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1. Multidecadal Modulation of the ENSO Teleconnection to Precipitation and Tree Growth Over Subtropical North America

   https://doi.org/10.1029/2018PA003510  

   Torbenson, M. C. A. ; Stahle, D. W. ; Howard, I. M. ; Burnette, D. J. ; Villanueva‐Díaz, J. ; Cook, E. R. ; Griffin, D. ( May 2019 , Paleoceanography and Paleoclimatology)

   The teleconnection of the El Niño/Southern Oscillation (ENSO) to instrumental precipitation and temperature during the cool season over North America is strongest and most temporally stable in the TexMex sector of northern Mexico and the borderlands of southwestern United States. The ENSO impact on North American hydroclimate expands and contracts out of this region on multidecadal timescales, possibly associated with the positive and negative phases of the Atlantic Multidecadal Oscillation. A subset of tree‐ring chronologies from the TexMex sector also has the strongest and most stable ENSO signal detected in the North American network, similar to the strong ENSO signal measured in instrumental climate data from the same region. This subset of chronologies is used to reconstruct the multivariate ENSO index (MEI) as a measure of ENSO impact on North American hydroclimate during the instrumental and preinstrumental eras. The reconstruction exhibits improved fidelity in the frequency domain and better registration of spatial changes in ENSO signal over North America when compared to an MEI reconstruction based on all ENSO‐correlated tree‐ring chronologies irrespective of temporal stability of correlation. When correlated with gridded instrumental and tree‐ring reconstructed Palmer drought indices across North America, the stable MEI estimate reproduces the changes in spatial impact of ENSO signal measured with instrumental data, and it reveals similar multidecadal changes in prehistory, potentially linked to the Atlantic Multidecadal Oscillation. The Great Plains drought of the 1850s and 1860s may have been an example of this Pacific‐Atlantic configuration.

    

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2. Streamflow Variability Indicated by False Rings in Bald Cypress (Taxodium distichum (L.) Rich.)

   https://doi.org/10.3390/f11101100  

   Therrell, Matthew D. ; Elliott, Emily A. ; Meko, Matthew D. ; Bregy, Joshua C. ; Tucker, Clay S. ; Harley, Grant L. ; Maxwell, Justin T. ; Tootle, Glenn A. ( October 2020 , Forests)

   Despite growing in wet lowland and riparian settings, Taxodium distichum (L.) Rich. (bald cypress) has a strong response to hydroclimate variability, and tree ring chronologies derived from bald cypress have been used extensively to reconstruct drought, precipitation and streamflow. Previous studies have also demonstrated that false rings in bald cypress appear to be the result of variations in water availability during the growing season. In this study 28 trees from two sites located adjacent to the Choctawhatchee River in Northwestern Florida, USA were used to develop a false ring record extending from 1881 to 2014. Twenty false ring events were recorded during the available instrumental era (1931–2014). This record was compared with daily and monthly streamflow data from a nearby gage. All 20 of the false-ring events recorded during the instrumental period occurred during years in which greatly increased streamflow occurred late in the growing season. Many of these wet events appear to be the result of rainfall resulting from landfalling tropical cyclones. We also found that the intra-annual position of false rings within growth rings reflects streamflow variability and combining the false-ring record with tree ring width chronologies improves the estimation of overall summer streamflow by 14%. Future work using these and other quantitative approaches for the identification and measurement of false ring variables in tree rings may improve tree-ring reconstructions of streamflow and potentially the record of tropical cyclone rainfall events. 

