Tuesday, 11 January 2005
Dynamical mechanisms for monsoon changes during the mid-holocene
During the mid-Holocene, about 6 ka BP, the African monsoon was stronger and extended further north than in present-day climate. With 6 ka BP solar forcing but present-day SST and land surface configurations, PMIP-type experiments simulate enhanced monsoon circulation but precipitation fails to move as far north as paleoenvironmental data suggest. Earlier research showed land surface vegetation interaction and oceanic feedback are important to the northward extension of the Holocene monsoon rainfall, but GCMs produce different results in terms of rainfall distribution. For current climate, we have found dynamical mechanisms are important in setting the poleward extent of monsoon rainfall. This study attempts to examine how solar forcing and land-surface processes are mediated by atmosphere-ocean dynamical mechanisms for the Holocene monsoon changes. We use an intermediate complexity climate model, the quasi-equilibrium tropical circulation model (QTCM), combined with a slab mixed-layer ocean model. In addition to standard PMIP-type experiments where SST is fixed at present day values, we conduct simulations with different land-surface-vegetation configurations and with mixed-layer ocean coupling. Moist static energy budget analysis is performed to examine to the relative importance of each dynamical process in various model settings. A QTCM PMIP-type experiment produces the Holocene precipitation and temperature changes comparable to other GCMs. When surface albedo of north Africa is reduced to 0.2, a typical value for grassland, precipitation increases over a broad area in equatorial Africa south of 20N. This provides a case akin to interactive vegetation runs that can be used to understand the role of dynamical effects. Further northward extension of monsoon rainfall is produced when anomalous advection of moisture is reduced over the Africa continent, suggesting the "ventilation effect"---advection of moisture from outside the continent---is a key process in limiting moisture increase and thus in determining the position of monsoon rainfall.