Response of the African monsoon to orbital forcing and ocean feedbacks in the middle Holocene

Science

Volumn 278, Issue 5337, 1997, Pages 440-443

  Kutzbach, J E ^a   Liu, Z ^a  

^a UNIVERSITY OF WISCONSIN MADISON   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

AFRICAN MONSOON; CLIMATE MODEL; HOLOCENE CLIMATE; INSOLATION; ORBITAL FORCING; PRECIPITATION; SEA SURFACE TEMPERATURE;

ARTICLE; ATMOSPHERE; CLIMATE; PRIORITY JOURNAL; SEA; SEASON; TEMPERATURE;

EID: 0030661821     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.278.5337.440     Document Type: Article

References (36)

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20. This degree of monsoon enhancement was not found in earlier simulations of the climate of 6 or 9 ka that coupled an AGCM with a static mixed-layer ocean (27, 28). Our analysis of these earlier experiments and other simulations confirms that the SSTs and low-level atmospheric winds of the control simulation are substantially biased by the lack of ocean dynamics and that this bias in the control alters the atmospheric response to orbital forcing. Cooler equatorial waters and a more humid continental surface may also strengthen the monsoon by increasing the south-to-north gradient of equivalent potential temperature in the boundary layer over western North Africa [E. A. B. Eltahir and C. Gong, J. Clim. 9, 1030 (1996)]; however, the change of this gradient is small in our simulation. Warmer North Atlantic waters poleward of the tropics may also cause precipitation to increase in North Africa, based on the results of a climate model experiment [P. B. deMenocal and D. Rind, J. Geophys. Res. 98, 7265 (1993)]; this linkage may occur in our stimulation as well.
21. This degree of monsoon enhancement was not found in earlier simulations of the climate of 6 or 9 ka that coupled an AGCM with a static mixed-layer ocean (27, 28). Our analysis of these earlier experiments and other simulations confirms that the SSTs and low-level atmospheric winds of the control simulation are substantially biased by the lack of ocean dynamics and that this bias in the control alters the atmospheric response to orbital forcing. Cooler equatorial waters and a more humid continental surface may also strengthen the monsoon by increasing the south-to-north gradient of equivalent potential temperature in the boundary layer over western North Africa [E. A. B. Eltahir and C. Gong, J. Clim. 9, 1030 (1996)]; however, the change of this gradient is small in our simulation. Warmer North Atlantic waters poleward of the tropics may also cause precipitation to increase in North Africa, based on the results of a climate model experiment [P. B. deMenocal and D. Rind, J. Geophys. Res. 98, 7265 (1993)]; this linkage may occur in our stimulation as well.
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27. In the equatorial Atlantic, three core sites indicate warmer than present conditions, and 10 core sites indicate colder than present conditions with a range of anomalies between 2°C and -2°C for August (23). The sites indicating colder than present conditions do not agree in sign with the long time series site (22), which indicated warmer conditions coinciding with perihelion in northern summer. In the northern subtropics, a site near the Bahamas at 25°N shows a mid-Holocene maximum in annual temperature (an increase of ∼0.5°C) (24). Adjacent to the African coast, 20° to 25°N, two core sites indicate warmer than present conditions, and three core sites indicate colder than present conditions with a range of anomalies between 1.5° and -2.9°C for August (23). As in the equatorial Atlantic, the intercore differences are relatively large.
28. Areas of particular future interest include the Arctic, where sea-ice cover almost certainly decreased (3, 27, 28); the Pacific, where there is evidence of changed El Niño-Southern Oscillation behavior [D. H. Sandweiss, J. B. Richardson III, E. J. Reitz, H. B. Rollins, K. A. Maasch, Science 273, 1531 (1996)]; and the Indian Ocean, where changed upwelling in the Arabian Sea provided the first evidence for orbitally forced changes in monsoons [W. L. Prell, in Milankovitch and Climate, A. Berger et al., Eds. (Reidel, Hingham, MA, 1984), pp. 349-366]. Changes in SSTs in these other ocean basins may also influence the climate of the Atlantic-African sector [(17); C. K. Folland, T. N. Palmer, D. E. Parker, Nature 320, 602 (1986); T. N. Palmer, ibid. 322, 251 (1986); S. Curtis and S. Hastenrath, J. Geophys. Res. 100, 15835 (1995)]. On the basis of analogies with recent interannual variability [S. Hastenrath, Climate and Circulation of the Tropics (Reidel, Dordretcht, Netherlands, 1985)], the SST changes in the tropical Atlantic that were simulated at 6 ka could also lead to changes in the mid-Holocene climate of eastern tropical South America.
29. Areas of particular future interest include the Arctic, where sea-ice cover almost certainly decreased (3, 27, 28); the Pacific, where there is evidence of changed El Niño-Southern Oscillation behavior [D. H. Sandweiss, J. B. Richardson III, E. J. Reitz, H. B. Rollins, K. A. Maasch, Science 273, 1531 (1996)]; and the Indian Ocean, where changed upwelling in the Arabian Sea provided the first evidence for orbitally forced changes in monsoons [W. L. Prell, in Milankovitch and Climate, A. Berger et al., Eds. (Reidel, Hingham, MA, 1984), pp. 349-366]. Changes in SSTs in these other ocean basins may also influence the climate of the Atlantic-African sector [(17); C. K. Folland, T. N. Palmer, D. E. Parker, Nature 320, 602 (1986); T. N. Palmer, ibid. 322, 251 (1986); S. Curtis and S. Hastenrath, J. Geophys. Res. 100, 15835 (1995)]. On the basis of analogies with recent interannual variability [S. Hastenrath, Climate and Circulation of the Tropics (Reidel, Dordretcht, Netherlands, 1985)], the SST changes in the tropical Atlantic that were simulated at 6 ka could also lead to changes in the mid-Holocene climate of eastern tropical South America.
