Showing papers on "Forcing (mathematics) published in 2021"

Significant impact of forcing uncertainty in a large ensemble of climate model simulations.

Abstract: Forcing due to solar and volcanic variability, on the natural side, and greenhouse gas and aerosol emissions, on the anthropogenic side, are the main inputs to climate models Reliable climate model simulations of past and future climate change depend crucially upon them Here we analyze large ensembles of simulations using a comprehensive Earth System Model to quantify uncertainties in global climate change attributable to differences in prescribed forcings The different forcings considered here are those used in the two most recent phases of the Coupled Model Intercomparison Project (CMIP), namely CMIP5 and CMIP6 We show significant differences in simulated global surface air temperature due to volcanic aerosol forcing in the second half of the 19th century and in the early 21st century The latter arise from small-to-moderate eruptions incorporated in CMIP6 simulations but not in CMIP5 simulations We also find significant differences in global surface air temperature and Arctic sea ice area due to anthropogenic aerosol forcing in the second half of the 20th century and early 21st century These differences are as large as those obtained in different versions of an Earth System Model employing identical forcings In simulations from 2015 to 2100, we find significant differences in the rates of projected global warming arising from CMIP5 and CMIP6 concentration pathways that differ slightly but are equivalent in terms of their nominal radiative forcing levels in 2100 Our results highlight the influence of assumptions about natural and anthropogenic aerosol loadings on carbon budgets, the likelihood of meeting Paris targets, and the equivalence of future forcing scenarios

Influences of Recent Particle Formation on Southern Ocean Aerosol Variability and Low Cloud Properties

Abstract: Controls on pristine aerosol over the Southern Ocean (SO) are critical for constraining the strength of global aerosol indirect forcing. Observations of summertime SO clouds and aerosols in synopti...

Modelled land use and land cover change emissions - a spatio-temporal comparison of different approaches

Abstract: . Quantifying the net carbon flux from land use and land cover changes ( fLULCC ) is critical for understanding the global carbon cycle and, hence, to support climate change mitigation. However, large-scale fLULCC is not directly measurable and has to be inferred from models instead, such as semi-empirical bookkeeping models and process-based dynamic global vegetation models (DGVMs). By definition, fLULCC estimates are not directly comparable between these two different model types. As an important example, DGVM-based fLULCC in the annual global carbon budgets is estimated under transient environmental forcing and includes the so-called loss of additional sink capacity (LASC). The LASC results from the impact of environmental changes on land carbon storage potential of managed land compared to potential vegetation and accumulates over time, which is not captured in bookkeeping models. The fLULCC from transient DGVM simulations, thus, strongly depends on the timing of land use and land cover changes mainly because LASC accumulation is cut off at the end of the simulated period. To estimate the LASC, the fLULCC from pre-industrial DGVM simulations, which is independent of changing environmental conditions, can be used. Additionally, DGVMs using constant present-day environmental forcing enable an approximation of bookkeeping estimates. Here, we analyse these three DGVM-derived fLULCC estimations (under transient, pre-industrial, and present-day forcing) for 12 models within 18 regions and quantify their differences as well as climate- and CO2 -induced components and compare them to bookkeeping estimates. Averaged across the models, we find a global fLULCC (under transient conditions) of 2.0±0.6 PgC yr −1 for 2009–2018, of which ∼40 % are attributable to the LASC ( 0.8±0.3 PgC yr −1 ). From 1850 onward, the fLULCC accumulated to 189±56 PgC with 40±15 PgC from the LASC. Around 1960, the accumulating nature of the LASC causes global transient fLULCC estimates to exceed estimates under present-day conditions, despite generally increased carbon stocks in the latter. Regional hotspots of high cumulative and annual LASC values are found in the USA, China, Brazil, equatorial Africa, and Southeast Asia, mainly due to deforestation for cropland. Distinct negative LASC estimates in Europe (early reforestation) and from 2000 onward in the Ukraine (recultivation of post-Soviet abandoned agricultural land), indicate that fLULCC estimates in these regions are lower in transient DGVM compared to bookkeeping approaches. Our study unravels the strong dependence of fLULCC estimates on the time a certain land use and land cover change event happened to occur and on the chosen time period for the forcing of environmental conditions in the underlying simulations. We argue for an approach that provides an accounting of the fLULCC that is more robust against these choices, for example by estimating a mean DGVM ensemble fLULCC and LASC for a defined reference period and homogeneous environmental changes ( CO2 only).

