In 2016, the sea-ice around Antarctica shrunk to its lowest level since satellite records began. One of the key reasons for this sudden decrease after years of gradual expansion was an increase in the waviness of the jet stream, the high latitude westerly winds that circle the icy continent. This increased waviness brought warm air towards Antarctica, leading to a record-breaking reduction in sea ice.
Similar waviness in the Arctic jet stream has been blamed for stopping weather systems in their tracks in the Northern Hemisphere leading to a string of damaging extreme weather events over Europe, Asia, and North America.
Over time, observations from both the Northern and Southern Hemisphere show that some of the waves embedded in the jet streams are almost permanent features with peaks and troughs that remain in semi-fixed locations. For 50 years, scientists have assumed these almost permanent deviations in the jet were caused by the continents and huge land features such as the Andes, similar to the Himalayas in the north, which created strong and continuous updrafts that pushed the jets aside.
Now, new findings from a team of researchers at the ARC Centre of Excellence for Climate Extremes has uncovered that while this might be true in the Northern Hemisphere, there is another even more powerful force originating north of Australia that creates the almost permanent stationary wave in the southern jet – the Indo Pacific Warm Pool.
The findings, published today in Nature Geoscience, overturn a paradigm that has existed for 50 years and opens the possibility of improved polar predictions by observing temperature changes in this warm pool region north of Australia. The findings might also help improve forecasts of the systems that stall south of Australia and bring extreme weather events.
“The area of ocean north of Australia is one of warmest in the world and generates consistent strong convection that lifts warm air up to 11 kilometres into the atmosphere,” said CLEX lead author Rishav Goyal.
“This sets off a chain reaction high in the atmosphere that radiates thousands of kilometres southward until it collides with the polar jet, causing it to deviate polewards in certain regions. You can think of it like a giant rock rolling into a stream and creating waves.”
In 2016, concurrent with the Antarctic sea-ice contracting to its lowest level in decades, one of the strongest tropical convection events in the modern record occurred. This led to a cascading series of atmospheric disturbances that amplified the wave pattern in the jet stream, pushing warm air further south and helping trigger the rapid melt.
To get their results the scientists created alternate worlds using the kind of models that are used to make global weather and climate forecasts. This allowed the researchers to alter the distributions of land, mountains, and oceans, while the laws of physics remained unchanged.
Some worlds had continents that were completely flat, others were entirely ocean covered or included mountain ranges that jutted into the jet stream. The team also rearranged the land in the Southern Hemisphere, and the tropics, and changed the size of mountains, all in an effort to tease out where the real driver came from. And out of that process, it became clear that one patch of ocean held the key.
“We were surprised to find that the oceans north of Australia had more influence on the waviness of the Antarctic jet-stream – thousands of kilometres to the south – than the Andes; a massive range of mountains running along much of the length of South America,” said Mr Goyal.
“Discovering this link has important implications for predicting this wave pattern. Previously we thought it was largely internally generated variability in the regions close to Antarctica. Now we have the means to predict strong events by linking it to the tropical Pacific warm pool, and with that comes the potential for improved forecasting of extreme weather events.”
- Paper: Goyal, R., Jucker, M., Sen Gupta, A. et al. Zonal wave 3 pattern in the Southern Hemisphere generated by tropical convection. Nat. Geosci. (2021). https://doi.org/10.1038/s41561-021-00811-3