Climate Effects of Anthropogenic Aerosol Forcing on Tropical Precipitation and Circulations

ABSTRACT Aerosols are one of the key factors influencing the hydrological cycle and radiation balance of the climate system. Although most aerosols deposit near their sources, the induced cooling effect is on a global scale and can influence the tropical atmosphere through slow processes, such as air–sea interactions. This study analyzes several simulations of fully coupled atmosphere–ocean climate models under the influence of anthropogenic aerosols, with the concentrations of greenhouse gases kept constant. In the cooling simulations, precipitation is reduced in deep convective areas but increased around the edges of convective areas, which is opposite to the “rich-get-richer” phenomenon in global warming scenarios in the first-order approximation. Tropical convection is intensified with a shallower depth, and tropical circulations are enhanced. The anomalous gross moist stability (M′) mechanism and the upped-ante mechanism can be used to explain the dynamic and thermodynamic processes in the changes in tropical precipitation and convection. There is a northward cross-equatorial energy transport due to the cooler Northern Hemisphere in most of the simulations, together with the southward shift of the intertropical convergence zone (ITCZ) and the enhancement of the Hadley circulation. The enhancement of the Hadley circulation is more consistent between models than the changes of the Walker circulation. The change in the Hadley circulation is not as negligible as in the warming cases in previous studies, which supports the consistency of the ITCZ shift in cooling simulations.

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Implications of aerosol-induced snow darkening on regional hydroclimate over the Himalayas

10.5194/egusphere-egu21-11972 ◽

2021 ◽

Author(s):

Vijayakumar Sivadasan Nair ◽

Usha Keshav Hasyagar ◽

Surendran Nair Suresh Babu

Keyword(s):

South Asia ◽

Climate Model ◽

Hydrological Cycle ◽

Monsoon Season ◽

Radiation Balance ◽

Western Himalayas ◽

Radiative Effect ◽

Anthropogenic Aerosols ◽

Snow Albedo ◽

Snow Cover Extent

The snow-covered mountains of Himalayas are known to play a crucial role in the hydrology of South Asia and are known as the &#8220;Asian water tower&#8221;. Despite the high elevations, the transport of anthropogenic aerosols from south Asia and desert dust from west Asia plays a significant role in directly and indirectly perturbing the radiation balance and hydrological cycle over the region. Absorbing aerosols like black carbon (BC) and dust deposited on the snow surface reduces the albedo of the Himalayan snow significantly (snow darkening or snow albedo effect). Using a Regional Climate Model (RegCM-4.6.0) coupled with SNow, ICe and Aerosol Radiation (SNICAR) module, the implications of aerosol-induced snow darkening on the regional hydroclimate of the Himalayas are investigated in this study. The aerosols deposited on snow shows a distinct regional heterogeneity. The albedo reduction due to aerosols shows a west to east gradient during pre-monsoon season and this results in the positive radiative effect of about 29 Wm-2, 17 Wm-2 and 5 Wm-2 over western, central and eastern Himalayas respectively. The reduction in the snow albedo also results in the sign reversal of the aerosol direct radiative effect i.e., from warming to cooling at the top of the atmosphere during pre-monsoon season. The excess solar energy trapped at the surface due to snow darkening warms the surface (0.66-1.9 K) and thus decreases the snow cover extent significantly. This results in the reduction of the number of snow-covered days by more than a month over the western Himalayas and about 10 &#8211; 15 days over the central Himalayas. The early snowmelt due to aerosol-induced snow darkening results in the increase of runoff throughout the melting season. Therefore, the present study highlights the heterogeneous response of aerosol induced snow albedo feedbacks over the Himalayan region and its impact on the snowpack and hydrology, which has further implications on the freshwater availability over the region.

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Reply to “Comment on ‘Rethinking the Lower Bound on Aerosol Radiative Forcing’”

Journal of Climate ◽

10.1175/jcli-d-17-0034.1 ◽

2017 ◽

Vol 30 (16) ◽

pp. 6585-6589 ◽

Cited By ~ 4

Author(s):

Bjorn Stevens ◽

Stephanie Fiedler

Keyword(s):

