C41A-0475
Springtime Observations of Black Carbon in Arctic Snow across Northern Russia During IPY 2007-2008
Black carbon (BC) in snow at ppb levels can significantly reduce the visible and near IR albedo. The effect is
important for climate in regions where large areas of snow-covered surfaces are exposed to significant
sunlight. The initial study of Clarke and Noone (1985) across the western Arctic in 1983-84 indicated albedo
reduction of about 0-4 percent due to BC; however, their survey did not include results from the Russian
Arctic. During April and May of 2007 and 2008, as part of the International Polar Year Program, two
cooperative U.S.-Russian expeditions obtained the first set of BC observations at selected sites near the
communities of Naryan Mar, Vorkuta, Dikson, Khatanga, Tiksi, Chersky, Bilibino, and Pevek, spanning almost
the entire northern coastal zone of Russia. Samples were also obtained near Yakutsk, a sub-Arctic region of
boreal forest with a severe winter climate. This time period was chosen to provide access to the full winter
snowpack just prior to the onset of spring melt. This project is a critical component of a repeat and extension
of the original 1985 survey, which now includes sites spanning the entire Arctic. A discussion of this work is
the topic of an invited presentation by S. G. Warren in session "Snow and Ice Impurities as Climate Forcing
Agents and Records" (C04).
This project required access to restricted border regions of Russia, which was facilitated by the political
prominence of the IPY program in the Russian government. Generous logistical assistance and advice were
provided by Dr. V. N. Makarov of the Permafrost Institute in Yakutsk, Dr. Sergei Zimov of the Northeast
Scientific Station at Chersky, and the Hydrometeorological Service at Pevek.
Commercial air travel to the above-mentioned communities, in conjunction with local transportation, provided
access to the observation sites, which were located at distances of 15-100 km from local sources to sample
background levels of BC. At each site, snow samples and density profiles were taken from multiple levels in
the snowpack, providing values for the snow deposited throughout the cold season. The samples were
processed locally using the filtration method developed by Clarke and Noone to deposit the BC on 0.4-micron
nuclepore filters. BC content was estimated via visual comparison with a set of standard filters and later
verified via spectrophotometric techniques. Median values range from 20 to 30 ppb in general. These imply a
decrease in snow albedo in the range 0.002 to 0.03 depending on grain size. Higher values of pollution were
obtained near Tiksi, where dust and soil were included, and near Vorkuta, where local sources of coal
burning were exceptionally strong. The snow thickness in both years was typically in the range 25-40 cm, and
in many locations BC content was greatest near the surface, possibly because of dry deposition during
months with little snowfall. Values of 10 to 30 ppb are in general agreement with the model predictions of
Flanner et al. (2007).
http://www.atmos.washington.edu/sootinsnow
C41A-0476
Hydrologic response of the Greenland Ice sheet: the role of oceanographic warming
The response of the Greenland Ice Sheet to ongoing climate change remains an area of great uncertainty, with most previous studies having concentrated on the contribution of the atmosphere to the ice mass- balance signature. Here we systematically assess for the first time the influence of oceanographic changes on the Ice Sheet. The first part of this assessment involves a statistical analysis and interpretation of the relative changes and variations in sea-surface and air temperatures around Greenland for the period 1870- 2007. This analysis is based on HadISST1 and Reynolds OI.v2 sea-surface temperature analyses, in situ SST and deeper-ocean temperature series, surface-air-temperature records for key points located around the Greenland coast, and examination of atmospheric pressure and geopotential height from NCEP/NCAR reanalysis. Second, we carried out a novel sensitivity experiment in which SSTs were perturbed as input to a regional climate model, and document the resulting effects on simulated Greenland climate and surface mass balance. We conclude that sea-surface/ocean temperature forcing is not sufficient to strongly influence precipitation/snow accumulation and melt/runoff of the ice sheet. Additional evidence from meteorological reanalysis suggests that high Greenland melt anomalies of summer 2007 are likely to have been primarily forced by anomalous advection of warm air masses over the Ice Sheet and to have therefore had a more remote atmospheric origin. We also take a preliminary look at summer 2008 climate and melt anomalies over Greenland and their attribution. However, there is a striking correspondence between ocean warming and dramatic accelerations and retreats of key Greenland outlet glaciers in both Southeast and Southwest Greenland during the late 1990s and early 2000s.
C41A-0477
A Science Plan for Development of an Arctic System Model
In the last 50 years a wide variety of changes in the Arctic have been documented. Regardless of the driving forces, the combined observations and documentation suggest that the arctic system may be entering a state unprecedented in the history of civilization. The complex interplay of physical, chemical, biological and social processes interact to such a degree that it is not possible to understand future trajectories without developing holistic perspectives of the complete system. A central justification for developing an 'Arctic System Model' is to strengthen our understanding of the inter-connections among system components and related feedback processes, thereby enhancing the predictive capability required for societal planning and response to future change. A recent community workshop has identified the objectives and strategic elements that comprise a plan for Arctic System Model development and implementation. The objective encompasses our understanding of change, attribution of change, and effects of change. The plan includes the use of a limited area model, driven at the boundaries by a global model. The limited-area model approach allows for the use of computationally sophisticated algorithms and very high resolution to resolve processes parameterized in global models. The implementation strategy includes the utilization of ongoing efforts in component modeling, together with community oversight and a dedicated vehicle for the provision of coordination, support activities, and liaison with the observational and user communities.
