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110 OCEANOGRAPHY IN THE NEXT DECADE 



Processes 



The lateral boundaries and shelf-slope topography that charac- 

 terize continental margins substantially determine the nature of 

 coastal currents. For example, on a rotating planet, nearly steady 

 currents are constrained from crossing isobaths (lines of constant 

 depth). As a result, flow in the coastal ocean tends to parallel the 

 coast, and exchange between waters over the continental shelf 

 and the adjacent deep ocean is inhibited. Thus in many cases, 

 distinct shelf water masses form, and the shelf represents a par- 

 tially closed chemical and biological system. Fronts often mark 

 the boundaries between these coastal and oceanic systems, and 

 these fronts have their own important biological and atmospheric 

 effects. 



Wind-driven currents over continental shelves tend to be par- 

 ticularly energetic because the coastline interrupts water trans- 

 port in the turbulent layer in the upper ocean. This interruption 

 leads to a connection between surface winds and currents deeper in 

 the water column. The resulting currents flowing alongshore below 

 the turbulent surface layer dominate variability in most places over 

 the continental shelves. Wind-driven currents are understood well 

 enough that models are able to predict the speed and direction of 

 coastal currents, as shown by the close agreement between ob- 

 served and predicted currents shown in Figure 3-4. 



Of broader importance to coastal ecosystems is the related 

 onshore-offshore circulation, including the coastal upwelling of 

 cold, nutrient-rich subsurface waters. Their temperature leads to 

 the unusually cool, stable atmospheric conditions that character- 

 ize the U.S. West Coast during spring and summer. The upwelled 

 nutrients fuel marine plant growth, leading to high biomass throughout 

 the food web and some of the world's greatest fisheries, including 

 those off the West Coast and off the coast of Peru. Upwelling can 

 also intensify the transfer of organic materials from the surface to 

 the seafloor in such areas. For example, off Peru, as much as one- 

 half of the carbon fixed by phytoplankton production induced by 

 upwelling may be deposited on the bottom. Upwelling in the 

 coastal ocean can also be caused by factors other than wind. For 

 example, upwelling of nutrient-rich water along the inshore edge 

 of the Gulf Stream does much to stimulate productivity off the 

 southeastern coast of the United States, as determined by chloro- 

 phyll measurements (Figure 3-5). Whatever its cause, upwelling 

 contributes to the well-known high biological productivity of the 

 coastal ocean (Figure 3-6). Estuaries and coastal embayments, on 



