| 1. |
- Hou, Yandong, et al.
(författare)
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Sahara's surface transformation forced an abrupt hydroclimate decline and Neolithic culture transition in northern China
- 2024
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Ingår i: The Innovation. - 2666-6758. ; 5:1
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Tidskriftsartikel (refereegranskat)abstract
- The remote forcing from land surface changes in the Sahara is hypothesized to play a pivotal role in modulating the intensity of the East Asian summer monsoon (EASM) through ocean-atmospheric teleconnections. This modu-lation has far-reaching consequences, particularly in facilitating societal shifts documented in northern China. Here, we present a well-dated lake-level record from the Daihai Lake Basin in northern China, providing quantitative assessments of Holocene monsoonal precipitation and the consequent mi-grations of the northern boundary of the EASM. Our reconstruction, informed by a water-and-energy balance model, indicates that annual precipitation reached -700 mm during 8-5 ka, followed by a rapid decline to -550 mm be-tween 5 and 4 ka. This shift coherently aligns with a significant -300 km northwestward movement of the EASM northern boundary during the Middle Holocene (MH), in contrast to its current position. Our findings underscore that these changes cannot be entirely attributed to orbital forcing, as corrob-orated by simulation tests. Climate model simulations deployed in our study suggest that the presence of the Green Sahara during the MH significantly strengthened the EASM and led to a northward shift of the monsoon rainfall belt. Conversely, the Sahara's reversion to a desert landscape in the late Ho-locene was accompanied by a corresponding southward retraction of monsoon influence. These dramatic hydroclimate changes during -5-4 ka likely triggered or at least contributed to a shift in Neolithic cultures and societal transformation in northern China. With decreasing agricultural pro-ductivity, communities transitioned from millet farming to a mixed rainfed agriculture and animal husbandry system. Thus, our findings elucidate not only the variability of the EASM but also the profound implications of a remote forcing, such as surface transformations of the Sahara, on climatic changes and cultural evolution in northern China.
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| 2. |
- Axelsson, Josefine, 1992-
(författare)
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Interglacial climates in proxies and models : Utilizing sampled oxygen isotopes and model simulations to understand past Indian summer monsoon variability
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The study of Earth's climate system, including the mechanisms driving monsoon systems, is a key area of research within environmental sciences. Monsoons, vital for billions of people, are complex atmospheric phenomena influenced by various global factors, including orbital changes and natural climate variability. Among monsoon systems, the Indian summer Monsoon (ISM) is of particular interest due to its significant impact on the South Asian climate, agriculture, and water resources. Despite extensive study, comprehending the ISM's historical variability and its future implications remains a challenge. Utilizing natural archives like speleothems, along with stable water isotopes from precipitation and advanced climate model simulations, this thesis aims to decipher the ISM's responses to natural forcings across key interglacial periods—the Last Interglacial and the Holocene.Our findings indicate that the ISM's strength is critically influenced by slight variations in orbital configurations, leading to significant shifts in monsoon patterns. Our research also highlights the dual influence of local geographical features and distant atmospheric conditions on the ISM's annual variability. Most notably, we observed discrepancies between δ18O values obtained from isotope-enabled climate models and those derived from speleothems. This insight indicates that the models need refinement to accurately mirror the complexities observed in the proxy records and that the uncertainty parameter in speleothem records needs to be improved.The alignment between proxy and model data is crucial for a more accurate reconstruction of past climates and for enhancing the predictive capabilities of future monsoon behavior under changing climatic conditions. By advancing our knowledge of the ISM's past, we are better equipped to anticipate its future. To achieve that, this thesis stresses the importance of bridging the gap between proxy data insights and climate model simulations. This would not only enrich our historical climate knowledge but also inform future climate projections, highlighting the indispensable role of interdisciplinary research in climate science challenges.
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| 3. |
- Berntell, Ellen, et al.
(författare)
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Mid-Holocene West African monsoon rainfall enhanced in EC-Earth simulation with dynamic vegetation feedback
- 2024
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Ingår i: Climate Dynamics. - 0930-7575 .- 1432-0894.
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Tidskriftsartikel (refereegranskat)abstract
- Proxy records have shown that the Mid-Holocene was a period of humid conditions across West Africa, with an enhanced West African Monsoon (WAM) and vegetated conditions in areas currently characterized by desert, often referred to as the Green Sahara. However, General Circulation Models regularly struggle with recreating this strengthened Mid-Holocene monsoon in West Africa. Vegetation feedbacks has long been viewed as an essential process modulating the monsoon variability in West Africa, and simulations using prescribed vegetation to recreate a Green Sahara have shown a strengthened WAM and increased rainfall. However, simulations with prescribed vegetation in Sahara represent an idealized vegetation cover and do not take any environmental heterogeneity into account. Furthermore, this only represents a one-directional forcing by the vegetation on the climate rather than the full vegetation feedback. To address this, we have simulated the Mid-Holocene (similar to 6 ka) climate using the Earth System Model EC-Earth3-Veg. The results show that coupled dynamic vegetation reproduces an apparent enhancement of the WAM, with the summer rainfall in the Sahel region increasing by 15% compared to simulations with a prescribed modern vegetation cover. Vegetation feedbacks enhance the warming of the Sahara region, deepens the Sahara Heat Low, results in increased rainfall and strengthens monsoonal flow across West Africa. However, the enhancement is still below what can be viewed in proxy reconstructions, highlighting the role of model limitation and biases and the importance of investigating other processes, such as the interactive aerosol-albedo feedback.
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| 4. |
- Han, Zixuan, et al.
