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1. 21st century California drought risk linked to model fidelity of the El Niño teleconnection

   https://doi.org/10.1038/s41612-018-0032-x  

   Allen, Robert J. ; Anderson, Ray G. ( September 2018 , npj Climate and Atmospheric Science)

   Greenhouse gas induced climate change is expected to lead to negative hydrological impacts for southwestern North America, including California (CA). This includes a decrease in the amount and frequency of precipitation, reductions in Sierra snow pack, and an increase in evapotranspiration, all of which imply a decline in surface water availability, and an increase in drought and stress on water resources. However, a recent study showed the importance of tropical Pacific sea surface temperature (SST) warming and an El Niño Southern Oscillation (ENSO)-like teleconnection in driving an increase in CA precipitation through the 21st century, particularly during winter (DJF). Here, we extend this prior work and show wetter (drier) CA conditions, based on several drought metrics, are associated with an El Niño (La Niña)-like SST pattern. Models that better simulate the observed ENSO-CA precipitation teleconnection also better simulate the ENSO-CA drought relationships, and yield negligible change in the risk of 21st century CA drought, primarily due to wetting during winter. Seasonally, however, CA drought risk is projected to increase during the non-winter months, particularly in the models that poorly simulate the observed teleconnection. Thus, future projections of CA drought are dependent on model fidelity of the El Niño teleconnection. As opposed to focusing on adapting to less water, models that better simulate the teleconnection imply adaptation measures focused on smoothing seasonal differences for affected agricultural, terrestrial, and aquatic systems, as well as effectively capturing enhanced winter runoff.

    

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2. Holocene El Niño–Southern Oscillation variability reflected in subtropical Australian precipitation

   https://doi.org/10.1038/s41598-019-38626-3  

   Barr, C. ; Tibby, J. ; Leng, M. J. ; Tyler, J. J. ; Henderson, A. C. G. ; Overpeck, J. T. ; Simpson, G. L. ; Cole, J. E. ; Phipps, S. J. ; Marshall, J. C. ; et al ( February 2019 , Scientific Reports)

   The La Niña and El Niño phases of the El Niño-Southern Oscillation (ENSO) have major impacts on regional rainfall patterns around the globe, with substantial environmental, societal and economic implications. Long-term perspectives on ENSO behaviour, under changing background conditions, are essential to anticipating how ENSO phases may respond under future climate scenarios. Here, we derive a 7700-year, quantitative precipitation record using carbon isotope ratios from a single species of leaf preserved in lake sediments from subtropical eastern Australia. We find a generally wet (more La Niña-like) mid-Holocene that shifted towards drier and more variable climates after 3200 cal. yr BP, primarily driven by increasing frequency and strength of the El Niño phase. Climate model simulations implicate a progressive orbitally-driven weakening of the Pacific Walker Circulation as contributing to this change. At centennial scales, high rainfall characterised the Little Ice Age (~1450–1850 CE) in subtropical eastern Australia, contrasting with oceanic proxies that suggest El Niño-like conditions prevail during this period. Our data provide a new western Pacific perspective on Holocene ENSO variability and highlight the need to address ENSO reconstruction with a geographically diverse network of sites to characterise how both ENSO, and its impacts, vary in a changing climate.

    

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3. On the Interpretation of the ENSO Signal Embedded in the Stable Isotopic Composition of Quelccaya Ice Cap, Peru

   https://doi.org/10.1029/2018JD029064  

   Hurley, J. V. ; Vuille, Mathias ; Hardy, Douglas R. ( January 2019 , Journal of Geophysical Research: Atmospheres)

   Theδ¹⁸O signal in ice cores from the Quelccaya Ice Cap (QIC), Peru, corresponds with and has been used to reconstruct Niño region sea surface temperatures (SSTs), but the physical mechanisms that tie El Niño–Southern Oscillation (ENSO)‐related equatorial Pacific SSTs to snowδ¹⁸O at 5,680 m in the Andes have not been fully established. We use a proxy system model to simulate how QIC snowδ¹⁸O varies by ENSO phase. The model accurately simulates higher and lowerδ¹⁸O values during El Niño and La Niña, respectively. We then explore the relative roles of ENSO forcing on different components of the forward model: (i) the seasonality and amount of snow gain and loss at the QIC, (ii) the initial water vaporδ¹⁸O values, and (iii) regional temperature. Most (more than two thirds) of the ENSO‐related variability in the QICδ¹⁸O can be accounted for by ENSO's influence on South American summer monsoon (SASM) activity and the resulting change in the initial water vapor isotopic composition. The initial water vaporδ¹⁸O values are affected by the strength of upstream convection associated with the SASM. Since convection over the Amazon is enhanced during La Niña, the water vapor over the western Amazon Basin—which serves as moisture source for snowfall on QIC—is characterized by more negativeδ¹⁸O values. In the forward model, higher initial water vaporδ‐values during El Niño yield higher snowδ¹⁸O at the QIC. Our results clarify that the ENSO‐related isotope signal on Quelccaya should not be interpreted as a simple temperature response.

