Blocking, turning and bunching: the hard life of waves riding on currents

Without currents, the life of waves would be simple: waves are borne from the wind and radiate away as swell when the wind dies until they eventually themselve die when breaking on the shoreline. With currents in the way things can be much more chaotic and result in very unusual sea states that are dreaded by mariners or leisure boaters: short choppy seas, very high waves in opposing currents... In theory this is fairly well known, but in practice it is hard to put all the pieces of the theoretical puzzle to understand waves over currents. When it gets complicated this is where a numerical model can be useful. This is what was just done in a study published in the Journal of Physical Oceanography (December 2012), summarizing the current effects as three distinct contributions: blocking, turning and bunching. New measurements off the French Atlantic coasts are particularly revealing of the last two effects ...