Browsing College of Natural Science and Mathematics (CNSM) by Subject "Sea ice"
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Arctic sea ice trafficability: new strategies for a changing icescapeSea ice is an important part of the Arctic social-environmental system, in part because it provides a platform for human transportation and for marine flora and fauna that use the ice as a habitat. Sea ice loss projected for coming decades is expected to change ice conditions throughout the Arctic, but little is known about the nature and extent of anticipated changes and in particular potential implications for over-ice travel and ice use as a platform. This question has been addressed here through an extensive effort to link sea ice use and key geophysical properties of sea ice, drawing upon extensive field surveys around on-ice operations and local and Indigenous knowledge for the widely different ice uses and ice regimes of Utqiaġvik, Kotzebue, and Nome, Alaska. A set of nine parameters that constrain landfast sea ice use has been derived, including spatial extent, stability, and timing and persistence of landfast ice. This work lays the foundation for a framework to assess and monitor key ice-parameters relevant in the context of ice-use feasibility, safety, and efficiency, drawing on different remote-sensing techniques. The framework outlines the steps necessary to further evaluate relevant parameters in the context of user objectives and key stakeholder needs for a given ice regime and ice use scenario. I have utilized this framework in case studies for three different ice regimes, where I find uses to be constrained by ice thickness, roughness, and fracture potential and develop assessment strategies with accuracy at the relevant spatial scales. In response to the widely reported importance of high-confidence ice thickness measurements, I have developed a new strategy to estimate appropriate thickness compensation factors. Compensation factors have the potential to reduce risk of misrepresenting areas of thin ice when using point-based in-situ assessment methods along a particular route. This approach was tested on an ice road near Kotzebue, Alaska, where substantial thickness variability results in the need to raise thickness thresholds by 50%. If sea ice is thick enough for safe travel, then the efficiency of travel is relevant and is influenced by the roughness of the ice surface. Here, I develop a technique to derive trafficability measures from ice roughness using polarimetric and interferometric synthetic aperture radar (SAR). Validated using Structure-from-Motion analysis of imagery obtained from an unmanned aerial system near Utqiaġvik, Alaska, I demonstrate the ability of these SAR techniques to map both topography and roughness with potential to guide trail construction efforts towards more trafficable ice. Even when the ice is sufficiently thick to ensure safe travel, potential for fracturing can be a serious hazard through the ability of cracks to compromise load-bearing capacity. Therefore, I have created a state-of-the-art technique using interferometric SAR to assess ice stability with capability of assessing internal ice stress and potential for failure. In an analysis of ice deformation and potential hazards for the Northstar Island ice road near Prudhoe Bay on Alaska's North Slope I have identified a zone of high relative fracture intensity potential that conformed with road inspections and hazard assessments by the operator. Through this work I have investigated the intersection between ice use and geophysics, demonstrating that quantitative evaluation of a given region in the ice use assessment framework developed here can aid in tactical routing of ice trails and roads as well as help inform long-term strategic decision-making regarding the future of Arctic operations on or near sea ice.
Global and local contributors to the historical and projected regional climate change on the North Slope of AlaskaThis thesis includes four studies that explore and compare the impacts of four contributing factors resulting in regional climate change on the North Slope of Alaska based on a numerical simulation approach. These four contributing factors include global warming due to changes in radiative forcing, sea ice decline, earlier Arctic lake ice-off, and atmospheric circulation change over the Arctic. A set of dynamically downscaled regional climate products has been developed for the North Slope of Alaska over the period from 1950 up to 2100. A fine grid spacing (10 km) is employed to develop products that resolve detailed mesoscale features in the temperature and precipitation fields on the North Slope of Alaska. Processes resolved include the effects of topography on regional climate and extreme precipitation events. The Representative Concentration Pathway (RCP) 4.5 scenario projects lower rates of precipitation and temperature increase than RCP8.5 compared to the historical product. The increases of precipitation and temperature trends in the RCP8.5 projection are higher in fall and winter compared to the historical product and the RCP4.5 projection. The impacts of sea ice decline are addressed by conducting sensitivity experiments employing both an atmospheric model and a permafrost model. The sea ice decline impacts are most pronounced in late fall and early winter. The near surface atmospheric warming in late spring and early summer due to sea ice decline are projected to be stronger in the 21st century. Such a warming effect also reduces the total cloud cover on the North Slope of Alaska in summer by destabilizing the atmospheric boundary layer. The sea ice decline warms the atmosphere and the permafrost on the North Slope of Alaska less strongly than the global warming does, while it primarily results in higher