Simulation of the 1986–1987 El Niño and 1988 La Niña events with a free surface tropical Pacific Ocean general circulation model

Abstract

Observed atmospheric forcing fields over the period 1984–1989 force a free surface tropical Pacific Ocean general circulation model. Numerical simulation of the 1986–1987 El Niño and 1988 La Niña events is presented in the paper. Some quantitative comparisons between model time series and corresponding observations of sea level, and upper ocean current and temperature are made to verify the model performance. Diagnostic analyses of heat balance and available energy budget are given as well. The space‐time evolution of various model variables demonstrates that the model produces interannual variations with reasonable success. Beginning in mid‐1986, westerly wind over the western equatorial Pacific drives strong eastward surface currents which accomplish the massive transfer of warm surface water. The strong westerly wind in late 1986 excites the pronounced equatorial Kelvin waves, which propagate eastward toward the eastern and coastal Pacific where they depress the thermocline and raise sea level twice, and increase sea surface temperature. The eastern Pacific warming occurs primarily from the diminished cooling contribution of vertical advection, whereas in the central Pacific, eastward advection by anomalous zonal flows is the principal mechanism. The El Niño conditions in the eastern Pacific disappear in mid‐1987, whereas they remain in the central and western Pacific until early 1988. Subsequently, the tropical Pacific Ocean rebounds to significant La Niña conditions. Available energy (AE) has a good phase relationship with respect to other variables characterized by warm and cold conditions. AE is anomalously high prior to a warm event, accompanying conversion from kinetic energy (KE) to available potential energy (APE). During the development of El Niño, although relaxation of trade wind reduces input of wind energy, the appearance of westerly wind in the western Pacific leads to a sharp increase in KE. This excites excessive conversion from APE to KE through work done by the buoyancy force, corresponding to out‐of‐phase fluctuation between KE and APE. In the aftermath of the warm event, APE, as well as KE and available surface energy, is depleted and is extremely low. Development of La Niña is characterized by long and slow accumulation of APE with the transformation from KE to APE.