Variability Of Moisture Sources Research Articles

Moisture variability in northeast Tibet following the middle Miocene Climate transition

After the middle Miocene Climate transition, global cooling caused substantial hydrological changes in East Asia. Today, many lines of evidence indicate that inland East Asia experienced persistent aridification after the middle Miocene Climate transition. However, chemical weathering records from the South China Sea as well as late-Miocene paleolake expansions in northeastern Tibet contrast with the idea of long-term aridification after the middle Miocene Climate transition. Here, we present manganese-to-iron ratios and clay mineral contents of sediments formed approximately 14 to 10 million years ago from the Guide Basin, northeastern Tibetan Plateau, combined with previously published records (lithology, magnetic susceptibility, redness, and strontium content). Our results show that a paleolake in the Guide Basin expanded between approximately 11.4 and 10.5 million years ago, opposite to the long-term drying trend after the middle Miocene Climate transition. On the 100-kilo-year eccentricity scale, our results demonstrate alternating moisture source variability in northeast Tibet, with precipitation predominantly delivered via the westerlies and the East Asian summer monsoon. This variability could potentially be related to the movement of the westerly jet over the Tibetan Plateau. After comparison to climate model simulations, we attribute this movement to astronomical forcing and the waxing and waning of the Antarctic ice sheet.

Impact of Atmospheric Circulation Variability on U.S. Midwest Moisture Sources

Abstract Elevated spring and summer rainfall in the U.S. Midwest is often associated with a strong Great Plains low-level jet (GPLLJ), which transports moist air northward to the region from the Gulf of Mexico. While the intensity of hourly precipitation extremes depends on local moisture availability and vertical velocity, sustained moisture convergence on longer time scales depends on horizontal moisture advection from remote sources. Therefore, the magnitude of moisture convergence in the Midwest depends in part on the humidity in these moisture source regions. Past work has identified the time-mean spatial distribution of moisture sources for the Midwest and studied how this pattern changes in years with anomalous rainfall. Here, using reanalysis products and an Eulerian moisture tracking model, we seek to increase physical understanding of this moisture source variability by linking it to the GPLLJ, which has been studied extensively. We find that on interannual time scales, an anomalously strong GPLLJ is associated with a shift in the distribution of moisture sources from land to ocean, with most of the anomalous moisture transported to—and converged in—the Midwest originating from the Atlantic Ocean. This effect is more pronounced on synoptic time scales, when almost all anomalous moisture transported to the region originates over the ocean. We also show that the observed positive trend in oceanic moisture contribution to the Midwest from 1979 to 2020 is consistent with a strengthening of the GPLLJ over the same period. We conclude by outlining how projected changes in a region’s upstream moisture sources may be useful for understanding changes in local precipitation variability. Significance Statement In this work, we study how the origin of moisture that forms precipitation in the U.S. Midwest covaries with large-scale atmospheric circulation. Our results show that an intensification of the mean winds tends to increase the proportion of total rainfall that originates from the ocean. This analysis may help to constrain future projections of rainfall extremes in the central United States, as projected changes in humidity over the ocean are typically more robust and better understood than those over land.

Major moisture source patterns for extreme precipitation events over the Chinese Loess Plateau

AbstractExtreme precipitation (EP) has significant hydrological and environmental influences on the Chinese Loess Plateau (CLP) during the rainy season (June–September). In this study, we analysed the moisture sources for the EP events over the CLP during the rainy season. Three major moisture source patterns were identified through analysing the moisture source variability across the EP events with a newly developed method combining the Lagrangian moisture diagnostic and a clustering algorithm. South China and North China were determined as the most critical moisture source regions for the EP events over the CLP. The three moisture source patterns corresponded to three EP types, including the CLP‐A type with a high contribution from North China (23.55%, the relative moisture contribution) and a relatively low contribution from South China (13.77%), and the CLP‐B and CLP‐C types, which had limited contributions (10.64% and 13.28%) from North China in common, and exhibited moderately high (19.30%) and extremely high (30.59%) contributions from South China, respectively. The moisture transport pathways for the three EP types were quantitatively analysed in detail. The favourable circulation patterns for the three EP types were further illustrated to reveal the atmospheric conditions for the spatial variations in moisture source patterns. In addition, this study provides a new strategy to analyse the moisture source variability, especially when it is difficult to find an easily manipulated criterion to classify the precipitation events.

