Extratropical extended-range precursors near the tropopause preceding persistent strong precipitation in South China: a climatology

Abstract

The pre-summer rainy season (PSRS; April–June) is the period of most frequent strong precipitation in South China (SC). Persistent strong precipitation events (PSPEs) can cause greater loss of life and property than short-lived strong precipitation. This paper investigates extended-range signals near the tropopause preceding two persistent positive climatological intraseasonal oscillations (PPCISOs) in precipitation corresponding to two climatological PSPEs in SC during the PSRS, using the CISO analysis of isentropic potential vorticity (IPV). It is found that precursor signals can be better seen in IPV CISOs than in unfiltered IPV. The first rainfall PPCISO is caused by an eastward propagation of high-IPV CISOs along the dynamical tropopause originating from the Arctic region and west of the Tibetan Plateau. The second is the most persistent (13 days). Its high-IPV CISOs originate over the Arctic region and the tropical monsoon region, respectively, 20 or more days prior to the rainfall CISO peak. The west of the Tibetan Plateau and the northern region of the East Asian westerly jet near the tropopause are two transit points where extratropical high-IPV CISOs strengthen and then change their propagation direction. The southward high-IPV CISOs and the northward positive IPV CISO caused by the monsoon begin to interact about 10 days prior to rainfall peak. Since then, the interaction leads a narrow meridional positive PV channel on the inclined 345 K isentropic surface. A specific CISO pattern, characterized by an elongated high-IPV CISO isolated by three negative IPV CISOs (the South Asian, the Okhotsk and the western Pacific subtropical highs), is responsible for the long duration of the second rainfall PPCISO.