A new scientific review has revealed that the Southern Annular Mode (SAM), the Southern Hemisphere's most influential climate driver, is now in its most positive state in over 1,000 years. If greenhouse gas emissions continue to rise, this positive state is projected to persist throughout the 21st Century, with long-term implications for Antarctica and the Southern Ocean.
Published today in Nature Reviews Earth and Environment, the review was led by Monash University researcher, Dr Ariaan Purich, from Securing Antarctica's Environmental Future (SAEF), in collaboration with scientists from across the Australian climate and Antarctic science communities. The paper synthesises the current knowledge on the dynamics and projections of the SAM, and its impacts on Antarctica and the Southern Ocean.
Dr Purich said understanding how the SAM will change is essential to inform climate projections over the 21st Century and beyond.
"The SAM influences temperature, rainfall, ocean circulation and Antarctic ice. Because it affects so many parts of the climate system, we need to know how it will change so we can better anticipate the challenges facing Antarctica and the Southern Hemisphere in the decades ahead," Dr Purich said.
The SAM's three phases, neutral, positive and negative, describe the shifts in atmospheric pressure between Antarctica and the mid-latitudes, which influence the strength and position of the westerly winds that circle the Southern Ocean. When the SAM enters a positive phase, the westerly winds tighten and move towards Antarctica. When it moves into a negative phase, these winds expand back towards Australia.
SAM's influence can be significant. In the spring of 2022, a positive SAM contributed to the unusually high rainfall across southeastern Australia, leading to flooding across Victoria and the Melbourne region.
"Our review shows that since the mid-20th Century, the SAM has trended towards more frequent positive phases due to both ozone depletion and increasing greenhouse gas concentrations," Dr Purich said.
"With ozone levels now stabilising and projected to recover due to the success of the Montreal Protocol, greenhouse gases are becoming the dominant driver of the SAM's ongoing positive trend."
Modelling assessed in the review shows that under high emission pathways, SAM is likely to continue trending towards its positive phase over the coming century. However, under low emissions pathways, the SAM could stabilise by 2050 and begin returning to its historical range by 2100.
A persistent positive SAM is expected to reduce the Southern Ocean's ability to absorb excess carbon dioxide, causing our climate to become warmer. This is because stronger, southward-shifted winds bring more warm, nutrient-rich water to the surface, releasing more carbon from deep in the ocean into the atmosphere, in turn reducing the ocean's capacity as a net carbon sink. The review shows that the implications of a persistent positive SAM for the Antarctic Ice Sheet remain unclear. SAM-driven ocean circulation changes could accelerate melt beneath the ice shelves that buttress the ice sheet, while SAM-driven increased snowfall in some regions may offset these effects.
"The Antarctic Ice Sheet is the largest source of uncertainty in future sea-level projections. Understanding how SAM affects ocean circulation, ice shelf melt and snowfall is essential for improving projections needed to inform climate adaptation," Dr Purich said.
Dr Purich said Antarctic scientists are now reaching the point where they can model many of these processes, but significant gaps remain. More observations beneath Antarctic ice shelves and high-resolution model simulations are urgently needed to better understand these processes in order to refine projections of future changes.
The paper was a collaboration between scientists from across the Australian climate and Antarctic communities from programs including the ARC Centre for Climate Extremes, Australian Antarctic Program Partnership, ARC Australian Centre for Excellence in Antarctic Science and the ARC Centre of Excellence for 21st Century Weather.
