Pacific variability under present-day and Middle Miocene boundary conditions
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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.
KeywordsClimate variability ENSO PDO Middle Miocene MPI-ESM
This work has been accomplished within the project FOR 1070 Understanding Cenozoic Climate Cooling of the German Research Foundation (DFG). All model integrations were performed at the high- performance computing platforms of the German Climate Computing Centre (DKRZ) in Hamburg. We would like to thank two anonymous reviewers for their help to improve the manuscript. We also would like to thank Davide Zanchettin for many fruitful discussions and his useful comments on earlier versions of this manuscript. Also thanks to Jochem Marotzke for helping with the introduction. Lastly, MK thanks Romy Schirrmeister for inquiring a finished manuscript.
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