Near-surface Snow Research Articles

Soluble, light-absorbing species in snow at Barrow, Alaska

[1] As part of the international multidisciplinary Ocean - Atmosphere - Sea Ice - Snowpack (OASIS) program we analyzed more than 500 terrestrial (melted) snow samples near Barrow, AK between February and April 2009 for light absorption, as well as H2O2 and inorganic anion concentrations. For light absorption in the photochemically active region (300–450 nm) of surface snows, H2O2 and NO3− make minor contributions (combined < 9% typically), while HUmic LIke Substances (HULIS) and unknown chromophores each account for approximately half of the total absorption. We have identified four main sources for our residual chromophores (i.e., species other than H2O2 or NO3−): (1) vegetation and organic debris impact mostly the lowest 20 cm of the snowpack, (2) marine inputs, which are identified by high Cl− and SO42− contents, (3) deposition of diamond dust to surface snow, and (4) gas-phase exchange between the atmosphere and surface snow layers. The snow surfaces, and accompanying chromophore concentrations, are strongly modulated by winds and snowfall at Barrow. However, even with these physical controls on light absorption, we see an overall decline of light absorption in near-surface snow during the 7 weeks of our campaign, likely due to photo-bleaching of chromophores. While HULIS and unknown chromophores dominate light absorption by soluble species in Barrow snow, we know little about the photochemistry of these species, and thus we as a community are probably overlooking many snowpack photochemical reactions.

Possible contribution of triboelectricity to snow - air interactions

Environmental contextPolar near-surface snow can act as a chemical reactor that alters the composition and chemistry of snow and the overlying air. Although the mechanisms and driving forces of these reactions have long been debated, triboelectrification (production of electrostatic charges by friction) of snow by wind has not yet been considered as a factor. It is proposed that in polar regions, triboelectrification could significantly influence the composition and chemistry of snow. AbstractReactions that proceed in polar snow cover may significantly affect the chemistry of the overlying atmosphere, but mechanisms and driving forces of these reactions are still under discussion. The proposed hypothesis attempts to explain some experimental data that cannot be fully understood (e.g. the effect of wind on OH radicals, ozone and persistent organic pollutants levels) by taking into account the influence of electrical phenomena on the snow surface. We assumed that a combination of such factors as low humidity, high wind speed and low temperatures makes the influence of triboelectrification of snow significant for polar areas, where purity and the depth of snow cover prevent fast charge dissipation. The major points of the hypothesis are: (1) when the electric field reaches a value sufficient for the onset of corona discharge, various free radical processes are initiated resulting in changes in the concentrations of ozone, OH radical, nitrate, etc., and the decomposition of pollutant molecules; (2) the high electric field can stimulate transport of ions (such as bromide and nitrate) from the condensed phase to the gas phase; and (3) the ageing of charged snow can increase its electrical potential.

Point observations of liquid water content in wet snow – investigating methodical, spatial and temporal aspects

Abstract. Information about the volume and the spatial and temporal distribution of liquid water in snow is important for forecasting wet snow avalanches and for predicting melt-water run-off. The distribution of liquid water in snow is commonly estimated from point measurements using a "hand" squeeze test, or a dielectric device such as a "Snow Fork" or a "Denoth meter". Here we compare estimates of water content in the Swiss Alps made using the hand test to those made with a Snow Fork and a Denoth meter. Measurements were conducted in the Swiss Alps, mostly above tree line; more than 12 000 measurements were made at 85 locations over 30 days. Results show that the hand test generally over estimates the volumetric liquid water content. Estimates using the Snow Fork are generally 1 % higher than those derived from the Denoth meter. The measurements were also used to investigate temporal and small-scale spatial patterns of wetness. Results show that typically a single point measurement does not characterize the wetness of the surrounding snow. Large diurnal changes in wetness are common in the near-surface snow, and associated changes at depth were also observed. A single vertical profile of measurements is not sufficient to determine whether these changes were a result of a spatially homogeneous wetting front or caused by infiltration through pipes. Based on our observations, we suggest that three measurements at horizontal distances greater than 50 cm are needed to adequately characterize the distribution of liquid water through a snowpack. Further, we suggest a simplified classification scheme that includes five wetness patterns that incorporate both the vertical and horizontal distribution of liquid water in a snowpack.

