robust MU with a contribution of 20–90% occurs locally before three days of TCG. Meanwhile, remote sources contribute a moisture amount of ≤ 20%. Furthermore, the composite analysis of different diagnostic parameters confirms that identified MU at the local scale is attributed to an evaporation process in association with a dynamic uplifting mechanism caused by an anomalous cyclonic circulation, which provides moisture for TCG, while the horizontal advection is the mechanism, which gradually builds moisture required for the TCG from the remote sources.

We apply the Lagrangian-based moisture back trajectory method to two reanalysis datasets to determine the moisture sources for wet season precipitation over the Arabian Peninsula, defined as land on the Asian continent to the south of the Turkish border and west of Iran. To accomplish this, we make use of the evaporative source region between 65°W–120°E and 30°S–60°N, which is divided into twelve sub-regions. Our comparison of reanalyses and multiple observations allows us to validate datasets and highlight broad-scale similarities in characteristics, notwithstanding some inconsistencies in the southwest AP. The results indicate north-to-south spatiotemporal heterogeneity in the characteristics of dominant moisture sources. In the north, moisture for precipitation is mainly sourced from midlatitude land and water bodies, such as the Mediterranean and Caspian Seas. Areas further south are dependent on moisture transport from the Western Indian Ocean and parts of the African continent. The El Niño-Southern Oscillation (ENSO) exhibits an overall positive but sub-seasonally varying influence on the precipitation variability over the region, with noticeable moisture anomalies from all major source regions. A significant drying trend exists over parts of the Peninsula, which both reanalyses partially attribute to anomalies in the moisture advection from the Congo Basin and South Atlantic Ocean. However, considerable uncertainty in evaporation trends over the terrestrial evaporative sources in observations warrants additional modeling studies to further our understanding of key processes contributing to the negative trends.

Tracking and quantifying the moisture sources of precipitation in different drainage basins in the Tibetan Plateau (TP) help to reveal basin-scale hydrological cycle characteristics under the interactions between the westerlies and Indian summer monsoon (ISM) systems and to improve our understanding on the mechanisms of water resource changes in the ‘Asian Water Tower’ under climate changes. Based on a Eulerian moisture tracking model (WAM-2) and three atmospheric reanalysis products (ERA-I, MERRA-2, and JRA-55), the contributions of moisture sources to the precipitation in six major sub-basins in the TP were tracked during an approximately 35-year period (1979/1980–2015). The results showed that in the upper Indus (UI), upper Tarim River (UT), and Qaidam Basin (QB), the moisture sources mainly extended westward along the mid-latitude westerlies to the western part of the Eurasian continent. In contrast, in the Yarlung Zangbo River Basin (YB), inner TP (ITP), and the source area of three eastern rivers (TER, including the Nujiang River, Lancang River, and Yangtze River), the moisture sources extended both westward and southward, but mainly southward along the ISM. In winter and spring, all of the sub-basins were dominated by western moisture sources. In summer, the western sources migrated northward with the zonal movement of the westerlies, and simultaneously the southern sources of the YB, ITP, and TER expanded largely toward the Indian Ocean along the ISM. In autumn, the moisture sources of the UI, UT, and QB shrank to the western sources, and the moisture sources of the YB, ITP, and TER shrank to the central-southern TP and the Indian subcontinent. By quantifying the moisture contributions from multiple sources, we found that the terrestrial moisture dominated in all of the sub-basins, particularly in the UT and QB (62–73%). The oceanic contributions were relatively high in the UI (38–42%) and YB (38–41%). In winter, evaporation from the large western water bodies (such as the Mediterranean, Red Sea, and Persian Gulf) was significantly higher than that from the continental areas. This contributed to the peak (valley) values of the oceanic (terrestrial) moisture contributions to all of the sub-basins. In summer, the terrestrial moisture contributions to the UI, UT, and QB reached their annual maximum, but the abundant oceanic moisture transported by the ISM restrained the appearance of land source contribution peaks in the YB, ITP, and TER, resulting in almost equal moisture contributions in the YB from the ocean and land.

Despite the importance of the Tibetan Plateau (TP) to the surrounding water cycle, the moisture sources of the TP remain uncertain. In this study, the moisture sources of the TP are quantitatively identified based on a 33-year simulation with a horizontal resolution of 1.9° × 2.5° using the Community Atmosphere Model version 5.1 (CAM5.1), in which atmospheric water tracer technology is incorporated. Results demonstrate that the major moisture sources differ over the southern TP (STP) and northern TP (NTP). During the winter, Africa, the TP, and India are the dominant source regions, contributing nearly half of the water vapour over the STP. During the summer, the tropical Indian Ocean (TIO) supplies 28.5 ± 3.6% of the water vapour over the STP and becomes the dominant source region. The dominant moisture source regions of the water vapour over the NTP are Africa (19.0 ± 2.8%) during the winter and the TP (25.8 ± 2.4%) during the summer. The overall relative contribution of each source region to the precipitation is similar to the contribution to the water vapour over the TP. Like most models, CAM5.1 generally overestimates the precipitation over the TP, yielding uncertainty in the absolute contributions to the precipitation. Composite analyses exhibit significant variations in the TIO-supplied moisture transport and precipitation over the STP during the summer alongside anomalous TP heating. This relationship between moisture transport from the TIO and the TP heating primarily involves the dynamic change in the TIO-supplied moisture flux, which further controls the variation in the TIO-contributed precipitation over the STP.