C41A-0478
Response of lower trophic level production to long-term climate change in the southeastern Bering Sea
The ecosystem in the Bering Sea has undergone profound changes in response to climate regime shifts in
the past decades. The lower tropic level production is assessed with a vertically 1-D coupled ice-ocean
ecosystem model, which was applied to a NOAA/PMEL mooring from 1960 to 2005. The physical model is
forced by sea surface winds, heat and salt fluxes, tides, and sea ice. The biological model includes coupled
pelagic and ice algae components. Model results are validated well with daily mooring temperature,
fluorometer and daily SeaWiFS chl a data. There are two distinct ocean conditions and phytoplankton bloom
patterns related to the Pacific Decadal Oscillation (PDO) index regimes with warmer temperature and later
bloom of warm water phytoplankton species in PDO>1 years, and colder temperature and earlier bloom of
cold water phytoplankton species in PDO<-1 years. The phytoplankton production of different species
experienced dramatic changes after the 1976 climate shift, but the total annual net primary production (NPP)
remained flat over the past four decades under similar nutrients regulation. Climate shift also affected the
vertical distribution of lower trophic level production and energy flow to the upper ocean pelagic ecosystem or
the benthic community. There were a long-term PDO regime shift in 1976 and short-term PDO reversals in
1990s. Phytoplankton biomass responded to both short and long-term climate changes. Zooplankton
biomass responded to long-term climate shift as well but not to those short-term variations in 1990s. This
indicates that the lower the trophic level, the more sensitive to climate changes.
http://people.iarc.uaf.edu/~mbj
C41A-0479
East Greenland Glacier Dynamics: An Interdisciplinary Study of Helheim Glacier
Our study of Helheim glacier was initiated in 2006 primarily to investigate the source of glacial earthquakes. Glacial earthquakes appear to be sensitive to climate parameters and could potentially serve as an "early warning" for changes in the dynamics of the Arctic glaciers. However, glacial earthquakes are only useful for this purpose if we understand the mechanisms controlling them. Interdisciplinary project groups from Denmark, Spain, and the US have joined forces to cover a wide range of observables related to glacial earthquakes and glacier dynamics. The study involves seismology, geodesy, glaciology, and climatology. The seismic waves from the glacial earthquakes are detected at teleseismic distances as well as by regional and local seismographs in Greenland. The higher frequency waves from glacial earthquakes have only reasonable signal to noise ratios at distances less than a few hundred km away, whereas the lower frequencies survive to teleseismic distances. The velocity field of Helheim glacier is measured through three summer field seasons using high-rate GPS measurements directly on the ice. Automatic Weather Stations record a wide range of meteorological parameters for the purpose of looking for correlations between changes in glacier dynamics and changes in melting. Lidar data were collected in 2007 and interpreted jointly with ASTER data. It is difficult to penetrate the highly crevassed part of the glacier with radar, so in order to get a better estimate of the thickness of the ice a gravity profile was measured across Helheim glacier in 2008. The data and the preliminary results will be presented
C41A-0480
Circumpolar Active Layer Monitoring (CALM) Program: Accomplishments and Future Directions
The Circumpolar Active Layer Monitoring (CALM) program, established in the early 1990s, is designed to observe temporal and spatial variability of the active layer, near-surface permafrost parameters, and their response to changes and variations in climatic conditions. The CALM network involves 15 participating countries and is comprised of 168 sites distributed throughout the Arctic, parts of Antarctica, and several mountain ranges of the mid-latitudes. Owing to historical circumstances and logistical constraints, the distribution of sites is not uniform within the permafrost regions. The majority of the sites are in Arctic and Subarctic lowlands. At 77 sites, direct active-layer measurements are conducted on standard rectangular grids ranging from 10 x 10 m to 1 x 1 km. The locations of grids were selected to represent generalized surface and subsurface conditions characteristic of broad regions. The size of each grid reflects the level of local geographic variability. At 91 sites, active-layer values are inferred using soil temperature measurements from boreholes of variable depth. Approximately 60 CALM sites have continuous active-layer records longer than five years and 30 have ten-year records or longer. Auxiliary information includes air temperature, soil moisture, soil temperature at different depth, snow cover, soil composition, and landscape characterization. Several sites have records of frost heave and thaw subsidence that are contributing to a reconceptualization of the role of the active layer in global-change studies. Metadata include detailed site descriptions and photographs for each site. CALM is the world's primary source of information about the active layer. Data obtained from the network have been used in validation procedures for permafrost, hydrological, ecological, and climatic models, at a variety of geographic scales. This presentation summarizes decadal results and accomplishments of the CALM program and project future directions.
