Some historical, theoretical and applied aspects of quantitative water mass analysis

The concept of water masses is reviewed from the point of view of quantitat ive water mass analysis. A theoretical framework is presented which describ es the life history of water masses in terms of formation, consolidation, a ging and decay. Water masses are described as physical entities and compare d with their atmospheric counterparts (air masses). The classical temperature-salinity diagram is expanded into the mathematica l concept of water types in an n-dimensional parameter space. Water types a nd their standard deviations are introduced as the foundation for quantitat ive water mass analysis. The relationship between parameter space and physi cal space is established through the definition of water type density. Mode Waters are discussed as regions in physical space with a minimum in water type density. Some unresolved issues of the structure of the oceanic thermo cline are discussed in this context. The definition of water masses is extended to include water masses in the s urface mixed layer where air-sea exchange processes continuously modify wat er mass properties. The paper concludes with a brief discussion of the repr esentation of water masses and their evolution in numerical models.