Observations of a freshwater pulse induced by Typhoon Morakot off the northern coast of Taiwan in August 2009

Abstract

In this paper we describe large-scale impacts from a typhoon on the circulation over the continental shelf and slope north of Taiwan. Typhoon Morakot was a category 2 tropical storm that landed in central Taiwan, but caused destruction primarily in southern Taiwan from Aug. 8–10, 2009. The typhoon brought record-breaking rainfall; approximately 3 m accumulated over four days in southern Taiwan. River discharge on the west coast of Taiwan increased rapidly from Aug. 6–7 and peaked on Aug. 8, yielding a total volume 27.2 km3 of freshwater discharged off the west coast of Taiwan over five days (Aug. 6–10). The freshwater mixed with ambient seawater, and was carried primarily by the northeastward-flowing Taiwan Strait current to the sea off the northern coast of Taiwan. Two joint surveys each measured the hydrography and current velocity in the Taiwan Strait and off the northeastern coast of Taiwan roughly one week and two and a half weeks after Morakot. The first survey observed an Ω-shaped freshwater pulse off the northern tip of Taiwan, in which the salinity was ∼1 lower than the climatological mean salinity. The freshwater pulse met the Kuroshio and formed a density front off the northeastern coast of Taiwan. The hydrographic data obtained in the second survey suggested that the major freshwater pulse left the sea off the northern and northeastern coasts of Taiwan, which may have been carried by the Kuroshio to the northeast. Biogeochemical sampling conducted after Morakot suggested that the concentrations of nutrients in the upper ocean off the northern coast of Taiwan increased remarkably compared with their normal values. A typhoon-induced biological bloom is attributed to the inputs both from the nutrient-rich river runoff and upwelling of the subsurface Kuroshio water.