Interannual-Interdecadal Variation in Large-Scale Atmospheric Circulation and Extremely Wet and Dry Summers in China/Japan during 1951-2000 Part I: Spatial Patterns

摘要

This is the first part of a two-part study that seeks links between summer rainfall variability in China/Japan and the large-scale circulation over the East Asia/western Pacific region, in both space and time, for the period of 1951-2000. Part I focuses on the spatial patterns, while Part II on the dominant timescales with which some extremely wet and dry summers in these two countries may have occurred. In this part, we use the singular value decomposition (SVD) method to find the dominant covariance patterns of summer rainfall anomalies over 160 stations in China and 72 stations in Japan, and the regional 500 hPa geopotential height anomaly over 60°E-160°W, 20°S-70°N. The associated 850 hPa horizontal wind patterns are studied by linear regression against the temporal coefficients of these modes. For a positive temporal coefficient, SVD1 represents a north-south wave pattern: a blocking high over eastern Siberia, a southwestward advanced and intensified subtropical high, and an elongated mid-latitude low in between. This mode mainly represents wet/dry trends over the 50-year period in the following regions: wetter summers in the Yangtze River valley and southwestern Japan, and drier summers in northern China and the Kinki area of Japan after the climate regime shift in the late 1970s. SVD2 and SVD3 are a pair of north-south wave patterns over the East Asia/western Pacific region with their phases in quadrature. The dominance of SVD2 or SVD3 implies a shift of the main rain band in China across the Yangtze River in the north-south direction, in association with very different rainfall patterns in Japan-either most of Japan experiences above- or below-normal rainfall in the same summer due to SVD2, or rainfall anomalies are opposite in sign between the Pacific side and the Sea of Japan side due to SVD3. These three dominant SVD spatial patterns, with their positive and negative phases, represent six dominant patterns of the western Pacific subtropical high and the mid-latitude wave systems in the regional large-scale circulation, in association with six main rain patterns in the two countries. The temporal behaviors of these modes will be further studied with wavelet and composite analyses in Part II.