The observational records have shown a cooling of the Arctic lower stratospheric temperature (ALST) since 1980. However, it remains inconclusive whether the observed change is driven by external forcing or 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. Here we quantitatively estimate the contributions of external forcing and internal variability to the recently observed ALST cooling using large-ensemble simulations from five state-of-the-art Earth System Models. The results demonstrate that external forcing is the primary driver of stratospheric cooling, accounting for approximately 85% of the observed cooling in the Arctic lower stratosphere during 1980-2014. On the other hand, the Interdecadal Pacific Oscillation (IPO) is a leading mode of interdecadal climate variability associated with ALST change, accounting for approximately 13%. Furthermore, the IPO evolution substantially affects the near-term projection of the ALST, introducing large uncertainty to the near-term projection. Under the SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios, if the IPO turns into its warm phase in the next few decades, an amplitude of +2 standard deviation of the IPO trend will decrease the trend of ALST by nearly 32%, 46%, and 34%, respectively, compared to the original projections. In conclusion, this study emphasizes that accurately predicting the ALST requires a full consideration of the impact of the internal variability of the IPO.
 

stratospheric temperature, external forcing, climate variability, future projection, climate attribution, climate change