30. Areas of particular future interest include the Arctic, where sea-ice cover almost certainly decreased (3, 27, 28); the Pacific, where there is evidence of changed El Niño-Southern Oscillation behavior [D. H. Sandweiss, J. B. Richardson III, E. J. Reitz, H. B. Rollins, K. A. Maasch, Science 273, 1531 (1996)]; and the Indian Ocean, where changed upwelling in the Arabian Sea provided the first evidence for orbitally forced changes in monsoons [W. L. Prell, in Milankovitch and Climate, A. Berger et al., Eds. (Reidel, Hingham, MA, 1984), pp. 349-366]. Changes in SSTs in these other ocean basins may also influence the climate of the Atlantic-African sector [(17); C. K. Folland, T. N. Palmer, D. E. Parker, Nature 320, 602 (1986); T. N. Palmer, ibid. 322, 251 (1986); S. Curtis and S. Hastenrath, J. Geophys. Res. 100, 15835 (1995)]. On the basis of analogies with recent interannual variability [S. Hastenrath, Climate and Circulation of the Tropics (Reidel, Dordretcht, Netherlands, 1985)], the SST changes in the tropical Atlantic that were simulated at 6 ka could also lead to changes in the mid-Holocene climate of eastern tropical South America.
31. Areas of particular future interest include the Arctic, where sea-ice cover almost certainly decreased (3, 27, 28); the Pacific, where there is evidence of changed El Niño-Southern Oscillation behavior [D. H. Sandweiss, J. B. Richardson III, E. J. Reitz, H. B. Rollins, K. A. Maasch, Science 273, 1531 (1996)]; and the Indian Ocean, where changed upwelling in the Arabian Sea provided the first evidence for orbitally forced changes in monsoons [W. L. Prell, in Milankovitch and Climate, A. Berger et al., Eds. (Reidel, Hingham, MA, 1984), pp. 349-366]. Changes in SSTs in these other ocean basins may also influence the climate of the Atlantic-African sector [(17); C. K. Folland, T. N. Palmer, D. E. Parker, Nature 320, 602 (1986); T. N. Palmer, ibid. 322, 251 (1986); S. Curtis and S. Hastenrath, J. Geophys. Res. 100, 15835 (1995)]. On the basis of analogies with recent interannual variability [S. Hastenrath, Climate and Circulation of the Tropics (Reidel, Dordretcht, Netherlands, 1985)], the SST changes in the tropical Atlantic that were simulated at 6 ka could also lead to changes in the mid-Holocene climate of eastern tropical South America.
32. Areas of particular future interest include the Arctic, where sea-ice cover almost certainly decreased (3, 27, 28); the Pacific, where there is evidence of changed El Niño-Southern Oscillation behavior [D. H. Sandweiss, J. B. Richardson III, E. J. Reitz, H. B. Rollins, K. A. Maasch, Science 273, 1531 (1996)]; and the Indian Ocean, where changed upwelling in the Arabian Sea provided the first evidence for orbitally forced changes in monsoons [W. L. Prell, in Milankovitch and Climate, A. Berger et al., Eds. (Reidel, Hingham, MA, 1984), pp. 349-366]. Changes in SSTs in these other ocean basins may also influence the climate of the Atlantic-African sector [(17); C. K. Folland, T. N. Palmer, D. E. Parker, Nature 320, 602 (1986); T. N. Palmer, ibid. 322, 251 (1986); S. Curtis and S. Hastenrath, J. Geophys. Res. 100, 15835 (1995)]. On the basis of analogies with recent interannual variability [S. Hastenrath, Climate and Circulation of the Tropics (Reidel, Dordretcht, Netherlands, 1985)], the SST changes in the tropical Atlantic that were simulated at 6 ka could also lead to changes in the mid-Holocene climate of eastern tropical South America.
33. Areas of particular future interest include the Arctic, where sea-ice cover almost certainly decreased (3, 27, 28); the Pacific, where there is evidence of changed El Niño-Southern Oscillation behavior [D. H. Sandweiss, J. B. Richardson III, E. J. Reitz, H. B. Rollins, K. A. Maasch, Science 273, 1531 (1996)]; and the Indian Ocean, where changed upwelling in the Arabian Sea provided the first evidence for orbitally forced changes in monsoons [W. L. Prell, in Milankovitch and Climate, A. Berger et al., Eds. (Reidel, Hingham, MA, 1984), pp. 349-366]. Changes in SSTs in these other ocean basins may also influence the climate of the Atlantic-African sector [(17); C. K. Folland, T. N. Palmer, D. E. Parker, Nature 320, 602 (1986); T. N. Palmer, ibid. 322, 251 (1986); S. Curtis and S. Hastenrath, J. Geophys. Res. 100, 15835 (1995)]. On the basis of analogies with recent interannual variability [S. Hastenrath, Climate and Circulation of the Tropics (Reidel, Dordretcht, Netherlands, 1985)], the SST changes in the tropical Atlantic that were simulated at 6 ka could also lead to changes in the mid-Holocene climate of eastern tropical South America.
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36. We thank W. Xu, R. Gallimore, and P. Behling for assistance with model simulations and graphics; M. Kennedy for preparing the manuscript; and S. L. Thompson and D. Pollard for allowing us to use the National Center for Atmospheric Research (NCAR) Genesis climate model. Supported by grants to the University of Wisconsin-Madison by the NSF's Climate Dynamics program and Earth System History program and by the U.S. Department of Energy. NCAR, which is sponsored by the NSF, provided computer resources.