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3. Sub‐Seasonal Tree‐Ring Reconstructions for More Comprehensive Climate Records in U.S. West Coast Watersheds

   https://doi.org/10.1029/2020GL091598  

   Wise, Erika K. ( January 2021 , Geophysical Research Letters)

   Tree‐ring records of preinstrumental hydroclimate, which contribute needed context for understanding recent drought and flood events, typically provide one value per year that represents the entire year or one particular season. This may introduce a seasonal bias to the records or omit seasonally variable moisture. Here, I use tree‐ring records to reconstruct precipitation and runoff in 28 U.S. West Coast watersheds for running 1, 3, 6, and 12‐month intervals. When compared on a yearly basis, the Monthly and Four‐Season models have higher overall skill and better extreme capture in most basins than the Cool‐Warm and Annual models. The Monthly and Four‐Season versions also decrease model error in years with more intense precipitation and retain more variability in the preinstrumental period. Improved capture of year‐round moisture can provide a more complete representation of the preinstrumental past and strengthen capacity to study shorter‐duration and season‐specific events like atmospheric rivers.

    

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4. Six hundred years of South American tree rings reveal an increase in severe hydroclimatic events since mid-20th century

   https://doi.org/10.1073/pnas.2002411117  

   Morales, Mariano S. ; Cook, Edward R. ; Barichivich, Jonathan ; Christie, Duncan A. ; Villalba, Ricardo ; LeQuesne, Carlos ; Srur, Ana M. ; Ferrero, M. Eugenia ; González-Reyes, Álvaro ; Couvreux, Fleur ; et al ( July 2020 , Proceedings of the National Academy of Sciences)

   South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions. 

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5. Drought increase since the mid-20th century in the northern South American Altiplano revealed by a 389-year precipitation record

   https://doi.org/10.5194/cp-19-457-2023  

   Morales, Mariano S. ; Crispín-DelaCruz, Doris B. ; Álvarez, Claudio ; Christie, Duncan A. ; Ferrero, M. Eugenia ; Andreu-Hayles, Laia ; Villalba, Ricardo ; Guerra, Anthony ; Ticse-Otarola, Ginette ; Rodríguez-Ramírez, Ernesto C. ; et al ( January 2023 , Climate of the Past)

   Abstract. Given the short span of instrumental precipitationrecords in the South American Altiplano, longer-term hydroclimatic recordsare needed to understand the nature of climate variability and to improvethe predictability of precipitation, a key natural resource for thesocioeconomic development in the Altiplano and adjacent arid lowlands. Inthis region grows Polylepis tarapacana, a long-lived tree species that is very sensitive tohydroclimatic changes and has been widely used for tree-ring studies in thecentral and southern Altiplano. However, in the northern sector of thePeruvian and Chilean Altiplano (16–19∘ S)still exists a gap of high-resolution hydroclimatic data based on tree-ringrecords. Our study provides an overview of the temporal evolution of thelate-spring–mid-summer precipitation for the period 1625–2013 CE at thenorthern South American Altiplano, allowing for the identification of wet ordry periods based on a regional reconstruction from three P. tarapacana chronologies. Anincrease in the occurrence of extreme dry events, together with a decreasingtrend in the reconstructed precipitation, has been recorded since the 1970sin the northern Altiplano within the context of the last ∼4 centuries. The average precipitation over the last 17 years stands outas the driest in our 389-year reconstruction. We reveal a temporal andspatial synchrony across the Altiplano region of dry conditions since themid-1970s. Independent tree-ring-based hydroclimate reconstructions andseveral paleoclimatic records based on other proxies available for thetropical Andes record this synchrony. The influence of El Niño–SouthernOscillation (ENSO) on the northern Altiplano precipitation was detected byour rainfall reconstruction that showed past drier conditions in our studyregion associated with ENSO warm events. The spectral properties of therainfall reconstruction showed strong imprints of ENSO variability atdecadal, sub-decadal, and inter-annual timescales, in particular from thePacific NIÑO 3 sector. Overall, the recent reduction in precipitation incomparison with previous centuries, the increase in extreme dry events andthe coupling between precipitation and ENSO variability reported by thiswork is essential information in the context of the growing demand for waterresources in the Altiplano. This study will contribute to a betterunderstanding of the vulnerability and resilience of the region to theprojected evapotranspiration increase for the 21st century associated withglobal warming. 

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