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1. Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part I: Rainfall Response

   https://doi.org/10.1175/JCLI-D-22-0172.1  

   Bao, Yuntao ; Liu, Zhengyu ; He, Chengfei ( July 2023 , Journal of Climate)

   Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation and precipitationδ¹⁸O (δ¹⁸O_p) response have remained controversial, and their response mechanisms are unclear. In particular, it is not clear whether the regional precipitation represents the intensity of the millennial South American summer monsoon (SASM). Here, we study the TSA hydroclimate variability during the last deglaciation (20–11 ka ago) by combining transient simulations of an isotope-enabled Community Earth System Model (iCESM) and the speleothem records over the lowland TSA. Our model reasonably simulates the deglacial evolution of hydroclimate variables and water isotopes over the TSA, albeit underestimating the amplitude of variability. North Atlantic meltwater discharge is the leading factor driving the TSA’s millennial hydroclimate variability. The spatial pattern of both precipitation andδ¹⁸O_pshow a northwest–southeast dipole associated with the meridional migration of the intertropical convergence zone, instead of a continental-wide coherent change as inferred in many previous works on speleothem records. The dipole response is supported by multisource paleoclimate proxies. In response to increased meltwater forcing, the SASM weakened (characterized by a decreased low-level easterly wind) and consequently reduced rainfall in the western Amazon and increased rainfall in eastern Brazil. A similar dipole response is also generated by insolation, ice sheets, and greenhouse gases, suggesting an inherent stability of the spatial characteristics of the SASM regardless of the external forcing and time scales. Finally, we discuss the potential reasons for the model–proxy discrepancy and pose the necessity to build more paleoclimate proxy data in central-western Amazon.

   We want to reconcile the controversy on whether there is a coherent or heterogeneous response in millennial hydroclimate over tropical South America and to clearly understand the forcing mechanisms behind it. Our isotope-enabled transient simulations fill the gap in speleothem reconstructions to capture a complete picture of millennial precipitation/δ¹⁸O_pand monsoon intensity change. We highlight a heterogeneous dipole response in precipitation andδ¹⁸O_pon millennial and orbital time scales. Increased meltwater discharge shifts ITCZ southward and favors a wet condition in coastal Brazil. Meanwhile, the low-level easterly and the summer monsoon intensity reduced, causing a dry condition in the central-western Amazon. However, the millennial variability of hydroclimate response is underestimated in our model, together with the lack of direct paleoclimate proxies in the central-west Amazon, complicating the interpretation of changes in specific paleoclimate events and posing a challenge to constraining the spatial range of the dipole. Therefore, we emphasize the necessity to increase the source of proxies, enhance proxy interpretations, and improve climate model performance in the future.

    

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2. Monitoring of Sierra Nevada Caves Reveals the Potential for Stalagmites to Archive Seasonal Variability

   https://doi.org/10.3389/feart.2021.781526  

   Wortham, Barbara E. ; Montañez, Isabel P. ; Bowman, Kimberly ; Kuta, Daphne ; Contreras, Nora Soto ; Brummage, Eleana ; Pang, Allison ; Tinsley, John ; Roemer-Baer, Greg ( December 2021 , Frontiers in Earth Science)

   In the southwestern United States, California (CA) is one of the most climatically sensitive regions given its low (≤250 mm/year) seasonal precipitation and its inherently variable hydroclimate, subject to large magnitude modulation. To reconstruct past climate change in CA, cave calcite deposits (stalagmites) have been utilized as an archive for environmentally sensitive proxies, such as stable isotope compositions (δ¹⁸O, δ¹³C) and trace element concentrations (e.g., Mg, Ba, Sr). Monitoring the cave and associated surface environments, the chemical evolution of cave drip-water, the calcite precipitated from the drip-water, and the response of these systems to seasonal variability in precipitation and temperature is imperative for interpreting stalagmite proxies. Here we present monitored drip-water and physical parameters at Lilburn Cave, Sequoia Kings Canyon National Park (Southern Sierra Nevada), CA, and measured trace element concentrations (Mg, Sr, Ba, Cu, Fe, Mn) and stable isotopic compositions (δ¹⁸O, δ²H) of drip-water and for calcite (δ¹⁸O) precipitated on glass substrates over a two-year period (November 2018 to February 2021) to better understand how chemical variability at this site is influenced by local and regional precipitation and temperature variability. Despite large variability in surface temperatures and precipitation amount and source region (North Pacific vs. subtropical Pacific), Lilburn Cave exhibits a constant cave environment year-round. At two of the three sites within the cave, drip-water δ¹⁸O and δ²H are influenced seasonally by evaporative enrichment. At a third collection site in the cave, the drip-water δ¹⁸O responds solely to precipitation δ¹⁸O variability. The Mg/Ca, Ba/Ca, and Sr/Ca ratios are seasonally responsive to prior calcite precipitation at all sites but minimally to water-rock interaction. Lastly, we examine the potential of trace metals (e.g., Mn²⁺and Cu²⁺as a geochemical proxy of recharge and find that variability in their concentrations has high potential to denote the onset of the rainy season in the study region. The drip-water composition is recorded in the calcite, demonstrating that stalagmites from Lilburn Cave, and potentially more regionally, could record seasonal variability in weather even during periods of substantially reduced rainfall.

