Impact of the Megalake Chad on climate and vegetation during the late Pliocene and the mid-Holocene

Introduction Conclusions References


Geoscientific Instrumentation
Methods and Data Systems The Cryosphere This discussion paper is/has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP if available.

Introduction
The Chad basin is a vast endorheic basin located in North Central Africa, between 5 • and 25 • N (Fig. 1), covering approximately 2.5×10 6 km 2 . At present, the climate is subdesertic to soudanian, with precipitation ranging from 0 to 900 mm yr −1 . Wind regime is dominated by the Harmattan northeasterly wind during the dry season (Novem-evidenced by seasonal fluctuations as well as by historical and geological archives. Since 1960, lake Chad shrunk from 25 000 to 6000 km 2 on average, mainly because of a decrease in rainfall and associated Chari-Logone river flow to the lake (Olivry et al., 1996). At longer time-scales like precession cycles, lake Chad extent has proven to vary considerably. The presence of a Megalake Chad (MLC) in the mid-Holocene is 10 widely attested from lake deposits and from coastal morphosedimentary structures (e.g. Tilho, 1925;Pias and Guichard, 1957;Grove and Pullan, 1963;Maley, 1977;Kusnir and Moutaye, 1997;Ghienne et al., 2002;Schuster et al., 2003Schuster et al., , 2005Schuster et al., , 2009Drake and Bristow, 2006;Leblanc et al., 2006;Bouchette et al., 2010). This large water body reached more than 350 000 km 2 (Ghienne et al., 2002;Schuster et al., 2005; 15 Leblanc et al., 2006) with wind-driven hydrodynamics (Bouchette et al., 2010). Similar MLC episodes are thought to have occurred during the Miocene and the Pliocene (Schuster et al., 2001(Schuster et al., , 2006Griffin, 2006), supposedly linked to a northward shift of the west-african monsoon system. In particular, sediments from the northern Chad basin describe lacustrine occurrences from 7 to 3 Ma (Schuster et al., 2001(Schuster et al., , 200620 Lebatard et al., 2010), associated to the presence of fossil vertebrate fauna and two hominids (Sahelanthropus Tchadensis, 7 Ma, Brunet et al., 2002, andAustralopithecus Bahrelghazali, 3.6 Ma, Brunet et al., 1995), in a mosaic vegetation landscape (Brunet et al., 1997). Climate models for the mid-Holocene (Braconnot et al., 2007) and the Pliocene (Haywood et al., 2013) generally simulate more precipitation in North Africa, 25 allowing drainage systems reactivation and large lakes development for these periods. Investigating potential climate and vegetation perturbations induced by open continental water surfaces is the subject of many studies (e.g. Coe and Bonan, 1997;Broström et al., 1998) (25 000 km 2 ) on climate was investigated by Lauwaet et al. (2012). Using a mesoscale regional atmospheric model coupled to a Soil-Vegetation-Atmosphere Transfer (SVAT) model, they found that total precipitation amounts were hardly affected by the presence of a lake Chad compared to no lake or to a small one (6000 km 2 ). A filled preindustrial lake Chad increases precipitation over the lake and its eastern border, but large-scale 5 atmospheric processes are not affected, and far field impact is unclear and variable (Lauwaet et al., 2012). The impact of a MLC on climate has been tested several times with numerical simulations as well, but only in the mid-Holocene or preindustrial climates. Broström et al. (1998) used an Atmospheric General Circulation Model (AGCM) to study the impact of vegetation, of a MLC and wetlands on North African climate dur- 10 ing the mid-Holocene, but using present-day sea-surface temperatures (SSTs). They found that vegetation forced the African monsoon roughly 300 km northward, while MLC and wetlands only produced local changes, with no northward shift of the monsoon. Sepulchre et al. (2009) and Krinner et al. (2012) investigated African climate responses to different surface conditions using the LMDZ4 AGCM including a lake sur- 15 face module (Krinner, 2003). Sepulchre et al. (2009) carried out sensitivity tests with a MLC imposed in a preindustrial climate, and found that it had a negative feedback on the Chad basin water balance, deep convection processes being weakened by the cold water surface of the MLC, preventing precipitation and associated local water recycling. Krinner et al. (2012), using 9 kyr SSTs previously simulated with an AOGCM, 20 accounting for mid-Holocene lakes (including MLC), wetlands and vegetation feedback all over northern Africa, found a similar negative feedback of MLC over its surface, but suggested that at larger scale, these surface conditions locally more than doubled the simulated precipitation rates outside the lake Chad basin, in the dry areas of Central and Western Sahara.

