No abstract

The frequency and location distribution of tropical depressions (TDs) from 1979 to 2017 in the South China Sea (SCS) are statistically analyzed based on the best track data of tropical cyclones (TCs) from the Shanghai Typhoon Institute, China Meteorological Administration (CMA-STI). ECMWF interim reanalysis data (ERA-Interim) are used to investigate the reasons for the weakening of TDs in this study. The results show that there are 4.8 TDs formed in the SCS per year. And these TDs can be separated into 3.2 developing cases (DTDs) and 1.6 non-developing cases (NTDs) according to whether they intensify into tropical storms. Further objective classification by the multivariable-time empirical orthogonal function (MVT-EOF) method finds that the weakening cases in the positive-PC1 (the first principle component) mode occur in May-September, with the reason for weakening being a shortage of moisture. The decrease of westerly wind south of the NTDs reduces the water vapor transportation from Indian Ocean. Binary TCs in the northwestern Pacific acquire water vapor from the eastern boundary of the SCS NTDs. Meanwhile, the weak high-level divergence and low-level convergence are not enough for the accumulation of local moisture and maintaining local convections inside the NTDs. The weakening cases in negative-PC1 mode occur in October-December with the reason for weakening being the invasion of cold air from the north. Strong cold air advection in the lower troposphere increases the vertical wind shear in front of the NTDs, sharply reduce sensible and latent heat flux as well. Seasonal dependence exists in the causes of the SCS NTDs weakening.

Extratropical cyclones strongly impact the coupled atmosphere-ocean-sea ice environment in the Southern Ocean, playing a vital role in maintaining energy balance between the polar and tropical regions through the transport of moisture, heat, and momentum. These cyclones not only lead to severe weather events such as heavy precipitation, strong winds, and temperature extremes (Gobato & Heidari, 2020;Shaw et al., 2016) but also significantly impact the ocean system. For instance, extreme cyclones can lead to changes in the distribution of sea ice, as mentioned in recent studies (