The term v^ • V(T + q) includes two components: the moisture transport v^ • Vq and the temperature transport v^ • VT. To examine the effects of these two components respectively, v^ • VT and v^ • Vq are suppressed separately in Figs. 6(d) and 6(e). When v^ • VT is suppressed, the Asian summer monsoon rain zone does extend a little northward, but concentrates over the southeastern part of Asia [Fig. 6(d)]. The amplitude of the monsoon rainfall does not show any sign of enhancement. When v^ • Vq is suppressed, on the other hand, the change of the Asian summer monsoon is substantial [Fig. 6(e)]. The monsoon rainfall increases significantly, and the rain zone extends northward not only over the southeastern part of Asia but also South Asia. The pattern of the Asian summer monsoon rainfall in Fig. 6(e) is very similar to the pattern shown in Fig. 6(c) when both moisture and temperature transports are suppressed, except for the rainfall over East Asia. This implies that the ventilation mechanism in the Asian summer monsoon is contributed mainly by the moisture transport rather than the temperature transport.
To examine the IRH mechanism, the ¡3 effect is suppressed by dropping the terms that contain f — fo in the momentum equation, where f0
is a reference value of Coriolis parameter f. Figure 6(f) shows that the Asian summer monsoon rain zone extends much farther northward, particularly over the western part of the Asian continent. The Rossby-wave-induced subsidence does affect the northward extent of the Asian summer monsoon rain zone. However, the amplitude of the monsoon rainfall does not change too much. Over higher latitudes, between 40°N and 50°N in particular, the precipitation is still relatively low, which may be due to the ventilation by cross-continental westerly flow. When both of the ventilation and IRH mechanisms are suppressed, the rainfall over the target region becomes very similar to what Fnet indicates [Fig. 6(g)]. In other words, Fnet initiates the summer monsoon rainfall, but the ventilation and IRH mechanisms determine the northern boundary of the Asian summer monsoon rain zone and induce the east-west asymmetry. We further examine the effect of a local Hadley circulation. In this experiment, we suppress only the term (f — f0)k x v, not (f — fo)k x v', where v is zonally averaged across the target region and v' is the departure from this zonal average. Figure 6(h) shows a similar pattern of the summer monsoon rainfall to Fig. 6(g). With or without the local Hadley circulation, the summer monsoon rainfall does not change too much. This implies that descent in a local Hadley circulation does not limit the northward extent of the summer monsoon rain zone.
5.3. Land-ocean heating contrast
Two variables that affect land-ocean heating contrast are examined here: surface albedo and the Tibetan Plateau. Surface albedo can be affected by the surface type, such as snow cover. Many studies (e.g. Bamzai and Shukla, 1999; Dickson, 1984; Douville and Royer, 1996; Hahn and Shukla, 1976; Kripalani et al., 1996; Liu and Yanai, 2002; Meehl, 1994; Yanai and Li, 1994) discussed an inverse relationship between the Eurasian snow cover and the succeeding Asian summer monsoon. Strong snowfall in the previous winter and spring cools the Asian continent and delays the Asian summer monsoon onset in the coming season. Besides snow cover, vegetation is also related to surface albedo. For instance, desert regions have higher surface albedo, while grass has lower surface albedo.
Figure 7(a) shows the rainfall changes when the surface albedo is reduced over Eurasia: the less the surface albedo, the stronger the monsoon rainfall. The cyclonic summer monsoon circulation is enhanced when the surface albedo is small, while the Pacific subtropical high extends more westward. The monsoon rain zone is then pushed northward, especially over the eastern part of the Asian continent, and so the tilting angle of the monsoon rain zone becomes larger. Meanwhile, the upper tro-pospheric temperature (200-500 hPa) over the Asian continent becomes warmer, and so the meridional temperature gradient is enhanced [Fig. 7(b)]. The maximum warm tropospheric temperature anomalies are over the east coast of the Asian continent, with a little eastward shift relative to the area with the changed surface albedo, which is over the entire Eurasian continent. This is due to the temperature advection by the cross-continental flow, which cools the troposphere over the western part of the continent and heats the troposphere over the eastern part of the continent [Fig. 7(c)]. This increased meridional temperature gradient is associated with the strengthened Asian summer monsoon rainfall and circulation (CHOU; Li and Yanai, 1996; Webster et al., 1998). The positive moisture transport is also consistent with the increased rainfall over the eastern part of the continent. This indicates that the enhanced monsoon rainfall is associated with the feedback of the monsoon circulation via the horizontal moisture transport. Besides positive precipitation anomalies, negative precipitation anomalies are also found over tropical Africa and the western North Pacific including the South China Sea, just south of the enhanced monsoon rainfall. The negative rainfall anomalies over Africa are associated with the IRH mechanism: a Rossby-wave-induced subsidence (Rodwell and Hoskins, 1996, 2001) and the feedback of the anomalous moisture transport [Fig. 7(d)]. The negative rainfall anomalies over the western North Pacific, on the other hand, are induced by dry advection associated with the strengthened Pacific subtropical high.
Next we examine the effect of the Tibetan Plateau. Here we focus only on the thermal effect of the Tibetan Plateau, an elevated heating source, and exclude the mechanical effect. Thus, some caveats, such as the turning flow around the Plateau and the associated advection, may not be correctly simulated. However, we should still be able to examine the land-ocean heating contrast induced by the Tibetan Plateau in such experiments. When the mountains are uplifted, the monsoon rainfall is enhanced and the rain zone extends farther north [Fig. 8(a)]. The upper tropospheric temperature increases over the Asian continent with a more concentrated warming pattern than that in the surface albedo experiments. A maximum warming is found over the eastern part of the
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