Extended-Range Forecast for the Low-Frequency Oscillation of Temperature and Low-Temperature Weather over the Lower Reaches of the Yangtze River in Winter

Basing from observational data, this study analyzed the variations in ISO (intraseasonal oscillation) of the daily temperatures and its relationships to the low temperature in December–February of 1979/1980–2017/2018 over the lower reaches of the Yangtze River (LYR). The daily temperatures over the LYR in December–February are mainly of periodic oscillations of 15–25 d, 25–40 d, and 50–70 d, and the interannual variation in the intensity of its 25–40-day oscillation has a strong positive correlation with the number of low-temperature days in December–February. The real-time low-frequency components of daily temperature in the LYR and the principal components of 850-hPa low-frequency temperature in eastern Asia from 2001 to 2018 were used to establish the time-varying extended complex autoregressive (ECAR) model on an extended-range forecast of the 25–40-day low-frequency temperature over the LYR in winter. Real-time SSA (singular spectrum analysis) filtering with the T-EOF (temporal empirical orthogonal functions) extension can effectively inhibit the end effects of the traditional SSA and enhance the real-time signal of ISO. A 17-year independent real-time extended-range forecast was conducted on the extended-range forecast of the low-frequency component of the temperature over the LYR in December–February of 2001/2002–2017/2018. Experimental results show that the data-driven ECAR model has a good forecast skill at the lead time of approximately 26 days, and its forecast ability is superior to that of the traditional autoregressive model. Hence, the development and variation of the leading 25–40-day modes for the low-frequency temperatures at 850 hPa in eastern Asia and temporal evolutions of their relationships to the low-frequency components of the temperature over the LYR in winter supplied the valuable predicting background to determine the extended-range weather process in the persistent low temperature over the LYR at the 3–4-week leads.