• Past-present-future monsoon variability revealed by stable water isotopes, 2018-2020, Vetenskapsrådet
• Simulating green Sahara with an earth system model, 2018-2021, Vetenskapsrådet

Stabila vattenisotoper och modellering

Stabila vattenisotoper (SWI) möjliggör en djupgående utvärdering av den fysiska representationen av den hydrologiska cykeln inom modellen. För paleoklimatstudier ger modellering av klimatproxys som vatten oss ytterligare insikter om temperatur- och rumsliga isotop-temperaturförhållanden och effekterna av klimatförändringar.

Klimatrekonstruktioner med SWI

Observationer har kopplat paleoklimatiska isotopdata till icke-lokala mönster av klimatvariabilitet, särskilt till atmosfäriska cirkulationsmönster på mellanbreddgraderna och till variationer i tropikerna. Följaktligen kan paleoklimatrekonstruktioner bättre utnyttja isotopdata som proxyer genom att kombinera dem med klimatmodeller med isotopdiagnostik.

Pågående arbete

Jag arbetar för närvarande med projekt som fokuserar på data-modell-jämförelser mellan isotop-modeller och paleoklimatiska arkiv (fokus på stalagmiter). Huvudfokus ligger på det senaste årtusendet och görs både i global skala och på den indiska sommarmonsunregionen. Projekten involverar både statistiska bedömningar (globalt) och ett mål att producera klimatfältrekonstruktioner (Indian sommarmonsunen) med hjälp av dataassimileringstekniker och stalagmit-data.

 

I urval från Stockholms universitets publikationsdatabas

• Interglacial climates in proxies and models: Utilizing sampled oxygen isotopes and model simulations to understand past Indian summer monsoon variability

  2024. Josefine Axelsson.

  Avhandling (Dok)

  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.

  Läs mer om Interglacial climates in proxies and models

• A Precipitation Isotopic Response in 2014-2015 to Moisture Transport Changes in the Central Himalayas

  2023. Josefine Axelsson (et al.). Journal of Geophysical Research - Atmospheres 128 (13)

  Artikel

  The impact of moisture transport and sources on precipitation stable isotopes (d(18)O and d-excess) in the central Himalayas are crucial to understanding the climatic archives. However, this is still unclear due to the lack of in-situ observations. Here we present measurements of stable isotopes in precipitation at two stations (Yadong and Pali) in the central Himalayas during 2014-2015. Combined with simulations from the dispersion model FLEXPART, we investigate effects on precipitation stable isotopes related to changes in moisture sources and convections in the region, and possible influence by El Nino. Our results suggest that the moisture supplies related to evaporation over northeastern India and moisture losses related to convective activities over the Bay of Bengal (BoB) and Bangladesh region play important roles in changes in d(18)O and d-excess in precipitation in the Yadong Valley. Outgoing longwave radiation and moisture flux divergence analysis further confirm that the contribution from continental evaporation dominates the moisture supply in the central Himalayas with a lesser contribution from convection over the BoB during the 2015 monsoon season compared with 2014. A change in the altitude effect is observed in 2015, which is more significant than the temperature and precipitation amount effect during the observation period. These findings provide valuable insights into climatic interpretations of paleo-isotopic archives with an isotopic response to changes in moisture transport to the central Himalayas.

  Läs mer om A Precipitation Isotopic Response in 2014-2015 to Moisture Transport Changes in the Central Himalayas

• Holocene climate change in southern Oman deciphered by speleothem records and climate model simulations

