A current flowing beneath a surface current, usually in the opposite direction.

• A. Surface Current
• B. Undercurrent
• C. Transverse Current
• D. Sverdrup (sv)

Answer: (B)

Currents can differ with depth, so you can have a subsurface flow that moves in a different direction from the surface. An undercurrent describes water flowing beneath the surface current, often opposite to it. This vertical shear happens because the wind-driven motion is strongest near the surface and friction, pressure gradients, and density structure can drive a compensating flow at greater depths. That makes the deep flow the best fit for the description. It’s different from an undertow, which refers to a shoreward return flow after waves break on the beach, and from Sverdrup, which is a unit of transport rather than a type of current.