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 win... Abstract 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 win...