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Most state-of-art models project a reduced equatorial Pacific east-west temperature gradient and a weakened Walker circulation under global warming. However, the causes of this robust projection remain elusive. Here, we devise a series of slab ocean model experiments to diagnostically decompose the global warming response into the contributions from the direct carbon dioxide (CO2) forcing, sea ice changes, and regional ocean heat uptake. The CO2forcing dominates the Walker circulation slowdown through enhancing the tropical tropospheric stability. Antarctic sea ice changes and local ocean heat release are the dominant drivers for reduced zonal temperature gradient over the equatorial Pacific, while the Southern Ocean heat uptake opposes this change. Corroborating our model experiments, multimodel analysis shows that the models with greater Southern Ocean heat uptake exhibit less reduction in the temperature gradient and less weakening of the Walker circulation. Therefore, constraining the tropical Pacific projection requires a better insight into Southern Ocean processes.
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Pleistocene Sunda Shelf submersion-exposure cycles initiate vegetation Walker Circulation feedback
Recent research has found that the subsiding Sunda Shelf (Southeast Asia) was permanently exposed prior to ca. 400 ka with initial submersion-exposure cyclicity, associated with interglacial-glacial sea-level cycles, beginning between 400 and 240 ka. We analyzed the impact submersion-exposure cycles on regional environment and climate through a 640 k.y. leaf-wax carbon isotope (δ13Cwax) reconstruction at Andaman Sea Site U1448, representing relative changes in C3/C4 plant abundances. Prior to ca. 250 ka, the Sunda region was inhabited by a stable C3 (forest) biome, after which submersion-exposure cycles initiated with the deglacial sea-level rise at ca. 250 ka. During subsequent glacial-age sea-level drops, the newly exposed shelf was rapidly colonized by C4 grasses, followed by slow transitions back to C3 forests, representing a tenfold increase in the variability of C3/C4 vegetation in the Sunda region. The C3/C4 regime shift since 250 ka is coherent across the Southeast (SE) Asia peninsula and Sunda Shelf and is coincident with a shift in the east-west sea-surface temperature gradient in the equatorial Pacific Ocean. We hypothesize that the expansion of C4 grasslands promoted and sustained drier glacial-age climates over SE Asia via a feedback mechanism that contributed to weakening the ascending branch of the east-west atmospheric circulation in the equatorial Pacific region known as the Walker Circulation. Our results indicate that the Sunda Shelf region has a larger influence on Walker Circulation than is seen in current paleoclimate simulations.
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- Award ID(s):
- 2126815
- PAR ID:
- 10446602
- Date Published:
- Journal Name:
- Geology
- ISSN:
- 0091-7613
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G51412.1
