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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.
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This content will become publicly available on July 1, 2026
Severe Convective Weather Outbreaks on 10 and 15 December 2021: Large-Scale Antecedent Conditions
Abstract The first 2 weeks of December 2021 were exceptionally active for severe convective storms across the central and eastern United States. While previous work has indicated that this was related to the existence of a negative phase of the Pacific–North American pattern, we demonstrate that such a pattern was configured via dynamical linkages between multiple extratropical cyclogenesis events in the western North Pacific, the recurvature of Typhoon Nyatoh, and the subsequent phase evolution of the North Pacific jet. These processes were found to aid in the excitation of Rossby wave packets and the amplification of upper-level flow downstream over the Pacific, ultimately configuring synoptic-scale weather regimes supportive of anomalous high-frequency and high-intensity severe convective weather in the contiguous United States. In addition, abnormally warm Gulf of America/Gulf of Mexico sea surface temperatures, aided by a period of antecedent synoptic-scale subsidence, played a critical role in enhancing convective instability in the surface warm sector. This work underscores the importance of cataloging these events for purposes of examining (and potentially enhancing) predictability. Significance StatementThe first half of December 2021 recorded one of the most active cool-season severe weather periods in the United States, resulting in two billion-dollar convective outbreaks on 10 and 15 December. This study links these extreme events to upstream dynamical processes over the North Pacific, including extratropical cyclogenesis, the recurvature of Typhoon Nyatoh, and the retraction of the North Pacific jet. These processes amplified downstream flow and configured synoptic environments favorable for severe weather across the United States. Additionally, anomalously warm Gulf of America/Gulf of Mexico sea surface temperatures enhanced convective instability. By identifying these key precursors, this work highlights the potential for improved anticipation of extended-range severe weather likelihood—particularly during the cool season—when such events remain rare but highly impactful.
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- Award ID(s):
- 2039384
- PAR ID:
- 10627918
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Monthly Weather Review
- Volume:
- 153
- Issue:
- 7
- ISSN:
- 0027-0644
- Page Range / eLocation ID:
- 1171 to 1194
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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