Mitchell B. Gregory

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B. Gregory

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  • Article
    Biological structure and seasonality in the Japan/East Sea
    (Oceanography Society, 2006-09) Ashjian, Carin J. ; Arnone, Robert ; Davis, Cabell S. ; Jones, Burton ; Kahru, Mati ; Lee, Craig M. ; Mitchell, B. Gregory
    The Japan/East Sea (JES) contains several oceanic regions separated by dynamic boundaries. These distinct regions, and the physical features that establish and maintain the boundaries between the regions, have significant impacts on its ocean biology. Until recently, most studies of the biology of the JES have focused on nearshore regions, with few detailed studies of the interior of the JES or the dynamic features that define the different regions. In addition, the classic sampling methods used in previous work have not allowed high-resolution studies of biological-physical interactions associated with key dynamic mesoscale frontal zones, quasi-synoptic surveys of water column and biological structure in three dimensions, or broad-scale description of the seasonal cycles in the different biogeographic regions of the JES.
  • Preprint
    Winter mesoscale circulation on the shelf slope region of the southern Drake Passage
    ( 2013-02-20) Zhou, Meng ; Zhu, Yiwu ; Measures, Christopher I. ; Hatta, Mariko ; Charette, Matthew A. ; Gille, Sarah T. ; Frants, Marina ; Jiang, Mingshun ; Mitchell, B. Gregory
    An austral winter cruise in July-August 2006 was conducted to study the winter circulation and iron delivery processes in the Southern Drake Passage and Bransfield Strait. Results from current and hydrographic measurements revealed a circulation pattern similar to that of the austral summer season observed in previous studies: The Shackleton Transverse Ridge (STR) in the southern Drake Passage blocks a part of the eastward Antarctic Circumpolar Current (ACC) which forces the ACC to detour southward, produces a Taylor Column over the STR, and forms an ACC jet within the Shackleton Gap, a deep channel between the STR and the shelf of Elephant Island. Observations show that to the west of the STR, the Upper Circumpolar Deep Water (UCDW) intruded onto the shelf around the South Shetland Islands while to the east of the STR, shelf waters were transported off the northern shelf of Elephant Island. Along a similar west-east transect approximately 50 km off the shelf, the northward transport of shelf waters was approximately 2.4 and 1.2 Sv in the austral winter and summer, respectively. The waters around Elephant Island primarily consist of the UCDW that has been modified by local cooling and freshening, unmodified UCDW that has recently intruded onto the shelf, and Bransfield Current water that is a mixture of shelf and Bransfield Strait waters. Weddell Sea outflows were observed which affect the hydrography and circulation in the Bransfield Strait and indirectly affect the circulation patterns in the southern Drake Passage and around Elephant Island. Two Fe enrichment and transport mechanisms are proposed that intrusions of the UCDW onto the northern shelf region of the South Shetland Islands is considered as the results of Ekman pumping due to prevailing westerly wind in the region while the offshelf transport of shelf waters in the shelf region east of Elephant Island is due to acquisition of positive vorticity by shelf waters from horizontal mixing with onshelf intruded ACC waters.
  • Article
    Satellite detection of dinoflagellate blooms off California by UV reflectance ratios
    (University of California Press, 2021-06-09) Kahru, Mati ; Anderson, Clarissa ; Barton, Andrew D. ; Carter, Melissa L. ; Catlett, Dylan ; Send, Uwe ; Sosik, Heidi M. ; Weiss, Elliot L. ; Mitchell, B. Gregory
    As harmful algae blooms are increasing in frequency and magnitude, one goal of a new generation of higher spectral resolution satellite missions is to improve the potential of satellite optical data to monitor these events. A satellite-based algorithm proposed over two decades ago was used for the first time to monitor the extent and temporal evolution of a massive bloom of the dinoflagellate Lingulodinium polyedra off Southern California during April and May 2020. The algorithm uses ultraviolet (UV) data that have only recently become available from the single ocean color sensor on the Japanese GCOM-C satellite. Dinoflagellates contain high concentrations of mycosporine-like amino acids and release colored dissolved organic matter, both of which absorb strongly in the UV part of the spectrum. Ratios <1 of remote sensing reflectance of the UV band at 380 nm to that of the blue band at 443 nm were used as an indicator of the dinoflagellate bloom. The satellite data indicated that an observed, long, and narrow nearshore band of elevated chlorophyll-a (Chl-a) concentrations, extending from northern Baja to Santa Monica Bay, was dominated by L. polyedra. In other high Chl-a regions, the ratios were >1, consistent with historical observations showing a sharp transition from dinoflagellate- to diatom-dominated waters in these areas. UV bands are thus potentially useful in the remote sensing of phytoplankton blooms but are currently available only from a single ocean color sensor. As several new satellites such as the NASA Plankton, Aerosol, Cloud, and marine Ecosystem mission will include UV bands, new algorithms using these bands are needed to enable better monitoring of blooms, especially potentially harmful algal blooms, across large spatiotemporal scales.