On the role of oceanic entrainment temperature (T_e) in decadal changes of El Niño/Southern Oscillation

Abstract. The role of decadal changes in ocean thermal structure in modulating El Niño/Southern Oscillation (ENSO) properties was examined using a hybrid coupled model (HCM), consisting of a statistical atmospheric model and an oceanic general circulation model (OGCM) with an explicitly embedded empirical parameterization for the temperature of subsurface water entrained into the mixed layer (T_e), which was constructed via an EOF analysis of model-based historical data. Using the empirical T_e models constructed from two subperiods, 1963–1979 (T_e⁶³⁻⁷⁹) and 1980–1996 (T_e⁸⁰⁻⁹⁶), the coupled system exhibits striking different properties of interannual variability, including oscillation periods and the propagation characteristic of sea surface temperature anomalies (SSTAs) along the equator. In the T_e⁶³⁻⁷⁹ run, the model features a 2–3 yr oscillation and a westward propagation of SSTAs along the equator, while in the T_e⁸⁰⁻⁹⁶ run, it is characterized by a 4–5 yr oscillation and an eastward propagation. Furthermore, a Lag Covariance Analysis (LCOA) was utilized to illustrate the leading physical processes responsible for decadal change in SST. It is shown that the change in the structure of T_e acts to modulate the relative strength of the zonal advective and thermocline feedbacks in the coupled system, leading to changes in ENSO properties. Two additional sensitive experiments were conducted to further illustrate the respective roles of the changes in ocean mean states and in T_e in modulating ENSO behaviors. These decadal changes in the simulated ENSO properties are consistent with the observed shift occurred in the late 1970s and a previous simulation performed with an intermediate coupled model (ICM) described in Zhang and Busalacchi (2005), indicating a dominant role T_e plays in decadal ENSO changes.