Morocco, being part of the Mediterranean basin, is characterized by a semi-arid climate heavily affected by climate change, spatial heterogeneity of the water resources along with its high demand. As the region heavily relies on precipitation to supply surface and groundwater, the restraints are a capital threat to its availability. Therefore, conducting studies on the spatio-temporal variations of precipitation ought to be a necessity. Herein, we present the results of our study conducted in the High Oum-Er Rbiaa catchment in the Moroccan Middle Atlas Mountains. Spatial and temporal monitoring of precipitation isotopic composition during a full hydrological year indicated a wide variation of isotopic values ranging from – 3.7‰ to – 13.1‰ and from – 17.1‰ to – 78.2‰ for δ18O and δ2H, between winter and summer respectively. Deuterium excess was marked by a strong seasonality, ranging from -9.2‰ to 27.8‰, with a mean value of 14.8‰. The preliminary local meteoric water line (LMWL: δ2H = 6.58 × δ18O + 8.3 (R2 = 0.92)) reflects the significant evaporation effect, whereas the high d-excess values might also suggest Mediterranean-sourced moisture. Global reanalysis data of the vertical integral of moisture flux combined with air masses trajectories simulations, confirm substantial Mediterranean-sourced moisture during the rainiest events in 2020–2021, as shown by the depleted isotope values (– 9.5 < δ18O (‰) < – 13.1; – 55 < δ2H (‰) < – 78.2) and high d-excess (18 < d-excess (‰) < 27). Contrariwise, summer precipitation displays weak and heterogenous moisture sources indicative of continental influence, as reflected by the enriched isotope values and high d-excess (0 < d-excess (‰) < 5) likely due to sub-cloud evaporation process. The findings allowed to emphasize the influence of large-scale water vapor transport of oceanic moisture contributing to winter precipitation within the region, whereas summer precipitation is affected by cloud microphysical processes generating local rainfall.

We explore the South Atlantic as a moisture source for South America and its relationship with the precipitation variability in southeastern South America (SESA) during the 1982–2015 period. Based on diagnostic calculation with a Semi-Lagrangian analytical model, three regions of the South Atlantic acting as moisture sources for South America were studied: the Tropical Atlantic (15° N–5° S), Subtropical Atlantic (30° S–5° S), and Southwestern Atlantic (21° S–50° S; 30° W to the further west). The Tropical and Subtropical Atlantic are important sources of moisture for the Amazon, and occasionally for SESA. The Southwestern Atlantic contributes mainly locally, although in summer it also has a role in increasing precipitation over Uruguay and southern Brazil. Approximately 17% of the observed precipitation over the La Plata basin comes from the three regions identified as moisture source in the South Atlantic. Sea surface temperature variability is related to the moisture contribution from the South Atlantic to the continent. In summer, a significant positive correlation between the sea surface temperature leading-mode and the precipitation contribution from the Tropical Atlantic and La Plata Basin is found. A significant negative correlation between the sea surface temperature leading-mode and contribution in terms of precipitation from the Southwest Atlantic was found, as warm anomalies are associated with an anomalous cyclonic circulation over the Southwest Atlantic that favors moisture transport to SESA. Finally, the study of individual precipitation events identified contributions from the Subtropical and Southwest Atlantic to particular daily precipitation events in SESA. Climatological contributions from the Southwestern Atlantic are low, however, in events such as these, their contribution can increase up to 40% on the synoptic scale.

The Vietnamese Mekong Delta (VMD) is the most productive region in Vietnam in terms of agriculture and aquaculture. Unsurprisingly, droughts have been a prevalent concern for stakeholders across the VMD over the past decades. However, the VMD precipitation moisture sources and their dominant factors during drought conditions were not well understood. Using the ERA5 reanalysis data as inputs, the Water Accounting Model‐2layers (WAM‐2layers), a moisture tracking tool, was applied to identify the VMD precipitation moisture sources from 1980 to 2020. The modeling simulation indicates that the moisture sources transported from the upwind regions dominate the VMD precipitation by 60.4%–93.3%, and the moisture source areas vary seasonally with different monsoon types. Results of the causal inference algorithms indicate that the humidity and wind speed in the upwind area are the dominant factors for driving moisture transport and determining the amount of VMD precipitation in dry and wet seasons, respectively. The local atmospheric conditions may also have a causal effect on moisture recycling. During the drought events in 2015–2016 and 2019–2020, the reduced moisture transport in the 2016 dry season was mainly caused by the anomalies in both humidity and wind speed, while the below average moisture sources in the 2020 dry season were dominated by humidity. In the 2019 wet season, an anomaly in wind speed led to a decrease in the tracked moisture. These findings are of great significance for understanding the moisture sources of precipitation and further improving drought prediction in the VMD. Key Points Precipitation moisture sources for the Vietnamese Mekong Delta primarily originate from the external area, accounting for over 60% Based on causality algorithms, humidity and wind speed are the dominant factors of moisture transport in dry and wet seasons, respectively The large‐scale forcings and local atmospheric instability also have effects on moisture transport and recycling