Browsing by Author "Xu, Chengzhu"

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    Effects of Seasonal Ice Coverage on the Physical Oceanographic Conditions of the Kitikmeot Sea in the Canadian Arctic Archipelago
    (Taylor and Francis, 2021-08-31) Xu, Chengzhu; Mikhael, Wahad; Myers, Paul G.; Else, Brent; Sims, Richard P.; Zhou, Qi
    The Kitikmeot Sea is a semi-enclosed, east–west waterway in the southern Canadian Arctic Archipelago (CAA). In the present work, the ice conditions, stratification, and circulation of the Kitikmeot Sea are diagnosed using numerical simulations with a 1/12° resolution. The physical oceanographic conditions of the Kitikmeot Sea are different from channels in the northern CAA due to the existence of a substantial ice-free period each year. The consequences of such ice conditions are twofold. First, through fluctuations of external forcings, such as solar radiation and wind stress, acting directly or indirectly on the sea surface, the seasonal ice coverage leads to significant seasonal variations in both stratification and circulation. Our simulation results suggest that such variations include freshening and deepening of the surface layer, in which salinity can reach as low as 15 during the peak runoff season, and significantly stronger along-shore currents driven directly by the wind stress during the ice-free season. The second consequence is that the sea ice is not landfast but can move freely during the melting season. By analyzing the relative importance of thermodynamic (freezing and/or melting) and dynamic (ice movement) processes to the ice dynamics, our simulation results suggest that there is a net inflow of sea ice into the Kitikmeot Sea, which melts locally each summer. The movement of sea ice thus provides a significant freshwater pathway, which contributes approximately 14 km3 yr−1 of fresh water to the Kitikmeot Sea, on average, equivalent to a third of freshwater input from runoff from the land.
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    Reduced-order representation of stratified wakes by proper orthogonal decomposition utilizing translational symmetry
    (Springer, 2021-02-11) Halawa, Basem; Xu, Chengzhu; Zhou, Qi
    Visualizations of reduced-order representations of stratified wakes of Reynolds number Re∈{5,25,100}×103 are presented at a fixed internal Froude number. The reduced-order representations are constructed by applying proper orthogonal decomposition (POD) to numerical datasets that are high-resolution, three-dimensional and time-dependent. Due to the transient nature of the flow, the dynamics to be represented are highly non-stationary, posing a challenge for the effectiveness of POD. The translational symmetry inherent in the computational configuration is utilized for the POD analysis. This technique turns out to be effective in terms of improving the convergence of energy content represented by the POD modes and enhancing the interpretability of the temporal dynamics. Individual POD modes representing distinct dynamics of various scales are visualized. In the turbulent region, visualizations of the reconstructed vertical vorticity fields suggest that the dominant length scale of flow structures decreases with the modal index. For internal wave motions, visualizations of the reconstructed vertical velocity fields show the opposite trend, as the wavelength of internal waves observed in the wake’s ambient increases with the modal index. The temporal coefficients for a given mode are observed to vary minimally between Re=2.5×104 and 105, suggesting a potential asymptote of the large-scale temporal dynamics in terms of Reynolds number.