ORCID Profile
0000-0001-9598-6869
Current Organisation
University of Reading
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Publisher: American Geophysical Union (AGU)
Date: 10-2016
DOI: 10.1002/2016JD025480
Abstract: Knowledge about long‐term changes in climate extremes is vital to better understand multidecadal climate variability and long‐term changes and to place today's extreme events in a historical context. While global changes in temperature and precipitation extremes since the midtwentieth century are well studied, knowledge about century‐scale changes is limited. This paper analyses a range of largely independent observations‐based data sets covering 1901–2010 for long‐term changes and interannual variability in daily scale temperature and precipitation extremes. We compare across data sets for consistency to ascertain our confidence in century‐scale changes in extremes. We find consistent warming trends in temperature extremes globally and in most land areas over the past century. For precipitation extremes we find global tendencies toward more intense rainfall throughout much of the twentieth century however, local changes are spatially more variable. While global time series of the different data sets agree well after about 1950, they often show different changes during the first half of the twentieth century. In regions with good observational coverage, gridded observations and reanalyses agree well throughout the entire past century. Simulations with an atmospheric model suggest that ocean temperatures and sea ice may explain up to about 50% of interannual variability in the global average of temperature extremes, and about 15% in the global average of moderate precipitation extremes, but local correlations are mostly significant only in low latitudes.
Publisher: American Geophysical Union (AGU)
Date: 12-2021
DOI: 10.1029/2021EF002249
Abstract: Climate variability in the Pacific has an important influence on climate around the globe. In the period from 1981 to 2012, there was an observed large‐scale cooling in the Pacific. This cooling projected onto the negative phase of the Pacific Decadal Oscillation (PDO) and contributed to a slowdown in the rate of near‐surface temperature warming. However, this cooling pattern is not simulated well by the majority of coupled climate models and its cause is uncertain. We use large multi‐model ensembles from the sixth Climate Model Intercomparison Project, and an ensemble of simulations with HadGEM3‐GC3.1‐LL that is specifically designed to s le the range of uncertainty in historical anthropogenic aerosol forcing, to revisit the role of external forcings. We show that anthropogenic aerosols can drive an atmospheric circulation response via an increase in North Pacific sea level pressure and contribute to a negative PDO during this period in several global climate models. In HadGEM3, this increase in North Pacific sea‐level pressure is associated with an anomalous Rossby Wave train across the North Pacific, which is also seen in observations. Our results provide further evidence that anthropogenic aerosols may have contributed to observed cooling in the Pacific in this period. However, the simulated cooling in response to aerosol forcing is substantially weaker than the warming induced by greenhouse gases, resulting in simulations that are warming faster than observations, and further highlighting the need to understand whether models correctly simulate atmospheric circulation responses.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-15360
Abstract: Evidence from model simulations has suggested that anthropogenic aerosols may have forced multidecadal variability in a range of North Atlantic variables including sea surface temperatures, ocean circulation, and sea ice. However, many questions remain concerning the importance of anthropogenic aerosols in driving past changes in the North Atlantic climate system. The pathways via which changes in aerosol and aerosol precursor emissions, and oxidant levels, influence climate are complex. They involve both chemical and physical processes, and likely include changes in clouds, radiation, surface temperatures, atmospheric and oceanic circulation, and Arctic sea ice. This complexity is an important factor in the large uncertainty surrounding the role of anthropogenic aerosol in North Atlantic climate change, and was one of the major motivations for the UK& #8217 s North Atlantic Climate System Integrated Study (ACSIS). ACSIS was a multidisciplinary research programme conducted over the period 2016-2022, delivered by a consortium of seven UK institutions. This presentation draws together findings from the programme to provide an overall synthesis of what was learned in ACSIS about the role of anthropogenic aerosol in North Atlantic climate change. Remaining uncertainties, the potential for observational constraints, and opportunities for future work will also be discussed.& ACSIS made extensive use of simulations conducted for CMIP6, particularly historical simulations, and attribution experiments included in AerChemMIP and DAMIP. Additional sensitivity experiments with HadGEM3-GC3.1 and UKESM1 were used to quantify the effects of uncertainty in aerosol forcing in the absence of the additional uncertainty associated with model differences, to decompose the aerosol forcing, and to better illustrate the role of aerosol in recent changes.& As aerosol emissions increased (1850-1985), North Atlantic CDNC increased. Emissions of ozone precursors, and resulting changes in OH, contributed to this trend. This led to downwelling surface shortwave decreases across the North Atlantic, which drove colder surface temperatures, increased sea ice extent, and increased mean sea level pressure. In contrast, the eastern subpolar gyre warmed, likely due to increased ocean heat convergence due to the increase in the AMOC.& As local aerosol emissions fell (1986-2014) much of the reverse occurred. Downwelling surface shortwave increased across the North Atlantic, predominantly over land, driving warmer surface temperatures and reduced sea ice extent. The eastern subpolar gyre cooled. However, the role of aerosol in this later period is less clear due to a dominance of temperature-mediated cloud feedbacks over aerosol forcing, AMOC related feedbacks, and a changing aerosol forcing pattern.
