When easterly winds weaken over the tropical Pacific Ocean, a string of weather extremes unfolds all over the globe, with impacts ranging from flooding in South American deserts to reduced monsoon rains in Indonesia and India. This shift in wind and water currents, known as the El Niño–Southern Oscillation (ENSO), will become more intense if global temperatures continue to rise.
Research now has revealed that projected changes to this global weather maker will also influence the remote Southern Ocean. Using the latest climate models, scientists have shown that enhanced El Niño events will likely speed the heating of deep-ocean waters around Antarctica, with the potential for accelerated melting of the continent’s land-held ice.
Scientists are concerned about how stronger El Niño events could affect the Antarctic because of the potential for sea level rise. The Antarctic Ice Sheet holds about 60% of the world’s freshwater—enough to raise global sea levels by around 70 meters.
Satellite measurements have shown that the Antarctic Ice Sheet has been melting steadily over the past 2 decades. But owing to the remoteness of Antarctica and the challenge of collecting data directly from the ice or surrounding ocean, scientists still don’t fully understand the processes influencing Antarctic ice loss, leading to uncertainties in projections of global sea level rise.
Changes to ENSO complicate sea level projections further. “We know very little about how ENSO intensification will impact the Antarctic climate and, in turn, how that could drive ice loss,” said Wenju Cai, a climate scientist at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia’s national science agency.
In a study published in Nature Climate Change, Cai and his colleagues used climate models to make the first assessments of how ENSO intensification could affect Antarctic climate.
A Range of Climate Models
According to Cai, most climate models project an increase in ENSO variability by 2100, indicating that both phases of ENSO, warm El Niño and cool La Niña, will likely become stronger and more frequent in the future.
“ENSO influences many aspects of the Antarctic climate, driven by the changes it exerts on atmospheric and ocean circulation around the continent,” said study coauthor Ariaan Purich, a climate scientist with Monash University in Australia.
In their study, the researchers investigated how more frequent ENSO events could affect the Southern Ocean over the next century, assuming a business-as-usual scenario in which greenhouse gas emissions remain uncurbed.
To do this, they analyzed projected changes to sea ice, ocean temperatures, winds, and heat fluxes across a group of 31 global climate models produced as part of the Coupled Model Intercomparison Project Phase 6 (CMIP6).
The researchers assessed the spread of ENSO variability changes across the models, looking for reproducible patterns that could explain how ENSO will affect the temperature of the Southern Ocean through changes to high-latitude winds and ocean circulation.
“Simulating the impact of climate change on ENSO is a challenging task in itself,” said Mat Collins, a climate scientist with the University of Exeter who was not involved in the study. “The novelty here is that [the researchers] bring together a range of climate models to disentangle complex climate responses.”
The researchers found that an increase in ENSO variability over the next century will lead to changes in circulation in the Southern Ocean, causing accelerated warming of Antarctica’s deeper waters. The reason is that the slower easterly winds that blow over the region will reduce the upwelling of deep warm waters.
The analysis also showed that although the ocean surface will warm more slowly, the deeper ocean around Antarctica will warm more quickly, exposing the ice shelves that fringe the continent to heat from below.
These shelves do the important job of holding back glaciers on land. Deep-ocean warming could destabilize and melt the floating ice, allowing land-held ice sheets to slide more easily into the ocean, raising sea level. The researchers cannot say at this stage how deep-ocean warming will affect ice loss because the CMIP6 models don’t include interactive ice shelves or ice sheets.
“Scientists have known from satellite observations that Antarctic ice shelves typically lose mass during El Niño events,” said Helen Fricker, a glaciologist with the Scripps Institution of Oceanography who was not involved in the study. The loss occurs because an influx of warm water melts the ice shelves from below, she explained. “What’s new is that we now have a preview of how they could respond as El Niño intensifies.”
Fricker added that continued efforts to collect observational data from the Antarctic are needed to complete the picture of ocean and ice shelf interactions. Purich agreed. “This is an important part of the puzzle,” she said. “But there are still many unknowns surrounding the processes influencing ocean circulation and ice melt on the Antarctic shelf.” The researchers said they plan to build ice sheet models into future analyses to help them get a better handle on how the ocean warming they observed could affect rates of ice melt.
—Erin Martin-Jones (@Erin_M_J), Science Writer
Correction, 28 March 2023: An earlier version of this article incorrectly stated that westerly winds weaken during an El Niño event. Easterly winds weaken during El Niño.