Abstract: In this paper, adiabatic density surface, neutral density surface and potential density surface are compared. The adiabatic density surface is defined as the surface on which a water parcel can move adiabatically, without changing its potential temperature and salinity. For a water parcel taken at a given station and pressure level, the corresponding adiabatic density surface can be determined through simple calculations. This family of surface is neutrally buoyant in the world ocean, and different from other surfaces that are not truly neutrally buoyant. In order to explore mixing path in the ocean, a mixing ratio m is introduced, which is defined as the portion of potential temperature and salinity of a water parcel that has exchanged with the environment during a segment of migration in the ocean. Two extreme situations of mixing path in the ocean are m=0 (no mixing), which is represented by the adiabatic density curve, and m=1, where the original information is completely lost through mixing. The latter is represented by the neutral density curve. The reality lies in between, namely, 0<m<1. In the turbulent ocean, there are potentially infinite mixing paths, some of which may be identified by using different tracers (or their combinations) and different mixing criteria. Searching for mixing paths in the real ocean presents a great challenge for further research.

Key words: adiabatic density surface, neutral density surface, potential density surface, mixing path