Heat conduction is a heat transfer phenomenon when there is no macroscopic motion in the medium, which can occur in solids, liquids and gases, but strictly speaking, only in solids is pure heat conduction, in fluids (generally refers to liquids and gases) In another case, even if the fluid is in a static state, natural convection will occur due to the density difference caused by the temperature gradient
thermal convection and heat conduction may occur simultaneously in the fluid.
The high temperature water in the upper layer of the ocean and the low temperature water in the lower layer are exchanged due to the temperature difference