RSS 1.0Atmospheric Sciences
Recent Submissions
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Climate variability, trends, and impacts on the Yukon-Kuskokwim Delta with insights into relationship-building to enhance climate scienceClimate change impacts in the Arctic and Alaska vary widely, providing opportunities to study regional complexities. This thesis, guided by insights from Yup'ik Elders from the Yukon-Kuskokwim Delta, explores the significant impacts of climate change and examines four key topics: climate-vegetation connections, shifting hydroclimate regimes, the role of large-scale climate patterns in tundra wildfires, and fostering community relationships. The first paper investigates tundra vegetation productivity trends which have decreased in the Yukon-Kuskokwim Delta despite increasing temperatures during the growing season, contrasting with other Arctic tundra regions. Using available long-term climate datasets, a coherent multi-decadal pattern involving spring sea-ice concentration in the East Bering Sea, growing season temperatures, and tundra productivity is revealed. This finding highlights that low-frequency variability can obscure long-term climate relationships. The second paper examines moisture dynamics in the Yukon-Kuskokwim Delta, revealing significant variability in moisture-related climatic factors. A comprehensive analysis of atmospheric data indicates a shifting hydroclimate regime in the Yukon-Kuskokwim Delta, with decreasing large-scale precipitation, increasing convective precipitation and evaporation, and fewer synoptic storms. The study underscores that warmer conditions and changing precipitation patterns can alter vegetation and overall landscape vulnerability to climatic changes. The third paper delves into early-season climate drivers of tundra wildland fires in the Yukon-Kuskokwim Delta. It establishes a baseline climatology for early fire seasons, connecting warmer temperatures and earlier snow-off dates with increased fire activity. It identifies May and June temperatures exceeding 15°C and snow-off dates before May 8 as key indicators for large fire seasons, emphasizing that lightning stroke counts crucially influence the area burned. Contrary to previous research, this study finds that May and June precipitation and spring sea-ice concentration are not primary drivers of tundra fires in the region. The findings suggest a future increase in fire frequency due to projected climate conditions. The fourth topic synthesizes perspectives on regional climate research, relationshipbuilding, and community engagement in Alaska. It discusses the importance of focusing on smaller regions for climate studies to build relevant, credible, and legitimate scientific endeavors with local communities. It emphasizes the value of cultural humility, self-reflection, and active community participation for effective climate science communication and relationship-building. This piece also addresses the systemic challenges of community work within academia and highlights the significance of volunteerism to enhance community-based climate research. Collectively, these papers underscore the intricate relationships between climate variables, tundra landscapes, and wildfire dynamics in Alaskan tundra regions, and the need for region-specific research approaches. The research concludes that community collaboration and culturally sensitive approaches are essential for impactful climate science.
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Current exposure of Yukon Flats tribal villages' residents to PM₂.₅ from natural and anthropogenic sources: establishing baselines for climate change adaptation and resilienceHow healthy is the air in the villages during the summer fire seasons? Why does Fort Yukon always seem to be colder than the surrounding villages in winter and spring? How healthy is the air we breathe in our homes and workplaces? These are but a few of the questions asked by Alaska's Eastern Interior residential village's Indigenous Tribal Governments. A tribal-owned network of aerosol monitors and meteorological stations was installed at Ts'aahudaaneekk'onh Denh, Gwichyaa Zheh, Jałgiitsik, and Danzhit Khànlaj̜j̜ in the Yukon Flats, Alaska. To assess the exposure of residents in rural communities in the Yukon Flats to particulate matter of 2.5 [micro]m or less in diameter (PM2.5), both indoor and outdoor concentration observations were carried out from spring 2017 through to August 2019. Surface-based-temperature inversions occurred under calm wind conditions due to surface radiative cooling. In May, local emissions governed air quality with worst conditions related to road and river dust. As the warm season progressed, worst air quality was due to transport of pollutants from upwind wildfires. Absorption of solar radiation in the smoke layer and upward scattering enhanced stability and fostered the persistence of the surface-based-temperature inversions. Under weak large-scale forcing mountain-valley circulations develop that are driven by the differences in insolation. During the long dark nights, surface radiative cooling occurs in the near-surface layer of the mountain slopes of the Brooks, Ogilvie and White Mountains Ranges and at the bottom of the valley. Here surface-based-temperature inversion - known as roof-top inversions - form, while the cold air drains from the slopes. A frontal wedge forms when the cold air slides over the relatively colder air in the valley. Drainage of cold air from the Brooks Range governed the circulation and cold air pooling in the valley. At the site, which is closest to the mountains, concentrations marginally changed in the presence of temperature inversions. Indoor concentrations were measured at 0.61 m in homes and at 1.52 m heights both in homes and office/commercial buildings. Air quality was better at both heights in cabins than frame homes both during times with and without surface-based-temperature inversions. During summer indoor concentrations reached unhealthy for sensitive groups to hazardous conditions for extended times that even exceeded the high outdoor concentrations. Indoor and outdoor concentrations were strongest related for office/commercial buildings, followed by frame houses and cabins. These are but a few of the answers found in this research of meteorology effects, unhealthy locations for breathing PM2.5 air outdoors and in homes.