Seasonal Variations in Moisture Origin Explain Spatial Contrast in Precipitation Isotope Seasonality on Coastal Western Greenland

AbstractIncreased precipitation in the Arctic is a robust feature across model simulations of the coming century, driven by intensification of meridional moisture transport and enhanced local evaporation in the absence of sea ice. These mechanisms are associated with distinct, seasonal, spatial, and, likely, precipitation isotope (δ2HPrecip) expressions. Historical observations of δ2HPrecip reveal a contrast in seasonality between southwestern and northwestern coastal Greenland: δ2HPrecip in northwestern Greenland varies in phase with local temperature, whereas δ2HPrecip in southwestern Greenland is decoupled from local temperature and exhibits little seasonal variation. We test the hypothesis that reduced δ2HPrecip seasonality in southwestern Greenland relative to northwestern Greenland results from dynamic moisture source variations, by diagnosing monthly average moisture sources to three sink regions (Kangilinnguit, Ilulissat, and Qaanaaq) using the Water Accounting Model‐2layers model. All domains demonstrate strong intra‐annual moisture source variations. Moisture to the southernmost region is sourced most remotely in summer and most locally in winter, associated with stronger cooling from the source in summer than winter, promoting more negative δ2HPrecip and counteracting local temperature‐driven seasonality. In comparison, moisture transport distance to the northernmost region is relatively constant, as local sea ice restricts northward migration of the winter moisture source. We simulate seasonal patterns in δ2HPrecip in a simple Rayleigh model, which confirm the importance of source temperature and starting isotopic compositions in determining δ2HPrecip for these regions. δ2HPrecip sensitivity to moisture source variability suggests these coastal Arctic settings may yield paleoclimate records sensitive to the moisture transport processes predicted to amplify future precipitation.

The role of land and ocean evaporation on the variability of precipitation in the Yangtze River valley

Abstract. The Yangtze River valley (YRV) experiences large intraseasonal and interannual precipitation variability, which is mainly due to East Asian monsoon influence. The East Asian monsoon is caused by interaction of many processes in the coupled land–atmosphere–ocean system. To better understand YRV precipitation variability in this complex system, we have studied the precipitation moisture sources and their connection to YRV precipitation. We obtained the moisture sources by using the European Centre for Medium-Range Weather Forecasts' (ECMWF) ERA-Interim reanalysis dataset, the FLEXible PARTicle dispersion model (FLEXPART), and the WaterSip moisture source diagnostic. The variability of moisture sources reflects the variability of YRV precipitation. Intraseasonal variations of moisture sources include a shift of the most important source regions as the monsoon progresses. Interannual variability of the moisture sources shows that sources which are less important climatologically are closely connected to variations of the driest and wettest years. Our results show that land directly contributes 58 % of moisture for YRV precipitation during 1980–2016, whereas the ocean contributes 42 % in direct transport. While the importance of the ocean as a moisture source is often emphasized, our results underscore the importance of the process of continental recycling and the role of land moisture sources.