  2023. Ye Tian (et al.). Nature Communications 14 (1)

  Artikel

  Qunf Cave oxygen isotope (& delta;O-18(c)) record from southern Oman is one of the most significant of few Holocene Indian summer monsoon cave records. However, the interpretation of the Qunf & delta;O-18(c) remains in dispute. Here we provide a multi-proxy record from Qunf Cave and climate model simulations to reconstruct the Holocene local and regional hydroclimate changes. The results indicate that besides the Indian summer monsoon, the North African summer monsoon also contributes water vapor to southern Oman during the early to middle Holocene. In principle, Qunf & delta;O-18(c) values reflect integrated oxygen-isotope fractionations over a broad moisture transport swath from moisture sources to the cave site, rather than local precipitation amount alone, and thus the Qunf & delta;O-18(c) record characterizes primary changes in the Afro-Asian monsoon regime across the Holocene. In contrast, local climate proxies appear to suggest an overall slightly increased or unchanged wetness over the Holocene at the cave site. Southern Oman speleothem oxygen isotope and multi-proxy data reveal diverse changes in the Afro-Indian summer monsoon circulations and local hydroclimate conditions during the Holocene, confirming climate model simulations.

  Läs mer om Holocene climate change in southern Oman deciphered by speleothem records and climate model simulations

• Investigating stable oxygen and carbon isotopic variability in speleothem records over the last millennium using multiple isotope-enabled climate models

  2022. Janica C. Bühler (et al.). Climate of the Past 18 (7), 1625-1654

  Artikel

  The incorporation of water isotopologues into the hydrology of general circulation models (GCMs) facilitates the comparison between modeled and measured proxy data in paleoclimate archives. However, the variability and drivers of measured and modeled water isotopologues, as well as the diversity of their representation in different models, are not well constrained. Improving our understanding of this variability in past and present climates will help to better constrain future climate change projections and decrease their range of uncertainty. Speleothems are a precisely datable terrestrial paleoclimate archives and provide well-preserved (semi-)continuous multivariate isotope time series in the lower latitudes and mid-latitudes and are therefore well suited to assess climate and isotope variability on decadal and longer timescales. However, the relationships of speleothem oxygen and carbon isotopes to climate variables are influenced by site-specific parameters, and their comparison to GCMs is not always straightforward.

  Here we compare speleothem oxygen and carbon isotopic signatures from the Speleothem Isotopes Synthesis and Analysis database version 2 (SISALv2) to the output of five different water-isotope-enabled GCMs (ECHAM5-wiso, GISS-E2-R, iCESM, iHadCM3, and isoGSM) over the last millennium (850–1850 CE). We systematically evaluate differences and commonalities between the standardized model simulation outputs. The goal is to distinguish climatic drivers of variability for modeled isotopes and compare them to those of measured isotopes.

  We find strong regional differences in the oxygen isotope signatures between models that can partly be attributed to differences in modeled surface temperature. At low latitudes, precipitation amount is the dominant driver for stable water isotope variability; however, at cave locations the agreement between modeled temperature variability is higher than for precipitation variability. While modeled isotopic signatures at cave locations exhibited extreme events coinciding with changes in volcanic and solar forcing, such fingerprints are not apparent in the speleothem isotopes. This may be attributed to the lower temporal resolution of speleothem records compared to the events that are to be detected. Using spectral analysis, we can show that all models underestimate decadal and longer variability compared to speleothems (albeit to varying extents).

  We found that no model excels in all analyzed comparisons, although some perform better than the others in either mean or variability. Therefore, we advise a multi-model approach whenever comparing proxy data to modeled data. Considering karst and cave internal processes, e.g., through isotope-enabled karst models, may alter the variability in speleothem isotopes and play an important role in determining the most appropriate model. By exploring new ways of analyzing the relationship between the oxygen and carbon isotopes, their variability, and co-variability across timescales, we provide methods that may serve as a baseline for future studies with different models using, e.g., different isotopes, different climate archives, or different time periods.

