The changes taking place in the subsurface layers beneath the surface water of the oceans are affected by the following factors- (a) The difference in the amount of heat absorbed directly at different depths, (b) the effect of heat conduction, (c) the horizontal displacement of water masses and the consequent changes in the ocean currents (d) the effect of vertical motions produced in the ocean water.
Observations made in the Monterey Bay of California made it clear that the annual variations of temperature taking place in the sub-layers of the oceans are caused by the changes in the ocean currents, the seasonal changes of the atmospheric temperature, the upwelling of water from the deeper layers of the oceans, and the transference of heat by conduction.
However, when observations were made in the Kuro Shio to the south of Japan, it was found that the annual variations of temperature in the sub-layers of the ocean water are mainly caused by the transfer of surface temperature to the sub-layers by the process of conduction.
It was also observed that with increasing depth the annual temperature variations gradually decrease. As a matter of fact, the annual temperature variations are found up to a depth of 100 meters beneath the surface.
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However, under special circumstances in certain areas the annual temperature variations are observed up to 300 meters beneath the surface water of the oceans.