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Mesoscale variations of biogeochemical properties in the Sargasso Sea

Mesoscale variations of biogeochemical properties in the Sargasso Sea
Mesoscale variations of biogeochemical properties in the Sargasso Sea
A 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 18°C 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 euphotic 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).
0148-0227
13381-13394
McGillicuddy, D.J.
d2ccd105-b0e7-47b1-8ae8-0a89feeb9df2
Johnson, R.
b34c3641-2bda-412f-a9ce-59dabbad7b10
Siegel, D.A.
739675b1-9eee-41bb-b9fe-ad98dfa78005
Michaels, A.F.
4e298b07-6355-4565-9e28-eb8436ab3d85
Bates, N.R.
954a83d6-8424-49e9-8acd-e606221c9c57
Knap, A.H.
efc1dabf-e1e7-4e17-84a4-af0a25962747
McGillicuddy, D.J.
d2ccd105-b0e7-47b1-8ae8-0a89feeb9df2
Johnson, R.
b34c3641-2bda-412f-a9ce-59dabbad7b10
Siegel, D.A.
739675b1-9eee-41bb-b9fe-ad98dfa78005
Michaels, A.F.
4e298b07-6355-4565-9e28-eb8436ab3d85
Bates, N.R.
954a83d6-8424-49e9-8acd-e606221c9c57
Knap, A.H.
efc1dabf-e1e7-4e17-84a4-af0a25962747

McGillicuddy, D.J., Johnson, R., Siegel, D.A., Michaels, A.F., Bates, N.R. and Knap, A.H. (1999) Mesoscale variations of biogeochemical properties in the Sargasso Sea. Journal of Geophysical Research, 104 (C6), 13381-13394. (doi:10.1029/1999JC900021).

Record type: Article

Abstract

A 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 18°C 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 euphotic 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).

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More information

Published date: 15 June 1999
Organisations: Ocean Biochemistry & Ecosystems

Identifiers

Local EPrints ID: 358351
URI: http://eprints.soton.ac.uk/id/eprint/358351
ISSN: 0148-0227
PURE UUID: 7bd2bc44-b4e2-4f21-bdc4-9e21a5f657d3

Catalogue record

Date deposited: 03 Oct 2013 16:08
Last modified: 16 Jul 2019 21:21

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