understood. This will permit estimates of current shear, convergence, and 

 divergence as well as nutrient and plankton transport for various oceanic 

 regions. These processes all affect the productivity of the oceans and are 

 impractical to measure globally by conventional means. 



The third ancillary satellite sensor which can significantly improve our 

 understanding of the processes affecting the productivity of the oceans is 

 the scatterometer. It measures the off nadir backscattered radar return, 

 largely from capillary waves which respond in heiqht directly with the 

 surface wind stress or wind speed. Wind-driven divergence zones can 

 produce upwelling of cool, nutrient-rich water to initiate and sustain 

 phytoplankton blooms, and wind-driven convergence zones can concentrate 

 bouyant food resources (e.g. phytoplankton, detritus, seaweed), attracting 

 fish. Since the surface currents are primarily wind-driven, a better 

 understanding of the wind field will also improve models of the current 

 field and of the density structure in the euphotic zone (depth of the mixed 

 layer, thermocline stability, etc.), affecting the vertical distribution 

 of shelf phytoplankton and productivity in response to wind events (Walsh, 

 1976). 



The prospects of a scatterometer flying in the last part of this decade are 

 good. One is scheduled to be aboard ERS-1 (a European Space Agency satel- 

 lite), and NASA is studying the feasibility of flying a scatterometer 

 "piggyback" on an available polar orbiter. A dedicated altimetric 

 satellite (TOPEX) is a prospective mission of the Environmental Observa- 

 tion Division of NASA, with a possible launch date late in the decade. The 

 footprint (pixel size) of passive microwave radiometers, which provide sea 

 surface temperature measurements through the clouds is generally too large 

 (e.g., 100 km) to be of much utility to ocean productivity studies. These 

 sensors also suffer from inaccuracies within 600 km of land (sidelobe 

 returns), where the most dynamic productivity processes (e.g., coastal 

 upwelling) occur. 



A-5 



