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Terrestrial proxies of wind direction spanning the last deglaciation suggest easterly winds were present near the Laurentide Ice Sheet margin in the North American midcontinent. However, the existence and spatial extent of such easterly winds have not been investigated with transient paleoclimate model simulations, which could provide improved dynamical context for interpreting the causes of these winds. Here we assess near-surface winds near the retreating southern Laurentide Ice Sheet margin using iTRACE, a transient simulation of deglacial climate from 20–11 ka. Near the south-central margin, simulated near-surface winds are northeasterly to easterly through the deglaciation, due to katabatic flow off the ice sheet and anticyclonic circulation. As the ice sheet retreats and the Laurentide High moves northeastward and weakens, near-surface northeasterly winds weaken. Meltwater fluxes also influence temperature and sea level pressure over the North Atlantic, leading to easterly wind anomalies over eastern to midwestern North America. The agreement between proxy and model wind directions is promising, although simulated easterly to northeasterly winds extend too far south in iTRACE relative to the proxy data. Agreement is also strongest in winter, spring, and fall, suggesting these may have been seasons with greater aeolian activity. 

Abstract The hydrologic cycle is a fundamental component of the climate system with critical societal and ecological relevance. Yet gaps persist in our understanding of water fluxes and their response to increased greenhouse gas forcing. The stable isotope ratios of oxygen and hydrogen in water provide a unique opportunity to evaluate hydrological processes and investigate their role in the variability of the climate system and its sensitivity to change. Water isotopes also form the basis of many paleoclimate proxies in a variety of archives, including ice cores, lake and marine sediments, corals, and speleothems. These records hold most of the available information about past hydrologic variability prior to instrumental observations. Water isotopes thus provide a ‘common currency’ that links paleoclimate archives to modern observations, allowing us to evaluate hydrologic processes and their effects on climate variability on a wide range of time and length scales. Building on previous literature summarizing advancements in water isotopic measurements and modeling and describe water isotopic applications for understanding hydrological processes, this topical review reflects on new insights about climate variability from isotopic studies. Our intention is to highlight new work and opportunities to enhance our understanding and predictive skill, rather than be exhaustive. We offer a set of recommendations to advance observational and model-based tools for climate research, and highlight opportunities to better constrain climate sensitivity and identify anthropogenically-driven hydrologic changes within the inherently noisy background of natural climate variability. 

Oceanic islands support unique biotas but often lack ecological redundancy, so that the removal of a species can have a large effect on the ecosystem. The larger islands of the Galápagos Archipelago once had one or two species of giant tortoise that were the dominant herbivore. Using paleoecological techniques, we investigate the ecological cascade on highland ecosystems that resulted from whalers removing many thousands of tortoises from the lowlands. We hypothesize that the seasonal migration of a now-extinct tortoise species to the highlands was curtailed by decreased intraspecific competition. We find the trajectory of plant community dynamics changed within a decade of the first whaling vessels visiting the islands. Novel communities established, with a previously uncommon shrub, Miconia , replacing other shrubs of the genera Alternanthera and Acalypha . It was, however, the introduction of cattle and horses that caused the local extirpation of plant species, with the most extreme impacts being evident after c. 1930. This modified ecology is considered the natural state of the islands and has shaped subsequent conservation policy and practice. Restoration of El Junco Crater should emphasize exclusion of livestock, rewilding with tortoises, and expanding the ongoing plantings of Miconia to also include Acalypha and Alternanthera . 

Abstract ENSO and the mean zonal sea surface temperature gradient (dSST) of the tropical Pacific are important drivers of global climate and vary on decadal to centennial time scales. However, the relationship between dSST and ENSO cannot be assessed with the short instrumental record, and is uncertain in proxy data, with intervals of both stronger and weaker ENSO postulated to occur with overall strong dSST in the past. Here we assess the ENSO–dSST relationship during the last millennium using general circulation models (GCMs) participating in phase 3 of the Paleoclimate Modeling Intercomparison Project. Last millennium GCM simulations show diversity in the strength and direction of the ENSO–dSST relationship. Yet, the models that best simulate modern tropical Pacific climate frequently have a more negative ENSO–dSST correlation. Thus, last millennium tropical Pacific climate simulations support the likelihood of enhanced ENSO during decadal to centennial periods of reduced tropical Pacific dSST. However, the alternating directional ENSO–dSST relationship in all model simulations suggests that this relationship is not constant through time and is likely controlled by multiple mechanisms. 

Abstract The chronology and cause of millennial depositional oscillations within last glacial loess of the Central Lowlands of the United States are uncertain. Here, we present a new age model that indicates the Peoria Silt along the Illinois River Valley accumulated episodically from ~28,500 to 16,000 cal yr BP, as the Lake Michigan Lobe margin fluctuated within northeastern Illinois. The age model indicates accelerated loess deposition coincident with regional glacial advances during the local last glacial maximum. A weakly developed paleosol, the Jules Geosol, represents a period of significantly slower deposition, from 23,700 to 22,000 cal yr BP. A gastropod assemblage-based reconstruction of mean July temperature shows temperatures 6–10 ° C cooler than modern during Peoria Silt deposition. Stable oxygen and carbon isotope values (δ 18 O and δ 13 C) of gastropod carbonate do not vary significantly across the pedostratigraphic boundary of the Jules Geosol, suggesting slower loess accumulation was a result of reduced glacial sediment supply rather than direct climatic factors. However, a decrease in δ 18 O values occurred between 26,000 and 24,000 cal yr BP, synchronous with the Lake Michigan Lobe’s southernmost advance. This δ 18 O decrease suggests a coupling of regional summer hydroclimate and ice lobe position during the late glacial period.