Snow/atmosphere coupled simulation at Dome C, Antarctica

AbstractUsing a snow/atmosphere coupled model, the evolution of the surface and near-surface snow temperature is modeled at Dome C, Antarctica, during the period 20–30 January 2010. Firstly, the detailed multilayer snow model Crocus is run in stand-alone mode, with meteorological input forcing data provided by local meteorological observations. The snow model is able to simulate the evolution of surface temperature with good accuracy. It reproduces the observed downward propagation of the diurnal heatwave into the upper 50 cm of the snowpack reasonably well. Secondly, a fully coupled 3-D snow/atmosphere simulation is performed with the AROME regional meteorological model, for which the standard single-layer snow parameterization is replaced by Crocus. In spite of a poor simulation of clouds, the surface and near-surface snow temperatures are correctly simulated, showing neither significant bias nor drifts during the simulation period. The model reproduces particularly well the average decrease of the diurnal amplitude of air temperature from the surface to the top of the 45 m instrumented tower. This study highlights the potential of snow/atmosphere coupled models over the Antarctic plateau and the need to improve cloud microphysics and data assimilation over polar regions.

Gullies, polygons and mantles in Martian permafrost environments: cold desert landforms and sedimentary processes during recent Martian geological history

Abstract A range of cold desert landforms are found on the Martian surface that have been interpreted to indicate prevailing frozen and hyper-arid conditions for at least the past several million years. These cold desert conditions are punctuated by brief periods of localized surficial liquid water flow. Sediment transport pathways operate under these conditions of extreme cold and aridity and the processes involved generate permafrost landforms that are recognizable from spacecraft at local, regional and global scales. Thermal-contraction-crack polygons are associated with hemisphere-spanning mantle units that contain excess ice in the immediate subsurface. Sublimation is the dominant phase transition rather than melting under present Martian conditions. Evidence is presented for melting of near-surface snow, frost and/or ground ice in protected gully alcove microclimates during the most recent several million years.

A one-dimensional heat transfer model of the Antarctic Ice Sheet and modeling of snow temperatures at Dome A, the summit of Antarctic Plateau

A vertical one-dimensional numerical model for heat transferring within the near-surface snow layer of the Antarctic Ice Sheet was developed based on simplified parameterizations of associated physical processes for the atmosphere, radiation, and snow/ice systems. Using the meteorological data of an automatic weather station (AWS) at Dome A (80°22′S, 70°22′E), we applied the model to simulate the seasonal temperature variation within a depth of 20 m. Comparison of modeled results with observed snow temperatures at 4 measurement depths (0.1, 1, 3, 10 m) shows good agreement and consistent seasonal variations. The model results reveal the vertical temperature structure within the near-surface snow layer and its seasonal variance with more details than those by limited measurements. Analyses on the model outputs of the surface energy fluxes show that: 1) the surface energy balance at Dome A is characterized by the compensation between negative net radiation and the positive sensible fluxes, and 2) the sensible heat is on average transported from the atmosphere to the snow, and has an evident increase in spring. The results are considered well representative for the highest interior Antarctic Plateau.

