Abstract. Anthropogenic aerosols (AAs) induce global and regionaltropospheric circulation adjustments due to the radiative energyperturbations. The overall cooling effects of AA, which mask a portion ofglobal warming, have been the subject of many studies but still have largeuncertainty. The interhemispheric contrast in AA forcing has also beendemonstrated to induce a major shift in atmospheric circulation. However,the zonal redistribution of AA emissions since start of the 20th century, with anotable decline in the Western Hemisphere (North America and Europe) and acontinuous 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-ensemblesimulations with various combinations of external forcings to quantify theradiative and circulation responses due to the spatial redistribution of AAforcing during 1980–2020. In particular, we focus on the distinct climateresponses 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 the1980s leads to a weakening negative radiative forcing over the WHmid-to-high latitudes and an enhancing negative radiative forcing over theEH 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_EastFF1920and Fix_WestFF1920) are performed to separate the roles ofzonally 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 drivingatmospheric circulation and surface temperature responses during the recentdecades highlight the importance of considering zonally asymmetric forcings(west to east) and also their meridional locations within the NH (tropicalvs. extratropical).
more »
« less
Nonlinear Climate Responses to Increasing CO2 and Anthropogenic Aerosols Simulated by CESM1
Atmospheric CO2 and anthropogenic aerosols (AA) have increased simultaneously. Because of their opposite radiative effects, these increases may offset each other, which may lead to some nonlinear effects. Here the seasonal and regional characteristics of this nonlinear effect from the CO2 and AA forcings are investigated using the fully coupled Community Earth System Model. Results show that nonlinear effects are small in the global mean of the top-of-the-atmosphere radiative fluxes, surface air temperature, and precipitation. However, significant nonlinear effects exist over the Arctic and other extratropical regions during certain seasons. When both forcings are included, Arctic sea ice in September–November decreases less than the linear combination of the responses to the individual forcings due to a higher sea ice sensitivity to the CO2-induced warming than the sensitivity to the AA-induced cooling. This leads to less Arctic warming in the combined-forcing experiment due to reduced energy release from the Arctic Ocean to the atmosphere. Some nonlinear effects on precipitation in June–August are found over East Asia, with the northward-shifted East Asian summer rain belt to oppose the CO2 effect. In December–February, the aerosol loading over Europe in the combined-forcing experiment is higher than that due to the AA forcing, resulting from CO2-induced circulation changes. The changed aerosol loading results in regional thermal responses due to aerosol direct and indirect effects, weakening the combined changes of temperature and circulation. This study highlights the need to consider nonlinear effects from historical CO2 and AA forcings in seasonal and regional climate attribution analyses.
more » « less- NSF-PAR ID:
- 10126974
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 33
- Issue:
- 1
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- p. 281-301
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)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 forcingand 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.more » « less
-
Response of Meridional Wind to Greenhouse Gas Forcing, Arctic Sea-Ice Loss, and Arctic AmplificationAbstract Under increasing greenhouse gases, the Arctic warms about twice as fast as elsewhere, known as Arctic amplification (AA). AA weakens meridional temperature gradients and is hypothesized to weaken zonal wind and cause wavier circulation with stronger meridional wind ( υ ) over northern mid-to-high latitudes. Here model simulations are analyzed to examine the υ response to increased CO 2 and AA alone. Total υ changes are found to be dominated by the effect of increased CO 2 without AA, with a zonal wavenumber-4 (wavenumber-3) change pattern over the northern (southern) extratropics that generally enhances current υ and results partly from changes in zonal temperature gradients. The extratropical υ change patterns are quasi-barotropic and are more pronounced during boreal winter. The CO 2 forcing also causes baroclinic υ changes over the tropics tied to convection changes. The impact of AA on υ is mainly over the northern extratropics and is opposite to the effect of increased CO 2 but with smaller magnitude. An eastward shift (∼5° longitude) and an amplitude increase (∼1 m s −1 ) in the climatology of the northerlies over Europe caused mainly by CO 2 forcing contribute to the drying in southern Europe, while both AA and CO 2 forcing enhance the climatology of the northerlies over East Asia. Over the northern mid-to-high latitudes, Arctic sea ice loss and AA enhance the land–ocean thermal contrast in winter, while increased CO 2 alone weakens it, resulting in opposite changes in zonal temperature gradients and thus υ . Different warming rates over land and ocean also contribute to the intermodel spread in υ response patterns among climate models. Significance Statement Meridional wind ( υ ) greatly contributes to thermal and moisture advection due to large meridional gradients in these fields. It is hypothesized that the enhanced Arctic warming under anthropogenic global warming could weaken meridional temperature gradients, decelerate westerly jets, and cause wavier circulation with stronger υ over northern extratropics. Using novel climate model simulations, we found that the effect of increased CO 2 without AA determines the total υ changes. AA generally weakens the climatological υ , contrary to the direct effect of increased CO 2 . The υ changes are small relative to its climatology but may have large impacts on regional climate over central Europe, East Asia, and interior North America. More research is needed to examine the mechanisms causing regional υ changes.more » « less
-
more » « less
Abstract The processes controlling idealized warming and cooling patterns are examined in 150-yr-long fully coupled Community Earth System Model, version 1 (CESM1), experiments under abrupt CO2forcing. By simulation end, 2 × CO2global warming was 20% larger than 0.5 × CO2global cooling. Not only was the absolute global effective radiative forcing ∼10% larger for 2 × CO2than for 0.5 × CO2, global feedbacks were also less negative for 2 × CO2than for 0.5 × CO2. Specifically, more positive shortwave cloud feedbacks led to more 2 × CO2global warming than 0.5 × CO2global cooling. Over high-latitude oceans, differences between 2 × CO2warming and 0.5 × CO2cooling were amplified by familiar linked positive surface albedo and lapse rate feedbacks associated with sea ice change. At low latitudes, 2 × CO2warming exceeded 0.5 × CO2cooling almost everywhere. Tropical Pacific cloud feedbacks amplified the following: 1) more fast warming than fast cooling in the west, and 2) slow pattern differences between 2 × CO2warming and 0.5 × CO2cooling in the east. Motivated to quantify cloud influence, a companion suite of experiments was run without cloud radiative feedbacks. Disabling cloud radiative feedbacks reduced the effective radiative forcing and surface temperature responses for both 2 × CO2and 0.5 × CO2. Notably, 20% more global warming than global cooling occurred regardless of whether cloud feedbacks were enabled or disabled. This surprising consistency resulted from the cloud influence on non-cloud feedbacks and circulation. With the exception of the tropical Pacific, disabling cloud feedbacks did little to change surface temperature response patterns including the large high-latitude responses driven by non-cloud feedbacks. The findings provide new insights into the regional processes controlling the response to greenhouse gas forcing, especially for clouds.