Energy Budget Constraints on the Time History of Aerosol Forcing and Climate Sensitivity

Abstract: An observationally-constrained time series of historical aerosol effective radiative forcing (ERF) from 1750 to 2019 is developed in this paper. We find that the time history of aerosol ERFs diagno...

Ocean fronts and eddies force atmospheric rivers and heavy precipitation in western North America

Abstract: Atmospheric rivers (ARs) are responsible for over 90% of poleward water vapor transport in the mid-latitudes and can produce extreme precipitation when making landfall. However, weather and climate models still have difficulty simulating and predicting landfalling ARs and associated extreme precipitation, highlighting the need to better understand AR dynamics. Here, using high-resolution climate models and observations, we demonstrate that mesoscale sea-surface temperature (SST) anomalies along the Kuroshio Extension can exert a remote influence on landfalling ARs and related heavy precipitation along the west coast of North America. Inclusion of mesoscale SST forcing in the simulations results in approximately a 40% increase in landfalling ARs and up to a 30% increase in heavy precipitation in mountainous regions and this remote impact occurs on two-week time scales. The asymmetrical response of the atmosphere to warm vs. cold mesoscale SSTs over the eddy-rich Kuroshio Extension region is proposed as a forcing mechanism that results in a net increase of moisture flux above the planetary boundary layer, prompting AR genesis via enhancing moisture transport into extratropical cyclones in the presence of mesoscale SST forcing.

The unidentified eruption of 1809: a climatic cold case

Abstract: . The “1809 eruption” is one of the most recent unidentified volcanic eruptions with a global climate impact. Even though the eruption ranks as the third largest since 1500 with a sulfur emission strength estimated to be 2 times that of the 1991 eruption of Pinatubo, not much is known of it from historic sources. Based on a compilation of instrumental and reconstructed temperature time series, we show here that tropical temperatures show a significant drop in response to the ∼ 1809 eruption that is similar to that produced by the Mt. Tambora eruption in 1815, while the response of Northern Hemisphere (NH) boreal summer temperature is spatially heterogeneous. We test the sensitivity of the climate response simulated by the MPI Earth system model to a range of volcanic forcing estimates constructed using estimated volcanic stratospheric sulfur injections (VSSIs) and uncertainties from ice-core records. Three of the forcing reconstructions represent a tropical eruption with an approximately symmetric hemispheric aerosol spread but different forcing magnitudes, while a fourth reflects a hemispherically asymmetric scenario without volcanic forcing in the NH extratropics. Observed and reconstructed post-volcanic surface NH summer temperature anomalies lie within the range of all the scenario simulations. Therefore, assuming the model climate sensitivity is correct, the VSSI estimate is accurate within the uncertainty bounds. Comparison of observed and simulated tropical temperature anomalies suggests that the most likely VSSI for the 1809 eruption would be somewhere between 12 and 19 Tg of sulfur. Model results show that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure. While spatial correlations between the N-TREND NH temperature reconstruction and the model simulations are weak in terms of the ensemble-mean model results, individual model simulations show good correlation over North America and Europe, suggesting the spatial heterogeneity of the 1810 cooling could be due to internal climate variability.

