Response of tropical Pacific interannual variability to decadal entrainment temperature change in a hybrid coupled model

The role of decadal changes in ocean thermal structure, as observed in the tropical Pacific in the late 1970s, in modulating El Nino/Southern Oscillation (ENSO) is examined using a hybrid coupled model (HCM), consisting of an atmospheric general circulation model (AGCM; ECHAM4.5) and an intermediate ocean model (IOM) with an empirical parameterization for the temperature of subsurface water entrained into the mixed layer (T-e), which is constructed via a singular value decomposition (SVD) analysis of historical data. A standard HCM run produces irregularity of ENSO. Two perturbation runs are conduced by introducing the decadal changes in T-e estimated from two subperiods before and after the climate shift (T-e(63-79) and T-e(80-96)), respectively. Together with previous studies using an intermediate coupled model (ICM) consisting of the same IOM, different modulating effects of T-e in the ocean and stochastic forcing in the atmosphere are demonstrated on the properties of ENSO. The former is responsible for changes in the phase propagation of ENSO, while the latter can contribute to the amplitude and period modulation.