PARTICLE DYNAMICS AS CONTROLLED BY THE FLOW-FIELD OF THE EASTERN EQUATORIAL PACIFIC

Profiles made during the JGOFS EqPac October 1992 time series cruise t o the equator with a Large Aggregate Profiling System (LAPS) recorded the concentration and size distribution of particles in the marine sno w size range (> 0.5 mm diameter). Profiles were made routinely at loca l midnight during the twenty day occupation of the time series station on the equator. The LAPS data set, when combined with the CTD/transmi ssometer data set from the EqPac program's intensive profiling operati on, reveals a complex dynamic of particle production and aggregation d riven by the equatorial flow field. During the cruise, the influence o f 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 s hoaling of the thermocline and a minimum in the aggregate volume conce ntration. Subsequently, the particle load in the surface water increas ed followed by an increase in the aggregate abundance. Quasi-oligotrop hic conditions on the equator (subsurface particle and aggregate maxim a 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 temper ature and salinity fields are ascribed to the passage of a TIW; in seq uence: the trailing edge, the northwestward-flowing cool cusp water an d the convergent front. Current meters at the equator recorded a rotat ional flow at 80 m, with a shift from southeastward to northeastward f low 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 si gnature of this return flow is reflected in the decrease in the vertic al gradients of nutrients and oxygen at the equator with respect to th e 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 a nd Leinen, 1996). The return flow of particles to the equator is refle cted in the maximum in accumulation rate near the equator. The longter m influence of TIW's is reflected in the asymmetry of the accumulation rates across the equator as described by Murray and Leinen (1996), wi th the maximum accumulation rate found south of the equator, a local m inimum in accumulation rate at approximately 2 degrees N, and a local maximum near 4 degrees N. (C) 1998 Elsevier Science Ltd.