Publisher: Copernicus GmbH
Date: 07-09-2022
DOI: 10.5194/GMD-2022-113
Abstract: Abstract. Many CMIP6 models exhibit a substantial cold bias in global mean surface temperature (GMST) in the latter part of the 20th century. An overly strong negative aerosol forcing has been suggested as a leading contributor to this bias. An updated configuration of UKESM1, UKESM1.1, has been developed with the aim of reducing the historical cold bias in this model. Changes implemented include an improved representation of SO2 dry deposition along with several other smaller modifications to the aerosol scheme and a retuning of some uncertain parameters of the fully coupled Earth System Model. The Diagnostic, Evaluation and Characterization of Klima (DECK) experiments, a 6-member historical ensemble and a subset of future scenario simulations are completed. In addition, the total anthropogenic effective radiative forcing (ERF), its components and the effective and transient climate sensitivities are also computed. The UKESM1.1 pre-industrial climate is warmer than UKESM1 by up to 0.75 K and a significant improvement in the historical GMST record is simulated with the magnitude of the cold bias reduced by over 50 %. The warmer climate increases ocean heat uptake in the northern hemisphere oceans and reduces Arctic sea ice in better agreement with observations. Changes to the aerosol and related cloud properties are the key drivers of the improved GMST simulation despite only a modest change in aerosol ERF (+0.08 Wm-2). The total anthropogenic ERF increases from 1.76 Wm-2 in UKESM1 to 1.84 Wm-2 in UKESM1.1. The effective climate sensitivity (5.27 K) and transient climate response (2.64 K) remain largely unchanged from UKESM1 (5.36 K and 2.76 K respectively).
Publisher: IOP Publishing
Date: 09-2015
Publisher: Wiley
Date: 05-12-2022
Publisher: Wiley
Date: 11-04-2022
Publisher: Wiley
Date: 09-08-2022
Publisher: Wiley
Date: 10-08-2022
Publisher: Springer Science and Business Media LLC
Date: 25-11-2021
Publisher: American Meteorological Society
Date: 12-2015
DOI: 10.1175/JCLI-D-14-00753.1
Abstract: This study examines trends in the area affected by temperature and precipitation extremes across five large-scale regions using the climate extremes index (CEI) framework. Analyzing changes in temperature and precipitation extremes in terms of areal fraction provides information from a different perspective and can be useful for climate monitoring. Trends in five temperature and precipitation components are analyzed, calculated using a new method based on standard extreme indices. These indices, derived from daily meteorological station data, are obtained from two global land-based gridded extreme indices datasets. The four continental-scale regions of Europe, North America, Asia, and Australia are analyzed over the period from 1951 to 2010, where sufficient data coverage is available. These components are also computed for the entire Northern Hemisphere, providing the first CEI results at the hemispheric scale. Results show statistically significant increases in the percentage area experiencing much-above-average warm days and nights and much-below-average cool days and nights for all regions, with the exception of North America for maximum temperature extremes. Increases in the area affected by precipitation extremes are also found for the Northern Hemisphere regions, particularly Europe and North America.
Publisher: American Geophysical Union (AGU)
Date: 07-2023
DOI: 10.1029/2022JC018725
Abstract: The causes of decadal variations in global warming are poorly understood, however it is widely understood that variations in ocean heat content (OHC) are linked with variations in surface warming. To investigate the forced response of OHC to anthropogenic aerosols (AA), we use an ensemble of historical simulations, which were carried out using a range of anthropogenic aerosol forcing magnitudes in a CMIP6‐era global circulation model. We find that the centennial scale linear trends in historical OHC are significantly sensitive to AA forcing magnitude (−3.0 ± 0.1 × 10 5 (J m −3 century −1 )/(W m −2 ), R 2 = 0.99), but interannual to multi‐decadal variability in global OHC appear largely independent of AA forcing magnitude. Comparison with observations find consistencies in different depth ranges and at different time scales with all but the strongest aerosol forcing magnitude, at least partly due to limited observational accuracy. We find that OHC is sensitive to aerosol forcing magnitude across much of the tropics and sub‐tropics, and stronger negative forcing induces more ocean cooling. The polar regions and North Atlantic show the strongest heat content trends, and also show the strongest dependence on aerosol forcing magnitude. However, the OHC response to increasing aerosol forcing magnitude in the North Atlantic and Southern Ocean is either dominated by internal variability, or strongly state dependent, showing different behavior in different time periods. Our results suggest the response to aerosols in these regions is a complex combination of influences from ocean transport, atmospheric forcings, and sea ice.