Temperatures, heating rates and vapour pressures in near-surface snow at the South Pole

AbstractA finite-volume model is used to simulate 9 years (1995–2003) of snow temperatures at the South Pole. The upper boundary condition is skin-surface temperature derived from routine upwelling longwave radiation measurements, while the lower boundary condition is set to the seasonal temperature gradient at 6.5 m depth, taken from prior measurements at the South Pole. We focus on statistics of temperature, heat fluxes, heating rates and vapour pressures in the top metre of snow, but present results from the full depth of the model (6.5 m). The monthly mean net heat flux into the snow agrees with results from previous studies performed at the South Pole. On shorter timescales, the heating rates and vapour pressures show large variability. The net heat flux into the snow, which is between ±5 W m−2 in the monthly mean, can be greater than ±20 W m−2 on hourly timescales. On sub-daily timescales, heating rates exceed 40 K d−1 in the top 10 cm of the snow. Subsurface temperatures, and therefore heating rates, are more variable during winter (April–September) due to increased synoptic activity and the presence of a strong, surface-based, atmospheric temperature inversion. The largest vapour pressures (60–70 Pa) and vertical gradients of vapour pressure are found in the top metre of snow during the short summer (December–January). In contrast, during the long winter, the low temperatures result in very small vapour pressures and insignificant vapour-pressure gradients. The high summertime vapour-pressure gradients may be important in altering the isotopic composition of snow and ice on the Antarctic plateau.

The local insolation signature of air content in Antarctic ice. A new step toward an absolute dating of ice records

An accurate chronology of ice cores is still needed for interpreting the paleoclimatic record and to understand the relation between insolation and climate. A new domain of research in this area has been stimulated by the work of M. Bender linking the record of N 2/O 2 ratio in the air trapped in the ice with the local insolation. Here we investigate the potential of the air content of polar ice, V, as another and complementary ice proxy of local insolation. We propose that the long-term changes in air content recorded in ice from the high Antarctic plateau is dominantly imprinted by the local summer insolation. The V measurements covering the last 440,000 yr and obtained along the EPICA DC (EDC) ice core are presented. 86% of the variance observed in the V record can be explained neither by air pressure nor by temperature changes, and then should reflect properties influencing the porosity at close-off other than temperature. A wavelet analysis indicates a dominant obliquity period (around 41 ka) over the last 440 ka. We propose a mechanism, which can account for the observed anti-correlation between local insolation and V and explain how the local insolation, via the integrated summer insolation, can affect the near-surface snow structure and consequently the porosity at close-off. The V and the integrated summer insolation changes show high coherency over all the record and a variable phase relationship with a maximum phase difference of about 4000 yr around 200 ka ago. The phase difference may reflect the difference between the EDC2 chronology based on an inverse dating method and an accurately dated insolation curve. Our work, by proposing a new and complementary ice proxy for local insolation should eventually contribute to the establishment of an absolute dating of the ice paleo-record.

Seasonal variations in the soluble ion content of snow at Summit. Greenland: Constraints from three years of daily surface snow samples

Daily samples of the surface snow at Summit, Greenland were collected from June 1997 to April 1998 and then from August 2000 to August 2002. Concentrations of nine soluble ions (only eight in the first year) were determined in order to assess the validity of seasonal variations in snow composition at this site inferred from earlier snowpit and core studies. Strong and consistently sharp spring (April) peaks in dust, and broader summer (June–August) enhancements of NH 4 + and excess Cl - in the surface snow fully support the timing of these signals inferred from the pit profiles. Sea-salt reached maximum concentrations in the surface snow in late winter (February–March), based on averaging all three years of monthly means, but showed different patterns in winter each of the years. This is also consistent with the variable late-winter to spring timing inferred from pits. Simulated snowpit profiles constructed from the surface snow samples compared well with the ion profiles recovered from well dated snowpits sampled in this investigation, suggesting that early postdepositional changes do not greatly impact the glaciochemical records preserved at Summit. Nitrate in the real snowpits was approximately 25% lower than in simulated pits, this was the worst agreement for any ion but is consistent with several processes being known to deplete NO 3 - from near-surface snow.