Twentieth Century ◽

Lower Bound ◽

Radiative Forcing ◽

Climate Models ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

The North ◽

Aerosol Forcing

Kretzschmar et al., in a comment in 2017, use the spread in the output of aerosol–climate models to argue that the models refute the hypothesis (presented in a paper by Stevens in 2015) that for the mid-twentieth-century warming to be consistent with observations, then the present-day aerosol forcing, [Formula: see text] must be less negative than −1 W m−2. The main point of contention is the nature of the relationship between global SO2 emissions and [Formula: see text] In contrast to the concave (log-linear) relationship used by Stevens and in earlier studies, whereby [Formula: see text] becomes progressively less sensitive to SO2 emissions, some models suggest a convex relationship, which would imply a less negative lower bound. The model that best exemplifies this difference, and that is most clearly in conflict with the hypothesis of Stevens, does so because of an implausible aerosol response to the initial rise in anthropogenic aerosol precursor emissions in East and South Asia—already in 1975 this model’s clear-sky reflectance from anthropogenic aerosol over the North Pacific exceeds present-day estimates of the clear-sky reflectance by the total aerosol. The authors perform experiments using a new (observationally constrained) climatology of anthropogenic aerosols to further show that the effects of changing patterns of aerosol and aerosol precursor emissions during the late twentieth century have, for the same global emissions, relatively little effect on [Formula: see text] These findings suggest that the behavior Kretzschmar et al. identify as being in conflict with the lower bound in Stevens arises from an implausible relationship between SO2 emissions and [Formula: see text] and thus provides little basis for revising this lower bound.

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Thermodynamic and dynamic responses of the hydrological cycle to solar dimming

10.5194/acp-2016-886 ◽

2016 ◽

Author(s):

Jane E. Smyth ◽

Rick D. Russotto ◽

Trude Storelvmo

Keyword(s):

Hydrological Cycle ◽

Global Climate ◽

Hadley Circulation ◽

Climate Impacts ◽

Dynamic Responses ◽

Tropical Precipitation ◽

Solar Geoengineering ◽

Intercomparison Project ◽

Rainfall Anomalies ◽

Precipitation Changes

Abstract. The fundamental role of the hydrological cycle in the global climate system motivates thorough evaluation of its responses to climate change and mitigation. The Geoengineering Model Intercomparison Project (GeoMIP) is a global collaboration that aims to assess the climate impacts of solar geoengineering, a proposal to counteract global warming with a reduction of incoming solar radiation. We assess the mechanisms underlying the rainfall response to a simplified simulation of solar dimming in the suite of GeoMIP models and identify robust features. While solar geoengineering restores preindustrial temperatures, the global hydrology is altered. Tropical precipitation changes dominate the response across the model suite. The models indicate a range of possibilities for the hydrological response, and in most cases, both thermodynamic and non-thermodynamic mechanisms drive precipitation minus evaporation changes in the geoengineered simulations relative to the preindustrial. Shifts of the Hadley circulation cells cause greater rainfall anomalies than local changes in relative humidity or the Clausius-Clapeyron scaling of precipitation minus evaporation. The variations among models in the movement of the intertropical convergence zone highlights the need for cautious consideration and continued study before any implementation of solar geoengineering.

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Dynamic and energetic constraints on the modality and position of the intertropical convergence zone in an aquaplanet

10.5194/egusphere-egu2020-4903 ◽

2020 ◽

Author(s):

Ori Adam ◽

Hilla Gerstman

Keyword(s):

Energy Transport ◽

Hadley Circulation ◽

Coupled System ◽

Ocean Energy ◽

Ocean Stratification ◽

Tropical Precipitation ◽

Double Peaks ◽

Root Relation ◽

Convergence Zones ◽

Asymmetric Heating

The tropical zonal-mean precipitation distribution can vary between single and double peaks, which are associated with intertropical convergence zones (ITCZs). Here, the meridional modality and the sensitivity to hemispherically-asymmetric heating of tropical precipitation is studied in an idealized GCM with parameterized wind-driven ocean energy transport (OET). In the idealized model, transitions from unimodal to bimodal distributions are driven by equatorial ocean upwelling and cooling which inhibits equatorial precipitation. For sufficiently strong cooling, the circulation bifurcates to anti-Hadley circulation (AHC) in the deep tropics, with a descending branch near the equator and off-equatorial double ITCZs. The intensity of the AHC is limited by a negative feedback: the AHC drives westerly surface winds which balance the easterly stress (and hence equatorial upwelling) required for its maintenance. The modality of the precipitation affects the response to asymmetric heating: For weak ocean stratification, OET damps shifts of the tropical precipitation centroid but amplifies shifts of precipitation peaks. For strong ocean stratification, which leads to double ITCZs, asymmetric heating leads to relative intensification of the ITCZ in the warming hemisphere, but the positions of the double ITCZs are insensitive to changes in the asymmetric heating and ocean stratification. The dynamic feedbacks of the coupled system damp the slope of the atmospheric energy transport (AET) near the equator. This justifies a cubic root relation between the cross-equatorial AET and the position of the ITCZ, which captures migrations of the ITCZ significantly better than the commonly-used linear relation.