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The present study aims at investigating the contribution of the MLC on climate and vegetation in two different contexts, the mid-Holocene and the late Pliocene warm period. Considering MLC occurrences during the Mio-Pliocene and the availability of boundary conditions for the simulation of the mid-Piacenzian warm period (mPWP) Introduction climate, we have the opportunity to test the impact of a MLC in a new climatic context. We investigate the MLC impact on climate and vegetation in the mid-Holocene (6 kyr BP) and in two configurations of the mPWP using the LMDZ4 AGCM including a lake surface module (Krinner, 2003). It is possible to study the regional impact of a MLC thanks to the zoomed grid of the LMDZ model. While the African Humid Pe-5 riod of the mid-Holocene is well driven by orbital forcing, the mPWP time slice is much longer (from 3.3 to 3 Ma) and contains several orbital cycles. Indeed, it is known that precession can favor a northern position of the ITCZ, thus deeply modifying the monsoon pattern during the mid-Holocene (Marzin and Braconnot, 2009), coinciding with the last occurrence of the MLC. Before 2.8 Ma, African monsoon is thought to have been controlled by precession as well, with higher humidity when perihelion coincides with boreal summer (deMenocal et al., 1995(deMenocal et al., , 2004. Considering that the impact of a MLC might be different in different climatic backgrounds, and varying with monsoon intensity, two climatic configurations of the late Pliocene are investigated: one uses SSTs calculated in the frame of the PlioMIP, i.e. using late Pliocene boundary con- 15 ditions and pre-industrial orbital configurations, the other uses SSTs calculated using late Pliocene boundary conditions and an orbital configuration with perihelion in summer, corresponding to the maximum of insolation at 30 • N between 3.3 to 3 Ma. These climate simulations will help clarify the feedback role of the MLC on climate, by testing this impact in a new climatic context, the late Pliocene. The simulated climates are 20 then used to force the vegetation model BIOME4 (Kaplan et al., 2003), which is widely used to simulate paleovegetation with GCM ouputs (e.g. Harrison et al., 2003) and accounts for CO 2 concentration variations. This study investigates the local and regional feedbacks of the MLC on climate and vegetation, in different climatic backgrounds and orbital configurations for which results are compared in order to exhibit common robust features and if possible background climate-dependent features. In this aspect, it is the first time that the impact of the MLC is investigated in the late Pliocene climate. Introduction

LMDZ4 LAKE atmospheric model
We use the LMDZ4 Atmospheric General Circulation Model (AGCM) (Hourdin et al., 2006) including a water surface module (Krinner, 2003) composed of 8 layers, which represent the thermal and hydrological processes occurring above and beneath con-5 tinental water surface. The model explicitly represents the penetration and absorption of sunlight into the lake levels, heat conduction, convective overturning, water phase changes, sensible and latent turbulent surface heat fluxes and water balance (Krinner, 2003). Lake surface is fixed and is an additional surface type to the LMDZ4 scheme already including ocean, sea ice, land ice and land. In this study, vegetation is prescribed through albedo and roughness boundary conditions, and surface scheme is a bucket model. Although water balance of the lake is calculated, the absence of a surface water routing scheme makes it inappropriate to use the model for prediction of lake level and extent. This model can then only be used as a predictor of lake impact on climate.
Resolution is zoomed to ∼ 100 km on the lake Chad region, and mean grid spacing is The atmosphere has 19 vertical layers. Parameterization of convection is the Emanuel scheme (see Hourdin et al., 2006). Previous studies used this model to investigate the climatic impact of MLC in a preindustrial context  and during the mid-Holocene (Krinner et al., 2012).