C41A-0481
Ice-Tethered Profiler Contributions to the Arctic Observing Network
One of the hoped-for legacies of the International Polar Year is a sustained observational program such as the Arctic Observing Network to document and build understanding of future climate and ecosystem change. In the spirit of the now-operational international Argo float program, investigators from North America, Europe and Japan are collaborating to deploy drifting, ice-based observatory instrument systems on and below floes in the Arctic to sample the polar atmosphere-ice-ocean system and to make the resulting data available to researchers world-wide in real time. One element of these observatories is the WHOI Ice-Tethered Profiler, first deployed in August 2004. The ITP consists of a surface float and electronics package that sits atop an ice floe, a weighted, plastic-jacketed wire-rope tether extending from the surface float through the ice and down to 750-800 m depth, and a profiling vehicle with sensor package that moves up and down the tether. To date, 30 ITP systems (funded by research programs in 5 countries) have been deployed in the Arctic that together have returned more than 10,000 high-vertical-resolution temperature and salinity profiles spanning approximately 7 to 760 m depth over all seasons. Examples of the science being conducted with these data will be presented, along with performance statistics for the ITP instruments and engineering improvements/enhancements that are being implemented. Plans for sustaining the ITP contribution to the Arctic Observing Network will also be reviewed and future international collaborations invited.
C41A-0482
Sensitivity of a Numerical Ice Sheet Model to Interannual Climate Forcing and Interactive Atmosphere-Ice Sheet Coupling.
Numerical ice sheet model (ISM) simulations are usually forced in a way that neglects both interannual atmospheric variability and two-way interactive feedbacks between the cryosphere and the atmosphere. We test the sensitivity of an ISM to both of these limiting assumptions through a series of idealized simulations of the Laurentide ice sheet. Lessons learned will be valuable to ongoing efforts that aim to incorporate interactive ice sheets in earth system models. Experiments in which the ISM was forced by increasingly complete portions of an interannually varying coupled climate model representation of conditions at the Last Glacial Maximum indicate a relatively low ISM sensitivity to the persistent component of ENSO. In contrast we observed a dramatic shift in the simulated equilibrium LGM ice sheet geometry when North American climate variations were incorporated directly as a looping interannual forcing time series; this appears to be due to a dynamically mediated feedback at the ice sheet margin that can be activated in susceptible regions by transient cold anomalies in the interannual climate forcing record. Idealized deglaciation simulations indicate that the use of one-way (non-interactive) atmospheric forcing fields can lead to a significant underestimation of the melt response in the ISM on millennial timescales. We demonstrate that a realistic deglaciation timescale can be achieved using two-way (interactive) asynchronous ice-atmosphere coupling scheme with a seasonal ice albedo parameterization that accounts for the observed darkening of ice in the moist summertime ablation zone.
C41A-0483
A GCM Investigation of Snow Cover-Atmosphere-Ocean Interactions in the Northern Hemisphere Using an Interactive Slab Ocean Model
Snow cover anomalies on both continental and hemispheric scales affect interannual and interseasonal Northern Hemisphere climate variability. Previous modeling and model intercomparison studies have investigated snow cover representation in GCMs and the potential impacts of anomalous snow cover on the atmosphere and atmospheric teleconnection modes. Many studies have investigated the impact of hemispheric snow cover on the atmosphere and identified Eurasia as a key forcing region due to the magnitude of snowmass, however those which consider North American snow cover have generally found a weaker relationship with downstream climate. These studies have exclusively been carried out using a fixed ocean boundary, forced by historical sea surface temperature values which cannot be modified by atmospheric processes. This study investigates the impact of realistic Northern Hemisphere snow cover and depth perturbations on modes of atmospheric variability using NCAR's Community Atmospheric Model (CAM) in conjunction with an interactive limited layer slab ocean model. Realistic snow cover and depth anomalies are based on gridded in situ measurements. Initial results show differences between an atmosphere only and atmosphere with slab ocean model in snow water equivalent and lower atmospheric temperature. Preliminary results from anomalous snow cover scenarios are also discussed.
C41A-0484
The Nature of the Decadal Variability of Surface Climate Over the North Atlantic Ocean --- --- Coupled Ocean-Atmosphere-Sea Ice Interaction Organized by Damped Ocean Mode
The nature of the observed 11-14 year decadal variability of the surface climate over the North Atlantic Ocean is investigated using the Fast Ocean Atmosphere Model (FOAM) and its data-atmosphere configuration. A 14-16 year damped ocean mode, characterized by the decadal variability of Atlantic Meridional Overturning Circulation (AMOC), is found to be able to organize coupled ocean–-atmosphere–- sea ice interaction in the North Atlantic and produce the decadal variability of the surface climate in this region. In the coupled simulation, two physical processes were found to be critical. The SAT-convection feedback is the local effect of SAT on oceanic convections in the Labrador Sea. Under cold episodes of surface climate in the North Atlantic, the damped ocean mode is pushed to its "positive" mode when the cold air induces stronger convective activity in the Labrador Sea. The stronger convections produce more vigorous AMOC and more heat transport into the subpolar North Atlantic and generates warmer SST and SAT. This gives rise to the warm episodes of surface climate. The sea ice-convection feedback is the effect of salinity anomaly on oceanic convections. The aforementioned warmer SAT induces more sea ice melting and results in low sea surface salinity in the northern subpolar gyre, especially in Irminger Sea. After the low salinity is transported into Labrador Sea, it suppresses local convective activity and acts jointly with SAT-convection feedback to switch the damped ocean mode into its "negative" mode. The damped ocean mode was uncovered in the stochastic atmospheric simulation and found to be responsible for the decadal time scale in the coupled simulations. However, the stochastic atmospheric simulation fails maintaining the decadal variability of ocean temperature and salinity in the North Atlantic both at surface and subsurface. In fact, given the insurmountable damping effect of the stochastic atmosphere, it is impossible to sustain the decadal variability of surface climate without any atmospheric response. Therefore it is coupled ocean--atmosphere--sea ice interaction organized by the damped ocean mode that generates the decadal variability of the surface climate in the North Atlantic Ocean.
