The Interdecadal Variation of the Western Pacific Subtropical High as Measured by 500 hPa Eddy Geopotential Height

The interdecadal variation of the summer western Pacific subtropical high (WPSH) during 1948-2009 is investigated in this study. Compared with most previous works, which focused on the 500 hPa geopotential height, the interdecadal variation of the horizontal winds, relative vorticity, and eddy geopotential height over the western Pacific are all analyzed. The weakened anticyclone and decreased negative relative vorticity at middle-low levels over the western Pacific suggest that the WPSH weakened during 1979-2009 relative to 1948-78. After subtracting the zonal belt mean height between 0 degrees and 40 degrees N, the 500 hPa eddy geopotential height with significant negative anomalies over the western Pacific can correctly depict this weakened interdecadal variation of the WPSH. The illusory westward extension signal reflected by the 500 hPa geopotential height may derive from the significant increment of the geopotential height at middle and lower latitudes in the late 1970s under global warming.