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BIOME4 is an equilibrium biogeography model developed by Kaplan et al. (2003), based on BIOME3 (Haxeltine and Prentice, 1996). It is widely used for the simulation of equilibrium vegetation in past climates, including the pre-Quaternary climates of the Tortonian (Pound et al., 2011) and Piacenzian (Salzmann et al., 2008;Kamae and Ueda, 2012) and the more recent mid-Holocene (Kaplan et al., 2003;Krinner et  Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | natural vegetation is determined by the ranking of 12 plant functional types (PFTs) which have different bioclimatic limits (temperature resistance, moisture requirement, sunshine amount). Competition between the PFTs is a function of net primary productivity (NPP) and uses an optimization algorithm to calculate the maximum sustainable leaf area index (LAI). NPP, LAI and mean annual soil moisture are then classified empir-5 ically to determine the predominant biome (Haxeltine and Prentice, 2003). Input data of the model include the mean monthly climate (temperature, precipitation, sunshine and minimum temperature), soil physical properties and atmospheric CO 2 concentration.

Experimental design and boundary conditions
3.1 AGCM experiments 10 Three climatic configurations are investigated: two are late Pliocene (mPWP) experiments (Plio and Plio max simulations), one is mid-Holocene (6 kyr, MidHol simulations). The two late Pliocene experiments differ by the imposed SSTs and orbital configuration, which are described in Sect. 3.1.2. For each simulation, integration time is 70 yr, and climatologies are performed on the last 50 yr. To study the impact of the lake 15 on these past climates, we carry out two simulations for each climatic configuration (except the control): one with imposed MLC surface (Fig. 1, "lake" simulations), the other one with a single grid point defined as a lake surface, which roughly corresponds to the present-day lake Chad. Lake depth is fixed to 20 m at the beginning of the simulation. In total, six climatic simulations plus the control run were carried out. All these simulated 20 climate anomalies are then used to force the BIOME4 vegetation model. A summary of the boundary conditions for each simulation can be found in Table 1. Orbital parameters for each simulation can be found in 2009), following the guidelines of PlioMIP (Haywood et al., 2010), except for Plio max experiments. For this last experiment, ice-free Greenland topography is calculated with the Grisli ice-sheet model (Ritz et al., 2001) to take into account isostatic rebound, and Pliocene anomalous topography is used everywhere else.

Mid-Holocene experiments
For the mid-Holocene experiments (MidHol and MidHol lake), we use anomalies of SSTs and sea-ice cover calculated with the IPSL-CM5A AOGCM, in the PMIP3 framework (Fig. 2d). This AOGCM simulation is described in Kageyama et al. (2012). Contrary to Pliocene SSTs, MidHol SSTs show a decrease in temperature of about 0.5 to 1 • C in the Gulf of Guinea. Orbital configuration is as stated in PMIP3 for the mid-

15
Holocene (Table 2). CO 2 and ice sheets are preindustrial. A first simulation with LMDZ4 was carried out with the control vegetation, from which climatic variables were used to calculate a vegetation distribution for the mid-Holocene with the BIOME4 model. The resulting vegetation, in which the Saharan desert is smaller than at present, is used to calculate albedo and roughness for the mid-Holocene simulations.

