Transient modulation of Kuroshio upper layer flow by directly impinging typhoon Morakot in east of Taiwan in 2009

This study deals with the modulation of the Kuroshio upper layer flow (KULF) in response to the passage of Typhoon Morakot in 2009, using Regional Oceanic Modeling System (ROMS) and in situ measurements from Argos drifters and Argo floats. The analysis of the simulated current fields near the typhoon track revealed an intermittency phenomenon of the KULF, which was almost shut down for at least 6 hours. The process begun 2 days prior to the approach of typhoon center due to blockage of the KULF by the steadily northerly winds, and lasted for more than 2 days, simultaneously shifting the Kuroshio main stream (KMS) path. When the Morakot gradually moved closely to the Kuroshio, the KMS shifted vertically from the surface layer to deeper layer of 50 - 100 m depth, and the maximum current speed in the KMS decreased from more than 1.3 m s(-1) to less than 1.1 m s(-1). When the Morakot center approached about 100 km to the original position of the Kuroshio, the KULF spread eastward for 1.5 degrees at 24 degrees N. When the Morakot center moved to the original position of the KMS, the Kuroshio abruptly rushed with a maximum speed near 1.4 m s(-1). Meanwhile, an offshore cool jet originating from southeastern tip of Taiwan was generated and extended northward along the Kuroshio. In the cool jet, the lowest temperature reached about 5 degrees C lower than the ambient waters. Modeled current variations and the cool jet during the Morakot passage were validated by in situ measurements.