Internal ocean-atmosphere variability drives megadroughts in Western North Ameirca - PDF

Multidecadal droughts that occurred during the Medieval Climate Anomaly represent an important target for validating the ability of climate models to adequately characterize drought risk
over the near-term future. A prominent hypothesis is that these megadroughts were driven by a centuries-long radiatively forced shift in the mean state of the tropical Pacific Ocean. Here we use a novel combination of spatiotemporal tree ring reconstructions of Northern Hemisphere hydroclimate to infer the atmosphere-ocean dynamics that coincide with megadroughts over the American West and find that these features are consistently associated with 10–30 year periods of frequent cold El Niño–Southern Oscillation conditions and not a centuries-long shift in the mean of the tropical Pacific Ocean. These results suggest an important role for internal variability in driving past megadroughts. State-of-the-art climate models from the Coupled Model Intercomparison Project Phase 5, however, do not simulate a consistent association between megadroughts and internal variability of the tropical Pacific Ocean, with implications for our confidence in megadrought risk projections.

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The improbable but unexceptional occurrence of megadrought clustering in the American West during the Medieval Climate Anomaly - PDF

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The five most severe and persistent droughts in the American West(AW) during the Common Era occurred during a 450 year period known as the Medieval Climate Anomaly (MCA—850–1299 C.E.). Herein we use timeseries modeling to estimate the probability of such a period of hydroclimate change occurring. Clustering of severe and persistent drought during an MCA-length period occurs in approximately 10% of surrogate timeseries that were constructed to have the same characteristics as a tree-ring derived estimate of AW hydroclimate variability between 850 and 2005 C.E. Periods of hydroclimate change like the MCA are thus expected to occur in the AW, although not frequently, with a recurrence interval of approximately 11 000 years. Importantly, a shift in mean hydroclimate conditions during the MCA is found to be necessary for drought to reach the severity and persistence of the actual MCA megadroughts. This result has consequences for our understanding of the atmosphere-ocean dynamics underlying the MCA and a persistently warm Atlantic Multidecadal Oscillation is suggested to have played an important role in causing megadrought clustering during this period.

REFERENCE

Coats, S., J. E. Smerdon, K. B. Karnauskas and R. Seager, 2016: The improbable but unexceptional occurrence of megadrought clustering in the American West during the Medieval Climate Anomaly. Environmental Research Letters, 11(7), doi:10.1088/1748-9326/11/7/074025.

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Winter-to-summer precipitation phasing in southwestern North America: A multicentury perspective from paleoclimatic model-data comparisons - PDF

The phasing of winter-to-summer precipitation anomalies in the North American monsoon (NAM) region 2 (113.25°W–107.75°W, 30°N–35.25°N—NAM2) of southwestern North America is analyzed in fully coupled simulations of the Last Millennium and compared to tree ring reconstructed winter and summer precipitation variability. The models simulate periods with in-phase seasonal precipitation anomalies, but the strength of this relationship is variable on multidecadal time scales, behavior that is also exhibited by the reconstructions. The models, however, are unable to simulate periods with consistently out-of-phase winter-to-summer precipitation anomalies as observed in the latter part of the instrumental interval. The periods with predominantly in-phase winter-to-summer precipitation anomalies in the models are significant against randomness, and while this result is suggestive of a potential for dual-season drought on interannual and longer time scales, models do not consistently exhibit the persistent dual-season drought seen in the dendroclimatic reconstructions. These collective findings indicate that model-derived drought risk assessments may underestimate the potential for dual-season drought in 21st century projections of hydroclimate in the American Southwest and parts of Mexico.

REFERENCE

Coats, S., J.E. Smerdon, R. Seager, D. Griffin and B.I. Cook, 2015: Winter-to-summer precipitation phasing in southwestern North America: A multicentury perspective from paleoclimatic model-data comparisons. J. Geo. Res.: Atmos.,120, 8052-8064, doi:10.1002/2015JD023085.

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North American Pancontinental Droughts in Model Simulations of the Last Millennium - PDF

Pancontinental droughts in North America, or droughts that simultaneously affect a large percentage of the geographically and climatically distinct regions of the continent, present significant on-the-ground management challenges and, as such, are an important target for scientific research. The methodology of paleoclimate-model data comparisons is used herein to provide a more comprehensive understanding of pancontinental drought dynamics. Models are found to simulate pancontinental drought with the frequency and spatial patterns exhibited by the paleoclimate record. They do not, however, agree on the modes of atmosphere–ocean variability that produce pancontinental droughts because simulated El Niño–Southern Oscillation (ENSO), Pacific decadal oscillation (PDO), and Atlantic multidecadal oscillation (AMO) dynamics, and their teleconnections to North America, are different between models and observations. Despite these dynamical differences, models are able to reproduce large-magnitude centennial-scale variability in the frequency of pancontinental drought occurrence—an important feature of the paleoclimate record. These changes do not appear to be tied to exogenous forcing, suggesting that simulated internal hydroclimate variability on these time scales is large in magnitude. Results clarify our understanding of the dynamics that produce real-world pancontinental droughts while assessing the ability of models to accurately characterize future drought risks.

REFERENCE

Coats, S., B. Cook, J. Smerdon, and R. Seager, 2015: North American Pancontinental Droughts in Model Simulations of the Last Millennium. J. Climate, 28, 5, 2025-2043, doi:10.1175/JCLI-D-14-00634.1.

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The temporal stationarity of the teleconnection between the tropical Pacific Ocean and North America (NA) is analyzed in atmosphere-only, and coupled last-millennium, historical, and control runs from the Coupled Model Intercomparison Project Phase 5 data archive. The teleconnection, defined as the correlation between December-January-February (DJF) tropical Pacific sea surface temperatures (SSTs) and DJF 200 mb geopotential height, is found to be nonstationary on multidecadal timescales. There are significant changes in the spatial features of the teleconnection over NA in continuous 56-year segments of the last millennium and control simulations. Analysis of atmosphere-only simulations forced with observed SSTs indicates that atmospheric noise cannot account for the temporal variability of the teleconnection, which instead is likely explained by the strength of, and multidecadal changes in, tropical Pacific Ocean variability. These results have implications for teleconnection-based analyses of model fidelity in simulating precipitation, as well as any reconstruction and forecasting efforts that assume stationarity of the observed teleconnection. Citation: Coats, S., J. E. Smerdon, B. I. Cook, and R. Seager (2013), Stationarity of the tropical pacific teleconnection to North America in CMIP5/PMIP3 model simulations, Geophys. Res. Lett., 40, 4927–4932, doi:10.1002/grl.50938.

REFERENCE

Coats, S., J.E. Smerdon, B.I. Cook and R. Seager, 2013: Stationarity of the tropical pacific teleconnection to North America in CMIP5/PMIP3 model simulations. Geophys. Res. Lett., 40, 4927–4932, doi:10.1002/grl.50938.