392 



58 OCEANOGRAPHY IN THE NEXT DECADE 



interaction with the fields of biology and chemistry is particu- 

 larly rewarding. The rates of heat, water, gas, and momentum 

 exchange across the ocean-atmosphere interface must be estimated 

 better from easily measured or calculated environmental param- 

 eters (e.g., wind, temperature differences across the air-sea inter- 

 face, v/aves, and stability of the mixed layer). 



Because photosynthesis involves the conversion of carbon di- 

 oxide and nutrients into living material and oxygen, the euphotic 

 zone is an immediate sink for atmospheric carbon dioxide. The 

 flow and mixing of water masses and the transport of nutrients 

 and particles that control phytoplankton populations depend on 

 physical oceanographic processes. Understanding the mixed layer 

 is a complex problem involving studies in marine biology, chem- 

 istry, and physics. In a crude sense, the mixed layer can be re- 

 garded as controlled by a set of chemical reactions in which bio- 

 logical processes determine most of the reaction rates (nutrient 

 fixation and regeneration) and physical processes (advection, mix- 

 ing, and particle sinking) determine the rates at which reactants 

 and products are provided to or removed from the system. 



The physical oceanographer's approach to studying the surface 

 mixed layer involves measurement of currents and horizontal varia- 

 tions to determine advection, microstructure measurements to study 

 turbulent fluxes, and measurements of deeper properties to infer 

 vertical flows. Chemical oceanographers study the latter process 

 using time-dependent tracers to estimate the vertical path of wa- 

 ter masses and observe changes along it. Both methods can be 

 strengthened by a model that integrates the measurements. 



Thermohaline Circulation In a few limited regions of the ocean, 

 a combination of low temperature and high salinity produces dense 

 surface water that flows into the deep ocean and spreads laterally 

 to initiate global-scale thermohaline circulation. Deep-reaching 

 convection occurs in the northern North Atlantic Ocean and around 

 Antarctica. These water masses spread throughout the ocean and 

 force deep ocean water, which has been made more buoyant by 

 the downward diffusion of heat, to upwell slowly. Eventually, the 

 upweiled water migrates back to the sinking regions to complete 

 a thermohaline circulation ceil (see Gordon et al., 1992). Water 

 masses formed in different regions vary in terms of temperature, 

 salinity, nutrient concentrations, and stored carbon content. The 

 relative contribution from each source region determines the ocean's 

 average temperature, salinity, and other properties, such as carbon 

 storage. In addition, this downward flow of surface water pro- 