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Dynamic and Energetic Constraints on the Modality and Position of the Intertropical Convergence Zone in an Aquaplanet

Journal of Climate ◽

10.1175/jcli-d-20-0128.1 ◽

2021 ◽

Vol 34 (2) ◽

pp. 527-543

Author(s):

Ori Adam

Keyword(s):

Energy Transport ◽

Surface Wind ◽

Hadley Circulation ◽

Coupled System ◽

Precipitation Distribution ◽

Ocean Stratification ◽

Tropical Precipitation ◽

Wide Range ◽

Convergence Zones ◽

Asymmetric Heating

AbstractThe tropical zonal-mean precipitation distribution varies between having single or double peaks, which are associated with intertropical convergence zones (ITCZs). Here, the effect of this meridional modality on the sensitivity of the ITCZ to hemispherically asymmetric heating is studied using an idealized GCM with parameterized Ekman ocean energy transport (OET). In the idealized GCM, transitions from unimodal to bimodal distributions are driven by equatorial ocean upwelling and cooling, which inhibits equatorial precipitation. For sufficiently strong equatorial cooling, the tropical circulation bifurcates to anti-Hadley circulation in the deep tropics, with a descending branch near the equator and off-equatorial double ITCZs. The intensity and extent of the anti-Hadley circulation is limited by a negative feedback: westerly geostrophic surface wind tendency in its poleward-flowing lower branches balances the easterly stress (and hence equatorial upwelling) required for its maintenance. For weak ocean stratification, which goes along with unimodal or weak bimodal tropical precipitation distribution, OET damps shifts of the tropical precipitation centroid but amplifies shifts of precipitation peaks. For strong ocean stratification, which goes along with pronounced double ITCZs, asymmetric heating leads to relative intensification of the precipitation peak in the warming hemisphere, but negligible meridional shifts. The dynamic feedbacks of the coupled system weaken the gradient of the atmospheric energy transport (AET) near the equator. This suggests that over a wide range of climates, the ITCZ position is proportional to the cubic root of the cross-equatorial AET, as opposed to the commonly used linear relation.

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What Surface Observations Are Important for Separating the Influences of Anthropogenic Aerosols from Other Forcings?

Journal of Climate ◽

10.1175/jcli-d-15-0667.1 ◽

2016 ◽

Vol 29 (11) ◽

pp. 4165-4184 ◽

Cited By ~ 2

Author(s):

Xiaoqin Yan ◽

Timothy DelSole ◽

Michael K. Tippett

Keyword(s):

Spatial Structure ◽

Surface Temperature ◽

Sea Level ◽

Climate Models ◽

Accurate Estimate ◽

Forced Response ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Sea Level Pressure ◽

Level Pressure

Abstract This paper shows that joint temperature–precipitation information over a global domain provides a more accurate estimate of aerosol forced responses in climate models than does any other combination of temperature, precipitation, or sea level pressure. This fact is demonstrated using a new quantity called potential detectability, which measures the extent to which a forced response can be detected in a model. In particular, this measure can be evaluated independently of observations and therefore permits efficient exploration of a large number of variable combinations before performing optimal fingerprinting on observations. This paper also shows that the response to anthropogenic aerosol forcing can be separated from that of other forcings using only spatial structure alone, leaving the time variation of the response to be inferred from data, thereby demonstrating that temporal information is not necessary for detection. The spatial structure of the forced response is derived by maximizing the signal-to-noise ratio. For single variables, the north–south hemispheric gradient and equator-to-pole latitudinal gradient are important spatial structures for detecting anthropogenic aerosols in some models but not all. Sea level pressure is not an independent detection variable because it is derived partly from surface temperature. In no case does sea level pressure significantly enhance potential detectability beyond that already possible using surface temperature. Including seasonal or land–sea contrast information does not significantly enhance detectability of anthropogenic aerosol responses relative to annual means over global domains.

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Evaluating the large-scale hydrological cycle response within the PlioMIP2 ensemble

10.5194/cp-2021-72 ◽

2021 ◽

Author(s):

Zixuan Han ◽

Qiong Zhang ◽

Qiang Li ◽

Ran Feng ◽

Alan M. Haywood ◽

...

