Two phytoplankton blooms near Strait generated by Lingering Typhoon Parma

Hui Zhao1, Guoqi Han2, Shuwen Zhang1, and Dongxiao Wang3 1College of Ocean and Meteorology, Guangdong Ocean University, Zhanjiang 524088, 2 Northwest Atlantic Fisheries Centre, Fisheries and Oceans Canada, St. John’s, NL, Canada 3. State Key Laboratory of Tropical Oceanography(LTO),SCSIO, CAS

Introduction A. WS,EKV, Sep 15-30, 2009 B. WS, EKV, OCT 4-5, 2009 Typhoons or tropical cyclones occur frequently in the South China Sea (SCS), over 7 times 22N annually on average. Due to limit of nutrients, cyclones and typhoons have important effects on chlorophyll-a (Chl-a) and phytoplankton blooms in oligotrophic ocean waters of the SCS. 20N Typhoons in the region, with different translation speeds and intensities, exert diverse impacts

on intensity and area of phytoplankton blooms. However, role of longer-lingering weak cyclones 18N played phytoplankton biomass was seldom investigated in SCS. Parma was one slow-moving and relatively weak (≤Ca. 1), while lingering near the 16N northern Luzon Island for about 7 days in an area of 3°by 3°(Fig. 1). This kind of long lingering typhoons are rather infrequent in the SCS, and their influences on phytoplankton 118E 120E122E 124E 118E 120E122E 124E Fig. 2 Surface Wind Vectors (m s-1 , respectively) and Ekman Pumping blooms have seldom been evaluated. In this work, we investigate two phytoplankton blooms Velocity (EPV) (color shaded in 10-4 m s-1). (A) Before Typhoon; (B). During (one offshore and the other nearshore) north of Luzon Island and the impacts of typhoon’s Fig 1 Track and intensity of Typhoon Parma (2009) in the Study area. The 3 asterisks, Typhoon. Red Box: a sampling region of EPV (119-120E,19.5-20.5N); Black box in Fig 2: a sampling region of wind speed ( 119-121E, 19-21N). translation speed and intensity during Parma, using satellite observations and in-situ data. CTD stations.

Results and Discussion

A. Comparison of Ocean Conditions before/after the Typhoon Intrusion B. Possible Factors inducing phytoplankton blooms

A. CDOM ‹Storm-induced rainfall a. 15 -30 Sep, 2009 b.08-15Oct, 2009 c. 16-23 Oct, 2009 22N accumulation of 662 mm over the Fig 3 Images (a) before Typhoon Parma northern Luzon Island during 20N and (b and c) after typhoon Parma. (A) Parma’s intrusion. Color dissolved organic material (CDOM) 18N (unitless) and (B) Chlorophyll-a (Chl-a) -3 ‹Cumulative vertical displacement 16N (mg m ). The Offshore box (119-120.5E, 19.8-20.9N) and the nearshore box (120- of ~47.5m based on the area- B. Chl‐a 122.3E, 18.3-19.3N) in Fig 3Bb are 22N a. 15 -30 Sep, 2009 b. 08-15Oct, 2009 c. 16-23 Oct, 2009 averaged EPV . sampling regions of Chl-a and CDOM. 20N ‹Significant reverse direction after the typhoon intrusion 18N ‹ Succession of Chl-a and CDOM during the typhoon:normal, 16N increase and decay. 118E 120E122E 124E 118E 120E122E 124E118E 120E122E 124E Fig 5. Time series of Ocean conditions for the chosen (A) Rainfall a. 15 -30 Sep, 2009 b. 02-09Oct, 2009 (mm/day) 22N regions/location. See Fig. 2 and 4 for the sampling 20N Fig 4. (A) TRMM rainfall (mm d-1); (B) locations. GHRSST L4 RSS MW IR OI SST; (C) 18N Absolute geostrophic current (GC) (m s- 1). (a) before Typhoon Parma and (b) 16N during/after Typhoon Parma. The box in (A): Black boxes in (A-B) delineates Fig. 6 Time series of Chl-a and CDOM. (A, (B) SST a. 15 -30 Sep, 2009 b. 04-12Oct, 2009 22N sampling regions of TRMM rainfall (16- C): the offshore box in Fig. 3Bb; and (B, D) 19N, 120-122E) and SST (118.5- the nearshore box in Fig. 3Bb. Note: the 20N 120.5E,18.5-21N) , respectively; Asterisk in (C) is the sampling point of the GC blue shade is unreliable due to their 18N (8.8N, 121.6E). sparse distribution.

25SEP 25SEP 16N ‹Offshore Chl-a bloom appears ‹ Evident changes of rainfall, SST (C) GC a. Sep 16-30, 2009 b. OCT 7-14, 2009 immediately after the typhoon 22N and Geostrophic current occur in intrusion. The maximum of CDOM the regions during pre-/post- lag one week after the bloom. 20N typhoon intrusion. ‹Simmilarly, the maximum of the 18N nearshore CDOM appears two 0.8m/s 0.8m/s P week after the typhoon. E P 16N S E 525SEP S 2 2525SEP 118E 120E122E 124E118E 120E122E 124E

Which of wind-induced mixing and upwelling plays more important role in the offshore bloom? Fig. 7 Vertical profiles of (A) „Nitrate indicates low nutrients in the upper 50-m layer, suggesting mixing shallower than 50 m may exert less the climatological nitrates in influence on nutrients increase of upper layer. September, (B) temperature ( C), (C) salinity (psu) and (D) „Mixed layer depth reaches only 50m after the typhoon intrusion. Density (σt). „As implied in Fig 7, Wind-induced vertical displacement is about 50m. Probably, wind-induced upwelling may play more important role in the offshore bloom.

Conclusions Two typhoon-induced phytoplankton blooms were observed near Luzon Strait where Parma was during the weak-intensity and slow-moving stage: one offshore west of the Strait and other nearshore north of Luzon Island. The CDOM maxima in two bloom areas lagged the Chl-a maxima by ~8 days. The offshore bloom was likely triggered by wind-induced upwelling of nutrients and mixing entrainment in view of Parma’s longer lingering time, with the former playing a more important role. The nearshore bloom was likely due to the increased discharge from the River Estuary supported by typhoon-induced strong precipitation and favorable variation of current directions during the typhoon. References 1. S Babin，T. D. Dickey, et al. J. Geophys. Res., 109, C03043. (2004) 3. I. Lin. J. Geophys. Res., 117, C03039. (2012) 2. I. Lin, W. Liu, et al. Geophys. Res. Lett., 30(13), 1718, (2003). 4. H Zhao , D Tang, et al. Mar. Ecol. Prog. Ser., 365:57-65. (2008).

Acknowledgments The present research was supported by the National Natural Science Foundation of China (project number: 41006070), and the Canadian Space Agency Government Related Initiative Program.