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1. Last millennium hurricane activity linked to endogenous climate variability

   https://doi.org/10.1038/s41467-024-45112-6  

   Yang, Wenchang; Wallace, Elizabeth; Vecchi, Gabriel A; Donnelly, Jeffrey P; Emile-Geay, Julien; Hakim, Gregory J; Horowitz, Larry W; Sullivan, Richard M; Tardif, Robert; van_Hengstum, Peter J; et al (December 2024, Nature Communications)

   Abstract Despite increased Atlantic hurricane risk, projected trends in hurricane frequency in the warming climate are still highly uncertain, mainly due to short instrumental record that limits our understanding of hurricane activity and its relationship to climate. Here we extend the record to the last millennium using two independent estimates: a reconstruction from sedimentary paleohurricane records and a statistical model of hurricane activity using sea surface temperatures (SSTs). We find statistically significant agreement between the two estimates and the late 20th century hurricane frequency is within the range seen over the past millennium. Numerical simulations using a hurricane-permitting climate model suggest that hurricane activity was likely driven by endogenous climate variability and linked to anomalous SSTs of warm Atlantic and cold Pacific. Volcanic eruptions can induce peaks in hurricane activity, but such peaks would likely be too weak to be detected in the proxy record due to large endogenous variability. 

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2. Surface climate signals transmitted rapidly to deep North Atlantic throughout last millennium

   https://doi.org/10.1126/science.adf1646  

   Lu, Wanyi; Oppo, Delia W.; Gebbie, Geoffrey; Thornalley, David J. (November 2023, Science)

   Instrumental observations of subsurface ocean warming imply that ocean heat uptake has slowed 20th-century surface warming. We present high-resolution records from subpolar North Atlantic sediments that are consistent with instrumental observations of surface and deep warming/freshening and in addition reconstruct the surface-deep relation of the last 1200 years. Sites from ~1300 meters and deeper suggest an ~0.5 degrees celsius cooling across the Medieval Climate Anomaly to Little Ice Age transition that began ~1350 ± 50 common era (CE), whereas surface records suggest asynchronous cooling onset spanning ~600 years. These data suggest that ocean circulation integrates surface variability that is transmitted rapidly to depth by the Atlantic Meridional Ocean Circulation, implying that the ocean moderated Earth’s surface temperature throughout the last millennium as it does today. 

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3. Last Millennium Reanalysis with an expanded proxy database and seasonal proxy modeling

   https://doi.org/10.5194/cp-15-1251-2019  

   Tardif, Robert; Hakim, Gregory J.; Perkins, Walter A.; Horlick, Kaleb A.; Erb, Michael P.; Emile-Geay, Julien; Anderson, David M.; Steig, Eric J.; Noone, David (January 2019, Climate of the Past)

   null (Ed.)

   Abstract. The Last Millennium Reanalysis (LMR) utilizes an ensemble methodology to assimilate paleoclimate data for the production of annually resolved climate field reconstructions of the Common Era. Two key elements are the focus of this work: the set of assimilated proxy records and the forward models that map climate variables to proxy measurements. Results based on an updated proxy database and seasonal regression-based forward models are compared to the LMR prototype, which was based on a smaller set of proxy records and simpler proxy models formulated as univariate linear regressions against annual temperature. Validation against various instrumental-era gridded analyses shows that the new reconstructions of surface air temperature and 500 hPa geopotential height are significantly improved (from 10 % to more than 100 %), while improvements in reconstruction of the Palmer Drought Severity Index are more modest. Additional experiments designed to isolate the sources of improvement reveal the importance of the updated proxy records, including coral records for improving tropical reconstructions, and tree-ring density records for temperature reconstructions, particularly in high northern latitudes. Proxy forward models that account for seasonal responses, and dependence on both temperature and moisture for tree-ring width, also contribute to improvements in reconstructed thermodynamic and hydroclimate variables in midlatitudes. The variability of temperature at multidecadal to centennial scales is also shown to be sensitive to the set of assimilated proxies, especially to the inclusion of primarily moisture-sensitive tree-ring-width records. 

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4. Persistent high latitude amplification of the Pacific Ocean over the past 10 million years

   https://doi.org/10.1038/s41467-022-35011-z  

   Liu, Xiaoqing; Huber, Matthew; Foster, Gavin L.; Dessler, Andrew; Zhang, Yi Ge (December 2022, Nature Communications)

   Abstract While high latitude amplification is seen in modern observations, paleoclimate records, and climate modeling, better constraints on the magnitude and pattern of amplification would provide insights into the mechanisms that drive it, which remain actively debated. Here we present multi-proxy multi-site paleotemperature records over the last 10 million years from the Western Pacific Warm Pool (WPWP) – the warmest endmember of the global ocean that is uniquely important in the global radiative feedback change. These sea surface temperature records, based on lipid biomarkers and seawater Mg/Ca-adjusted foraminiferal Mg/Ca, unequivocally show warmer WPWP in the past, and a secular cooling over the last 10 million years. Compiling these data with existing records reveals a persistent, nearly stationary, extratropical response pattern in the Pacific in which high latitude (~50°N) temperatures increase by ~2.4° for each degree of WPWP warming. This relative warming pattern is also evident in model outputs of millennium-long climate simulations with quadrupling atmospheric CO 2 , therefore providing a strong constraint on the future equilibrium response of the Earth System. 

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5. South American monsoon intensification during the last millennium driven by joint Pacific and Atlantic forcing

   https://doi.org/10.1126/sciadv.ado9543  

   Lyu, Zhiqiang; Vuille, Mathias; Goosse, Hugues; Orrison, Rebecca; Novello, Valdir F; Cruz, Francisco W; Stríkis, Nicolás M; Cauhy, Julio (September 2024, Science Advances)

   The South American summer monsoon (SASM) profoundly influences tropical South America’s climate, yet understanding its low-frequency variability has been challenging. Climate models and oxygen isotope data have been used to examine the SASM variability over the last millennium (LM) but have, at times, provided conflicting findings, especially regarding its mean-state change from the Medieval Climate Anomaly to the Little Ice Age. Here, we use a paleoclimate data assimilation (DA) method, combining model results and δ¹⁸O observations, to produce a δ¹⁸O-enabled, dynamically coherent, and spatiotemporally complete austral summer hydroclimate reconstruction over the LM for tropical South America at 5-year resolution. This reconstruction aligns with independent hydroclimate and δ¹⁸O records withheld from the DA, revealing a centennial-scale SASM intensification during the MCA-LIA transition period, associated with the southward shift of the Atlantic Intertropical Convergence Zone and the strengthening Pacific Walker circulation (PWC). This highlights the necessity of accurately representing the PWC in climate models to predict future SASM changes. 

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