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1. North Atlantic meltwater during Heinrich Stadial 1 drives wetter climate with more atmospheric rivers in western North America.

   Oster, Jessica L; Macarewich, Sophia; Lofverstrom, Marcus; de_Wet, Cameron; Montañez, Isabel; Lora, Juan; Skinner, Christopher; Tabor, Clay (November 2023, Science advances)

   Atmospheric rivers (ARs) bring concentrated rainfall and flooding to the western United States (US) and are hypothesized to have supported sustained hydroclimatic changes in the past. However, their ephemeral nature makes it challenging to document ARs in climate models and estimate their contribution to hydroclimate changes recorded by time-averaged paleoclimate archives. We present new climate model simulations of Heinrich Stadial 1 (HS1; 16,000 years before the present), an interval characterized by widespread wetness in the western US, that demonstrate increased AR frequency and winter precipitation sourced from the southeastern North Pacific. These changes are amplified with freshwater fluxes into the North Atlantic, indicating that North Atlantic cooling associated with weakened Atlantic Meridional Overturning Circulation (AMOC) is a key driver of HS1 climate in this region. As recent observations suggest potential weakening of AMOC, our identified connection between North Atlantic climate and northeast Pacific AR activity has implications for future western US hydroclimate. 

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2. The Influence of Carbon Dioxide and Precession on Western North American Hydroclimate and Pacific Sea Surface Temperature During the Holocene

   https://doi.org/10.1029/2024PA005079  

   Cheung, Anson H; Du, Xiaojing; Parish, Meredith C; Vachula, Richard S; Fox‐Kemper, Baylor; Herbert, Timothy D (July 2025, Paleoceanography and Paleoclimatology)

   Abstract Model‐based projections of hydroclimate in western North America (wNA) remain uncertain and depend on how Pacific sea surface temperature (SST) will evolve in the future. However, whether climate models can accurately capture Pacific SST changes and its relationship with wNA hydroclimate in the future remains elusive. Here, we use a synthesis of proxy records and idealized model simulations to elucidate the spatiotemporal evolution and the forcings that drive wNA hydroclimate and Pacific SST during the Holocene (past ∼11,000 years), when the boundary conditions are different from the present. We find that wNA hydroclimate and Pacific SST co‐evolved during the Holocene, where wNA became wetter while the eastern equatorial Pacific and the north Pacific became warmer toward the present. We attribute changes in wNA hydroclimate to precession and carbon dioxide changes, but we are unable to attribute Pacific SST changes unambiguously to any forcing. Our analysis offers a framework to understand the relationship between wNA hydroclimate and Pacific SST and provides an empirical assessment of how these two regions are related over time. 

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3. The Hydrologic Cycle and Atmospheric Rivers in CESM2 Simulations of the Last Glacial Maximum

   https://doi.org/10.1029/2023GL104805  

   Lora, J. M.; Skinner, C. B.; Rush, W. D.; Baek, S. H. (September 2023, Geophysical Research Letters)

   Abstract Proxy reconstructions and model simulations of precipitation during Earth's glacial periods suggest that the locations and mechanisms of atmospheric moisture transport have changed considerably during Earth's past. We investigate the hydroclimate of the Last Glacial Maximum (LGM) using simulations with the Community Earth System Model, with a focus on the extratropics and the influence of atmospheric rivers (ARs), a key driver of modern‐day moisture transport globally. Mean and extreme precipitation increase significantly over southwestern Patagonia, Iberia, and southwestern North America—mid‐latitude regions affected by ARs in the modern climate—despite overall decreases elsewhere. In each, the associated moisture transport changes are different, with increased transport and AR activity mainly occurring in the North Atlantic. The overall LGM response is dominated by the response to ice sheets, with other forcings causing additional cooling and drying over the extratropics and a strong decrease of moisture transport over the subpolar North Atlantic. 

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4. Deglacial Temperature and Carbonate Saturation State Variability in the Tropical Atlantic at Antarctic Intermediate Water Depths

   https://doi.org/10.1029/2023PA004674  

   Oppo, D W; Lu, W; Huang, K‐F; Umling, N E; Guo, W; Yu, J; Curry, W B; Marchitto, T M; Wang, S (September 2023, Paleoceanography and Paleoclimatology)

   Abstract Variations in the Atlantic Meridional Overturning Circulation (AMOC) redistribute heat and nutrients, causing pronounced anomalies of temperature and nutrient concentrations in the subsurface ocean. However, exactly how millennial‐scale deglacial AMOC variability influenced the subsurface is debated, and the role of other deglacial forcings of subsurface temperature change is unclear. Here, we present a new deglacial temperature reconstruction, which, with published records, helps assess competing hypotheses for deglacial warming in the upper tropical North Atlantic. Our record provides new evidence of regional subsurface warming in the western tropical North Atlantic within the core of modern Antarctic Intermediate Water (AAIW) during Heinrich Stadial 1 (HS1), an early deglacial interval of iceberg discharge into the North Atlantic. Our results are consistent with model simulations that suggest subsurface heat accumulates in the northern high‐latitude convection regions and along the upper AMOC return path when the AMOC weakens, and with warming due to rising greenhouse gases. Warming of AAIW may have also contributed to warming in the tropics at modern AAIW depths during late HS1. Nutrient andreconstructions from the same site suggest a link between AMOC intensity and the northward extent of AAIW in the northern tropics across the deglaciation and on millennial time scales. However, the timing of the initial deglacial increase in AAIW to the northern tropics is ambiguous. Deglacial trends and variability ofin the upper North Atlantic have likely biased temperature reconstructions based on the elemental composition of calcitic benthic foraminifera. 

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5. Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

   https://doi.org/10.1175/JPO-D-20-0060.1  

   Sun, Shantong; Thompson, Andrew F.; Eisenman, Ian (August 2020, Journal of Physical Oceanography)

   null (Ed.)

   Abstract Climate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate. 

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