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Synoptic climatology of the eastern Brooks Range, Alaska: a data legacy of the international geophysical year"Data from three International Geophysical Year (1957-1958) expeditions and one International Hydrological Decade National Science Foundation project (1969-1972) to the eastern Alaska North Slope have been rescued and made available in digital form: Chamberlain Glacier, Lake Peters, and McCall Glacier. Comparisons between these sites and US and Canadian Weather Service stations within 500km of McCall Glacier were conducted to determine the broad temperature climatology of the region. McCall Glacier is generally a swing site, and the climatology of the region often was linked most closely to the Beaufort Sea coast, though on some occasions, was more closely related to the Mackenzie River Delta and on other occasions, to the Interior. These early data represent an important addition to the Arctic data legacy by allowing a more complete climate record to be developed that focuses on a region demonstrably sensitive to climate change and yet lacking in data. Key words: glacier, meteorology, International Geophysical Year, Alaska, McCall Glacier, Brooks Range, data rescue"--Leaf iii
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Theoretical and experimental investigations of resonance fluorescence lidar for measurements of N₂ in the auroral atmosphere"In this thesis, a series of experimental and theoretical studies of the resonance fluorescence lidar system at Poker Flat Research Range (located in Chatanika, Alaska) for use in obtaining measurements of aurorally produced molecular nitrogen ions (N₂) are presented. Obtaining measurements of N₂ is made challenging by both the operational performance of the resonance lidar system and the high degree of geophysical variability inherent in the aurora. Analyses are conducted of measurements obtained by the operational sodium and iron resonance lidar systems to verify the lidar system performance. To increase the strength and quality of the lidar measurements, the telescope in the lidar receiver system was upgraded from a 0.6 in Newtonian telescope to a Cassegrain telescope with a 1.02 m diameter primary mirror. Lidar measurements from the system operating with this telescope are presented and compared to previous measurements to confirm an improvement to the overall operation. A spectroscopic analysis of the laser dye used in the previous development of the molecular nitrogen resonance lidar system is conducted to determine the cause of decreased lidar system performance at the operational wavelength relevant for studies of N₂. A total of ten laser dyes are tested in the dye laser system. Based upon the performance of these dyes in the resonance lidar system, it is concluded that successful measurements of the strongest emission band in N⁺₂ are unlikely due the transmittance of the diffraction grating at the relevant wavelength and low system efficiency in the dye laser. Therefore, the resonance lidar system is being developed to obtain measurements of the second strongest band of emissions in N⁺₂ . To assess the capabilities of this system to obtain statistically significant measurements of aurorally produced N⁺₂, the expected resonance lidar signal is simulated by modifying an existing model. It is found that to obtain N⁺₂ resonance lidar measurements of reasonable strength with the current operational system, the data would be obtained at high resolution and post integrated over selected temporal and spatial ranges"--Leaf iii
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Investigation of the impact of ship emissions on atmospheric composition and deposition into remote, coastal landscapes of Southwest Alaska"Every summer season, the Gulf of Alaska experiences an influx of shipping traffic, yet ship emissions are only modestly regulated allowing for substantial amounts of pollutants to be released. These pollutants can be transformed and transported affecting atmospheric composition and deposition of contaminants in even remote landscapes. The fully coupled Weather Research and Forecasting meteorology and chemistry transport model, WRFChem, is used to simulate physical and chemical processes such as transport, transformation, and deposition. Model simulations for a tourist season are performed without and with the inclusion of a ship emission inventory developed for this study. Ship emissions are shown to significantly increase both primary and secondary pollutant concentrations in the Gulf of Alaska causing reduced visibility and contributing greatly to accumulated deposition into coastal ecosystems. Complex topography also plays a role in regions most affected by ship emissions including Prince William Sound and the Kenai Peninsula. Meteorological conditions govern the temporal evolution of air quality and deposition throughout the season. Evaluation of WRFChem with meteorological observations reveals that it well captures the synoptic situation during the season. WRFChem underestimates aerosol concentrations, but aerosol monitoring sites are sparse within the Gulf of Alaska and may not accurately reflect overall performance"--Leaf iii
