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1. Multiple Ocean Equilibria and Decoupling of Miocene Atmospheric pCO ₂ and Regional Temperatures

   https://doi.org/10.1029/2025PA005126  

   Lee, Daeun; Sarr, Anta‐Clarisse; Acosta, R Paul; Poulsen, Christopher J (May 2025, Paleoceanography and Paleoclimatology)

   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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2. Toward Predicting Changes in the Land Monsoon Rainfall a Decade in Advance

   https://doi.org/10.1175/JCLI-D-17-0521.1  

   Wang, B; Li, J; Cane, MA; Liu, J; Webster, PJ; Xiang, B; Kim, H-M; Cao, J; Ha, K-J (January 2018, Journal of climate)

   Predictions of changes of the land monsoon rainfall (LMR) in the coming decades are of vital importance forsuccessful sustainable economic development. Current dynamic models, though, have shown little skill in thedecadal prediction of the Northern Hemisphere (NH) LMR (NHLMR). The physical basis and predictability forsuch predictions remain largely unexplored. Decadal change of the NHLMR reflects changes in the total NHcontinental precipitation, tropical general circulation, and regional land monsoon rainfall over northern Africa,India, East Asia, and North America. Using observations from 1901 to 2014 and numerical experiments, it isshown that the decadal variability of the NHLMR is rooted primarily in (i) the north–south hemispheric thermalcontrast in the Atlantic–Indian Ocean sector measured by the North Atlantic–south Indian Ocean dipole(NAID) sea surface temperature (SST) index and (ii) an east–west thermal contrast in the Pacific measured by anextended El Niño–Southern Oscillation (XEN) index. Results from a 500-yr preindustrial control experimentdemonstrate that the leading mode of decadal NHLMR and the associated NAID and XEN SST anomalies maybe largely an internal mode of Earth’s climate system, although possibly modified by natural and anthropogenicexternal forcing. A 51-yr, independent forward-rolling decadal hindcast was made with a hybrid dynamic con-ceptual model and using the NAID index predicted by a multiclimate model ensemble. The results demonstratethat the decadal changes in the NHLMR can be predicted approximately a decade in advance with significantskills, opening a promising way forward for decadal predictions of regional land monsoon rainfall worldwide. 

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3. The Impact of AMOC SST Fingerprints on Tropical Storm Risk Along the U.S. East & Gulf Coasts and Latin America

   https://doi.org/10.22541/essoar.175883392.28753009/v1  

   Karnauskas, Kristopher B; Kramer, Sydney M; Donnelly, Jeffrey P (September 2025, ESS Open Archive)

   Tropical storms pose a significant risk to coastal populations, including those throughout the Caribbean and along the Atlantic and Gulf coasts of North America. The impact of climate change on tropical storms is multifaceted, and patterns of sea surface temperature (SST) change may play a role in shaping future tropical storm risk. While the SST fingerprints associated with changes in the Atlantic Meridional Overturning Circulation (AMOC) may be uncertain, the North Atlantic Warming Hole (NAWH) and enhanced SST warming near the Gulf Stream are robust features of both past and projected future climate change. Here we use the Community Earth System Model version 2 (CESM2) to highlight the remote contributions of both of these potential SST fingerprints of AMOC decline to changes in tropical cyclone (TC) genesis potential in the Atlantic basin, and thus to uncertainty in future coastal climate risk. Both the NAWH and enhanced warming near the Gulf Stream lead to significant changes in TC genesis potential, particularly in the western North Atlantic (between Bermuda and the Bahamas), the northeastern Gulf of Mexico and the Caribbean Sea, where changes are on the order of ±10% over the full Atlantic hurricane season, with considerably stronger responses focused in the two halves of the season. Diagnosis of the Genesis Potential Index (GPI) indicates that changes in mid-tropospheric humidity and vertical wind shear are the most important factors driving these responses. The simulated changes in GPI occur in regions of considerable historical TC genesis, highlighting the need to further understand the historical and projected future patterns of SST change in the North Atlantic Ocean, including their relationship to AMOC and its potential decline. 

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4. Mechanisms of Low-Frequency Variability in North Atlantic Ocean Heat Transport and AMOC

   https://doi.org/10.1175/JCLI-D-20-0614.1  

   Oldenburg, Dylan; Wills, Robert C.; Armour, Kyle C.; Thompson, LuAnne; Jackson, Laura C. (March 2021, Journal of Climate)

   null (Ed.)

   Abstract Ocean heat transport (OHT) plays a key role in climate and its variability. Here, we identify modes of low-frequency North Atlantic OHT variability by applying a low-frequency component analysis (LFCA) to output from three global climate models. The first low-frequency component (LFC), computed using this method, is an index of OHT variability that maximizes the ratio of low-frequency variance (occurring at decadal and longer timescales) to total variance. Lead-lag regressions of atmospheric and ocean variables onto the LFC timeseries illuminate the dominant mechanisms controlling low-frequency OHT variability. Anomalous northwesterly winds from eastern North America over the North Atlantic act to increase upper ocean density in the Labrador Sea region, enhancing deep convection, which later increases OHT via changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The strengthened AMOC carries warm, salty water into the subpolar gyre, reducing deep convection and weakening AMOC and OHT. This mechanism, where changes in AMOC and OHT are driven primarily by changes in Labrador Sea deep convection, holds not only in models where the climatological (i.e., time-mean) deep convection is concentrated in the Labrador Sea, but also in models where the climatological deep convection is concentrated in the Greenland-Iceland-Norwegian (GIN) Seas or the Irminger and Iceland Basins. These results suggest that despite recent observational evidence suggesting that the Labrador Sea plays a minor role in driving the climatological AMOC, the Labrador Sea may still play an important role in driving low-frequency variability in AMOC and OHT. 

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5. Mechanisms of Decadal North Atlantic Climate Variability and Implications for the Recent Cold Anomaly

   https://doi.org/10.1175/JCLI-D-20-0464.1  

   Årthun, Marius; Wills, Robert C.; Johnson, Helen L.; Chafik, Léon; Langehaug, Helene R. (May 2021, Journal of Climate)

   null (Ed.)

   Abstract Decadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability. 

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