Investigations of meltwater refreezing and density variations in the snowpack and firn within the percolation zone of the Greenland ice sheet

AbstractThe mass balance of polythermal ice masses is critically dependent on the proportion of surface-generated meltwater that subsequently refreezes in the snowpack and firn. In order to quantify this effect and to characterize its spatial variability, we measured near-surface (&lt;10 m) snow and firn densities at an elevation of ~1945ma.s.l. in the percolation zone of the Greenland ice sheet in spring and autumn 2004. Results indicate that local snowpack depth above the previous end-of-summer 2003 melt surface increased by ±5% (7.6 cm) from spring to autumn while, over the same period, snowpack density increased by &gt;26%, resulting in a 32% increase in net accumulation. This ‘seasonal densification’ increased at lower elevations, rising to 47% 10 km closer to the ice-sheet margin at 1860ma.s.l. Density/depth profiles from nine sites within 1 km2 at ~1945ma.s.l. reveal complex stratigraphies that change over short spatial scales and seasonally. We conclude that estimates of mass-balance change cannot be calculated solely from observed changes in surface elevation, but that near-surface densification must also be considered. However, predicting spatial and temporal variations in densification may not be straightforward. Further, the development of complex firn-density profiles both masks discernible annual layers in the near-surface firn and ice stratigraphy and is likely to introduce error into radar-derived estimates of surface elevation.

Antarctic Surface and Subsurface Snow and Ice Melt Fluxes

Abstract This paper presents modeled surface and subsurface melt fluxes across near-coastal Antarctica. Simulations were performed using a physical-based energy balance model developed in conjunction with detailed field measurements in a mixed snow and blue-ice area of Dronning Maud Land, Antarctica. The model was combined with a satellite-derived map of Antarctic snow and blue-ice areas, 10 yr (1991–2000) of Antarctic meteorological station data, and a high-resolution meteorological distribution model, to provide daily simulated melt values on a 1-km grid covering Antarctica. Model simulations showed that 11.8% and 21.6% of the Antarctic continent experienced surface and subsurface melt, respectively. In addition, the simulations produced 10-yr averaged subsurface meltwater production fluxes of 316.5 and 57.4 km3 yr−1 for snow-covered and blue-ice areas, respectively. The corresponding figures for surface melt were 46.0 and 2.0 km3 yr−1, respectively, thus demonstrating the dominant role of subsurface over surface meltwater production. In total, computed surface and subsurface meltwater production values equal 31 mm yr−1 if evenly distributed over all of Antarctica. While, at any given location, meltwater production rates were highest in blue-ice areas, total annual Antarctic meltwater production was highest for snow-covered areas due to its larger spatial extent. The simulations also showed higher interannual meltwater variations for surface melt than subsurface melt. Since most of the produced meltwater refreezes near where it was produced, the simulated melt has little effect on the Antarctic mass balance. However, the melt contribution is important for the surface energy balance and in modifying surface and near-surface snow and ice properties such as density and grain size.

Formaldehyde and hydrogen peroxide in air, snow and interstitial air at South Pole

Abstract Average H2O2 (HCHO) mixing ratios measured above the snowpack at South Pole (SP) were 278 pptv (103 pptv) in December 2000 and between 4 and 43 times (1.4–2.6 times) the value estimated from gas-phase photostationary state (PSS) model calculations. The larger difference is realized if dry deposition of both species is included in the model. H2O2 and HCHO fluxes from the snowpack were independently determined from gradient measurements in the air above the snow surface, from firn-air measurements and from the temporal concentration changes in near-surface snow. On average, the snowpack at SP was releasing on the order of 1×1013 and 2×1012 molecules m−2 s−1 of H2O2 and HCHO, respectively, in December 2000. This is consistent with the volumetric fluxes needed for the PSS model to reproduce the observed atmospheric mixing ratios of both H2O2 and HCHO. The highly elevated levels of both species found in firn air further support the above estimates. In the case of HCHO, it was also shown that there was good agreement between the measured flux and the physical air–snow exchange model as driven by changes in snow temperature from winter to summer. Shading experiments suggest that the net production of HCHO within the snow by heterogeneous photochemical processes may have exceeded photochemical destruction by no more than 20% of the measured fluxes. The very rapid changes observed in atmospheric HCHO, which are also seen in NO and OH, can be understood in terms of dynamical processes that lead to rapid changes in the atmospheric mixing depth.