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North Atlantic air-sea interactions driven by atmospheric and oceanic stochastic forcing in a simple box model"North Atlantic (NA) variability has wide-spread implications locally and globally. This study investigates mechanisms driving NA variability using a simple box model incorporating time evolution of interacting upper ocean temperature anomalies, horizontal (Gyre) and vertical (meridional overturning circulation, or MOC) circulation, driven by surface air temperature, wind, and Labrador Sea temperature forcings. Simulated upper ocean responses to external atmospheric forcing result in solutions with redder spectra than solutions by white noise atmospheric forcing, implying that the ocean acts as a low-pass filter to this external forcing. Simulated ocean dynamic response may be viewed as a response to a cumulative atmospheric forcing over an interval defined by system damping properties. A strong anti-correlation links simulated MOC and Gyre circulation intensity suggesting a mechanism, in which system heat balance is maintained via communication between the dynamic components, (e.g. excess of heat supply from a stronger Gyre circulation would be balanced by lack of heat from a weaker MOC circulation and vise versa). Wind was the dominant forcing for NA upper ocean temperature anomalies and the intensity of MOC and Gyre circulations. Further investigations of NA variability mechanisms are important as they have serious implications on global heat transport"--Leaf iii.
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An observational study of the surface-based radiation temperature inversion in Fairbanks, Alaska"Steep, surface-based inversions are a common occurrence in the valleys of interior Alaska due to the frequent occurrence of thermodynamically stable air masses caused in large part by the large negative net surface radiation balance present in high-latitude regions during wintertime. The inversion typically does not exceed heights of several hundred meters and can exhibit temperature ranges of up to 30°C from valley bottoms up to the top of the inversion. The main objective of this project was to improve the understanding of the surface-based inversion's temporal evolution by analyzing a suite of extra upper-air balloon launches released during clear and calm nights in Fairbanks. A series of radiosonde launches at high temporal frequency were conducted over seven case-study days spanning spring 2009 and fall/winter 2010. These case studies each present a unique story due to variability with respect to the effects of incoming solar radiation, snow cover, and winds on the development of the surface-based inversion. Several generalizations emerged which provided insight into the development of the surface based inversion with respect to these variables. In general, the surface temperature decreased most rapidly just after sunset. After this initial cooling pulse at the surface the profile begins to cool vertically. Another feature that appeared on several launches was when surface cooling appeared to cease, suggesting that some sort of temperature minimum had been attained. Cases without snow on the ground had larger surface temperature changes than those with snow cover. The presence of snow cover limits the diurnal surface temperature drop due to reflectivity of radiation which would heat up the surface during the day. This unique data set of two to three hour radiosonde launches could be the basis of future projects in air quality studies, modeling studies, and micrometeorological studies"--Leaf iii
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Synoptic drivers of storm surge in Kotzebue Sound"Coastal flooding due to storm surge is a serious danger for communities and industry in western Alaska. The purpose of this study is to gain an understanding of which storm tracks lead to the greatest surge, whether certain areas are more vulnerable than others due to local scale features, and what currents are associated with surge events. Kotzebue Sound was chosen as the area of focus on the basis of physical and social factors. Physically, its bathymetry, topography, and coastal orientation make it particularly susceptible to surge. Socially, this region possesses a range of affected coastal groups, including heavy industry and subsistence communities. The study was performed using the ADvanced CIRCulation (ADCIRC) coastal circulation and surge model forced with idealized storm scenarios. Results from the study suggest the utility of applying a surge model to learn about water run up, set down and induced current on the Alaskan coast. Local flooding peculiarities were found in Hotham Inlet, which could have potential impacts on a sensitive ecosystem. Noticeable differences were found in surge magnitude and distribution between storms of different ground track speed, as well as small changes in track location"--Leaf iii
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Use of Hurst and Renyi analysis to detect and characterize Pacific decadal oscillation impacts on climate variability in AlaskaWhile climate systems are known be nonlinear, most statistical tools used to study climate are linear. Two nonlinear analyses are introduced for indicating predictability in climate studies: Hurst analysis and Renyi analysis, the advantages of which are illustrated by applying both to characterize Alaska climate time series 'dynamics' or temporal evolution. These methods are also applied to reanalysis and model data to compare with the observational analysis. Hurst analysis is used to calculate long term predictability in data on a scale of five to 15 years; Renyi analysis is used to quantify the degree of order on a time scale of two to 15 days. The analyses revealed that temperature may be more statistically predictable in certain areas of Alaska during the positive phase of the Pacific Decadal Oscillation (PDO). Circulation effects associated with the PDO shift are found to plausibly cause the change in randomness of the SAT data.
