Sailors know the southern mid-latitudes experience some of the strongest storms on Earth. Now, scientists say, there's an atmospheric pattern hidden in the region dubbed the Roaring Forties and Furious Fifties by ship crews — the first-ever mid-latitude climate cycle ever found.
A 20- to 30-day oscillation shows up in an atmospheric cycle called the baroclinic annular mode, or BAM, researchers report today (Feb. 6) in the journal Science. This pattern influences storm strength in the Southern Ocean, or the body of water surrounding Antarctica, including how much heat storms carry and how much rain and snow they drop.
"This is clearly a robust phenomenon," said Steven Feldstein, a climate scientist at Penn State University, who was not involved in the study. "It shows up in both climate models and in the real world."
Climate scientists already rely on well-known tropical atmospheric patterns for forecasting weather and climate change. These pulses in the sky help move heat and moisture around the planet in predictable ways. Some last just 40 to 70 days, others take years. One example is the two- to seven-year El Niño/La Niña Southern Oscillation, which drives drought and rainfall patterns around the world, and influences the number of Atlantic hurricanes.
But despite searching for decades, until now, no one has discovered atmospheric oscillations in the either the northern or southern mid-latitudes.
Lead study author David Thompson, of Colorado State University, identified the new cycle by poring through satellite data from the past 30 years. First, he and co-author and Jonathan D. Woodworth found a 25- to 30-day oscillation in storminess in the Southern Ocean, which they describe in a paper accepted to the Journal of Atmospheric Sciences. [The World's 10 Biggest Oceans and Seas]
Then, Thompson and Elizabeth Barnes, also at Colorado State, showed how the BAM cycle originates, as reported in Science. The key is a north-south temperature imbalance. Heat builds up in the lower southern latitudes — the swath that includes the coastlines of South America, Africa and Australia — but there's a time lag as storms carry the heat toward Antarctica. That time lag sets up a feedback loop driven by the atmosphere's uneven heating, the researchers show. The BAM oscillation, essentially, is the temperature imbalance flipping back and forth between higher and lower mid-latitudes.
"[This] periodicity clearly has potentially profound implications for understanding and predicting Southern Hemisphere climate variability over broad spatial scales," the authors write in Science.
The discovery of a Southern Ocean storm cycle could prove useful in predicting future climate variability, such as the Southern Hemisphere's response to climate change, Thompson writes in the forthcoming Journal of Atmospheric Sciences article.
But for forecasting weather, the cycle would first have to be linked to regional effects, Feldstein told Live Science. "This phenomenon is averaged over the mid-latitudes and entire Southern Hemisphere, but for forecasting, you're interested in something regional," Feldstein said.
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