Decadal ENSO Modulation of the Last Millennium Simulation

In this study, we classified two types of El Niño-Southern Oscillation (ENSO) events within the decadal ENSO amplitude modulation cycle using the long-term CGCM simulations – pre-industrial and the last millennium simulations. The drivers of climate over the last millennium (850-1850 AD) are chiefly orbital, solar, volcanic, changes in land use/land cover and some variation in greenhouse gas levels. Therefore, we can expect the examination of ENSO modulation with or without a natural external climate forcing using both long-term simulations. First, we defined two climate states - strong and weak ENSO amplitude periods – and separated the characteristics of ENSO that occurred in both periods. In the case of pre-industrial simulation, there are two major features in the characteristics of ENSO: the first is the asymmetric spatial structure between El Niño and La Niña events; the second is that the El Niño-La Niña asymmetry is reversed during strong and weak ENSO amplitude periods. However, there does not exist a modulation of spatial structure according to the amplitude modulation in the last millennium simulation. This is because the two-way interaction between ENSO and tropical Pacific decadal variability is changed under the forced climate simulation.