publications

2024

1. 

   Interannual ENSO diversity, transitions, and projected changes in observations and climate models

   Mandy B Freund, Josephine R Brown, Andrew G Marshall, Carly R Tozer, Benjamin J Henley, James S Risbey, Nandini Ramesh, Ruby Lieber, and S Sharmila

   Environmental Research Letters, Sep 2024

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   Diverse characteristics of El Niño Southern Oscillation (ENSO) events challenge the traditional view of tropical coupled ocean-atmosphere systems. The probability of a transition from one type of event to another is influenced by multiple factors of which many are projected to change. Here we assess the likelihood of ENSO transitions in observations and climate models, including a distinction between events that peak in the Eastern Pacific (EP) and Central Pacific (CP). We find that the initial ENSO state influences the likelihood of certain transitions and that some transitions are not physically possible or stochastically likely. For example, transitions to CP events are more likely than EP events except from a neutral state. We also find that El Niños tend to occur as singular events compared to La Niñas. While consecutive El Niño and La Niña events of EP type are possible, opposing EP events do not occur in succession. We identify several transitions likely driven by internal dynamical processes including neutral conditions to El Niño, CP El Niño to another El Niño, EP El Niño to CP La Niña, CP La Niña to CP El Niño and La Niña, and EP La Niña to neutral and CP El Niño. Projections of future transitions show an increased probability of transitions to CP El Niño events while transitions to EP La Niña events become less frequent under a high-emissions scenario. Accordingly, transitions to these events become more and less likely, respectively. We also find changes in the likelihood of specific transitions in a warming world: consecutive CP El Niño events become more likely while EP El Niño events become less likely to transition into CP La Niña events. These changes are expected to occur as early as 2050 with some changes to be accelerated by the end of the 21st century.

2. Intensification and Poleward Shift of Compound Wind and Precipitation Extremes in a Warmer Climate

   Delei Li, Jakob Zscheischler, Yang Chen, Baoshu Yin, Jianlong Feng, Mandy B Freund, Jifeng Qi, Yuchao Zhu, and Emanuele Bevacqua

   Geophysical Research Letters, Sep 2024

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   Compound wind and precipitation extremes (CWPEs) can severely impact natural andsocioeconomic systems. However, our understanding of CWPE future changes, drivers, and uncertainties undera warmer climate is limited. Here, by analyzing the event both on oceans and landmasses via state‐of‐the‐artclimate model simulations, we reveal a poleward shift of CWPE occurrences by the late 21st century, withnotable increases at latitudes exceeding 50° in both hemispheres and decreases in the subtropics around 25°.CWPE intensification occurs across approximately 90% of global landmasses, and is most pronounced under ahigh‐emission scenario. Most changes in CWPE frequency and intensity (about 70% and 80%, respectively)stem from changes in precipitation extremes. We further identify large uncertainties in CWPE changes, whichcan be understood at the regional level by considering climate model differences in trends of CWPE drivers.These results provide insights into understanding CWPE changes under a warmer climate, aiding robustregional adaptation strategy development

3. Distinct geographical and seasonal signals in two tree-ring based streamflow reconstructions from Tasmania, southeastern Australia

   Kathryn J Allen, Danielle C Verdon-Kidd, Mandy B Freund, Carly R Tozer, Jonathan G Palmer, Philippa A Higgins, Krystyna M Saunders, and Patrick J Baker

   Journal of Hydrology: Regional Studies, Sep 2024

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   Study focus We present two new tree-ring based inflow reconstructions from western Tasmania in southeastern Australia.The warm season reconstruction (Dec–Feb) extends from 1030–2007 CE and explains up to 42% of the variance in instrumental flow, while the cool season (JA) extends from 1550–2007 CE and explains 27% of instrumental flow variance. Key features include an extended pluvial period in the 11th Century and a protracted dry period in 1500CE, neither of which are represented in the DJF instrumental record. Decreasing JA flow since the 19th Century is consistent with a local sediment-based hydroclimate record. New hydrological insights for the region The reconstructions confirm that the instrumental data do not capture how protracted past low or high flow periods have been. It is therefore important to consider pre-instrumental flow data when planning for the future. The reconstructions provide new insights into regional variability through their association with the Subtropical Ridge (STR) and the Southern Annular Mode (SAM). Differing spatial signatures of the seasonal reconstructions, and their associations with season-specific impacts of STR and SAM, highlight the need for caution when considering the use of remote hydroclimate proxy records with strong seasonal signatures. The reconstructions suggest that extrapolation of seasonally defined reconstructions to represent annual flow for regions beyond the extent of their spatial footprint may be problematic.