Changes in the Total Solar Irradiance and climatic effects

Abstract: The correlation between the averaged reconstructed March temperature record for Kyoto, Japan, and the reconstructed Total Solar Irradiance (TSI) over 660 years from 1230 to 1890 gives evidence with 98% probability that the Little Ice Age with four cold periods is forced by variations of TSI. If the correlation is restricted to the period 1650–1890, with two cold periods in the 17th and 19th century and for which two independent reconstructed March temperature records are available, the probability of solar forcing increases to 99.99%. As solar irradiance variations have a global effect there has to be a global climatic solar forcing impact. However, by how much global temperature were lower during these minima and with what amplitude TSI was varying is not accurately known. The two quantities, global temperature and TSI, are linked by the energy equilibrium equation for the Earth system. The derivation of this equation with respect to a variation of the solar irradiance has two terms: A direct forcing term, which can be derived analytically and quantified accurately from the Stefan-Boltzmann law, and a second term, describing indirect influences on the surface temperature. If a small TSI variation should force a large temperature variation, then it has to be the second indirect term that strongly amplifies the effect of the direct forcing. The current knowledge is summarized by three statements:During the minima periods in the 13th, 15/16th, 17th, and 19th centuries the terrestrial climate was colder by 0.5–1.5 °C;Indirect Top-down and Bottom-up mechanisms do not amplify direct forcing by a large amount, i.e. indirect solar forcing is of the same magnitude (or smaller) as direct solar forcing;The radiative output of the Sun cannot be lower by more than 2 Wm−2 below the measured present-day TSI value during solar cycle minimum.These three statements contradict each other and it is concluded that at least one is not correct. Which one is a wrong statement is presently not known conclusively. It is argued that it is the third statement and it is speculated that over centennial time scales the Sun might vary its radiance significantly more than observed so far during the last 40 years of space TSI measurements. To produce Maunder minimum type cold climate excursions, a TSI decrease of the order of 10 Wm−2 is advocated.

ISMIP6-based projections of ocean-forced Antarctic Ice Sheet evolution using the Community Ice Sheet Model

Abstract: . The future retreat rate for marine-based regions of the Antarctic Ice Sheet is one of the largest uncertainties in sea-level projections. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) aims to improve projections and quantify uncertainties by running an ensemble of ice sheet models with atmosphere and ocean forcing derived from global climate models. Here, the Community Ice Sheet Model (CISM) is used to run ISMIP6-based projections of ocean-forced Antarctic Ice Sheet evolution. Using multiple combinations of sub-ice-shelf melt parameterizations and calibrations, CISM is spun up to steady state over many millennia. During the spin-up, basal friction parameters and basin-scale thermal forcing corrections are adjusted to optimize agreement with the observed ice thickness. The model is then run forward for 550 years, from 1950–2500, applying ocean thermal forcing anomalies from six climate models. In all simulations, the ocean forcing triggers long-term retreat of the West Antarctic Ice Sheet, especially in the Filchner–Ronne and Ross sectors. Mass loss accelerates late in the 21st century and then rises steadily for several centuries without leveling off. The resulting ocean-forced sea-level rise at year 2500 varies from about 150 to 1300 mm, depending on the melt scheme and ocean forcing. Further experiments show relatively high sensitivity to the basal friction law, moderate sensitivity to grid resolution and the prescribed collapse of small ice shelves, and low sensitivity to the stress-balance approximation. The Amundsen sector exhibits threshold behavior, with modest retreat under many parameter settings but complete collapse under some combinations of low basal friction and high thermal forcing anomalies. Large uncertainties remain, as a result of parameterized sub-shelf melt rates, simplified treatments of calving and basal friction, and the lack of ice–ocean coupling.

Understanding the combined effects of global warming and anthropogenic aerosol forcing on the South Asian monsoon