The EPICA ice core from Dronning Maud Land: first results from stable-isotope measurements

AbstractThe European Project for Ice Coring in Antarctica (EPICA) focuses on the drilling of two deep ice cores, the first at Dome C and the second at Kohnen station (75°00’ S, 0°04’ E) in Dronning Maud Land (DML). This paper deals with stable-isotope records from ice cores drilled in DML. In the first season, the deep EPICA DML core reached a depth of 450 m, recovering ice approximately 7000 years old. Generally, the δ18O record indicates a stable Holocene climate and shows low variability. However, during the last 4000 years (based on a preliminary time-scale) the δ18O values decrease continuously by about 0.6%, and the deuterium excess values increase by about 0.5%. The correlation between δ18O and the deuterium excess d is investigated for a 50m long core section and the near-surface snow. High-pass filtered profiles are positively correlated, whereas the correlation between low-pass filtered profiles is negative. A post-depositional effect due to diffusion processes can be seen in a sub-annually resolved profile from snow-pit samples. Changes in the seasonality of the evolution of the snow cover and the consequences for stable-isotope content are demonstrated with data from ice core B31.

Variability of snow layer hardness by aspect and prediction using meteorological factors

The majority of slab avalanche accidents occur when the victim triggers the slide. Slab hardness is an important property affecting skier-triggered avalanches because hardness partially determines whether sufficient stress reaches the weak layer to cause failure and/or fracture. This study examines how new and old snow layer hardness varies with aspect and which meteorological variables most influence those changes. Slab hardness was measured with a ram penetrometer on north and south aspects from January through March, 2000 at Jackson Hole Mountain Resort and Grand Teton National Park, Wyoming. Continuous weather data were obtained from weather stations at Jackson Hole Mountain Resort. Analyses were carried out on new and older near-surface snow layers. New snow layer hardness increased most rapidly on the south aspect due to accelerated settlement and densification from warming by incoming shortwave radiation. With the exception of the surface layer, old snow layers, 2 months after deposition, became harder on the north aspect in comparison to the south aspect. A temperature index was calculated for the south and north aspects to describe the delayed effect of increasing temperature on increasing hardness through sintering, settlement, and densification. The south temperature index, maximum daily temperature, and the interaction between maximum daily temperature and incoming shortwave radiation were the most significant predictors of new snow layer hardness on the south aspect. The north temperature index, maximum daily temperature, and the previous day's wind speed were the most significant predictors of new snow layer hardness on the north aspect. The temperature index was the only significant predictor of old snow layer hardness on both the north and south aspects. The results of this research suggest that it may be possible to use meteorological factors to predict changes in snow hardness, which is an important component in predicting skier-triggered avalanches.

Seasonal changes in snow surface roughness characteristics at Summit, Greenland: implications for snow and firn ventilation

AbstractAir–snow transfer processes impact both ice-core interpretation and exchange affecting atmospheric chemistry. An understanding of seasonal differences in the character of the surface snow will facilitate evaluation of possible preferential seasonal exchange of reactive chemical species. Both diffusive processes and advective (ventilation) processes can serve to alter the physical, chemical and isotopic character of snow and firn. In this paper, we examine measurements of surface roughness over the course of a year at Summit, Greenland, and the implications for snow and firn ventilation. At Summit, during the winter-over experiment, summer and fall sastrugi amplitudes were approximately 5 cm and had smoothly curved profiles. the average amplitudes experienced mild increases in January, but by the end of February through March the amplitude increased to approximately 20 cm, and the profiles exhibited more abrupt geometries. Calculations are performed to show the potential impact of the changing roughness on interstitial ventilation rates in the snow, assuming that the permeability profile does not change in time. Under high winds, ventilation velocities in the near-surface snow can be up to 3 cms–1 in the winter, compared to 1 cms–1 in the summer. the frequency of 12 ms–1 winds in the summer, however, is less than in the winter. Under low-wind conditions, the summer roughness causes ventilation rates that are comparable to diffusion rates. However, in winter even 5 ms–1 wind conditions can cause the interstitial airflow due to ventilation to exceed the diffusion rates.