BIOME4 simulations
Monthly mean temperature, precipitation, sunshine and annual minimum temperature anomalies to the control were computed from the last 50 yr of each climate simulation and were added to the modern climatology from Leemans and Cramer (1991). This anomaly procedure is widely used (e.g. Kaplan et al., 2003;Haywood et al., 2009) Fig. 3. The model overestimates annual mean precipitation in the equatorial zone (from 0 • to 7 • N), but well represents the transition from soudanian to desertic zones (from 10 • N to the Sahara desert), although with a slight underesti-10 mation. Regarding monsoon precipitation, the model is in good agreement with GPCP data on the equatorial zone, but from 10 • to 25 • N, precipitation is also slightly underestimated, particularly from 12 • to 16 • N (−1 mm day −1 ). It means that for the sahelian zone, the annual amount of precipitation is correctly represented but is too widely distributed throughout the year. Summer precipitation response to a given perturbation 15 might thus be slightly underestimated in the model.

North African rainfall in the late Pliocene and mid-Holocene
Over North Africa, simulated summer temperature and precipitation anomalies for the mid-Holocene without a MLC surface are comparable to the PMIP2 multi-model mean (Braconnot et al., 2007). Simulated annual temperature and precipitation anomalies for 20 Plio simulation are comparable to the multi-model mean for Pliocene coupled model experiments (Haywood et al., 2013). This gives us good confidence regarding the use of this model for these paleoclimates. Mean annual precipitation in function of latitude for each simulation is shown on Fig. 4. First, all simulations are more humid than the control between 10 • to 20 • N. Plio configuration is generally more humid than the control,  (Fig. 4, red line). Plio max configuration is characterized by a much broader ITCZ, hence reduced precipitation in the equatorial zone between 3 • to 12 • , but much wetter tropics and subtropics from 12 • to 30 • N (Fig. 4, blue line) compared to the control run. MidHol simulation is more humid than the control except slightly drier from 6 • to 9 • N. MidHol simulation is also more humid than Plio from 10 • 5 to 20 • N. Despite these changes, the Chad basin is still influenced by southwesterly monsoon winds in summer (Fig. 5, left) and northeastern Harmattan wind in winter for all simulations (Fig. 5, right). During summer months, in Plio simulation, there is an increase in precipitation over the eastern part of the Chad basin, and over central Africa compared to the control run. 10 In Plio max simulation, summer monsoon over west Africa is shifted several degrees northward, while over central and eastern Africa, precipitation increases particularly between 15 • to 22 • N. Precipitation is increased by more than 5 mm d −1 over the central eastern Chad basin. In the MidHol simulation, patterns are similar to Plio max, but with lesser intensity (up to +2.5 mm d −1 in the Chad basin). West African summer mon-15 soon is shifted several degrees northward, with local increases up to 3.5 mm d −1 over west Africa. Summer precipitation also increases in the eastern part of the Chad basin, up to 2 mm d −1 . Concerning winter monsoon (Fig. 5,right), precipitation increases in the Plio simulation. In contrast, in Plio max and MidHol simulations, precipitation front is narrower and shifted southward, though with lesser intensity in the MidHol simula-20 tion. A common feature of these simulations is the northward shift of the ITCZ during summer months compared to the control run, controlled by the reduced latitudinal temperature gradient in the Northern Hemisphere (Davis and Brewer, 2009), with increased southwesterlies over Central to Eastern parts of the Sahara (Fig. 5, left). Similarly to the present, precipitation falls on the Chad basin only during summer months. Increased 25 precipitation over the Chad basin in all the simulations agree with the possibility of a MLC in the basin. We will now investigate the impact of such a water surface area on climate and vegetation, in order to understand its role in the development of sustainable conditions for hominids presence.