4. Sommertrockenheit in Nordostdeutschland im europäischen historischen Kontext–Einblicke durch stabile Isotope in Baumjahrringen.

   Gerhard Helle, Mandy B Freund, Viorica Nagavciuc, Ingo Heinrich, and Daniel Balanzategui

   Sep 2024
5. A multi-model likelihood analysis of unprecedented extreme rainfall along the east coast of Australia

   Damien B Irving, James S Risbey, Dougal T Squire, Richard Matear, Carly Tozer, Didier P Monselesan, Nandini Ramesh, P Jyoteeshkumar Reddy, and Mandy B Freund

   Meteorological Applications, Sep 2024

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   A large stretch of the east coast of Australia experienced unprecedented rainfall and flooding over a two-week period in early 2022. It is difficult to reliably estimate the likelihood of such a rare event from the relatively short observational record, so an alternative is to use data from an ensemble prediction system (e.g., a seasonal or decadal forecast system) to obtain a much larger sample of simulated weather events. This so-called ‘UNSEEN’ method has been successfully applied in several scientific studies, but those studies typically rely on a single prediction system. In this study, we use data from the Decadal Climate Prediction Project to explore the model uncertainty associated with the UNSEEN method by assessing 10 different hindcast ensembles. Using the 15-day rainfall total averaged over the river catchments impacted by the 2022 east coast event, we find that the models produce a wide range of likelihood estimates. Even after excluding a number of models that fail basic fidelity tests, estimates of the event return period ranged from 320 to 1814 years. The vast majority of models suggested the event is rarer than a standard extreme value assessment of the observational record (297 years). Such large model uncertainty suggests that multi-model analysis should become part of the standard UNSEEN procedure.

6. On the archetypalflavours’, indices and teleconnections of ENSO revealed by global sea surface temperatures

   Didier P Monselesan, James S Risbey, Benoit Legresy, Sophie Cravatte, Bastien Pagli, Takeshi Izumo, Christopher C Chapman, Mandy B Freund, Abdelwaheb Hannachi, Damien Irving, and  others

   arXiv preprint arXiv:2406.08694, Sep 2024
7. Historical and Future Asymmetry of ENSO Teleconnections with Extremes

   Ruby Lieber, Josephine Brown, Andrew King, and Mandy B Freund

   Journal of Climate, Aug 2024

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   El Niño-Southern Oscillation (ENSO) is the dominant source of climate variability globally. Many of the most devastating impacts of ENSO are felt through extremes. Here we present and describe a spatially complete global synthesis of extreme temperature and precipitation relationships with ENSO. We also investigate how these relationships evolve under a future warming scenario under high greenhouse gas emissions using fourteen models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble. Firstly, we demonstrate that models broadly capture observed ENSO teleconnections to means and extremes using the Twentieth Century Reanalysis version 3 (20CRv3). The models project that more regions will experience an amplification of the historical ENSO teleconnection with mean temperature and precipitation than a dampening under a high-emissions climate projection. The response of the ENSO teleconnection with extremes is very similar to the mean response, with even larger changes in some regions. Hence, regions that are predicted to experience an amplification of the ENSO teleconnection under future warming can also expect a comparable amplification in the intensity of extremes. Furthermore, models that suggest greater amplification of ENSO amplitude also tend to exhibit greater intensification of teleconnections. Future changes in regional climate variability may be better constrained if changes in ENSO itself are better understood.

