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The end of the rainy (Baiu) season in the Kanto region and northern Japan is induced by a "convection jump," known as the abrupt enhancement of convective activity (precipitation) over the northeast of the Philippines in late July. The intrusion of cyclonic unstable airmass (high-potential-vorticity (PV) airmass) from the upper-tropospheric mid-latitude modulates the convective activity over the subtropical western North Pacific, including the northeast of the Philippines. However, the influence of extratropical upper-tropospheric variations on the convection jump has not been fully discussed.

In a published in the Journal of the Meteorological Society of Japan. Ser. II, researchers at University of Tsukuba analyzed 20 typical convection jump years. When the convection jump occurs, the westerly winds meander largely over the east of Japan and the upper-tropospheric large-scale high-PV airmass intrudes around the occurrence region of the convection jump.

Using a method that can detect and track cyclonic circulation, the advection of the high-PV airmass is interpreted as the cutoff lows migrating from the upper-tropospheric planetary-scale trough. Furthermore, the intrusion of the high-PV airmass influences the enhancement of dynamical ascent and plays an encouraging role in the occurrence and maintenance of the convection jump.

These results show that the convection jump is influenced by the dynamical effects of the high-PV airmass intrusion from the upper-tropospheric mid-latitudes as well as the previously revealed coupled atmosphere-ocean system in the tropics related to the warm sea surface temperature.

Further investigation of the seasonal evolution from the rainy season to the mid-summer from the perspective of the interactions between the tropics and mid-latitudes will contribute to improving the accuracy of seasonal forecasts and our understanding of climate systems.