A MEETING PLACE OF GREAT OCEAN CURRENTS - SHIPBOARD OBSERVATIONS OF ACONVERGENT FRONT AT 2-DEGREES-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 i ts surrounding environment. The front was a component of a tropical in stability 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 oxyge n, which suggests that the water had only been exposed at the sea surf ace for a period of a few weeks. Although the atmospheric exposure tim e was short, the effects of biological activity could be detected in e nhanced concentrations of total (dissolved plus suspended particulate) organic carbon concentration, proving that TOC can be produced quickl y in response to changing environmental conditions. The front itself w as dominated by the accumulation of a ''patch'' of buoyant diatoms Rhi zosolenia castracanei concentrated in the top centimeters of the warm surface water north of the front, and elevated chlorophyll concentrati ons 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 tha n in situ photosynthesis. Photosynthetic uptake of carbon appears to h ave occurred in patch waters, but without corresponding uptake of fixe d 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 at mospheric ventilation). The subduction of nutrient-rich equatorial sur face water below the front was detected 100 km north of the front in t he signatures of temperature, salinity and ammonium. (C) 1998 Elsevier Science Ltd.