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A comparison of surface moisture budget and structural equation models in high latitudes: evapotranspiration and atmospheric driversArctic soil moisture is one of the most impactful and unknown aspects of the Arctic climate system. As the climate changes, surface soil moisture can impact water supplies, wildfire risk, and vegetation stress, all of which have consequences for terrestrial ecosystems and human activities. The present analysis is intended to (1) document seasonal and interannual variations of surface moisture fluxes in the Arctic region and (2) clarify the drivers of variations of net Precipitation minus Evapotranspiration (P-ET) across Arctic tundra and boreal vegetation and permafrost status. Forty-five flux tower sites were examined across boreal and tundra ecosystems across the Arctic and sub-arctic. The surface moisture budget at boreal forest sites in permafrost areas generally shows a moisture deficit in late spring and early summer, followed by a moisture surplus from late summer into autumn. The annual net P-ET is generally positive but can vary interannually by more than an order of magnitude. A factor analysis found the primary drivers of variations in evapotranspiration to be radiative fluxes, air temperature, and relative humidity, while a path analysis found windspeed to have the largest independent influence on evapotranspiration. Overall, the ET at boreal forest sites shows a stronger dependence on relative humidity, and ET at tundra sites shows the stronger dependence on air temperature. These differences imply that tundra sites are more temperature-limited and boreal sites are more humidity-dependent. Relative to nearby unburned sites, the recovery time of ET after disturbance by wildfire was found to vary from several years on the Alaska tundra to nearly a decade in the Alaska boreal forest.
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Exploring the use of machine learning for daily fire growth prediction in AlaskaWildfire is a natural but often hazardous part of the Alaskan ecosystems. Physically based wildfire models range from simple relationships used for rapid, in-situ fire behavior analysis to complex weather models used for prediction over several days and weeks. Physical models in Alaska, however, often struggle to integrate weather forecast information to make predictions beyond just a few days. The random forest model explored here is able to leverage an array of variables to identify days of enhanced and reduced satellite fire detections. Peaks and lulls in activity are accurately identified, though exact magnitudes are often incorrect, especially when wildfire suppression efforts occurred. This study emphasizes the use of reanalysis weather variables in addition to antecedent fire activity, highlighting the usefulness of variables like vapor pressure deficit for use in quantitative prediction. By applying weather forecast data, the model generated simulated wildfire forecasts. These forecasts show some success at identifying peaks and lulls in fire activity. Effective lead time varied widely ranging between 1 and 10 days, mostly dependent on the weather model performance. By providing specific timing and using real ensemble forecasts for medium term prediction, a model likes this fills a potential open niche in fire predictive services. Machine learning may be especially useful for its relative efficiency and ease of automation.
