Shifts In Circulation Research Articles

Quantifying Surface Shelf Water Export in the Southern Middle Atlantic Bight Using a Lagrangian Particle Tracking Approach

AbstractShelf water is influenced by atmospheric forcing, river outflows, and the open ocean. Studying its variability is crucial for understanding anthropogenic impacts on coastal oceans and their transport to the open ocean. In the Middle Atlantic Bight (MAB), the interaction of the Gulf Stream with shelf/slope circulation leads to some of the complex exchanges between the shelf and open ocean along the U.S. East Coast. This study employs a Lagrangian particle tracking approach, grounded in a high‐resolution, data‐assimilative ocean reanalysis, to examine the export pathways of surface shelf water in the MAB. We analyzed over 700 daily images of simulated particle distributions using image clustering techniques. This revealed three distinct export patterns: abrupt entrainment to the Gulf Stream, gradual entrainment, and southern transport. Each pattern was observed roughly equally during the study period from January 2017 to December 2018. The observed export patterns are closely linked to the coastal circulation dynamics near Cape Hatteras. Understanding the timing and duration of these patterns is vital for assessing water quality and predicting the settlement of species that spawn in the region. Our study further underscores the influence of tropical cyclones, including Hurricanes Jose, Maria, and Chris, on these export patterns. These extreme weather events lead to significant shifts in coastal circulation near Cape Hatteras.

Mid-Holocene climate at mid-latitudes: assessing the impact of Saharan greening

Abstract. During the first half of the Holocene (11 000 to 5000 years ago), the Northern Hemisphere experienced a strengthening of the monsoonal regime, with climate reconstructions robustly suggesting a greening of the Sahara region. Palaeoclimate archives also show that this so-called African humid period (AHP) was accompanied by changes in climate conditions at middle to high latitudes. However, inconsistencies still exist in reconstructions of the mid-Holocene (MH) climate at mid-latitudes, and model simulations provide limited support in reducing these discrepancies. In this paper, a set of simulations performed using a climate model are used to investigate the hitherto unexplored impact of Saharan greening on mid-latitude atmospheric circulation during the MH. Numerical simulations show Saharan greening has a year-round impact on the main circulation features in the Northern Hemisphere, especially during boreal summer (when the African monsoon develops). Key findings include a westward shift in the global Walker Circulation, leading to modifications in the North Atlantic jet stream in summer and the North Pacific jet stream in winter. Furthermore, Saharan greening modifies atmospheric synoptic circulation over the North Atlantic, enhancing the effect of orbital forcing on the transition of the North Atlantic Oscillation phase from predominantly positive to negative in winter and summer. Although the prescription of vegetation in the Sahara does not improve the proxy–model agreement, this study provides the first constraint on the influence of Saharan greening on northern mid-latitudes, opening new opportunities for understanding MH climate anomalies in regions such as North America and Eurasia.

Novel Atmospheric Dynamics Shape the Inner Edge of the Habitable Zone around White Dwarfs

White dwarfs offer a unique opportunity to search nearby stellar systems for signs of life, but the habitable zone around these stars is still poorly understood. Since white dwarfs are compact stars with low luminosity, any planets in their habitable zone should be tidally locked, like planets around M dwarfs. Unlike planets around M dwarfs, however, habitable white dwarf planets have to rotate very rapidly, with orbital periods ranging from hours to several days. Here we use the ExoCAM global climate model to investigate the inner edge of the habitable zone around white dwarfs. Our simulations show habitable planets with ultrashort orbital periods (P ≲ 1 day) enter a “bat rotation” regime, which differs from typical atmospheric circulation regimes around M dwarfs. Bat rotators feature mean equatorial subrotation and a displacement of the surface’s hottest regions from the equator toward the midlatitudes. We qualitatively explain the onset of bat rotation using shallow water theory. The resulting circulation shifts increase the dayside cloud cover and decrease the stratospheric water vapor, expanding the white dwarf habitable zone by ∼50% compared to estimates based on 1D models. The James Webb Space Telescope should be able to quickly characterize bat rotators around nearby white dwarfs thanks to their distinct thermal phase curves. Our work underlines that tidally locked planets on ultrashort orbits may exhibit unique atmospheric dynamics, and guides future habitability studies of white dwarf systems.

Distribution of and Relationships between Epidemiological and Clinicopathological Parameters in Canine Leishmaniosis: A Retrospective Study of 15 Years (2009-2023).

