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.

Monsoon variability over the past 150,000 years

Present-day production of CaCO3 in tne world ocean is calculated to be about 5 billion tons (bt) per year, of which about 3 bt accumulate in sediments; the other 40% is dissolved. Nearly half of the carbonate sediment accumulates on reefs, banks, and tropical shelves, and consists largely of metastable aragonite and magnesian calcite. Deep-sea carbonates, predominantly calcitic coccoliths and planktonic foraminifera, have orders of magnitude lower productivity and accumulation rates than shallow-water carbonates, but they cover orders of magnitude larger basin area. Twice as much calcium is removed from the oceans by present-day carbonate accumulation as is estimated to be brought in by rivers and hydrothermal activity (1.6 bt), suggesting that outputs have been overestimated or inputs underestimated, that one or more other inputs have not been identified, and/or that the oceans are not presently in steady state. One “missing” calcium source might be groundwater, although its present-day input is probably much smaller than that of rivers. If, as seems likely, CaCO3 accumulation presently exceeds terrestial and hydrothermal input, this imbalance presumably is offset by decreased accumulation and increased input during lowered sea level: shallow-water accumulation decreases by an order of magnitude with a 100 m drop in sea level, while groundwater influx increases because of heightened piezometric head and the diagenesis of metastable aragonite and magnesian calcite from subaerially exposed shallow-water carbonates.

Production and accumulation of calcium carbonate in the ocean: Budget of a nonsteady state

The Croll–Milankovitch astronomical theory of climate1–3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession, although nonlinearity had to be assumed for eccentricity4,5. Moreover, interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6,000–5,000 yr is seen between an astronomical climate index and terrestrial climate indicators, dominated by the northern ice-sheet dynamics. Climate proxy data of low latitudes are less directly dependent on the ice sheets. Here it is shown that during the past 464,000 yr, African monsoons signalled by the East Mediterranean sapropels were heaviest always and only when a northern summer monsoon index, computed from the orbital variation of insolation, reached maximum values. This occurred during all the interglacials, but also twice during glacial periods. Thus tropical aridity is not the single climatic pattern of glacial periods. An immediate terrestrial climate response to orbital variations of insolation is established.

African monsoons, an immediate climate response to orbital insolation

Mediterranean Quaternary sapropels, an immediate response of the african monsoon to variation of insolation

Sediments in the Arabian Sea provide biological, biogeochemical and lithogenic evidence of past changes in the Indian Ocean summer monsoon winds. For the past 350,000 years, this system has been externally forced by cyclical changes in solar radiation, and internally phase-locked to the transport of latent heat from the southern subtropical Indian Ocean to the Tibetan Plateau. In contrast to the results of general circulation models, these geological data suggest that the climate change associated with variability in global ice volume is not a primary factor in determining the strength and timing of the monsoon winds.

James L. Cullen

Papers

Microfossil evidence for changing salinity patterns in the bay of Bengal over the last 20 000 years

Planktonic foraminifera of the northern Indian Ocean: Distribution and preservation in surface sediments

The Brown University Foraminiferal Data Base (BFD)