2023

1. Variability and long-term change in Australian monsoon rainfall: A review

   Hanna Heidemann, Tim Cowan, Benjamin J Henley, Joachim Ribbe, Mandy B Freund, and Scott Power

   Wiley Interdisciplinary Reviews: Climate Change, Aug 2023

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   The Australian monsoon delivers seasonal rain across a vast area of the continent stretching from the far northern tropics to the semi-arid regions. This article provides a review of advances in Australian monsoon rainfall (AUMR) research and a supporting analysis of AUMR variability, observed trends, and future projections. AUMR displays a high degree of interannual variability with a standard deviation of approximately 34% of the mean. AUMR variability is mostly driven by the El Niño-Southern Oscillation (ENSO), although sea surface temperature anomalies in the tropical Indian Ocean and north of Australia also play a role. Decadal AUMR variability is strongly linked to the Interdecadal Pacific Oscillation (IPO), partially through the IPO’s impact on the strength and position of the Pacific Walker Circulation and the South Pacific Convergence Zone. AUMR exhibits a century-long positive trend, which is large (approximately 20 mm per decade) and statistically significant over northwest Australia. The cause of the observed trend is still debated. Future changes in AUMR over the next century remain uncertain due to low climate model agreement on the sign of change. Recommendations to improve the understanding of AUMR and confidence in AUMR projections are provided. This includes improving the representation of atmospheric convective processes in models, further explaining the mechanisms responsible for AUMR variability and change. Clarifying the mechanisms of AUMR variability and change would aid with creating more sustainable future agricultural systems by increasing the reliability of predictions and projections.

2. 

   European tree-ring isotopes indicate unusual recent hydroclimate

   Mandy B Freund, Gerhard Helle, Daniel F Balting, Natasha Ballis, Gerhard H Schleser, and Ulrich Cubasch

   Communications earth & environment, Aug 2023

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   In recent decades, Europe has experienced more frequent flood and drought events. However, little is known about the long-term, spatiotemporal hydroclimatic changes across Europe. Here we present a climate field reconstruction spanning the entire European continent based on tree-ring stable isotopes. A pronounced seasonal consistency in climate response across Europe leads to a unique, well-verified spatial field reconstruction of European summer hydroclimate back to AD 1600. We find three distinct phases of European hydroclimate variability as possible fingerprints of solar activity (coinciding with the Maunder Minimum and the end of the Little Ice Age) and pronounced decadal variability superimposed by a long-term drying trend from the mid-20th century. We show that the recent European summer drought (2015–2018) is highly unusual in a multi-century context and unprecedented for large parts of central and western Europe. The reconstruction provides further evidence of European summer droughts potentially being influenced by anthropogenic warming and draws attention to regional differences.

3. ENSO diversity, transitions and projected changes

   Mandy B Freund, Josephine Brown, Andrew Marshall, Carly Tozer, Benjamin Henley, James Risbey, and Sur Sharmila

   AGU23, Aug 2023
4. A past and present perspective on the European summer vapour pressure deficit

   Viorica Nagavciuc, Simon LL Michel, Daniel F Balting, Gerhard Helle, Mandy B Freund, Gerhard H Schleser, David N Steger, Gerrit Lohmann, and Monica Ionita

   Climate of the Past Discussions, Aug 2023

2022

1. A past, present and future perspective on the European summer vapour pressure deficit

   Daniel Balting, Simon Michel, Viorica Nagavciuc, Gerhard Helle, Mandy B Freund, Gerhard H Schleser, David Steger, Gerrit Lohmann, and Monica Ionita

   Earth System Science Data Discussions, Aug 2022
2. Past and future rainfall changes in the Australian midlatitudes and implications for agriculture

   Katharina Waha, John Clarke, Kavina Dayal, Mandy B Freund, Craig Heady, Irene Parisi, and Elisabeth Vogel

