Decadal Modulation of ENSO and Linkages to Pacific Decadal Variability: Analysis of a 1300-year CCSM4 Control Simulation

Decadal modulation in the amplitude and other properties of El Nino-Southern Oscillation (ENSO) is analyzed in a 1300-year preindustrial control simulation of the Community Climate System Model version 4 (CCSM4). The CCSM4 displays notable improvements in simulating tropical Pacific mean climate and variability compared to its predecessor CCSM3. The amplitude of ENSO in CCSM4 exhibits pronounced decadal-interdecadal variability, in agreement with the analysis of observational data and paleoclimate proxy records. Decadal changes in the ENSO amplitude are closely linked to decadal changes in the background mean state, suggestive of their positive feedback. During the phase of strong ENSO, zonal gradients of sea surface temperature (SST) and thermocline weaken, the intertropical convergence zone is displaced southward, and the North Pacific subtropical high strengthens as part of the positive phase of the North Pacific Oscillation (NPO). These changes are associated with the second leading EOF mode of tropical Pacific decadal variability. The leading EOF mode, which resembles the so-called Pacific Decadal Oscillation, is not correlated with the ENSO amplitude but with the relative frequency of El Nino and La Nina. The two leading EOF modes of tropical Pacific decadal variability in CCSM4 are compared with those in a 500-year control simulation of the atmospheric component model coupled to a slab ocean model. It is suggested that the second mode in CCSM4 arises from a combination of stochastic NPO forcing and dynamical air-sea interactions in the equatorial Pacific. The first mode, on the other hand, appears to be linked to stochastic variability of the Pacific-South American pattern.