Significance Statement We analyze the processing controlling idealized warming and cooling under abrupt CO2forcing using a modern and highly vetted fully coupled climate model. We were especially interested to compare simulations with and without cloud radiative feedbacks. Notably, 20% more global warming than global cooling occurred regardless of whether cloud feedbacks were enabled or disabled. This surprising consistency resulted from the cloud influence on forcing, non-cloud feedbacks, and circulation. With the exception of the tropical Pacific, disabling cloud feedbacks did little to change surface temperature response patterns including the large high-latitude responses driven by non-cloud feedbacks. The findings provide new insights into the regional processes controlling the response to greenhouse gas forcing, especially for clouds. When combined with estimates of cooling at the Last Glacial Maximum, the findings also help rule out large (4+ K) values of equilibrium climate sensitivity.
-
null (Ed.)Abstract. The Arctic is warming 2 to 3 times faster than the global average, partly due to changes in short-lived climate forcers (SLCFs) including aerosols. In order to study the effects of atmospheric aerosols in this warming, recent past (1990–2014) and future (2015–2050) simulations have been carried out using the GISS-E2.1 Earth system model to study the aerosol burdens and their radiative and climate impacts over the Arctic (>60∘ N), using anthropogenic emissions from the Eclipse V6b and the Coupled Model Intercomparison Project Phase 6 (CMIP6) databases, while global annual mean greenhouse gas concentrations were prescribed and kept fixed in all simulations. Results showed that the simulations have underestimated observed surface aerosol levels, in particular black carbon (BC) and sulfate (SO42-), by more than 50 %, with the smallest biases calculated for the atmosphere-only simulations, where winds are nudged to reanalysis data. CMIP6 simulations performed slightly better in reproducing the observed surface aerosol concentrations and climate parameters, compared to the Eclipse simulations. In addition, simulations where atmosphere and ocean are fully coupled had slightly smaller biases in aerosol levels compared to atmosphere-only simulations without nudging. Arctic BC, organic aerosol (OA), and SO42- burdens decrease significantly in all simulations by 10 %–60 % following the reductions of 7 %–78 % in emission projections, with the Eclipse ensemble showing larger reductions in Arctic aerosol burdens compared to the CMIP6 ensemble. For the 2030–2050 period, the Eclipse ensemble simulated a radiative forcing due to aerosol–radiation interactions (RFARI) of -0.39±0.01 W m−2, which is −0.08 W m−2 larger than the 1990–2010 mean forcing (−0.32 W m−2), of which -0.24±0.01 W m−2 was attributed to the anthropogenic aerosols. The CMIP6 ensemble simulated a RFARI of −0.35 to −0.40 W m−2 for the same period, which is −0.01 to −0.06 W m−2 larger than the 1990–2010 mean forcing of −0.35 W m−2. The scenarios with little to no mitigation (worst-case scenarios) led to very small changes in the RFARI, while scenarios with medium to large emission mitigations led to increases in the negative RFARI, mainly due to the decrease in the positive BC forcing and the decrease in the negative SO42- forcing. The anthropogenic aerosols accounted for −0.24 to −0.26 W m−2 of the net RFARI in 2030–2050 period, in Eclipse and CMIP6 ensembles, respectively. Finally, all simulations showed an increase in the Arctic surface air temperatures throughout the simulation period. By 2050, surface air temperatures are projected to increase by 2.4 to 2.6 ∘C in the Eclipse ensemble and 1.9 to 2.6 ∘C in the CMIP6 ensemble, compared to the 1990–2010 mean. Overall, results show that even the scenarios with largest emission reductions leads to similar impact on the future Arctic surface air temperatures and sea-ice extent compared to scenarios with smaller emission reductions, implying reductions of greenhouse emissions are still necessary to mitigate climate change.more » « less
