CSIR NET June 2016 Q66: Physical Oceanography
The main (permanent) oceanic thermocline is a mid-depth layer of rapid temperature decrease with depth; its vertical thickness varies systematically with latitude.
Near the equator the thermocline is relatively shallow and compressed by a thin warm surface layer, while towards the poles it weakens and eventually disappears (polar waters are cold throughout the water column, with no thermocline). Between these extremes, at mid-to-high latitudes around 60°, the thermocline layer occupies its greatest vertical thickness before vanishing poleward. Option 3 is correct as per the official key.
1) Incorrect — near the equator the thermocline is present but comparatively thin. 2) Incorrect — at 30°, thickness is still increasing, not yet maximal. 3) Marked correct per official key — thermocline attains maximum thickness around 60° latitude before disappearing poleward. 4) Incorrect — at the poles, the water column is isothermal and no thermocline exists.