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1. Demystifying the drivers of the spring warming asymmetry between Eurasia and North America

   https://doi.org/10.1126/sciadv.adu2364  

   Ding, Xiuyuan; Chen, Gang; Wang, Yuan; Sun, Lantao (May 2025, Science Advances)

   In spring, global warming exhibits prominent zonal asymmetry at continental scales, with Eurasia warming three times faster than North America during 1979–2021. Meanwhile, snow loss is also highly asymmetric. These changes are critical for regional agriculture and water management, yet the roles of specific forcings behind them remain unclear. Based on hierarchical large-ensemble climate model simulations, ~32 ± 28% of the surface air temperature (SAT) asymmetric trend and 16 ± 13% of the snow cover asymmetric trend between Eurasia and North America are attributable to tropical Pacific variability. Single-forcing experiments reveal that anthropogenic aerosols can induce a comparable asymmetry, accounting for 34 ± 23% (24 ± 17%) of observed SAT (snow cover) asymmetric trends. However, their effects are largely masked by the greenhouse gas forcing. As anthropogenic aerosol emissions are expected to decline, the current warming asymmetry may reverse in the future. 

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2. Drought and Flood Extremes on the Amazon River and in Northeast Brazil, 1790–1900

   https://doi.org/10.1175/JCLI-D-23-0146.1  

   Granato-Souza, Daniela; Stahle, David W (October 2023, Journal of Climate)

   Abstract Recent severe droughts, extreme floods, and increasing differences between seasonal high and low flows on the Amazon River may represent a twenty-first-century increase in the amplitude of the hydrologic cycle over the Amazon Basin. These precipitation and streamflow changes may have arisen from natural ocean–atmospheric variability, deforestation within the drainage basin of the Amazon River, or anthropogenic climate change. Tree-ring reconstructions of wet-season precipitation extremes, substantiated with historical accounts of climate and river levels on the Amazon River and in northeast Brazil found in the Brazilian Digital Library, indicate that the recent river-level extremes on the Amazon may have been equaled or possibly exceeded during the preinstrumental nineteenth century. The “Forgotten Drought” of 1865 was the lowest wet-season rainfall total reconstructed with tree-rings in the eastern Amazon from 1790 to 2016 and appears to have been one of the lowest stream levels observed on the Amazon River during the historical era according to first-hand descriptions by Louis Agassiz, his Brazilian colleague João Martins da Silva Coutinho, and others. Heavy rains and flooding are described during most of the tree-ring-reconstructed wet extremes, including the complete inundation of “First Street” in Santarem, Brazil, in 1859 and the overtopping of the Bittencourt Bridge in Manaus, Brazil, in 1892. These extremes in the tree-ring estimates and historical observations indicate that recent high and low flow anomalies on the Amazon River may not have exceeded the natural variability of precipitation and streamflow during the nineteenth century. Significance StatementProxy tree-ring and historical evidence for precipitation extremes during the preinstrumental nineteenth century indicate that recent floods and droughts on the Amazon River may have not yet exceeded the range of natural hydroclimatic variability. 

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3. The Strength of Coupling to the Southern Ocean Modulates Tropical Eastern Pacific Variability and Forced Response

   https://doi.org/10.1175/JCLI-D-25-0226.1  

   Espinosa, Zachary I; Zelinka, Mark D; Bitz, Cecilia M; Armour, Kyle C (March 2026, Journal of Climate)

   Abstract Despite rising global-mean temperatures, large parts of the Southern Ocean and tropical eastern Pacific Ocean have cooled during the satellite era. These regions may be linked by teleconnections, with Southern Ocean cooling contributing to tropical eastern Pacific cooling. We demonstrate that, on average, state-of-the-art Earth system models (ESMs) underestimate the magnitude of interaction between the Southern Ocean and tropical eastern Pacific Ocean. The strength of the teleconnection is shown to be mediated by the magnitude of the positive cloud–sea surface temperature (SST) feedback in the subtropical eastern Pacific Ocean and the strength of the wind–evaporation–SST (WES) feedback. We link excessive precipitation in the tropical Pacific south of the equator to the strength of the Southern Ocean–eastern Pacific teleconnection. This model bias, known as the double intertropical convergence zone (ITCZ), is shown to be related to erroneous convection south of the equator, weakened cross-equatorial trade winds, and unfavorable meteorological conditions for marine boundary layer subtropical clouds. We postulate there is a two-way interaction, in which a double-ITCZ occurs with weaker cloud–SST and WES feedbacks, which in turn impact local SSTs and amplify the double-ITCZ. Models with a stronger Southern Ocean to tropical Pacific teleconnection tend to exhibit more multidecadal variability in the Walker circulation, ITCZ, and west–east equatorial SST gradient, as well as greater delayed warming in the tropical eastern Pacific Ocean resulting from delayed Southern Ocean warming under greenhouse gas forcing. These results provide insight into why ESMs struggle to replicate observed tropical Pacific temperature trend patterns and point to ITCZ location as a key target for improvement in future model development. Significance StatementThe key advancement of this study is to demonstrate that, on average, state-of-the-art Earth system models underestimate the magnitude of interaction between the Southern Ocean and tropical east Pacific. As a result, historical cooling in the Southern Ocean may explain a larger fraction of observed east Pacific cooling than previously appreciated. Initial evidence suggests unrealistic precipitation simulated by models in the southeast equatorial Pacific may result in a “blocking” of high latitude influence due to its impact on the magnitude of the cloud–SST feedback and response of easterly trade winds. These results improve our understanding of the processes controlling the Southern Ocean–eastern Pacific teleconnection and provide a guide for future model development and climate trend attribution. 

