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.