scholarly journals Anthropogenic aerosol effects on tropospheric circulation and sea surface temperature (1980–2020): separating the role of zonally asymmetric forcings

2021 ◽  
Vol 21 (24) ◽  
pp. 18499-18518
Author(s):  
Chenrui Diao ◽  
Yangyang Xu ◽  
Shang-Ping Xie
Keyword(s):  
Surface Temperature ◽  
Radiative Forcing ◽  
Western Hemisphere ◽  
Hadley Cell ◽  
Jet Stream ◽  
High Latitudes ◽  
Aerosol Forcing ◽  
Eastern Hemisphere ◽  
Northeastern Pacific ◽  
Tropospheric Circulation

Abstract. Anthropogenic aerosols (AAs) induce global and regional tropospheric circulation adjustments due to the radiative energy perturbations. The overall cooling effects of AA, which mask a portion of global warming, have been the subject of many studies but still have large uncertainty. The interhemispheric contrast in AA forcing has also been demonstrated to induce a major shift in atmospheric circulation. However, the zonal redistribution of AA emissions since start of the 20th century, with a notable decline in the Western Hemisphere (North America and Europe) and a continuous increase in the Eastern Hemisphere (South Asia and East Asia), has 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 radiative and circulation responses due to the spatial redistribution of AA forcing during 1980–2020. In particular, we focus on the distinct climate responses due to fossil-fuel-related (FF) aerosols emitted from the Western Hemisphere (WH) versus the Eastern Hemisphere (EH). The zonal (west to east) redistribution of FF aerosol emission since the 1980s leads to a weakening negative radiative forcing over the WH mid-to-high latitudes and an enhancing negative radiative forcing over the EH at lower latitudes. Overall, the FF aerosol leads to a northward shift of the Hadley cell and an equatorward shift of the Northern Hemisphere (NH) jet stream. Here, two sets of regional FF simulations (Fix_EastFF1920 and Fix_WestFF1920) are performed to separate the roles of zonally asymmetric aerosol forcings. We find that the WH aerosol forcing, located in the extratropics, dominates the northward shift of the Hadley cell by inducing an interhemispheric imbalance in radiative forcing. On the other hand, the EH aerosol forcing, located closer to the tropics, dominates the equatorward shift of the NH jet stream. The consistent relationship between the jet stream shift and the top-of-atmosphere net solar flux (FSNTOA) gradient suggests that the latter serves as a rule-of-thumb guidance for the expected shift of the NH jet stream. The surface effect of EH aerosol forcing (mainly from low- to midlatitudes) is confined more locally and only induces weak warming over the northeastern Pacific and North Atlantic. In contrast, the WH aerosol reduction leads to a large-scale warming over NH mid-to-high latitudes that largely offsets the cooling over the northeastern Pacific due to EH aerosols. The simulated competing roles of regional aerosol forcings in driving atmospheric circulation and surface temperature responses during the recent decades highlight the importance of considering zonally asymmetric forcings (west to east) and also their meridional locations within the NH (tropical vs. extratropical).

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

2021 ◽  
Author(s):  
Chenrui Diao ◽  
Yangyang Xu ◽  
Shang-Ping Xie
Keyword(s):  
Radiative Forcing ◽  
Western Hemisphere ◽  
Hadley Cell ◽  
High Latitudes ◽  
Aerosol Forcing ◽  
Eastern Hemisphere ◽  
Aerosol Emission ◽  
Tropospheric Circulation ◽  
Cooling Effects

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.

scholarly journals The Large-Scale Dynamical Response of Clouds to Aerosol Forcing

2017 ◽  
Vol 30 (21) ◽  
pp. 8783-8794 ◽  
Author(s):  
Brian Soden ◽  
Eui-Seok Chung
Keyword(s):  
Atmospheric Circulation ◽  
Radiative Forcing ◽  
Large Scale ◽  
Energy Transport ◽  
Hadley Cell ◽  
Dynamical Response ◽  
Aerosol Radiative Forcing ◽  
Aerosol Forcing ◽  
Meridional Displacement ◽  
Cloud Response