Abstract: There is growing evidence of the influence of northern hemisphere (NH) anthropogenic aerosols (AA) on the observed regional climate change over Asia in the recent few decades. While the radiative effects of AA can regionally offset the greenhouse gas (GHG) forcing, their combined impacts on the South Asian monsoon (SAM) are not adequately understood. The present study examines the individual and combined effects of GHG and AA forcing on the SAM precipitation response, based on numerical experiments conducted using the IITM Earth System Model version2 (IITM-ESMv2). We performed the following three sets of 50-year simulations based on the CMIP6 forcing (a) AER: where the atmospheric CO2 concentration is fixed to the pre-industrial level (i.e., year 1850) and AA levels correspond to the year 2005 (b) GHG: CO2 concentration corresponds to the year 2005 and AA levels correspond to the year 1850 (c) COMB: Both CO2 concentration and AA levels correspond to the year 2005. The three experiments are compared against the pre-industrial control (PICTL) run of the IITM-ESMv2. An intensification of SAM precipitation is noted in GHG relative to PICTL, whereas AER exhibits a decrease of SAM precipitation and weakening of monsoon circulation in response to the AA forcing. The results show that absorption of shortwave radiation above the atmospheric boundary layer over the Asian region creates surface radiation deficit and stabilizes the lower troposphere, leading to a slowdown of monsoonal winds, reduced evaporation over the Indian Ocean and decreased moisture convergence over South and Southeast Asia. A striking finding from our analysis is the reinforced suppression of SAM precipitation and organized monsoon convection in COMB, which results from an enhanced inter-hemispheric asymmetry in radiative forcing under the combined influence of the elevated CO2 and AA forcing. Furthermore, both the AER and COMB experiments reveal a widespread suppression of large-scale organized monsoon convective activity on sub-seasonal time-scales over South and Southeast Asia.

Evolution of the East Asian winter land temperature trends during 1961–2018: role of internal variability and external forcing

Abstract: Detecting the contributions of internal variability and external forcing to the evolution of surface air temperature (SAT) trend at regional scales is a challenge. Based on the observations and large-ensemble simulations of climate models, we estimate the contribution of the internal and forced components to the evolution of East Asian winter land SAT (EAWT) during 1961–2018. Although the external forcing induced EAWT trends show a slow increase, both the total and internally generated EAWT trends exhibit a decrease with the extension of the time period, suggesting a critical role of internal variability in the evolution of the EAWT trends. The internal variability contributes to about 70% of total EAWT trends during 1961–1995. With the extension of the time period, the contribution of internal variability decreases, whereas the contribution of external forcing gradually grows to dominate the EAWT trends. Based on the dynamical adjustment method, we identify that the internal dynamics and forced thermodynamics account for a majority of internal and forced EAWT variations, respectively. We further identify that the multidecadal fluctuation of internal component of autumn Arctic sea ice is a critical precursor of the internal variability, especially the internal dynamically induced EAWT variations, through triggering a meridional stationary Rossby wave response in the following boreal winter. Our findings provide an insight into the understanding of the present and future climate change over East Asia.

Low-latitude forcing: A new insight into paleo-climate changes.

Abstract: Pinxian Wang1,* 1State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China *Correspondence: pxwang@tongji.edu.cn Received: May 23, 2021; Accepted: July 14, 2021; Published Online: July 16, 2021; https://doi.org/10.1016/j.xinn.2021.100145 a 2021 This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Citation: Wang P. (2021). Low-latitude forcing: A new insight into paleo-climate changes. The Innovation 2(3), 100145.

NAO predictability from external forcing in the late 20th century

Abstract: The North Atlantic Oscillation (NAO) is predictable in climate models at near-decadal timescales. Predictive skill derives from ocean initialization, which can capture variability internal to the climate system, and from external radiative forcing. Herein, we show that predictive skill for the NAO in a very large uninitialized multi-model ensemble is commensurate with previously reported skill from a state-of-the-art initialized prediction system. The uninitialized ensemble and initialized prediction system produce similar levels of skill for northern European precipitation and North Atlantic SSTs. Identifying these predictable components becomes possible in a very large ensemble, confirming the erroneously low signal-to-noise ratio previously identified in both initialized and uninitialized climate models. Though the results here imply that external radiative forcing is a major source of predictive skill for the NAO, they also indicate that ocean initialization may be important for particular NAO events (the mid-1990s strong positive NAO), and, as previously suggested, in certain ocean regions such as the subpolar North Atlantic ocean. Overall, we suggest that improving climate models’ response to external radiative forcing may help resolve the known signal-to-noise error in climate models.

