Armitage Thomas

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Armitage
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Thomas
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  • Article
    Retrieving sea ice drag coefficients and turning angles from in situ and satellite observations using an inverse modeling framework
    (American Geophysical Union, 2019-08-14) Heorton, Harold ; Tsamados, Michel ; Cole, Sylvia T. ; Ferreira, Ana M. G. ; Berbellini, Andrea ; Fox, Matthew ; Armitage, Thomas
    For ice concentrations less than 85%, internal ice stresses in the sea ice pack are small and sea ice is said to be in free drift. The sea ice drift is then the result of a balance between Coriolis acceleration and stresses from the ocean and atmosphere. We investigate sea ice drift using data from individual drifting buoys as well as Arctic‐wide gridded fields of wind, sea ice, and ocean velocity. We perform probabilistic inverse modeling of the momentum balance of free‐drifting sea ice, implemented to retrieve the Nansen number, scaled Rossby number, and stress turning angles. Since this problem involves a nonlinear, underconstrained system, we used a Monte Carlo guided search scheme—the Neighborhood Algorithm—to seek optimal parameter values for multiple observation points. We retrieve optimal drag coefficients of CA=1.2×10−3 and CO=2.4×10−3 from 10‐day averaged Arctic‐wide data from July 2014 that agree with the AIDJEX standard, with clear temporal and spatial variations. Inverting daily averaged buoy data give parameters that, while more accurately resolved, suggest that the forward model oversimplifies the physical system at these spatial and temporal scales. Our results show the importance of the correct representation of geostrophic currents. Both atmospheric and oceanic drag coefficients are found to decrease with shorter temporal averaging period, informing the selection of drag coefficient for short timescale climate models.
  • Article
    Analysis of the Beaufort Gyre freshwater content in 2003-2018
    (American Geophysical Union, 2019-12-11) Proshutinsky, Andrey ; Krishfield, Richard A. ; Toole, John M. ; Timmermans, Mary-Louise ; Williams, William J. ; Zimmermann, Sarah ; Yamamoto-Kawai, Michiyo ; Armitage, Thomas ; Dukhovskoy, Dmitry S. ; Golubeva, Elena ; Manucharyan, Georgy E. ; Platov, Gennady A. ; Watanabe, Eiji ; Kikuchi, Takashi ; Nishino, Shigeto ; Itoh, Motoyo ; Kang, Sung-Ho ; Cho, Kyoung-Ho ; Tateyama, Kazutaka ; Zhao, Jing
    Hydrographic data collected from research cruises, bottom‐anchored moorings, drifting Ice‐Tethered Profilers, and satellite altimetry in the Beaufort Gyre region of the Arctic Ocean document an increase of more than 6,400 km3 of liquid freshwater content from 2003 to 2018: a 40% growth relative to the climatology of the 1970s. This fresh water accumulation is shown to result from persistent anticyclonic atmospheric wind forcing (1997–2018) accompanied by sea ice melt, a wind‐forced redirection of Mackenzie River discharge from predominantly eastward to westward flow, and a contribution of low salinity waters of Pacific Ocean origin via Bering Strait. Despite significant uncertainties in the different observations, this study has demonstrated the synergistic value of having multiple diverse datasets to obtain a more comprehensive understanding of Beaufort Gyre freshwater content variability. For example, Beaufort Gyre Observational System (BGOS) surveys clearly show the interannual increase in freshwater content, but without satellite or Ice‐Tethered Profiler measurements, it is not possible to resolve the seasonal cycle of freshwater content, which in fact is larger than the year‐to‐year variability, or the more subtle interannual variations.