A baroclinic fluid is defined as that state of a fluid for which the isopycnal surfaces are inclined with respect to the isobaric surfaces, which implies that isnot a function of the pressure alone. This condition represents the most general situation of which the two preceding states are special cases.
In general, the density of a fluid mixture is a function of temperature and concentration of solute as well as pressure. In the ocean the isothermal surfaces and surfaces of constant salinty (isohaline surfaces) are inclined with respect to the level surfaces, and isobaric surfaces. This implies that the isopycnal surfaces are also inclined with respect to the isobaric surfaces.
The baroclinic situation is illustrated schematically in Fig. 3.08-1(c). This may be compared with the special cases illustrated in diagrams (a) and (b) of the same figure. The spacing of standard isobaric surfaces in the baroclinic situation differs somewhat from that of the preceding special case.
The conditions in the ocean are such that the temperature and salinity approach uniformity at great depth. Consequently, the density is essentially determined by pressure alone in the deep layers and therefore quasi-barotropic conditions prevail in this region of the ocean.
In the region of the thermocline where the thermal gradients of the sea are most pronounced the condition of baroclinicity is the most extreme.
A very important difference between the baroclinic situation and the barotropic or homogeneous situation is that in the baroclinic case the isobaric surfaces are not parallel at all depths. A change in inclination of the isobaric surfaces must always exist if the isopycnal surfaces are inclined with respect to the isobaric surfaces. In the ocean it is common to find the baroclinic situation in the surface layers with the barotropic conditions being approached at great depth.