Orographic mechanical and surface thermal effects of the Tibetan–Iranian Plateau on extratropical intraseasonal waves in boreal summer: numerical experiments

Abstract

The intensity and location of boreal summer extratropical intraseasonal oscillations along the subtropical westerly jet (EISO-SJ) are crucial in triggering and distributing extreme events over Eurasia. Based on numerical experiments, this study distinguishes the orographic mechanical and surface thermal forcing of the Tibetan–Iranian Plateau (i.e. TIP-MF and TIP-TF) on EISO-SJ. The TIP-MF primarily modulates the amplitude of EISO-SJ that strengthens over the upstream and weakens over the downstream. Comparatively, the TIP-TF not only reduces/increases the intensity of EISO-SJ over the TIP upstream/downstream, but also significantly migrates the track of EISO-SJ northward. Further analysis demonstrates that the changes of the westerly jet, eddy energy propagation and energy conversion are consistent with the track and amplitude changes of EISO-SJ. This study indicates the variations of the TIP surface sensible heating in interannual variation and global change, as well as the terrain uplift of the TIP in paleoclimate influence on the mid-latitude subseasonal variation.

Keywords

• orographic mechanical effect
• thermal effect
• extratropical intraseasonal oscillation
• numerical experiment