Tropical cyclone influence on the long-term variability of Philippine summer monsoon onset

Hisayuki Kubota,Ryuichi Shirooka,Jun Matsumoto,Esperanza O. Cayanan,Flaviana D. Hilario

doi:10.1186/s40645-017-0138-5

Abstract

The long-term variability of Philippine summer monsoon onset from 1903 to 2013 was investigated. The onset date is defined by daily rainfall data at eight stations in the northwestern Philippines. Summer monsoons tended to start earlier in May after the mid-1990s. Other early onset periods were found during the 1900s, 1920s, and 1930s, and an interdecadal variability of summer monsoon onset was identified. Independent surface wind data observed by ships in the South China Sea (SCS) revealed prevailing westerly wind in May during the early monsoon onset period. To identify atmospheric structures that trigger Philippine summer monsoon onset, we focused on the year 2013, conducting intensive upper-air observations. Tropical cyclone (TC) Yagi traveled northward in the Philippine Sea (PS) in 2013 and triggered the Philippine monsoon onset by intensifying moist low-level southwesterly wind in the southwestern Philippines and intensifying low-level southerly wind after the monsoon onset in the northwestern Philippines. The influence of TC was analyzed by the probability of the existence of TC in the PS and the SCS since 1951, which was found to be significantly correlated with the Philippine summer monsoon onset date. After the mid-1990s, early monsoon onset was influenced by active TC formation in the PS and the SCS. However, the role of TC activity decreased during the late summer monsoon periods. In general, it was found that TC activity in the PS and the SCS plays a key role in initiating Philippine summer monsoon onset.

Highlights

The Philippines is an archipelago located in the western rim of the western tropical Pacific
Rainfall at the eastern Philippine stations increased in October from the northern station of Aparri, propagating southward in November and reaching a maximum in January at the southern station of Hinatuan, associated with the winter monsoon (Fig. 3c, d) of the winter monsoon occurred in December in northern Luzon Island and tended to be delayed moving southward
We focus on the change in atmospheric structure during Philippine summer monsoon onset in 2013 using intensive upper-air observations, which are included in the reanalysis data

Summary

Introduction

The Philippines is an archipelago located in the western rim of the western tropical Pacific. There are distinct summer monsoons (local name Habagat) on the western side and winter monsoons (local name Amihan) on the eastern side of the country associated with the seasonal shift of major wind direction (Flores and Balagot 1969). The seasonal change of wind direction characterizes the climate phenomenon referred to as a “monsoon” (Ramage 1971). The thermal contrast between land and ocean contributes to this seasonal variation of winds in Asia (Murakami and Matsumoto 1994). Seasonal wind (monsoon) is closely linked to the seasonal variation of rainfall; namely, the Asian summer monsoon system is characterized by seasonal wind and rainfall. The Asian summer monsoon has several subsystems. Over the oceanic region of the PS including the Philippine region, the western North Pacific (WNP) summer monsoon dominates (Murakami and Matsumoto 1994)

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