Final answer:
The layer in the oceans that undergoes heating and cooling as seasons change is the mixed layer, approximately 100-200 meters thick, followed by the thermocline.
Explanation:
The layer in the oceans subject to heating and cooling as the seasons change is the mixed layer. This layer is typically around 100-200 meters thick in mid-latitude regions. Below the mixed layer, there is a rapid decline in temperature known as the thermocline, where the temperature decreases with depth.
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