Seminars - Oceanography
Thursday, February 13, 2020. Spanagel Hall, Room 316. 12:00
Double-diffusive convection may occur where temperature and salinity both increase with depth, such as in the Arctic Ocean. Double-diffusive convection is identifiable by its distinct staircase structure, consisting of thick mixed layers separated by high-gradient interfaces in temperature and salinity. In the Arctic Ocean, these staircases are widely present in the interior basin and responsible for transporting heat upwards to the overlying sea ice cover. However, they are largely absent around basin boundaries; this is likely due to the effect of intermittent turbulence. Recent echosounder (acoustic) observations of the Arctic Ocean provide a high-resolution method of visualizing an individual staircase evolve in both space and time. By analyzing these acoustic observations, we track the spatial/temporal evolution of individual interfaces in a double-diffusive staircase. A comparison between measurements of temperature/salinity and reflected acoustic signals, as well as a simple mathematical model, suggest that the magnitude of these reflected acoustic signals is proportional to the strength of the stratification for a given interface. Further, the acoustic data are sufficiently high resolution that individual interface thicknesses may be resolved. These results indicate that acoustic measurements may be used to infer mixing levels in double-diffusive staircases and understand staircase persistence and evolution in a setting of weak background turbulence.
Thursday, July 25, 2019. Spanagel Hall, Room 316. 12:00
Four low-power, autonomous Antarctic Precipitation Systems (APSs) were installed on the Ross Ice Shelf, Antarctica for year-round in situ measurement of precipitation. The APS sites were installed in December 2017 as a part of the United States Antarctic Program (USAP). The precipitation is being measured using an Ott Pluvio2 weighing precipitation gauge installed inside a double-alter wind shield. Additional measurements, such as snow height, wind speed, particle counts, and videos, are included in the APS sites to provide supporting observations. The precipitation measurements, and supporting observations, are providing a “ground truth” in understanding precipitation and snow accumulation in Antarctica. The measurements of liquid-water-equivalent (LWE) are compared to the results from numerical weather prediction models and global reanalyses on an event-by-event basis. Additionally, changes in snow height, measured using two different methods, will be compared to occurrences of precipitation in low, medium, and high wind speed cases. The results provide insight on the capability and validity of the precipitation estimates being made with numerical models and the ability to understand snow accumulation at a given location.
Changes in atmospheric circulation over the Amundsen Sea region have been shown to have possible connections to the thinning of the West Antarctic Ice Sheet. An evaluation of 35 years of ERA-Interim reanalysis data using the method of Self-Organizing Maps (SOMs) is presented to identify changes in the atmospheric circulation. A primer on the application of SOMs to atmospheric and climate studies will also be covered.
Thursday, May 23, 2019 Spanagel Hall, Room 316, 12:00
Muddy marine sediments are elastic materials through which worms extend burrows by fracture. Infauna, animals living in marine sediments, modify sediment structure by burrowing, constructing burrows and tubes, and irrigating burrows. These activities can change the bulk porosity and density as well as create heterogeneity in sediment structure. Elastic fracture depends on two sediment material properties: fracture toughness and stiffness. Variability in the ratio of these properties has been shown to affect the behavior of burrowing worms. Very little data exists, however, on how these properties vary in the natural environment. The goal of this research is to determine how infauna modify both acoustic and geotechnical properties, as well as how these properties relate to each other. We have identified several problems with previous methods of measuring sediment fracture toughness and have developed and tested an instrument that addresses these problems. We test the hypothesis that these activities alter sound speed and attenuation in sediments by manipulating homogenized sediments to mimic animal activities. Each of these activities or functions is performed by multiple species of animals that comprise a functional group. Our results will help identify functional groups that have important impacts on sediment acoustics and will be used to interpret field data in which deviations from predicted sound speed and attenuation are correlated with different and diverse communities of infauna.
Monday, May 20, 2019. Spanagel Hall, Room 316. 12:00
Internal waves are oscillatory motions of a density-stratified fluid. They are ubiquitous in Earth's oceans and atmosphere, transporting momentum and energy and playing an important role in ocean dynamics and climate. In varying stratification, internal waves transfer energy to harmonic modes. This nonlinear process may contribute to the transfer of internal wave energy from large to small scales in the ocean. This seminar will introduce basic concepts of internal waves and explore harmonic generation in variable stratification using weakly nonlinear theory, laboratory experiment, and numerical simulation.
Wednesday, March 27, 2019. Spanagel Hall, Room 316 15:00
Linear structures, sometimes hundreds of meters in length, have recently been observed in seafloor imagery and bathymetry collected with both synthetic aperture sonar (SAS) and multibeam echosounder systems (MBES). This phenomenon is not due to the true morphology of the seafloor, but is caused by water column features related to the breaking of internal waves on a seafloor slope. Changes observed in acoustic intensity and bathymetry estimates are caused by the focusing of sound through structures with lower sound speed. In terms of seafloor mapping, these topography-mimicking features will impact the interpretation of imagery, may complicate the production of mosaics, and have the potential to cause bathymetric uncertainties exceeding International Hydrographic Organization standards. In terms of object detection, shadow zones may obscure targets of interest. In this talk it will be shown that these features may not be uncommon using examples of data collected with several different SAS and MBES systems in a variety of locations.
