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Mesoscale Variations of Biogeochemical Properties in the Sargasso SeaA mesoscale resolution biogeochemical survey was carried out in the vicinity of the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time-series Study (BATS) site during the summer of 1996. Real-time nowcasting and forecasting of the flow field facilitated adaptive sampling of several eddy features in the area. Variations in upper ocean nutrient and pigment distributions were largely controlled by vertical isopycnal displacements associated with the mesoscale field. Shoaling density surfaces tended to introduce cold, nutrient-rich water into the euphotic zone, while deepening isopycnals displaced nutrient-depleted water downward. Chlorophyll concentration was generally enhanced in the former case and reduced in the latter. Eddy-induced upwelling at the base of the euphotic zone was affected by features of two different types captured in this survey-, (1) a typical mid-ocean cyclone in which doming of the main thermocline raised the near-surface stratification upward and (2) a mode water eddy composed of a thick lens of 18C water, which pushed up the seasonal thermocline and depressed the main thermocline. Model hindcasts using all available data provide a four-dimensional context in which to interpret temporal trends at the BATS site and two other locations during the 2 weeks subsequent to the survey. Observed changes in near-surface structure at the BATS site included shoaling isopycnals, increased nutrient availability at the base of the euphotic zone, and enhanced chlorophyll concentration within the cuphotic zone. These trends are explicable in terms of a newly formed cyclone that impinged upon the site during this time period. These observations reveal that eddy upwelling has a demonstrable impact on the way in which the nitrate-density relationship changes with depth from the aphotic zone into the euphotic zone. A similar transition is present in the BATS record, suggesting that eddy-driven upwelling events are present in the time series of upper ocean biogeochemical properties. The variability in main thermocline temperature and nitrate in this synoptic spatial survey spans the range observed in these quantities in the 10-year time series available at BATS to date (1988-1998).
Document ID
20020071030
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
McGillicuddy, D. J.
(Woods Hole Oceanographic Inst. MA United States)
Johnson, R.
(Bermuda Biological Station for Research, Inc. Ferry Reach, Bermuda)
Siegel, D. A.
(California Univ. Santa Barbara, CA United States)
Michaels, A. F.
(University of Southern California Avalon, CA United States)
Bates, N. R.
(Bermuda Biological Station for Research, Inc. Ferry Reach, Bermuda)
Knap, A. H.
(Bermuda Biological Station for Research, Inc. Ferry Reach, Bermuda)
Date Acquired
August 20, 2013
Publication Date
June 15, 1999
Publication Information
Publication: Journal of Geophysical Research
Volume: 104
Issue: C6
Subject Category
Oceanography
Funding Number(s)
CONTRACT_GRANT: NAG5-6455
Distribution Limits
Public
Copyright
Other

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