    

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4. El Niño–Southern Oscillation signal in a new East Antarctic ice core, Mount Brown South

   https://doi.org/10.5194/cp-17-1795-2021  

   Crockart, Camilla K. ; Vance, Tessa R. ; Fraser, Alexander D. ; Abram, Nerilie J. ; Criscitiello, Alison S. ; Curran, Mark A. ; Favier, Vincent ; Gallant, Ailie J. ; Kittel, Christoph ; Kjær, Helle A. ; et al ( January 2021 , Climate of the Past)

   Abstract. Paleoclimate archives, such as high-resolution ice core records, provide ameans to investigate past climate variability. Until recently, the Law Dome(Dome Summit South site) ice core record remained one of fewmillennial-length high-resolution coastal records in East Antarctica. A newice core drilled in 2017/2018 at Mount Brown South, approximately 1000 kmwest of Law Dome, provides an additional high-resolution record that willlikely span the last millennium in the Indian Ocean sector of EastAntarctica. Here, we compare snow accumulation rates and sea saltconcentrations in the upper portion (∼ 20 m) of three MountBrown South ice cores and an updated Law Dome record over the period1975–2016. Annual sea salt concentrations from the Mount Brown South siterecord preserve a stronger signal for the El Niño–Southern Oscillation(ENSO; austral winter and spring, r = 0.533, p < 0.001, Multivariate El Niño Index) compared to a previously defined Law Dome record of summer sea salt concentrations (November–February, r = 0.398, p = 0.010, SouthernOscillation Index). The Mount Brown South site record and Law Dome recordpreserve inverse signals for the ENSO, possibly due to longitudinalvariability in meridional transport in the southern Indian Ocean, althoughfurther analysis is needed to confirm this. We suggest that ENSO-related seasurface temperature anomalies in the equatorial Pacific drive atmosphericteleconnections in the southern mid-latitudes. These anomalies areassociated with a weakening (strengthening) of regional westerly winds tothe north of Mount Brown South that correspond to years of low (high) seasalt deposition at Mount Brown South during La Niña (El Niño)events. The extended Mount Brown South annual sea salt record (whencomplete) may offer a new proxy record for reconstructions of the ENSO overthe recent millennium, along with improved understanding of regionalatmospheric variability in the southern Indian Ocean, in addition to thatderived from Law Dome. 

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5. The Lake Ice Continuum Concept: Influence of Winter Conditions on Energy and Ecosystem Dynamics

   https://doi.org/10.1029/2020JG006165  

   Cavaliere, E. ; Fournier, I. B. ; Hazuková, V. ; Rue, G. P. ; Sadro, S. ; Berger, S. A. ; Cotner, J. B. ; Dugan, H. A. ; Hampton, S. E. ; Lottig, N. R. ; et al ( November 2021 , Journal of Geophysical Research: Biogeosciences)

   Millions of lakes worldwide are distributed at latitudes or elevations resulting in the formation of lake ice during winter. Lake ice affects the transfer of energy, heat, light, and material between lakes and their surroundings creating an environment dramatically different from open‐water conditions. While this fundamental restructuring leads to distinct gradients in ions, dissolved gases, and nutrients throughout the water column, surprisingly little is known about the resulting effects on ecosystem processes and food webs, highlighting the lack of a general limnological framework that characterizes the structure and function of lakes under a gradient of ice cover. Drawing from the literature and three novel case studies, we present the Lake Ice Continuum Concept (LICC) as a model for understanding how key aspects of the physical, chemical, and ecological structure and function of lakes vary along a continuum of winter climate conditions mediated by ice and snow cover. We examine key differences in energy, redox, and ecological community structure and describe how they vary in response to shifts in physical mixing dynamics and light availability for lakes with ice and snow cover, lakes with clear ice alone, and lakes lacking winter ice altogether. Global change is driving ice covered lakes toward not only warmer annual average temperatures but also reduced, intermittent or no ice cover. The LICC highlights the wide range of responses of lakes to ongoing climate‐driven changes in ice cover and serves as a reminder of the need to understand the role of winter in the annual aquatic cycle.

    

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