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https://doi.org/10.5194/egusphere-2026-2082
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2026-2082
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
21 Apr 2026
| 21 Apr 2026
Status: this preprint is open for discussion and under review for The Cryosphere (TC).
Brief Communication: The Interdecadal Pacific Oscillation is not a Reliable Indicator of the Timing of an Ice-Free Arctic
Abstract. The Arctic is transitioning towards a seasonally ice-free state over the next few decades, with internal variability determining precisely when. The phase of the Interdecadal Pacific Oscillation (IPO) has been proposed to reduce internal variability uncertainty in ice-free predictions. Using a selection of 8 large ensembles from the Coupled Model Intercomparison Project - Phase 5 (CMIP5) and CMIP6, starting from current mean sea ice extent, we find no agreement that the IPO affects ice-free timing. However, a majority of large ensembles indicate the IPO can affect the rate and spatial distribution of Arctic sea ice decline over the next two decades.
How to cite. Wyburn-Powell, C. and Jahn, A.: Brief Communication: The Interdecadal Pacific Oscillation is not a Reliable Indicator of the Timing of an Ice-Free Arctic, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-2082, 2026.
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Latest update: 13 May 2026
Christopher Wyburn-Powell
CORRESPONDING AUTHOR
Department of Atmospheric and Oceanic Science and Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder Colorado
Alexandra Jahn
Department of Atmospheric and Oceanic Science and Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder Colorado
Short summary
The Arctic Ocean is expected to become seasonally ice-free within a few decades. Previous research using one climate model showed that a major climate pattern called the Interdecadal Pacific Oscillation (IPO) could narrow predictions of when this will happen. Using a broader selection of the latest climate models, we show the IPO is not a reliable predictor of ice-free timing. However, the IPO may still influence the rate and location of Arctic sea ice reduction over the next two decades.
The Arctic Ocean is expected to become seasonally ice-free within a few decades. Previous...
This short paper is an important contribution. It takes as a starting point the work of Screen and Deser, which suggested with a single model that the IPO could be a useful constraint on the timing of an ice-free Arctic. As with all such potential constraints, it is valuable to test their robustness across multiple models. This submission tests the constraint across 8 models and finds that it is not robust. I support publication after revision and/or suitable responses to the following points:
Line 60: I don’t understand the rationale for the ensemble pooling. The text states “to incorporate multiple initial conditions”, however, as far as I understand it, these are not independent runs but multiple samples from the same ensemble member. Unlike a distinct ensemble member, which is free to evolve differently to all other members, these pooled samples are not independent and are simply offset by a few years. This process seems to artificially inflate the ensemble size, without providing independent samples, and needs better justification.
Line 76: Does this increase in the percentage of LEs showing significant divergence come from the alternative IPO definition and/or the pooling of initial states? If the latter, I’m concerned that significance is inflated artificially and has limited physical meaning. Does the significance testing assume each sample is independent? They aren’t (see above). Besides, regardless of the significance or not of the divergence, the more important point seems to be that the divergence is small around the time of first ice-free conditions, which is independent of the pooling (as the samples still have the same timing of ice-free conditions). Basically, I’m wondering if the pooling is necessary, even if it could be justified.
Section 3.2: I think this section could be clearer. There are lots of kind-of “what if” statements and the key result – which I think is it that there is no consistent lagged relationship between IPO and ice-free conditions across models (in fact, in most models there is no relationship at all) – gets a bit lost. I think this point could be made more succinctly and clearly, alongside some simplification of Figure 3 (see comment below).
Figure 3: It’s unclear what value panels I-P add, and they are not directly referred to in the text. If I understand correctly, the correlation shown in panels I-P can already been inferred from the blue lines in panels A-H. Panels A-H include a second metric for ice-free, which is not used (or introduced) before. Is this necessary? What is the added value of the red line? In such a short note, I’m in favour of parsimony: keeping things as simple as possible.
Conclusions: The lack of robustness of the constraint across models is an important result, but there is scope for deeper consideration of the causes of this lack of robustness, its implications, and potential ways forward. The IPO constraint can only ever reduce the model uncertainty due to internal variability. Its utility is dependent on the timeframe for an ice-free Arctic, which also depends on the simulated forced response. In this regard, it might not be appropriate to consider all models as equal. There is no effort to critique if any of models have (un)realistic forced responses, for example, by looking at their historical trends or sea-ice sensitivity (sea-ice loss per degree of global warming). At the very least, this requires some thought and discussion. A more sophisticated approach might be to first constrain the model spread in the forced response, before considering if internal variability can additionally constrain the spread around this forced response.
Other minor points:
Typo line 18: “showed the IPO has been shown”
Figure 1: might be helpful to list the number of members on the titles
Line 134: “at today’s lead times” confusing and probably not needed. The sentence makes (more) sense without this bit.