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Atmospheric humidity and soil moisture in the Amazon forest are tightly coupled to the region's water balance, or the difference between two moisture fluxes, evapotranspiration minus precipitation (ET-P). However, large and poorly characterized uncertainties in both fluxes, and in their difference, make it challenging to evaluate spatiotemporal variations of water balance and its dependence on ET or P. Here, we show that satellite observations of the HDO/H2O ratio of water vapor are sensitive to spatiotemporal variations of ET-P over the Amazon. When calibrated by basin-scale and mass-balance estimates of ET-P derived from terrestrial water storage and river discharge measurements, the isotopic data demonstrate that rainfall controls wet Amazon water balance variability, but ET becomes important in regulating water balance and its variability in the dry Amazon. Changes in the drivers of ET, such as above ground biomass, could therefore have a larger impact on soil moisture and humidity in the dry (southern and eastern) Amazon relative to the wet Amazon.

Stable water isotopes (δ18Ow and δDw) have been successfully implemented for the first time in a high-resolution model of the Mediterranean Sea (NEMO-MED12). In this numerical study, model results are compared with available in situ observations to evaluate the model performance of the present-day distribution of stable water isotopes and their relationship with salinity on a sub-basin scale. There is good agreement between the modelled and observed distributions of δ18Ow in the surface water. The model successfully simulates the observed east–west gradient of δ18Ow characterising surface, intermediate, and deep waters. The results also show good agreement between the simulated δDw and the in situ data. The δDw shows a strong linear relationship with δ18Ow (r2=0.98) and salinity (r2=0.94) for the whole Mediterranean Sea. Moreover, the modelled relationships between δ18Ow and salinity agree well with observations, with a weaker slope in the eastern basin than in the western basin. We investigate the relationship of the isotopic signature of the planktonic foraminifera shells (δ18Oc) with temperature and the influence of seasonality. Our results suggest a more quantitative use of δ18O records, combining reconstruction with modelling approaches. [ABSTRACT FROM AUTHOR]

Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2-3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought.

The concept of the "Asian monsoon" masks the existence of two separate summer rainfall régimes: Convective storms over India, Bangladesh, and Nepal (the South Asian monsoon) and frontal rainfall over China, Japan, and the Korean Peninsula (the East Asian monsoon). In addition, the Himalayas and other orography, including the Arakan Mountains, Ghats, and Yunnan Plateau, create smaller precipitation domains with abrupt boundaries. A mode of continental precipitation variability is identified that spans both South and East Asia during July and August. Point-to-point correlations and EOF analysis with Asian Precipitation-Highly-Resolved Observational Data Integration Toward Evaluation of the Water Resources (APHRODITE), a 57-yr rain gauge record, show that a dipole between the Himalayan foothills (+) and the "monsoon zone" (central India, -) dominates July-August interannual variability in South Asia, and is also associated in East Asia with a tripole between the Yangtze corridor (+) and northern and southern China (-). July-August storm tracks, as shown by lag-lead correlation of rainfall, remain mostly constant between years and do not explain this mode. Instead, it is proposed that interannual change in the strength of moisture transport from the Bay of Bengal to the Yangtze corridor across the northern Yunnan Plateau induces widespread precipitation anomalies. Abundant moisture transport along this route requires both cyclonic monsoon circulation over India and a sufficiently warm Bay of Bengal, which coincide only in July and August. Preliminary results from the LMDZ version 5 (LMDZ5) model, run with a zoomed grid over Asia and circulation nudged toward the ECMWF reanalysis, support this hypothesis. Improved understanding of this coupling may help to project twenty-first-century precipitation changes in East and South Asia, home to over three billion people.

In order to complement the picture of the atmospheric water cycle in the Southern Ocean, we have continuously monitored water vapor isotopes since January 2020 on Amsterdam Island in the Indian Ocean. We present here the first 2-year long water vapor isotopic record at this site. We show that the water vapor isotopic composition largely follows the water vapor mixing ratio, as expected in marine boundary layers. However, we detect 11 periods of a few days where there is a strong loss of correlation between water vapor δ18 O and water vapor mixing ratio as well as abrupt negative excursions of water vapor δ18O. These excursions often occur toward the end of precipitation events. Six of these events show a decrease in gaseous elemental mercury, suggesting subsidence of air from a higher altitude. Our study aims to further explore the mechanism driving these negative excursions in water vapor δ18O. We used two different models to provide a data–model comparison over this 2-year period. While the European Centre Hamburg model (ECHAM6-wiso) at 0.9° was able to reproduce most of the sharp negative water vapor δ18 O excursions, hence validating the physics process and isotopic implementation in this model, the Laboratoire de Météorologie Dynamique Zoom model (LMDZ-iso) at 2° (3°) resolution was only able to reproduce seven (one) of the negative excursions, highlighting the possible influence of the model resolution for the study of such abrupt isotopic events. Based on our detailed model–data comparison, we conclude that the most plausible explanations for such isotopic excursions are rain–vapor interactions associated with subsidence at the rear of a precipitation event. [ABSTRACT FROM AUTHOR]

