Feedbacks between climate and surface water in northern Africa during the middle Holocene

Observations indicate that the area of lakes and wetlands in northern Africa was considerably greater during the middle Holocene than at present. Simulations are designed to examine whether expanded surface waters may have had a significant impact on the strength of the summer monsoon of northern Africa. Three experiments with the National Center for Atmospheric Research community climate model (NCAR CCM3) are analyzed, a modern and two middle Holocene (6000 years before present) simulations, one with and one without prescribed expanded surface water. There is a significant increase in the strength of the summer monsoon in the middle Holocene simulation due to the enhanced seasonal insolation cycle. The addition of surface waters result in a June, July, and August mean increase in the net surface radiation (5%), an increase in the latent heat flux (30%), a decrease in the sensible heat flux (10%), and an increase in the near-surface specific humidity (>5%) compared to the middle Holocene simulation without surface water changes. The changes in these simulated climate variables are comparable in scale to changes due to orbital forcing alone. The expanded surface waters result in a cooling of the atmosphere and anticyclonic flow over the large water bodies in summer relative to the simulation without surface water changes. The combination of increased atmospheric moisture and altered circulation results in significant changes to the precipitation distribution in northern Africa including a small increase in the zonal mean July precipitation to the north of the lakes and a decrease to the south. The geographic distribution of the precipitation with surface water changes is qualitatively in better agreement with observations than the distribution with orbital forcing alone but still does not fully match the expansion implied by observations nor the expansion required to produce the simulated middle Holocene surface waters used in this study. The results of this study suggest that surface waters were an important factor in the climate of northern Africa during the middle and early Holocene and that they must be included for accurate simulation of this climate.

[1]  C. Haynes,et al.  Holocene palaeoecology of the eastern Sahara; Selima Oasis , 1989 .

[2]  James J. Hack,et al.  Description of the NCAR Community Climate Model (CCM3). Technical note , 1996 .

[3]  Val L. Eichenlaub.  Weather and climate of the Great Lakes region , 1979 .

[4]  J. Kutzbach,et al.  Monsoon Climate of the Early Holocene: Climate Experiment with the Earth's Orbital Parameters for 9000 Years Ago , 1981, Science.

[5]  Steven W. Hostetler,et al.  Interactive coupling of a lake thermal model with a regional climate model , 1993 .

[6]  F. Gasse,et al.  Freshwater to marine-like environments from Holocene lakes in northern Sahara , 1985, Nature.

[7]  Julia Walker,et al.  The effect of soil moisture on circulation and rainfall in a tropical model , 1977 .

[8]  P. Carbonel,et al.  La dépression de Taoudenni (Sahara malien) à l'Holocène , 1987 .

[9]  J. Kutzbach,et al.  The Sensitivity of the African-Asian Monsoonal Climate to Orbital Parameter Changes for 9000 Years B.P. in a Low-Resolution General Circulation Model. , 1982 .

[10]  Y. Sud,et al.  A study of the influence of surface albedo on July circulation in semi‐arid regions using the glas GCM , 1982 .

[11]  H. Pachur,et al.  Wadi Howar: Paleoclimatic Evidence from an Extinct River System in the Southeastern Sahara , 1987, Science.

[12]  A. T. Grove,et al.  Environmental and climatic implications of late Quaternary lake-level fluctuations in Africa , 1976, Nature.

[13]  D. Rowell,et al.  The influence of soil wetness distribution on short‐range rainfall forecasting in the West African Sahel , 1990 .

[14]  B. Bonan,et al.  A Land Surface Model (LSM Version 1.0) for Ecological, Hydrological, and Atmospheric Studies: Technical Description and User's Guide , 1996 .

[15]  J. Shukla,et al.  Influence of Land-Surface Evapotranspiration on the Earth's Climate , 1982, Science.

[16]  A. Pitman,et al.  Land‐surface schemes for future climate models: Specification, aggregation, and heterogeneity , 1992 .

[17]  D. Legates,et al.  Mean seasonal and spatial variability in gauge‐corrected, global precipitation , 1990 .

[18]  Robert M. Chervin,et al.  On Determining the Statistical Significance of Climate Experiments with General Circulation Models , 1976 .

