The Interdecadal Pacific Oscillation (IPO) Plays a Nonnegligible Role in the Arctic Lower-Stratospheric Temperature Change

Abstract Stratospheric temperature trend is a fingerprint for anthropogenic climate change. Since 1980, observational records have shown a negative trend in the Arctic lower-stratospheric temperature (ALST). However, it remains inconclusive to what extent the observed changes are driven by external forcing versus internal variability, and which modes of interdecadal internal variability significantly affect ALST. Moreover, providing a solid answer to how ALST will change in the near future remains a challenge. Using five large-ensemble simulations, this study shows that both external forcing and internal variability associated with the interdecadal Pacific oscillation (IPO) modulate the ALST changes. Among, the positive–negative phase transition of the IPO from 1980 to 2014 enhances both the upward propagation of planetary waves into the stratosphere and the strength of the Brewer–Dobson (BD) circulation, contributing to the Arctic stratospheric warming. The quantitative results suggest that the phase transition of IPO could offset approximately 15% (4%–26%) of the Arctic lower-stratospheric cooling. Furthermore, the IPO evolution substantially affects the near-term projection of the ALST, introducing large uncertainty to the near-term projection. In conclusion, the findings highlight the combined effect of external forcing and internal variability, especially that relating to IPO, significantly influences ALST changes. Significance Statement It remains unclear which interdecadal internal variability dominates the Arctic lower-stratospheric temperature (ALST) change in the recent decade, and whether its influence will continue or weaken soon. In this study, using five sets of large-ensemble models, we provide robust evidence that interdecadal Pacific oscillation (IPO)-related internal variability significantly influences both the historical changes and near-term projections of ALST, introducing substantial uncertainty in future projections. Meanwhile, we also propose the potential physical mechanisms through which the IPO influences ALST changes and quantify the relative contributions of external forcing and IPO-related internal variability. These research findings will enhance our understanding of how external forcing and internal variability influence the Arctic stratospheric temperature.