C41A-0485
Running hot and cold: Decadal fluctuations in snow cover and planetary wave forcing limit global warming in late boreal winter.
A warming trend in global surface temperatures over the last forty years has been well established, consistent with anthropogenic increases in greenhouse gases. Over the last two decades, this trend appears to have accelerated. In contrast to this general behavior, however, here we show that trends during the boreal cold months in the recent period have developed a marked asymmetry, with vigorous warming in October-December followed by a reversal to a neutral/cold trend in January-March. This observed asymmetry in the cold half of the boreal year is linked to a two-way stratosphere-troposphere interaction, which is strongest in the Northern Hemisphere during late winter and is linked to variability in Eurasian land surface conditions during autumn. This link has been demonstrated for year- to- year variability and used to improve seasonal-timescale winter forecasts; here, this coupling is shown to strongly modulate the warming trend, with implications for decadal-scale temperature projections.
C41A-0486
Increasing Anthropogenic Activities and Climatic Changes and Implications on Western Himalayan Snow Cover
In the last two decades, anthropogenic activities in the Indo-Gangetic basin, located south of the Himalayan region has increased enormously. Further, the westerly winds bring dust from the Arabian peninsula during pre-monsoon season in the Indo-Gangetic basin, and often snow cover of the Himalayan region gets blanketed by the dust. Dust deposition reduces albedo of snow surface that enhances solar absorption. Decrease in snow albedo is clearly indicated from long term changes in snow cover extent of the Himalayan region, especially in the western parts. The spatial distributions of snow cover along the eastern and western parts of the Himalayan region in the last three decades will be presented. The increasing aerosol loading observed from multi sensor data will be discussed in view of the increasing anthropogenic emissions and role of pre-monsoon dust events in changing the regional climatic conditions.
C41A-0487
Teleconnection Pathways Linking The Winter Pacific ¡§C North American Pattern To North American Spring Snow Depth
The wintertime Pacific-North American (PNA) teleconnection pattern has previously been shown to influence springtime snow depth over portions of North America, and possible thermodynamic and climatic drivers behind these regional teleconnections have been explored. However, this regional emphasis may not portray the complete snow cover response associated with the hemispheric-scale PNA pattern, which motivates this investigation of the underlying pathway and physical mechanisms responsible for PNA ¡§Csnow depth linkages over all of North America. A newly released, long-term, high-resolution gridded snow depth dataset covering all of North America facilitates our continental-scale analysis. An empirical orthogonal function (EOF) filtering process is used to identify and isolate the interannual PNA signal in the snow depth field; then linear and partial correlations are employed to investigate the physical mechanisms that link winter PNA with spring snow depth. During positive winter PNA years, the enhanced PNA pressure centers lead to warmer temperatures and less precipitation over distinct regions within North America. The temperature and precipitation pathways act independently and in different geographical regions, and together they serve to reduce winter snow depth across much of North America. Winter anomalies in the snow depth field then tend to persist into spring. Our previous understanding of the dynamic mechanisms responsible for the precipitation and temperature pathways, involving moisture transport and cold air intrusions, is clarified and refined, and shown to extend to continental snow depth anomalies. These new insights will help to effectively incorporate snow cover anomalies and feedbacks into hydroclimatic forecasting and climate change predictions.
C41A-0488
Spring Snow Depth Response to Winter Climate Modes
In this study we investigate the response of spring snow depth to winter modes of atmospheric variability. Due to the modulation of the surface energy balance by snow cover, the potential for snow-albedo feedback, the difficulties encountered in modeling many of these processes, and the differences between snow depth versus snow extent variations, a thorough understanding of the snow depth-climate linkage is of great importance to understand climate variability. The recent release of a century-long, daily snow depth data provides an opportunity to investigate this relationship at continental-scale (North America) as opposed to the earlier regional-scale studies (mostly constrained in western North America). We conclude that winter snow depth responds to temperature anomalies induced by winter North Atlantic Oscillations (NAO) and to snowfall variability associated with winter Pacific/North American pattern (PNA); and that winter snow depth variations persist into spring. Clear mechanistic pathways of these teleconnections are discussed.
