University of Tsukuba finds interaction between tropics and mid-high latitudes brings an end to the rainy season in Japan

A research group including Professor Hiroaki Ueda and Graduate Student Ryouta Nakanishi of the Faculty of Life and Environmental Sciences at the University of Tsukuba, clarified the mechanism behind the convection jump phenomenon. In this phenomenon, the rainfall area over the ocean east of the Philippines suddenly expands to the northeast in late July, which occurs due to the influence of the mid-latitude upper troposphere. This was found by analyzing 20 cases of convection jump events that occurred in the past. The results showed the importance of the interaction between the tropics and mid and high latitudes in the formation of the summer climate and are expected to contribute to improving the accuracy of seasonal forecasts and understanding of the phenomenon. The results were published in the December 10 issue of the Journal of the Meteorological Society of Japan, an international journal published by the Meteorological Society of Japan.

A convection jump is a phenomenon in which an area of active cumulus convection activity (the Inter-Tropical Convergence Zone) suddenly expands northeastward in late July over the ocean east of the Philippines. This phenomenon brings about a dramatic end of the rainy season in Japan north of the Kanto region through teleconnections or linked atmospheric events. The mechanism of this development has been explained by a coupled atmospheric and oceanic action related to high sea-surface temperatures in the subtropical zone. Other effects, especially those of high-potential vorticity air masses flowing from mid and high latitudes to the subtropical zone, remained unknown. Potential vorticity is a physical quantity that indicates the direction and stability of the vortex of an air mass. This quantity increases as the absolute vorticity, which is a sum of planetary vorticity and relative vorticity, increases and as the stability of an air mass decreases.

In this study, the research group aimed to clarify the mechanism of inflow of high-potential-vorticity air masses in the upper troposphere from mid to high latitudes when a convection jump occurs and their effect on increased convection activity. The study selected 20 cases of confirmed convection jump events from atmospheric data over the past 48 years. Long-term reanalysis data incorporating observations and cold vortex indices were used in the analysis.

The results show that convection jumps triggered the following: westerly winds meandered significantly over the seas east of Japan; the breaking of Rossby waves, which reverses the north-south gradient of vorticity that normally is large at high latitudes, occurred; and a sustained and large-scale inflow of high-potential-vorticity air masses from mid to high latitudes occurred in the upper troposphere near the convection jump region.

In this research, to investigate the relationship between the inflow of high-potential-vorticity air masses and cumulus convection activity resulting from the occurrence of convection jumps, the researchers analyzed the potential vorticity balance and Q vector, a physical quantity that diagnoses the dynamic effect on the upward flow. They found that the sustained inflow of high-potential-vorticity air masses near a region where the convection jump occurs as a result of the breaking of Rossby waves creates a condition requiring a dynamic upflow, which contributes to strengthening and maintaining the upward flow associated with the occurrence of the convection jump. The inflow of high-potential-vorticity air masses was found to dynamically strengthen and maintain the upward flow and affect the activation of cumulus convective activity in the region where the convection jump occurred.

Ueda said, "The convection jump phenomenon was discovered in 1995, and the mechanism has been studied for 30 years. Following tropical and subtropical processes such as the prior emergence of warm water pools (1996) and their association with teleconnections (2009), the current study clarified the influence of high latitudes on tropical convection activity. This discovery provides a perspective that was initially unimaginable and reaffirms the depth and potential of research into climate dynamics."

Journal Information
Publication: Journal of the Meteorological Society of Japan
Title: Summertime Convection Jump over the Subtropical Western North Pacific and its Relation to Rossby Wave Breaking near the Asian Jet Exit Region
DOI: 10.2151/jmsj.2025-001