Role of Large-Scale Circulation in Triggering Foehns in the Hokuriku District of Japan during Midsummer

摘要

The relationships between large-scale circulation and foehns observed during midsummer in Hokuriku district, located on the Japan Sea side of central Japan, are examined using Japanese long-term reanalysis project data, with additional data from the Japan Meteorological Agency climate data assimilation system. All foehn events are classified into two types: a tropical cyclone (TC)-induced foehn and an extratropical cyclone (EC)induced foehn. The occurrence of the TC type is attributed to a combination of a typhoon and its induced teleconnection pattern, the Pacific-Japan (PJ) pattern, in the lower troposphere. Local intensification of the North Pacific high just east of Japan, accompanied by the dominance of the PJ pattern, can in turn force a typhoon track to shift westward. The northward migration of the typhoon along the western periphery of the locally enhanced high strengthens a zonal pressure gradient across central Japan, thus producing a foehn. In contrast, an upper-level teleconnection along the Asian jet serves as a prominent trigger of the occurrence of an EC-type foehn. Stationary Rossby wave packets propagating eastward along the upper-level Asian waveguide facilitate not only the westward development of the North Pacific high but also the development of an extratropical cyclone in the vicinity of the Japan Sea by leading to the equatorward advection of higher potential vorticity from high latitudes. Both developments are crucial for the reinforcement of a northwest-southeast pressure gradient in the lower troposphere around Japan, thus providing a favorable condition for a foehn event.