C41A-0489
Modelling the Effects of Temperature and Cloud Cover Change on Mountain Permafrost Distribution, Northwest Canada
Spatial models of permafrost probability for three study areas in northwest Canada between 59°N and 61°N were perturbed to investigate climate change impacts. The models are empirical-statistical in nature, based on basal temperature of snow (BTS) measurements in winter, and summer ground-truthing of the presence or absence of frozen ground. Predictions of BTS values are made using independent variables of elevation and potential incoming solar radiation (PISR), both derived from a 30 m DEM. These are then transformed into the probability of the presence or absence of permafrost through logistic regression. Under present climate conditions, permafrost percentages in the study areas are 44% for Haines Summit, British Columbia, 38% for Wolf Creek, Yukon, and 69% for part of the Ruby Range, Yukon (Bonnaventure and Lewkowicz, 2008; Lewkowicz and Bonaventure, 2008). Scenarios of air temperature change from -2K (approximating Neoglacial conditions) to +5K (possible within the next century according to the IPCC) were examined for the three sites. Manipulations were carried out by lowering or raising the terrain within the DEM assuming a mean environmental lapse rate of 6.5K/km. Under a -2K scenario, permafrost extent increased by 22-43% in the three study areas. Under a +5K warming, permafrost essentially disappeared in Haines Summit and Wolf Creek, while in the Ruby Range less than 12% of the area remained perennially frozen. It should be emphasized that these model predictions are for equilibrium conditions which might not be attained for several decades or longer in areas of cold permafrost. Cloud cover changes of -10% to +10% were examined through adjusting the partitioning of direct beam and diffuse radiation in the PISR input field. Changes to permafrost extent were small, ranging from -2% to -4% for greater cloudiness with changes of the opposite magnitude for less cloud. The results show that air temperature change has a much greater potential to affect mountain permafrost distribution in the long-term than the probable range of cloud cover changes. Modelled results for the individual areas respond according to the hypsometry of the terrain and the relative strength of elevation and PISR in the regression models. This study indicates that significant changes to the distribution and extent of mountain permafrost in northwest Canada can be expected in the next few decades. References Bonnaventure, P.P. and Lewkowicz, A.G. (2008). Mountain permafrost probability mapping using the BTS method in two climatically dissimilar locations, northwest Canada. Canadian Journal of Earth Sciences, 45, 443-455. Lewkowicz, A.G. and Bonnaventure, P.P. (2008). Interchangeability of local mountain permafrost probability models, northwest Canada. Permafrost and Periglacial Processes, 19, 49-62.
C41A-0490
Interactions between the atmospheric flow and ice sheets
A barotropic quasi-geostrophic model is employed to examine interactions between the atmospheric circulation and the surface mass balance of a single idealised ice sheet. In particular, the influence of temperature anomalies, induced by topographically-forced stationary waves, and upslope precipitation on the surface mass balance is examined for a wide range of zonal wind speeds and ice sheet extents. It is found that the stationary-wave feedback can be positive as well as negative depending on the ice sheet extent and the zonal wind. For the range of realistic wind speeds, ice growth is supported by the stationary-wave feedback for large ice sheets, and for low and high (but not intermediate) zonal wind speeds. Maximum growth is obtained for the high wind case, owing to increased upslope precipitation. The sign-reversal of the stationary-wave feedback may be of relevance for the evolution of Pleistocene ice sheets.
C41A-0491
The role of the ice-shelves in Heinrich events: from a conceptual to a 3D model
It is now often accepted that Heinrich events are associated with internal oscillations of the Northern Hemisphere ice sheets leading to periodical large surges of ice in the ocean. However, no consensus has yet been obtained about the origin of these oscillations. These large-scale surges likely occur with large basal sliding leading to fast flowing ice streams. In sediment zones, this process is favoured by strongly reduced basal dragging. However, thanks to the behavior of the ice shelves in the Antarctic Peninsula, and due to the buttressing effect of the ice shelves, we know now that basal melting may also play a significant role in the production of ice surges. In this study, we investigate trough a conceptual and a 3D thermo-mechanical ice- sheet model including representations of grounding and floating ice and ocean, the ability to produce instabilities of the Northern ice sheets. A series of tests has been conducted to analyze the role of the presence of soft-saturated sediment, the amplitude of basal sliding in grounded areas, the role of basal melting under the ice shelves and the ocean response.
