CSIR NET Dec 2015 Q136: Physical Oceanography

A longshore current is a nearshore current that flows parallel to the coastline within the surf zone. It is generated when waves approach the shoreline at an oblique angle: as these waves enter shallowing water, wave refraction (the bending of wave crests due to the wave's speed decreasing in shallower water) causes the wave crests to become progressively more aligned with the bathymetric contours (and hence more parallel to shore) as they approach the beach, but a residual component of wave-induced momentum remains directed along the shore. This continuous input of momentum parallel to the coastline drives a persistent current flowing parallel to the coast -- the longshore current -- matching option 3 (parallel to the coast, forms by wave refraction).

Option 1 (perpendicular to the coast, forms by wave refraction) is incorrect in direction; longshore currents flow along (parallel to), not perpendicular to, the coastline, even though refraction is indeed the relevant wave process. Option 2 (perpendicular to the coast, forms by wave diffraction) is incorrect both in direction and mechanism; diffraction (wave bending around obstacles/into shadow zones) is not the primary process generating longshore currents. Option 3 is correct: longshore currents flow parallel to the shoreline and are generated by the along-shore momentum imparted by obliquely incident, refracting waves. Option 4 (parallel to the coast, forms by wave diffraction) correctly identifies the flow direction but misattributes the causal wave process; it is refraction of obliquely approaching waves, not diffraction, that generates the longshore current.