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Investigations on the impacts of ship traffic in the Bering Sea on aerosol optical depth using automatic identification systems data, and MODIS collection 6.1 dataIncreasing Arctic shipping requires study of the increasing aerosol emissions impact on aerosol optical depth (AOD) in the Bering Sea and the Bering Strait. This study used Moderate Resolution Imaging Spectroradiometer (MODIS) 550 nm AOD 3 km products to study the seasonal variability over the length of the Arctic shipping season, from June to October, over 2011 and 2014. Bucket resampling was used to project and downscale the MODIS AOD to a 0:25 by 0:25 grid during overpasses. An overpass is dened as consecutive MODIS granules from both the Terra and Aqua satellites. Ship positional data obtained from automatic identication systems (AIS) was aggregated to hourly data. Collocation of ships and AOD from overpasses were determined for all quarter degree grid cells and times. Area-weighted means for both grid-cells with ship occurrence and without ships were determined for each month. AOD decreases with increasing time in the shipping season due to the increasing frequency of low pressure system and hence aerosol removal via washout and scavenging. Comparison of the AOD of 2011 and 2014 revealed that the position of the Aleutian Low not only strongly aects the sample size, but also AOD over the Bering Sea. The sea-ice area seemed to be without notable impact on the number of ships and AOD. A weak positive correlation was found between AOD and the number of ships present in the same grid cell during a overpass for 7 out of the 10 months. A strong skewing towards October occurred in 2011 due to a strong positive correlation of the number of data points.
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Meteorological drivers of lightning in Alaska on seasonal and sub-seasonal timescalesWildfire has long been a part of the Boreal forest ecosystem in Alaska and the increasing number of large fire seasons over the past 2 decades has had substantial economic and health impacts. Boreal wildfires are expected to continue to increase over the next century in part due to a projected increase in lightning. This motivates developing lightning outlooks to inform fire management decisions regarding the economic allocation of shared firefighting resources including but not limited to personnel and air tankers. The goal of this research is to identify key meteorological parameters associated with lightning processes on a stroke-by-stroke spatial scale and at hourly-to-sub-seasonal timescales. This is a first step towards developing robust lightning outlooks. In order to identify the key parameters, lightning data for the Alaska Lightning Detection Network was paired with hourly European Center Reanalysis Version 5 (ERA5) over the 2012-2019 study period. This data was analyzed on the scale of Alaska Fire Service Predictive Service Areas (PSAs) and three sub-seasons of the Alaska fire season. This strategy helped to identify regional and sub-seasonal variability and made the research operationally relevant. Key results from this research include the following. The majority of lightning occurs in the duff driven sub-season across all PSAs. Lightning, particularly in the Interior PSAs, follows a diurnal pattern with lightning on average beginning earlier in the day in the eastern portion of Alaska and later in the day in the western portion of Alaska. This distinctive pattern is not as well defined in the Coastal PSAs. Results also suggested that dry lightning may be more prevalent in portions of the western Interior than in other regions of Alaska. Lightning events were more common under specific atmospheric flow directions at 500 and 700 hPa, where these directions varied by PSA. Northeasterly and northwesterly flow aloft were most favorable for lightning in the Tanana Valley West PSA, while southerly flow aloft was more favorable for lightning in the North Slope and Upper Yukon PSAs. Finally, easterly flow was a more common pattern during lightning strokes in the Seward Peninsula and Kuskokwim Valley PSAs.
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Gravity and mountain waves and their phenomena in Fairbanks, Alaska: a comparative case studyThe area of Fairbanks, Alaska is in a valley surrounded by mountains from the Alaska Range to the South, the Brooks Range to the north, and the eastern mountain range that extends into Canada. The topographical nature of this subarctic area brings unique atmospheric features to the Fairbanks area, such as temperature inversions and mountain wave perturbation. This thesis will examine two case studies of gravity wave phenomena; one a mountain wave from December 2017, the other a non-mountainous gravity wave from December 2016. Data was collected from radiosondes and Global Data Assimilation Model (GDAS) maps, with the former smoothed for comparative purposes. Profiles of the atmosphere were created to see direct changes that mountain waves create on atmospheric parameters and the subarctic valley area. Methods were explored to separate mountain wave buoyancy effects from other atmospheric buoyancy effects, and then used to compare mountain wave buoyancy effects with frontal motion buoyancy effects. In all cases, the polar jet stream was found to have signicant influence on gravity wave effects in the Fairbanks area. Attention to the polar jet stream location can help predict mountain wave effects and associated atmospheric perturbations. Weather phenomena localized by altitude due to gravity waves were also identified. These include localized short-term surface pressure systems on the day of gravity wave cases. Mountain waves were found to be strongly linked to synoptic temperature events in Fairbanks, Alaska, and the mountain wave case was found to have more pronounced atmospheric effects than the gravity wave case.
