The Effect of the Galapagos Islands on ENSO in Forced Ocean and Hybrid Coupled Models

An ocean general circulation model (OGCM) of the tropical Pacific Ocean is used to examine the effects of the Galapagos Islands on the El Nino-Southern Oscillation (ENSO). First, a series of experiments is conducted using the OGCM in a forced context, whereby an idealized El Nino event may be examined in cases with and without the Galapagos Islands. In this setup, the sensitivity of the sea surface temperature (SST) anomaly response to the presence of the Galapagos Islands is examined. Second, with the OGCM coupled to the atmosphere via zonal wind stress, experiments are conducted with and without the Galapagos Islands to determine how the Galapagos Islands influence the time scale of ENSO.In the forced setup, the Galapagos Islands lead to a damped SST anomaly given an identical zonal wind stress perturbation. Mixed layer heat budget calculations implicate the entrainment mixing term, which confirms that the difference is due to the Galapagos Islands changing the background mean state, that is, the equatorial thermocline as diagnosed in a previous paper. In the hybrid coupled experiments, there is a clear shift in the power spectrum of SST anomalies in the eastern equatorial Pacific. Specifically, the Galapagos Islands lead to a shift in the ENSO time scale from a biennial to a quasi-quadrennial period. Mechanisms for the shift in ENSO time scale due to the Galapagos Islands are discussed in the context of well-known paradigms for the oscillatory nature of ENSO.