Leishmaniosis is a vector-borne disease caused by protozoan parasites of the genus Leishmania, which are zoonotic and have an important impact on animal and public health globally. Between 2009 and 2023, blood samples from domestic dogs with clinical suspicion of leishmaniosis were received from 286 veterinary medical centres throughout mainland Portugal. An enzyme-linked immunosorbent assay (ELISA) was utilised to detect antibodies against Leishmania infantum antigens. Additionally, a complete blood count and tests for total proteins, urea, creatinine and alanine aminotransferase, as well as protein electrophoresis, were also performed. No significant relationship between sex and breed was observed. The age distribution was bimodal, with the highest prevalence of disease occurring at 2-5 years of age and a secondary peak occurring at 6 years or over (p < 0.001). No statistical correlation was observed between creatinine and urea across the ELISA serological groups. In contrast, both the gamma globulin levels (r = 0.45; p < 0.001) and the albumin/globulin ratio (r = -0.36; p < 0.001) exhibited moderate correlations with the ELISA. These findings support recent seroprevalence studies in dogs, with some geographical areas in Northern Portugal exhibiting the highest values, which may be the result of geographical shifts in parasite circulation due to climate change.

Climate Change Impacts on Precipitation Dynamics in the Southern Marmara Region of Turkey

Understanding the dynamics of precipitation patterns is crucial for effective water management strategies, especially in regions vulnerable to the impacts of climate change. This study investigates the projected changes in annual and seasonal precipitation across the Southern Marmara Region of Turkey by comparing the averages of the reference period (1971-2000) with those of the future period (2061-2090). Employing multiple climate models (GFDL, HADGEM, and MPI) and Representative Concentration Pathways (RCP4.5 and RCP8.5), the analysis includes Mann-Kendall trend tests and Sen's slope method to determine trends in precipitation patterns. Key findings reveal significant variability in precipitation projections among different models and scenarios, with implications for water resource management, agriculture, and ecosystem resilience in provinces such as Çanakkale, Balıkesir, Bursa, Bilecik, and Yalova. According to the annual rainfall change rates relative to the reference period, Balıkesir province stands out as the most resilient province against climate change with average rates of 8.81% and 7.09% under the HADGEM and MPI model simulations, respectively. Regarding seasonal variations, Bilecik province is expected to experience a significant decrease in rainfall, reaching up to -53.78% under the MPI RCP8.5 scenario. In terms of within-period changes in annual rainfall values, the strongest declining trend was identified with Z=-2.03 in Bilecik province under the MPI RCP8.5 scenario conditions by the Mann-Kendall test. On the other hand, for seasonal variations, Bursa province demonstrates the most robust decreasing trend under the GFDL RCP4.5 conditions (Z=-2.89). The study emphasizes the importance of considering spatially varying precipitation patterns and potential shifts in atmospheric circulation for sustainable water resource management amidst climate variability and change in the Southern Marmara region. These findings provide critical insights for policymakers and stakeholders involved in developing adaptive strategies to address the challenges posed by future climate scenarios.

Anti-phase glacier fluctuations on the millennial-scale on the southern Tibetan Plateau and New Zealand during the last glacial period

The sparsity of high-resolution 10Be chronologies in the Northern Hemisphere makes it difficult to determine whether the global fluctuations in glacier extent during the last glacial occurred simultaneously. This limits our understanding of the past mechanisms driving global climate changes. We compiled and reanalyzed 476 10Be exposure ages from the southern Tibetan Plateau for the last glacial and compared them with coeval 10Be ages from the Southern Alps. The results indicate an anti-phase relationship between the glacier expansions in these two regions on the millennial-scale during the last glacial period. Comparison with a glacier-derived temperature reconstruction suggests that maintaining a stable glacier state in both regions required a similar cooling magnitude. We speculate that variability of the Atlantic Meridional Overturning Circulation, which induced poleward/equatorward shifts in atmospheric and oceanic circulation, was the primary driving mechanism of this anti-phase relationship. Our findings provide glacial geological evidence to support the bipolar seesaw hypothesis.