Potential Impacts of Supersonic Aircraft Emissions on Ozone and Resulting Forcing on Climate: An Update on Historical Analysis

Abstract: The overall demands by the public for air travel, the aspiration for more intercontinental travel, and the desire for shorter flight times have all increased in the past few decades. As a result, various companies and organizations around the world have been reconsidering development of supersonic aircraft for the business jet and commercial airline markets (e.g., NASA, Aerion, Spike Aerospace, and Boom Technology). Commercial fleets of supersonic transport (SST) aircraft were first considered in the 1970s (Climate Impact Assessment Program, 1975) and then again in the 1990s (Kawa et al., 1999). The cruise altitudes flown by supersonic aircraft depends on the design and speed of the aircraft with faster aircraft flying at higher altitudes. Supersonic aircraft would fly at higher altitudes than the current fleet of subsonic aircraft, with their emissions primarily being in the stratosphere.

Anthropogenic Aerosol effects on Tropospheric Circulation and Sea Surface Temperature (1980–2020): Separating the role of Zonally Asymmetric Forcings

Abstract: . Anthropogenic Aerosols (AA) induce global and regional tropospheric circulation adjustments due to the radiative energy perturbations. The overall cooling effects of AA since the pre-industrial (PI) era, to mask a portion of global warming, have been the subject of many studies with large uncertainty remaining. The interhemispheric contrast in AA forcing has also been demonstrated to induce a major shift in atmospheric circulation. The zonally heterogeneous changes in AA emissions since the late 20th century, with a notable decline in the Western Hemisphere and continuous increase in the Eastern Hemisphere, received less attention. Here we utilize four sets of single-model initial-condition large-ensemble simulations with various combinations of external forcings to quantify the different radiative and circulation responses due to aerosol emissions changes during 1980-2020. In particular, we focus on the distinct climate responses to Fossil-Fuel (FF) related aerosol from Western Hemisphere (WH) versus Eastern Hemisphere (EH). The zonal and meridional redistribution of FF aerosols from WH to EH results in negative radiative forcing over Asia and positive radiative forcing over North America and Europe. This leads to a counterclockwise anomaly of zonal mean stream function over the tropics (i.e. a northward shift of Hadley cell) and stronger equatorward shift of the Northern Hemisphere (NH) jet stream, consistent with the thermal wind argument with the gradient of surface air temperature (SAT) as a predictive metric. Two sets of regional FF simulations (Fix_EastFF1920 and Fix_WestFF1920) are performed and reveals the dominating role of WH forcing due to aerosol reduction in the NH. The Aerosol reduction over WH mid-to-high latitudes dominates the warming over NH mid-to-high latitudes. The increased aerosol over the EH low-to-mid latitudes is confined more locally but also induces slight warming over the northeastern Pacific and North Atlantic. The competing role of FF forcing originating from EH and WH in shaping tropospheric circulation and surface climate response indicates the importance of both zonal and meridional distribution of aerosol forcing within the NH, and previous idealized models that only consider the zonal difference of aerosol emission may oversimplify the real aerosol forcing.

Last Glacial Maximum (LGM) climate forcing and ocean dynamical feedback and their implications for estimating climate sensitivity

Abstract: . Equilibrium climate sensitivity (ECS) has been directly estimated using reconstructions of past climates that are different than today's. A challenge to this approach is that temperature proxies integrate over the timescales of the fast feedback processes (e.g., changes in water vapor, snow, and clouds) that are captured in ECS as well as the slower feedback processes (e.g., changes in ice sheets and ocean circulation) that are not. A way around this issue is to treat the slow feedbacks as climate forcings and independently account for their impact on global temperature. Here we conduct a suite of Last Glacial Maximum (LGM) simulations using the Community Earth System Model version 1.2 (CESM1.2) to quantify the forcing and efficacy of land ice sheets (LISs) and greenhouse gases (GHGs) in order to estimate ECS. Our forcing and efficacy quantification adopts the effective radiative forcing (ERF) and adjustment framework and provides a complete accounting for the radiative, topographic, and dynamical impacts of LIS on surface temperatures. ERF and efficacy of LGM LIS are −3.2 W m −2 and 1.1, respectively. The larger-than-unity efficacy is caused by the temperature changes over land and the Northern Hemisphere subtropical oceans which are relatively larger than those in response to a doubling of atmospheric CO2 . The subtropical sea-surface temperature (SST) response is linked to LIS-induced wind changes and feedbacks in ocean–atmosphere coupling and clouds. ERF and efficacy of LGM GHG are −2.8 W m −2 and 0.9, respectively. The lower efficacy is primarily attributed to a smaller cloud feedback at colder temperatures. Our simulations further demonstrate that the direct ECS calculation using the forcing, efficacy, and temperature response in CESM1.2 overestimates the true value in the model by approximately 25 % due to the neglect of slow ocean dynamical feedback. This is supported by the greater cooling (6.8 ∘ C) in a fully coupled LGM simulation than that (5.3 ∘ C) in a slab ocean model simulation with ocean dynamics disabled. The majority (67 %) of the ocean dynamical feedback is attributed to dynamical changes in the Southern Ocean, where interactions between upper-ocean stratification, heat transport, and sea-ice cover are found to amplify the LGM cooling. Our study demonstrates the value of climate models in the quantification of climate forcings and the ocean dynamical feedback, which is necessary for an accurate direct ECS estimation.

