DYNAMICS OF THE TRANSITION ZONE IN COASTAL ZONE COLOR SCANNER SENSED OCEAN COLOR IN THE NORTH PACIFIC DURING OCEANOGRAPHIC SPRING

A transition zone in phytoplankton concentration running across the No rth Pacific basin at 30-degrees to 40-degrees north latitude correspon ds to a basin-wide front in surface chlorophyll observed in a composit e of coastal zone color scanner (CZCS) images for May, June, and July 1979-1986. This transition zone with low chlorophyll to the south and higher chlorophyll to the north can be simulated by a simple model of the concentration of phytoplankton, zooplankton, and dissolved nutrien t (nitrate) in the surface mixed layer of the ocean applied to the Nor th Pacific basin for the climatological conditions during oceanographi c springtime (May, June, and July). The model is initialized with a 1- degrees x 1-degrees gridded estimate of wintertime (February, March, a nd April) mixed layer nitrate concentrations calculated from an extens ive nutrient database and a similarly gridded mixed layer,depth data s et. Comparison of model predictions with CZCS data provides a means of evaluating the dynamics of the transition zone. We conclude that in t he North Pacific, away from major boundary currents and coastal upwell ing zones, wintertime vertical mixing determines the total nutrient av ailable to the plankton ecosystem in the spring. The transition zone s een in basin-scale CZCS images is a reflection of the geographic varia tion in the wintertime mixed layer depth and the nitracline, leading t o a latitudinal gradient in phytoplankton chlorophyll.