The potential recording of North Ionian Gyre’s reversals as a decadal signal in sea level during the instrumental period

In recent decades, the north Ionian Sea, central Mediterranean Sea, has witnessed shifts in surface current circulation from cyclonic to anticyclonic and vice versa at the quasi-decadal scale, a phenomenon termed the North Ionian Gyre (NIG) reversal. This process impacts parameters such as sea level by altering thermohaline properties and redistributing water masses at the sub-basin scale. Previous studies have shown that during anticyclonic (cyclonic) phases, the Ionian sea-level trend is falling (rising), the opposite of what is observed in other Mediterranean sub-basins. Assuming that sea level records reversals, this study employed signal decomposition to analyze satellite altimetry data and tide gauge observations across the region, some dating back to the 1900s. A distinct quasi-decadal periodicity emerges as the second dominant oscillatory mode from all independent observations, aligning its peaks and troughs (i.e., changes in sea-level trend) with known NIG reversals and associated changes in the state of North Ionian vorticity. Furthermore, this mode from altimetry data clearly depicts the spatial variability of sea level attributed to the opposite NIG states. This periodicity appears coherent among different sub-basins within the domain, especially in the eastern Mediterranean, with a shared generalized out of phase and weakening occurred from the 1930s to the 1950s. This study presents a century-scale reconstruction of NIG reversals, contributing to the understanding of this phenomenon prior to 1987 using direct observations from sea-level data.

Impact of southern annular mode on the Indian Ocean surface waves

AbstractInterannual climate modes, especially the southern annular mode (SAM), significantly influence the wave climate modulation of the Indian Ocean (IO). The present study, aligned with previous research, identifies two crucial swell generation regions in the IO: the extratropical southern Indian Ocean (ETSI) and the tropical southern Indian Ocean (TSIO). The SAM, governing Southern Ocean surface winds, significantly shapes wave generation in these zones, thereby dominantly regulating IO wave conditions. Positive SAM phases shifts the westerlies poleward creating significant negative anomalies in the northern ETSI, reducing wave generation and swell propagation into the northern IO (NIO) basins, while the positive wind anomalies in the western TSIO creates a new swell generation area that directs swells towards the Arabian Sea, elevating wave heights there during monsoons. Conversely, negative SAM phases enhance TSIO easterlies, making it the primary IO swell source, increasing swell activity in the Bay of Bengal, notably during premonsoon and monsoon seasons. SAM impact expands beyond swells, influencing NIO wave climate by altering wind seas through Hadley cell (HC) circulation shifts. Positive SAM phases trigger NIO and midlatitude anomalous warming, intensifying HC and NIO surface winds in the next season, thereby affecting convection and subsequent sea surface temperature (SST) anomaly changes. The “SAM positive anomaly wind‐SST oscillations (SPAWSO)” pattern emerges, where warm SST anomalies in DJF and JJA precede increased surface winds in MAM and SON, thus increasing the wind‐sea conditions in the NIO. SPAWSO, hence, acts as a delayed mode, season‐dependent positive air–sea interaction cycle linked to positive SAM phases, significantly impacting NIO's wind‐sea dynamics. Thus the present study provides better insights into the long‐term wave prediction accuracy in the IO by considering the direct swell influence and SPAWSO‐driven wind‐sea changes, aiding preparations for changing wave dynamics and their impacts.

Prevalence of asymptomatic Leishmania infection and knowledge, perceptions, and practices in blood donors in mainland Portugal

BackgroundAsymptomatic infection is the most common outcome of exposure to Leishmania parasites. In the Mediterranean region, where Leishmania infantum is endemic, studies on the prevalence of asymptomatic infection have often relied on serological testing in blood donors. In Spain, regional studies have shown seroprevalence in blood donors between 1 and 8%; in Portugal, values of 0 and 2% were suggested by two localized studies, in different populations. The purpose of this study was (i) to estimate the prevalence of asymptomatic Leishmania infection in blood donors in mainland Portugal, and (ii) to study the association between the detection of antibodies to Leishmania and sociodemographic factors, and also the knowledge, perceptions and practices (KPP) of the blood donors regarding leishmaniasis.MethodsA cross-sectional study targeted the population of people who donated blood in mainland Portugal. Participants, distributed proportionally by municipality and aged between 18 and 65 years, were selected randomly in 347 blood collection points between February and June 2022, and completed a sociodemographic and a KPP questionnaire. Detection of anti-Leishmania antibodies in serum was performed using an ELISA commercial kit. Individual KPP scores were calculated by adding grades defined for each question.ResultsGlobally, 201/3763 samples were positive. The estimated national true seroprevalence was 4.8% (95% CI 4.1–5.5%). The proportion of positive results was significantly different between NUTS (Nomenclature of Territorial Units for Statistics) regions. Models suggested that seropositivity was significantly higher in male sex, people older than 25 years, or residing in the Centro NUTS2 region, but not in dog owners nor people with lower KPP scores. Overall, 72.3% of participants had previously heard of leishmaniasis and, in multivariate analysis, a higher Knowledge score was associated with age 25–40 years, female sex, ownership of dogs, and higher education.ConclusionsGlobal estimated true seroprevalence (4.8%) was similar to previous regional studies in blood donors in neighboring Spain. Higher seroprevalence values in the NUTS2 Centro region were consistent with incidence data from humans and seroprevalence studies in dogs. On the other hand, the low values in the Alentejo and the high values in the northern subregions may be the result of geographical shifts in parasite circulation due to climate change and should prompt localized and integrated, vector, canine, and human research, following a One Health approach.Graphical

