© Graphic: Helge Arz, IOWSchematic depiction of changes in the ocean-atmosphere system in the South Pacific in comparison, throughout the precession cycles (21,000 years).
Changes in the southern westerly wind belt produce fundamental effects on the intensity and position of the Antarctic Circumpolar Current, which is the world's largest ocean current and shapes ocean circulation worldwide. In this regard, one key factor is the wind-driven upwelling of CO2-rich deep-water masses, which, due to their comparative warmth, influences both the stability of the West Antarctic Ice Sheet and the carbon budget of the Southern Ocean.
On the basis of sediment cores, the team of researchers investigated precipitation-driven changes in sediment input in the Pacific off the coast of Chile. Assessing the past 1 million years, they identified what are known as precession cycles: changes caused by natural variations in the Earth's orbital parameters; in this case, cyclical changes in the rotation of its axis that occurred roughly every 21,000 years. Changes in these and other orbital cycles are generally considered to be a major driver for the alternation between extended glacials and interglacials over the past million years.
© Graphic: Helge Arz IOW, Frank Lamy AWIChanges in fluvial sediment input off the coast of northern Chile over the past 1 million years and comparison to changes in the precession of the Earth’s axis.
The nature of the phenomenon was ultimately deciphered with the aid of American climate modeller John Chiang (University of California, Berkeley): "Our climate models show that the precipitation changes recorded in the sediment cores are connected to the configuration of high-altitude winds over the subtropical Pacific. In this regard, the division of the high-altitude wind changes into a northern, subtropical branch, a middle branch, and a subpolar branch in the course of the 21,000-year cycle."
"The study provides the first evidence of long-term changes in the high-altitude winds of the southern westerly wind belt over the South Pacific," explains Dr Frank Lamy. "Our findings indicate closer atmospheric ties between the tropics and mid to high latitudes than in other sectors of the Southern Hemisphere, with consequences for global overturning circulation and the storage of atmospheric CO2 in the ocean."
The team's findings are also important with regard to understanding current and especially future large-scale climate mechanisms in the comparatively under-researched Southern Hemisphere. One crucial aspect is the coupling of the tropical Pacific with the source of the global climate phenomenon El Niño Southern Oscillation (ENSO) and the West Antarctic. The data shows that the West Antarctic Ice Sheet's high sensitivity to ENSO in the Pacific sector, which can be seen in satellite observations made over the past few decades, is most likely also significant over much longer time scales. "A change in the high-altitude winds over the South Pacific in response to the increased frequency and intensity of El Niño events that many climate models predict would reduce the stability of the West Antarctic Ice Sheet, while also negatively impacting CO2 storage in the South Pacific," says Lamy, putting the findings in perspective.
Original publication
The study will be published on November 5, 2019 as an open access article in the online portal PNAS.
The original title is: Frank Lamy, John C.H. Chiang, Gema Martínez-Méndez, Mieke Thierens, Helge W. Arz, Joyce Bosmans, Dierk Hebbeln, Fabrice Lambert, Lester Lembke-Jene, Jan-Berend Stuut: Precession modulation of the South Pacific westerly wind belt over the past million years, Proceedings of the National Academy of Sciences of the United States of America (PNAS) /doi.org/10.1073/pnas.1905847116
Reader Comments
to our Newsletter