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The Azores High (AH), a subtropical ridge in the atmosphere over the North Atlantic comprising one node of the North Atlantic Oscillation (NAO) system, has a dominant influence on the weather and climate of the Iberian Peninsula and northwest Africa. The behavior of the entire NAO system over the last millennium has been the subject of much debate in both proxy- and model-based studies. Many studies have focused on the behavior of the entire NAO system, but we focus solely on the behavior of the AH due to its proximity to this region. Other proxies from this region, mainly from Spain and Morocco, have provided details about atmospheric dynamics yet spatiotemporal gaps remain. In this study, we present a continuous, sub-decadally-resolved composite stalagmite carbon isotopic record from three partially overlapping stalagmites from Buraca Gloriosa (BG) cave, western Portugal, situated within the center of the AH, that preserves evidence of regional hydroclimate variability from approximately 800 CE to the present. This composite record, developed from U-Th dating and laminae counting paired with carbon isotopes, primarily reflects effective moisture in western Portugal. Given the close pairing of AH behavior (intensity, size, and location) and moisture transport in this region, the BG composite record allows for a thorough analysis of AH behavior over time. Multidecadal to centennial scale variability in the BG record and state-of-the-art last millennium climate model simulations show considerable coherence with precipitation-sensitive records from Spain and Morocco that, like BG, are strongly influenced by the intensity, size, and location of the AH. Synthesis of model output and proxy data suggests that western Portugal was persistently dry during much of the Medieval Climate Anomaly (MCA; ~850-1250 CE) and Modern era (1850 CE-present) and experienced wetter conditions during Little Ice Age (LIA; ~1400-1850 CE). Even considering age uncertainties from the Iberian Peninsula and northwest Africa proxy records, the apparent timing in the transition from a relatively dry MCA to a wetter LIA is spatially variable across this region, likely due to the non-stationary behavior of the AH system. 

Warming in the North Atlantic Ocean has been heterogeneous in recent decades, with locations along the eastern United States seaboard (northwestern Atlantic) seeing some of the largest and fastest warming in the last 100 years. In order to provide a longer temporal context for these changes, we are in the process of developing several master shell growth chronologies and associated geochemical records from theMid-Atlantic coast using the shells of the long-lived marine bivalve Arctica islandica. Based on the shell collection locations (shelf regions offOcean City, Maryland in ~ 61 m water depth and Long Island, New York in ~47 m water depth) and shell geochemistry measurements, we will be able to better ascertain hydrographic spatial and temporal variability of subtropical Atlantic water moving northward through time. These findings will be integrated with similar sclerochronology datasets previously published from the Gulf of Maine region and several others from theMid-Atlantic region that are currently being constructed. Collectively, this network of sclerochronology records will allow us to better characterize changes in the northwestern Atlantic and provide hydrographic insights beyond the relatively short instrumental record and evaluate potential dynamical forcings through time. 

The Azores High (AH), a subtropical ridge in the atmosphere over the North Atlantic comprising one node of the North Atlantic Oscillation (NAO) system, has a dominant influence on the weather and climate of the Iberian Peninsula and northwest Africa. The behavior of the entire NAO system over the last millennium has been the subject of much debate in both proxy- and model-based studies. Many studies have focused on the behavior of the entire NAO system, but we focus solely on the behavior of the AH due to its proximity to this region. Other proxies from this region, mainly from Spain and Morocco, have provided details about atmospheric dynamics yet spatiotemporal gaps remain. In this study, we present a continuous, sub-decadally-resolved composite stalagmite carbon isotopic record from three partially overlapping stalagmites from Buraca Gloriosa (BG) cave, western Portugal, situated within the center of the AH, that preserves evidence of regional hydroclimate variability from approximately 800 CE to the present. This composite record, developed from U-Th dating and laminae counting paired with carbon isotopes, primarily reflects effective moisture in western Portugal. Given the close pairing of AH behavior (intensity, size, and location) and moisture transport in this region, the BG composite record allows for a thorough analysis of AH behavior over time. Multidecadal to centennial scale variability in the BG record and state-of-the-art last millennium climate model simulations show considerable coherence with precipitation-sensitive records from Spain and Morocco that, like BG, are strongly influenced by the intensity, size, and location of the AH. Synthesis of model output and proxy data suggests that western Portugal was persistently dry during much of the Medieval Climate Anomaly (MCA; ~850-1250 CE) and Modern era (1850 CE-present) and experienced wetter conditions during Little Ice Age (LIA; ~1400-1850 CE). Even considering age uncertainties from the Iberian Peninsula and northwest Africa proxy records, the apparent timing in the transition from a relatively dry MCA to a wetter LIA is spatially variable across this region, likely due to the non-stationary behavior of the AH system. 