C41A-0492
The Dynamical Response to Snow Cover Perturbations in a Large Ensemble of Atmospheric GCM Integrations
Variability in the extent of fall season snow cover over the Eurasian sector has been linked in observations to
a teleconnection with the winter Northern Annular Mode pattern. Current generation GCMs do not capture
this snow-circulation connection as part of their internal dynamics. So we instead explore the dynamics of this
snow-circulation connection by analyzing the response to externally imposed snow anomalies in a GCM
simulation. We here investigate a 100-member ensemble of transient integrations with the GFDL AM2
atmospheric general circulation model. The model is perturbed with a simple persisted snow anomaly over
Siberia and integrated from October through December. Strong surface cooling occurs above the anomalous
Siberian snow cover, which produces a tropospheric form stress anomaly associated with vertical
propagation of wave activity. This wave activity response drives wave-mean flow interaction in the lower
stratosphere and subsequent downward propagation of a negative phase Northern Annular Mode response
back into the troposphere. A wintertime coupled stratosphere-troposphere response to fall season snow
forcing is also found to occur even when the snow forcing itself does not persist into winter. Finally, the
response to snow forcing is compared in versions of the same model with and without a well-resolved
stratosphere. The version of the model with the well-resolved stratosphere exhibits a faster and weaker
response to snow forcing and this difference is tied to the unrealistically fast timescale of fall season internal
variability in that model.
http://pjk.atmosp.physics.utoronto.ca/preprints
C41A-0493
Influence of the annual Arctic Oscillation on the negative correlation between Okhotsk Sea ice and Amur River discharge
Freshwater discharging from Amur River is an important factor controlling the formation of sea ice because it causes a large stratification that suppresses deep convection and promotes freezing. Newly obtained observational discharge data reveal the cause of a significant negative correlation between Amur River discharge and Okhotsk Sea ice at multiyear timescales. The annually integrated Arctic Oscillation (AO) influences both summer discharge and winter ice. Summer discharge is larger and winter ice is reduced during positive AO years. Annual AO also influences the annual horizontal moisture flux convergence in the river basin. When the annual AO is positive, the annual mean air temperatures are warm over Eurasia, particularly over the Amur River basin and the Okhotsk. Consequently, autumn SSTs are warmer in the Okhotsk Sea. The warmer autumn SSTs suppress ice formation during the following winter. Freshwater from the river is not the main control of multiyear ice variability. Consideration of the annual AO provides a new look at climate system persistence at multiseasonal scales.
C41A-0494
Does ice accretion temperature and geothernmal heating pace the change from ~40 kyr to ~100 kyr glacial cycles?
The growth and decay of ice sheets are clearly linked to cycles of the Earth's orbit, particularly the ~41 kyr obliquity cycle. Early Pleistocene ice ages appear to have grown and decayed on a timescale close to the obliquity period, while the period for late Pleistocene ice ages averages closer to 100 kyr. The thermal state of an ice sheet may control whether the climatic changes produced by orbital changes are can lead to the termination of an ice age. Numerous model studies indicate that the termination of ice ages requires glacial flow that is faster than expected for cold-based ice sheets. Fast flow likely requires that the base of the ice sheet be partially melted. Large ice sheets probably accrete cold, but geothermal heating can melt the base of an ice sheet if the surface of the ice is not too cold (as it may be in Antarctica) or if the accretion rate is not too high (as it may be in Greenland). The timescale for basal melting depends on the initial temperature structure of the ice sheet and on the rate of geothermal heating. For example the melting timescale depends on the square of the initial temperature of the ice sheet base. Although some workers assume parameters that give a melting timescale of less than 10 kyrs, reasonable values of ice temperature and terrestrial heatflow could give a timescale several times longer. One-dimensional numerical ice sheet flow models with a rate of ice accretion/ablation paced at 41 kyrs show a periodicity of the glacial cycles that depends on the period of basal melting. If the melting period is short (say <~30 kyrs) then the base of the ice sheet will be partially melted in time for the obliquity maximum that could produce a termination. The ice sheet then disappears at this first obliquity maximum after ice sheet initiation because it can flow fast enough to melt at lower latitudes and elevations. For longer basal melting periods the entire ice sheet may not be warm enough to flow fast enough to lead to termination at the first obliquity maximum. Thus, the ice sheet could last more than one obliquity cycle. The warmer ambient and ice accretion temperatures of the early Pleistocene could have led to a short time of basal melting leading to glacial cycles that match the 41 kyr obliquity cycle. The cooler temperatures of the late Pleistocene could have led to a longer time for basal melting so that part of the ice sheet was too cold to flow fast and melt easily at the time of the first obliquity maximum. This could have led to ice sheets that lasted for more than one obliquity cycle and so late Pleistocene glacial cycles of ~80 or more kyrs.
