Sea ice coverage in Antarctica quickly shrank to a record low in late 2016 and remained well below average.
But what is behind this continuous dramatic melting and low ice cover?
Research published earlier this month suggests that a combination of natural variability in the atmosphere and ocean was to blame, although man-induced climate change may also play a role.
What happened to the Antarctic sea ice in 2016?
Antarctic sea ice is frozen sea water, usually less than a few feet thick. It differs from ice shelves, which are formed by glaciers, float in the sea and are up to one kilometer thick.
Sea ice cover in Antarctica is crucial to the global climate, and marine ecosystems and satellites have been monitoring it since the late 1970s. In contrast to the Arctic, sea ice around Antarctica was slowly expanding (see the figure below).
However, by the end of 2016, the Antarctic sea ice melted dramatically and rapidly, reaching a record low. This has aroused the interest of climate scientists, because such large, unexpected, and rapid changes are rare. Sea ice coverage is still well below average now.
We wanted to know what caused this unprecedented decline in Antarctic sea ice and what changes in the system sustained these declines. We also wanted to know if this was a temporary change or the beginning of a long-term decline, as predicted by climate models. Finally, we wanted to know if man-induced climate change contributed to these record lows.
The sea ice cover around Antarctica varies greatly from one year or decade to the next. In fact, the ice cover of the Antarctic sea reached a record level in 2014.
This provided a clue. As sea ice coverage year by year and decade to decade varies greatly, this can mask the melting of sea ice for a longer period due to anthropogenic warming.
The next clue was in broken records far from Antarctica. In the spring of 2016, sea-surface temperatures and rainfall in the East Indian Ocean were at record levels. This was in association with a strongly negative event of the Indian Ocean Dipole (IOD), which brought warmer waters to the northwest of Australia.
Although IOD events influence rainfall in southeastern Australia, we have found (using statistical analyzes and experiments with climate models) that it has promoted a pattern in the winds over the Southern Ocean which was particularly favorable to the decrease in sea ice.
These surface winds that blow from the north not only pushed the sea ice to the Antarctic continent, but also warmed, helping to melt the ice of the sea.
These northern winds combined almost perfectly with the main regions where the sea ice diminished.
Atmospheric circulation and sea ice concentration during September to October 2016. The figure above shows the wind anomaly from September to October (vectors, upper right scale, m / s) at the bottom of the atmosphere; the red shading shows a warmer air flow to the north, and the blue shading represents the flow to the south. The bottom figure shows the extent of the sea ice: green represents more sea ice than the average, and purple shows regions of sea ice reduction.
Although previous studies have linked this wind pattern to declining sea ice, our studies are the first to champion the dominant role of the Eastern Tropical Indian Ocean in conducting it.
But this was not the only factor.
Later, in 2016, the typical western winds that surround Antarctica weakened to record lows. This caused the surface of the ocean to warm up, promoting less coverage of sea ice.
Weaker winds began at the top of the atmosphere over Antarctica, in the region known as the stratospheric polar vortex. We believe that this sequential occurrence of tropical and then stratospheric influences contributed to the record decline in 2016.
Taken together, the evidence we present corroborates the idea that the rapid decline of the Antarctic sea ice in late 2016 was largely due to natural climate variability.
The current state of the Antarctic sea ice
Since then, sea ice has remained far below average in association with the hottest ocean temperatures around Antarctica.
We argue that these are the product of stronger than normal winds over the last 15 years or so around Antarctica, expelled again from the tropics. These stronger winds induced a response in the ocean, with the hotter submerged water moving toward the surface over time.
The combination of record tropical sea surface temperatures and weakened western winds in 2016 warmed all 600m of water in most regions of the Antarctic Ocean around Antarctica. These warmer ocean temperatures have maintained the reduced extent of sea ice.
The extent of the Antarctic sea ice is recording a record start in the new year. He suggests that the rapid initial decline seen at the end of 2016 was not an isolated event, and when combined with the decadal-temporal warm-up of the upper Southern Ocean, could mean reducing the extent of sea ice for some time.
We argue that what we are seeing so far can be understood in terms of natural variability superimposed on a long signal of man-induced warming.
This is because records of precipitation and ocean temperature observed in the eastern tropical Indian Ocean that led to the initial decline of sea ice in 2016 probably had some contribution to climate change.
This warming and the recovery of the ozone hole in Antarctica can also affect surface wind patterns in the coming decades.
Such changes may be fueling the effects of climate change that are beginning to emerge in the Antarctic region. However, limited data logging and high variability indicate that it is too early to say
We would like to acknowledge the role of our co-authors S Abhik, Cecilia M Bitz, Christine TY Chung, Alice Du Vivier, Harry H Hendon, Marika M Holland, Eun-Pa Lim, LuAnne Thompson, Peter van Rensch and Dongxia Yang in contributing to the research discussed in this article.