All aboard! Earth system investigations with the CH2O-CHOO TRAIN v1.0

Abstract. Models of the carbon cycle and climate on geologic (&gt;104-year) timescales have improved tremendously in the last 50 years due to parallel advances in our understanding of the Earth system and the increase in computing power to simulate its key processes. Still, balancing the Earth system's complexity with a model's computational expense is a primary challenge in model development. Simulations spanning hundreds of thousands of years or more generally require a reduction in the complexity of the climate system, omitting features such as radiative feedbacks, shifts in atmospheric circulation, and the expansion and decay of ice sheets, which can have profound effects on the long-term carbon cycle. Here, we present a model for climate and the long-term carbon cycle that captures many fundamental features of global climate while retaining the computational efficiency needed to simulate millions of years of time. The Carbon–H2O Coupled HydrOlOgical model with Terrestrial Runoff And INsolation, or CH2O-CHOO TRAIN, couples a one-dimensional (latitudinal) moist static energy balance model of climate with a model for rock weathering and the long-term carbon cycle. The CH2O-CHOO TRAIN is capable of running million-year-long simulations in about 30 min on a laptop PC. The key advantages of this framework are (1) it simulates fundamental climate forcings and feedbacks; (2) it accounts for geographic configuration; and (3) it is flexible, equipped to easily add features, change the strength of feedbacks, and prescribe conditions that are often hard-coded or emergent properties of more complex models, such as climate sensitivity and the strength of meridional heat transport. We show how climate variables governing temperature and the water cycle can impact long-term carbon cycling and climate, and we discuss how the magnitude and direction of this impact can depend on boundary conditions like continental geography. This paper outlines the model equations, presents a sensitivity analysis of the climate responses to varied climatic and carbon cycle perturbations, and discusses potential applications and next stops for the CH2O-CHOO TRAIN.

Exploring the ocean with sound: Telltale acoustic signatures of a changing ocean

Our ocean plays a major role in climate regulation, hosts vast biodiversity, sustains economies, ensures food security, and supports international shipping, tourism, recreation, and human health and security. Yet, the ocean is experiencing unprecedented changes: sea level rise, coral bleaching, ocean acidification, anoxia, accelerated melting of the major ice sheets, shifts in ocean circulation, pollution, overfishing, and increased industrialization. Sound plays a vital role in exploring, understanding, and monitoring these changes over a range of spatial and temporal scales. In this talk, I will focus on the multitude of approaches taken to using sound to understand our changing ocean, the advances in platforms used to enable these studies, the expansion of global ocean observing systems that include measurements of sound as an essential ocean variable, and the enormous complexities in interpreting acoustic signals in a dynamic and evolving ocean landscape. I will end by discussing the opportunities and obstacles associated with globalizing approaches for using sound to improve ocean stewardship.

Atmospheric Response to a Collapse of the North Atlantic Circulation under a Mid-Range Future Climate Scenario: A Regime Shift in Northern Hemisphere Dynamics