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Mesoscale modeling study of a polar low in the Chukchi and Beaufort SeasPolar lows are intense mesoscale maritime cyclones, often associated with strong winds that can damage high-latitude coastal environments and infrastructure. These systems have been historically infrequent in the Chukchi and Beaufort seas, but this behavior is expected to change along with the amplified changes in Arctic climate. This study investigates the unusual occurrence of a polar low in this region on October 9-10, 2009. Sensitivity experiments with the Weather Research and Forecasting model indicate that using ERA-Interim as large-scale forcing and performing spectral nudging at all simulation hours yield the most realistic simulation. The simulations are highly sensitive to physical parameterizations, where Morrison rnicrophysics and Yonsei University boundary layer produce the smallest errors. Surface forcings were not important for the polar low development and their influence could not extend above 850 hPa due to a stable lower atmosphere. A convergence zone between the Aleutian Low and the Beaufort High established a southerly flow that created favorable conditions by continuously adverting heat and moisture from lower latitudes. The polar low had a hybrid development and was likely triggered by the interaction between a deep-penetrating upper-level potential vorticity anomaly and a low-level baroclinic zone, which were driven northward by the jet stream.
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Climate drivers of Interior Alaska wildland fireThis study focused on the climate drivers of wildfire in Interior Alaska that occurred in summer season, JJA, during periods in 1994 to 2017. Analysis results presented in this paper provide identify links between meteorological variables and area burned, in the context of spatial and temporal variability at the PSA level. Warmer temperatures caused higher chance of wildland fires as in summer 2004 (26797 km2) where the temperature reached the highest levels compared to all years of study. In addition, this study has shown that temperatures have the same seasonal cycle in all PSAs level; where the temperature increase begins in June, peaks in July and then gradually decline, consistent with the fire season. Although precipitation limits the increase in forest fires, the accompanying lightning increases the chance fires which gives precipitation a double role in influencing the risk of fire. This can be seen clearly in both Upper Yukon valley (AK02) and Tanana Zone South (AK03S) where the largest number of lightning strikes over Interior Alaska occur (17000 and 11000 strikes, respectively). In addition, these two PSAs have the greatest area burned (1441.2 and 1112.4 km2).There is an upward trend in both temperature and precipitation in all months especially in May and September which indicates a decline in the snow season and an increase in the length of the fire season. A similar pattern was documented between PSAs in eastern versus western Alaska. Eastern PSAs receive the highest amount of precipitation in July, (AK01W , AK01E, AK02, AK03N, AK03S) , and western PSAs in August, (AK04, AK05, AK07). The years 2004, 2015, 2005 and 2009 display the largest values for area burned with extremely warm and dry condition especially in 2004 with approximately 26797 km2 (6.6 m acres).
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Characteristics of Arctic storms and their influence on surface climateImpacts of intense synoptic storms on Chukchi Sea and Beaufort Sea surface environmental conditions are examined, focusing on storms moving into the regions with northward and eastward pathways. Both storms alter the prevailing northeasterly wind to southerly and southwesterly wind. The storms moving from the East Siberian Sea that follow a west to east route are most active in summer and have the longest duration. Increasing southwesterly wind plays a key role in the decline of thin sea ice within the warm season. Storms traveling from the relatively warm Pacific Ocean into the Arctic over the Bering Strait are more common in winter, and are typically more intense than the summer storms that propagate west to east. Downward longwave radiation increases considerably with the passage of intense winter storms over the ice-covered Chukchi Sea; the sea ice concentration decreases accordingly. The impact of different sea ice conditions on Arctic synoptic storm systems