Evidence for link between modelled trends in Antarctic sea ice and underestimated westerly wind changes

Abstract

Despite global warming, total Antarctic sea ice coverage increased over 1979–2013. However, the majority of Coupled Model Intercomparison Project phase 5 models simulate a decline. Mechanisms causing this discrepancy have so far remained elusive. Here we show that weaker trends in the intensification of the Southern Hemisphere westerly wind jet simulated by the models may contribute to this disparity. During austral summer, a strengthened jet leads to increased upwelling of cooler subsurface water and strengthened equatorward transport, conducive to increased sea ice. As the majority of models underestimate summer jet trends, this cooling process is underestimated compared with observations and is insufficient to offset warming in the models. Through the sea ice-albedo feedback, models produce a high-latitude surface ocean warming and sea ice decline, contrasting the observed net cooling and sea ice increase. A realistic simulation of observed wind changes may be crucial for reproducing the recent observed sea ice increase.

Sea Ice Trends in Climate Models Only Accurate in Runs with Biased Global Warming

“…Consistent with this, a number of previous studies have suggested that internal climate variability could explain the difference between the observed sea ice trend and the ensemblemean simulated trend in each hemisphere (Holland et al 2008;Kay et al 2011;Stroeve et al 2012;Polvani and Smith 2013;Mahlstein et al 2013;Turner et al 2013;Swart and Fyfe 2013;Zunz et al 2013;IPCC 2013;Fan et al 2014;Notz 2014;Gagné et al 2015;Goosse and Zunz 2014;Swart et al 2015;Purich et al 2016;Jones et al 2016).…”

“…This implies that if the CMIP5 models are correct, then the probability that the Arctic sea ice would retreat as fast as observed is 11%, and the probability that the Antarctic sea ice would expand as fast as observed is 2.5%. These results are approximately similar to previous studies that found that, after accounting for simulated internal variability, the models and observations are statistically consistent in the Arctic (Stroeve et al 2012;Notz 2014;Swart et al 2015) and marginally consistent in the Antarctic (Swart and Fyfe 2013;Turner et al 2013;Zunz et al 2013;Purich et al 2016;Jones et al 2016).…”

“…Although the center of the Antarctic distribution moves closer to the observed value, the width of the distribution decreases sufficiently to cause the percentage of runs in the distribution that have Antarctic sea ice expansion as large as the observations to drop from 1.6% (Figure 1f) to 0.37% ( Figure 3d). Note that these results are qualitatively consistent with the sea ice sensitivities reported by Purich et al (2016) and Stroeve and Notz (2016).…”

“…Therefore, this bias may be related to issues in the atmosphere, ocean, or sea ice model components that are connected to the simulated sea ice changes or to the level of global warming. For example, several studies have identified model biases related to global warming trends (e.g., IPCC 2013; Kosaka and Xie 2013) and local processes that influence sea ice (Rampal et al 2011;Jahn et al 2012;Mahlstein and Knutti 2012;Bintanja et al 2013;Mahlstein et al 2013;Zunz et al 2013;Uotila et al 2014;Haumann et al 2014;Purich et al 2016;Jones et al 2016). Additional studies have suggested that polar teleconnections may also have an important influence on sea ice trends in each hemisphere (Meehl et al 2016;Screen and Francis 2016).…”

“…Most CMIP5 models do not simulate this trend (Figures 1c,f) (e.g., Turner et al 2013;Zunz et al 2013;IPCC 2013), contributing to a consensus view that there is "low confidence" in the scientific understanding of the observed Antarctic sea ice expansion (IPCC 2013). Nonetheless, a number of recent studies have argued that the models are still at least marginally consistent with observations when the range of internal climate variability is considered (Turner et al 2013;Swart and Fyfe 2013;Zunz et al 2013;Mahlstein et al 2013;Polvani and Smith 2013;Goosse and Zunz 2014;Gagné et al 2015;Fan et al 2014;Turner et al 2015;Purich et al 2016;Jones et al 2016). In this view, the observed Antarctic sea ice expansion is the result of internal climate variability overwhelming the sea ice retreat that would have occurred due to climate forcing.…”