The WNPSM is weak during the decay of an El Niño, whereas the ISM tends to abate when an El Niño develops.
The WNP monsoon has leading spectral peaks at 50 and 16 months, whereas the Indian monsoon displays a primary peak around 30 months. An anomalous WNPSM exhibits a prominent meridional coupling among the Australian high, cross-equatorial flows, WNP monsoon trough, WNP subtropical high, east Asian subtropical front, and Okhotsk high. So the WNPSM index also provides a measure for the east Asian summer monsoon. A weak WNPSM features suppressed convection along 10°–20°N and enhanced rainfall along the mei-yu/baiu front. Thus, two circulation indices are necessary, which measure the variability of the ISM and WNPSM, respectively. Letter “A” and “C” represent anticyclone and cyclone, respectivelyĪnalyses of 50-yr NCEP–NCAR reanalysis data reveal remarkably different interannual variability between the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) in their temporal–spatial structures, relationships to El Niño, and teleconnections with midlatitude circulations. The lower-level and upper-level circulation anomalies are denoted by solid and dashed line, respectively. Schematic diagrams showing the major circulation anomalies associated with (a) a strong Indian summer monsoon and (b) a strong western North Pacific summer monsoon. The dashed horizontal lines indicate correlation of 95% confidence level Lag correlation of the seasonal mean Niño-3.4 SST anomalies with reference to the IMI (solid) and WNPSMI (dashed) at the summer of year (0) for the period of (a) 1948–97, (b) 1958–76, and (c) 1979–97. The wind scale is displayed at the upper right of (a), (b), and (c) Shading denotes regions of difference at 95% confidence level. The contour interval for the height is 4 m. The rainfall data cover 1979–96 onlyĬomposite difference of (a) 200-hPa wind, (b) 500-hPa geopotential height, and (c) 850-hPa wind between strong and weak monsoon years with respect to the IMI. The contour interval is 1 mm day −1 for the rainfall and 0.2☌ for SST. The smooth solid and dashed curves are for the red noise spectrum and its 90% confidence levelĬomposite temporal evolution of the monthly mean (a) IMI and (b) WNPMI for strong and weak monsoons from the year before (−1) to the year after (+1) the summer monsoonĬomposite difference of (a) summer rainfall and (b) sea (land) surface temperature between the strong and weak monsoon years with respect to the IMI. Spectra of the monthly mean (a) IMI and (b) WNPMI for the period of 1948–97. 3 except for the western North Pacific–east Asian monsoon region and the WNPMI For comparison, the normalized IMI is plotted in (b) using bar charts The wind scales are displayed at the upper-right corners of (a) and (b). (a) The leading multivariate EOF mode and (b) time coefficient of 850-hPa winds in the south Asian monsoon region for summer of 1948–97. The solid boxes denote regions where the zonal winds are used to define the monsoon circulation indices (refer to the text for details) Shaded boxes indicate the regions for the rainfall indices, CI1 and CI2. Schematic diagram for the definition of the monsoon circulation indices, IMI and WNPMI. Intensities of the monthly mean maximum rain rate for the two centers are given in the plot at the upper-right corner The locations of the maximum rain rate are indicated by circled bold italic. The contours denote rain rate of 12 mm day −1. Annual march of the rain-rate centers over the Indian and the western Pacific Oceans.
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