A meeting place of great ocean currents:: shipboard observations of a convergent front at 2°N in the Pacific

We present a synthesis of physical, chemical and biological shipboard observations of a convergent front at 2 degrees N, 140 degrees W and its surrounding environment. The front was a component of a tropical instability wave generated by shear between westward-flowing equatorial waters to 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 total (dissolved plus suspended particulate) organic carbon concentration, proving that TOC 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. (C) 1998 Elsevier Science Ltd.