C AdvT hPa diff

Figure 8. As in Fig. 7, but for differences between the experiments with and without the Tibetan Plateau. The dashed line on the Asian continent indicates the range of the Tibetan Plateau.

latitudes are also enhanced, and the anomalies extend westward to those in the Atlantic. These easterlies are associated with the North Pacific subtropical high and transport dry air to suppress convection over the south coast of Asia and the Indian Ocean. Overall, the changes in precipitation and winds are similar to those in the previous experiments for examining surface albedo and topography effects (Figs. 7 and 8), but with a much broader scale. Further examining the horizontal advection terms, the moisture transport -{vVq) is also the dominant factor for the precipitation anomalies [Figs. 9(d) and 9(e)].

In a more realistic case, such as ENSO, the effect of ocean heat transport is also examined. By specifying anomalous Q flux, typical El Niño-like SST anomalies are simulated [Fig. 10(a)] and these SST anomalies create longitudinal gradients of SST in the tropics. The associated tropospheric temperature anomalies also show a longitudinal gradient anomaly over the north of 20°N [Fig. 10(b)]. Because of wave dynamics in the tropics (Chiang and Sobel, 2002; Su and Neelin, 2002), widespread warm tropospheric temperature anomalies (Wallace et al., 1998) reduce the meridional tropos-pheric temperature gradient over the Asian continent. The anomalous meridional and longitudinal tropospheric temperature gradients induce a weak anomalous anticyclonic circulation over the Asian continent and reduce the strength of the Asian summer monsoon [Fig. 10(c)] when one is judging from a monsoon index that is often used to present the Asian summer monsoon as a whole (Webster and Yang, 1992). However, this anomalous anticyclonic circulation exists only over the north of 20° N. Over the south of 20°N, another anticyclonic circulation anomaly is found over South Asia and the Indian Ocean. This anomalous anticyclone is associated with a Rossby wave response to the weaker convection induced by the cold SST anomalies over the western Pacific and the Indian Ocean. The cold SST anomalies reduce the precipitation over the Indian Ocean. The decreased monsoon heating induces an anticy-clonic circulation anomaly associated with the Rossby wave to its west. The anticyclonic circulation anomaly over South Asia and the Indian Ocean pushes the monsoon rain zone northward. Thus, the monsoon rain zone extends northward when the meridional tropospheric temperature gradient and the summer monsoon circulation are weakened. This implies that local SST is important to the subregional monsoon system when the tropospheric temperature gradient does not change too much. Further examining the horizontal advection terms, — (v • Vq) is still the dominant effect on the positive precipitation

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