The state and fate of lake ice thickness in the Northern Hemisphere

Abstract: Lake ice thickness (LIT) is important for regional hydroclimate systems, lake ecosystems, and human activities on the ice, and is thought to be highly susceptible to global warming. However, the spatiotemporal variability in LIT is largely unknown due to the difficulty in deriving in situ measurements and the lack of an effective remote sensing platform. Despite intensive development and applications of lake ice models driven by general circulation model output, evaluation of the global LIT is mostly based on assumed “ideal” lakes in each grid cell of the climate forcing data. A method for calculating the actual global LIT is therefore urgently needed. Here we use satellite altimetry to retrieve ice thickness for 16 large lakes in the Northern Hemisphere (Lake Baikal, Great Slave Lake, and others) with an accuracy of ∼0.2 m for almost three decades. We then develop a 1-D lake ice model driven primarily by remotely sensed data and cross-validated with the altimetric LIT to provide a robust means of estimating LIT for lakes larger than 50 km2 across the Northern Hemisphere. Mean LIT (annual maximum ice thickness) for 1313 simulated lakes and reservoirs covering ∼840,000 km2 for 2003–2018 is 0.63 ± 0.02 m, corresponding to ∼485 Gt of water. LIT changes are projected for 2071–2099 under RCPs 2.6, 6.0, and 8.5, showing that the mean LIT could decrease by ∼0.35 m under the worst concentration pathway and the associated lower ice road availability could have a significant impact on socio-economic activities.

The Arctic Polar Vortex Response to Volcanic Forcing of Different Strengths

Abstract: Large explosive volcanic eruptions can inject sulfur into the stratosphere, which forms sulfate aerosols and leads to global mean surface temperature reductions and changes in atmospheric circulation in the following years (Robock, 2000; Timmreck, 2012). Particular attention has been paid to a possible strengthening of the polar vortex in the Arctic winter stratosphere because via stratosphere-troposphere dynamical coupling this has been linked to tropospheric circulation changes leading, for instance, to a surface winter warming signal in Northern Eurasia (Robock & Mao, 1992; Shindell et al., 2004), although the latter has been disputed recently (Polvani et al., 2019).

Asymmetrical response of summer rainfall in East Asia to CO2 forcing

Abstract: Understanding the regional hydrological response to varying CO2 concentration is critical for cost-benefit analysis of mitigation and adaptation polices in the near future. To characterize summer monsoon rainfall change in East Asia in a changing CO2 pathway, we used the Community Earth System Model (CESM) with 28 ensemble members in which the CO2 concentration increases at a rate of 1% per year until its quadrupling peak, i.e., 1468 ppm (ramp-up period), followed by a decrease of 1% per year until the present-day climate conditions, i.e., 367 ppm (ramp-down period). Although the CO2 concentration change is symmetric in time, the amount of summer rainfall anomaly in East Asia is increased 42% during a ramp-down period than that during a ramp-up period when the two periods of the same CO2 concentration are compared. This asymmetrical rainfall response is mainly due to an enhanced El Nino-like warming pattern as well as its associated increase in the sea surface temperature in the western North Pacific during a ramp-down period. These sea surface temperature patterns enhance the atmospheric teleconnections and the local meridional circulations around East Asia, resulting in more rainfall over East Asia during a ramp-down period. This result implies that the removal of CO2 does not guarantee the return of regional rainfall to the previous climate state with the same CO2 concentration.

