Water Vapor–Forced Greenhouse Warming over the Sahara Desert and the Recent Recovery from the Sahelian Drought

Evan, Amato T.; Flamant, Cyrille; Lavaysse, Christophe; Kocha, Cécile; Saci, Azzedine

doi:10.1175/jcli-d-14-00039.1

Cited by 94 publications

(100 citation statements)

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“…The impact of dust may therefore be greater than previously believed. Recent studies have proposed a water vapour positive feedback mechanism driving decadal variations in SHL intensity, implicated in the recent recovery of Sahelian rainfall (Evan et al, 2015b). Our results are consistent with this but strongly suggest that variability in dust loading should be considered in explaining variability and change in the SHL, reinforcing the need for high-quality long-term aerosol observations.…”

Section: Discussionsupporting

confidence: 89%

“…In comparison to the observational analysis of M16, we see some important differences; notably, we see a greater surface net warming effect of water vapour and as a result negligible, not positive, atmospheric radiation convergence. Nevertheless, our estimate of the sensitivity of surface longwave radiation to changes in CIWV of 1.1 W kg −1 is at the lower end of the range (1.0-3.6 W kg −1 ) estimated by Evan et al (2015b), from observations and RT simulations, suggesting the role of water vapour in driving longer-term interannual to decadal heating of the SHL may not be as pronounced as previously suggested.…”

Section: Water Vapourcontrasting

confidence: 53%

“…Thus, there is a net atmospheric cooling up to −0.25 K day −1 and strong heating up to 2.5 K day −1 near the surface as a result of the increase in moisture. The atmospheric cooling in the longwave causes surface warming, which is suggested to be linked with the intensification of the Saharan heat low region (Evan et al, 2015b). The reversed heating rate profiles in the layer between 500 and 400 hPa are because the mean moisture profile in this layer is larger during the dry days, and vice versa (Fig.…”

Section: Water Vapourmentioning

confidence: 99%

“…Chauvin et al, 2010;Couvreux et al, 2010;Lafore et al, 2010;Parker et al, 2005;Peyrille and Lafore, 2007;Sultan and Janicot, 2003;Thorncroft and Blackburn, 1999;Xue et al, 2010). Notably, the intensification of the SHL in recent decades has been linked to the recovery of the Sahelian rainfall from the multi-decadal drought of the 1970-1990s, partly through a water vapour positive feedback process, in which radiative warming from increasing water vapour strengthens the SHL, which enhances the moist lowlevel monsoon flow, driving greater water vapour transport into the SHL and further warming (Dong and Sutton, 2015;Evan et al, 2015b;Lavaysse et al, 2016) with an implied enhanced west African monsoon.…”

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confidence: 99%

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The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

et al. 2018

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Abstract. The Saharan heat low (SHL) is a key component of the west African climate system and an important driver of the west African monsoon across a range of timescales of variability. The physical mechanisms driving the variability in the SHL remain uncertain, although water vapour has been implicated as of primary importance. Here, we quantify the independent effects of variability in dust and water vapour on the radiation budget and atmospheric heating of the region using a radiative transfer model configured with observational input data from the Fennec field campaign at the location of Bordj Badji Mokhtar (BBM) in southern Algeria (21.4 • N, 0.9 • E), close to the SHL core for June 2011. Overall, we find dust aerosol and water vapour to be of similar importance in driving variability in the top-of-atmosphere (TOA) radiation budget and therefore the column-integrated heating over the SHL (∼ 7 W m −2 per standard deviation of dust aerosol optical depth -AOD). As such, we infer that SHL intensity is likely to be similarly enhanced by the effects of dust and water vapour surge events. However, the details of the processes differ. Dust generates substantial radiative cooling at the surface (∼ 11 W m −2 per standard deviation of dust AOD), presumably leading to reduced sensible heat flux in the boundary layer, which is more than compensated by direct radiative heating from shortwave (SW) absorption by dust in the dusty boundary layer. In contrast, water vapour invokes a radiative warming at the surface of ∼ 6 W m −2 per standard deviation of column-integrated water vapour in kg m −2 . Net effects involve a pronounced net atmospheric radiative convergence with heating rates on average of 0.5 K day −1 and up to 6 K day −1 during synoptic/mesoscale dust events from monsoon surges and convective cold-pool outflows ("haboobs"). On this basis, we make inferences on the processes driving variability in the SHL associated with radiative and advective heating/cooling. Depending on the synoptic context over the region, processes driving variability involve both independent effects of water vapour and dust and compensating events in which dust and water vapour are co-varying. Forecast models typically have biases of up to 2 kg m −2 in column-integrated water vapour (equivalent to a change in 2.6 W m −2 TOA net flux) and typically lack variability in dust and thus are expected to poorly represent these couplings. An improved representation of dust and water vapour and quantification of associated radiative impact in models is thus imperative to further understand the SHL and related climate processes.