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4. Impacts of Different Anthropogenic Forcings on Tropical Cyclone Frequency in Large Ensemble Climate Model Simulations

   https://doi.org/10.1175/JCLI-D-25-0442.1  

   Henry, Sarah; Wang, Zhuo; Min, Seung-Ki; Ramsay, Hamish (October 2026, Journal of Climate)

   Abstract To better understand the effects of anthropogenic forcings on tropical cyclone (TC) activity, we investigate the individual impacts of anthropogenic aerosols (AAER), greenhouse gases (GHG), and biomass burning aerosols (BMB) on TC frequency using Community Earth System Model, version 2 (CESM2), large ensemble simulations. Changes in TC genesis are estimated using a dynamic genesis potential index (DGPI), where distinct impacts from AAER and GHG are revealed. Increasing GHG emissions result in a decrease and southward shift in DGPI over the North Atlantic and eastern North Pacific since 1940, whereas the time-dependent AAER emissions lead to a decrease and southward shift in DGPI before 1980s and an increase and northward shift thereafter. In addition, BMB emission increases lead to a slight increase in TC frequency in the North Atlantic. Various dynamic and thermodynamic parameters act cooperatively to change TC frequency associated with GHG and AAER over the North Atlantic, while the increase in DGPI associated with BMB is mainly due to the changes in dynamic variables. AAER effects largely dominate DGPI changes in earlier decades, but GHG effects become increasingly dominant in recent decades. Near-future projections, characterized by continued increases in GHG emissions and steady or declined AAER emissions, suggest decreased DGPI over the North Atlantic and eastern North Pacific driven primarily by GHG forcing. A comparison between DGPI and an alternative, thermodynamic genesis potential index (GPI) shows similar increases or decreases in TC genesis potential with quantitative differences. However, comparisons between CESM2, CESM1, and six CMIP6 models show considerable intermodel spread in DGPI response, highlighting model uncertainties and the need for caution when applying and interpreting multimodel ensemble means. Significance StatementThis study explores how different types of human-caused emissions—greenhouse gases, anthropogenic aerosols, and biomass burning aerosols—affect tropical cyclone frequency. Using Community Earth System Model, version 2 (CESM2), model simulations, we find that greenhouse gases lead to fewer tropical cyclones and a southward shift in their formation since the 1940s. In contrast, the time-dependent anthropogenic aerosol emissions reduce cyclone frequency before the 1980s but contribute to an increase afterward, along with a northward shift in cyclone formation. Anthropogenic aerosols have had a stronger influence on tropical cyclone frequency in earlier decades, while greenhouse gases have become the dominant driver, leading to the emergence of the anthropogenic signals in tropical cyclone frequency. However, large differences in historical attributions and near-future projections exist between different models. 

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5. Gross discrepancies between observed and simulated 20th-21st-century precipitation trends in Southeastern South America

   https://doi.org/10.1175/JCLI-D-20-0746.1  

   Varuolo-Clarke, Arianna M.; Smerdon, Jason E.; Williams, A. Park; Seager, Richard (May 2021, Journal of Climate)

   null (Ed.)

   Abstract Southeastern South America (SESA; encompassing Paraguay, Southern Brazil, Uruguay, and northern Argentina) experienced a 27% increase in austral summer precipitation from 1902-2019, one of the largest observed trends in seasonal precipitation globally. Previous research identifies Atlantic Multidecadal Variability and anthropogenic forcing from stratospheric ozone depletion and greenhouse gas emissions as key factors contributing to the positive precipitation trends in SESA. We analyze multi-model ensemble simulations from Phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) and find that not only do Earth System Models simulate positive SESA precipitation trends that are much weaker over the historical interval, but some models persistently simulate negative SESA precipitation trends under historical forcings. Similarly, 16-member ensembles from two atmospheric models forced with observed historical sea surface temperatures never simulate precipitation trends that even reach the lower bound of the observed trend’s range of uncertainty. Moreover, while future 21 st -century projections from CMIP6 yield positive ensemble mean precipitation trends over SESA that grow with increasing greenhouse-gas emissions, the mean forced response never exceeds the observed historical trend. Pre-industrial control runs from CMIP6 indicate that some models do occasionally simulate centennial-scale trends in SESA that fall within the observational range, but most models do not. Results point to significant uncertainties in the attribution of anthropogenically forced influences on the observed increases in precipitation over SESA, while also suggesting that internal decadal-to-centennial variability of unknown origin and not present in state-of-the-art models may have also played a large role in generating the 20 th -21 st -century SESA precipitation trend. 

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