  Läs mer om Investigating stable oxygen and carbon isotopic variability in speleothem records over the last millennium using multiple isotope-enabled climate models

• Seasonal to decadal variations of precipitation oxygen isotopes in northern China linked to the moisture source

  2024. Fangyuan Lin (et al.). npj Climate and Atmospheric Science 7

  Artikel

  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 (δ¹⁸O_p) 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 δ¹⁸O_p in northern China. Our data indicate that during the non-monsoon season, the δ¹⁸O_p 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 δ¹⁸O_p. Intensified upstream convection along the NIO moisture transport pathway results in a more negative summer δ¹⁸O_p compared to moisture transported from the PO and LLA regions. Our analysis shows a decadal shift in summer δ¹⁸O_p 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 δ¹⁸O_p variability can be recorded in the natural archives from the region, which can provide valuable insights into understanding past climate variability.

  Läs mer om Seasonal to decadal variations of precipitation oxygen isotopes in northern China linked to the moisture source

• EC-Earth Simulations Reveal Enhanced Inter-Hemispheric Thermal Contrast During the Last Interglacial Further Intensified the Indian Monsoon

  2022. Kaiqi Chen (et al.). Geophysical Research Letters 49 (6)

  Artikel

  Paleoclimate proxy data indicate a stronger Indian summer monsoon (ISM) during the Last Interglacial (LIG) than in the present day. This is largely attributed to orbital forcing induced high seasonal and latitudinal insolation anomalies in the Northern Hemisphere during LIG. According to the general circulation model EC-Earth3, the simulated ISM rainfall is increased by approximately 28% during the LIG compared to the pre-industrial period as a result of the orbital forcing and the amplified land-sea contrast due to both local and remote ocean feedbacks. Although the LIG is often portrayed as a potential analogue of future warmer climates, our study suggests that the enhanced inter-hemispheric thermal gradient during the LIG strengthened the ISM, in opposition to the observed weakening of ISM under present-day warming.

  Läs mer om EC-Earth Simulations Reveal Enhanced Inter-Hemispheric Thermal Contrast During the Last Interglacial Further Intensified the Indian Monsoon

• Simulating the mid-Holocene, last interglacial and mid-Pliocene climate with EC-Earth3-LR

  2021. Qiong Zhang (et al.). Geoscientific Model Development 14 (2), 1147-1169

  Artikel

  As global warming is proceeding due to rising greenhouse gas concentrations, the Earth system moves towards climate states that challenge adaptation. Past Earth system states are offering possible modelling systems for the global warming of the coming decades. These include the climate of the mid-Pliocene (similar to 3 Ma), the last interglacial (similar to 129-116 ka) and the mid-Holocene (similar to 6 ka). The simulations for these past warm periods are the key experiments in the Paleoclimate Model Intercomparison Project (PMIP) phase 4, contributing to phase 6 of the Coupled Model Intercomparison Project (CMIP6). Paleoclimate modelling has long been regarded as a robust out-of-sample test bed of the climate models used to project future climate changes. Here, we document the model setup for PMIP4 experiments with EC-Earth3-LR and present the large-scale features from the simulations for the mid-Holocene, the last interglacial and the mid-Pliocene. Using the pre-industrial climate as a reference state, we show global temperature changes, large-scale Hadley circulation and Walker circulation, polar warming, global monsoons and the climate variability modes - El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). EC-Earth3-LR simulates reasonable climate responses during past warm periods, as shown in the other PMIP4-CMIP6 model ensemble. The systematic comparison of these climate changes in past three warm periods in an individual model demonstrates the model's ability to capture the climate response under different climate forcings, providing potential implications for confidence in future projections with the EC-Earth model.

  Läs mer om Simulating the mid-Holocene, last interglacial and mid-Pliocene climate with EC-Earth3-LR

• The Role of El Niño in Driving Drought Conditions over the Last 2000 Years in Thailand

  2020. Katherine Power (et al.). Quaternary 3 (2)

  Artikel

  Irregular climate events frequently occur in Southeast Asia due to the numerous climate patterns combining. Thailand sits at the confluence of these interactions, and consequently experiences major hydrological events, such as droughts. Proxy data, speleothem records, lake sediment sequences and tree ring chronologies were used to reconstruct paleo drought conditions. These trends were compared with modelled and historic El Nino Southern Oscillation (ENSO) data to assess if the ENSO climate phenomena is causing droughts in Thailand. Drought periods were found to occur both during El Nino events and ENSO neutral conditions. This indicates droughts are not a product of one climate pattern, but likely the result of numerous patterns interacting. There is uncertainty regarding how climate patterns will evolve under climate change, but changes in amplitude and variability could potentially lead to more frequent and wider reaching hydrological disasters. It is vital that policies are implemented to cope with the resulting social and economic repercussions, including diversification of crops and reorganisation of water consumption behaviour in Thailand.