Holocene changes in biome size and tropical cyclone activity around the Northern South China Sea

The South China Sea (SCS), characterized by a large continental shelf, is located at the edge of the Asian monsoon domain. In this study, two marine sediment cores from the northern SCS (NSCS) continental slope were investigated to construct composite vegetation and precipitation isotopic composition records based on the δ13C and δD values of plant-wax n-alkanes throughout the Holocene (last 11,200 years; i.e. 11.2 ka). The composite δ13Cwax record indicates an overall predominance of C3 vegetation over the last 11.2 ka. Before 8 ka BP, higher δ13Cwax values are attributed to preferential wax input from grassland and wetland biomes on the exposed continental shelf. After the inundation of the shelf by eustatic sea level rise until ca. 8 ka BP grassland and wetland biomes suffered a major size reduction and arboreal vegetation became better represented in the δ13Cwax record. The composite temperature corrected δDwax-T record suggests that moisture source variability drove precipitation isotopic composition changes during the Holocene. Lower δDwax-T values before 8.3 ka BP are interpreted as a larger moisture contribution by Pacific Ocean tropical cyclones, whereas higher δDwax-T values after 8.5 ka BP are interpreted as a larger moisture contribution from the Indian Ocean summer monsoon. Higher incidence of tropical cyclones in the NSCS during the Early Holocene was related to a temporary westward shift of the Western Pacific Warm Pool and enhanced insolation over the Northern Hemisphere. Both external and internal forcing mechanisms regulated moisture source changes in East Asia during the Holocene.

Moisture Sources for East Asian Precipitation: Mean Seasonal Cycle and Interannual Variability

ABSTRACT This study investigates the moisture sources that supply East Asian (EA) precipitation and their interannual variability. Moisture sources are tracked using the Water Accounting Model-2layers (WAM-2layers), based on the Eulerian framework. WAM-2layers is applied to five subregions over EA, driven by the ERA-Interim reanalysis from 1979 to 2015. Due to differences in regional atmospheric circulation and in hydrological and topographic features, the mean moisture sources vary among EA subregions. The tropical oceanic source dominates southeastern EA, while the extratropical continental source dominates other EA subregions. The moisture sources experience large seasonal variations, due to the seasonal cycle of the EA monsoon, the freeze–thaw cycle of the Eurasian continent, and local moisture recycling over the Tibetan Plateau. The interannual variability of moisture sources is linked to interannual modes of the coupled ocean–atmosphere system. The negative phase of the North Atlantic Oscillation increases moisture transport to northwestern EA in winter by driving a southward shift in the midlatitude westerly jet over the Mediterranean Sea, the Black Sea, and the Caspian Sea. Atmospheric moisture lifetime is also reduced due to the enhanced westerlies. In summers following El Niños, an anticyclonic anomaly over the western North Pacific increases moisture supplied from the South China Sea to the southeastern EA and shortens the traveling distance. A stronger Somali Jet in summer increases moisture to the Tibetan Plateau and therefore increases precipitation over the eastern Tibetan Plateau. The methods and findings in this study can be used to evaluate hydrological features in climate simulations.

Speleothem Paleoclimatology for the Caribbean, Central America, and North America

Speleothem oxygen isotope records from the Caribbean, Central, and North America reveal climatic controls that include orbital variation, deglacial forcing related to ocean circulation and ice sheet retreat, and the influence of local and remote sea surface temperature variations. Here, we review these records and the global climate teleconnections they suggest following the recent publication of the Speleothem Isotopes Synthesis and Analysis (SISAL) database. We find that low-latitude records generally reflect changes in precipitation, whereas higher latitude records are sensitive to temperature and moisture source variability. Tropical records suggest precipitation variability is forced by orbital precession and North Atlantic Ocean circulation driven changes in atmospheric convection on long timescales, and tropical sea surface temperature variations on short timescales. On millennial timescales, precipitation seasonality in southwestern North America is related to North Atlantic climate variability. Great Basin speleothem records are closely linked with changes in Northern Hemisphere summer insolation. Although speleothems have revealed these critical global climate teleconnections, the paucity of continuous records precludes our ability to investigate climate drivers from the whole of Central and North America for the Pleistocene through modern. This underscores the need to improve spatial and temporal coverage of speleothem records across this climatically variable region.

