Connections between Two rare Freezing Rain Events over the Jianghuai Region and Anomalous Tropical Air-Sea conditions and Stratospheric Polar Vortex

Based on the ERA5 atmospheric reanalysis dataset, daily precipitation data from Climate Prediction Center (CPC) and extended reconstructed global monthly mean sea surface temperature (SST) from National Oceanic and Atmospheric Administration (NOAA) during 1981-2024, a comparative analysis of two rare freezing rain events occurred over the Jianghuai region during early 2010 and 2024 (hereafter Event 1 and Event 2) is conducted. The analysis focuses on the differences in the observed characteristic, atmospheric circulation patterns and temperature stratification associated with these two freezing rain events. The connections between the tropical ocean-atmospheric forcings and stratospheric polar vortex anomalies with these two events, as well as their possible effects are further explored. The results are as follows: (1) The range and duration of Event 2 were larger and longer than those of Event 1. These two freezing rain events occurred under similar circulation patterns, characterized by the convergence of cold air from the north, and the warm moisture transported by the southwestern flow in front of the trough and the periphery of the Western Pacific subtropical high. The stable and sustained transport of warm moisture was crucial for the establishment and maintenance of the melting layer and the inversion layer during both events. (2) The concurrent El Ni?o events in the tropical Pacific favored the establishment of a vertical temperature configuration with cooling in the middle levels and warming in the lower levels over the Jianghuai region. Meanwhile, the intensified western north Pacific anticyclone anomalies in lower levels enhanced moisture transport, providing favorable large-scale background conditions for these two freezing rain events. (3) The deep convection associated with the active Madden‐Julian oscillation (MJO) over the western Pacific triggered strong anomalous anticyclonic circulations over the northwestern Pacific, providing abundant moisture transport to the Jianghuai region for occurrence of the two freezing rain events. This facilitated the establishment and maintenance of the melting layer and inversion layer in the lower levels. (4) In the early stages of the two freezing rain events, stratospheric sudden warming (SSW) events resulted in the deformation and southward displacement of the stratospheric polar vortex to the Asia. This led the cold air outbreaks by modulating the mid-high latitude tropospheric circulation, strongly converged with warm and moist air over the Jianghuai region. As a result, a stable and sustained inversion structure was established, accompanied by intense convergence of low-level horizontal wind fields and upward motion, facilitating the occurrence of both freezing rain events. In general, during these two unexpectable freezing rain events over the Jianghuai region, the combined effects of tropical ocean-atmospheric forcings and the deformation of the stratospheric polar vortex played a significant role in the occurrence of these events. Therefore, MJO and SSW events should be considered for short-term predictions of freezing rain processes in the Jianghuai region.