Scott St. George

The continuum of hydroclimate variability in western North America during the last millennium

AbstractThe distribution of climatic variance across the frequency spectrum has substantial importance for anticipating how climate will evolve in the future. Here power spectra and power laws (β) are estimated from instrumental, proxy, and climate model data to characterize the hydroclimate continuum in western North America (WNA). The significance of the estimates of spectral densities and β are tested against the null hypothesis that they reflect solely the effects of local (nonclimate) sources of autocorrelation at the monthly time scale. Although tree-ring-based hydroclimate reconstructions are generally consistent with this null hypothesis, values of β calculated from long moisture-sensitive chronologies (as opposed to reconstructions) and other types of hydroclimate proxies exceed null expectations. Therefore it may be argued that there is more low-frequency variability in hydroclimate than monthly autocorrelation alone can generate. Coupled model results archived as part of phase 5 of the Coupled ...

The seasonality of precipitation signals embedded within the North American Drought Atlas

We examine how the seasonality of precipitation signals embedded within the North American Drought Atlas varies across the continent. Instrumental records of average summer (JJA) Palmer Drought Severity Index (PDSI) are characterized by major regional differences in the relative importance of precipitation during summer and winter (DJF). The Atlas, which is based on a network of drought-sensitive tree-ring records, is able to reproduce the main geographic patterns of these biases, but tree-ring reconstructions exaggerate the influence of seasonal precipitation anomalies in the southwestern United States and northern Mexico (towards a stronger winter signal) and western Canada (towards a stronger summer signal). Drought reconstructions from the Southwest and Tex-Mex regions are tuned mainly to winter precipitation and display strong teleconnections to both El Niño and La Niña. In contrast, winter precipitation signals are either weak or absent in drought reconstructions from northwestern North America, and tree-ring estimates of PDSI show a much less robust association with the El Niño-Southern Oscillation. Geographical differences in the relative strength of seasonal precipitation signals are likely due to (i) local factors that influence tree growth but are not incorporated into the PDSI algorithm and (ii) real differences in regional climatology. These seasonal biases must be taken into account when comparing drought reconstructions across North America, when comparing tree-ring PDSI to drought records developed from other proxies or when attempting to use the Drought Atlas to link past droughts to potential forcing mechanisms.

The tree-ring record of drought on the Canadian Prairies

Ring-width data from 138 sites in the Canadian Prairie Provinces and adjacent regions are used to estimate summer drought severity during the past several hundred years. The network was divided into five regional groups based on geography, tree species, and length of record: the eastern Rockies, northern Saskatchewan, central Manitoba, southern Manitoba, and northwestern Ontario. Regional tree-ring records are primarily related to summer moisture and drought conditions, and are less responsive to droughts caused by deficits in winter precipitation. These summer-sensitive data are not linearly related to major modes of climate variability, including ENSO and the Pacific decadal oscillation (PDO), which primarily affect the climate of western Canada during winter. Extended drought records inferred from tree rings indicate that drought on the Canadian Prairies has exhibited considerable spatial heterogeneity over the last several centuries. For northern Saskatchewan and northwestern Ontario, intervals of persistently low tree growth during the twentieth century were just as long as or longer than low-growth intervals in the eighteenth or nineteenth centuries. Longer records from southern Alberta suggest that the most intense dry spell in that area during the last 500 yr occurred during the 1720s. At the eastern side of the prairies, the longest dry event is centered around 1700 and may coincide with low lake stands in Manitoba, Minnesota, and North Dakota. Although the Canadian Prairies were dry at times during the 1500s, there is no regional analog to the sixteenth-century ‘‘megadroughts’’ that affected much of the western United States and northern Mexico.