   Climatic Change, Aug 2022

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   Annual rainfall and the seasonality of rainfall during a year are important drivers of agricultural productivity and profitability in Australian agriculture. Historic trend detection can give insights into significant and prolonged changes that might continue in the future and are of relevance to agriculture. Here we complement the analysis of historic data with climate projections from global climate models. We use gridded and station rainfall data for three study areas in the Australian midlatitudes, between 24°S and 35°S. Total summer and winter rainfall, annual total rainfall and annual rainfall extreme indices are calculated for the period 1907 to 2018. Historic trends are analysed with statistical significance tests of linear trends and rainfall deciles. Future trends are analysed for annual, summer and winter rainfall for three time periods, 2020–2039, 2040–2059 and 2060–79, as the ensemble of 25–37 global climate models. Summer rainfall in the Western Australia wheat belt increased by 0.18–0.21 mm per year. Winter rainfall decreased by 0.42–0.43 mm per year. Parts of northern New South Wales (NSW) experienced an exceptionally dry decade, 2011–2020 with summer rainfall 50–200 mm below the long-term average. Future rainfall projections for the wheat belt show a strong declining trend, irrespective of the climate scenario, while in the other two regions, increases and decreases are possible. We confirm previous findings of declines in winter rainfall in the Western Australia wheat Belt but do not detect any such changes in Eastern Australia. Some of the observed long periods of dry summers in northern NSW are rather unprecedented. Future studies should repeat the trend analysis for Eastern Australia as the data becomes available. Apart from climate indices, agrometeorological indices for specific agricultural commodities should be developed and used in trend analysis.

3. ENSO teleconnections more uncertain in regions of lower socioeconomic development

   Ruby Lieber, Andrew King, Josephine Brown, Linden Ashcroft, Mandy B Freund, and Celia McMichael

   Geophysical Research Letters, Aug 2022

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   The El Niño Southern Oscillation (ENSO) impacts climate variability globally and can influence extreme climate and weather events. We quantify the uncertainty in ENSO’s atmospheric teleconnections with extremes using the Twentieth Century Reanalysis, showing that uncertainty estimates vary regionally over the historical period. Uncertainty is found to be greater in regions of lower socioeconomic development. This can be linked to the limited availability of observational data in these regions as well as difficulties constraining tropical climate dynamics in global gridded atmospheric data sets. Poorer locations face greater challenges due to lack of understanding of past variability limiting confidence in regional projections.

2021

1. 

   Central Pacific El Niño as a precursor to summer drought-breaking rainfall over southeastern Australia

   Mandy B Freund, Andrew G. Marshall, Matthew C. Wheeler, and Jaclyn N. Brown

   Geophysical Research Letters, Aug 2021

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   Using an extended 120-year record of El Niño events, we distinguish between central Pacific (CP) and eastern Pacific (EP) types to show that the strength of CP events is a factor in the amplitude and sign of the impact on rainfall over southeastern Australia. Both weak and strong CP events cause widespread rainfall deficits in Australia during the onset phase from April to September. However, this relationship reverses over southeastern Australia including the Murray Darling Basin river catchment region for the strongest CP events after October, leading to positive rainfall anomalies during the mature phase of strong CP El Niños. This reversal can be explained by a change in the circulation over eastern Australia from drier, more westerly orientated flow to moister, more easterly onshore flow. These findings may help with seasonal prediction efforts to predict drought-breaking rain such as occurred in early 2020.

2. Large-scale climate signals of a European oxygen isotope network from tree rings

   Daniel F Balting, Monica Ionita, Martin Wegmann, Gerhard Helle, Gerhard H Schleser, Norel Rimbu, Mandy B Freund, Ingo Heinrich, Diana Caldarescu, and Gerrit Lohmann

   Climate of the Past, Aug 2021

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   We investigate the climate signature of δ18O tree-ring records from sites distributed all over Europe covering the last 400 years. An empirical orthogonal function (EOF) analysis reveals two distinct modes of variability on the basis of the existing δ18O tree-ring records. The first mode is associated with anomaly patterns projecting onto the El Niño–Southern Oscillation (ENSO) and reflects a multi-seasonal climatic signal. The ENSO link is pronounced for the last 130 years, but it is found to be weak over the period from 1600 to 1850, suggesting that the relationship between ENSO and the European climate may not be stable over time. The second mode of δ18O variability, which captures a north–south dipole in the European δ18O tree-ring records, is related to a regional summer atmospheric circulation pattern, revealing a pronounced centre over the North Sea. Locally, the δ18O anomalies associated with this mode show the same (opposite) sign with temperature (precipitation). Based on the oxygen isotopic signature derived from tree rings, we argue that the prevailing large-scale atmospheric circulation patterns and the related teleconnections can be analysed beyond instrumental records.

