A Meeting Place of Great Ocean Currents:
Shipboard Observations of a Convergent Front
at 2°N in the Pacific

David Archer,1 Jim Aiken,2 William Balch,3 Dick Barber,4
John Dunne,5 Pierre Flament,6 Wilford Gardner,7 Chris Garside,3
Catherine Goyet,8 Eric Johnson,9 David Kirchman,10 Michael McPhaden,11
Jan Newton,5 Edward T. Peltzer,8 Leigh Welling,12 Jacques White,13
and James Yoder14

 
1:Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA.
2:Plymouth Marine Laboratory, Plymouth P11 3DH, UK.
3:Bigelow Laboratory for Ocean Science, McKown Point, West Boothbay Harbor, ME 04575, USA.
4:Duke University Marine Laboratory, Pivers Island, Beaufort, NC 28516, USA.
5:School of Oceanography, WB-10, University of Washington, Seattle, WA 98195, USA.
6:Department of Oceanography, University of Hawaii, Manoa, Honolulu, HI 96822, USA.
7:Department of Oceanography, Texas A & M University, College Station, TX 77843, USA.
8:Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
9:Joint Institute for the Study of the Atmosphere and Oceans, University of Washington, Seattle, WA 98195, USA.
10:College of Marine Sciences, University of Delaware, Lewes, DE 19958, USA.
11:Pacific Marine environmental Laboratory, NOAA, 7600 Sand Point Way NE, Seattle, WA 98115, USA.
12:College of Oceanography, Oregon State University, Corvallis, OR 97331, USA.
13:People for Puget Sound, 1326 Fifth Avenue, Seattle, WA 98101, USA.
14:Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA.

Deep-Sea Research II (1997) 44: 1827-1849.

Received: 13 May 1997.
Accepted: 9 July 1997.
Published: September-October 1997.


ABSTRACT

We present a synthesis of physical, chemical, and biological shipboard observations of a convergent front at 2°N 140°W and its surrounding environment. The front was a component of a tropical instability wave generated by the convergence of cold equatorial waters from the south and warmer equatorial counter current water to the north. Surface waters on the cold side were undersaturated with oxygen, which suggests that the water had only been exposed at the sea surface for a period of a few weeks. Although the atmospheric exposure time was short, the effects of biological activity could be detected in enhanced concentrations of dissolved organic carbon concentration, proving that DOC can be produced quickly in response to changing environmental conditions. The front itself was dominated by the accumulation of a "patch" of buoyant diatoms Rhizosolenia castracanei concentrated in the top centimeters of the warm surface water north of the front, and elevated chlorophyll concentrations were observed from the air over a spatial scale of order 10-20 km northward from the front.

The nitrogen budget and thorium data suggest that a significant fraction of the elevated POC, and virtually all of the PON, arrived in the patch waters as imported particles rather than in situ photosynthesis. Photosynthetic uptake of carbon appears to have occurred in patch waters, but without corresponding uptake of fixed nitrogen (an uncoupling of the usual Redfield stoichiometry). Solute chemistry of the patch appears to be controlled by turbulent mixing, which flushes out patch waters on a time scale of days (faster than atmospheric ventilation). The subduction of nutrient-rich equatorial surface water below the front was detected 100 km north of the front in the signatures of temperature, salinity and ammonium.


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