83rd Annual

Wednesday, 12 February 2003: 2:30 PM
Biological responses of the sea to typhoons
I-I Lin, National Taiwan University, Taipei, Taiwan; and W. T. Liu
Poster PDF (587.0 kB)
Typhoon landfall is devastating to the economy and may cause human lives. Yet typhoon may also enhance other forms of life the primary biological productivity in the ocean. Documenting the biological response by ships with prescribed tracks and by buoys with fixed location is difficult because of the transient and severe nature of the typhoons. Two spaceborne microwave sensors, QuikSCAT and the Tropical Rain Measuring Mission Microwave Imager (TMI) allow us to measure ocean surface wind vectors and sea surface temperature (SST) under the cloud cover of typhoons, day and night, and the Sea Viewing Wide-Field-of-View Sensor (SeaWifs) shows us the ocean color, and the biological activities it denotes, after the typhoon passage.

In early July 2000, QuikSCAT observed typhoon Kai-Tak passing slowly through the northern part of South China Sea (SCS), which is a semi-enclosed marginal sea. TMI and SeaWifs showed a 9C cooling of SST and 100 time increases (from 0.1 to 10 mg/m3) in chlorophyll-a (indicating biological activities). The curl of wind stress computed from QuikSCAT showed that the cooling and biological bloom were preceded by the sharp increase in Ekman depth (reaching below the climatological nutricline) and Ekman pumping velocity. The Kai-Tak passage was found to generate 2-4% of the annual new production SCS. This observation of strong episodic nutrient injection through vertical mixing across the nutricline driven by Ekman pumping may help to resolve the old controversy arisen from the deficiency of the known mechanisms of nutrient supply in sustaining the amount new production in oligotropic oceans.

In August of 2000, super-typhoon Bilis left a cold wake in the deep water east of Taiwan as observed by TMI, but with no increase in biological activities near the surface as observed by SeaWifs. A weaker typhoon Prapirron followed Bilis a week later with observed increase in biological activities in the cold wake northeast of Taiwan. Climatological in situ measurements show that in the ocean east of Taiwan, the nutrients lie deep. They do not rise to the surface even with the passage of super-typhoon. However, in the water northeast of Taiwan, nutrients are near the surface and they rise to the surface even with the passage of a weak typhoon

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