Yushan is the most famous location for snow in Taiwan, while snowfall in the subtropical zone is rare. When it is snowing in Yushan, people are experiencing unusually cold and wet weather elsewhere in Taiwan. In this study, Yushan snowfall activity from 1979 to 2009 and the related atmosphere circulation were examined with the Taiwan Central Weather Bureau's Yushan weather station observations and the National Centers for Environmental Prediction/Department of Energy (NCEP/DOE) reanalysis atmospheric data.
To provide a quantitative measure of snowfall events, a snowfall activity index (SAI) was defined in this study. The time series of yearly SAIs shows that Yushan snowfall activity for an active year, such as 1983 (SAI=39 153) was similar to 118 times larger than for an inactive year, such as 1999 (SAI=331).
Our analyses show that snowfall activity is closely related to the position of the East Asian Trough (EAT) and the strength of the West Pacific High (WPH). In active years, when the EAT shifted eastward and the strength of WPH increased, an anomalous anticyclone occurred in the West Pacific. This anticyclone introduced anomalous southwesterly flows along the southeastern cost of mainland China and over Taiwan, resulting in a wetter-than-normal atmosphere that favored snowfall. Alternatively, in inactive years, a drier-than-normal atmosphere resulted in sluggish snowfall seasons.
