Objective delineation of subseasonal-to-seasonal stages in the WNPSH annual cycle

Abstract

The western North Pacific subtropical high (WNPSH) is a widely recognized circulation system during the boreal summer. Despite the extensive knowledge about the WNPSH’s impacts on the summertime weather patterns, a complete picture of its annual evolution has been lacking so far. This study employs a data-driven classification technique, the Self-organizing Map, to systematically identify important stages over the WNPSH annual cycle. In good agreement with the existing literature, the derived WNPSH annual cycle comprises stages synchronizing well with the subseasonal‐to‐seasonal (S2S) variations of monsoonal precipitation and circulation patterns. The interaction between the WNPSH and the Aleutian Low plays a crucial role in shaping the annual cycle of the WNPSH. Examining the WNPSH's interannual variability unveils significant correlations between the length of the WNPSH stages and the strength of the East Asia Monsoon and ENSO. More specifically, the frequency of occurrence of the Winter WNPSH stage is associated with a concurrent La Niña episode, while the Mid-summer WNPSH stage is favored by the preceding La Niña condition. The result reveals that the frequency of occurrence of Mid-summer WNPSH stage is influence by atmospheric circulation induced preceding negative ENSO event, which is the complementary part of the Pacific–East Asian Teleconnection theory. The Mid-summer WNPSH stage unveils a higher frequency of tropical cyclone genesis than usual. The proposed approach to deriving WNPSH stages and onsets will be useful for operational diagnosis, subseasonal-to-seasonal forecasting and hazard preparedness, including cold surge, Meiyu and tropical cyclones.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Mengqian LU (Supervisor) & Jianping GAN (Supervisor)