C41A-0495
Comparison of AIRS and IASI Surface Observations of DomeC in Antarctica with Surface Temperatures Reported by AWS8989
The decrease of the ice in the Antarctic indicates that the land and the ocean along the coastline are warming up. Representative numbers for warming at the surface further inland are much more complicated due to the vast size of the continent. The Automated Weather Station AWS8989 has been reporting temperatures from Concordia Station on DomeC in Antarctica every 10 minutes since 1996. AWS8989 is located about 1 mile from the power plant at Concordia Station. We compare the surface temperatures at DomeC deduced from Atmospheric Infrared Sounder (AIRS) and Infrared Atmospheric Sounding Interferometer (IASI) data to the surface temperature reported by Automated Weather Station AWS8989 for the year between May 1, 2007 and April 30, 2008. AIRS and IASI measure the mean skin brightness temperature in a 50-km-radius circle from DomeC, while the AWS reports the temperature of the air at 3 meters above the surface. The AIRS and IASI measurements agree within 50 mK over the entire temperature range from 190 K in the winter to 245 K in the summer, but consistently report a colder temperature than the AWS8989. The warm bias of AWS8989 is season dependent, changing from 1.5 K warm in the winter to 5.5 K warm in the summer. Comparison of AIRS data in 2005 with a temporary Italian AWS (Aumann et al. 2006) and located several miles upwind from the power station, showed no significant temperature bias throughout the year 2005. The warm readings of AWS8989 are likely due the combination of a season-independent 1.5 K warm calibration bias in the AWS8989 sensor plus thermal contamination of the AWS8989 site. This heat island effect ranges from near zero during the low-activity winter months to about 4 K during the summer months with the highest activity at Concordia Station. The fact that activities at DomeC are increasing makes surface temperature trends from AWS8989 suspect. AIRS and IASI are hyperspectral infrared sounders designed in support of weather forecasting and climate research. AIRS was launched in May 2002 on the EOS Aqua spacecraft into a 704 km altitude polar sun- synchronous orbit with a 1:30 PM ascending node and is expected to provide data through 2015. IASI was launched in October 2006 into an 825 km altitude polar sun-synchronous orbit with a 9:30 AM ascending node with an expected 5-year lifetime.
C41A-0496
Glaciochemical evidence in an East Antarctica ice core of a recent (A.D. 1450-1850) neoglacial episode
Chemical analysis of a shallow (82.5 m) ice core from a location (DT263) in the essentially unexplored areas of Princess Elizabeth Land, East Antarctica has been used to construct a continuous, high resolution 780- year (A.D. 1207-1996) glaciochemical record. During the twentieth century, the snow accumulation rates and the concentrations of most chemical species in snow appear to be normal with short-term variations, indicating relatively stable and warm climatic conditions. The period of A.D. 1450-1850 in this record is characterized by sharply reduced snow accumulation rates and decreased concentrations of several reversibly deposited chemical species. These characteristics are consistent with colder climatic conditions and suggest that this is likely a neoglacial episode. The timing of this neoglacial episode coincides with that of the Little Ice Age (LIA), a relatively cold period in the Northern Hemisphere between the beginning of the fifteenth century and the end of the nineteenth century. However, existing Antarctic ice core records do not support a consistent, LIA-type episode between the fifteenth century and the twentieth century across the continent. Rather, the DT263 record suggests that colder and drier conditions prevailed during the LIA time period at the eastern Indian Ocean sector of East Antarctica.
C41A-0497
Surface and Mid-tropospheric Climate Change in Antarctica
Near-surface air temperatures and 500-hPa temperatures over Antarctica for 1960-2007 have been reconstructed over the entire continent using manned station observations and radiosonde records, respectively, from the READER database maintained by British Antarctic Survey. The 50-year trends found in our near-surface temperature reconstruction agree with recent work by others using a variety of spatial extrapolation techniques. It is found that the statistically significant Antarctic Peninsula near-surface warming on an annual basis has spread into West Antarctica reaching as far as east as the Pine Island Bay-Thwaites Glacier region. The warming is most marked in recent years with 2007 being the warmest year in the 1960- 2007 interval. In contrast to the western (eastern) Antarctic Peninsula warming which is maximized in winter (summer), the warming over West Antarctica is maximized in the spring (SON) and in that season statistically significant warming stretches across all of West Antarctica and into northern Victoria Land. Weak near- surface warming is found over East Antarctica and the continent as a whole on an annual basis although continental warming in the spring is statistically significant and driven largely by the strong and widespread changes in West Antarctica. The 1960-2007 500-hPa temperature reconstruction is compared to the changes described by Turner et al. (2005), who found strong winter warming in radiosonde records over Antarctica for 1971-2003 but noted greater uncertainty over West Antarctica where there are few observational constraints.
C41A-0498
Response of Antarctic Peninsula Mesoscale Cyclones to the Southern Hemisphere Annular Mode: Potential Linkages to Climate Change
The Western Antarctic Peninsula (WAP) is a region that has undergone significant climate change. The WAP region has experienced the largest regional climate warming over the past half century. There has been a concomitant trend in the Southern Annular Mode (SAM) toward a positive polarity: several modeling studies have revealed that a combination of springtime ozone depletion and greenhouse gas increases are primarily responsible. Here we investigate possible linkages with mesoscale cyclone activity in the WAP region, as a potential climate system feedback mechanism. We have analyzed 7066 satellite images from the U.S. Antarctic Program satellite tracking facility at Palmer Station, Antarctica, comprising between 2-14 overpasses per day during 1991-94 (a time interval of large SAM variability). These images allow us to reliably identify and track mesoscale cyclones. The frequency of mesoscale cyclones in the WAP region during is found to be correlated with the SAM index, most strongly during winter and spring. Also, during periods of positive SAM index polarity there is a shift in the storm tracks to favor more east-bound trajectories, consistent with strengthening of circumpolar westerlies. The presence of mesoscale cyclones is associated with positive near-surface-air temperature anomalies in the WAP region year-round, largest during winter. Mesoscale cyclones may therefore have a significant indirect effect on climate change in the WAP region.
