Tropical Pacific Decadal Variability (TPDV) directly affects ecosystems across the planet. It affects billions of people worldwide through its modulation of e.g., precipitation, drought and food risk, sea-level, and it is linked to decadal changes in the impact of ENSO on agricultural production and streamflow. TPDV modulates sea-ice in the Arctic and Antarctic, and the rate at which Earth is warming in response to anthropogenic greenhouse gas emissions. TPDV may also be linked to the sensitivity of the planet to climate increases in atmospheric carbon dioxide concentrations. Improved understanding of TPDV could therefore reduce the large uncertainty in estimates of this sensitivity, thereby helping society respond more effectively and at much lower cost to the myriad of threats posed by climate change.

Here we review the current state-of-knowledge of TPDV, its spatial and temporal characteristics, it’s many proposed mechanisms, and the current ability of state-of-the-art modelling and prediction systems to simulate and predict TPDV. The review is based on analyses of a wide, diverse, and in some cases contradictory array of evidence from historical records, instrumental and paleoclimate data, theory, mathematical models of Earth's climate, and decadal prediction systems. We outline the natural (e.g., internal variability in the tropical and extra-tropical Pacific, influences from other oceans; explosive volcanic eruptions) and anthropogenic (e.g. emissions of greenhouse gases and sulphate aerosols) processes posited as causing or strongly influencing TPDV, and the interactions between them. We also assess the degree of confidence we have in the proposed mechanisms and the extent to which the processes may provide a degree of predictability. We conclude with recommendations to advance our understanding of TPDV and our ability to predict it.