From Glossary of Meteorology
The spatial gradient of potential temperature on an isopycnal surface.
If thermoclinicity is zero, all fluid parcels in an isopycnal surface have the same potential temperature and are interchangeable. If thermoclinicity is nonzero, fluid parcels in the same isopycnal surface can have different potential temperature T and salinity S. Exchange of such parcels will lead to large T/S differences within the surface.
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