Radiative kernels are used to quantify the instantaneous radiative forcing of aerosols and the aerosol-mediated cloud response in coupled ocean–atmosphere model simulations under both historical and future emission scenarios. The method is evaluated using matching pairs of historical climate change experiments with and without aerosol forcing and accurately captures the spatial pattern and global-mean effects of aerosol forcing. It is shown that aerosol-driven changes in the atmospheric circulation induce additional cloud changes. Thus, the total aerosol-mediated cloud response consists of both local microphysical changes and nonlocal dynamical changes that are driven by hemispheric asymmetries in aerosol forcing. By comparing coupled and fixed sea surface temperature (SST) simulations with identical aerosol forcing, the relative contributions of these two components are isolated, exploiting the ability of prescribed SSTs to also suppress changes in the atmospheric circulation. The radiative impact of the dynamical cloud changes is found to be comparable in magnitude to that of the microphysical cloud changes and acts to further amplify the interhemispheric asymmetry of the aerosol radiative forcing. The dynamical cloud response is closely linked to the meridional displacement of the Hadley cell, which, in turn, is driven by changes in the cross-equatorial energy transport. In this way, the dynamical cloud changes act as a positive feedback on the meridional displacement of the Hadley cell, roughly doubling the projected changes in cross-equatorial energy transport compared to that from the microphysical changes alone.

scholarly journals Using an AGCM to Diagnose Historical Effective Radiative Forcing and Mechanisms of Recent Decadal Climate Change

2014 ◽  
Vol 27 (3) ◽  
pp. 1193-1209 ◽  
Author(s):  
Timothy Andrews
Keyword(s):  
Climate Change ◽  
Sea Ice ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Radiative Forcing ◽  
General Circulation ◽  
Circulation Model ◽  
Sea Surface ◽  
Aerosol Forcing ◽  
Effective Radiative Forcing

Abstract An atmospheric general circulation model is forced with observed monthly sea surface temperature and sea ice boundary conditions, as well as forcing agents that vary in time, for the period 1979–2008. The simulations are then repeated with various forcing agents, individually and in combination, fixed at preindustrial levels. The simple experimental design allows the diagnosis of the model’s global and regional time-varying effective radiative forcing from 1979 to 2008 relative to preindustrial levels. Furthermore the design can be used to (i) calculate the atmospheric model’s feedback/sensitivity parameters to observed changes in sea surface temperature and (ii) separate those aspects of climate change that are directly driven by the forcing from those driven by large-scale changes in sea surface temperature. It is shown that the atmospheric response to increased radiative forcing over the last 3 decades has halved the global precipitation response to surface warming. Trends in sea surface temperature and sea ice are found to contribute only ~60% of the global land, Northern Hemisphere, and summer land warming trends. Global effective radiative forcing is ~1.5 W m−2 in this model, with anthropogenic and natural contributions of ~1.3 and ~0.2 W m−2, respectively. Forcing increases by ~0.5 W m−2 decade−1 over the period 1979–2008 or ~0.4 W m−2 decade−1 if years strongly influenced by volcanic forcings—which are nonlinear with time—are excluded from the trend analysis. Aerosol forcing shows little global decadal trend due to offsetting regional trends whereby negative aerosol forcing weakens in Europe and North America but continues to strengthen in Southeast Asia.

Influences of ENSO-Induced Indo–Western Pacific SST Anomalies on Extratropical Atmospheric Variability during the Boreal Summer

2005 ◽  
Vol 18 (15) ◽  
pp. 2922-2942 ◽  
Author(s):  
Ngar-Cheung Lau ◽  
Ants Leetmaa ◽  
Mary Jo Nath ◽  
Hai-Lan Wang
Keyword(s):  
Surface Temperature ◽  
Western Hemisphere ◽  
Western Pacific ◽  
Boreal Summer ◽  
Mean Flow ◽  
Enso Events ◽  
Eastern Hemisphere ◽  
Simulated Precipitation ◽  
Atmospheric Changes ◽  
Sst Anomalies

Abstract The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Niño–Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo–western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these global-scale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the “atmospheric bridge” mechanism. Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the IWP region, which are closely related to the underlying SST changes. Diagnosis of the output from the GCM integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high-frequency eddies at locations farther downstream. The geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the Western Hemisphere. Such local eddy–mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the IWP region. Analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in ENSO events could also generate zonally symmetric perturbations in midlatitudes. The model-simulated precipitation and surface temperature anomalies in the North American sector in response to SST changes in IWP suggest an increased frequency of droughts and heat waves in that region during the summer season after warm ENSO events.

scholarly journals Understanding the Time Scales of the Tropospheric Circulation Response to Abrupt CO2 Forcing in the Southern Hemisphere: Seasonality and the Role of the Stratosphere

2017 ◽  
Vol 30 (21) ◽  
pp. 8497-8515 ◽  
Author(s):  
Kevin M. Grise ◽  
Lorenzo M. Polvani
Keyword(s):  
Surface Temperature ◽  
Southern Hemisphere ◽  
Time Scales ◽  
Time Scale ◽  
Lower Stratosphere ◽  
Hadley Cell ◽  
Global Mean Surface Temperature ◽  
Tropospheric Circulation ◽  
Dry Zone ◽  
Global Mean