The Indian summer monsoon (ISM), which today supplies ~75% of annual precipitation to South Asia, has been reconstructed across previous centuries using a variety of hydroclimate-sensitive proxies. In some of these cases, ISM variability far exceeds that observed in the century-and-a-half-long instrumental record. Understanding the origins of these events is best addressed by developing a wide-ranging, multi-proxy network of high-resolution ISM reconstructions. In Nepal, ISM variability has been examined through tree rings, glacial ice, and lake sediments, but no stalagmite isotopic records of ISM rainfall have yet been published. Here we present a sub-decadally-resolved, precisely-dated, composite aragonite stalagmite record of ISM variability from Siddha Baba cave, central Nepal, for the last 2.7 kyr. A rainwater sampling program near the cave site, and a published study from Kathmandu (Adhikari et al., 2020), 150 km to the southeast, reveal that rainfall amount explains little of the observed variance in d18O values. Local hydroclimate is thus reconstructed from stalagmite 13C values, which we interpret as reflecting prior aragonite precipitation driven by changes in effective precipitation above the cave. ISM variability is apparent across a number of time scales, including centennial periods of reduced or enhanced rainfall coincident with societally-relevant precipitation regimes identified at other sites across South Asia. These include the Neo-Assyrian drought in the eastern Mediterranean and Middle East (2.7-2.5 kyr BP; Kathayat et al., 2019), the Mauria Empire (2.1-1.9 kyr BP), and the Guge Kingdom (0.9-0.3 kyr BP) pluvials in India and Tibet (Kathayat et al., 2017). A secular shift toward drier conditions since 0.5 kyr BP in the Siddha Baba record tracks the 18O records from Dasuopu glacier, Nepal Himalaya, and Sahiya cave, North India. Numerous multidecadal oscillations are also evident, including markedly wetter conditions during the 18th century, in the late Little Ice Age, apparent in the Dasuopu and Sahiya records. References Adhikari et al. (2020) Tellus B: Chem. Phys. Meteor., 72, 1-17. Kathayat et al. (2017) Sc. Adv., 7, e1701296. Kathayat et al. (2019) Sci. Adv., 5, eaax6656. 

Anthropogenic climate change is expected to alter global hydrological regimes in the near future, resulting in significant changes to water availability. However, the magnitude of such changes will vary regionally. The Iberian Peninsula, and specifically Portugal, has been identified by climate model projections as an area where climate change will increase drought frequency and severity. Climate in the Iberian Peninsula is impacted by both internal and external climate modes, potentially producing different precipitation patterns within a small geographic region. Thus, the development of regional highly resolved paleoclimate records from Portugal is critical for improving the predictive capability of regional climate models under future warming scenarios and to determine the extent to which different teleconnection patterns are influencing hydroclimate. Here we present a near annually resolved stable carbon isotope (δ13C) and oxygen (δ18O) isotope time-series from three stalagmites from the Algarve region of southern Portugal from two caves within 2.3 km of each other. U/Th dating indicates that our composite record spans the last millennia continuously through 2019 CE. Two stalagmites (GIA-19-1 and C-18-1) stopped growing around 1550 CE, during a dry interval, and sample GIA-19-2 grew continuously since the 17th century. GIA-19-2, with sub-annual resolution, is compared to modern instrumental records to evaluate the influence of specific environmental controls, including temperature and precipitation amounts. Isotope data from all three stalagmites exhibit substantial multidecadal variability indicating relatively wet and dry intervals. Based on our initial results, it is likely that both temperature and precipitation amount effects are the dominant controls on isotopic variability in these stalagmites. Comparison of the GIA-19-2 oxygen isotope time-series with the instrumental record and reconstructed index of the East Atlantic (EA) pattern (1650 CE to present) shows strong coherence with a reconstructed EA index (1650-2018 CE) and an instrumental EA index (1950 to present). Hence, variability in Southern Portuguese hydroclimate associated with the EA mode should also be considered by policy makers planners as they prepare for future warming and associated water stresses.