Publisher: Bureau of Meteorology, Australia
Date: 12-2014
DOI: 10.22499/2.6404.002
Publisher: Copernicus GmbH
Date: 07-09-2022
Publisher: Copernicus GmbH
Date: 23-06-2020
DOI: 10.5194/ACP-2020-478
Abstract: Abstract. Anthropogenic aerosols are dominant drivers of historical monsoon rainfall change. However, large uncertainties in the radiative forcing associated with anthropogenic aerosol emissions, and the dynamical response to this forcing, lead to uncertainty in the simulated monsoon response. We use historical simulations in which aerosol emissions are scaled by factors from 0.2 to 1.5 to explore the monsoon sensitivity to aerosol forcing uncertainty (−0.38 W m−2 to −1.50 W m−2). Hemispheric asymmetry in emissions generates a strong relationship between scaling factor and both hemispheric temperature contrast and meridional location of tropical rainfall. Increasing the scaling from 0.2 to 1.5 reduces the global monsoon area by 3 % and the global monsoon intensity by 2 % over the period 1950–2014, and switches the dominant influence on the 1950–1980 monsoon rainfall trend between greenhouse gas and aerosol. Regionally, aerosol scaling has a pronounced effect on Northern Hemisphere monsoon rainfall.
Publisher: Copernicus GmbH
Date: 03-12-2020
DOI: 10.5194/ACP-20-14903-2020
Abstract: Abstract. Anthropogenic aerosols are dominant drivers of historical monsoon rainfall change. However, large uncertainties in the radiative forcing associated with anthropogenic aerosol emissions, as well as the dynamical response to this forcing, lead to uncertainty in the simulated monsoon response. We use historical simulations from the “SMURPHS” project, run using HadGEM3-GC3.1, in which the time-varying aerosol emissions are scaled by factors from 0.2 to 1.5 to explore the monsoon sensitivity to historical aerosol forcing uncertainty (present-day versus preindustrial aerosol forcing in the range −0.38 to −1.50 W m−2). The hemispheric asymmetry in emissions generates a strong relationship between scaling factor and both hemispheric temperature contrast and meridional location of tropical rainfall. Averaged over the period 1950–2014, increasing the scaling factor from 0.2 to 1.5 reduces the hemispheric temperature contrast by 0.9 ∘C, reduces the tropical summertime land–sea temperature contrast by 0.3 ∘C and shifts tropical rainfall southwards by 0.28∘ of latitude. The result is a reduction in global monsoon area by 3 % and a reduction in global monsoon intensity by 2 %. Despite the complexity of the monsoon system, the monsoon properties presented above vary monotonically and roughly linearly across scalings. A switch in the dominant influence on the 1950–1980 monsoon rainfall trend between greenhouse gases and aerosol is identified as the scalings increase. Regionally, aerosol scaling has a pronounced effect on Northern Hemisphere monsoon rainfall, with the strongest influence on monsoon area and intensity located in the Asian sector, where local emissions are greatest.
Publisher: American Geophysical Union (AGU)
Date: 26-01-2023
DOI: 10.1029/2022EF002995
Abstract: Anthropogenic aerosol emissions from North America and Europe have strong effects on the decadal variability of the West African monsoon (WAM). Anthropogenic aerosol effective radiative forcing is model dependent, but the impact of such uncertainty on the simulation of long‐term WAM variability is unknown. We use an ensemble of simulations with HadGEM3‐GC3.1 that span the most recent estimates in simulated anthropogenic aerosol effective radiative forcing. We show that uncertainty in anthropogenic aerosol radiative forcing leads to significant uncertainty at simulating multi‐decadal trends in West African precipitation. At the large scale, larger forcing leads to a larger decrease in the interhemispheric temperature gradients, in temperature over both the North Atlantic Ocean and northern Sahara. There are also differences in dynamic changes specific to the WAM (locations of the Saharan heat low and African Easterly Jet, of the strength of the West African westerly jet, and of African Easterly Wave activity). We also assess effects on monsoon precipitation characteristics and temperature. We show that larger aerosol forcing results in a decrease of the number of rainy days and of heavy and extreme precipitation events and warm spells. However, simulated changes in onset and demise dates do not appear to be sensitive to the magnitude of aerosol forcing. Our results demonstrate the importance of reducing the uncertainty in anthropogenic aerosol forcing for understanding and predicting multi‐decadal variability in the WAM.