[19]  L. Picon,et al.  Effect of a Change of the Surface Albedo of the Sahel on Climate , 1986 .

[20]  J. Kutzbach,et al.  The Influence of Changing Orbital Parameters and Surface Boundary Conditions on Climate Simulations for the Past 18 000 Years , 1986 .

[21]  J. Kutzbach,et al.  Simulation of Late Permian Climate and Biomes with an Atmosphere-Ocean Model: Comparisons with Observations , 1993 .

[22]  P. Rowntree,et al.  Simulations of the Saharan atmosphere—dependence on moisture and albedo , 1986 .

[23]  Syukuro Manabe,et al.  The Effect of Soil Moisture on the Short-Term Climate and Hydrology Change—A Numerical Experiment , 1984 .

[24]  Gordon B. Bonan,et al.  Sensitivity of a GCM simulation to subgrid infiltration and surface runoff , 1996 .

[25]  Jack Kornfield,et al.  A Comparative Study of the Effects of Albedo Change on Drought in Semi-Arid Regions. , 1977 .

[26]  Michael T. Coe,et al.  Simulating Continental Surface Waters: An Application to Holocene Northern Africa , 1997 .

[27]  Gordon B. Bonan,et al.  Sensitivity of a GCM simulation to inclusion of inland water surfaces , 1995 .

[28]  A. Pitman.  A simple parameterization of sub-grid scale open water for climate models , 1991 .

[29]  J. Cogley.  Runoff from the World's Landmasses: Amounts and Uncertainties at 2-degree Resolution , 1995 .

[30]  N. Noblet,et al.  Sensitivity of simulated Asian and African summer monsoons to orbitally induced variations in insolation 126, 115 and 6 kBP , 1996 .

[31]  F. Street-Perrott.  Lake levels and climate reconstruction , 1985 .

[32]  S. Harrison,et al.  Simulated climate and biomes of Africa during the late quaternary: comparison with pollen and lake status data , 1998 .

[33]  A. T. Grove,et al.  Global Maps of Lake-Level Fluctuations since 30,000 yr B.P. , 1979, Quaternary Research.

[34]  J. Riser,et al.  Holocene lake deposits and palaeoenvironments in Central Sahara, Northeastern Mali , 1981 .

[35]  Steven W. Hostetler,et al.  Use of a regional atmospheric model to simulate lake-atmosphere feedbacks associated with Pleistocene Lakes Lahontan and Bonneville , 1992 .

[36]  F. Giorgi,et al.  Lake-Atmosphere Feedbacks Associated with Paleolakes Bonneville and Lahontan , 1994, Science.

[37]  S. Harrison,et al.  An evaluation of the simulated water balance of Eurasia and northern Africa at 6000 y BP using lake status data , 1996 .

[38]  F. Giorgi,et al.  Toward the Simulation of the Effects of the Great Lakes on Regional Climate , 1993 .

[39]  J. Mitchell,et al.  Climate simulations for 9000 years before present: Seasonal variations and effect of the Laurentide ice sheet , 1988 .

[40]  J. Charney.  Dynamics of deserts and drought in the Sahel , 1975 .

[41]  Gordon B. Bonan,et al.  Land-atmosphere CO2 exchange simulated by a land surface process model coupled to an atmospheric general circulation model , 1995 .

[42]  Michael J. McPhaden,et al.  A comparison of tropical Pacific surface wind analyses , 1989 .

[43]  Stanley A. Changnon,et al.  Review of the influences of the Great Lakes on weather , 1972 .

[44]  J. Kutzbach,et al.  Vegetation and soil feedbacks on the response of the African monsoon to orbital forcing in the early to middle Holocene , 1996, Nature.

[45]  J. Mitchell,et al.  Milankovitch and albedo forcing of the tropical monsoons: a comparison of geological evidence and numerical simulations for 9000 yBP , 1990, Transactions of the Royal Society of Edinburgh: Earth Sciences.

[46]  A. Lézine,et al.  Pollen and hydrological evidence for the interpretation of past climates in tropical west Africa during the holocene , 1989 .

[47]  Yongkang Xue,et al.  The Influence of Land Surface Properties on Sahel Climate. Part 1: Desertification , 1993 .