Abstract
The thermal effect of the Tibetan–Iranian Plateau (TIP) on the Asian summer monsoon and the role of air–sea interactions over the Indian Ocean in regulating the effects of the TIP are explored. The results demonstrate that the direct thermal effect of the TIP produces a lower troposphere cyclonic circulation in the area surrounding the TIP and increases the continental precipitation over South and East Asia. It also decreases the precipitation over the tropical Indian Ocean and increases the sea-surface temperature (SST) of the tropical Indian Ocean with a large gradient zone located along 10°N but decreases SST of the western coast of Indonesia. In the lower troposphere, the air–sea interaction induced by the TIP thermal forcing produces an anticyclonic circulation surrounding the TIP and a stronger westerly flow to the south of the anticyclone. A circulation dipole thus forms to the south of the TIP. Together with this horizontal dipole, a meridional circulation dipole is generated to the south of the TIP, which is characterized by strong air ascent from 10 to 15°N where a strong westerly flow occurs, and the descent of air over the southern slope of the TIP and south Hemisphere. These results demonstrate that the indirect effect of the air–sea interaction over the Indian Ocean induced by the TIP thermal forcing is to counteract its direct effect on the Asian summer monsoon. The uncertainty of this indirect effect is also discussed.
Highlights
More detailed topographic experiments have been carried out by Kitoh (2004, 2010), who assessed the effects of different mountain heights in a coupled GCMs (CGCMs) and found that the Asian summer monsoon (ASM) precipitation moves gradually inland when forced by progressive mountain uplift
Two atmospheric general circulation models (AGCMs) experiments (CON_OBSST and TIPNS_OBSST) were carried out using the SAMIL2 component, which used the same setting as the control experiment (CON) and TIPNS runs, except that the sea-surface temperature (SST) and sea ice were forced by the observed climatological monthly mean
To obtain the three-dimensional features of ocean temperature and current, in Fig. 5 we show the spatial characteristics of the ocean temperature and current differences between the CON and TIPNS runs at depths of 5, 25, 45, and 85 m, representing the near surface, the middle of the mixed layer, the intersection between the mixed layer and the thermocline, and the thermocline, respectively
Summary
The Tibetan Plateau (TP), the largest highland in the world, is located in the central and eastern parts of the Eurasian continent and has a strong effect on the climate of Asia (Manabe and Terpstra 1974; Tao and Ding 1981; Huang 1985; Zhao and Chen 2001; Wu et al 2007, 2012a; Wang et al 2008; Zhou et al 2009; Liu et al 2012; Boos and Kuang 2010, 2013). More detailed topographic experiments have been carried out by Kitoh (2004, 2010), who assessed the effects of different mountain heights in a CGCM and found that the ASM precipitation moves gradually inland when forced by progressive mountain uplift In this scenario, the Pacific subtropical anticyclone and the associated trade winds become stronger. Using a regional model, Wang et al (2018) found TP heating could cool the SST over northern Indian Ocean by enhancing southwestly winds Despite these studies, the responses of the SST and the associated changes in the mixed layer to the uplifted thermal effects of the TP remain unclear.
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