Influence of Typhoon Matsa on Phytoplankton Chlorophyll-a off East China

Abstract

Typhoons can cause strong disturbance, mixing, and upwelling in the upper layer of the oceans. Rich nutrients from the subsurface layer can be brought to the euphotic layer, which will induce the phytoplankton to breed and grow rapidly. In this paper, we investigate the impact of an intense and fast moving tropical storm, Typhoon Matsa, on phytoplankton chlorophyll-a (Chl-a) concentration off East China. By using satellite remote sensing data, we analyze the changes of Chl-a concentration, Sea Surface Temperature (SST) and wind speed in the pre- and post-typhoon periods. We also give a preliminary discussion on the different responses of the Chl-a concentration between nearshore and offshore waters. In nearshore/coastal regions where nutrients are generally rich, the Chl-a maximum occurs usually at the surface or at the layer close to the surface. And, in offshore tropical oligotrophic oceans, the subsurface maxima of Chl-a exist usually in the stratified water column. In an offshore area east of Taiwan, the Chl-a concentration rose gradually in about two weeks after the typhoon. However, in a coastal area north of Taiwan high Chl-a concentration decreased sharply before landfall, rebounded quickly to some degree after landfall, and restored gradually to the pre-typhoon level in about two weeks. The Chl-a concentration presented a negative correlation with the wind speed in the nearshore area during the typhoon, which is opposite to the response in the offshore waters. The phenomena may be attributable to onshore advection of low Chl-a water, coastal downwelling and intensified mixing, which together bring pre-typhoon surface Chl-a downward in the coastal area. In the offshore area, the typhoon may trigger increase of Chl-a concentration through uptake of nutrients by typhoon-induced upwelling and entrainment mixing.

Fig 1. Map of the study domain and the two selected areas (nearshore: 26°N~28°N, 120.5°E~122.5°E, offshore: 22.5°N~24.5°N, 124.5°E~126.5°E).

The track and intensity (the maximum sustained wind (MSW) speed) of Typhoon Matsa are also shown.

Fig 2. Sea-surface wind from August 1 to 6, 2005 (arrows represent the wind directions).

Fig 3. Time series of sea-surface winds spatially averaged for the nearshore (open dots) and offshore (solid dots) boxes from July 29 to August 12, respectively.

Fig 4. SST from August 1 to 6, 2005.

Fig 5. SST in the nearshore (open dots) and offshore (solid dots) boxes from July 29 to August 12, 2005.

Fig 6. Sea surface Chl-a concentration during the pre- and post-typhoon periods.

Fig 7. Sea surface Chl-a concentrations in the nearshore area (open dots) and the offshore area (solid dots).

Chl-a data are not available for the offshore area on 4 August or 16 August, or in the offshore area on 20 August, 2005.

Fig 8. SST, Chl-a concentration and wind speed in the nearshore area during the typhoon.

Fig 9. Ekman pumping velocity (EPV, positive upward) in the offshore area during the typhoon.

Fig 10. Surface Ekman current and Ekman pumping velocity (EPV) from August 3~6, 2005.

Surface Ekman current vectors (arrows represent current speeds in m·s^–1 and their directions) and EPV (color shaded in 10⁻⁴ m·s^–1).

Fig 11. Coastal upwelling velocity for the nearshore box (positive upward), derived from Eq (4) by assuming a cross-shelf scale of 200 km.

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