Nature says South American and Asian monsoons were generally out of phase.
Nature published a peer-reviewed paper examining 450,000 years of monsoon patterns using cave-mineral records from the Amazon Basin. Its authors say the South American and Asian monsoons were largely out of phase but sometimes synchronized, including as ice ages ended.
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On Oct. 14, Nature published a peer-reviewed study that reconstructs 450,000 years of monsoon behavior. The researchers built the record from oxygen isotopes preserved in speleothems, mineral deposits that grow in caves, from the Amazon Basin.
That Southern Hemisphere record let the team compare the South American Summer Monsoon with long records of the Asian monsoon in the Northern Hemisphere.
The two monsoon records were generally “strikingly out of phase,” the authors report. In other words, their major shifts usually moved in opposite directions rather than together.
The team also found occasional exceptions. Its synchronicity threshold was crossed during 10 events, including glacial terminations. Those episodes show that parts of the monsoon system can move together during periods of rapid climate change, even though that was not the dominant pattern across the full record.
Both monsoon records varied with Milankovitch forcing, the slow changes in Earth’s orbit and orientation that alter how sunlight is distributed, as well as with internal climate conditions such as ice cover and sea-surface temperatures.
For the synchronous episodes, the researchers say greenhouse-gas concentrations, cross-equatorial temperature gradients, and sea-surface temperatures in the Atlantic and Pacific probably played a role. The paper does not assign a single cause to those events.
The authors argue that the global monsoon can reorganize when planetary boundary conditions change quickly. Their 450,000-year Southern Hemisphere record gives researchers a longer baseline for testing how the global monsoon responds to external forcing and internal climate shifts.
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