Multiscale Energy Conversion Characteristics of Two Extreme Wet-Cold Events in South China and Their Differential Impacts on Water Vapor Transport

Based on ERA5 reanalysis data, the multiscale window transform and the localized multiscale energy analysis are employed to investigate the multiscale energy conversion mechanisms and their differential impacts on persistent water vapor transport during two extreme wet-cold events in southern China, one spanning January to early February 2008 and the other occurring in January 2016. The results show that, from 25 January to 2 February 2008, water vapor transport was dominated by the low-frequency-scale (8–64 days). The various terms in the energy budget largely canceled each other out, so the low-frequency-scale circulation system was persistently maintained. The southern branch trough and the western Pacific subtropical high remained persistently anomalously strong over the mid- to low latitudes, so that a quasi-stationary water vapor transport corridor was established from the Indian Ocean and the western Pacific to southern China. The low-frequency-scale moisture convergence zone closely overlapped with the precipitation area, leading to persistent rainfall. During 20–22 January 2016, water vapor transport was jointly regulated by the low-frequency-scale and synoptic-scale (<8 days) processes. Within the low-frequency-scale moisture convergence zone east of the southern branch trough at low latitudes, the meridional wind and positive temperature perturbations were simultaneously maximized. Their alignment with the background meridional temperature gradient triggered baroclinic instability. Available potential energy was transferred from the background-scale to the low-frequency-scale, and the low-frequency-scale kinetic energy was subsequently enhanced through buoyancy conversion and work done by the pressure gradient force. This intensification strengthened the southwesterly water vapor transport ahead of the trough into southern China and provided a favorable low-frequency-scale background. Meanwhile, synoptic?scale disturbances at mid?latitudes were intensified by baroclinic instability and buoyancy conversion. The synoptic-scale high-pressure anomaly moved southeastward to the eastern coast and then stalled. Through this process, water vapor from the western Pacific, together with the Indian Ocean moisture that had been transported to southwestern China by the low-frequency-scale flow, was converged into South China. A sustained water vapor supply was achieved by the synergistic interaction of the two scales. Thus, persistent water vapor transport in extreme wet-cold events over southern China can be dominated not only by the low-frequency-scale circulation, but synoptic-scale disturbances can also play a key role under favorable background conditions.