Heterodynes dominate precipitation isotopes in the East Asian monsoon region, reflecting interaction of multiple climate factors

For the past decade, East Asian monsoon history has been interpreted in the context of an exceptionally well-dated, high-resolution composite record of speleothem oxygen isotopes (δ18Ocave) from the Yangtze River Valley. This record is characterized by a unique spectral response, with variance concentrated predominantly within the precession band and an enigmatic lack of variance at the eccentricity and obliquity bands. Here we examine the spectral characteristics of all existing >250-kyr-long terrestrial water isotope records in Asia, including a new water isotope record using leaf wax hydrogen isotope ratios from the Chinese Loess Plateau. There exist profound differences in spectral characteristics among all orbital-scale Asian water isotope records. We demonstrate that these differences result from latitudinal gradients in the influence of the winter and summer monsoons, both of which impact climate and water isotopes throughout East Asia. Water isotope records therefore do not reflect precipitation during a single season or from a single circulation system. Rather, water isotope records in East Asia reflect the complex interplay of oceanic and continental moisture sources, operating at multiple Earth-orbital periods. These non-linear interactions are reflected in water isotope spectra by the presence of heterodynes. Although complex, we submit that water isotope records, when paired with rapidly developing isotope-enabled model simulations, will have the potential to elucidate mechanisms causing seasonal precipitation variability and moisture source variability in East Asia.

Twentieth-century sea ice variability in the Weddell Sea and its effect on moisture transport: Evidence from a coastal East Antarctic ice core record

High-resolution study of deuterium excess (d-excess), sea salt sodium (ss-Na+), and methane sulfonic acid (MSA) in an ice core from coastal Dronning Maud Land (cDML), East Antarctica, revealed the history of moisture transport and sea ice extent (SIE) during the last century. Backward wind trajectories show that air parcels were mainly derived from the Weddell Sea region. The d-excess profile shows a dramatic shift from an average value of 8‰ during 1905–1920 to −1‰ during ~1940 and thereafter positive excursion during 1940–1980. The dramatic shift during 1920–1940 has been attributed to the reduced moisture supply from low/mid-latitude to Antarctica associated with shifting of Southern Annular Mode (SAM) from positive to negative mode. The ss-Na+ flux profile shows systematic positive excursion during 1940–1980 which coincide with that of the d-excess profile. The MSA flux shows a negative excursion during 1950–1965, overlapping with the period of positive excursions in ss-Na+ and d-excess profiles. The concomitant increase of ss-Na+ and d-excess values and positive excursions during 1940–1980 indicate higher SIE. Based on significant correlation between the Na+ flux and satellite-derived winter SIE record of the Weddell Sea, ~10% increase in SIE is estimated compared to its average value of the last century. The power spectrum analysis of d-excess and ss-Na+ flux shows a significant periodicity at ~3.5 years, exactly matching with that of the winter SIE in the Weddell Sea. Wavelet analysis of SAM index and Southern Oscillation Index (SOI) shows the highest common power in 4 to 8 year band, overlapping with the periods of higher SIE and opposite phase in 10 to 16 year band, overlapping with the periods of higher d-excess. This study highlights the role of SAM and its teleconnection to El Niño Southern Oscillations (ENSO) in controlling sea ice and moisture source variability in annual to decadal scale in the coastal regions of Antarctica.

