Dynamic and Thermal Effects of the Tibetan and Iranian Plateaus on the Northward-Propagating Intraseasonal Oscillation during Boreal Summer

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Abstract This study investigates the dynamic and thermal effects of the Tibetan and Iranian Plateaus (TIP) on the characteristics of the northward-propagating boreal summer intraseasonal oscillation (BSISO) and reveals associated physical mechanisms. The relative importance of the Tibetan Plateau (TP) and Iranian Plateau (IP) to the northward propagation of BSISO is also detected. The findings are based on comparative analyses of the control run and six sensitivity experiments using the atmospheric model component of CAS FGOALS-f3-L (the low-resolution version of the Chinese Academy of Sciences Flexible Global Ocean–Atmosphere–Land System model, finite-volume version 3). It is demonstrated that the absence of TIP surface sensible heat or terrain restricts the occurrence of upper-level easterly and lower-level westerly anomalies, as well as moisture convergence over the Asian summer monsoon (ASM) region, which leads to a weakened easterly vertical wind shear with a narrowed range and the generation of a negative meridional specific humidity gradient in the northern Bay of Bengal. These mean-state changes are not conducive to the formation of positive barotropic vorticity together with planetary boundary layer moisture convergence to the north of the convective center, thus impeding the northward propagation of the BSISO. Results indicate that the combined TIP area, via its dynamic and thermal effects, modulates the northward-propagating BSISO by directly affecting the vertical wind shear and meridional moisture gradient in the northern Indian Ocean. By contrast, the TP plays a decisive role in the BSISO northward propagation due to the limited dynamic and thermal effects of IP on the background circulation and water vapor convergence over the ASM region.