The role of Barents–Kara sea ice loss in projected polar vortex changes

Abstract

Abstract. The Northern Hemisphere stratospheric polar vortex (SPV) plays a key role in mid-latitude weather and climate. However, in what way the SPV will respond to global warming is not clear, with climate models disagreeing on the sign and magnitude of projected SPV strength change. Here we address the potential role of Barents and Kara (BK) sea ice loss in this. We provide evidence for a non-linear response of the SPV to global mean temperature change, which is coincident with the time the BK seas become ice-free. Using a causal network approach, we demonstrate that climate models show some partial support for the previously proposed link between low BK sea ice in autumn and a weakened winter SPV but that this effect is plausibly very small relative to internal variability. Yet, given the expected dramatic decrease in sea ice in the future, even a small causal effect can explain all of the projected ensemble-mean SPV weakening, approximately one-half of the ensemble spread in the middle of the 21st century, and one-third of the spread at the end of the century. Finally, we note that most models have unrealistic amounts of BK sea ice, meaning that their SPV response to ice loss is unrealistic. Bias adjusting for this effect leads to pronounced differences in SPV response of individual models at both ends of the spectrum but has no strong consequences for the overall ensemble mean and spread. Overall, our results indicate the importance of exploring all plausible implications of a changing Arctic for regional climate risk assessments.

North Atlantic Oscillation in winter is largely insensitive to autumn Barents-Kara sea ice variability

“…Many different physical mechanisms have been proposed to explain the ice-NAO relationship and other related Arctic-mid-latitude teleconnections (5)(6)(7). Common among these mechanisms, however, are important roles for turbulent heat fluxes associated with sea ice loss, atmospheric blocking patterns over the Urals, and a weakening of the stratospheric polar vortex, as identified in both observational and modeling studies (8)(9)(10)(11)(12)(13)(14)(15).…”

Towards a Robust Detection of Interannual Ensemble Forecast Signals over the North Atlantic and Europe using Atmospheric Circulation Regimes

“…The Bayes factor is the ratio of the probability of the data given a hypothesis for two different hypotheses H 1 and H 2 , i.e. P (D|H 1 )/P (D|H 2 ) (Kass and Raftery, 1995) and has been recently used in climate studies (Kretschmer et al, 2020). Here, the first hypothesis H 1 is the linear regression model and the second hypothesis H 2 is a constant occurrence rate following the overall value.…”

Long Range Prediction and the Stratosphere

“…Subsequent modelling studies with larger samples of simulations have provided mixed results (Zhang et al 2018;Dai and Song 2020;Smith et al 2021) and some argued that the atmospheric response to sea ice is weak and that while the sensitivity to Barents-Kara sea ice may be stronger, the stratospheric response in particular is highly variable (McKenna et al 2017). While there may well be a longer-term effect via the stratosphere from sea ice decline (Sun et al, 2015;Screen and Blackport 2019;Kretschmer et al, 2020), sensitivity of the response to the background state complicates the issue (Labe et al, 2019;Smith et al 2017), as do possible confounding influences from the tropics (Warner et al 2020) and to date there is no clear consensus for strong enough year to year effects to provide significant seasonal predictability.…”

“…In this case it seems that the response of the mid-latitude circulation involves a negative shift in the Arctic Oscillation (Screen et al, 2018;Zappa et al, 2018;McKenna et al, 2018). This could again be amplified by interaction with the stratosphere as some studies suggest that the stratospheric response is necessary for a large surface response (Kim et al, 2014), while others highlight that the stratospheric interaction is sensitive to the regional pattern of sea ice decline (McKenna et al, 2018), and still others show evidence of non-linear stratospheric, and stratosphere-mediated surface response (Manzini et al, 2018), coincident with the time when Barents and Kara seas become ice-free (Kretschmer et al, 2020). Furthermore, studies also indicate that the surface climate response to sea ice decline depends systematically on the phase of the stratospheric QBO (Labe et al, 2019).…”