Atmospheric forcing of the oceanic semidiurnal tide

The principal solar semidiurnal tide (S 2 ) in the ocean is forced by the pressure loading of the atmospheric thermal tide as well as by the gravitational tidal potential. This paper examines the effects of adding the atmospheric S 2 forcing to a forward tide model. When the model is forced only by the gravitational potential, the S 2 relative elevation error with respect to pelagic tide gauges is anomalously poor. After atmospheric S 2 forcing is added, the relative error reduces to levels seen in other tidal constituents. In the global average, the atmospherically forced S 2 ocean tide is 14.7 percent as large as the gravitationally forced S 2 tide, and differs by about 109.4° in phase, consistent with the relative amplitudes and phases of the atmospheric and gravitational S 2 forcings. Because the S 2 air tide is periodic, the oceanic S 2 tide represents a particularly clean test of the ability of numerical models to successfully replicate the oceanic response to atmospheric pressure loading.