This study examines the time scales of the Southern Hemisphere (SH) tropospheric circulation response to increasing atmospheric CO2 concentrations in models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). In response to an abrupt quadrupling of atmospheric CO2, the midlatitude jet stream and poleward edge of the Hadley circulation shift poleward on the time scale of the rising global-mean surface temperature during the summer and fall seasons but on a much more rapid time scale during the winter and spring seasons. The seasonally varying time scales of the SH circulation response are closely tied to the meridional temperature gradient in the upper troposphere–lower stratosphere and, in particular, to temperatures in the SH polar lower stratosphere. During summer and fall, SH polar lower-stratospheric temperatures cool on the time scale of warming global surface temperatures, as the lifting of the tropopause height with tropospheric warming is associated with cooling at lower-stratospheric levels. However, during winter and spring, SH polar lower-stratospheric temperatures cool primarily from fast time-scale radiative processes, contributing to the faster time-scale circulation response during these seasons. The poleward edge of the SH subtropical dry zone shifts poleward on the time scale of the rising global-mean surface temperature during all seasons in response to an abrupt quadrupling of atmospheric CO2. The dry zone edge initially follows the poleward shift in the Hadley cell edge but is then augmented by the action of eddy moisture fluxes in a warming climate. Consequently, with increasing atmospheric CO2 concentrations, key features of the tropospheric circulation response could emerge sooner than features more closely tied to rising global temperatures.

scholarly journals A Limited Role for Unforced Internal Variability in Twentieth-Century Warming

2019 ◽  
Vol 32 (16) ◽  
pp. 4893-4917 ◽  
Author(s):  
Karsten Haustein ◽  
Friederike E. L. Otto ◽  
Victor Venema ◽  
Peter Jacobs ◽  
Kevin Cowtan ◽  
...  
Keyword(s):  
Twentieth Century ◽  
Surface Temperature ◽  
Radiative Forcing ◽  
Low Frequency ◽  
Internal Variability ◽  
External Forcing ◽  
Anthropogenic Aerosol ◽  
Transient Climate Response ◽  
The North ◽  
Aerosol Forcing

AbstractThe early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is still debated. Using a two-box impulse response model, we demonstrate that multidecadal ocean variability was unlikely to be the driver of observed changes in global mean surface temperature (GMST) after AD 1850. Instead, virtually all (97%–98%) of the global low-frequency variability (>30 years) can be explained by external forcing. We find similarly high percentages of explained variance for interhemispheric and land–ocean temperature evolution. Three key aspects are identified that underpin the conclusion of this new study: inhomogeneous anthropogenic aerosol forcing (AER), biases in the instrumental sea surface temperature (SST) datasets, and inadequate representation of the response to varying forcing factors. Once the spatially heterogeneous nature of AER is accounted for, the MC period is reconcilable with external drivers. SST biases and imprecise forcing responses explain the putative disagreement between models and observations during the EW period. As a consequence, Atlantic multidecadal variability (AMV) is found to be primarily controlled by external forcing too. Future attribution studies should account for these important factors when discriminating between externally forced and internally generated influences on climate. We argue that AMV must not be used as a regressor and suggest a revised AMV index instead [the North Atlantic Variability Index (NAVI)]. Our associated best estimate for the transient climate response (TCR) is 1.57 K (±0.70 at the 5%–95% confidence level).

scholarly journals A network-based detection scheme for the jet stream core

2017 ◽  
Vol 8 (1) ◽  
pp. 75-89 ◽  
Author(s):  
Sonja Molnos ◽  
Tarek Mamdouh ◽  
Stefan Petri ◽  
Thomas Nocke ◽  
Tino Weinkauf ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
Probability Distributions ◽  
Reanalysis Data ◽  
Western Hemisphere ◽  
Jet Stream ◽  
Jet Streams ◽  
Detection Scheme ◽  
The North ◽  
Eastern Hemisphere ◽  
Subtropical Jet

Abstract. The polar and subtropical jet streams are strong upper-level winds with a crucial influence on weather throughout the Northern Hemisphere midlatitudes. In particular, the polar jet is located between cold arctic air to the north and warmer subtropical air to the south. Strongly meandering states therefore often lead to extreme surface weather. Some algorithms exist which can detect the 2-D (latitude and longitude) jets' core around the hemisphere, but all of them use a minimal threshold to determine the subtropical and polar jet stream. This is particularly problematic for the polar jet stream, whose wind velocities can change rapidly from very weak to very high values and vice versa. We develop a network-based scheme using Dijkstra's shortest-path algorithm to detect the polar and subtropical jet stream core. This algorithm not only considers the commonly used wind strength for core detection but also takes wind direction and climatological latitudinal position into account. Furthermore, it distinguishes between polar and subtropical jet, and between separate and merged jet states. The parameter values of the detection scheme are optimized using simulated annealing and a skill function that accounts for the zonal-mean jet stream position (Rikus, 2015). After the successful optimization process, we apply our scheme to reanalysis data covering 1979–2015 and calculate seasonal-mean probabilistic maps and trends in wind strength and position of jet streams. We present longitudinally defined probability distributions of the positions for both jets for all on the Northern Hemisphere seasons. This shows that winter is characterized by two well-separated jets over Europe and Asia (ca. 20° W to 140° E). In contrast, summer normally has a single merged jet over the western hemisphere but can have both merged and separated jet states in the eastern hemisphere. With this algorithm it is possible to investigate the position of the jets' cores around the hemisphere and it is therefore very suitable to analyze jet stream patterns in observations and models, enabling more advanced model-validation.

