Scientists have shed more light on the puzzling contradiction of increasing Antarctic sea ice in a warming world.
In a new paper published in Nature Geoscience this week, researchers from Monash University and the US-based National Center for Atmospheric Research pointed to climate variability and a complex series of flow-on effects between the tropical Pacific and Ross Sea.
Reasons why the extent of Antarctic sea ice has increased since satellite records began in 1979 have long confounded climate scientists.
The Antarctic situation sits in stark contrast with the Arctic, where ice is rapidly melting, and is not reflected in climate model predictions.
In the new study, the researchers report that patterns of climate variability and corresponding changes in water temperatures in the tropical Pacific Ocean, and the Amundsen Sea off the coast of west Antarctica, have changed ocean circulation in the Ross Sea, driving an increase in sea ice.
As the increase had accelerated since the 1990s, what's called the Interdecadal Pacific Oscillation (IPO) -- an internally-generated mode of climate variability -- had transitioned from positive to negative.
This had led to a an average cooling of tropical Pacific sea surface temperatures, a slowdown of the global warming trend, and a deepening of the Amundsen Sea Low near Antarctica, that had contributed to regional circulation changes in the Ross Sea region and expansion of sea ice.
This negative phase of the IPO was characterised by anomalies similar to the observed sea-level pressure and near-surface wind changes near Antarctica since 2000, that were conducive to expanding Antarctic sea ice extent, particularly in the Ross Sea region in all seasons.
The authors explained that the atmospheric circulation changes were shown to be mainly driven by precipitation and convective heating anomalies related to the IPO in the equatorial eastern Pacific -- with additional contributions from effects in the South Pacific convergence zone and tropical Atlantic regions.
Unravelling the mystery has been a major focus of Victoria University climate scientist Dr James Renwick, who was not involved in the new study.
"As a lot of people have commented over the last few years, the sea ice trend has a lot to do with trends in the surface winds -- whether they've been pushing the ice around, pushing the heat towards the ice and melting, or pulling cold off the continent and freezing the water," he said.
"And they've been pointing out, basically, that the Interdecadal Pacific Oscillation has driven a fair bit of the trend, actually."
Dr Renwick said there had been a large period spanning from the late 1970s through to the end of the last century that had been dominated by El Nino climate systems, but up until perhaps a year ago, the trend had reversed, with mainly La Nina systems.
"The bottom line is we have seen this trend toward more sea ice and Antarctica since the late 1970s, which was when the satellite record began, but also when the IPO flipped positive last.
"And that's been associated with a decrease in sea ice near the peninsula, but an increase near the Ross Sea, plus a few other places around the hemisphere."
Interestingly, there was a good chance the trend might have peaked in 2014, when the sea ice extent reached more than 20 million square kilometres.
Dr Renwick said the record for 2015 was below average, and expected the total for this year would not be much above average.
Antarctic voyage uncovers toothfish insights
Meanwhile, at the edge of the advancing winter sea ice in the Ross Sea, a first-ever fisheries survey has uncovered new secrets of the Antarctic toothfish.
Researchers onboard the New Zealand commercial fishing vessel Janas, an ice strengthened longline vessel owned and operated by Talley's New Zealand, collected the embryos of Antarctic toothfish for the first time by using plankton nets to sample down to a depth of 500m.
They have also fertilised eggs from captured adults in spawning condition, which provided a known start time to observe developmental rate.
The new study was part of a collaborative survey funded by the Ministry for Primary Industries, the Commission of the Conservation of Antarctic Marine Living Resources (CCAMLR) and Talley's to study the reproduction of Antarctic toothfish, their population distribution, and their role in the Ross Sea ecosystem.
NIWA fisheries scientist Dr Steve Parker said the discovery of developing toothfish embryos was important for several reasons.
"Finding them documents the spawning season of the fish, confirms some areas where spawning was suspected to occur, and most importantly, provides information about the depth at which the drifting eggs reside in the water column," he said.
"This new information can be integrated with models of ocean currents to predict where the eggs and larvae will be transported as they develop and grow to become part of the juvenile population."
Antarctic toothfish spawned under sea ice that extended more than 1000km from the continent during the winter, and protecting them for most of the year from Weddell seals and killer whales, but also from scientists and fishers.
The winter voyage to the Ross Sea had taken more than a year to plan, and those aboard had endured near constant darkness, vast expanses of sea ice, fierce seas, and temperatures far below zero.
All fishing in the Ross Sea is highly regulated by CCAMLR through catch limits, full observer coverage and closed areas.
The new information would be used to improve stock assessment and ecosystem models and advance management of the fishery.