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FUTURE DIRECTIONS IN OCEAN SCIENCES 59 



vides a link between the atmosphere and the deep ocean. Thus a 

 better understanding of the global climate system requires a de- 

 tailed understanding of the thermohaline circulation, its vulner- 

 ability to change, and the processes that govern water mass for- 

 mation rates. Once these factors are understood, they can be 

 represented in global ocean and climate models. 



There is evidence that surface salinity fluctuations in the high- 

 latitude North Atlantic Ocean control the thermohaline circula- 

 tion by altering North Atlantic Deep Water (NADW) formation. 

 One possible mechanism to slow NADW formation is capping of 

 the ocean surface with low-salinity water, such as Arctic Ocean 

 waters (Weyl, 1968). During the last century, there were at least 

 two episodes of low surface salinity water in the northern North 

 Atlantic Ocean [the latter during the late 1960s and 1970s is re- 

 ferred to as the great salinity anomaly (Dickson €t al., 1988)] that 

 drastically reduced or stopped convection and NADW production. 

 Changes in the twentieth century are very small compared to 

 suspected changes in NADW formation rates during the swings 

 between glacial and interglacial periods (Boyle, 1990). 



The Indian Ocean is a strongly evaporative ocean. Lacking a 

 northern polar region, the tropical heating of the Indian Ocean 

 cannot be vented by flow to the north. Strong evaporation ui the 

 Red Sea and Persian Gulf forms warm salty water that, like the 

 Mediterranean outflow, is small in volume but adds significant 

 heat and salt to the deep ocean. The role of the Indian Ocean in 

 larger-scale thermohaline circulation remains unclear and should 

 be studied in the coming years. 



The sea ice cover of the Southern Ocean acts to decouple the 

 ocean from the atmosphere, limiting cooling of the ocean by the 

 polar atmosphere. The insulating blanket of sea ice protects the 

 ocean from the cold atmosphere. The extreme seasonality and 

 rapid spring melting of the Southern Ocean sea ice cover suggest 

 that the heat earned into the surface layer by the upwelling of 

 deep water is a key in understanding the Southern Ocean sea ice 

 budget; the buildup of heat within the mixed layer under the 

 winter ice cover induces melting even before solar radiation melts 

 the ice from above. Ocean heat flux also limits sea ice thickness 

 during the winter to less than 1 meter, in contrast to the 3-meter 

 ice of the more stable Arctic Ocean. 



Heat and Freshwater Fluxes The ocean interacts with the atmo- 

 sphere in affecting the heat and freshwater fluxes that control the 

 climate system. Estimates of the fluxes to and from the ocean 