Keyword(s):

Large Scale ◽

Hydrological Cycle ◽

Atmospheric Transport ◽

Hadley Circulation ◽

Dynamic Effect ◽

Walker Circulation ◽

Future Climate Change ◽

Atmospheric Energy ◽

Budget Analysis ◽

Thermodynamic Effect

Abstract. The mid-Pliocene (~ 3 million years ago) is one of the most recent warm periods with high CO2 concentrations in the atmosphere and resulting high temperatures and is often cited as an analog for near-term future climate change. Here, we apply a moisture budget analysis to investigate the response of the large-scale hydrological cycle at low latitudes within a 13-model ensemble from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The results show that increased atmospheric moisture content within the mid-Pliocene ensemble (the thermodynamic effect) results in wetter conditions over the deep tropics, i.e., the Pacific intertropical convergence zone (ITCZ) and the Maritime Continent, and drier conditions over the subtropics. The thermodynamic effect is to some extent offset by a dynamic effect involving a northward shift of the Hadley circulation that dries the deep tropics and moistens the subtropics in the Northern Hemisphere (i.e., the subtropical Pacific). From the perspective of Earth’s energy budget, the enhanced southward cross-equatorial atmospheric transport (0.22 PW), induced by the hemispheric asymmetries of the atmospheric energy, favors an approximately 1° northward shift of the ITCZ. The shift of the ITCZ reorganizes atmospheric circulation, favoring a northward shift of the Hadley circulation. In addition, the Walker circulation consistently shifts westward within PlioMIP2 models, leading to wetter conditions over the northern Indian Ocean. The PlioMIP2 ensemble highlights that an imbalance of interhemispheric atmospheric energy during the mid-Pliocene could have led to changes in the dynamic effect, offsetting the thermodynamic effect and hence altering mid-Pliocene hydroclimate cycling.

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Decomposing local and remote surface temperature impacts of Asian aerosols

10.5194/egusphere-egu21-13320 ◽

2021 ◽

Author(s):

Joonas Merikanto ◽

Kalle Nordling ◽

Petri Räisänen ◽

Jouni Räisänen ◽

Declan O'Donnell ◽

...

Keyword(s):

Surface Temperature ◽

Energy Transport ◽

East Asian ◽

Temperature Response ◽

Longwave Radiation ◽

Shortwave Radiation ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Clear Sky ◽

Temperature Responses

We investigate how a regionally confined radiative forcing of South and East Asian aerosols translate into local and remote surface temperature responses across the globe. To do so, we carry out equilibrium climate simulations with and without modern day South and East Asian anthropogenic aerosols in two climate models with independent development histories (ECHAM6.1 and NorESM1). &#160;We run the models with the same anthropogenic aerosol representations via MACv2-SP (a simple plume implementation of the 2nd version of the Max Planck Institute Aerosol Climatology). This leads to a near identical change in instantaneous direct and indirect aerosol forcing due to removal of Asian aerosols in the two models. We then robustly decompose and compare the energetic pathways that give rise to the global and regional surface temperature effects in the models by a novel temperature response decomposition method, which translated the changes in atmospheric and surface energy fluxes into surface temperature responses by using a concept of planetary emissivity. &#160;We find that the removal of South and East Asian anthropogenic aerosols leads to strong local warming &#160;response from increased clear-sky shortwave radiation over the region, combined with opposing warming and cooling responses due to changes in cloud longwave and shortwave radiation. However, the local warming response is strongly modulated by the changes in horizontal atmospheric energy transport. Atmospheric energy transport and changes in clear-sky longwave radiation redistribute the surface temperature responses efficiently across the Northern hemisphere, and to a lesser extent also over the Southern hemisphere. The model-mean global surface temperature response to Asian anthropogenic aerosol removal is 0.26&#177;0.04 &#176;C (0.22&#177;0.03 for ECHAM6.1 and 0.30&#177;0.03 &#176;C for NorESM1) of warming. Model-to-model differences in global surface temperature response mainly arise from differences in longwave cloud (0.01&#177;0.01 for ECHAM6.1 and 0.05&#177;0.01 &#176;C for NorESM1) and shortwave cloud (0.03&#177;0.03 for ECHAM6.1 and 0.07&#177;0.02 &#176;C for NorESM1) responses. The differences in cloud responses between the models also dominate the differences in regional temperature responses. In both models, the Northern hemispheric surface warming amplifies towards the Arctic, where the total temperature response is highly seasonal and modulated by seasonal changes in oceanic heat exchange and clear-sky longwave radiation.We estimate that under a strong Asian aerosol mitigation policy tied with strong greenhouse gas mitigation (Shared Socioeconomic Pathway 1-1.9) the Asian aerosol reductions can add around 8 years&#8217; worth of current day global warming during the next few decades.

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Aerosol indirect effects in a multi-scale aerosol-climate model PNNL-MMF

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-3399-2011 ◽

2011 ◽

Vol 11 (1) ◽

pp. 3399-3459 ◽

Cited By ~ 5

Author(s):

M. Wang ◽

S. Ghan ◽

M. Ovchinnikov ◽

X. Liu ◽

R. Easter ◽

...