Publisher: Elsevier BV
Date: 09-2018
Publisher: Copernicus GmbH
Date: 20-03-2023
Abstract: Abstract. Many Coupled Model Intercomparison Project phase 6 (CMIP6) models have exhibited a substantial cold bias in the global mean surface temperature (GMST) in the latter part of the 20th century. An overly strong negative aerosol forcing has been suggested as a leading contributor to this bias. An updated configuration of UK Earth System Model (UKESM) version 1, UKESM1.1, has been developed with the aim of reducing the historical cold bias in this model. Changes implemented include an improved representation of SO2 dry deposition, along with several other smaller modifications to the aerosol scheme and a retuning of some uncertain parameters of the fully coupled Earth system model. The Diagnostic, Evaluation and Characterization of Klima (DECK) experiments, a six-member historical ensemble and a subset of future scenario simulations are completed. In addition, the total anthropogenic effective radiative forcing (ERF), its components and the effective and transient climate sensitivities are also computed. The UKESM1.1 preindustrial climate is warmer than UKESM1 by up to 0.75 K, and a significant improvement in the historical GMST record is simulated, with the magnitude of the cold bias reduced by over 50 %. The warmer climate increases ocean heat uptake in the Northern Hemisphere oceans and reduces Arctic sea ice, which is in better agreement with observations. Changes to the aerosol and related cloud properties are a driver of the improved GMST simulation despite only a modest reduction in the magnitude of the negative aerosol ERF (which increases by +0.08 W m−2). The total anthropogenic ERF increases from 1.76 W m−2 in UKESM1 to 1.84 W m−2 in UKESM1.1. The effective climate sensitivity (5.27 K) and transient climate response (2.64 K) remain largely unchanged from UKESM1 (5.36 and 2.76 K respectively).
Publisher: Elsevier BV
Date: 09-2015
Publisher: American Geophysical Union (AGU)
Date: 06-2020
DOI: 10.1029/2019MS001995
Publisher: Wiley
Date: 06-07-2022
Publisher: American Meteorological Society
Date: 06-2019
Abstract: Studies have recently reported statistically significant relationships between observed year-to-year spring Antarctic ozone variability and the Southern Hemisphere annular mode and surface temperatures in spring–summer. This study investigates whether current chemistry–climate models (CCMs) can capture these relationships, in particular, the connection between November total column ozone (TCO) and Australian summer surface temperatures, where years with anomalously high TCO over the Antarctic polar cap tend to be followed by warmer summers. The interannual ozone–temperature teleconnection is examined over the historical period in the observations and simulations from the Whole Atmosphere Community Climate Model (WACCM) and nine other models participating in the Chemistry–Climate Model Initiative (CCMI). There is a systematic difference between the WACCM experiments forced with prescribed observed sea surface temperatures (SSTs) and those with an interactive ocean. Strong correlations between TCO and Australian temperatures are only obtained for the uncoupled experiment, suggesting that the SSTs could be important for driving both variations in Australian temperatures and the ozone hole, with no causal link between the two. Other CCMI models also tend to capture this relationship with more fidelity when driven by observed SSTs, although additional research and targeted modeling experiments are required to determine causality and further explore the role of model biases and observational uncertainty. The results indicate that CCMs can reproduce the relationship between spring ozone and summer Australian climate reported in observational studies, suggesting that incorporating ozone variability could improve seasonal predictions however, more work is required to understand the difference between the coupled and uncoupled simulations.
