Invited review articlePacific decadal climate variability: Indices, patterns and tropical-extratropical interactions

- Review of patterns and indices of PDV using observations and palaeoclimate data
- IPO, PDO and SPDO related to, affected by, but distinct from ENSO in extratropics
- New stochastic tool provides probabilistic estimate of recent PDV state.
- Bi-hemispheric tropical-extratropical interactions might explain coherence IPO/PDO.
- Intense research focus needed on PDV observations, palaeoclimate and modelling

Pacific decadal variability (PDV) plays a critical role in the climate system. Here I present a review of indices and patterns of decadal climate variability in the Pacific from observations and palaeoclimate reconstructions. I examine the spatial characteristics of Pacific sea surface temperature variability and the metrics used to track observations of PDV. I find commonalities between the PDV patterns, the Interdecadal Pacific Oscillation (IPO) and its North and South Pacific counterparts, the Pacific Decadal and South Pacific Decadal Oscillations (PDO and SPDO). I present a tool to provide probabilistic quantification of the recent state of the IPO, and use the tool to provide reliable estimates of IPO state up to 2 years prior to the present. The tool indicates a probability of 80-90% that the IPO remained in its negative state until 2014-2015. I review palaeoclimate reconstructions of the IPO and PDO, and outline advances and challenges in our pre-instrumental understanding of PDV. I draw attention to a Pacific-wide tropical-extratropical mechanism that suggests that the cool and warm phases of PDV are not driven by tropical or extratropical variability alone, but are instead the result of continuous tropical-extratropical interactions on decadal timescales. I conclude by noting key sources of remaining uncertainty and emphasising the need to better understand decadal variability. This will occur through continual improvements in observations, an expansion of palaeoclimate exploration and data collection, and renewed efforts in model development.