Keyword(s):

Indirect Effects ◽

Large Scale ◽

Climate Models ◽

Climate Model ◽

Global Climate ◽

Global Climate Models ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Indirect Effects ◽

Multi Scale

Abstract. Much of the large uncertainty in estimates of anthropogenic aerosol effects on climate arises from the multi-scale nature of the interactions between aerosols, clouds and large-scale dynamics, which are difficult to represent in conventional global climate models (GCMs). In this study, we use a multi-scale aerosol-climate model that treats aerosols and clouds across multiple scales to study aerosol indirect effects. This multi-scale aerosol-climate model is an extension of a multi-scale modeling framework (MMF) model that embeds a cloud-resolving model (CRM) within each grid cell of a GCM. The extension allows the explicit simulation of aerosol/cloud interactions in both stratiform and convective clouds on the global scale in a computationally feasible way. Simulated model fields, including liquid water path (LWP), ice water path, cloud fraction, shortwave and longwave cloud forcing, precipitation, water vapor, and cloud droplet number concentration are in agreement with observations. The new model performs quantitatively similar to the previous version of the MMF model in terms of simulated cloud fraction and precipitation. The simulated change in shortwave cloud forcing from anthropogenic aerosols is −0.77 W m−2, which is less than half of that in the host GCM (NCAR CAM5) (−1.79 W m−2) and is also at the low end of the estimates of most other conventional global aerosol-climate models. The smaller forcing in the MMF model is attributed to its smaller increase in LWP from preindustrial conditions (PI) to present day (PD): 3.9% in the MMF, compared with 15.6% increase in LWP in large-scale clouds in CAM5. The much smaller increase in LWP in the MMF is caused by a much smaller response in LWP to a given perturbation in cloud condensation nuclei (CCN) concentrations from PI to PD in the MMF (about one-third of that in CAM5), and, to a lesser extent, by a smaller relative increase in CCN concentrations from PI to PD in the MMF (about 26% smaller than that in CAM5). The smaller relative increase in CCN concentrations in the MMF is caused in part by a smaller increase in aerosol lifetime from PI to PD in the MMF, a positive feedback in aerosol indirect effects induced by cloud lifetime effects. The smaller response in LWP to anthropogenic aerosols in the MMF model is consistent with observations and with high resolution model studies, which may indicate that aerosol indirect effects simulated in conventional global climate models are overestimated and point to the need to use global high resolution models, such as MMF models or global CRMs, to study aerosol indirect effects. The simulated total anthropogenic aerosol effect in the MMF is −1.05 W m−2, which is close to the Murphy et al. (2009) inverse estimate of −1.1 ± 0.4 W m−2 (1σ) based on the examination of the Earth's energy balance. Further improvements in the representation of ice nucleation and low clouds are needed.

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The Influence of Anthropogenic Aerosols on the Aleutian Low

10.5194/egusphere-egu2020-21680 ◽

2020 ◽

Author(s):

William Dow ◽

Amanda Maycock ◽

Marcus Lofverstrom

Keyword(s):

Diabatic Heating ◽

Climate Models ◽

Equation Model ◽

Aleutian Low ◽

Anthropogenic Aerosols ◽

Primitive Equation ◽

Anthropogenic Aerosol ◽

The Pacific ◽

Coupled Climate Models ◽

Major Mode

There is an incomplete understanding of the mechanisms that govern the Pacific Decadal Oscillation (PDO), a&#160;major mode of climate variability&#160;that plays a key role in the evolution of&#160;global&#160;climate on decadal time-scales. Recent research has suggested that regional anthropogenic aerosol (AA) emissions could modulate the behaviour of the PDO, including the transition to a negative PDO phase starting in the late 1990s (Smith et al., 2016). However, other studies have questioned whether this connection is robust (Oudar et al., 2018). East Asia is a region of particular focus, where&#160;AA&#160;emissions having increased in recent decades (Bartlett et al., 2017). Here we combine analysis of an ensemble of&#160;coupled climate models running idealised&#160;AA perturbation&#160;experiments and a steady-state primitive equation model (LUMA) forced by diabatic heating anomalies to examine whether AA&#160;emissions&#160;influence the behaviour of the Aleutian low - a climate feature closely associated with the PDO &#160;- and if so, test the posited&#160;teleconnection&#160;mechanisms&#160;proposed by&#160;Smith et al. (2016).&#160;We further compare&#160;the response of the Aleutian low to well mixed&#160;greenhouse&#160;gases to examine if AAs and GHGs influence the Aleutian low in a similar manner.

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