ENSO has been characterized as a "chaotic" or "stochastic" (randomly forced) dynamical system with "broadband" spectral peaks. Previous studies have shown that ENSO variability is also sensitive to changes in climate forcing conditions. Periods of high and low variability in ENSO strength correspond to different regime behaviors in many concurrent tropical Pacific phenomena. We will analyze the Last Millennium Ensemble to investigate different periods of ENSO regimes and corresponding mean state changes in the Tropical Pacific. We will also analyze the forced response versus the internal variability in the Tropical Pacific system in this context.