Thermodynamic concepts used in physical oceanography

Abstract. The thermodynamic concepts that are used in physical oceanography are reviewed, including how the First Law of Thermodynamics is derived, and introducing the several different types of salinity. Different temperature-like variables are discussed, leading to potential enthalpy and Conservative Temperature because of the need to accurately quantify the ocean's role in transporting heat. A key aspect of a thermodynamic variable is the extent of its non-conservation when mixing occurs at a given pressure. Methods are presented that quantify the amount of this non-conservation of several thermodynamic variables, and these are illustrated in the context of the global ocean. There has been confusion in the literature about the meaning of the salinity and temperature variables carried by ocean models, and here we explain why even in older ocean models that use the EOS-80 equation of state (rather than TEOS-10), the model's salinity is Preformed Salinity and the model's temperature variable is Conservative Temperature. The thermodynamic reasoning that leads to the concept of neutral surfaces is reviewed, along with thermobaricity, cabbeling, the dianeutral motion caused by the ill-defined nature of neutral surfaces, and Neutral Surface Planetary Potential Vorticity.