    

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3. California margin temperatures modulate regional circulation and extreme summer precipitation in the desert Southwest

   https://doi.org/10.1088/1748-9326/acfd43  

   Bhattacharya, Tripti ; Feng, Ran ; Maupin, Christopher R. ; Coats, Sloan ; Brennan, Peter R. ; Carter, Elizabeth ( October 2023 , Environmental Research Letters)

   In August 2022, Death Valley, the driest place in North America, experienced record flooding from summertime rainfall associated with the North American monsoon (NAM). Given the socioeconomic cost of these type of events, there is a dire need to understand their drivers and future statistics. Existing theory predicts that increases in the intensity of precipitation is a robust response to anthropogenic warming. Paleoclimatic evidence suggests that northeast Pacific (NEP) sea surface temperature (SST) variability could further intensify summertime NAM rainfall over the desert southwest. Drawing on this paleoclimatic evidence, we use historical observations and reanalyzes to test the hypothesis that warm SSTs on the southern California margin are linked to more frequent extreme precipitation events in the NAM domain. We find that summers with above-average coastal SSTs are more favorable to moist convection in the northern edge of the NAM domain (southern California, Arizona, New Mexico, and the southern Great Basin). This is because warmer SSTs drive circulation changes that increase moisture flux into the desert southwest, driving more frequent precipitation extremes and increases in seasonal rainfall totals. These results, which are robust across observational products, establish a linkage between marine and terrestrial extremes, since summers with anomalously warm SSTs on the California margin have been linked to seasonal or multi-year NEP marine heatwaves. However, current generation earth system models (ESMs) struggle to reproduce the observed relationship between coastal SSTs and NAM precipitation. Across models, there is a strong negative relationship between the magnitude of an ESM’s warm SST bias on the California margin and its skill at reproducing the correlation with desert southwest rainfall. Given persistent NEP SST biases in ESMs, our results suggest that efforts to improve representation of climatological SSTs are crucial for accurately predicting future changes in hydroclimate extremes in the desert southwest.

    

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4. The Unprecedented Character of California's 20th Century Enhanced Hydroclimatic Variability in a 600‐Year Context

   https://doi.org/10.1029/2022GL099582  

   Zamora‐Reyes, Diana ; Broadman, Ellie ; Bigio, Erica ; Black, Bryan ; Meko, David ; Woodhouse, Connie A. ; Trouet, Valerie ( October 2022 , Geophysical Research Letters)

   Recently, year‐to‐year swings in California winter precipitation extremes have resulted in drought, wildfires, and floods causing billions of dollars in damage. These recent precipitation swings represent an increasing trend in variability of California's hydroclimate over the past decades. Here, we put this trend in a longer‐term context using tree‐ring‐based precipitation, streamflow, and snow water equivalent reconstructions. We show that the statewide rise in hydroclimate variability in the 20th century is driven by an increasing trend in the magnitude of wet extremes. A prior period of strong variability in the 16th century, in contrast, is related to an increasing trend in the magnitude of dry extremes. Our results are consistent with climate model simulations that suggest an increasingly volatile future for California's hydroclimate and highlight the importance of collaboration between scientists and water resource managers to incorporate this increased variability into their decision‐making and planning, acknowledging higher risks for compound events.

    

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5. Evidence for Decreased Precipitation Variability in the Yucatán Peninsula During the Mid‐Holocene

   https://doi.org/10.1029/2021PA004219  

   Serrato Marks, Gabriela ; Medina‐Elizalde, Martín ; Burns, Stephen ; Weldeab, Syee ; Lases‐Hernandez, Fernanda ; Cazares, Gabriela ; McGee, David ( May 2021 , Paleoceanography and Paleoclimatology)

   The Yucatán Peninsula (YP) has a complex hydroclimate with many proposed drivers of interannual and longer‐term variability, ranging from coupled ocean–atmosphere processes to frequency of tropical cyclones. The mid‐Holocene, a time of higher Northern Hemisphere summer insolation, provides an opportunity to test the relationship between YP precipitation and ocean temperature. Here, we present a new, ∼annually resolved speleothem record of stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg/Ca and Sr/Ca) ratios for a section of the mid‐Holocene (5.2–5.7 kyr BP), before extensive agriculture began in the region. A meter‐long stalagmite from Río Secreto, a cave system in Playa del Carmen, Mexico, was dated using U–Th geochronology and layer counting, yielding multidecadal age uncertainty (median 2SD of ±70 years). New proxy data were compared to an existing late Holocene stalagmite record from the same cave system, allowing us to examine changes in hydrology over time and to paleoclimate records from the southern YP. The δ¹⁸O, δ¹³C, and Mg/Ca data consistently indicate higher mean precipitation and lower precipitation variability during the mid‐Holocene compared to the late Holocene. Despite this reduced variability, multidecadal precipitation variations were persistent in regional hydroclimate during the mid‐Holocene. We therefore conclude that higher summer insolation led to increased mean precipitation and decreased precipitation variability in the northern YP but that the region is susceptible to dry periods across climate mean states. Given projected decreases in wet season precipitation in the YP’s near future, we suggest that climate mitigation strategies emphasize drought preparation.

    

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