Observation of 2009 Typhoon Morakot induced excess freshwater pulse in Taiwan surrounding seas

Typhoon Morakot was a category 2 typhoon hit southern Taiwan and brought record-breaking rainfall, ~4 m at some spots, during 8-10 August 2009. The estimate volume of freshwater discharged into Taiwan Strait amounts up to 32 cubic km from the major rivers in Taiwan’s west coast. The excess freshwater pulse mixed with ambient seawater and was transported by Taiwan Strait current. Two joint hydrographic surveys using two research vessels each were conducted in the Taiwan Strait and the sea northeast of Taiwan roughly 5 days and 2.5 weeks after Morakot, which captured the unusual mixed water. During the first survey, a Ω-shaped diluted water plume with salinity ~1 psu less than climatology, 33-33.5, was found off northern tip of Taiwan. The diluted plume met the Kuroshio in the sea northeast of Taiwan forming strong salinity front and thus strong velocity vertical shear which may drag the Kuroshio landward onto the East China Sea shelf. The hydrography obtained in the second survey suggests that the diluted plume merged to and was carried by the Kuroshio towards northeast. The Kuroshio presented a meander like path northeast of Taiwan. A cold anomaly which is frequently called cold-dome appeared in the upper layer off northeastern Taiwan. The typhoon-caused excess freshwater pulse must induce considerable baroclinic effect along the coast and may affect current structure in coastal region. It may cause biological blooms in the upper layer due to its nutrient richness and the physical processes in the vertical. Although the record-breaking rainfall-caused freshwater pulse was a once every 50 years event, it may become more frequent hereafter due to global climate change.