(författare)
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Impacts of Mid-Pliocene Ice Sheets and Vegetation on Afro-Asian Summer Monsoon Rainfall Revealed by EC-Earth Simulations
- 2024
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 51:2
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Tidskriftsartikel (refereegranskat)abstract
- The impact of mid-Pliocene boundary conditions on Afro-Asian summer monsoon (AfroASM) rainfall is examined using the fully coupled Earth System Model EC-Earth3-LR. Our focus lies on the effects of varying CO2 concentration, diminished ice sheets and vegetation dynamics. We find that the enhanced AfroASM rainfall is predominantly caused by the “warmer-gets-wetter” mechanism due to elevated CO2 levels. Additionally, the ice sheet, similar in size to that of the mid-Pliocene era, creates several indirect effects. These include sea ice-albedo feedback and inter-hemispheric atmosphere energy transport. Such influences result in the southward shift of Hadley circulation and formation of Pacific-Japan pattern, leading to reduced rainfall in North African and South Asian monsoon regions but increased rainfall in East Asian monsoon region. Interestingly, while dynamic vegetation feedback has a minimal direct effect on AfroASM rainfall, it significantly influences rainfall in the mid-high latitudes of the North Hemisphere by enhancing water vapor feedback.
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| 5. |
- Lin, Fangyuan, 1996-, et al.
(författare)
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Seasonal to decadal variations of precipitation oxygen isotopes in northern China linked to the moisture source
- 2024
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Ingår i: npj Climate and Atmospheric Science. - 2397-3722. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- A precise characterization of moisture source and transport dynamics over the inland margins of monsoonal China is crucial for understanding the climatic significance of precipitation oxygen isotope (δ18Op) variability preserved in the regional proxy archives. Here, we use a general circulation model with an embedded water-tagging module to quantify the role of moisture dynamics on the seasonal to decadal variations of δ18Op in northern China. Our data indicate that during the non-monsoon season, the δ18Op variability is dominated by the temperature effect. Conversely, in the summer monsoon season, the moisture contributions from the low-latitude land areas (LLA), the Pacific Ocean (PO), and the North Indian Ocean (NIO) override the temperature effect and influence the summer δ18Op. Intensified upstream convection along the NIO moisture transport pathway results in a more negative summer δ18Op compared to moisture transported from the PO and LLA regions. Our analysis shows a decadal shift in summer δ18Op around the mid-1980s, marking changes in the relative contribution of oceanic moisture from PO and NIO in response to changes in the atmospheric circulation patterns influenced by the Pacific Decadal Oscillation. We suggest that such decadal-scale δ18Op variability can be recorded in the natural archives from the region, which can provide valuable insights into understanding past climate variability.
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| 6. |
- Long, Jingchao, et al.
(författare)
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Large-scale photovoltaic solar farms in the Sahara affect solar power generation potential globally
- 2024
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Ingår i: Communications Earth & Environment. - 2662-4435. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- Globally, solar projects are being rapidly built or planned, particularly in high solar potential regions with high energy demand. However, their energy generation potential is highly related to the weather condition. Here we use state-of-the-art Earth system model simulations to investigate how large photovoltaic solar farms in the Sahara Desert could impact the global cloud cover and solar generation potential through disturbed atmospheric teleconnections. The results indicate negative impacts on solar potential in North Africa (locally), Middle East, Southern Europe, India, Eastern China, Japan, Eastern Australia, and Southwestern US, and positive impacts in Central and South America, the Caribbean, Central & Eastern US, Scandinavia and South Africa, reaching a magnitude of ±5% in remote regions seasonally. Diagnostics suggest that large-scale atmospheric circulation changes are responsible for the global impacts. International cooperation is essential to mitigate the potential risks of future large-scale solar projects in drylands, which could impact energy production.
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| 7. |
- Yang, Kunpeng, et al.
(författare)
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North Atlantic Ocean–Originated Multicentennial Oscillation of the AMOC : A Coupled Model Study
- 2024
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 37:9, s. 2789-2807
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Tidskriftsartikel (refereegranskat)abstract
- Using a CESM1 control simulation, we conduct a follow-up study to advance our earlier theoretical research on the multicentennial oscillation (MCO) of the Atlantic meridional overturning circulation (AMOC). The modeled AMOC MCO primarily arises from internal oceanic processes in the North Atlantic, potentially representing a North Atlantic Ocean–originated mode of AMOC multicentennial variability (MCV) in reality. Specifically, this AMOC MCO is mainly driven by salinity variation in the subpolar upper North Atlantic, which dominates local density variation. Salinity anomaly in the subpolar upper ocean is enhanced by the well-known positive salinity advection feedback that is realized through anomalous advection in the subtropical to subpolar upper ocean. Meanwhile, mean advection moves salinity anomaly in the subtropical intermediate ocean northward, weakening the subpolar upper salinity anomaly and leading to its phase change. The salinity anomalies have a clear three-dimensional life cycle around the North Atlantic. The mechanism and time scale of the modeled AMOC MCO are consistent with our earlier theoretical studies. In the theoretical model, artificially deactivating either the anomalous or mean advection in the AMOC upper branch prevents it from exhibiting AMOC MCO, underscoring the indispensability of both the anomalous and mean advections in this North Atlantic Ocean–originated AMOC MCO. In our coupled model simulation, the South Atlantic and Southern Oceans do not exhibit variabilities synchronous with the AMOC MCO; the Arctic Ocean’s contribution to the subpolar upper salinity anomaly is much weaker than the North Atlantic. Hence, this North Atlantic Ocean–originated AMOC MCO is distinct from the previously proposed Southern Ocean–originated and Arctic Ocean–originated AMOC MCOs.
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