Wednesday, March 20, 2019. Spanagel Hall 231. 12:00
Seeing the Light: A 42-year Journey Wtih Bioluminescence
RDML (ret) Tom Donaldson, (former CO, CNMOC)
Thursday, October 25, 2018. Spanagel Hall 316, 12:00
Wind-blown sand occurs on Mars despite wind speeds rarely exceeding predicted thresholds for motion. This paradox has riddled the planetary community for decades, framing the desert landscape of Mars as relatively inactive when compared to deserts on Earth. To address this conundrum, we made new wind tunnel observations in simulated Martian conditions and employed new methods to reassess the Martian threshold. Here, we find the frequency of sand motion on Mars comparable to transport on Earth.
We show that threshold wind speeds necessary to initiate sand transport on Mars are 100-300% slower than previously thought, and Martian winds exceed the threshold over 6% of Curiosity’s 2-year wind record with 96% of winds capable of sustaining transport once initiated. This Earth-like transport on Mars gives rise to a picture in which frequent aeolian sand transport over millions of years was capable of transforming the martian landscape from one carved by water to one sculpted by wind.
Wednesday, October 3, 2018, Spanagel 316. 12:00
Yield-stress behavior in granular materials (e.g., sand or soils) is relevant to a wide variety of engineering and geophysical processes. One notable example is along riverbeds, where sediment transport occurs only above a minimum fluid flow rate. Sediment transport is a complex process, so simplifying assumptions are often necessary. Previous studies have typically used a detailed treatment of the fluid mechanics with a simpler description of the grains. In this talk, I will describe the results of numerical simulations modeling sediment transport where we take the opposite approach, focusing on the grains instead of the fluid. We find that a grain-focused approach gives significant insight into the shape of the Shields curve, a century-old collection of experimental and field data measuring the onset of sediment transport as a function of grain size. We also find that grain rearrangements become spatially correlated over arbitrarily large distances near the yield stress, implying that a collective description of grains is necessary to understand the onset of sediment transport.
Wednesday, August 29, 2018
The Labrador Sea is one of the few regions which experience deep convection. The water mass transformation which occurs here is of great importance to the global overturning circulation. As the Labrador Sea is relatively inhospitable to directly survey, particularly during the convective winter period, numerical modelling is an excellent tool to explore the processes which occur within. I will present the numerical modelling framework used at the University of Alberta to simulate the Atlantic and Arctic Oceans, as well as a few specific configurations. Afterwards I will discuss my current research which revolves around factors that influence the stratification of the Labrador Sea, specifically freshwater transport and atmospheric variability. I will conclude with some preliminary results from our current sub-mesoscale (1 km) simulation in the Labrador Sea.
We examined the freshwater transport from the boundary into the interior of the Labrador Sea. By calculating the cross-isobath freshwater transport for three water masses, we are able to better understand the regions where freshwater enters the interior of the Labrador Sea. We find that the west coast of Greenland supplies the majority of freshwater to the interior of the Labrador Sea; other regions either act as a sink or supply a very small amount of freshwater. The salty water masses, Labrador Sea Water and Irminger Water, tend to have onshore transport and act to promote a freshening of the Labrador Sea.
We also examine the role of atmospheric variability on the Labrador Sea. From using four different atmospheric forcing datasets to drive our numerical simulation, we calculate how the various air-sea fluxes result with changes in stratification, mixed layer depths, and Labrador Sea Water production. We find that relatively small differences in atmospheric forcing can result with significant changes in heat loss from the Labrador Sea, resulting with dramatic changes in Labrador Sea Water production.
Tuesday, July 17, 2018
Water from Siberian shelf seas penetrates most depth of the connected deep basins and thus contributes to the ventilation, properties and transport of chemical constituents. In this presentation data from 2008 and 2014 are used to assess the biogeochemical processes on the shelf and utilizing the resulting chemical signatures to trace the exchange along the continental margin from the Laptev Sea to the Herald Canyon in the Chukchi Sea.
The result show the importance of microbial degradation of organic matter in determining the partial pressure of carbon dioxide and thus also ocean acidification. The source of organic matter is both terrestrial, added by river runoff as well as by coastal erosion, and marine from primary production by phytoplankton. The dissolved organic carbon is largely degraded in the surface water and thus interacts with the atmosphere, while the particular mostly degrades at the sediment surface. The decay products of the latter are thus added to the bottom water, a water that often has had its salinity increased
by brine addition from sea ice production. In regions of relatively high surface water salinity and large sea ice production the bottom water salinity can reach levels that make it penetrate large depths of the deep basins and thus contribute to ocean ventilation and transport of chemical constituents. In some of the water layers the signature of the shelf processes can be traced all the way to the exit points of the Arctic Ocean, i.e. the Fram Strait and the Canadian Arctic Archipelago.