2020

1. Insights from CMIP6 for Australia’s future climate

   Michael Richard Grose, Sugata Narsey, FP Delage, Andrew J Dowdy, Margot Bador, Ghyslaine Boschat, Christine Chung, JB Kajtar, Surendra Rauniyar, Mandy B Freund, and  others

   Earth’s Future, Aug 2020

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   Outputs from new state-of-the-art climate models under the Coupled Model Inter-comparison Project phase 6 (CMIP6) promise improvement and enhancement of climate change projections information for Australia. Here we focus on three key aspects of CMIP6: what is new in these models, how the available CMIP6 models evaluate compared to CMIP5, and their projections of the future Australian climate compared to CMIP5 focussing on the highest emissions scenario. The CMIP6 ensemble has several new features of relevance to policymakers and others, for example, the integrated matrix of socioeconomic and concentration pathways. The CMIP6 models show incremental improvements in the simulation of the climate in the Australian region, including a reduced equatorial Pacific cold tongue bias, slightly improved rainfall teleconnections with large-scale climate drivers, improved representation of atmosphere and ocean extreme heat events, as well as dynamic sea level. However, important regional biases remain, evident in the excessive rainfall over the Maritime Continent and rainfall pattern biases in the nearby tropical convergence zones. Projections of Australian temperature and rainfall from the available CMIP6 ensemble broadly agree with those from CMIP5, except for a group of CMIP6 models with higher climate sensitivity and greater warming and increase in some extremes after 2050. CMIP6 rainfall projections are similar to CMIP5, but the ensemble examined has a narrower range of rainfall change in austral summer in Northern Australia and austral winter in Southern Australia. Overall, future national projections are likely to be similar to previous versions but perhaps with some areas of improved confidence and clarity.

2. Large scale climate signals of a European oxygen isotope network from tree-rings-predominantly caused by ENSO teleconnections?

   Daniel F Balting, Monica Ionita, Martin Wegmann, Gerhard Helle, Gerhard H Schleser, Norel Rimbu, Mandy B Freund, Ingo Heinrich, Diana Caldarescu, and Gerrit Lohmann

   Clim Past Discuss, Aug 2020

3. 

   Warming patterns affect El Niño diversity in CMIP5 and CMIP6 models

   Mandy B Freund, Josephine R Brown, Benjamin J Henley, David J Karoly, and Jaclyn N Brown

   Journal of Climate, Aug 2020

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   Given the consequences and global significance of El Niño–Southern Oscillation (ENSO) events it is essential to understand the representation of El Niño diversity in climate models for the present day and the future. In recent decades, El Niño events have occurred more frequently in the central Pacific (CP). Eastern Pacific (EP) El Niño events have increased in intensity. However, the processes and future implications of these observed changes in El Niño are not well understood. Here, the frequency and intensity of El Niño events are assessed in models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), and results are compared to extended instrumental and multicentury paleoclimate records. Future changes of El Niño are stronger for CP events than for EP events and differ between models. Models with a projected La Niña–like mean-state warming pattern show a tendency toward more EP but fewer CP events compared to models with an El Niño–like warming pattern. Among the models with more El Niño–like warming, differences in future El Niño can be partially explained by Pacific decadal variability (PDV). During positive PDV phases, more El Niño events occur, so future frequency changes are mainly determined by projected changes during positive PDV phases. Similarly, the intensity of El Niño is strongest during positive PDV phases. Future changes to El Niño may thus depend on both mean-state warming and decadal-scale natural variability.