The Physical Climate at Global Warming Thresholds as seen in the UK Earth System Model

Abstract: A key goal of the 2015 Paris Climate Agreement is to keep global mean temperature change at 2°C and if possible under 1.5°C by the end of the century. To investigate the likelihood of achieving this target, we calculate the year of exceedance of a given Global Warming Threshold (GWT) temperature across thirty-two CMIP6 models for Shared Socioeconomic Pathway (SSP) and radiative forcing combinations included in the Tier 1 ScenarioMIP simulations. Threshold exceedance year calculations reveal that a majority of CMIP6 models project warming beyond 2°C by the end of the century under every scenario or pathway apart from the lowest emission scenarios considered, SSP1-1.9 and SSP1-2.6 which is largely a function of the ScenarioMIP experiment design. The UK Earth System Model (UKESM1) ScenarioMIP projections are analysed in detail to assess the regional and seasonal variations in climate at different warming levels. The warming signal emerging by mid-century is identified as significant and distinct from internal climate variability in all scenarios considered and includes warming summers in the Mediterranean, drying in the Amazon and heavier Indian monsoons. Arctic sea-ice depletion results in prominent amplification of warming and tropical warming patterns emerge which are distinct from interannual variability. Climate changes projected for a 2°C warmer world are in almost all cases exacerbated with further global warming (e.g. to a 4°C warmer world).

How Does Indian Monsoon Regulate the Northern Hemisphere Stationary Wave Pattern

Abstract: The northern hemisphere summer climate is strongly affected by a circum-global stationary Rossby wave train, which can be manifested by the first EOF mode of the geopotential height at 200 hPa. Interannual variation of this Northern Hemisphere wave (NHW) pattern has a significant impact on remarkably warm surface temperature anomalies over the North Atlantic, Northeast Europe, East Asia to Central-North Pacific, and America, particularly in 2018 and 2010. The NHW pattern is likely generated by atmospheric diabatic heating and vorticity forcing: diabatic heating is mainly confined in the Indian summer monsoon (ISM) precipitation region, whereas the anti-cyclonic vorticity forcing is distributed in the globe. The ISM is a well-known diabatic heat source; however, the main source of vorticity forcing has not been established. In general, the tropical vorticity anomaly comes from diabatic heating-induced atmospheric waves and randomly generated inherent internal waves. The Linear Baroclinic Model experiment reveals that the NHW pattern can be generated by the westward propagating tropical waves generated by the ISM diabatic heat forcing.

Impacts of multi-layer overlap on contrail radiative forcing

Abstract: Condensation trails (“contrails”) which form behind aircraft are estimated to cause on the order of 50 % of the total climate forcing of aviation, matching the total impact of all accumulated aviation-attributable CO 2 The climate impacts of these contrails are highly uncertain, in part due to the effect of overlap between contrails and other cloud layers Although literature estimates suggest that overlap could change even the sign of contrail radiative forcing (RF), the impacts of cloud–contrail overlaps are not well understood, and the effect of contrail–contrail overlap has never been quantified In this study we develop and apply a new model of contrail radiative forcing which explicitly accounts for overlap between cloud layers Assuming maximum possible overlap to provide an upper bound on impacts, cloud–contrail overlap is found to reduce the shortwave-cooling effect attributable to aviation by 66 % while reducing the longwave-warming effect by only 37 % Therefore, on average in 2015, cloud–contrail overlap increased the net radiative forcing from contrails We also quantify the sensitivity of contrail radiative forcing to cloud cover with respect to geographic location Clouds significantly increase warming at high latitudes and over sea, transforming cooling contrails into warming ones in the North Atlantic corridor Based on the same data, our results indicate that disregarding overlap between a given pair of contrail layers can result in longwave and shortwave radiative forcing being overestimated by up to 16 % and 25 %, respectively, with the highest bias observed at high optical depths ( > 04) and high solar zenith angles ( > 75 ∘ ) When applied to estimated global contrail coverage data for 2015, contrail–contrail overlap reduces both the longwave and shortwave forcing by ∼ 2 % relative to calculations which ignore overlap The effect is greater for longwave radiation, resulting in a 3 % net reduction in the estimated RF when overlap is correctly accounted for This suggests that contrail–contrail overlap radiative effects can likely be neglected in estimates of the current-day environmental impacts of aviation However, the effect of contrail–contrail overlap may increase in the future as the airline industry grows into new regions

