Observed monsoon precipitation suppression caused by anomalous interhemispheric aerosol transport

Ajoku, Osinachi; Norris, Joel R.; Miller, Arthur J.

doi:10.1007/s00382-019-05046-y

Cited by 13 publications

(16 citation statements)

References 43 publications

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“…Model experiments and aircraft measurements in the framework of DAC-CIWA have shown a considerable contribution to air pollution along the Guinea coast in addition to the rapidly increasing local emissions Haslett et al, 2019a). Ajoku et al (2020) related daily variations in this aerosol plume during August to daily variations in rainfall over SWA and found a clear negative impact, most significantly over the eastern part of our study domain, where the AOD trends are largest. However, as already discussed in the introduction, one has to be cautious to claim a direct causality here as the daily AOD variations are associated with significant changes in circulation.…”

Section: Satellite-based Aerosol Estimatesmentioning

confidence: 56%

“…Given typical mean values of about 0.4 near the Guinea coast (not shown), this value underlines the dramatic increase in pollution over this large area. This phenomenon has been described extensively in the literature and is related to the increase in biomass-burning aerosol from Central Africa during the local dry season (Mari et al, 2011;Andela et al, 2014). A number of recent field campaigns in the south-east Atlantic targeted this aerosol layer and its interaction with clouds specifically (Formenti et al, 2019, and references therein).…”

Section: Satellite-based Aerosol Estimatesmentioning

confidence: 87%

“…The impact of this on precipitation is complex and depends on the cloud type and meteorological setting. Ajoku et al (2020) recently produced daily composites for August 2003-2015 based on aerosol optical depth (AOD) estimates from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) over the Guinea coastal zone (5-10 • N, 10 • W-10 • E), which is affected by the advection of burning aerosol from mostly manmade agricultural and forest fires in Central Africa during this period (Giglio et al, 2006;Zuidema et al, 2016;Das et al, 2017). Comparing precipitation over SWA on clean and polluted days suggests a suppressing effect of the aerosol, but it is difficult to establish a causal relationship due to strong covariance with meteorological variables, in particular lowlevel wind that changes from southerly on polluted days to easterly on clean days.…”

Section: Introductionmentioning

confidence: 99%

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The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

et al. 2021

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Abstract. Southern West Africa has one of the fastest-growing populations worldwide. This has led to a higher water demand and lower air quality. Over the last 3 decades, most of the region has experienced decreasing rainfall during the little dry season (LDS; mid-July to end of August) and more recently also during the second rainy season (SRS; September–October), while trends during the first rainy season (FRS; mid-May to mid-July) are insignificant. Here we analyse spatio-temporal variations in precipitation, aerosol, radiation, cloud, and visibility observations from surface stations and from space to find indications for a potential contribution of anthropogenic air pollution to these rainfall trends. The proposed mechanism is that the dimming of incoming solar radiation by aerosol extinction contributes to reducing vertical instability and thus convective precipitation. To separate a potential aerosol influence from large-scale climatic drivers, a multilinear-regression model based on sea-surface temperature (SST) indices is used. During both LDS and SRS, weakly statistically significant but accelerating negative rainfall trends unrelated to known climatic factors are found. These are accompanied by a strong increase in pollution over the upstream tropical Atlantic caused by fire aerosol from Central Africa, particularly during the LDS. Over southern West Africa, no long-term aerosol records are available, inhibiting a direct quantification of the local man-made effect. However, significant decreases in horizontal visibility and incoming surface solar radiation are strong indicators for an increasing aerosol burden, in line with the hypothesized pollution impact on rainfall. The radiation trend is further enhanced by an increase in low-level cloudiness. The large spatial extent of potentially aerosol-related trends during the LDS is consistent with the stronger monsoon flow and less wet deposition during this season. Negligible aerosol impacts during the FRS are likely due to the high degree of convective organization, which makes rainfall less sensitive to surface radiation. The overall coherent picture and the accelerating trends – some of which are concealed by SST effects – should alarm policymakers in West Africa to prevent a further increase in air pollution as this could endanger water supply and food and energy production for a large and growing population.

