Paleoclimatic records adjacent to India and Africa show major variability that is related to large fluctuations in the wind and precipitation fields associated with monsoonal circulations. Much of the variability occurs at orbital periodicities, and all of the paleoclimatic time series show four monsoon maxima that occur during interglacial conditions and coincide with precession maxima and maxima of northern hemisphere summer radiation. During glacial conditions, paleoclimatic records are less distinct and show more individual variability. To identify the processes causing changes in monsoon circulation, we used 13 general circulation model (National Center for Atmospheric Research (NCAR) community climate model (CCM)) simulations that incorporate a large range of solar radiation and surface (modern to full glacial) boundary conditions. The spatial patterns of climate variables and their zonal and regional averages revealed that under interglacial conditions increased northern hemisphere solar radiation produced a larger land-ocean pressure gradient, stronger winds, and greater precipitation over southern Asia and North Africa. Under glacial conditions, however, the monsoon is weakened in southern Asia (decreased winds and precipitation), but precipitation is increased in the equatorial west Indian Ocean and equatorial North Africa. Sensitivity coefficients are used to estimate the change in model-simulated precipitation (ΔP) relative to the changes in northern hemisphere summer radiation (ΔS) and glacial age boundary conditions (ΔGBC); the coefficients are then used with time series of ΔS and ΔGBC to simulate past precipitation (ΔP) for a specific area. Simulated records of ΔP for southern Asia and equatorial North Africa over the past 150,000 years show four monsoon maxima that are related to solar radiation maxima and are observed in the paleoclimatic data. The simulations also indicate that southern Asia is drier than today (weaker monsoon) for the period with extensive glacial boundary conditions, especially between 75,000 and 15,000 years ago. Conversely, equatorial North Africa is simulated to be wetter than today during glacial conditions. Both areas show stronger monsoons with increasing solar radiation during interglacial conditions. The agreement of simulated and observed paleoclimatic time series suggests that both orbitally produced solar radiation changes and glacial age boundary condition changes are necessary to explain the major regional features of monsoon climates at millenial or longer time scales. For southern Asia and equatorial North Africa the influence of these two factors is approximately additive.
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