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Friday, February 8, 2019.  Spanagel Hall, Room 316.  12:00  Postponed

Internal Waves in Variable Stratification

Dr. Scott Wunsch, Johns Hopkins University

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.

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Thursday, October 25, 2018.   Spanagel Hall 316,  12:00

New Thresholds Predict Earth-like Transport on Mars

Dr. C. Swann, NRL Stennis

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.

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Wednesday, October 3, 2018, Spanagel 316. 12:00

Yielding in granular materials, from riverbeds to renormalization group

Prof. Abe Clark , Department of Physics, NPS

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.

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Wednesday, August 29, 2018

Numerical Modelling in the Labrador Sea: Factors Which Influence Stratification and Deep Convection

Clark Pennelly and Paul G. Myers, Dept. of Earth and Atmospheric Sciences, University of Alberta, Canada

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.

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Tuesday, July 17, 2018

Shelf – basin interaction along the Siberian continental margin – information from chemical tracers

Leif G. Anderson, University of Gothenburgh, Sweden

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.

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