Snow grain-size measurements in Antarctica

AbstractGrain-size is an important but not well-known characteristic of snow at the surface of Antarctica. In the past, grain-size has been reported using various methods, the reliability, reproducibility and intercomparability of which is not warranted. In this paper, we present and recommend, depending on available logistical support, three techniques of snow-grain sampling and/or imaging in the field as well as an original digital image-processing method, which we have proved provides reproducible and intercomparable measures of a snow grain-size parameter, the mean convex radius. Results from more than 500 samples and 3000 images of snow grains are presented, which yield a still spatially limited yet unprecedentedly wide picture of near-surface snow grain-size distribution from fieldwork in Antarctica. In particular, except at sites affected by a very particular meteorology, surface grains in the interior of the ice sheet are uniformly small (0.1–0.2 mm). The climate-related increase of grain-size with depth through metamorphism is, as expected, not spatially uniform. Our Antarctic snow grain-size database will continue to grow as field investigations bring new samples, images and measures of snow grain.

The effect of wind on the snow cover

AbstractResults of wind-tunnel investigations of the propagation of air within snow showed that the process responsible for causing horizontal air flux in the pore space of snow was not necessarily related to specific surface topography. The reasons for this flux formation are: (i) the near-surface wind characteristics, such as the frequency of the pressure variation related to the turbulent character of the near-surface wind; (ii) the structure of the near-surface snow, responsible for the air exchange between the atmosphere and the pore space; and (iii) the temperature difference between the snow and the atmosphere, forming the air-density difference which augments or decreases the air exchange. This paper summarizes the experimentally observed effects that the wind and the air flux generated in the pore space had on snow. These included isothermal temperature distributions throughout the artificial snow cover, snow-density change and snow recrystallization.

Seasonal differences in surface energy exchange and accumulation at Summit, Greenland

AbstractThe 1997–98 Summit (Greenland) winter-over experiment was conducted to investigate the seasonal changes that might affect snow-air transfer processes and snow chemistry for polar ice-core interpretation This paper discusses meteorological measurements that were obtained during the experiment We use the measurements in energy-balance modeling to investigate seasonal differences in the snow-air energy exchange, and to investigate the timing of snow accumulation. We found that the surface energy exchange has distinct seasonal differences. The winter (November-February) has both the coldest average temperatures of the year and the largest temperature variations. The winter also has both the largest peak wind speeds and the longest periods of sustained high winds. Most of the water-vapor transport across the snow-air interface occurs in the summer, indicating that summer may be the primary season for near-surface snow metamorphism. Snow-depth sounder results indicate that snowfall occurs throughout the year at Summit, and thus that the ice-core record may not be affected by large seasonal gaps in the precipitation if accumulation patterns have not changed. The changes in air temperature, wind speed and radiation cause clear seasonal differences in the surface energy balance and snow-surface characteristics that are likely to cause seasonal changes in air-snow transfer processes and snow chemistry as well.

Temperature history and accumulation timing for the snowpack at GISP2, central Greenland

AbstractPrevious research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated thatδD andδ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of theδDrecord at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2data. The slope of the multi-yearTvsδcorrelation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.

Temperature history and accumulation timing for the snowpack at GISP2, central Greenland

AbstractPrevious research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated that δD and δ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of the δD record at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2 data. The slope of the multi-year T vs δ correlation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.