This study investigates the responses of upper-level frontogenesis in the middle latitudes to the Arctic Stratospheric Polar Vortex (SPV) changes during winter, and explores the underlying physical mechanisms. Through diagnostics of upper-level frontogenesis function and identification of synoptic-scale upper-level fronts, the results reveal that during weak SPV years, upper-level frontogenesis intensifies significantly in the 30°–50° N latitude band, particularly over North America, the North Atlantic, and Northeast Asia, while decreasing between 50° and 60° N. The physical mechanisms involve three main processes. Firstly, SPV weakening is accompanied by planetary wave divergence in 30°–50° N, which accelerates the westerly jet and increases potential vorticity (PV) gradients in this region, corresponding to enhanced frontogenesis in this area. Conversely, planetary wave convergence around 50°–60° N weakens westerlies and reduces PV gradients, weakening upper-level frontogenesis there. Secondly, SPV weakening induces significant deepening of both the North American and East Asian troughs, enhancing cold air advection in the trough regions, which drives a shift of the thermally direct circulation toward the anticyclonic side in jet entrance areas. On the other hand, the intensified subsidence in the frontal warm sector promotes frontogenesis by enhancing isentropic surface tilting and atmospheric baroclinicity. Lastly, the accelerated westerly flow in middle latitudes (30°–50° N) intensifies geostrophic forcing in jet entrance regions. This enhanced forcing markedly strengthens the thermally direct circulation, further creating favorable conditions for upper-level frontogenesis.
Du et al. (Thu,) studied this question.