Signatures of high-magnitude 19th-century floods in Quercus macrocarpa tree rings along the Red River, Manitoba, Canada

Quercus macrocarpa (Michx.) growing along the Red River, Manitoba, Canada, contain an anatomical signature related to high-magnitude 19th-century floods. Tree-ring samples were collected from 194 Q. macrocarpa over a 100 km transect along the Red River valley. The combined tree-ring record extends from A.D. 1463 to A.D. 1999; sample depth between 1463 and 1650 is limited and made up exclusively of subfossil logs derived from alluvial deposits. Thirteen trees from four sites contain annual rings with reduced earlywood-vessel transverse areas that reflect flooding during the tree9s growing season. Flood rings in 1826 are present in 24% of Q. macrocarpa samples and are coincident with the largest flood observed in the Red River valley. Flood rings in 1852 are exhibited in 5.9% of samples and correspond with the second largest Red River flood. These results confirm that Quercus species adapt to prolonged inundation by reducing the transverse area of their earlywood vessels and suggest that anatomical signatures in riverbank trees may be used to identify and delineate high-magnitude paleofloods for low-gradient rivers.

The Magnitude of Decadal and Multidecadal Variability in North American Precipitation

Abstract The authors use singular spectrum analysis to investigate the relative magnitude of decadal to multidecadal (D2M) variability in annual and seasonal precipitation anomalies across North America. In most places, decadal (10–20 yr) or multidecadal (20–50 yr) variability makes up less than 10% of the total variance in either annual or seasonal precipitation, with interannual variability or secular trends having much greater importance. Decadal variability is most prominent (contributing 25%–30% of the total variance) in Minnesota and northern California during winter, and the central Rocky Mountains in autumn. Eastern Quebec is the only major region where precipitation exhibits significant variance in the multidecadal band. Precipitation across much of Canada exhibits significant variance at extremely low frequencies (greater than 50 yr), but variability at these time scales cannot be separated from secular trends because of the limited length of instrumental climate records. Decadal signals in the ...

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

A review of flood records from tree rings

Palaeohydrology is now recognized as a valuable approach to characterize the hazards posed by flooding. Tree rings have emerged as an important source of evidence for paleohydrological studies, and, since the 1960s, have been used to document the occurrence of past floods. In this progress report we outline the major contributions of tree-ring records to flood research. By reviewing the key advances in this field, documenting different research trajectories, and highlighting recent developments, we make an argument in favor of more extensive use of tree rings in flood analyses. We show how tree-ring data have been applied to risk assessment and outline how the widespread distribution of flood-affected trees can be used to improve the understanding of flood processes. In addition, we outline new approaches and future perspectives for the inclusion of woody vegetation in hazard assessments, and end with new thematic perspectives.

Tree Rings as Paleoflood and Paleostage Indicators

Each year, floods cause enormous damage to property and kill thousands of people around the world. During the 1990s alone, freshwater flooding affected more than 1.4 billion people and caused about 100,000 deaths (Jonkman 2005). Worldwide, insured losses due to floods topped US

Paleoenvironmental Perspectives on Drought in Western Canada — Introduction

2 billion in 2008 (SwissRe 2009), making them the second-most expensive type of natural catastrophe (exceeded only by damages caused by tropical storms). In addition to the threats they pose to human communities, major floods are also important geological and biogeochemical agents that influence rates of erosion and sediment transport (Molnar 2001), redistribute organic matter and nutrients to downstream reaches (Velasco et al. 2006) and homogenize ecological processes and biological communities within floodplain systems (Thomaz et al. 2007).

A Robust Null Hypothesis for the Potential Causes of Megadrought in Western North America

Paleohydrology uses indirect evidence to describe the behaviour of hydrological or hydroclimatic systems prior to the initiation of direct monitoring. This evidence can be derived from either human sources or natural archives. The articles in this issue show how paleoenvironmental data may be used to: i) place recent observations within a context of the past several hundred years; ii) evaluate the impact of human modification of hydrological systems; and iii) examine the reliability of hypothesized connections between regional hydroclimate and remote climate forcings. As a group, these papers illustrate the dynamic and changing nature of the hydrology of western Canada, and provide a long-term perspective that can be crucial for good stewardship of water resources.