Eddy-resolving simulations reveal amplified marine cold spells in the Southern Ocean under greenhouse warming

Dear Editor,

Extreme ocean temperatures can exceed species' thermal tolerances and reorganize marine ecosystems. While marine heatwaves (MHWs) are projected to intensify under greenhouse warming, the future of marine cold spells (MCSs) remains poorly understood. Here, we use eddy-resolving climate simulations to show that, contrary to expectations from mean warming, the intensity and frequency of MCSs increase across the Southern Ocean (SO) under greenhouse forcing. In the high-resolution configuration, cumulative MCS intensity locally strengthens by more than a factor of three, driven by enhanced northward eddy cold advection. Strengthened westerlies intensify Ekman transport and steepen meridional temperature gradients, increasing eddy kinetic energy and favoring equatorward transport of colder high-latitude waters. In contrast, a low-resolution configuration with parameterized eddies simulates a decline in MCSs because eddy compensation enhances southward heat transport. These contrasting responses demonstrate that resolving mesoscale dynamics fundamentally alters projections of cold extremes. Amplified cold spells in the SO reveal that intensified variability, rather than mean cooling, can generate paradoxical cold extremes under climate change, with important implications for marine ecosystems.

Long-term climate change is accompanied by extreme events, which can have devastating impacts on the Earth system. Because of greenhouse warming, extreme ocean temperatures, such as MHWs, are expected to increase and induce growing environmental impacts. As the counterpart of MHWs, extreme cold ocean temperatures, namely MCSs, could also cause ecological shifts in the species composition and migration pattern. For example, those in the Agulhas Current and East Australian Current increase the risk of cold mortality and alter the migration pathways for bull sharks. An MCS event near southeast Australia in March 2017 caused the mass death of warm-water fishes. An MCS near Western Australia over 2016–2019 aided in the recovery of several benthic fish species that were severely affected by extreme MHW events during 2011–2013. Although generally MCSs have diminished in the historical period, observational datasets reveal growing MCSs, particularly in the SO. Furthermore, the SO experienced a net cooling from 1982 to 2011 despite global warming. It is unknown whether this perplexing phenomenon in the SO will persist into the future or if it is a unique response of the SO to greenhouse warming.

To examine the issue, we use an eddy-resolving climate simulation (abbreviated as HR hereafter; see climate simulations in the supplemental methods) spanning from historical to a future rapid-warming scenario, recognizing that eddies and fronts prevail in the SO. For comparison, we use an analog version but with a low resolution (abbreviated as LR hereafter; see climate simulations in the supplemental methods) that is consistent with most state-of-the-art climate simulations in terms of model resolution and dynamics.

A key aspect of studying future MCSs is defining a threshold for them (see marine cold spells in the supplemental methods). It is usually defined as the 10th percentile of local temperature within a baseline period (e.g., 31 years). If the baseline is fixed from a historical period, the resulting MCSs include effects from both extreme temperature variability and a gradually warming climate. Calculation relative to the long-term trend assumes that climate change could be adapted through evolution and migration.1 In this case, abrupt shifts to cold spells could have a severe impact on organisms because the temperatures exceed thermal tolerance. Therefore, we define the threshold of MCSs using a 31-year moving baseline period around the target year in order to remove the effect of rising mean temperature and focus on extreme variability.