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Section: Satellite-based Aerosol Estimatesmentioning

confidence: 56%

Section: Satellite-based Aerosol Estimatesmentioning

confidence: 87%

Section: Introductionmentioning

confidence: 99%

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The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

et al. 2021

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“…A detailed description of daily aerosol from Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), precipitation from Global Precipitation Climatology Project. Version 1.3 (GPCP 1.3) and Clouds and Earth's Radiant Energy System (CERES) cloud data sources as well as how composite and statistical analysis for dirty and clean aerosol days are used in this study can be found in Ajoku et al (2019). In brief, box average aerosol values over our GoG region were calculated and divided into pentiles.…”

Section: Data Sources and Methodsmentioning

confidence: 99%

Impacts of aerosols produced by biomass burning on the stratocumulus‐to‐cumulus transition in the equatorial Atlantic

Ajoku

Miller

Norris

2021

Atmospheric Science Letters

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The impact of aerosols produced by biomass burning on the stratocumulus-tocumulus transition (SCT) in the equatorial Atlantic is studied using satellitebased and reanalysis data for the month of June. The month of June is highlighted because it represents monsoon onset as well as the largest sea surface temperature gradient in the summer, which is the peak season of tropical African biomass burning. Boundary layer deepening and increasing temperatures put the location of the SCT within the Gulf of Guinea. Satellite retrievals indicate that the bulk of aerosols occur near 1,500 m in altitude, either above or below the boundary layer depending on latitudinal position. Changes in smoke loading over the Gulf of Guinea due to greater transport from southern Africa leads to increases in low-level cloud cover above cloud decks and decreases when mixed within the boundary layer. Further south, we find significant changes to temperature, cloud top height, tropospheric stability and moisture availability near maximum aerosol loading. In addition, changes in vertical velocity during dirty conditions further reinforce changes in tropospheric stability. These effects combine to shorten the SCT in space during increased aerosol loading episodes.

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“…In addition, the FT‐BBA plumes lead to an ∼2.4 K d −1 increase in the average instantaneous SW heating rate in cloudy conditions, which can reach ∼6.0 K d −1 during extreme cases (Table S2). This increase in SW heating can reduce local relative humidity, implying a negative SDE over the ocean where absorbing BBA layers frequently exist above persistent stratus cloud decks (Ajoku et al, 2020; Herbert et al, 2020; Wilcox, 2012). In addition, the significant radiative impact of BBA highlighted in this study could also have some large implications on the stratus‐to‐cumulus cloud transition as shown by Deetz et al (2018) during DACCIWA.…”

Section: Implications For Atmospheric Radiationmentioning

confidence: 99%

Unexpected Biomass Burning Aerosol Absorption Enhancement Explained by Black Carbon Mixing State

Denjean

Brito

Libois

et al. 2020

Geophysical Research Letters

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Direct and semi-direct radiative effects of biomass burning aerosols (BBA) from southern and central African fires are still widely debated, in particular because climate models have been unsuccessful in reproducing the low single scattering albedo in BBA over the eastern Atlantic Ocean. Using state-of-the-art airborne in situ measurements and Mie scattering simulations, we demonstrate that low single scattering albedo in well-aged BBA plumes over southern West Africa results from the presence of strongly absorbing refractory black carbon (rBC), whereas the brown carbon contribution to the BBA absorption is negligible. Coatings enhance light absorption by rBC-containing particles by up to 210%. Our results show that accounting for the diversity in black carbon mixing state by combining internal and external configurations is needed to accurately estimate the optical properties and henceforth the shortwave direct radiative effect and heating rate of BBA over southern West Africa. Plain Language Summary Extensive seasonal fires over southern and central Africa result in the transport of massive amounts of biomass burning aerosols over huge areas of the eastern Atlantic Ocean. Recent field observations highlight that biomass burning aerosols transported from the coast of southern Africa to the far north over southern West Africa were characterized by low single scattering albedo. This finding is of paramount interest, because radiative heating within the absorbing aerosol layer is hypothesized to affect the low cloud deck over this specific region and may ultimately influence the large-scale circulation. However, debate remains about the causes of the low single scattering albedo by biomass burning aerosols, causing ambiguous parameterizations of their optical properties in climate models. Here we present simultaneous airborne measurements of the composition and optical properties of biomass burning aerosols transported over southern West Africa. We show that black carbon particles dominated the light absorption by biomass burning aerosols at mid-visible wavelengths. Our findings indicate that the black carbon mixing state plays a significant role in the aerosol optical properties and may be an important modulator to be considered in climate models for simulating direct and semi-direct radiative effects of biomass burning aerosol over southern West Africa.

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Observed monsoon precipitation suppression caused by anomalous interhemispheric aerosol transport

Cited by 13 publications

References 43 publications

The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

The potential of increasing man-made air pollution to reduce rainfall over southern West Africa

Impacts of aerosols produced by biomass burning on the stratocumulus‐to‐cumulus transition in the equatorial Atlantic

Unexpected Biomass Burning Aerosol Absorption Enhancement Explained by Black Carbon Mixing State

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