Temporal and spatial variation of the transitional climate zone in summer during 1961–2018

Abstract

© 2020 Royal Meteorological Society The transitional climate zone (TCZ) over East Asia is an interface area between monsoon circulation and westerly circulation, making it highly susceptible to natural disasters and climate change. This study explores the spatial–temporal variation of the summer (May–September) TCZ during 1961–2018 and its response to monsoon rainy season. The summer Aridity Index (AI) instead of annual amount is used to recognize summer TCZ, with similar domain produced. However, differing from the annual total, the summer evaporation remains stable during the whole period, despite global warming. Hence, the summer TCZ has experienced little significant long-term trends toward wetter or drier, since it is predominantly driven by precipitation that shows no clear tendency. The EOF decomposition reveals three key patterns accounting for 58.2% of the total variance: spatially uniform, south–north dipole and meridional tripole, which shows the complexity of spatial-temporal variation in TCZ. TCZ is characterized by interdecadal mutations and periodical oscillations of 3 or 8 years. There exist remarkable 3 years interannual periods, with the greatest contribution stemming from the south-central part of TCZ. In comparison, the northeast part exhibits a significant interdecadal band of about 8 years, which promotes the regime shift from wet to dry in the late 1990s over TCZ. Furthermore, the onset, retreat, and duration of the rainy season of TCZ in response to summer monsoon are examined, based on EEMD filtered 4 mm/day precipitation. All three indicators are subjected to strong interannual dispersion, without any significant overall trends. Only the retreat date of the western part of TCZ (100–110°E) is significantly correlated with the onset date. Dry-wet condition in TCZ is more closely associated with the duration length of the monsoon influence. And we also notice that the westmost part of TCZ shows a greater sensitivity to monsoon activity than the east part.