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-12547
Abstract: & & In recent years, there has been increasing interest in how possible future climates at different stabilised, policy relevant global warming levels above pre-industrial might look like. Modelling groups are designing novel climate model simulations to investigate these questions and help answer important questions on the linearity of future climate change across warming levels, tipping points, and climate extremes, among others. A key question is how these projected changes are dependent on scenario choices, particularly the role of future anthropogenic aerosol emissions.& & & & Here, we present the results of new & #8220 quasi-stable& #8221 climate model simulations with UKESM1.0. Six multi-century simulations have been run under fixed forcings, branching-off from ScenarioMIP simulations of the same model. These simulations explore a range of global warming levels, from approximately 1.5 to 5& #176 C above pre-industrial. In addition, they also explore the role of different balances of forcings for achieving the same target warming level, in particular different combinations of greenhouse gas concentrations and anthropogenic aerosols. In this presentation, we describe how the climate evolves in each of these simulations. We focus on two key aspects: & ) differences between a more stable climate vs. transient climate change at the same warming level and 2) importance of scenario differences, in particular differences in anthropogenic aerosol emissions at the same warming level.& & & & We discuss various aspects of how climate changes in each of the above simulations, including climate extremes, which arguably are one of the most important aspects to consider when assessing the socio-economic impacts of possible future climate conditions at different warming levels and under different scenarios.& &
Publisher: Wiley
Date: 12-2019
Publisher: Elsevier BV
Date: 06-2019
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-1628
Abstract: & & An increase in European and North American anthropogenic aerosol emissions in the 1970s and 1980s led to a decrease in Sahel precipitation during the same time. Although significant, the effect of anthropogenic aerosols on Sahel precipitation is uncertain across a set of CMIP6 single-forcing simulations. However, understanding the cause of this uncertainty in simulated effects of anthropogenic aerosols on West African precipitation in CMIP6 models is difficult, largely due to the relatively small number of large-ensembles with single-forcing simulations. Here, we use a single-model ensemble that spans much of the range in anthropogenic aerosol effective radiative forcing from the CMIP5 and CMIP6 multi-model ensembles. The simulations are performed with HadGEM3-GC3.1 and the different forcings are achieved by scaling emissions in anthropogenic aerosols. We show that changes in anthropogenic aerosols have strong effects on the drivers of the West African monsoon, and on precipitation extremes. Further, we show that the magnitude and even the occurrence of the West African drought (1970s-1980s) strongly depend on the simulated effective anthropogenic aerosol radiative forcing in the model simulations. Our results show that a better understanding of the effects of anthropogenic aerosols on climate is necessary to improve predictions of decadal trends in Sahel precipitation.& & &
Publisher: American Geophysical Union (AGU)
Date: 25-06-2020
DOI: 10.1029/2019GL085806
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-14454
Abstract: The 2015 Paris Agreement adopted by 192 parties states the goal of limiting global warming to well below 2, preferably 1.5 degrees Celsius above pre-industrial levels. These goals imply an ambition to stay at or below these levels long-term. Evidence is beginning to emerge that regional patterns of change at given global warming levels (GWLs) can be very different between transiently warming through given GWLs and stabilising at those same GWLs.In this presentation, we explore regional climate change across multiple variables, with a particular focus on regional precipitation change. Using a novel ensemble of six 500-years long fixed concentration simulations across various levels of warming between 1.5 and 5 degrees above pre-industrial with the CMIP6-generation Earth System Model UKESM1.0, we show that precipitation trends opposite in sign to transient climate change projections occur in several regions at the same GWLs. Such differences have important implications for climate change risk assessments and adaptation discussions, which typically only include transient projections. Here, we provide ex les where a transient and stabilised climate differ and discuss the possible mechanisms driving these differences.&
Publisher: American Meteorological Society
Date: 03-11-2016
Abstract: The skill of eight climate models in simulating the variability and trends in the observed areal extent of daily temperature and precipitation extremes is evaluated across five large-scale regions, using the climate extremes index (CEI) framework. Focusing on Europe, North America, Asia, Australia, and the Northern Hemisphere, results show that overall the models are generally able to simulate the decadal variability and trends of the observed temperature and precipitation components over the period 1951–2005. Climate models are able to reproduce observed increasing trends in the area experiencing warm maximum and minimum temperature extremes, as well as, to a lesser extent, increasing trends in the areas experiencing an extreme contribution of heavy precipitation to total annual precipitation for the Northern Hemisphere regions. Using simulations performed under different radiative forcing scenarios, the causes of simulated and observed trends are investigated. A clear anthropogenic signal is found in the trends in the maximum and minimum temperature components for all regions. In North America, a strong anthropogenically forced trend in the maximum temperature component is simulated despite no significant trend in the gridded observations, although a trend is detected in a reanalysis product. A distinct anthropogenic influence is also found for trends in the area affected by a much-above-average contribution of heavy precipitation to annual precipitation totals for Europe in a majority of models and to varying degrees in other Northern Hemisphere regions. However, observed trends in the area experiencing extreme total annual precipitation and extreme number of wet and dry days are not reproduced by climate models under any forcing scenario.
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Andrea Dittus.