4. The shared socio-economic pathway (SSP) greenhouse gas concentrations and their extensions to 2500

   Malte Meinshausen, Zebedee RJ Nicholls, Jared Lewis, Matthew J Gidden, Elisabeth Vogel, Mandy B Freund, Urs Beyerle, Claudia Gessner, Alexander Nauels, Nico Bauer, and  others

   Geoscientific Model Development, Aug 2020

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   Anthropogenic increases in atmospheric greenhouse gas concentrations are the main driver of current and future climate change. The integrated assessment community has quantified anthropogenic emissions for the shared socio-economic pathway (SSP) scenarios, each of which represents a different future socio-economic projection and political environment. Here, we provide the greenhouse gas concentrations for these SSP scenarios – using the reduced-complexity climate–carbon-cycle model MAGICC7.0. We extend historical, observationally based concentration data with SSP concentration projections from 2015 to 2500 for 43 greenhouse gases with monthly and latitudinal resolution. CO2 concentrations by 2100 range from 393 to 1135 ppm for the lowest (SSP1-1.9) and highest (SSP5-8.5) emission scenarios, respectively. We also provide the concentration extensions beyond 2100 based on assumptions regarding the trajectories of fossil fuels and land use change emissions, net negative emissions, and the fraction of non-CO2 emissions. By 2150, CO2 concentrations in the lowest emission scenario are approximately 350 ppm and approximately plateau at that level until 2500, whereas the highest fossil-fuel-driven scenario projects CO2 concentrations of 1737 ppm and reaches concentrations beyond 2000 ppm by 2250. We estimate that the share of CO2 in the total radiative forcing contribution of all considered 43 long-lived greenhouse gases increases from 66 % for the present day to roughly 68 % to 85 % by the time of maximum forcing in the 21st century. For this estimation, we updated simple radiative forcing parameterizations that reflect the Oslo Line-By-Line model results. In comparison to the representative concentration pathways (RCPs), the five main SSPs (SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) are more evenly spaced and extend to lower 2100 radiative forcing and temperatures. Performing two pairs of six-member historical ensembles with CESM1.2.2, we estimate the effect on surface air temperatures of applying latitudinally and seasonally resolved GHG concentrations. We find that the ensemble differences in the March–April–May (MAM) season provide a regional warming in higher northern latitudes of up to 0.4 K over the historical period, latitudinally averaged of about 0.1 K, which we estimate to be comparable to the upper bound (∼5 % level) of natural variability. In comparison to the comparatively straight line of the last 2000 years, the greenhouse gas concentrations since the onset of the industrial period and this studies’ projections over the next 100 to 500 years unequivocally depict a “hockey-stick” upwards shape. The SSP concentration time series derived in this study provide a harmonized set of input assumptions for long-term climate science analysis; they also provide an indication of the wide set of futures that societal developments and policy implementations can lead to – ranging from multiple degrees of future warming on the one side to approximately 1.5 ∘C warming on the other.

5. Hydroclimate extremes in a north Australian drought reconstruction asymmetrically linked with Central Pacific Sea surface temperatures

   KJ Allen, Mandy B Freund, Jonathan G Palmer, R Simkin, L Williams, M Brookhouse, Edward R Cook, S Stewart, and Patrick J Baker

