Antarctic sea-ice expansion and Southern Ocean cooling linked to tropical variability

Abstract

A variety of hypotheses for explaining the observed Antarctic sea ice expansion over the period of continuous satellite monitoring and corresponding model-observation discrepancy have been proposed, but the issue remains unresolved. Here, by comparing multiple Large Ensembles of model simulations with available observations, we show that Antarctic sea ice has expanded due to ocean surface cooling associated with multi-decadal variability in the Southern Ocean that temporarily outweighs the opposing forced response, along with subiceshelf melting and stratospheric ozone depletion. In both observations and model simulations, the multi-decadal variability in the Southern Ocean is closely linked to internal variability in the tropics, especially in the Pacific, via atmospheric teleconnections. The linkages are, however, distinctly weaker in simulations than in observations, accompanied by a distinctly stronger global-mean warming response in simulations resulting from model biases and weaker tropical internal variability. Thus, the forced response dominates in simulations, resulting in apparent model-observation discrepancy.

The Antarctic ozone hole and the pattern effect on climate sensitivity

“…The times when this SAM index is ±1 SD away from its mean value are used to composite the SST. The SAM index is weakly negatively correlated with the Niño 3.4 index of tropical SST variability, which indicates that La Niña events are correlated with enhanced westerlies at 60 ∘ S. This is consistent with our hypothesis, but it is also consistent with a hypothesis that La Niña events drive SAM events ( 52 ). El Niño–Southern Oscillation (ENSO) is known to initiate anomalies that propagate from the tropical western Pacific into the SO ( 53 – 56 ), where they can influence the SAM.…”

“…It has been proposed that freshwater input to the SO from melting of Antarctic ice sheets and shelves could lead to cooling of the SO ( 57 ), especially as the SO continues to warm. Cooling of the tropical and southern Pacific Ocean could be related to natural decadal variability of the ocean–atmosphere system ( 52 , 58 ). It is possible that our removal of the effect of ENSO by linear regression does not entirely remove the effect of ENSO on the southern Pacific Ocean.…”

Systematic Climate Model Biases in the Large‐Scale Patterns of Recent Sea‐Surface Temperature and Sea‐Level Pressure Change

“…It remains an open question whether the differences in recent multi‐decadal trends between observations and models resulted from anomalous multi‐decadal variability or from aspects of the forced climate response not captured by models. Some studies suggest that these difference in Pacific and Southern Ocean trends could have resulted from internal atmosphere‐ocean variability (Chung et al., 2019, 2022; Olonscheck et al., 2020; Watanabe et al., 2021; L. Zhang et al., 2019; Zhao & Allen, 2019), while others suggest they result in part from missing forcings or model biases in the pattern of response to forcing (Bintanja et al., 2013; Coats & Karnauskas, 2018; Kohyama et al., 2017; Kostov et al., 2018; Schneider & Deser, 2018; Seager et al., 2019, 2022; Suarez‐Gutierrez et al., 2021; Wills et al., 2020). It is critical to distinguish between these hypotheses in order to predict future SST trends and the associated atmospheric circulation changes.…”

Antarctic Ice‐Sheet Meltwater Reduces Transient Warming and Climate Sensitivity Through the Sea‐Surface Temperature Pattern Effect

“…If our results hold in other models, this finding suggests that the near‐future warming projections by current GCMs may be overestimated. Furthermore, many studies attribute the recent observed SST trend pattern (with cooling in the tropical eastern Pacific and the Southern Ocean) to internal variability (e.g., the negative phase of Inter‐decadal Pacific Oscillation), and therefore hypothesize a reversal of SST trends in these regions to appear in the coming decades (Chung et al., 2022; Watanabe et al., 2021). However, our simulations show that a similar historical SST pattern can arise with sufficient Antarctic meltwater forcing and that this SST pattern can persist into the 21st century in the presence of increasing Antarctic meltwater input.…”