Abstract Climate models project a future weakening of the Atlantic meridional overturning circulation (AMOC), but the impacts of this weakening on climate remain highly uncertain. A key challenge in quantifying the impact of an AMOC decline is in isolating its influence on climate, relative to other changes associated with increased greenhouse gases. Here we isolate the climate impacts of a weakened AMOC in the broader context of a warming climate using a unique ensemble of Shared Socioeconomic Pathway (SSP) 2–4.5 integrations that was performed using the Climate Model Intercomparison Project phase 6 (CMIP6) version of the NASA Goddard Institute for Space Studies ModelE (E2.1). In these runs internal variability alone results in a spontaneous bifurcation of the ocean flow, wherein 2 out of 10 ensemble members exhibit an entire AMOC collapse, while the other 8 members recover at various stages despite identical forcing of each ensemble member and with no externally prescribed freshwater perturbation. We show that an AMOC collapse results in an abrupt northward shift and strengthening of the Northern Hemisphere (NH) Hadley cell (HC) and intensification of the northern midlatitude eddy-driven jet. We then use a set of coupled atmosphere–ocean abrupt CO2 experiments spanning the range 1 times to 5 times CO2 (1x to 5xCO2) to show that this response to an AMOC collapse results in a nonlinear shift in the NH circulation moving from 2xCO2 to 3xCO2. Slab-ocean versions of these experiments, by comparison, do not capture this nonlinear behavior. Our results suggest that changes in ocean heat flux convergences associated with an AMOC collapse—while highly uncertain—can result in profound changes in the NH circulation and continued efforts to constrain the AMOC response to future climate change are needed.

Drivers of δ18O Variability Preserved in Ice Cores From Earth's Highest Tropical Mountain

AbstractIn 2019, four ice cores were recovered from the world's highest tropical mountain, Nevado Huascarán (Cordillera Blanca, Peru; 9.11°S, 77.61°W). Composite hydroclimate records of the two Col cores (6,050 masl) and the two Summit cores (6,768 masl) are compared to gridded gauge‐analysis and reanalysis climate data for the most recent 60‐year. Spatiotemporal correlation analyses suggest that the ice core oxygen stable isotope (δ18O) record largely reflects tropical Pacific climate variability, particularly in the NINO3.4 region. By extension, the δ18O record is strongly related to rainfall over the Amazon Basin, as teleconnections between the El Niño Southern Oscillation and hydrological behavior are the main drivers of the fractionation of water isotopes. However, on a local scale, modulation of the stable water isotopes appears to be more closely governed by upper atmospheric temperatures than by rainfall amount. Over the last 60 years, the statistical significance of the climate/δ18O relationship has been increasing contemporaneously with the atmospheric and oceanic warming rates and shifts in the Walker circulation. Isotopic records from the Summit appear to be more sensitive to large‐scale temperature changes than the records from the Col. These results may have substantial implications for modeling studies of the behavior of water isotopes at high elevations in the tropical Andes.

Indonesian Throughflow Partitioning between Leeuwin and South Equatorial Currents

Abstract Indonesian Throughflow (ITF) waters move along multiple pathways within the Indian Ocean. The western route is within the thermocline of the South Equatorial Current (SEC), and the southern route is via injection into the Leeuwin Current (LC) along western Australia. We use gridded Argo data to examine heat content anomaly (HCa) within three boxes in the eastern Indian Ocean, one adjacent to the ITF outflow from the Indonesian Seas (ITF box), the second in the eastern portion of the SEC (SEC box), and the third in the LC (LC box). Although interannual HCa variability in the SEC and ITF boxes is well correlated, a large increase in HCa within the ITF box does not appear in the SEC box in 2011 but is evident in the LC box. The 2011 change in the SEC–LC partitioning is investigated using GODAS reanalysis by examining the strength of the SEC and LC during a 2009 HCa increase within the ITF box and the subsequent increase in 2011. During 2009, a strong SEC and weakened LC spread the increased ITF HCa into the central Indian Ocean, whereas a weak SEC and strengthened LC during 2011 transmit the HCa signal to the south. Near-surface winds and mean sea level pressure from NCEP–NCAR reanalysis reveal that Ningaloo Niño events led to shifts in ocean circulation during 2000 and 2011. LC and SEC exports show a high negative correlation at interannual time scales, indicating that a reduction of outflow from one pathway is partially compensated by an increase from the other.

Biomarker Records of Environmental Shifts on the Labrador Shelf During the Holocene