scholarly journals Why Does Aerosol Forcing Control Historical Global-Mean Surface Temperature Change in CMIP5 Models?

2015 ◽  
Vol 28 (17) ◽  
pp. 6608-6625 ◽  
Author(s):  
Leon D. Rotstayn ◽  
Mark A. Collier ◽  
Drew T. Shindell ◽  
Olivier Boucher
Keyword(s):  
Greenhouse Gases ◽  
Surface Temperature ◽  
Radiative Forcing ◽  
Dominant Factor ◽  
Cmip5 Models ◽  
Surface Temperature Change ◽  
Enhanced Sensitivity ◽  
Aerosol Forcing ◽  
Global Mean Surface Temperature ◽  
Global Mean

Abstract Linear regression is used to examine the relationship between simulated changes in historical global-mean surface temperature (GMST) and global-mean aerosol effective radiative forcing (ERF) in 14 climate models from CMIP5. The models have global-mean aerosol ERF that ranges from −0.35 to −1.60 W m−2 for 2000 relative to 1850. It is shown that aerosol ERF is the dominant factor that determines intermodel variations in simulated GMST change: correlations between aerosol ERF and simulated changes in GMST exceed 0.9 for linear trends in GMST over all periods that begin between 1860 and 1950 and end between 1995 and 2005. Comparison of modeled and observed GMST trends for these time periods gives an inferred global-mean aerosol ERF of −0.92 W m−2. On average, transient climate sensitivity is roughly 40% larger with respect to historical forcing from aerosols than well-mixed greenhouse gases. This enhanced sensitivity explains the dominant effect of aerosol forcing on simulated changes in GMST: it is estimated that 85% of the intermodel variance of simulated GMST change is explained by variations in aerosol ERF, but without the enhanced sensitivity less than half would be explained. Physically, the enhanced sensitivity is caused by a combination of 1) the larger concentration of aerosol forcing in the midlatitudes of the Northern Hemisphere, where positive feedbacks are stronger and transient warming is faster than in the Southern Hemisphere, and 2) the time evolution of aerosol forcing, which levels out earlier than forcing from well-mixed greenhouse gases.

scholarly journals A network-based detection scheme of the jet stream core

2016 ◽  
Author(s):  
Sonja Molnos ◽  
Tarek Mamdouh ◽  
Stefan Petri ◽  
Thomas Nocke ◽  
Tino Weinkauf ◽  
...  
Keyword(s):  
Western Hemisphere ◽  
Jet Stream ◽  
Data Sets ◽  
Jet Streams ◽  
Detection Scheme ◽  
The North ◽  
Eastern Hemisphere ◽  
Subtropical Jet ◽  
Latitudinal Position ◽  
Upper Level

Abstract. The polar and subtropical jet streams are strong upper-level winds with a crucial influence on weather throughout the Northern Hemisphere mid-latitudes. In particular, the polar jet is located between cold Arctic air to the North and warmer sub-tropical air to the South. Strongly meandering states therefore often lead to extreme surface weather. So far algorithms to detect jets' core around the hemisphere, including strong North-South undulations, are lacking. We develop a network-based scheme using Dijkstra's shortest path algorithm to detect the polar and subtropical jet stream core. This algorithm considers not only the commonly used wind strength for core detection but also takes wind direction and climatological latitudinal position into account. Furthermore, it distinguishes between polar and subtropical jet, and between separate and merged jet states. The detection scheme is optimized using simulated annealing and compared against an algorithm developed by Rikus (2015). After the successful optimization process we apply our scheme to climatology data and analyse seasonal data sets. We present probabilistic, regionally distinct positions for both jets for all seasons. This shows that winter is characterized by two well separated jets at mean longitudes of 20° S and 140° N. In contrast, summer normally has a single merged jet over the western Hemisphere and both merged and separated jet states possible in the eastern Hemisphere. With this algorithm it is possible to investigate the position of the jets' cores around the hemisphere and is therefore well suitable for analyses of jet stream patterns in observations or models.

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