Emma Watson | USGS Science

USGS2020

Dynamics, variability, and change in seasonal precipitation reconstructions for North America

Cool and warm season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December-April totals and 547 different chronologies correlated with May-July totals. These discrete seasonal predictor chronologies are not significantly correlated with the alternate season and the reconstructions calibrate at least 40% of the variance in both December-April and May-July precipitation totals over a large portion of North America for up to 2,000-years. Validation statistics computed on independent instrumental precipitation data from 1901-1927 indicate that the December-April reconstructions are reliable over most of the western and southern United States and northcentral Mexico, and the May-July estimates are valid over most of the United States, southwest Canada, and northeast Mexico. The strong continent wide El Niño/Southern Oscillation (ENSO) signal embedded in the cool season reconstructions, and the Arctic Oscillation signal registered by the warm season estimates, both faithfully reproduce the sign, intensity, and spatial patterns of these ocean-atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide new insight into decadal droughts and pluvials. They indicate that the 16th century megadrought, the most severe and sustained North American drought of the past 500-years, was the combined result of three distinct seasonal droughts each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic Multidecadal Oscillation. Significant 200- to 500-year long trends toward increased precipitation have been detected in the cool and warm season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the East that began due to natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

Journal of Climate↗

USGS2011

The unusual nature of recent snowpack declines in the North American cordillera

In western North America, snowpack has decl in ed in recent decades, and fur the r losses are projected through the 21st century. Here, we evaluate the uniqueness of recent declines us in g snowpack reconstructions from 66 tree-r in g chronologies in key run of f-generat in g areas of the Colorado, Columbia, and Missouri River dra in ages. Over the past millennium, late 20th century snowpack reductions are almost unprecedented in magnitude across the north ern Rocky Mounta in s and in the ir north -south synchrony across the cordillera . Both the snowpack declines and the ir synchrony result from unparalleled spr in gtime warm in g that is due to positive re in forcement of the anthropogenic warm in g by decadal variability. The in creas in g role of warm in g on large-scale snowpack variability and trends foreshadows fundamental impacts on streamflow and water supplies across the western United States.

Science↗