Pacific variability under present-day and Middle Miocene boundary conditions

We use the coupled climate model MPI-ESM to show that for higher CO₂ levels the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) merge into a single mode of Pacific variability, regardless of present-day or Middle Miocene (~15 Ma) topographic boundary conditions. Hence, topographic differences—determining the landscape of past climates—play a smaller role for Pacific variability than previously thought. We attribute the single variability mode to resonance between these two oscillation patterns. In order to estimate the strength of the resonance we compute the spectral power of the ENSO and PDO time series and their coherence. We find that for both Middle Miocene and present-day topographic conditions, higher CO₂ forcing leads to stronger resonance between ENSO and PDO. Our results show that (1) stronger CO₂ forcing enhances Pacific variability resulting in stronger “atmospheric bridge” and that (2) past climates are likely to exhibit Pacific variability corresponding either to ENSO, PDO, or our proposed single mode.