Dynamics, Variability, and Change in Seasonal Precipitation Reconstructions for North America

Abstract

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 chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both 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 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 insight into decadal droughts and pluvials. They indicate that the sixteenth-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 200–500-yr-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 eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

Tree Rings and Observations Suggest No Stable Cycles in Sierra Nevada Cool‐Season Precipitation

“…The RWI chronologies were calculated from raw measurements of tree-ring widths and these datasets were mostly obtained from the International Tree-Ring Databank (ITRDB). The network of RWI chronologies considered as potential predictors here is essentially the same as considered by Williams et al (2020), which is an update and extension of the network used for prior gridded drought reconstructions (Cook et al, 2010b;Stahle et al, 2016Stahle et al, , 2020. While many of the RWI records used in our study are known to be strong proxies for growing-season soil moisture, interannual variability in warm-season soil moisture across much of the study region, and the Sierra Nevada in particular, is dominated by cool-season precipitation (St. George & Ault, 2014;.…”

Informing Seasonal Proxy‐Based Flow Reconstructions Using Baseflow Separation: An Example From the Potomac River, United States

“…The final predictor pools contain EW, LW, and TRW records. To minimize exaggeration of season‐to‐season persistence, no TRW chronology could be considered for both cool and warm season (Stahle et al., 2020; Torbenson & Stahle, 2018). If a chronology was significantly correlated with aggregates of both seasons, it was considered a predictor variable for the season in which it displayed the highest correlation.…”

“…Traditional dendroclimatic studies have focused on identifying the strongest climate signal in the tree‐ring record (Cook et al., 1999), often an aggregated soil moisture variable such as growing‐season Palmer Drought Severity Index (PDSI, Palmer, 1965). However, this aggregate signal is driven by different seasonal precipitation components depending on species and local climatology (St. George et al., 2010; Stahle et al., 2020). In recent decades, an increasing number of seasonally resolved hydroclimate reconstructions have been produced (e.g., Stahle et al., 2009; Torbenson & Stahle, 2018).…”

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

“…Most tree‐ring sites do not strongly record climate in the late summer and autumn (Wise & Dannenberg, 2019) due to the cessation of cambial activity (Fritts, 1976), an issue frequently encountered in dendroclimate studies aimed at seasonal reconstructions. The North American Seasonal Drought Atlas (Stahle et al., 2020) reconstructs cool and warm seasons as December‐April and May‐July, respectively, omitting the third of the year from August‐November. Ziaco et al.…”