  Läs mer om The Role of El Niño in Driving Drought Conditions over the Last 2000 Years in Thailand

• Mass Balance Sensitivity and Future Projections of Rabots Glaciär, Sweden

  2021. Moon Taveirne (et al.). Climate 9 (8)

  Artikel

  Glacier mass balance is heavily influenced by climate, with responses of individual glaciers to various climate parameters varying greatly. In northern Sweden, Rabots Glaciär's mass balance has decreased since it started being monitored in 1982. To relate Rabots Glaciär's mass balance to changes in climate, the sensitivity to a range of parameters is computed. Through linear regression of mass balance with temperature, precipitation, humidity, wind speed and incoming radiation the climate sensitivity is established and projections for future summer mass balance are made. Summer mass balance is primarily sensitive to temperature at -0.31 m w.e. per degrees C change, while winter mass balance is mainly sensitive to precipitation at 0.94 m w.e. per % change. An estimate using summer temperature sensitivity projects a dramatic decrease in summer mass balance to -3.89 m w.e. for the 2091-2100 period under climate scenario RCP8.5. With large increases in temperature anticipated for the next century, more complex modelling studies of the relationship between climate and glacier mass balance is key to understanding the future development of Rabots Glaciär.

  Läs mer om Mass Balance Sensitivity and Future Projections of Rabots Glaciär, Sweden

• Regional and Local Impacts of the ENSO and IOD Events of 2015 and 2016 on the Indian Summer Monsoon-A Bhutan Case Study

  2021. Katherine Power (et al.). Atmosphere 12 (8)

  Artikel

  The Indian Summer Monsoon (ISM) plays a vital role in the livelihoods and economy of those living on the Indian subcontinent, including the small, mountainous country of Bhutan. The ISM fluctuates over varying temporal scales and its variability is related to many internal and external factors including the El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). In 2015, a Super El Nino occurred in the tropical Pacific alongside a positive IOD in the Indian Ocean and was followed in 2016 by a simultaneous La Nina and negative IOD. These events had worldwide repercussions. However, it is unclear how the ISM was affected during this time, both at a regional scale over the whole ISM area and at a local scale over Bhutan. First, an evaluation of data products comparing ERA5 reanalysis, TRMM and GPM satellite, and GPCC precipitation products against weather station measurements from Bhutan, indicated that ERA5 reanalysis was suitable to investigate ISM change in these two years. The reanalysis datasets showed that there was disruption to the ISM during this period, with a late onset of the monsoon in 2015, a shifted monsoon flow in July 2015 and in August 2016, and a late withdrawal in 2016. However, this resulted in neither a monsoon surplus nor a deficit across both years but instead large spatial-temporal variability. It is possible to attribute some of the regional scale changes to the ENSO and IOD events, but the expected impact of a simultaneous ENSO and IOD events are not recognizable. It is likely that 2015/16 monsoon disruption was driven by a combination of factors alongside ENSO and the IOD, including varying boundary conditions, the Pacific Decadal Oscillation, the Atlantic Multi-decadal Oscillation, and more. At a local scale, the intricate topography and orographic processes ongoing within Bhutan further amplified or dampened the already altered ISM.

  Läs mer om Regional and Local Impacts of the ENSO and IOD Events of 2015 and 2016 on the Indian Summer Monsoon-A Bhutan Case Study

Visa alla publikationer av Josefine Axelsson vid Stockholms universitet