A new University of Washington (UW) study shows that the lack of Antarctic sea ice in 2016 was in part due to a unique one-two punch from atmospheric conditions both in the tropical Pacific Ocean and around the South Pole.
The study, published in the current issue of Geophysical Research Letters, tries to probe the dramatic drop in Antarctic sea ice almost a year ago, during the Southern Hemisphere spring, which brought its maximum area down to its lowest level in 40 years of record keeping.
Actually, while winter sea ice in the Arctic is declining so dramatically that ships can now navigate those waters without any icebreaker escort, the scene in the Southern Hemisphere is very different, that until last year, sea ice area around Antarctica has actually increased slightly in winter.
The area of sea ice around Antarctica at its peak in late 2016 was 2 million square kilometers, or about 800,000 square miles, less than the average from the satellite record.
As statistically it would be expected to occur randomly once every 300 years, the record low was not predicted by climate scientists, prompting UW researchers to look at the bigger picture in ocean and atmospheric data to explain why it happened.
The previous year, 2015-16, had a very strong El Nino in the tropical Pacific Ocean, causing unusually warm ocean temperatures in Antarctica's eastern Ross, Amundsen and Bellingshausen seas. Nicknamed the "Godzilla El Nino," the event was similar to monster El Ninos in 1982-83 and 1997-98.
Unlike the 1997-98 event, however, the 2015-16 warm phase of ocean water was only followed by a relatively weak La Nina in 2016. When no strong La Nina materialized, the UW researchers found that these unusually warm surface pools lingered longer than usual and affected freeze-up of seawater the following season.
In addition, observations show that the winds circling Antarctica were unusually weak in 2016, meaning they did not push sea ice away from the Antarctic coast to make room for the formation of new ice.
Malte Stuecker, a UW postdoctoral researcher in atmospheric sciences and corresponding author of the paper, noted that "this combination of factors, all these things coming together in a single year, was basically the 'perfect storm,' for Antarctic sea ice."
The researchers analyzed 13,000 years of climate model simulations to study how these unique conditions would affect the sea ice. Taken together, the El Nino pattern and Southern Ocean winds explain about two-thirds of the 2016 decline. The rest may be due to unusually big storms, which a previous paper suggested had broken up ice floes.