ENSO Simulated by Intermediate Coupled Models and Evaluated with Observations over 1970–98. Part I: Role of the Off-Equatorial Variability

Abstract

ENSO simulations are investigated in 30-yr integrations of various intermediate coupled models and compared with observed SST, wind, and thermocline depth anomalies over the tropical Pacific. The Cane and Zebiak model simulates warm events with a period close to the observations, but with westerlies that are located 30° east of them and thermocline anomalies in the western Pacific that are much shallower. Between two warm events, the model simulates a series of three weak and short cold SST peaks and hardly ever simulates easterlies. The SST in the eastern equatorial Pacific is not sensitive to thermocline depth anomalies, but to the anomalous downwelling of surface currents induced by Ekman shear. The model simulates a pair of very strong cyclonic wind stress curl anomalies on both sides of the equator in the eastern off-equatorial domain between 7° and 15° lat. These are necessary to maintain the oscillatory regime—so are the ocean meridional Rossby modes higher than 5. The thermocline zonal slopes required to balance the off-equatorial curl anomalies are about three times steeper than the ones required to balance the zonal stress along the equator. Thus the off-equator exerts an excess of zonal pressure, which by continuity affects the equatorial ocean and plays a crucial role in reversing and triggering the growing events. Six months after the warm peaks, the whole ocean between 15°S and 15°N is significantly upwelled. The equatorial oceanic heat content is recharged from the south prior to a warm event. Contrary to simulations when the model is driven by observed wind anomalies, increasing the friction in the baroclinic ocean does not decrease the off-equatorial variability but significantly alters the low-frequency oscillations that are no longer ENSO-like. Introducing the parameterization of subsurface temperature derived from hydrographic profiles in the ocean component does not allow the coupled model to recover cold events as in a forced context. Introducing the parameterization of convection derived from high-cloud temperature measurements is the most effective improvement, but results still poorly agree with observations and are in contrast with the simulations driven by observed SST, biased toward westerlies in the central Pacific, upwelled thermocline in the west, and warm SST in the east. Thus modifying the ocean component only or the atmosphere only does not have the same impact on simulations as in a forced context. The coupling allows new mechanisms to grow and govern the model behavior. One of them is the slow meridional oceanic mass adjusment in quasi-Sverdrup balance with the winds.