Variations of the Baiu Frontal Activity in the Western North Pacific

In a region to the east of 125°E in the western North Pacific, the Baiu front is defined as a boundary between the two air masses, i.e., the Ogasawara maritime tropical (OMT) air mass in the subtropical Pacific high and the Okhotsk maritime polar (OMP) air mass to the north. The equivalent potential temperature (EPT), which is calculated from temperature and humidity, is suitable to express such air masses, and its meridional gradient determines the boundary, i.e., the Baiu front. This work defines the strength of the Baiu frontal activity (BFA) as the size of the meridional gradient of EPT and examines the variability during the Baiu season measured by the northward shift of the Baiu front from 30° to 40°N. The climatological change of the strength of BFA is controlled by the covariability of OMT and OMP air masses. In the early half of the Baiu season, a distance between the two air masses is small, which results in the strong BFA. In the later half, the northward shift of the northern OMP air mass is accelerated and that of the OMT one slows down, leading the weakening of BFA. The small meridional gradient of EPT is also suitable for a rapid northward shift of the Baiu front or a sudden end of the Baiu season. The interannual variability of BFA is controlled by a balance of the two air masses. The associated anomalies in EPT appear in and around Japan, indicating which air mass is predominant there. The dominant periods are 3-4 years, but the variability has no correlation with the El Nino/Southern Oscillation. The interannual variation also has insignificant correlation with that of the onset and closing dates of the Baiu season. Not only tropical but also extratropical variations, or the covariability of these two variations are significant for the interannual variability of BFA.