AbstractThe ultimate demise of the Laurentide Ice Sheet (LIS) and the preceding and succeeding oceanographic changes along the western Labrador Sea offer insights critically important to improve climate predictions of expected future climate warming and further melting of the Greenland ice cap. However, while the final disappearance of the LIS during the Holocene is rather well constrained, the response of sea ice during the resulting meltwater events is not fully understood. Here, we present reconstructions of paleoceanographic changes over the past 9.3 Kyr BP on the northwestern Labrador Shelf, with a special focus on the interaction between the final meltwater event around 8.2 Kyr BP and sea ice and phytoplankton productivity (e.g., IP25, HBI III (Z), brassicasterol, dinosterol, biogenic opal, total organic carbon). Our records indicate low sea‐ice cover and high phytoplankton productivity on the Labrador Shelf prior to 8.9 Kyr BP, sea‐ice formation was favored by decreased surface salinities due to the meltwater events from Lake Agassiz‐Ojibway and the Hudson Bay Ice Saddle from 8.55 Kyr BP onwards. For the past ca. 7.5 Kyr BP sea ice is mainly transported to the study area by local ocean currents such as the inner Labrador and Baffin Current. Our findings provide new insights into the response of sea ice to increased meltwater discharge as well as shifts in atmospheric and oceanic circulation.

Weakened AMOC related to cooling and atmospheric circulation shifts in the last interglacial Eastern Mediterranean

There is limited understanding of temperature and atmospheric circulation changes that accompany an Atlantic Meridional Overturning Circulation (AMOC) slowdown beyond the North Atlantic realm. A Peqi’in Cave (Israel) speleothem dated to the last interglacial period (LIG), 129–116 thousand years ago (ka), together with a large modern rainfall monitoring dataset, serve as the base for investigating past AMOC slowdown effects on the Eastern Mediterranean. Here, we reconstruct LIG temperatures and rainfall source using organic proxies (TEX86) and fluid inclusion water d-excess. The TEX86 data show a stepwise cooling from 19.8 ± 0.2° (ca. 128–126 ka) to 16.5 ± 0.6 °C (ca. 124–123 ka), while d-excess values decrease abruptly (ca. 126 ka). The d-excess shift suggests that rainfall was derived from more zonal Mediterranean air flow during the weakened AMOC interval. Decreasing rainfall d-excess trends over the last 25 years raise the question whether similar atmospheric circulation changes are also occurring today.

Westward extension of summer atmospheric circulation over the North Pacific after the 1990s

This paper documents the westward extension of the summer atmospheric circulation over the North Pacific (NP circulation) around the 1990s and investigates the possible reason, based on the North Pacific index. The NP circulation was centered in the Northeast Pacific during 1961–1983 and in the central North Pacific during 1994–2016. Further results show that the westward movement of the NP circulation was closely linked with an intensification of the relationship between the NP circulation and the previous spring sea surface temperature of the midlatitude North Pacific (SST_NP) after the 1990s. The spring SST_NP anomalies exerted considerable impacts on surface heat flux (i.e., sensible heat flux and latent heat flux) and vertical motion anomalies over the central North Pacific during the following summer, facilitating a westward shift in the summer NP circulation. Additionally, the interannual variability of the sea level pressure over the central North Pacific increased after the 1990s, which was likely a contributing factor to the westward extension of the NP circulation.摘要本文研究了20世纪90年代前后夏季北太平洋大气环流 (NP circulation) 向西扩展, 并探讨了其可能的原因. 结果表明, 在1961–1983年期间NP中心主要位于东北太平洋, 而在1994–2016年期间NP中心发生西移, 至北太平洋中部. 进一步分析指出, 90年代以后NP活动中心的西移和春季中纬度北太平洋海温(SST_NP) 与NP关系的加强有关. 春季SST_NP异常通过引起后期夏季北太平洋中部地表热通量(即感热通量和潜热通量)和垂直运动异常, 有利于NP活动中心向西移动. 此外, 20世纪90年代以后, 北太平洋中部海平面气压的年际变率增加, 这可能是NP向西扩展的另一原因.

Spatial pattern and cause of centennial-scale hydroclimatic variability over eastern China during the last millennium