Impact of the MLC on climate
First, the presence of the MLC does not impact zonal average precipitation over the Northern African continent (Fig. 4). In all simulations there is no northward shift of the precipitation front induced by the presence of a MLC. This result is different from Krinner et al. (2012), in which an increase in precipitation due to MLC and wetlands 5 is depicted on the zonal average, especially from 15 • to 25 • N. Over the Chad basin, wind regime varies throughout the year, and precipitation only falls during the monsoon season under southwesterly winds conditions. During the rest of the year (October to May), the precipitation front does not reach the MLC, and the presence of the megalake does not generate any precipitation. Considering this aspect, we restrict our analysis to summer months means, which display the same patterns as annual means. Regarding the spatial distribution of precipitation (Fig. 6), the most striking impact of the MLC is a reduction of precipitation above the surface of the lake in the three climatic configurations (from −1 to −5 mm d −1 ). It is consistent with previous studies Krinner et al., 2012) and is well explained by the following: since the lake 15 is generally colder than the surrounding environment, low level air becomes cooler and denser (Fig. 7) and stabilizes the atmosphere, preventing deep convection and associated rainfall. This mechanism is observed in both Pliocene simulations and in the mid-Holocene one, suggesting it is a robust feature appearing in different background climates. In the meantime, convective activity increases around the MLC, especially to 20 the east. Southwesterly winds are enhanced above the surface of the lake due to the flat surface reduced roughness length (Figs. 6 and 8), generating convective activity at the northeast of the MLC, in the three simulations. Similarly, the effect of preindustrial lake Chad on climate is to increase convective activity downwind of the lake (Lauwaet et al., 2012). Moreover, the MLC, especially its northern part, is a center of higher 25 pressure (Fig. 7), generating clockwise winds that can be seen on Fig. 7, which are responsible for the increase of precipitation at the southeast of the lake in the three  (Fig. 6). Significant precipitation response outside the Chad basin is variable in the three simulations and generally not above 0.5 mm d −1 .

Biome distribution during the Pliocene and the mid-Holocene
Although there are major changes in the vegetation response between the three different climates, the MLC impact in each climatic context is small (Fig. 9). This result 5 depicts the climate as the main driver of vegetation pattern in the Chad basin during these periods. Plio simulations ( Fig. 9a and b) show an extent of tropical savanna up to 15 • N, and xerophytic shrubland up to 20 • N, which is comparable to the MidHol simulated vegetation ( Fig. 9e and f). Plio max displays a drastic reduction of the Saharan desert to the benefit of xerophytic shrubland, which reaches 23 • N on the western coast 10 to 27 • N on the eastern coast of Africa. Tropical woodland also covers an important area of central to eastern Africa between 11 • to 17 • N. A ∼ 1 • northward shift of tropical savanna and a small patch of tropical deciduous forest/woodland appear at the east of MLC in the Plio lake simulation (Fig. 9b). For Plio max simulations, a narrow band of tropical woodland can be seen at the southeast of the MLC, and tropical xerophytic 15 shruland becomes temperate in the northwest of the Chad basin. In this configuration, small changes outside of the Chad basin are visible and consist in an expense of temperate sclerophyll woodland and temperate xerophytic shrubland at the expense of desert on the west Mediterranean coast, and a slight northern shift of the desert boundary to the Northwest of the Chad basin, when the MLC is present (Fig. 9d). For 20 the mid-Holocene simulation at 6 kyr ( Fig. 9e and f), with or without MLC, the simulated climate is not humid enough to reproduce savanna biomes up to 20 • N, as suggested by vegetation reconstructions (Hoelzmann et al., 1998). This discrepancy is also seen in the MIROC model (e.g. Ohgaito et al., 2013). Changes induced by the MLC consist in the appearance of tropical woodland at the southwest of the MLC (Fig. 9f). 9,2013 Impact of the Megalake Chad on climate and vegetation

