Responses of ENSO and NAO to the external radiative forcing during the last millennium: Results from CCSM4 and MPI-ESM-P simulations

The decadal–centennial variations of the El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) are analyzed, based on the outputs of last millennium (LM) and Historical experiments (850–2005 AD) and control (CTL) experiments from two climate models Community Climate System Model, version 4 (CCSM4) and Max Planck Institute for Meteorology Paleoclimate Model (MPI-ESM-P). Variation of ENSO and NAO among the Medieval Warm Period (MWP, 1050–1150 AD), Little Ice Age (LIA, 1600–1700 AD) and 20th Century Warming (20CW, 1905–2005 AD) are focused. Significant bi-centennial and multi-decadal periods are detected in the ENSO and NAO series, respectively. Both models have exerted statistically-significant contrasts of ENSO and NAO phases among the three typical periods. The model-independent shifts of an El Niño-like state during the MWP, a La Niña-like state during the LIA and an El Niño-like state during the 20CW are captured. However, the NAO series display a model-dependence. CCSM4 features a negative-phased NAO during the LIA, while MPI-ESM-P is characterized by a positive-phased NAO in the same period. The shifts of the ENSO and NAO phases are mainly due to the external radiative forcing, including the solar-volcanic (SV) forcing and greenhouse gases (GHG) forcing.