Abstract 199

Abstract ID: 199

Intraseasonal oscillations and the subseasonal peak rainfall event in the eastern Philippines during 2017/18 winter and S2S prediction evaluation

Lead Author: Wayne Yuan-Huai Tsai
Department of Atmospheric Sciences, National Taiwan University, Taiwan

Keywords: S2S prediction, the Philippines’ climate, Subseasonal rainfall extremes, MJO, Equatorial Rossby Waves

Abstract: From December 13, 2017 to January 2, 2018, three tropical cyclones (TCs) successively made landfall in Mindanao and led to the most extreme wintertime (December – February) subseasonal peak rainfall event (SPRE) over the eastern Philippines (9°-14°N, 122°-127°E). The SPRE is defined by the maximum 15-day accumulated rainfall amount within the region during a time span of 90 days. The 15-day rainfall extremes over the eastern Philippines are associated with the environmental low-level cyclonic vorticity, which can be contributed to La Niña condition, Madden-Julian Oscillation (MJO), and equatorial Rossby (ER) waves. During the 15-day period of 2017/18 winter SPRE, the cyclonic vorticity phase of the westward propagation ER waves from the North Pacific occurred twice and the second episode occurred simultaneously with an MJO from the Indian Ocean across the Maritime Continent to the western Pacific. Favored by anomalous cyclonic vorticity and humidity produced by La Niña and MJO, the two ER waves enhanced and resulted in two TCs and led to the extreme SPRE. Based on the hindcast data in the S2S database, the 2017/18 SPRE and associated MJO and ER waves in ECMWF and NCEP models show skillful forecasts up to the extended range (11-day lead). For the SPRE in 2017/18 winter, the ECMWF model performs better than the model’s hindcast skill, whereas the NCEP model performs worse. The model with a better representation of SPRE timing and intensity has smaller phase and amplitude biases of tropical waves in the medium (5-10-day lead) and extended ranges. The forecast skill of the MJO and ER linked to ENSO phases will be discussed. The analysis procedure proposed in this study can be applied to monitor and predict the SPREs and their associated large-scale drivers in other regions.

Mong-Ming Lu (Department of Atmospheric Sciences, National Taiwan University)
Yin-Min Cho (Department of Atmospheric Sciences, National Taiwan University)
Chung-Hsiung Sui (Department of Atmospheric Sciences, National Taiwan University)