Uncertainty in Observational Estimates of the Aerosol Direct Radiative Effect and Forcing

Abstract: A lower bound on the uncertainty in observational estimates of the aerosol direct radiative effect (DRE; the direct interaction with solar radiation by all aerosols) and the aerosol direct radiative forcing [DRF; the radiative effect of just anthropogenic aerosols (RFari)] is quantified by making the optimistic assumption that global aerosol observations can be made with the accuracy found in the Aerosol Robotic Network (AERONET) sun photometer retrievals. The global-mean all-sky aerosol DRE uncertainty was found to be 1.1 W m−2 (one standard deviation). The global-mean all-sky aerosol DRF (RFari) uncertainty was determined to be 0.31 W m−2. The total uncertainty in both quantities is dominated by contributions from the aerosol single scattering albedo uncertainty. These uncertainty estimates were compared to a literature survey of mostly satellite-based aerosol DRE/DRF values. Comparisons to previous studies reveal that most have significantly underestimated the aerosol DRE uncertainty. Past estimates of the aerosol DRF uncertainty are smaller (on average) than our optimistic observational estimates, including the aerosol DRF uncertainty given in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). This disconnect between our observation-based uncertainty and that found in past aerosol DRF studies that rely, at least in part, on modeling is discussed. Also quantified is a potential reduction in the current observational uncertainty possible with a future generation of satellite observations that would leverage aerosol typing and more refined vertical information.

The Seasonal Cycle of Significant Wave Height in the Ocean: Local Versus Remote Forcing

Abstract: Significant wave height (SWH) stems from a combination of locally generated "wind-sea" and remotely generated "swell" waves In the Northern and Southern Hemispheres, wave heights typically undergo

Elucidating the Mechanisms Responsible for Hadley Cell Weakening Under 4 × CO 2 Forcing

Abstract: Climate models project that the Hadley circulation (HC) will widen and weaken by the end of the 21st century (IPCC, 2013; Vallis et al., 2015). These circulation changes will have large societal impacts, affecting tropical and subtropical precipitation (IPCC, 2014). Note that changes in the HC are fundamentally different between the two hemispheres. The expansion of the circulation is projected to occur mostly in the Southern Hemisphere (SH) (Grise et al., 2019; Vallis et al., 2015). Over recent decades, reanalyses show a modest expansion of the SH HC (Davis & Davis, 2018; Grise et al., 2019), mostly driven by ozone depletion in the second half of the 20th century (Polvani et al., 2011; Son et al., 2009). With the unabated emissions of greenhouse gases into the atmosphere, the expansion in the SH is projected to emerge out of the internal variability by the mid-21st century (Grise et al., 2019). Over the last decades, several mechanisms have been proposed to explain the expansion of the HC (arguing that is caused by changes in sea surface temperature, eddy phase speed, tropopause height, static stability, etc.). Recently, by examining the HC response in the abrupt 4 × CO2 experiment of the Coupled Model Intercomparison Project phase 5 (CMIP5), we showed that changes in static stability are the key factor in tropical expansion (Chemke & Polvani, 2019a), thus confirming previous studies who argued for its importance (Lu et al., 2008; Son et al., 2018; Vallis et al., 2015). It is also important to note that HC expansion in the Northern Hemisphere (NH) is not projected to emerge from the internal variability between now and the end of this century (Grise et al., 2019).