Seasonal Prediction of the Variation of the Winter Cloudy Day Frequency in Eastern China Based on the Tropical and Ex-tropical Influence Routes

The temporal-spatial characteristics of the leading mode of winter cloudy day frequency (CDF) across eastern China are revealed via Empirical Orthogonal Function (EOF) analysis of daily cloud cover obtained from 1078 gauge stations in eastern China from 1961 to 2003. We identified the two influence routes of this leading mode, which we used to conduct a physical-motivated empirical model to the seasonal forecast of the winter CDF in eastern China. The results demonstrate that: (1) The first EOF mode of winter CDF explains 59% of the total variance, which is significant and independent of the other modes. This mode primarily demonstrates a homogenous spatial pattern across eastern China with dominating interannual variability. In the positive phase of this mode, a significant lower-level anticyclonic circulation anomaly occurs across the North Pacific. The anomalous southerly wind across the western flank of the anticyclonic could transport water vapor from the tropical ocean to eastern China, resulting in higher CDF. (2) the preceding persistent North Pacific dipole (NPD) pattern during August and September, and lowering of sea level pressure across midlatitude North Atlantic (LPA) from September–November are the two independent drivers for the formation and variation of this mode. The cold SSTA in the western pole of the NPD is advected southward to the tropical western Pacific using the anomalous northerly of the local low-level anomalous cyclone, forming the Bjerknes feedback, which maintains and accelerates the “cold west warm east” zonal SSTA dipole pattern in the tropical Pacific. This tropical Pacific zonal SSTA pattern stimulates zonal convection dipole, which induces a meridional atmospheric teleconnection in the North Pacific. The anomalous North Pacific anticyclones’ southerly is conducive to more CDF in eastern China. The LPA demonstrates the transition of a quasi-stationary Rossby wave train in mid-high latitudes Eurasia from autumn to winter. In winter, the southerly on the west of the barotropic anticyclonic anomaly across Northeast Asia, the terminal of the Rossby wave train, could result in increased CDF in eastern China. (3) Based on these two independent routes of physical mechanisms from both tropics and ex-tropics, a physics-motivated empirical model is conducted, which demonstrates potential independent prediction skill during the ten years of 2004–2013. The results are essential references for operational departments on seasonal prediction.