Coastal-trapped waves

This article considers waves extending across the continental shelf and/or slope and having periods of the order of 1 day or longer. Their phase propagation is generally cyclonic, with the coast to the right in the Northern Hemisphere, a sense denoted “forward”; cross-slope displacements change water-column depth and relative vorticity, causing cross-slope movement of adjacent water columns. At short scales, energy propagation can be in the opposite “backward” sense. Strict trapping occurs only for periods longer than half a pendulum day; shorter-period waves leak energy to the deep ocean, albeit only slowly for some forms. The waves travel faster in stratified seas and on broad shelf-slope profiles; speeds can be affected, even reversed, by along-shelf flows and reverses of bottom slope. Large amplitudes and abrupt alongshore changes in topography cause distortion and transfers between wave modes. The waves form a basis for the behavior (response to forcing, propagation) of shelf and slope motion on scales of days and the shelf width. Hence, they are important in shelf and slope–sea responses to forcing by tides, winds (e.g., upwelling), density gradients, and oceanic features. Their propagation (distance before decay) implies nonlocal response (over a comparable distance), especially in the “forward” direction