137-5 An El Nino-like response during Heinrich Stadial 1 drives pluvial lake expansions in the Western North America

Session: Joint SGD-SEPM-IAS Focus on the Sedimentary Record of Climate Change

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The drought-prone southwestern US (SWUS) experienced exceptionally wet conditions during Heinrich Stadial 1 (HS1; 18-15 ka). Such a change has been hypothesized to be driven by deepening of the Aleutian Low, a southward shift of the jet stream, and increased atmospheric rivers in association with a slowdown of the Atlantic Meridional Overturning Circulation (AMOC). However, the pathways through which an AMOC-driven climate signal is propagated from the North Atlantic to the western US remain elusive. To address this question, we first establish the global hydroclimate response by comparing an ensemble of freshwater hosing simulations to a synthesis of global hydroclimate proxy data. The best-performing models exhibit an El Niño-like mean state change in the tropical Pacific, which is driven by a weakened meridional temperature gradient in the eastern tropical Pacific due to transmission of cooling from the tropical North Atlantic across the Isthmus of Panama. This El Niño-like response excites a northeastward-propagating rossby wave from the central Pacific, leading to a deepened Aleutian Low and enhanced southwesterly moisture transport to the SWUS. To further distinguish this mechanism, we run a set of sensitivity tests using an atmosphere-only model, which show that changes in tropical Pacific SST alone could account for the majority of precipitation increase observed in the SWUS. Our results demonstrate that a weaker AMOC influences SWUS hydroclimate via tropical teleconnection involving a zonally asymmetric change in the tropical Pacific. As the AMOC is projected to weaken in the future, the ability of models to correctly simulate the tropical Pacific response will be critical for accurately predicting future hydroclimate change in the western US.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8176

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