The Azores High (AH), a subtropical ridge in the atmosphere over the North Atlantic comprising one node of the North Atlantic Oscillation (NAO) system, has a dominant influence on the weather and climate of the Iberian Peninsula and northwest Africa. The behavior of the entire NAO system over the last millennium has been the subject of much debate in both proxy- and model-based studies. Many studies have focused on the behavior of the entire NAO system, but we focus solely on the behavior of the AH due to its proximity to this region. Other proxies from this region, mainly from Spain and Morocco, have provided details about atmospheric dynamics yet spatiotemporal gaps remain. In this study, we present a continuous, sub-decadally-resolved composite stalagmite carbon isotopic record from three partially overlapping stalagmites from Buraca Gloriosa (BG) cave, western Portugal, situated within the center of the AH, that preserves evidence of regional hydroclimate variability from approximately 800 CE to the present. This composite record, developed from U-Th dating and laminae counting paired with carbon isotopes, primarily reflects effective moisture in western Portugal. Given the close pairing of AH behavior (intensity, size, and location) and moisture transport in this region, the BG composite record allows for a thorough analysis of AH behavior over time. Multidecadal to centennial scale variability in the BG record and state-of-the-art last millennium climate model simulations show considerable coherence with precipitation-sensitive records from Spain and Morocco that, like BG, are strongly influenced by the intensity, size, and location of the AH. Synthesis of model output and proxy data suggests that western Portugal was persistently dry during much of the Medieval Climate Anomaly (MCA; ~850-1250 CE) and Modern era (1850 CE-present) and experienced wetter conditions during Little Ice Age (LIA; ~1400-1850 CE). Even considering age uncertainties from the Iberian Peninsula and northwest Africa proxy records, the apparent timing in the transition from a relatively dry MCA to a wetter LIA is spatially variable across this region, likely due to the non-stationary behavior of the AH system.
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Speleothem record of geomagnetic South Atlantic Anomaly recurrence
The diminishing strength of the Earth’s magnetic dipole over recent millennia is accompanied by the increasing prominence of the geomagnetic South Atlantic Anomaly (SAA), which spreads over the South Atlantic Ocean and South America. The longevity of this feature at millennial timescales is elusive because of the scarcity of continuous geomagnetic data for the region. Here, we report a unique geomagnetic record for the last ∼1500 y that combines the data of two well-dated stalagmites from Pau d’Alho cave, located close to the present-day minimum of the anomaly in central South America. Magnetic directions and relative paleointensity data for both stalagmites are generally consistent and agree with historical data from the last 500 y. Before 1500 CE, the data adhere to the geomagnetic model ARCH3K.1, which is derived solely from archeomagnetic data. Our observations indicate rapid directional variations (>0.1°/y) from approximately 860 to 960 CE and approximately 1450 to 1750 CE. A similar pattern of rapid directional variation observed from South Africa precedes the South American record by 224 ± 50 y. These results confirm that fast geomagnetic field variations linked to the SAA are a recurrent feature in the region. We develop synthetic models of reversed magnetic flux patches at the core–mantle boundary and calculate their expression at the Earth’s surface. The models that qualitatively resemble the observational data involve westward (and southward) migration of midlatitude patches, combined with their expansion and intensification.
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- NSF-PAR ID:
- 10082835
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 115
- Issue:
- 52
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- 13198 to 13203
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The Azores High (AH), a subtropical ridge in the atmosphere over the North Atlantic comprising one node of the North Atlantic Oscillation (NAO) system, has a dominant influence on the weather and climate of the Iberian Peninsula and northwest Africa. The behavior of the entire NAO system over the last millennium has been the subject of much debate in both proxy- and model-based studies. Many studies have focused on the behavior of the entire NAO system, but we focus solely on the behavior of the AH due to its proximity to this region. Other proxies from this region, mainly from Spain and Morocco, have provided details about atmospheric dynamics yet spatiotemporal gaps remain. In this study, we present a continuous, sub-decadally-resolved composite stalagmite carbon isotopic record from three partially overlapping stalagmites from Buraca Gloriosa (BG) cave, western Portugal, situated within the center of the AH, that preserves evidence of regional hydroclimate variability from approximately 800 CE to the present. This composite record, developed from U-Th dating and laminae counting paired with carbon isotopes, primarily reflects effective moisture in western Portugal. Given the close pairing of AH behavior (intensity, size, and location) and moisture transport in this region, the BG composite record allows for a thorough analysis of AH behavior over time. Multidecadal to centennial scale variability in the BG record and state-of-the-art last millennium climate model simulations show considerable coherence with precipitation-sensitive records from Spain and Morocco that, like BG, are strongly influenced by the intensity, size, and location of the AH. Synthesis of model output and proxy data suggests that western Portugal was persistently dry during much of the Medieval Climate Anomaly (MCA; ~850-1250 CE) and Modern era (1850 CE-present) and experienced wetter conditions during Little Ice Age (LIA; ~1400-1850 CE). Even considering age uncertainties from the Iberian Peninsula and northwest Africa proxy records, the apparent timing in the transition from a relatively dry MCA to a wetter LIA is spatially variable across this region, likely due to the non-stationary behavior of the AH system.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.1809197115