Spatiotemporal structure of the moisture sources feeding heavy precipitation events over the Sichuan Basin

ABSTRACTThe Sichuan Basin (SCB), in southwest China, is characterized by a high frequency of heavy precipitation (HP) events in the boreal summer. In this study, the top 50 HP events that occurred in the SCB were selected, based on daily precipitation gauge data during 1980–2013, and the moisture sources for these events have been identified. Taking advantage of the Lagrangian approach, we particularly examined the spatiotemporal structure of the moisture sources, both in their magnitude and temporal sequence. The results of this study can be summarized as follows: (1) the quantification of the contribution of the moisture sources indicated that the remote sources of moisture originating from the Tibetan Plateau, Indian Peninsula, Bay of Bengal and the Arabian Sea made considerable contributions to the moisture in the HP events; however, these areas were only the third most important factor in climatology, following the stronger influence of moisture uptake from the SCB local regions and the nearby Indo‐China Peninsula and South China Sea regions; (2) the time that moisture involved in the HP events varies between a few hours to more than a week before the HP events. Most of the moisture uptakes happened 1–5 days before the events, with maximum uptake occurring about 1 day before; (3) individual event‐to‐event variability of moisture sources was apparent. However, in contrast to the primary moisture contributors, this event‐to‐event variation depended largely on the externally imported moisture, particularly water vapor conveyed by both dominant southwest circulations and the moisture exported from the Tibetan Plateau, rather than the local SCB region. These results have established the atmospheric flow patterns that are prerequisites for HP events; meanwhile, regional moisture availability over the SCB region could also be an essential component of forecasting the HP events and needs to be better understood.

Variability of moisture sources in the Mediterranean region during the period 1980-2000

In this study, seasonal and interannual variability of the main atmospheric moisture sources over eight regions in the Mediterranean basin were investigated along a 21 year period. The Lagrangian dispersion model FLEXPART, developed by Stohl and James [2004, 2005], was applied to identify the contribution of humidity to the moisture budget of each region. This methodology is used to compute budgets of evaporation minus precipitation (E-P) by calculating changes in the specific humidity along backward trajectories, for the preceding 10 day periods. The results show clear seasonal differences in the moisture sources between wet and dry seasons. The Western Mediterranean Sea is the dominant moisture source for almost all the regions in the Mediterranean basin during the wet season, while the local net evaporation dominates during the dry season. The highest interannual variability is found in contributions to the Iberian Peninsula, Italy, and the Eastern Mediterranean. It is seen that the role of teleconnections is more limited than for the precipitation recorded in the region.

A Diagnosis of the 1979–2005 Extreme Rainfall Events in the Southeastern United States with Isentropic Moisture Tracing

Abstract A detailed analysis is performed to better understand the interannual and subseasonal variability of moisture sources of major recent dry (1980, 1990, and 2000) and wet (1994, 2003, and 2005) June–August (JJA) seasons in the southeastern United States. Wet (dry) JJAs show an increased (decreased) standard deviation of daily precipitation. Whereas most days during dry JJAs have little or no precipitation, wet JJAs contain more days with significant precipitation and a large increase of heavy (+10 mm) precipitation days. At least two tropical cyclone/depression landfalls occur in the southeastern United States during wet JJAs, whereas none occur during dry JJAs. The trajectory analysis suggests significant local recycling of moisture, implying that land surface feedback has the potential to enhance (suppress) precipitation anomalies during a wet (dry) JJA. Remote moisture sources during heavy precipitation events are very similar between wet and dry JJAs. The distinction between wet and dry JJAs lies in the frequency of heavy precipitation events. During the wet JJAs, heavy precipitation events contribute to more than half of the JJA precipitation total.

Interannual variability of Greenland winter precipitation sources: Lagrangian moisture diagnostic and North Atlantic Oscillation influence