C41A-0499
Multi-decadal surface temperature trends near the Ice Divide of East Antarctica using borehole firn temperature measurements and inversion method
The objective of this investigation is to detect multi-decadal surface temperature trends near the Ice Divide of East Antarctica. The interior of East Antarctic Ice Sheet (EAIS) remains as one of the least explored areas on earth. In recent years there have been several studies attempting to reconstruct the surface temperature history of Antarctica for the past 50 years using several approaches. However the lack of in situ data has hindered the science community in reaching a conclusive answer about Antarctic climate change, and in particular for the EAIS. In order to gain a better assessment of Antarctic climate change, additional data sources are needed to reduce the current uncertainty. Surface temperature inversion from firn temperature measurements will provide a source of climate reconstruction independent of firn chemistry, sparse weather data, satellite data or ice cores. During the Norwegian-U.S. IPY Scientific Traverse of East Antarctica, in austral summer of 2007-08, thermal-profiling units were installed at three locations (76.06 ° S, 22.46 ° E, Traverse site NUS07-2; 78.65 ° S, 35.64 ° E, NUS07-5, 126 km from Plateau Station; 82.07 ° S, 54.9 ° E, NUS07-7, 2 km from the Pole of Inaccessibility). Each unit consists of 16 PRTs (Platinum Resistance Thermometers) distributed between 0.2 and 90 m in depth. Wired PRTs were lowered into the borehole after an ice core was drilled and before the hole was back-filled with granulated snow to prevent air circulation and provide thermal conduction between PRTs and firn. Near-hourly data are being transmitted through ARGOS satellite telemetry system. The overall uncertainty in firn temperature measurement is between 0.02 and 0.03 ° C. Mean temperature gradients between -0.5 and -0.75 ° C were found between 16 and 90 m at three sites, with standard deviations less then 0.03 ° C. These gradients are larger than or about the same as previously published studies that modeled temperature profile under a steady-state climatic condition. Our results suggest a cooling to no significant trend near the Ice Divide of East Antarctica for the past several decades. Detailed analysis with the application of inversion method is ongoing to determine how the observed signals in the temperature profiles translate in to magnitudes and temporal scales of the surface temperature trends. A positive temperature gradient, suggesting recent warming, was identified for the top of the Greenland ice sheet (Summit station) and will be discussed in comparison.
C41A-0500
Recent Changes in Antarctic Temperature and Circulation Patterns: A Nonlinear Approach
Results from a study of recent July near-surface temperature anomaly patterns over Antarctica show a shift from a regime dominated by East Antarctic variability to one where anomalous warmth in West Antarctica is much more common. These changes are due in part to changes in the Amundsen Sea Low circulation. Early record (1979-1983) temperature variability derives primarily (64%) from patterns of strong warm and cold anomalies in East Antarctica with a small contribution from warm West Antarctic patterns (8%). By the end of the record (1998-2002), the warm/cold patterns from East Antarctica have dropped by half (32%) while warm patterns in West Antarctica have tripled (27%). For the warm (cold) East Antarctic patterns, anomalies exceed 8 K (-4 K) over substantial areas, while the warm West Antarctic anomalies exceed 4 K. These results derive from new climatologies from a 23-year (1979-2002) Polar MM5 dataset based on self-organizing maps (SOMs). SOMs provide a nonlinear, artificial neural networks-based approach to the analysis of complex geophysical datasets, e.g., atmospheric circulation. Unsupervised classification of data into a relatively small number of distinct, generalized SOM patterns, or modes, summarizes data variability into a two-dimensional, spatially organized (nonlinear) grid format. Climatologies of July tropospheric atmospheric circulation (e.g., standard geopotential heights) have been developed to further study this region and to help interpret these temperature results. For example, the change in West Antarctic temperatures appears related to increased frequency of strong Amundsen Sea Low patterns bringing warm air into this region. East Antarctic temperature change is more complex. Many other aspects of variability in these climatologies (e.g., interannual frequency change, pattern transitions) are also being investigated. Analysis of additional time periods (e.g., summer) is also planned. We anticipate that these analyses will bring new insights into the complex climate of this region at many timescales.
C41A-0501
The Influence of Antarctic Sea Ice on the Variability of the Southern Hemisphere Annular Mode.
The response of the atmospheric circulation to observed extremes of Antarctic sea ice concentration was simulated using the National Center for Atmospheric Research's Community Climate System Model. Three simulations, one each for minimum, maximum and average sea ice concentrations were run for 150 years. The results clearly demonstrate that south of 55S, under minimum sea ice conditions, temperatures were consistently warmer, and cooler under maximum sea ice conditions. Composite differences of temperature and pressure indicate an altering of the meridional temperature and pressure gradients. These are accompanied by changes in strength of the midlatitude westerlies and the polar easterlies and latitudinal shifts in the location of the midlatitude jet stream. These responses of the atmospheric circulation to the sea ice distribution indicate some influence of sea ice on the polarity of the Southern Hemisphere annular mode. The mechanism by which this can occur is discussed.