Characteristics and dynamics of two distinct quasi‐biweekly oscillations in the Tibetan Plateau summer monsoon

• Published in: Climate dynamics 2023-07, Vol.61 (1-2), p.91-109
• Main Authors: Yuan, Yunyang, Hsu, Pang-Chi, Li, Wenkai
• Format: Article
• Language: English
• Subjects: Advection; Anomalies; Barotropic mode; Blocking effects; China; Climatology; Convection; Diabatic heating; Dipoles; Dynamic meteorology; Earth and Environmental Science; Earth Sciences; Empirical analysis; Environmental aspects; Eurasia; Function analysis; Geophysics/Geodesy; landscapes; latitude; Low level; monsoon season; Monsoons; Movement; Oceanography; Orthogonal functions; Oscillations; Perturbation; Planetary waves; Rossby waves; Summer; Summer monsoon; Topographic effects; topography; Tropical convection; Tropical environments; Vorticity; Vorticity budget; Wave dynamics; Wave packets; Wave propagation; Wave trains; Wind; Winds
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-022-06575-9

The Tibetan Plateau (TP) summer monsoon (TPSM) is a unique monsoon system generated by the uplifted thermal forcing over the large-scale terrain. Considering both the dynamic and thermodynamic features of TPSM, we identified two distinct quasi-biweekly modes based on multivariate empirical orthogonal function analysis. The first mode shows a south–north-aligned pattern, related to interaction between the tropics and mid‐latitudes. In contrast, the second mode is generated by a southeastward‐propagating wave train crossing over Eurasia to form a west–east dipole distribution over the TP. The diagnostic results of a vorticity budget analysis and Rossby wave dynamics suggest that the first mode originates from the Mediterranean and propagates eastward along the westerly jet. As the upper-level vorticity anomaly approaches the northern TP, the vorticity advection induced by zonal background winds shows a leading phase to the southern sector of the TP, inducing southward movement. Once the southward-moving upper‐level vorticity arrives over the Indian peninsula, it is coupled with tropical convection disturbances in the low level, whereupon they together propagate northwestward. The wave train associated with the second mode originates from the North Atlantic. Its southeastward propagation is also related to the advection of vorticity perturbations caused by background westerly mean flows. Before reaching the western TP, the vorticity anomalies of the wave train are well-organized and quasi‐barotropic in structure. The blocking effect of the TP topography seems to disturb the wave structure. The movements of lower‐level vorticity and diabatic heating slow down and form a baroclinic structure over the TP.