We present a new Lagrangian diagnostic for identifying the sources of water vapor for precipitation. Unlike previous studies, the method allows for a quantitative demarcation of evaporative moisture sources. This is achieved by taking into account the temporal sequence of evaporation into and precipitation from an air parcel during transport, as well as information on its proximity to the boundary layer. The moisture source region diagnostic was applied to trace the origin of water vapor for winter precipitation over the Greenland ice sheet for 30 selected months with pronounced positive, negative, and neutral North Atlantic Oscillation (NAO) index, using the European Centre for Medium‐Range Weather Forecasts' ERA‐40 reanalysis data. The North Atlantic and the Nordic seas proved to be the by far dominant moisture sources for Greenland. The location of the identified moisture sources in the North Atlantic basin strongly varied with the NAO phase. More specifically, the method diagnosed a shift from sources north of Iceland during NAO positive months to a maximum in the southeastern North Atlantic for NAO negative months, qualitatively consistent with changes in the concurrent large‐scale mean flow. More long‐range moisture transport was identified during the NAO negative phase, leading to the advection of moisture from more southerly locations. Different regions of the Greenland ice sheet experience differing changes in the average moisture source locations; variability was largest in the north and west of Greenland. The strong moisture source variability for Greenland winter precipitation with the NAO found here can have a large impact on the stable isotope composition of Greenland precipitation and hence can be important for the interpretation of stable isotope data from ice cores. In a companion paper, the implications of the present results are further explored in that respect.

Modeling δ18O in precipitation over the tropical Americas: 1. Interannual variability and climatic controls

We use two atmospheric general circulation models (AGCMs), the ECHAM‐4 and the GISS II models, to analyze the interannual variability of δ18O in precipitation over the tropical Americas. Several different simulations with isotopic tracers forced with observed global sea surface temperatures (SST) between 1950 and 1998 reveal the influence of varying temperature, precipitation amount, and moisture source contributions on the predicted δ18O distribution. Observational evidence from climatic (NCEP‐NCAR) and sparse stable isotope (IAEA‐GNIP) data is used to evaluate model performance. The models capture the essential features of surface climate over the tropical Americas in terms of both their spatial and temporal characteristics. Using a low‐resolution model (GISS II), adjusted to provide a more realistic Andean topography, or a higher‐resolution model (ECHAM‐4 T106) leads to an improved δ18O distribution over the tropical Americas with an altitude effect comparable to observations. Water vapor transport and gradual rainout and increasingly depleted composition of water vapor along its trajectory are correctly simulated in both models, although the ECHAM model appears to underestimate the continentality effect over the Amazon basin. A significant dependence of δ18O on the precipitation amount is apparent in both models, in accordance with observations, while the influence of temperature seems to be less significant in most regions and is accurately reproduced by the ECHAM model only. Over most regions, however, the δ18O signal in precipitation is influenced by a combination of factors, such as precipitation amount, temperature, moisture source variability, and atmospheric circulation changes. Over parts of the tropical Americas, the δ18O signal is therefore also significantly correlated with ENSO because ENSO is an integrator of many factors affecting the δ18O composition of precipitation.

Modeling δ18O in precipitation over the tropical Americas: 2. Simulation of the stable isotope signal in Andean ice cores

We use the ECHAM‐4 and the GISS II atmospheric general circulation models (AGCM) with incorporated stable isotopic tracers and forced with observed global sea surface temperatures (SST) between 1979 and 1998, to simulate the δ18O signal in three tropical Andean ice cores, from Huascarán (Peru), Quelccaya (Peru), and Sajama (Bolivia). In both models, the simulated stable isotopic records compare favorably with the observational data, when the seasonality of precipitation and dry season loss due to sublimation and wind scour are taken into account. Our simulations indicate a significant influence of the local climatic conditions (temperature and precipitation amount) on the δ18O signal. Moisture source variability appears to be less of a factor on interannual timescales. Even though the moisture originates over the Amazon basin and the tropical Atlantic, correlation fields with National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) Reanalysis atmospheric variables and SST data indicate a dominant tropical Pacific control on interannual timescales. More enriched (depleted) δ18O values are associated with periods of warm (cold) conditions in the tropical Pacific. This is consistent with modern observational evidence, which shows that climate and atmospheric circulation in the tropical Andes are closely correlated with SST anomalies in the tropical Pacific domain on interannual to interdecadal timescales. The growing number of stable isotope records from tropical Andean ice cores may thus provide an important archive for reconstructing Pacific climate variability.