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The nature of Late Glacial millennial-scale climate variability in the central United States is not well understood owing to a paucity of high-resolution, precisely-dated records in this region. Here we present a discontinuous, composite stalagmite record from Cosmic Cavern (36.4˚N, 93.5˚W), located on the southwestern margin of the Ozark Plateau in northwestern Arkansas, that preserves evidence of environmental change coincident with Greenland (inter)stadials and Heinrich stadials from 54-14 ka (marine isotope stages 3-2). Among the four stalagmites that comprise this record, carbon isotopes replicate well. Oxygen isotope ratios in three of the four stalagmites also exhibit robust replication, but in the fourth, oxygen isotope ratios are offset from coeval stalagmites by 1.5 permil, and thus this stalagmite was excluded from the composite record. Variability in stalagmite carbon and oxygen isotopes are broadly coincident with, and characterized by the same asymmetry as, the Greenland Ice Sheet Project Two (GISP2) oxygen isotope record; pronounced positive carbon and oxygen isotope anomalies also mark Heinrich stadials. Elevated carbon isotope values likely reflect reductions in soil respiration rates and increased CO2 out-gassing in voids above the cave during colder, and perhaps drier, stadials. Shifts in stalagmite oxygen isotope ratios are interpreted as reflecting variations in air temperature, consistent with modern temperature effects on precipitation oxygen isotope values documented over the course of a multi-year (and still on-going) precipitation monitoring program. We investigate the impacts of precipitation seasonality and moisture source on oxygen isotopic variability using Coupled Model Intercomparison Project phase 5 (CMIP5) simulations, namely the historical, pre-industrial, and Last Glacial Maximum simulations of the MIROC Earth System Model. Particular focus is placed on quantifying changes in moisture source and transport in the model simulations and the roles played by the North Atlantic subtropical high pressure system to the south and east, and precipitation derived from a Pacific moisture source to the west.

The Azores High is a subtropical high-pressure ridge in the North Atlantic surrounded by anticyclonic winds that steer rain-bearing weather systems. The size and intensity of the Azores High modulate the oceanic moisture transport to Europe thereby affecting hydroclimate across western Europe, especially during wintertime. While changes in the North Atlantic storm track have been linked to the variability of the North Atlantic Oscillation (NAO), we focus on North Atlantic variability with a subtropical perspective by focusing on the Azores High independently of the Icelandic Low. The subtropical perspective provides a direct understanding of regional climate variability in the western Mediterranean and reveals dramatic changes to North Atlantic climate throughout the past century and can provide insight into the impact of future warming on the dynamics of the Azores High and associated hydroclimate. Here we show that winters with an extremely large Azores High are significantly more common in the industrial era (since 1850 CE) than in preindustrial times, resulting in anomalously dry conditions across the western Mediterranean, including the Iberian Peninsula. Climate model simulations of the past millennium indicate that the industrial-era expansion of the Azores High is unprecedented throughout the last millennium (since 850 CE), consistent with proxy evidence from Portugal. Azores High expansion emerges after the end of the Little Ice Age and strengthens into the 20th century consistent with anthropogenically-driven warming.

The subtropical dry zones, including the broader Mediterranean region, are likely to experience considerable changes in hydroclimate in a warming climate. An expansion of the atmosphere’s meridional overturning circulation, the Hadley circulation, over recent decades has been reported, with implications for regional hydroclimate. Yet, there exists considerable disagreement in magnitude and even sign of these trends among different metrics that measure various aspects of the Hadley circulation, as well as discrepancies in trends between different analysis periods and reanalysis products during the 20th century. In light of these uncertainties, it is therefore of interest to explore variability and trends in subtropical hydroclimate and its dominant driver, the Hadley Circulation. We focus on the North Atlantic sector and explore variability in the Azores High, the manifestation of the Hadley Circulation’s downward branch, and hydroclimate across the Iberian Peninsula using a combination of observational/reanalysis products, state-of-the-art climate model simulations, and hydroclimatically-sensitive stalagmite records over the past 1200 yr. The Last Millennium Ensemble (LME) with the Community Earth System Model provides thirteen transient simulations covering the period 850 to 2005 A.D. with prescribed external forcing (e.g. greenhouse gas, solar, volcanic, land use, orbital, and aerosol) and smaller subsets with individual forcing only. The LME is shown to accurately simulate the variability and trends in the Azores High when compared to observational records from the 20th century. We evaluate variability in the Azores High (e.g., size, intensity, position) in relation to other key metrics that measure different aspects of the Hadley circulation throughout the course of the last millennium, as well as during key periods, such as the Little Ice Age or Medieval Climate Anomaly. The smaller subsets of LME simulations with individual forcing factors (e.g., solar, volcanic) allow for an attribution of past changes in regional hydroclimate to external drivers. Results from the climate model simulations are compared with hydroclimate reconstructed from stalagmites from Portuguese caves.

The Azores High is a subtropical high-pressure ridge in the North Atlantic. During boreal winters, anticyclonic winds rotate around the Azores High, transporting moisture to Western Europe. Variability in the size and intensity of the Azores High thus corresponds to variability in hydroclimate across Western Europe. We use the Last Millennium Ensemble (LME), which is run using the Community Earth System Model (CESM) and features thirteen transient simulations covering the period 850 to 2005 A.D. with prescribed external forcing (e.g. greenhouse gas, solar, volcanic, land use, orbital, and aerosol). The LME is shown to accurately simulate the variability and trends in the Azores High when compared to observational records from the 20th century. The Azores High has grown in size during the Industrial Era. This growth is most dramatic when observing the frequency of winters during which the Azores High is extremely large. The LME shows more winters with an extremely large Azores High in the past 100 years than any other 100-year period in the last millennium. Using LME as well as other simulations from the Paleoclimate Modelling Intercomparison Project Phase III, the recent expansion of the Azores High is shown to be well outside the range of natural variability since 850 A.D. Individual forcing simulations within the LME provide smaller ensembles in which only one external forcing is varied. These experiments attribute Azores High expansion to the recent increase in atmospheric greenhouse gas concentrations. Recent hydroclimatic signals across Western Europe consistent with the Azores High variability are also discussed.