Journal of Glaciology, Vol. 56, No. 198, 2010 715 Ice layers as an indicator of summer warmth and atmospheric blocking in Alaska Eric P. KELSEY,1 Cameron P. WAKE,1 Karl KREUTZ,2 Erich OSTERBERG3 1Complex Systems Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire 03824, USA E-mail: [email protected] 2Climate Change Institute and Department of Earth Sciences, University of Maine, 303 Bryand Global Sciences Center, Orono, Maine 04469-5790, USA 3Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA ABSTRACT. 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. 1. INTRODUCTION 2. DATA AND METHODOLOGY Melt layers are frequently observed in ice cores extracted During May 2008, we collected samples from a 4.35 m deep from polar and alpine glaciers. Melting occurs when the air snow pit and then drilled and sampled a 18.77 m long, temperature rises above 08C at these locations. The annual 8.2 cm diameter firn core from the bottom of the pit melt-layer variability in firn and ice cores has been used as a approximately 1 km south of Kahiltna Pass (KPass; proxy for summer temperature variability on several glaciers 63.078 N, 151.178 W; Fig. 1) at 2970 m a.s.l. (700 hPa). including the Greenland ice sheet (Herron and others, 1981; The physical depth and thickness of ice layers in the core Langway and Shoji, 1990; Alley and Anandakrishnan, 1995; (there were no ice layers in the snow pit) were recorded Kameda and others, 1995; Rowe and others, 1995), Devon carefully in the field. Melt-layer thickness was calculated by Ice Cap, Canada (Koerner, 1977); Agassiz Ice Cap, Canada measuring the along-core thickness. If a lens or melt layer (Fisher and others, 1995); and the West Antarctic ice sheet had a variable thickness, an average thickness was recorded. (Das and Alley, 2008). For example, Herron and others No vertical ice pipes were observed. Density in the snow pit (1981) found annual percent melt increased during the was measured every 10 cm, and the length of each core Medieval Warm Period, decreased during the Little Ice Age segment was measured and weighed to calculate the density. and correlated well with d18O. Koerner and Fisher (1990) The snow pit was sampled continuously at 5 cm inferred mean summer season temperatures through the use resolution for glaciochemical analyses and was averaged of annual melt percent from an Agassiz Ice Cap core. At to match the 10 cm continuously sampled firn core. Firn- Siple Dome, Antarctica, where under the present climate the core samples were scraped with a ceramic blade under a temperature rises above freezing during only a small fraction class 100 High Efficiency Particle Air (HEPA) clean bench at of summers, and at several Arctic locations of high annual –208C in the clean ice processing facility at the University melt, melt layers represent ‘extreme temperatures’ and also of Maine and then melted in pre-cleaned 250 mL Nalgene reflect mean summer temperatures (Koerner, 1997; Das and bottles. All samples remained frozen until just prior to Alley, 2005). analysis. The samples were analyzed for dD at the A snow pit and shallow core examined during a University of Maine with a Mircomass Isoprime mass reconnaissance field season in the spring of 2008 to Kahiltna spectrometer and Eurovector PyrOH peripheral (Morrison Glacier on the west shoulder of Denali, Alaska, USA, and others, 2001). revealed tightly clustered melt layers that we interpret as Meteorological data used in our analysis are from the US representing summertime melting events. Here we examine National Centers for Environmental Prediction (NCEP)/US the relationship between mean and extreme summer National Center for Atmospheric Research (NCAR) reanalysis temperatures and ice-layer thickness to determine if a (Kalnay and others, 1996) 6 hourly data on a 2.58 Â 2.58 grid. longer-term record of summertime warmth may be available These data at 62.58 N, 150.08 W, 700 hPa, the closest from analysis of melt layers in a deep core drilled at this site. gridpoint to KPass, are used as a proxy for meteorological This study also examines the types of weather patterns that conditions at KPass. This temperature record was compared are responsible for very warm summer temperatures in with daily temperature recorded by a University of Maine central Alaska. meteorological station installed in May 2008 at Kahiltna base 716 Kelsey and others: Ice layers as atmospheric indicators camp (2195 m