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Abstract During the Middle Miocene Climate Transition (MMCT; ∼14.7–13.8 Ma), the global climate experienced rapid cooling, leading to modern‐like temperatures, precipitation patterns, and permanent ice sheets. However, proxy records indicate that atmospheric pCO2and regional climate conditions (SST, ice volume) were highly variable from 17 to 12.5 Ma and these changes were not always synchronous. Here, we report on a series of middle Miocene (∼16–12.5 Ma) simulations using the water isotope enabled earth system model (iCESM1.2) to explore the potential for multiple equilibrium states to explain the observed decoupling between pCO2and regional climates. Our simulations indicate that initial ocean conditions can significantly influence deep water formation in the North Atlantic and lead to multiple ocean equilibria. When the model is initiated from a cold state, residual cool surface water temperatures in the North Atlantic intensify Atlantic Meridional Ocean Circulation (AMOC) and inhibit Arctic sea‐ice formation. When initiated from a warm state, the AMOC remains weak. The different ocean states drive differences in equator‐to‐pole sea surface temperature gradients and sea ice distributions through heat redistribution changes. These equilibria cause variations in temperature gradients and sea ice distribution due to changes in heat redistribution. Additionally, changes in ocean circulation and a reduced temperature gradient in the North Atlantic increase North Atlantic precipitation when the AMOC is strong. These findings underscore the importance of the ocean's initial state in shaping regional climate responses to atmospheric pCO2, potentially explaining regional climate pattern variability observed during the Miocene.
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Pantropical Indo-Atlantic temperature gradient modulates multi-decadal AMOC variability in models and observations
Abstract Interconnections between ocean basins are recognized as an important driver of climate variability. Recent modeling evidence suggests that the North Atlantic climate can respond to persistent warming of the tropical Indian Ocean sea surface temperature (SST) relative to the rest of the tropics (rTIO). Here, we use observational data to demonstrate that multi-decadal changes in pantropical ocean temperature gradients lead to variations of an SST-based proxy of the Atlantic Meridional Overturning Circulation (AMOC). The largest contribution to this temperature gradient-AMOCconnection comes from gradients between the Indian and Atlantic Oceans. TherTIOindex yields the strongest connection of this tropical temperature gradient to theAMOC. Focusing on the internally generated signal in three observational products reveals that an SST-basedAMOCproxy index has closely followed low-frequency changes ofrTIOtemperature with about 26-year lag since 1870. Analyzing the pre-industrial control simulations of 44 CMIP6 climate models shows that theAMOCproxy index lags simulated mid-latitudeAMOCvariations by 4 ± 4 years. These model simulations reveal the mechanism connectingAMOCvariations to pantropical ocean temperature gradients at a 27 ± 2 years lag, matching the observed time lag in 28 out of the 44 analyzed models. rTIO temperature changes affect the North Atlantic climate through atmospheric planetary waves, impacting temperature and salinity in the subpolar North Atlantic, which modifies deep convection and ultimately the AMOC. Through this mechanism, observed internalrTIOvariations can serve as a multi-decadal precursor ofAMOCchanges with important implications forAMOCdynamics and predictability.
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- Award ID(s):
- 2053096
- PAR ID:
- 10517436
- Publisher / Repository:
- Nature group
- Date Published:
- Journal Name:
- npj Climate and Atmospheric Science
- Volume:
- 6
- Issue:
- 1
- ISSN:
- 2397-3722
- 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/10.1038/s41612-023-00489-x