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Section: Discussionsupporting

confidence: 89%

Section: Water Vapourcontrasting

confidence: 53%

Section: Water Vapourmentioning

confidence: 99%

mentioning

confidence: 99%

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The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

et al. 2018

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“…Based on the analyses of surface temperature modes and their relationship with precipitation evolution in the WAMME I experiment, we identify two summer temperature modes (Xue et al 2010a): the Sahara mode (mostly covering the Sahara area between 20°N and 30°N) and the Sahel Mode (mostly covering the Sahel area between 10°N and 20°N). The crucial role of the Sahara Heat Low in WAM development is well known (Lavaysse et al 2009: Biasutti et al 2009Evan et al 2015) and corresponds to the Sahara mode. Meanwhile, the WAMME I results also reveal the important role of another mode, the Sahel mode, and show that the summer WAM precipitation northward movement/retreat is closely associated with an enhanced/weakened Sahara mode and a weakened/enhanced Sahel mode.…”

Section: Impact Of Sst On Circulation and Surface Energy Balancementioning

confidence: 99%

West African monsoon decadal variability and surface-related forcings: second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II)

et al. 2016

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in the Sahel climate system at seasonal to decadal scales. The project's strategy is to apply prescribed observationally based anomaly forcing, i.e., "idealized but realistic" forcing, in simulations by climate models. The goal is to assess these forcings' effects in producing/amplifying seasonal and decadal climate variability in the Sahel between the 1950s and the 1980s, which is selected to characterize the great drought period of the last century. This is the first multi-model experiment specifically designed to simultaneously evaluate such relative contributions. The WAMME II models have consistently demonstrated that SST forcing is a major contributor to the twentieth century Sahel drought. Under the influence of the maximum possible SST forcing, the ensemble mean of WAMME II models can produce up to 60 % of the precipitation difference during the period. The present paper also addresses the role of SSTs Abstract The second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II) is designed to improve understanding of the possible roles and feedbacks of sea surface temperature (SST), land use land cover change (LULCC), and aerosols forcings This paper is a contribution to the special issue on West African climate decadal variability and its modeling, consisting of papers from the West African Monsoon Modeling and Evaluation (WAMME) and the African Multidisciplinary Monsoon Analyses (AMMA) projects, and coordinated by Yongkang Xue, Serge Janicot, and William Lau. Provided Funding information has to be tagged. Electronic supplementary materialThe online version of this article (doi:10.1007/s00382-016-3224-2) contains supplementary material, which is available to authorized users. 3in triggering and maintaining the Sahel drought. In this regard, the consensus of WAMME II models is that both Indian and Pacific Ocean SSTs greatly contributed to the drought, with the former producing an anomalous displacement of the Intertropical Convergence Zone before the WAM onset, and the latter mainly contributes to the summer WAM drought. The WAMME II models also show that the impact of LULCC forcing on the Sahel climate system is weaker than that of SST forcing, but still of first order magnitude. According to the results, under LULCC forcing the ensemble mean of WAMME II models can produces about 40 % of the precipitation difference between the 1980s and the 1950s. The role of land surface processes in responding to and amplifying the drought is also identified. The results suggest that catastrophic consequences are likely to occur in the regional Sahel climate when SST anomalies in individual ocean basins and in land conditions combine synergistically to favor drought.Keywords Sahel seasonal and decadal climate variability · Sahel drought, SST and land forcings · GCM

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Dust‐rainfall feedback in West African Sahel

D’Odorico

Bhattachan

et al. 2015

Geophysical Research Letters

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Drought persistence in West African Sahel has often been explained as an effect of positive vegetation-atmosphere feedback associated with surface albedo or the partitioning of solar radiation into sensible and latent heat fluxes. An often overlooked aspect of land-atmosphere coupling results from vegetation controls on dust emissions and the ability of mineral aerosols to suppress precipitation. Here we first consider the case of local (endogenous) dynamics within the Sahel, whereby enhanced dust emissions resulting from a decrease in vegetation partly suppress precipitation, thereby further reducing vegetation cover. We then account for teleconnections between Sahel precipitation and exogenous (i.e., Saharan) dust emissions due to an increase in Saharan wind speed in years of above average Sahel precipitation. We find that in both cases vegetation-climate dynamics may have two stable states, one with low precipitation and high concentration of atmospheric dust and the other with high precipitation and lower levels of atmospheric dust.

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Water Vapor–Forced Greenhouse Warming over the Sahara Desert and the Recent Recovery from the Sahelian Drought

Cited by 94 publications

References 50 publications

The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

West African monsoon decadal variability and surface-related forcings: second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II)

Dust‐rainfall feedback in West African Sahel

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