Sea Level Variations Due to Equatorial Rossby Waves Associated with El Nin˜o

Interannual variations of sea level in the tropical Pacific are examined in relation to the El Nin˜o phenomenon. The amplitude of equatorial Rossby waves forced by wind and the sea level due to the waves are calculated from the analytical solutions in a two-layer reduced-gravity ocean model using filtered monthly wind data during the period 1961–87. Sea level variations with interannual periods at Truk, Ponape, and Majuro islands, located around 7°N, as well as at Rabaul, Honiara, and Funafuti in the South Pacific, are almost explained by Rossby waves remotely forced by winds. Reflected Rossby waves generated at the eastern coast of the Pacific may not significantly be effective. The interannual Rossby wave components of sea level are produced by odd meridional-mode waves excited from the equatorially symmetric component wind, which is composed of wind data in the same hemisphere as the studied sea level grid. Therefore, the Rossby components in the North Pacific are not affected by wind in the South Pacific, and we can successfully calculate them from North Pacific winds only. The wind forcing of first-mode Rossby wave and, therefore, the sea level due to the wave are almost the same in both the South and North Pacific. On the other hand, the sea level due to the third and fifth modes is different, primarily due to the phase difference of the wind forcing of the modes. As a result, the Rossby component of equatorial sea level within 4° latitude, predominantly due to the first mode, is almost the same in the South and North Pacific, while that outside of 6°, primarily due to the third and fifth modes, is quite different: the fall of sea level in relation to El Nin˜o south of 6°S occurs with a time lag of a few months to a year relative to that north of 6°N.