seasonal variability of the positive temperature trend that is bigger in late fall and early winter than in other seasons. The ongoing and projected earlier melt of the Arctic lake ice also contributes to regional climate change on the Northern coast of Alaska, though only on a local and seasonal scale. Heat and moisture released from the opened lake surface primarily propagate downwind of the lakes. The impacts of the earlier lake ice-off on both the atmosphere and the permafrost underneath are comparable to those of the sea ice decline in late spring and early summer, while they are roughly six times weaker than those of sea ice decline in late fall and early winter. The permafrost warming resulted from the earlier lake ice-off is speculated to be stronger with more snowfall expected in the 21st century, while the overall atmospheric warming of global origin is speculated to continue growing. Two major Arctic summer-time climatic variability patterns, the Arctic Oscillation (AO) and the Arctic Dipole (AD), are evaluated in 12 global climate models in Coupled Model Intercomparison Program Phase 5 (CMIP5). A combined metric ranking approach ranks the models by the Pattern Correlation Coefficients (PCCs) and explained variances calculated from the model-produced summer AO and AD over the historical period. Higher-ranked models more consistently project a positive trend of the summer AO index and a negative trend of summer AD index in their RCP8.5 projections. Such long-term trends of large-scale climate patterns will inhibit the increase in air temperature while favoring the increase in precipitation on the North Slope of Alaska. In summary, this thesis bridges the gaps by quantifying the relative importance of multiple contributing factors to the regional climate change on the North Slope of Alaska. Global warming is the leading contributing factor, while other factors primarily contribute to the spatial and temporal asymmetries of the regional climate change. The results of this thesis lead to a better understanding of the physical mechanisms behind the climatic impacts to the hydrological and ecological changes of the North Slope of Alaska that have been become more severe and more frequent. They, together with the developed downscaling data products, serve as the climatic background information in such fields of study.
Mechanisms and implications of changes in the timing of ocean freeze-upThe shift to an Arctic seasonal sea ice cover in recent years motivates a deeper understanding of freeze-up processes and implications of a lengthened open water season. As the sea ice boundary between the Arctic ocean and atmosphere covers a smaller area, the effects of enhanced wind mixing become more pronounced. Winds are important for ocean circulation and heat exchange. Ultimately, they can influence when freeze-up can occur, or can break up new ice as it forms. The chapters of this thesis are motivated by the substantial social and geophysical consequences of a lengthening open water season and linked through discussion of what controls freeze-up timing. Implications of a declining sea ice cover as it pertains to the three Arctic Alaska coastal communities of Kotzebue, Shishmaref, and Utqiaġvik are explored in depth. Indices of locally-relevant metrics are developed by using physical climate-related thresholds found by other studies to impact Alaska communities and coastal erosion rates. This allows for a large-scale climate dataset to be used to define a timeseries of these indices for each community. We found a marked increase in the number of false freeze-ups and break-ups, the number of days too windy to hunt via subsistence boat, and in Utqiaġvik, an approximate tripling of erosion-capable wind events from 1979-2014. The WRF-downscaled ERA-Interim dataset (ERA-Interim for sea ice) was also used in the analysis of all chapters. The cumulative wind energy input into the upper ocean was calculated for the Chukchi, southern Beaufort, and northeast Bering Seas at time periods up to three months prior to freeze-up, and then correlated with the timing of freeze-up. We have found that increased wind energy input into the upper ocean 2-3 months prior to freeze-up is positively and most strongly correlated with the date of freeze-up in the Chukchi Sea. Analysis of wind climatology shows winds are increasing in the period prior to freeze-up as a delayed freeze-up moves into the fall storm season. A negative correlation is found in the Bering Sea over shorter timescales, suggesting that storms promote the arrival of sea ice there. Case studies are evaluated for the Chukchi Sea and Bering Sea, to illustrate mechanisms at play that cause the positive and negative correlations in these seas, respectively. Ice advection and high winds from northerly directions are shown to hasten the timing of freeze-up in the Bering Sea. In the Chukchi Sea, higher winds from the dominant northeasterly direction promote upwelling of warm and salty water up onto the shelf, which suggests a mechanism for why high winds are associated with a delayed freeze-up there. We next examine the effect of winds on freeze-up timing by using a 1-D vertical column model of the mixed layer. The model is initialized using temperature and salinity profiles obtained from a freeze-up buoy deployed in 2015 in the north-east part of the Chukchi Sea. The meteorological forcing used to drive the model experiments comes from a WRF-downscaled ERA-Interim Reanalysis dataset. Our results show that vertical wind-driven mixing leads to enhanced heat loss. In light of the previously found positive correlation between wind energy input and freeze-up timing, the mixing model results suggest horizontal advection not captured by the 1-D column model can dominate wind-driven vertical mixing to promote freeze-up.