Effect of the Arakan Mountains in the northwestern Indochina Peninsula on the late May Asian monsoon transition

By simulations using a global climate model with and without the Arakan Mountains in the northwest of Myanmar, we demonstrated that this mesoscale meridionally elongated mountain range has a substantial effect on anchoring and enhancing precipitation in the region during the Bay of Bengal (BoB) monsoon onset in late May. In this period, the presence of the Arakan Mountains in the model significantly improves the simulation of the thermal and dynamical atmospheric structure of the monsoon, by substantially enhancing precipitation, deepening the midtropospheric trough and the southwesterly flow over the BoB and strengthening the upper tropospheric anticyclone atop the trough. These mountain-induced changes essentially improve the simulation of the late May Asian summer monsoon transition. Prior to the BoB monsoon onset, the blocking and deflecting effects of the Arakan Mountains on the low-level flow are marked, which enhance the moisture convergence and also probably influence the oceanic forcing over the BoB. As well as the mountain effect on the moisture convergence, inclusion of the Arakan Mountains apparently initiates an upstream troughing effect, which in turn enhances the synoptic and large-scale circulation during the monsoon transition. Furthermore, the presence of the Arakan Mountains induces a large-scale wavelike perturbation, which likely leads to an improvement of Meiyu/Baiu simulation. This study reveals that a narrow mountain like the Arakan Mountains likely contributes markedly to the characteristics of the Asian summer monsoon during its early seasonal march. Such effects need to be reasonably resolved in the models.