The North Pacific Subtropical High Characterized Separately for June, July, and August: Zonal Displacement Associated with Submonthly Variability

This paper describes the interannual and submonthly variability (disturbances with periods of less than 1 month) of the North Pacific subtropical high (NPSH) and the characteristics of its westward extension and eastward retreat in June, July, and August from 1979 to 2001. The study was based on 6-hourly data from the European Center for Medium-Range Weather Forecasts 40-Year Reanalysis with T106 resolution, Climate Prediction Center Merged Analysis of Precipitation data, and typhoon track data provided by the Japan Meteorological Agency. In the western Pacific, the interannual and submonthly variability of the NPSH were smallest in June and largest in August. To examine the characteristics of the westward extension and eastward retreat of the NPSH, an NPSH index was defined as the monthly mean anomaly of geopotential height Q at 850 hPa averaged over the western edge (125 degrees-150 degrees E, 17 degrees-32 degrees N) of the NPSH. Using this index, five extreme years of westward extension and five of eastward retreat were extracted to represent positive and negative years, respectively. Composite differences were calculated by subtracting the values of negative years from those of positive years. Composite analyses based on the NPSH index revealed several characteristics of the NPSH and its surroundings, including large-scale circulation, stationary Rossby waves and small-scale disturbances. In June and July, Baiu. frontal activity, including meso-alpha-scale disturbances, was stronger when the NPSH extended westward (in positive years). In positive (negative) years, most typhoons occurred in the western Pacific (western to mid-Pacific). The composite difference of submonthly variability of Z at 850 hPa. in August indicated a broad distribution of negative anomalies over the western Pacific; the large-scale horizontal structure of these anomalies was similar to that for the composite difference of monthly mean Z at 850 hPa. The NPSH index and submonthly variability of Z at 850 hPa in the index area were significantly negatively correlated.