Discussion
Morphosedimentary archives indicate an active Holocene hydrographic system flowing from the East (south Ennedi) associated to small-scale paleodeltas which argue for riverine and lacustrine conditions (Ghienne et al., 2002;Schuster et al., 2005;Bouchette et al., 2010). For the Mio-Pliocene, fluvial deposits of the Koro-Toro, Kossom Bougoudi and Kollé sites are attributed to ephemeral floods (Schuster, 2002).
In a Pliocene context with present day orbital parameters, precipitation front reaches the Ennedi, but total rainfall amount remains small (250 mm yr −1 ). In Plio max configurations however, there is important precipitation over the Ennedi (600 mm yr −1 ), even though the MLC has a negative impact over precipitation in this area. The presence of 10 the MLC, by increasing precipitation in the valley down the Ennedi (Fig. 6b), could have increased the outflow of these potential paleorivers near their outlet into the lake. For the mid-Holocene, mean precipitation over the Ennedi is small (less than 200 mm yr −1 ). In any configuration, precipitation over the Ennedi is restricted to summer months. Simulations also show that during the Pliocene and mid-Holocene, wind regime throughout 15 the year is comparable to today, only with increased monsoon winds in summer. The MLC amplifies this effect because of its flat surface, helping southwesterly winds to reach the northern shore of the MLC. Similarly in winter, the Harmattan is increased over the lake surface. The results presented above, showing a weak impact of surface conditions on cli-20 mate are in agreement with several studies (Coe and Bonan, 1997;Broström et al., 1998). However, a recent study by Krinner et al. (2012, hereafter K12)  conditions lead to inhibition of deep convection over the MLC and wetlands, and weak redistribution of precipitation with no significant increase over the latitudinal average. In the control run, K12 obtain more precipitation in the Sahel compared to our study, thus matching observed precipitation in this area. Sahel precipitation is enhanced at 6 kyr and the presence of lake and wetlands increases precipitation on the latitudinal 5 average, which is not the case in our simulations. Although the setup of our study and K12 is different, the higher sensitivity of the model in K12 study highlights the importance of convection parameterizations in climate models (Chikira et al., 2006). In our simulations the megalake feedback is similar in the three climatic contexts, small biome changes outside the Chad basin are only noticeable in the Plio max configuration, pos-10 sibly suggesting a wider impact of the MLC in a more humid background climate. Regarding biome distribution during the Pliocene, tropical savanna is present on the eastern shore of the lake, even in the Plio lake configuration, i.e. with presentday orbital configuration, which is close to a glacial one in terms of precession angle, providing a habitable area for hominids during periods of relative droughts during the 15 Pliocene. However, the presence of tropical savanna in this area is not induced nor significantly favored by the MLC, tropical savanna and more humid biomes presence in this area is governed by global climate. Moreover, Saharan desert extent seems to greatly vary throughout the Pliocene, depending on precession. With present-day orbital parameters, its southern limit is around 20 • N, whereas in the Plio max configu-20 ration, it shifts up to 5 • northward, with tropical woodland reaching 18 • N.

Conclusions
The presence of a MLC during humid periods of the Cenozoic and Quaternary clearly impacts local precipitation and wind patterns. The most important feature is the suppression of deep convection above the surface of the lake. In summer, monsoon winds 25 are increased over the lake surface, carrying humidity to the northeast. Over the MLC atmospheric pressure is slightly higher, a fact which also impacts wind circulation and explains the precipitation increase at the southeast of the MLC. During the dry season, the presence of a MLC does not generate precipitation over the basin. These results are a common feature of all the simulations. Impact away from the Chad basin is very weak and variable. We conclude that while the MLC should be taken into account for its influence on climate in the Chad basin, its influence alone outside the basin is not 5 determinant. The presence of a MLC does not enhance a northward migration of the monsoon, in agreement with Broström et al. (1998). Regarding vegetation distribution, the presence of a MLC has little effect on biomes, although humid biomes are simulated in the Chad basin in all simulations, providing a sustainable environment for hominid populations, even if restricted to the southern part of the basin in the Plio sim-10 ulation. The fact that environment was favorable for hominids during the Late Pliocene, but that MLC was not a major contribution to these conditions reinforce the hypothesis that climate change is the driver for these conditions, rather than surface hydrology changes. Other megalakes in present day dry climates are documented for the Quaternary, such as megalakes Frome and Eyre in Australia (e.g., Alley 1998; Cohen et al.,