Moist zonally-symmetric models and their applications to West African monsoons

In this thesis, we have developed several zonally-symmetric models with explicit hydrological cycles to investigate important physical processes involving "monsoon-like" circulations, based on the assumption that actual monsoons can be simplified as twodimensional thermally-direct circulations. First, we examine the response of moist zonally-symmetric atmospheres to specified subtropical surface temperature perturbations. This is an extension of the work by Plumb and Hou (1992) to the case with moisture-dynamics feedback. We find that the inclusion of the explicit hydrological cycle does not influence the threshold behavior of the onset of thermally-direct circulations, pointed out by Plumb and Hou (1992). We also find that the steady state theory can be applied to transient surface temperature perturbations with seasonal time scales. Then, as an application, we study the sensitivity of West African monsoons to vegetation perturbations, under the condition of perpetual summer solar insolation. The numerical experiments demonstrate that West African monsoons and therefore rainfall depend critically on the location of the vegetation perturbations. While the magnitude of local rainfall is sensitive to changes in local vegetation, the location of the InTer-Convergence Zone (ITCZ) is not sensitive to changes in the vegetation northward or southward from the location of ITCZ in the control experiment. However, the location of the ITCZ is sensitive to changes of the vegetation distribution in the immediate vicinity of the location of the ITCZ in the control experiment. The study indicates that changes in vegetative cover along the border between the Sahara desert and West Africa (desertification) have a minor impact on the simulated monsoon circulation. On the other hand, the tropical African deforestation may cause the collapse of the monsoon circulation for the worst case scenario and have a dramatic impact on the West African regional rainfall. The ever-growing tropical deforestation is then likely to be another contributor to the African drought besides the well-documented sub-Saharan desertification. Finally, we examine the role of tropical Atlantic Sea Surface Temperature (SST) in the dynamics of monsoon interannual variability. We present a case study for the wettest (1994) and driest (1992) years of the 1990s so far. The observations suggest that the warm spring SST anomaly in 1994 may be responsible for the abundant West African rainfall in 1994. This hypothesis is then explored using our numerical model. It is found that spring (April-J une) SST anomalies are indeed capable of producing the observed rainfall anomaly pattern. The positive rainfall anomaly over the ocean in spring eventually propagates onto the land and persists two to three months even after the SST anomaly vanishes. The interactions between ocean, land and atmosphere are proven to be crucial for the rainfall anomaly propagation. The positive feedback between rainfall and soil moisture is responsible for sustaining the rainfall anomaly. Thesis Supervisor: R. Alan Plumb Title: Professor of Nleteorology Thesis Supervisor: Elfatih A.B. Eltahir Title: Assistant Professor of Hydrology