Based on a 6‐year long record (2014–2020) of the isotopic composition of rain (δ18Op) at Réunion Island (55°E, 22°S), in the South‐West Indian Ocean, this study shows that the annual isotopic composition of precipitation in this region is strongly controlled by the number of cyclones, the number of best‐track days, and the proportion of cyclonic rain during the year. Our results support the use of δ18Op in annual‐resolved tropical climate archives as a reliable proxy of past cyclone frequency. The influence of the proportion of cyclonic rain on the annual isotopic composition arises from the systematically more depleted precipitation and water vapor during cyclonic events than during less organized convective systems. The analysis of the daily to hourly isotopic composition of water vapor (δ18Ov) during low‐pressure systems and the reproduction of daily δ18Ov observations by AGCMs with a global medium to coarse resolution (LMDZ‐iso and ECHAM6‐wiso) suggest that during cyclonic periods the stronger depletion mainly arises from both enhanced large‐scale precipitation and water vapor‐rain interactions under humid conditions. Plain Language Summary: Water molecules have different forms: 2 atoms of hydrogen and 1 atom of oxygen 16 (majority form), or 1 atom of hydrogen, 1 atom of deuterium (2H) and 1 atom of oxygen 16, or 2 atoms of hydrogen and 1 atom of oxygen 18. The relative proportion of these different molecules in precipitation and water vapor is called isotopic composition. We have analyzed the isotopic composition of rain and water vapor at Réunion Island (55°E, 22°S), in the South‐West Indian Ocean, for 6 years (2014–2020). We show that the annual isotopic composition of precipitation in this region is a reliable indicator of the number of tropical cyclones (TCs) during the year. This opens the possibility to use annual‐resolved tropical climate archives of the isotopic composition of precipitation, like speleothems, to study how cyclone frequency in the Indian Ocean has varied in the past under different mean climates (warmer or colder). This study also seeks to understand why the isotopic composition of precipitation behaves in this way during TCs. We show that this is due to increased precipitation at the regional scale and enhanced exchanges of water molecules between rain and water vapor in the vicinity of TCs. Key Points: The annual isotopic composition of precipitation on Réunion Island is controlled by the number of cyclones per yearBoth precipitation and water vapor are systematically more depleted during cyclonic periodsMore large‐scale precipitation and rain–vapor interactions are responsible for the stronger isotopic depletion during tropical cyclones (TCs) [ABSTRACT FROM AUTHOR]

In order to complement the picture of the atmospheric water cycle in the Southern Ocean, we have continuously monitored water vapor isotopes since January 2020 in Amsterdam Island (37.7983 °S, 77.5378 °E) in the Indian Ocean. We present here the first 2-year-long water vapor isotopic record monitored on this site. We show that the vapor isotopic composition largely follows the vapor mixing ratio, as expected in marine boundary layers. However, we evidence 11 cold front periods of a few days where there is a strong loss of correlation between water vapor δ18O and mixing ratio. These periods are associated with abrupt negative excursions of water vapor δ18Ο, often occurring toward the end of precipitation events. Six of these events show a decrease in gaseous elemental mercury suggesting subsidence of air from higher altitude. Accurately representing the water isotopic signal during these cold fronts is a real challenge for the atmospheric components of Earth System models equipped with water isotopes. While the ECHAM6-wiso model was able to reproduce most of the sharp negative water vapor δ18O excursions, the LMDZ-iso model at 2° (3°) resolution was only able to reproduce 7 (1) of the negative excursions. Based on a detail model-data comparison, we conclude that the most plausible explanations for such isotopic excursions are rain-vapor interactions associated with subsidence at the rear of a precipitation event. [ABSTRACT FROM AUTHOR]

The Raman lidar technique as currently applied for the retrieval of WV mixing ratio profiles allows, in theory, to estimate HDO/H2O atmospheric profiles. The objective of this study is to develop a lidar simulator and to study the feasibility of the groundbased Raman lidar HDO measurement. Thus, the characteristics of a realistic lidar system for the estimation of HDO/H2O atmospheric profiles are investigated through a set of numerical simulations.

Stable water isotopes are natural tracers in the hydrological cycle and have been applied in hydrology, atmospheric science, ecology, and paleoclimatology. However, the factors controlling the isotopic distribution, both at spatial and temporal scales, are debated in East Asia. For the first time, we made large scale (order 10000 km) continuous observations of near-surface vapor isotopes across China in both pre-monsoon and monsoon seasons, using a newly-designed vehicle-based vapor isotope monitoring system. For both seasons, the observed variations along the sampling route are mainly due to spatial variations, and marginally influenced by synoptic-scale variations. The data thus documents the spatial and seasonal variability of vapor isotopes. The spatial variations of vapor δ18O are mainly controlled by Rayleigh distillation during the pre-monsoon period, but significantly influenced by different moisture sources, continental recycling processes and convection during moisture transport during the monsoon period. The seasonal variation of vapor δ18O reflects the influence of the summer monsoon convective precipitation in southern China, and a dependence on temperature in the North. The spatial and seasonal variations in d-excess reflect the different moisture sources and the influence of continental recycling. The isotope-incorporated global spectral model (Iso-GSM) successfully captures the spatial variation of vapor δ18O during the pre-monsoon period owing to the large latitudinal contrast in humidity and temperature, the overall performance is weaker during the monsoon period. These results provides an overview of the spatial distribution and seasonal variability of water isotopic composition in East Asia and their controlling factors, and emphasize the need to interpret proxy records in the context of the regional system and moisture sources. [ABSTRACT FROM AUTHOR]