Interannual Variability of the Wintertime Northern Branch High Ridge in the Subtropical Westerlies and Its Relationship with Winter Climate in China

The high ridge in the northern branch of the subtropical westerly (hereafter referred to as northern branch ridge) extends from the north of the Tibetan Plateau to the north of Barr Kashmir Lake (32.5-47.5 N, 70-95 E) during wintertime. The intensity index and zonal position index of the wintertime northern branch ridge (WNBR) are defined in this study by using the NCEP-NCAR reanalysis data and precipitation and temperature observations at 160 stations in China. Interannual variation features of the WNBR and its relation with precipitation and surface air temperature in China are discussed based on wavelet analysis, correlation analysis, and composite analysis. The results indicate that the intensity and zonal position of the WNBR exhibit significant interannual variability. The intensity of the WNBR gradually weakens and varies in periodic cycles of 4-6, quasi-2, and quasi-16 yr. Its zonal position shifts westward slightly and varies in periodic cycles of quasi-8 and quasi-16 yr. The WNBR is highly correlated with precipitation and surface air temperature in China. When the WNBR is strong (weak) and its zonal position shifts eastward (westward), winter precipitation in Northeast China and the northern region of Northwest China increases (decreases), whereas precipitation decreases in central China, South China, and eastern regions of Northwest and Southwest China. Meanwhile, surface air temperatures in most areas of China are higher (lower) than normal. Further investigation has revealed that the two indexes are closely related to variations in general atmospheric circulation, which explains why there exists a close linkage between the variation of the WNBR and climate in China. It is believed that the WNBR is also tied to the Rossby wave, the North Atlantic Oscillation, the East Asian trough, and the North Pacific jet stream.