Continuous, high-resolution paleoclimate records from the North Pacific region spanning the past 1500 years are rare; and the behavior of the Aleutian Low (ALow) pressure center, the dominant climatological feature in the Gulf of Alaska, remains poorly constrained. Here we present a continuous, 1500 year long, calibrated proxy record for the strength of the wintertime (December-March) ALow from the Mount Logan summit (PR Col; 5200 m asl) ice core soluble sodium time series. We show that ice core sodium concentrations are statistically correlated with North Pacific sea level pressure and zonal wind speed. Our

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Precipitation regimes are changing in response to climate change, yet understanding of how forest ecosystems respond to extreme droughts and pluvials remains incomplete. As future precipitation extremes will likely fall outside the range of historical variability, precipitation manipulation experiments (PMEs) are critical to advancing knowledge about potential ecosystem responses. However, few PMEs have been conducted in forests compared to short‐statured ecosystems, and forest PMEs have unique design requirements and constraints. Moreover, past forest PMEs have lacked coordination, limiting cross‐site comparisons. Here, we review and synthesize approaches, challenges, and opportunities for conducting PMEs in forests, with the goal of guiding design decisions, while maximizing the potential for coordination. We reviewed 63 forest PMEs at 70 sites world‐wide. Workshops, meetings, and communications with experimentalists were used to generate and build consensus around approaches for addressing the key challenges and enhancing coordination. Past forest PMEs employed a variety of study designs related to treatment level, replication, plot and infrastructure characteristics, and measurement approaches. Important considerations for establishing new forest PMEs include: selecting appropriate treatment levels to reach ecological thresholds; balancing cost, logistical complexity, and effectiveness in infrastructure design; and preventing unintended water subsidies. Response variables in forest PMEs were organized into three broad tiers reflecting increasing complexity and resource intensiveness, with the first tier representing a recommended core set of common measurements. Differences in site conditions combined with unique research questions of experimentalists necessitate careful adaptation of guidelines for forest PMEs to balance local objectives with coordination among experiments. We advocate adoption of a common framework for coordinating forest PME design to enhance cross‐site comparability and advance fundamental knowledge about the response and sensitivity of diverse forest ecosystems to precipitation extremes.

Stable water isotopes provide a means of tracing many hydrologic processes, including poorly understood dynamics like soil water interactions with the atmosphere. We present a four-year dataset of biweekly water isotope samples from eight fluxes and stores in a headwater catchment at the Hubbard Brook Experimental Forest, New Hampshire, USA. We use Dansgaard's deuterium excess (d) parameter to infer hydrologic processes that cause stable water isotope fractionation. Although we expected to observe a decrease in d from precipitation to soil water because of evaporation, instead we observed an increase, which suggests sub-canopy water vapour recycling (evapotranspiration and then re-condensation). However, the underlying mechanisms and spatial dynamics remain uncertain. The apparent recycling is most evident in the growing season; weak evidence suggests a similar process in the dormant season. Sub-canopy water recycling is a novel hydrologic process that should have implications for micro-meteorology and habitat provided by the forest sub-canopy environment.

The high accumulation rate and negligible amount of melt at Eclipse Icefield (3017 m) in the Saint Elias Range of Yukon, Canada, allows for the preservation of a high-resolution isotopic and glaciochemical records valuable for reconstruction of climatic variables. Each of the three Eclipse ice cores have a well-constrained depth-age scale with dozens of reference horizons over the twentieth century that permits an exceptional level of confidence in the results of the current calibration exercise. Stacked time series of accumulation and stable isotopes were divided into cold and warm seasons and seasons of extreme high and extreme low accumulation and stable isotope values (eight groups). For each group, season-averaged composites of 500-hPa geopotential height grids, and the individual seasons that constitute them, were analyzed to elucidate common anomalous flow patterns.This analysis shows that the most fractionated isotopes and lowest accumulation cold seasons reflect a more zonal height pattern in the North Pacific associated with negative Pacific-North American (PNA) and Pacific decadal oscillation (PDO) indices. Conversely, the least fractionated isotopes and highest accumulation cold seasons are associated with a positive PNA pattern. Although only a maximum of approximately 20% of the total number of accumulation and stable isotope seasons exhibit a relatively consistent relationship with 500-hPa geopotential height patterns, these results support the hypothesis that the most extreme accumulation and extreme isotope cold-season values in the Saint Elias Mountains are related to consistent atmospheric circulation and oceanic sea surface temperature patterns.

Samples were collected from a snow pit and shallow firn core near Kahiltna Pass (2970 m a.s.l.), Denali National Park, Alaska, USA, in May 2008. The record spans autumn 2003 to spring 2008 and reveals clusters of ice layers interpreted as summertime intervals of above-freezing temperatures. High correlation coefficients (0.75–1.00) between annual ice-layer thickness and regional summertime station temperatures for 4 years (n = 4) indicate ice-layer thickness is a good proxy for mean and extreme summertime temperatures across Alaska, at least over the short period of record. A Rex-block (aka high-over-low) pattern, a downstream trough over Hudson Bay, Canada, and an upstream trough over eastern Siberia occurred during the three melting events that lasted at least 2 weeks. About half of all shorter melting events were associated with a cut-off low traversing the Gulf of Alaska. We hypothesize that a surface-to-bedrock core extracted from this location would provide a high-quality record of summer temperature and atmospheric blocking variability for the last several hundred years.

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The formation of katabatic winds and pooling of cold air in mountain valleys impact air quality, precipitation type, and local ecosystem functions. Much is still poorly understood about the multiscale interaction of processes in a mature mixed-hardwood forest that cause the formation and evolution of cold-air pools (CAPs). Processes involved in the evolution of a CAP in the Hubbard Brook Experimental Forest valley in New Hampshire were investigated during a field campaign on 4–5 November 2015. Vertical profiles of temperature and humidity were measured along a 150-m-long tethered balloon in the center of the valley and were compared with temperature and wind observations on the surrounding slopes to identify and assess the impacts of multiscale processes on a CAP. A CAP formed rapidly during the afternoon of 4 November and attained its maximum depth of ~150 m by sunset. This maximum depth is likely a result of the topography of the valley. Warm-air advection (WAA) occurred during the second half of the night at high elevations, and warm air mixed downward into the valley. As a result, the vertical thermal gradient strengthened and static stability increased, which allowed the lowest part of the CAP to continue to radiatively cool while the upper part of the CAP was warmed and eroded by the WAA. Results suggest that the canopy acts as the primary cooling surface for air at night, which causes split katabatic flow: cold and fast flow above canopy and warmer and slower flow below canopy. Understanding these processes in sloped forests has implications for eddy covariance research and montane microclimates.