   Global and Planetary Change, Aug 2020

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   An understanding of tropical hydroclimate variability, the associated drivers and how it is likely to change is a major scientific and societal challenge that is acutely hampered by short instrumental records. We present a 246-year tree-ring drought reconstruction of the Standardised Precipitation Evaporation Index (SPEI) for monsoonal northern Australia for the end of the wet season (March–May; MAM). This reconstruction extends the instrumental record back by 150 years. Around one third of total annual rainfall falls during MAM, making it a crucial component of the monsoonal cycle. MAM is also the season most impacted by the differential decay process of Central Pacific (as opposed to Western Pacific) El Niño events that are linked with dry conditions over northern and northwestern Australia more generally. Our reconstruction therefore provides an opportunity to consider how central Pacific variability has modulated MAM hydroclimate in Australia’s monsoonal north over the past two and a half centuries. We found that MAM hydroclimate extremes in the region have a strong, but asymmetric relationship with central Pacific sea surface temperatures (SSTs) and ENSO indices. Extremely wet MAMs in monsoonal north Australia were associated with cooler SSTs, above average rainfall across much of Australia, and often coincided with La Niña events. The spatial relationship between dry extremes and Pacific SSTs during dry events was generally, but weakly, consistent with the SST signature of central Pacific El Niño events. The association between reconstructed dry extremes in the monsoonal north and dry conditions across the rest of Australia is also less extensive and weaker than for wet events. Results suggest that more extreme wet events in the Australian monsoonal north likely reflect cool central Pacific SSTs and later termination of the Australian monsoon. Consecutive years with extremely dry MAMs became more frequent over the latter part of the 20th Century while the probability of an extreme dry MAM followed by an extreme wet MAM the next year peaked in the mid 20th Century and has since declined.

6. The shared socio-economic pathway (SSP) greenhouse gas concentrations and their extensions to 2500. Geosci Model Dev 13 (8): 3571–3605

   M Meinshausen, ZRJ Nicholls, J Lewis, MJ Gidden, E Vogel, Mandy B Freund, U Beyerle, C Gessner, A Nauels, N Bauer, and  others

   Aug 2020

2019

1. 

   Higher frequency of Central Pacific El Niño events in recent decades relative to past centuries

   Mandy B Freund, Benjamin J Henley, David J Karoly, Helen V McGregor, Nerilie J Abram, and Dietmar Dommenget

   Nature Geoscience, Aug 2019

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   El Niño events differ substantially in their spatial pattern and intensity. Canonical Eastern Pacific El Niño events have sea surface temperature anomalies that are strongest in the far eastern equatorial Pacific, whereas peak ocean warming occurs further west during Central Pacific El Niño events. The event types differ in their impacts on the location and intensity of temperature and precipitation anomalies globally. Evidence is emerging that Central Pacific El Niño events have become more common, a trend that is projected by some studies to continue with ongoing climate change. Here we identify spatial and temporal patterns in observed sea surface temperatures that distinguish the evolution of Eastern and Central Pacific El Niño events in the tropical Pacific. We show that these patterns are recorded by a network of 27 seasonally resolved coral records, which we then use to reconstruct Central and Eastern Pacific El Niño activity for the past four centuries. We find a simultaneous increase in Central Pacific events and a decrease in Eastern Pacific events since the late twentieth century that leads to a ratio of Central to Eastern Pacific events that is unusual in a multicentury context. Compared to the past four centuries, the most recent 30 year period includes fewer, but more intense, Eastern Pacific El Niño events.

2. The SSP greenhouse gas concentrations and their extensions to 2500, Geosci. Model Dev. Discuss

   M Meinshausen, Z Nicholls, J Lewis, MJ Gidden, E Vogel, M Freund, U Beyerle, C Gessner, A Nauels, N Bauer, and  others

   Aug 2019

2018

1. 

   Australian rainfall and El Niño diversity: past variability and context for recent changes

   Mandy Barbara Freund

   University of Melbourne, Aug 2018

   Website

2017

1. Spatial and temporal agreement in climate model simulations of the Interdecadal Pacific Oscillation

   Benjamin J Henley, Gerald Meehl, Scott B Power, Chris K Folland, Andrew D King, Jaclyn N Brown, David J Karoly, Francois Delage, Ailie JE Gallant, Mandy B Freund, and  others

