Asymmetric Pacific variability in the Pliocene: An unchanged PDO relative to a suppressed ENSO

Currently, the El Niño Southern Oscillation (ENSO) dominates the climate variability in the Pacific basin, while the Pacific Decadal Oscillation (PDO) is the second dominant mode of variability. Both modes of variability are crucial due to their global roles. However, the response of PDO and ENSO to changes in forcing, such as increased atmospheric CO₂ concentrations or geography, is not fully understood. Past intervals of geological time offer valuable insights to understand their responses to different forcing. Here, we investigate PDO and ENSO behaviour and their relationship in the mid-Pliocene Warm Period (MP; ∼3 million years ago) using the Pliocene Model Intercomparison Project version 2 (PlioMIP2) climate model ensemble. Results show that in the MP, although ENSO variability is robustly suppressed, PDO variability remains similar to the pre-industrial (PI) in terms of spatial pattern and multi model mean (MMM) amplitude, although individual models differ. The predominant frequencies of variability in the Pacific basin shift from interannual (3–4 years) in the PI to decadal (15–35 years) in the MP. Relative to a weakened ENSO, PDO is a more important mode of Pacific variability in the MP. The linear relationship between PDO and ENSO decreases by only 18 % in the MP even though ENSO variability decreases by ∼30 %, suggesting that the majority of PDO variability is not directly explained by changes in ENSO variability. We suggest that these results are driven by MP boundary conditions other than elevated CO₂ levels.