A first order internal seiche. The undisturbed sea level is indicated by the yellow line, two layers of slightly different density are represented by different shades of blue. Four water particles are shown as an indication of water movement in the internal seiche.

Note the node in the centre (where the interface between the layers does not move up or down), and that water under and above the node moves only horizontally, while water at both ends of the basin moves vertically.

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