The Impact of the Central Asian Mountains on Downstream Storminess and Monsoon Onset

In the first part of the thesis, the role of the Central Asian mountains on North Pacific storminess is examined using an atmospheric general circulation model by varying the height and area of the mountains. A series of model integrations shows that the presence of the Central Asian mountains suppresses North Pacific storminess by 20-30% during boreal winter. Their impact on storminess is found to be small during other seasons. Two main causes of the reduced storminess are diagnosed. First, the decrease in storminess appears to be associated with a weakening of downstream eddy development. The mountains disorganize the zonal coherency of wave packets and refract them more equatorward. As the zonal traveling distance of wave packets gets substantially shorter, downstream eddy development gets weaker, leading to the weakening of transient eddy kinetic energy and storminess. Second, the Central Asian mountains suppress the global baroclinic energy conversion. The decreased baroclinic energy conversion, particularly over the Eastern Eurasian continent decreases the number of eddy disturbances entering into the Western North Pacific. The `barotropic governor' does not appear to be a dominant factor in explaining the results. In the second part of the thesis, the impact of the Tibetan Plateau on the onset of the South Asian summer monsoon is examined using an atmospheric general circulation model. A series of model integrations shows that rainfall increases downstream of the mountains, whereas upstream of the mountains experiences anomalous subsidence. In particular, the Tibetan Plateau triggers low-level cross-equatorial flow and substantial rainfall over the Bay of Bengal during the pre-monsoon season (April-May). While the low-level cross-equatorial flow begins in May, the monsoon's onset over the Arabian Sea is suppressed until early June. The delayed onset over the Arabian Sea is probably because of the anomalous subsidence induced by the earlier moist convection over the Bay of Bengal. It is suggested that the earlier rainfall over the Bay of Bengal could be the response of stationary waves to the orographic forcing, such as low-level cyclonic motions downstream of the Tibetan Plateau. Because low-level westerlies exist over the Southern Tibetan Plateau during pre-monsoon season, the stationary wave response to the mountains is likely to occur