Generation of the Quasi-Biweekly Oscillation in the Surface Potential Vorticity Over the Tibetan Plateau During Boreal Summer: A Case Study of 2014

Abstract

The atmospheric circulation around the Tibetan Plateau (TP) exhibits a substantial 10–20-day quasi–biweekly oscillation (QBWO), profoundly impacting weather and climate locally and remotely. Understanding the factors influencing the generation of QBWO over the TP (QBWO_TP) and its physical mechanism is crucial. This study has investigated the influence of multi–timescale and land–atmosphere interactions on the generation of the QBWO_TP in surface potential vorticity (SPV), a valuable tool for characterizing the mechanical and thermal variabilities in mountain forcing, based on a 2014 case study. Results indicate that in the free atmosphere, the summer monsoon onset over the Bay of Bengal induces a northward shift in the westerly jet toward the TP, manifested as an increase in low-frequency zonal winds. This shift facilitates the propagation of wave trains, leading to atmospheric quasi–biweekly potential temperature anomalies (θ_a) over the TP through a multi-timescale interaction. Additionally, the TP's surface thermal forcing and arrival of wave trains trigger anomalous upward motion and increase cloud cover. The resultant decrease in net short-wave radiations and increase in net long-wave radiations contribute to variations in surface potential temperature (θ_s) over the TP. As θ_a and θ_s evolve, the difference between them enlarges, resulting in the generation of the SPV QBWO_TP. Given the relationship between the QBWO_TP and downstream rainfall, this study could provide novel insights into understanding and predicting downstream rainfall QBWO.