a.s.l.; 11 km south-southeast of KPass). Both time series have a high autocorrelation, so the daily residuals were calculated from an 11 day running-mean temperature to minimize autocorrelation. The correlation coefficient between these daily datasets for 8 May 2008 to 4 May 2009 is 0.61 (p < 0.0001). Therefore, we are confident that the NCEP/NCAR reanalysis gridpoint data closely represent weather conditions at KPass. We assume that melting of any significance occurred only when the temperature at the gridpoint was above 08C, an obvious threshold that is supported experimentally (Das and Alley, 2005). The metric used here to estimate the magnitude of warmth above 08Cat KPass is ‘positive degree-days’ (PDD), daily cumulative degrees above 08C (at 6 hour resolution) divided by 4, similar to the PDD used by Das and Alley (2005) and analogous to ‘degree-days’ (e.g. Braithwaite, 1995; Aizen and others, 2000, 2002; Hock, 2003). These same metrics were calcu- lated for higher temperature thresholds from daily tempera- Fig. 1. Map of northwestern North America. All meteorological ture records of Alaskan meteorological stations (US National stations (triangles) mentioned in the text and ice-core locations Climatic Data Center) for comparison with the core record. (circles: Mount Logan (Holdsworth and others, 1992; Osterberg and Nearby Gulkana Glacier mass-balance data from the high- others, 2008), Eclipse (Yalcin and Wake, 2001; Yalcin and others, est-elevation observation point (Site D, 1850 m; US Geo- 2003, 2006a,b,c), King Col (Goto-Azuma and others, 2003) and logical Survey–Glacier and Snow Program of Alaska and Bona-Churchill (Thompson and others, 2004)) are labeled. Washington Science Centers) and weather-station (1480 m) data were similarly compared with KPass annual melt-layer second warmest. One below-average summer occurred thickness. These stations and all other sites mentioned in the between 2003 and 2007, which was in 2006 at –4.58C. text are shown in Figure 1. KPass daily temperature correlates well with regional Geopotential height patterns at 500 hPa every 6 hours temperatures (Yakutat 0.77, McGrath 0.81, Cantwell 0.83, (NCEP/NCAR reanalysis) were analyzed during times when Talkeetna 0.83), including Arctic stations (Barrow 0.80, 8 the gridpoint temperature exceeded 0 C, to elucidate any Bettles 0.83, Kotzebue 0.84), indicating regionally coherent persistent atmospheric patterns that the melt layers repre- air masses. sent. This mid-tropospheric pressure level was chosen Typical summer weather stays below freezing but is because of its excellent representation of the dominant punctuated by occasional periods of above-freezing weath- synoptic-scale weather patterns. Sounding profiles were er, referred to here as heat events (HEs). An HE is defined obtained from the University of Wyoming Department of here as any period of time where the temperature exceeds Atmospheric Science website. 08C. An inventory of HEs at KPass for 2003–07 is provided in Table 1. Some discontinuous HEs were grouped together if 3. LOCAL METEOROLOGY AND CLIMATE the temperature dropped slightly below freezing for a few 6 hour periods because one synoptic pattern was responsible Alaska exhibits a wide range of climates. The state is for each HE and intrapattern variability created the brief surrounded by oceans and seas on all sides except the east drops below 08C. where it borders Canada, and daily and annual temperature ranges are strongly subdued by the maritime influence near the coast. The interior has a continental climate with high temperatures regularly exceeding 258C in the summer, and 4. MELT CONCEPTUAL MODEL wintertime lows frequently dropping below –308C (Shulski Surface snowmelt can occur via above-freezing tempera- and Wendler, 2007). The dominant moisture source for most tures, rain and freezing rain, while albedo, relative humidity, of the state, including Denali National Park, is the Gulf of insolation and wind speed are all variables that can change Alaska (GoA), the nearby mid-latitudinal North Pacific melt efficiency. Owing to the lack of meteorological Ocean and the Bering Sea. Decadal-scale mean annual observations at KPass, it is assumed that the effects on and June–August (JJA) temperatures for the state of Alaska melting from all of these variables, except for temperature, are strongly associated with North Pacific sea-surface are relatively constant on an annual basis.