in autumn are investigated in the North Pacific and Atlantic sectors, based on the ten ensembles of hindcast simulations from coupled regional climate model HIRHAM-NAOSIM. In both the Pacific and Atlantic sectors, greater transfers of heat and moisture fluxes from the open ocean to the atmosphere occur in low sea ice years than in high sea ice years. The largest increase of upward heat fluxes and baroclinicity occurs over the Laptev, southern Chukchi and Beaufort Seas in the Pacific sector, and over the southern Greenland and Barents Seas in the Atlantic sector. Enhanced baroclinity plays a dominant role in the development of intense storm systems. Therefore, storms in reduced sea ice years are more intense than in enhanced sea ice years in both Atlantic and Pacific sectors. The storm count also increases over locations exhibiting high baroclinicity. Sea ice volume anomalies are significantly correlated with synoptic storm counts based on maximum covariance analysis (MCA) leading modes of covariance between sea ice volume and storm count over Pacific and Atlantic sectors are identified respectively. The results are consistent with our findings in the composite analysis. In the Pacific sector, the first pattern of the MCA demonstrates that increasing storm counts over the Laptev Sea corresponds to decreasing sea ice volume over that region. In the Atlantic sector, the decrease of sea ice volume is highly correlated with decreasing storm counts over the northern Greenland Sea. Connection of storm activity over the North Pacific Ocean with the tropical stratosphere quasi-biennial oscillation (QBO) is investigated following a composite analysis of intense storm vertical cross sections. An observed stronger potential vorticity anomaly of intense storms is associated with the QBO west phase and results in enhanced warm air advection near the surface. A warm core structure forms over the east or northeast direction relative to the surface low center, which bows the isentropes downward. Upward motion following the isentropes reduces the surface low pressure, which in turn, facilitate storms to keep propagating in east and northeast directions. Under the QBO east phase, a weak surface warm core forms to the southeast of the storm center, resulting in a slow development of the storms, and these storms tend to move southeastward.
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Lidar and satellite studies of noctilucent clouds over AlaskaThis thesis presents studies of noctilucent clouds (NLCs) occurring in the summer polar mesosphere over Alaska. Lidar observations of NLCs conducted at Poker Flat Research Range in Chatanika, Alaska (65° N, 147° W) from 1998-2019 are analyzed. The NLCs detected by lidar are characterized in terms of their brightness properties and duration. NLCs were detected on ~51% of the nights when lidar observations have been conducted during NLC season. The brighter NLCs are found to exist at lower altitudes, indicating a growth-sedimentation mechanism. Cloud Imaging and Particle Size (CIPS) data from the Aeronomy of Ice in the Mesosphere (AIM) satellite is used to examine NLC occurrence and brightness over the Alaska region (60-70° N, 130-170° W). In general, high frequency and brightness in the CIPS data corresponds to positive detections of NLCs by the lidar. Microwave Limb Sounder (MLS) temperature and water vapor data from the Aura satellite is used to investigate the meteorological environment of the NLCs observed by lidar at Chatanika. The occurrence of NLCs at Chatanika is found to be driven by the temperature relative to the frost point. Low temperatures relative to the frost point (> 4 K below) correspond to observations when NLCs were present. High temperatures relative to the frost point (> 8 K above) correspond to observations when NLCs were absent. The MLS data is also used to investigate the stability of an ice cloud at different latitudes (64.7°-70.3° N) relative to the equilibrium water vapor mixing ratio. The stability study suggests that the weakest NLCs detected by lidar at Chatanika were in subsaturated conditions, and it is likely that the NLCs formed over several hundred kilometers to the north of Chatanika. The Rayleigh three-channel receiver system was used to conduct NLC measurements during 2019. A technical overview of the three-channel system and the density and temperature retrieval methods is presented at the end of the thesis using observations from the winter of 2018 and the summer of 2019.