   Environmental Research Letters, Aug 2017

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   Accelerated warming and hiatus periods in the long-term rise of Global Mean Surface Temperature (GMST) have, in recent decades, been associated with the Interdecadal Pacific Oscillation (IPO). Critically, decadal climate prediction relies on the skill of state-of-the-art climate models to reliably represent these low-frequency climate variations. We undertake a systematic evaluation of the simulation of the IPO in the suite of Coupled Model Intercomparison Project 5 (CMIP5) models. We track the IPO in pre-industrial (control) and all-forcings (historical) experiments using the IPO tripole index (TPI). The TPI is explicitly aligned with the observed spatial pattern of the IPO, and circumvents assumptions about the nature of global warming. We find that many models underestimate the ratio of decadal-to-total variance in sea surface temperatures (SSTs). However, the basin-wide spatial pattern of positive and negative phases of the IPO are simulated reasonably well, with spatial pattern correlation coefficients between observations and models spanning the range 0.4–0.8. Deficiencies are mainly in the extratropical Pacific. Models that better capture the spatial pattern of the IPO also tend to more realistically simulate the ratio of decadal to total variance. Of the 13% of model centuries that have a fractional bias in the decadal-to-total TPI variance of 0.2 or less, 84% also have a spatial pattern correlation coefficient with the observed pattern exceeding 0.5. This result is highly consistent across both IPO positive and negative phases. This is evidence that the IPO is related to one or more inherent dynamical mechanisms of the climate system.

2. 

   Multi-century cool-and warm-season rainfall reconstructions for Australia’s major climatic regions

   Mandy B Freund, Benjamin J Henley, David J Karoly, Kathryn J Allen, and Patrick J Baker

   Climate of the Past, Aug 2017

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   Australian seasonal rainfall is strongly affected by large-scale ocean–atmosphere climate influences. In this study, we exploit the links between these precipitation influences, regional rainfall variations, and palaeoclimate proxies in the region to reconstruct Australian regional rainfall between four and eight centuries into the past. We use an extensive network of palaeoclimate records from the Southern Hemisphere to reconstruct cool (April–September) and warm (October–March) season rainfall in eight natural resource management (NRM) regions spanning the Australian continent. Our bi-seasonal rainfall reconstruction aligns well with independent early documentary sources and existing reconstructions. Critically, this reconstruction allows us, for the first time, to place recent observations at a bi-seasonal temporal resolution into a pre-instrumental context, across the entire continent of Australia. We find that recent 30- and 50-year trends towards wetter conditions in tropical northern Australia are highly unusual in the multi-century context of our reconstruction. Recent cool-season drying trends in parts of southern Australia are very unusual, although not unprecedented, across the multi-century context. We also use our reconstruction to investigate the spatial and temporal extent of historical drought events. Our reconstruction reveals that the spatial extent and duration of the Millennium Drought (1997–2009) appears either very much below average or unprecedented in southern Australia over at least the last 400 years. Our reconstruction identifies a number of severe droughts over the past several centuries that vary widely in their spatial footprint, highlighting the high degree of diversity in historical droughts across the Australian continent. We document distinct characteristics of major droughts in terms of their spatial extent, duration, intensity, and seasonality. Compared to the three largest droughts in the instrumental period (Federation Drought, 1895–1903; World War II Drought, 1939–1945; and the Millennium Drought, 1997–2005), we find that the historically documented Settlement Drought (1790–1793), Sturt’s Drought (1809–1830) and the Goyder Line Drought (1861–1866) actually had more regionalised patterns and reduced spatial extents. This seasonal rainfall reconstruction provides a new opportunity to understand Australian rainfall variability by contextualising severe droughts and recent trends in Australia.

3. A global multiproxy database for temperature reconstructions of the Common Era

   Scientific data, Aug 2017

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   Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

4. Data Descriptor: A global multiproxy database for temperature reconstructions of the Common Era

   Jens Zinke, Julien Emile-Geay, Darrell S Kaufman, Lucien Von Gunten, Kevin J Anchukaitis, Nerilie J Abram, Mark AJ Curran, Michael N Evans, Zhixin Hao, Helen V McGregor, and  others

   Aug 2017
5. Multi-century cool-and warm-season rainfall reconstructions for Australia’s major climatic regions, Clim. Past, 13, 1751–1770

   Mandy B Freund, BJ Henley, DJ Karoly, KJ Allen, and PJ Baker

   Aug 2017

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6. A global multiproxy database for temperature reconstructions of the Common Era, Sci. Data, 4, 170088

   Aug 2017

   Abs HTML

   

   Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.