A composite study of extratropical cyclones accompanied by split fronts in the Northwestern Pacific

The structure and environment of extratropical cyclones around East Asia that are accompanied by split fronts (SFs), hereafter referred to as SF cyclones, are examined using the Japanese 55‐year reanalysis dataset. The SFs are objectively and automatically detected with a thermal front parameter using equivalent potential temperature (). Previous statistical studies have shown that dry intrusion in the northern hemisphere occurs most frequently in winter; however, SFs occur more frequently in spring and autumn. In winter, SFs tend to occur at lower latitudes than in spring and autumn. A composite analysis in which the SF cyclones are simply superposed with respect to their centres indicates that the SFs tend to be located in the quadrants east and southeast of the cyclone centre, in accordance with previous case studies. For developing SF cyclones in the southern Northwestern Pacific (southern cyclone; SC) in autumn, a southward intrusion of low from the upper troposphere is the major contributor towards the formation of the SFs; for those in the northern Northwestern Pacific (northern cyclone; NC), a northward intrusion of low‐level high is the major contributor. For both SCs and NCs, high potential‐vorticity anomalies approach the cyclones from behind, suggesting that upper‐level troughs contribute to the formation of SFs by inducing ascent to the east and descent to the west of the upper‐level troughs.