The temperature of seawater immediately below the mixed layer changes rapidly with depth. This layer of rapid temperature change is called the thermocline. Compare the thermocline for the low latitudes (Fig. 1a) to the thermocline for the high latitudes (Fig. 2a). What differences do you notice?

Thermocline is generally defined as the distinct boundary that separates the warm, less dense water from the cold, denser water. When the sunlight is incident on the surface of the oceans and seas, it penetrates up to a certain depth and eventually heats the water. This results in the formation of hot and warm water in the upper portion of the ocean water body, and the deeper part remains cold as sunlight cannot penetrate into the deeper zone.

In the low latitude areas, mostly in the equator and the tropical region, it receives a large amount of sunlight, as a result of which the thermocline layer is very distinct and prominent, and is located at some depth below the ocean's surface.

But, in the high latitude areas, mostly in the areas of the arctic circle and arctic ocean, there occurs a lesser amount of sunlight. Due to this, the temperature remains relatively very cold, so the thermocline in this region is not very prominent, as the temperature difference between the upper surface warm water and the cold denser water is indistinct.

This difference in temperature is noticed when comparing the thermocline for the high and low latitude areas.