Terdiurnal atmospheric tides induce an S3 radiational ocean tide, similar to radiational tides S1 and S2 in the diurnal and semidiurnal bands. Although of small amplitude, the terdiurnal tide has some intriguing properties. The tide has an unusually pronounced seasonal variation, manifested by annual sidelines here denoted R3 and T3, which causes the tide to nearly vanish during times near an equinox. Forcing is generally largest in the winter hemisphere. Complicating matters, the two sideline frequencies coincide with those of nonlinear compound tides SK3 and SP3. Whether radiational tides or nonlinear tides (or both) are appearing at any given tide gauge can usually be determined by the relative ampli-tudes and phase differences of the two sidelines. The amplitudes of R3 and T3 are generally comparable; the amplitudes of SK3 and SP3 are not. Proper identification can lead to a small improvement in tidal prediction, but more importantly can lead to improved physical interpretation. An example from recent measurements under the Ross Ice Shelf bears on the role of nonlinearity in interactions between the ocean tide and the floating ice shelf.
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