Mechanical and thermal effects of Asian topography on northern East Asian low

Asian topography significantly affects regional and even global climate and circulation systems through mechanical and thermal forcing. Northern East Asia low (NEAL), one important mid-latitude circulation component of the East Asian summer monsoon system, can modulate regional climate remarkably. However, the effect of Asian topography on the NEAL has remained unknown. To address this gap, this study investigates the mechanical and thermal forcing effects of Asian topography on the NEAL based on the CAS FGOALS-f3-L model simulations from the CMIP6 Global Monsoon Model Inter-comparison Project (GMMIP) tier-1 and tier-3 experiments for 1979-2014. Results show that the Asian topography enhances and expands the NEAL in its southwestern part and also deforms it with a clockwise rotation, causing a southwest-northeast tilt in the NEAL's axis. The changes in NEAL are attributed to subtle coordination between the thermal and mechanical forcing effects of Asian topography. The thermal (mechanical) forcing cause two lower-tropospheric cyclonic (anticyclonic) anomalies in mid-high latitudes: one in and to the west of NEAL around the Mongolian Plateau and the other to the northeast around the Highlands surrounding the Sea of Okhotsk. Consequently, the thermal forcing enhances west NEAL and rotates the NEAL clockwise, leading eventually to a southwest-northeast tilt in the axis, while the mechanical forcing plays a role of binding the NEAL "pocket" in the north to render it to a closed cyclone/low. The combined effects of Asian topography on NEAL in the south are dominated by the thermal forcing and in the north by the mechanical forcing. Therefore, the southwestward expansion of NEAL and tilt in the axis are attributed to the thermal forcing, and the enhancement of NEAL to both the thermal and mechanical forcings. This study highlights the crucial effects of both thermal and mechanical forcings of Asian topography in mid-latitude circulations, which differs from the dominant role of thermal forcing in the tropical and subtropical circulations.