Particle dynamics as controlled by the flow field of the eastern equatorial Pacific
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abstract
Profiles made during the JGOFS EqPac October 1992 time series cruise to the equator with a Large Aggregate Profiling System (LAPS) recorded the concentration and size distribution of particles in the marine snow size range (> 0.5 mm diameter). Profiles were made routinely at local midnight during the twenty day occupation of the time series station on the equator. The LAPS data set, when combined with the CTD/transmissometer data set from the EqPac program's intensive profiling operation, reveals a complex dynamic of particle production and aggregation driven by the equatorial flow field. During the cruise, the influence of the passage of a Tropical Instability Wave (TIW) was observed in the transmissometer/aggregate and temperature/salinity data sets. A peak in particle production rate occurred in conjunction with the maximum shoaling of the thermocline and a minimum in the aggregate volume concentration. Subsequently, the particle load in the surface water increased followed by an increase in the aggregate abundance. Quasi-oligotrophic conditions on the equator (subsurface particle and aggregate maxima and decreased particle production rates) toward the end of the time series corresponded to the appearance of stratified low salinity water characteristic of more northerly conditions. The variations in temperature and salinity fields are ascribed to the passage of a TIW; in sequence: the trailing edge, the northwestward-flowing cool cusp water and the convergent front. Current meters at the equator recorded a rotational flow at 80 m, with a shift from southeastward to northeastward flow during this sequence. A simple conceptual model of meridional flow field is presented in which poleward transport of the upwelled water from the equator returns at shallow depths to mix with the EUC. The signature of this return flow is reflected in the decrease in the vertical gradients of nutrients and oxygen at the equator with respect to the poleward gradients, and a subsurface particle maximum below the EUC. The flow field's effect on the distribution of particles is reflected in the meridional pattern of the sediment accumulation rate (Murray and Leinen, 1996). The return flow of particles to the equator is reflected in the maximum in accumulation rate near the equator. The longterm influence of TIW's is reflected in the asymmetry of the accumulation rates across the equator as described by Murray and Leinen (1996), with the maximum accumulation rate found south of the equator, a local minimum in accumulation rate at approximately 2N, and a local maximum near 4N.
