Mechanisms of Interannual Variations of Equatorial Sea Level Associated with El Niño

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Abstract
Interannual variations of equatorial sea level in the Pacific were examined in relation to the EJ Niño phenomenon. The dynamic component of sea level due to equatorial Kelvin and Rossby waves was calculated from the analytical solutions in a two-layer reduced-gravity ocean model using filtered monthly wind data during the period 1961–1987. The thermal component of sea level was also estimated from SST data. Kelvin waves are strongly excited by wind between 160°E and 150°W, correlated with the occurrence of El Niño. The wind-forced Kelvin waves produce a rise of sea level during the El Niño events, while the reflected Kelvin and wind-forced Rossby waves produce a fall of sea level with a slight time lag behind the rise. The sum of them, the dynamic component of equatorial sea level, has less amplitude and an earlier phase than the observed sea level, and cannot fully explain the observations. The differences between the observations and the dynamic component of sea level are explained by the thermal component of sea level. This component is due to the variations of water temperature in the upper layer, which are caused by zonal advection due to the equatorial waves and by entrainment of deeper water. The zonal advection is the major factor in the region between 180° and 110°W, while the vertical entrainment is dominant east of 110°W. The strength of the entrainment is highly correlated with the upper-layer depth anomaly due to the equatorial waves. The thermal component of sea level is produced by the thermodynamic effect of the equatorial waves. Therefore, the interannual variations of equatorial sea level are created by both the dynamic and thermodynamic effects of the equatorial Kelvin and Rossby waves.

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