Instrumental and historic records have shown that the hydroclimatic variability on interannual- to decadal-timescales over eastern China is spatially different, with a meridional dipole or even tripole pattern. Yet, how this spatial pattern of hydroclimate change on centennial timescales looks like is still unclear due to the lack of well-reconstructed climate records from South China. Here, we present a precisely-dated, high-resolution δ18O record of stalagmite (JL2) from Jiuluo Cave, Guilin, South China, to elucidate the hydroclimatic variability in South China over the last millennium and thus explore the spatial pattern and cause of centennial-scale hydroclimatic variability across eastern China. Based on the previous results of modern cave monitoring carried out in the Guilin region, the δ18O of JL2 is carefully interpreted as a robust proxy for summer monsoonal rainfall change. It clearly shows that a decrease in monsoonal rainfall over South China occurred during the periods of 941–1250 CE (i.e. the Medieval Warm Period (MWP)) and 1550–1850 CE (late Little Ice Age (LIA)), while intensification of monsoonal rainfall occurred during 1250–1550 CE (early LIA) and since 1850 CE (the Current Warm Period). Comparison of the JL2 record with other well-reconstructed proxy records from different zones along the trajectory of the East Asia summer monsoon (EASM) allows us to depict the spatial variation in centennial-scale monsoonal rainfall over eastern China. We confirmed that a “wet northern and dry southern” pattern existed for the rainfall regime of eastern China during the MWP, and a “dry northern and wet southern” pattern during the early LIA. Furthermore, there was a tripole pattern showing dry conditions in both North and South China but wet condition in the Jianghuai area, central China during the late LIA. These spatial patterns of centennial-scale monsoonal rainfall variability were principally linked to changes in summer monsoon intensity and the long-live state of El Niño–Southern Oscillation (ENSO). We also found that solar activity could influence the monsoonal rainfall to some degrees, and reduced monsoonal rainfall often occurred during intervals of solar minimum. Nevertheless, the monsoonal rainfall change during the 14th century was ascribed to great shift in oceanic and atmospheric circulation rather than to variations in solar activity.

Extreme precipitation stable isotopic compositions reveal unexpected summer monsoon incursions in the Qilian Mountains

Isotope composition and moisture sources of precipitation are important for understanding water cycles and reconstructing paleoclimate. Based on 15-years' precipitation stable Isotope composition (δ18O and δ2H) from four stations of the Qilian Mountains, we found unique δ18O and δ2H features associated with the incursion of the summer monsoon over the Qilian Mountains, northwestern China. In 12 of the 15 years, similar seasonal variations of δ18O and δ2H confirmed a dominant source of moisture from Westerly circulation, and higher intercepts of the local meteoric water line (LMWL) indicated strong recycling of continental moisture. However, in August 2016 and 2018, extremely low slopes and intercepts of the LMWL, and more negative δ18O and δ2H revealed substantial contributions of the Asian summer monsoon to precipitation of the Qilian Mountains, with extremely heavy precipitation in August 2016. The column moisture flux, land-sea thermal contrast, correlations of precipitation δ18O with East Asian Summer Monsoon Index and Westerlies Index, HYSPLIT modeling results and precipitation δ18O along backward trajectories confirmed incursions of the summer monsoon in August 2016 and 2018. Our redefining of the boundary of the summer monsoon region confirmed the summer monsoon incursion zone can extend to the west of longitude 96°E and north of latitude 40°N in strong monsoon years, corresponding to boundaries of monsoon incursions in the mid-Holocene. Temperature correlated with precipitation δ18O at monthly and shorter time scales, but not for whole seasons or at yearly scale, revealing that summer monsoon incursions are therefore more likely than changing temperature to explain the multi-year cycles in the Qilian Mountains ice archives. Continent-scale shifts in atmospheric circulation strongly influence water resources in the Qilian mountains, and may change in frequency as climate warms. This study therefore has important implications for understanding water resources in the Qilian mountains in the past and into the future.

Intermodel Spread of Historical Indian Monsoon Rainfall Change in CMIP6: The Role of the Tropical Pacific Mean State

Abstract Robust projections of the Indian summer monsoon rainfall (ISMR) are critical as it provides 80% of the annual precipitation to more than 1 billion people who are very vulnerable to climate change. However, even over the historical period, state-of-the-art climate models have difficulties in reproducing the observed ISMR trends and are affected by a large intermodel spread, which questions the reliability of ISMR projections. Such uncertainty could come from internal variability or model biases. Here, we study the impact of the latter on the historical forced change of ISMR in 34 models from CMIP6. First, we show that models’ biases over India do not significantly impact how they simulate the historical change of ISMR. However, we do find statistically significant relationships between ISMR historical forced changes and remote rainfall and temperature biases within the tropics by using a maximum covariance analysis (MCA). Our results highlight the key role of tropical Pacific sea surface temperature (SST) mean state biases as an important source of intermodel spread in the ISMR change. The physical mechanisms underlying these statistical relationships between ISMR change and the intermodel spread of Pacific SST biases are finally explored. We found that models having El Niño/La Niña–like mean SST bias in the Pacific tend to exhibit El Niño/La Niña–like changes over the historical period, impacting ISMR through a shift in the Walker circulation and Rossby wave propagation across the Pacific.