Employing composite analyses of downstream circulations associated with multiple tropical storm cases in May from 1948 to 2020, supplemented by a case study and previous numerical experiments, this research reveals that the circulation over East Asia and the Western Pacific evolved significantly in response to tropical cyclone (TC) activity and in tandem with the TC–extratropical flow interactions in the North Indian Ocean (NIO). Crucially, the research identifies that the most pronounced response occurred over Japan, Sakhalin Island and the adjacent eastern oceanic regions. Under strong baroclinic conditions, an extratropical storm accompanied by heavy precipitation is thus most likely to form in these locales, induced by a severe tropical storm in the NIO. A secondary maximum in the circulation response is observed across the region extending from eastern to northeastern China.
Furthermore, this study explores the interactions of TCs with extratropical flow and establishes a direct dynamical linkage between these interactions and downstream system development within the framework of downstream baroclinic development (DBD). The divergent (or ageostrophic) outflow of a TC is widely recognized as a pivotal structure mediating the TC–extratropical flow interactions. Through observational analyses, this study delineates the structure of the TC outflow and reveals how it evolves, being consistent with previous numerical experiments. Building upon these findings, we characterize the structure of the ageostrophic geopotential flux vectors associated with the TC. These fluxes are transported downstream by the outflow and subsequently converge within the jet immediately downstream of the TC, amplifying the downstream ridge. This ridge, in conjunction with the adjacent downstream trough it induces, constitutes a ridge–trough couplet that subsequently promotes further downstream development and, under certain conditions, cyclogenesis.
 

tropical cyclone,downstream development,, outflow,, TC–extratropical flow interactions,,ageostrophic geopotential flux