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Impacts of storm on sea ice: from case study to climate scale analysisRecent studies have shown that intense and long-lasting storms potentially facilitate sea ice melting. Under the background of extratropical storm tracks poleward shift, significant reductions of Arctic sea ice coverage, and thinning of sea ice thickness over the last several decades, a better understanding on how storms impact sea ice mass balance is obviously of great importance to better predict future sea ice and the Arctic climate changes. This thesis presents a multi-scale study on how storms impact sea ice, consisting of three different parts of the effort. In the first part, we examined the impacts of the 2016 summer intense storm on sea ice changes over the Chukchi Sea using ship-borne observations. The results show that the intense storm can accelerate ice melt through enhanced upper-ocean mixing and upward heat transport. The satellite-observed long-term sea ice variations potentially can be impacted by many factors. In the second part, we first explore key physical processes controlling sea ice changes under no-storm condition. We examined and compared results from 25 sensitivity experiments using the NCAR's Community Earth System Model (CESM). We found that sea ice volume, velocity, and thickness are highly sensitive to perturbed air-ice momentum flux and sea ice strength. Increased sea ice strength or decreased air-ice momentum flux causes counter-clockwise rotation of the transpolar drift, resulting in an increase in sea ice export through Fram Strait and therefore reduction of the pan-Arctic sea ice thickness. Following four tracers released over the Arctic, we found the sea ice thickness distributions following those tracers are broader over the western Arctic and becomes narrower over the eastern Arctic. Additionally, thermodynamic processes are more dominant controlling sea ice thickness variations, especially over periphery seas. Over the eastern Arctic, dynamic processes play a more important role in controlling sea ice thickness variation. Previous studies show that thin ice responds to external perturbations much faster than the thick ice. Therefore, the impacts of storms on sea ice are expected to be different compared with the western/eastern Arctic and the entral/periphery seas. In the third part, we conduct a new composite analysis to investigate the storm impact on sea ice over seven regions for all storms spanning from 1979 to 2018. We focused on sea ice and storm changes over seven regions and found storms tend to have different short-term (two days before and after storm passage), mid-term (one-two weeks after storm passage), and long-term (from 1979 to 2018) impact on sea ice area over those regions. Over periphery seas (Chukchi, East Siberian, Laptev, Kara, and Barents Seas), storms lead to a short-term sea ice area decrease below the climatology, and a mid-term sea ice increase above the climatology. This behavior causes sea ice area to have a small correlation with the storm counts from 1979 to 2018, which suggest that storms have a limited long-term impact on sea ice area over periphery seas. Both the short term and mid-term storm impacts on sea ice area are confined within a 400 km radius circle with maximum impacts shown within a 200 km radius circle. Storms over the western Arctic (Chukchi, East Siberian, and Laptev Seas) have a stronger short-term and mid-term impact on sea ice area compared with the Eastern Arctic (Barents and Kara Seas). Storms over both Atlantic and Pacific entrance regions have a small impact on sea ice area, and storms over the Norwegian, Iceland, and Greenland Seas have the smallest impact on the sea ice area. Compared to the periphery seas, storms tend to have a stronger long-term impact on sea ice area over the central Arctic. The correlation coefficients between the storm count and sea ice area exceed 0.75.
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Response of major modes of eastern Arctic Ocean variability to climate changeThe Arctic Ocean plays a central role in ongoing climate change, with sea ice loss being the most prominent indicator. Recent observations showed that Atlantic inflows play an increasingly important role in the demise of sea ice. This encroaching atlantification of the eastern Arctic Ocean impacts the mean state and the variability of hydrography and current dynamics throughout the basin. Among the most energetic modes of variability are the seasonal cycle and high frequency semidiurnal (∼12-hourly) dynamics in the tidal and inertial frequency band. Limited observations indicated a substantial increase of both, hydrographic seasonal cycles as well as semidiurnal current dynamics in the eastern Arctic over the last decade. Using a uniquely comprehensive data set from an array of six moorings deployed across the eastern Eurasian Basin (EB) continental slope along the 125°E meridian between 2013 and 2015 within the NABOS project, we assess the state of hydrographic seasonal cycles in the eastern EB. Results show a complex pattern of seasonality with a remarkably strong (∆T=1.4°C), deep reaching (∼600 m) temperature signal over the continental slope and large-scale seasonal displacements of isopycnal interfaces. Seasonally changing background conditions are also the main source of variability of semidiurnal frequency band currents: During winter, vigorous baroclinic tidal currents whose amplitudes by far exceed predictions follow the vertical evolution of the pycnocline. During summer, extensive open-water periods additionally lead to strong wind-driven inertial currents in the upper ocean, routinely exceeding 30 cm/s far offshore in the deep basin. In order to obtain an Arctic-wide perspective on the impact of baroclinic tidal currents, a pan-Arctic tidal current atlas has been developed that synthesizes all available observations from the last 20 years. This atlas allows for in-depth studies of regional baroclinic tidal current variability as well as for validation of ocean and climate models, an essential step towards more accurate projections of the future Arctic Ocean state. Our findings from the eastern EB region already indicate a new, more dynamic state of the eastern Arctic Ocean with direct implications for the ecosystem and further sea-ice reduction.
