Observations and Dynamics of Semiannual and Annual Sea Levels near the Eastern Equatorial Indian Ocean Boundary

Monthly sea level records from the western coasts of Thailand and Indonesia are examined together with atmospheric pressure, wind data, and a linear model to document and understand eastern equatorial Indian Ocean semiannual and annual sea level fluctuations. The semiannual sea level fluctuations are approximately constant in amplitude and phase for six sea level stations on a 4000-km stretch of coastline extending from 8.12°S to 7.83°N. Semiannual zonal equatorial winds in the Indian Ocean basin remotely generate these sea level fluctuations. In sharp contrast to the semiannual sea level, the phase and amplitude of the annual sea level fluctuations vary strongly along the coast. The annual fluctuations are forced predominantly by the coastal alongshore monsoon wind. For the six months from April to September this monsoon wind has a southeasterly component toward India, raising coastal sea level in the Northern Hemisphere and lowering it in the Southern Hemisphere; during the other six months winds, and sea levels reverse. A simple linear model with vertical modes one and two of the equatorial Kelvin and first-meridional-mode Rossby waves forced by observed wind stress gives annual eastern boundary sea level results similar to those observed. In the semiannual case model and observed sea levels agree approximately in phase but not amplitude. This may be caused by the difficulty in calculating amplitude accurately near resonance—the equatorial Indian Ocean has a 6-month period basin mode for the second vertical mode.