Sen Jan and Chen-Tung Arthur Chen

2. 台灣-地球與海洋科學研究的樂園 規模6.2級的強震每年~3.6次! Internal tides induced vigorous temperature oscillations and potential biogeochemical effects in the southernmost coast of Taiwan (Nan Wan) 強勁的東北季風與沙塵暴! 北太平洋最強的海流-黑潮 年平均~3個颱風侵襲台灣! Sen Jan and Chen-Tung Arthur Chen Acknowledgements. National Science Council & Taiwan Power Company 巨大的內潮與內波!

3. Generation of Internal Tides Oscillated tidal currents Surface tide Stratified ocean p-q Shelf break q Shelf break Internal tides Bottom ridge w = ± N cosq N2 = -g/rodr/dz

4. SAR image 潮流 孤立內波 非線性內波 內潮

5. The largest internal waves in the World Ocean MODIS image South China Sea Sulu Sea SAR image (Liu et al., 2004) (http://earthobservatory.nasa.gov)

6. SPOT image Taiwan (http://www.csrsr.ncu.edu.tw/chin.ver/c5query/c5query.php)

7. 領航鯨 (Lien, 2007)

8. 150公尺的起伏 (Lien, 2007)

9. Potential Biological effects brought by internal waves/tides 4-d after spring tide (Moore and Lien, 2007. MMS) (Wang, Dai and Chen, 2007. GRL)

12. T, sea levels, and winds observations • Large daily T drops are easily seen: 4-10ºC in summer; 1-2ºC in winter. • The T drops help cap the summertime mean temperature at 27.6oC. • The daily T drops are closely correlated to tides and are likely not correlated to daily sea breeze. • Strong southwest winds, particularly after typhoons, may enhance T drop. Depth=20 m Near bottom T in the western coast 72 hr low pass filtered Sea levels in the western coast Wind sticks at a buoy weather station in eastern Bay

13. High-passed T and raw tidal sea levels in summer and winter Summer Spring Spring Neap Neap 2004 Winter • Spring-neap and seasonal variations of T drop are clear. • The minimum T of each drop normally occurs at ~2 h after a lower low water. Spring Neap Spring Neap 2004

14. T, S, NO3- and DNCN distributions in Nanwan Upper 25 m thick layer Station 5 L H H Degree of nutrients consumption (Feb 2001) (Chen, Hsing, and Wang, 2004. MC) (Chen et al., 2005. MEPS)

15. Internal tides? Nutrients pumping?

16. What is the driving mechanism for the upwelling? • Based on model results, Lee et al. (1997; 1999a; 1999b) attributed the upwelling to swift tidal current induced eddies in the Bay and the upwelling causes strong T drops. The center of the small-scale eddy is always upwelling. v>0 L ▽P CF Top view v<0 • The scenario of local eddy induced T drop is not convincing. Ebb flow ~100 cm/s not shown in observations Flood flow ? ? 6 h lag (Lee et al., 1999b)

17. To evaluate the driving mechanism for T drops • To estimate nutrients fluxes induced by internal tides Initial (T, S) profiles Horizontal resolution 1/12o, Vertical 51 s-layers (433x493x51 grids) Pycnocline depth is summer < winter

18. Isopycnal vertical displacement relative to 100 m depth layer propagation propagation generation

19. The timing of T-drops agrees with the observation and value of T decrease accounts for 80% of that of observation Model [Summer] Observation [Summer] Model-rpoduced T at the closest grid to Nanwan Bay account for 80% of the temperature drops Model [Winter] Observation [Winter] T drop in winter is not as clear as that in summer Days

20. Internal tide is the primary cause for the T drops Local or remote generation? The rate of barotropic to baroclinic energy conversion and baroclinic energy flux suggest the east flank of Heng-Chun Ridge is the most possible generation site

21. Model run without west ridge (Heng-Chun Ridge) No significant temperature drops. The 5-min resolution is not fine enough to resolve topography in the bay… The generation of internal tides over submarine ridges or continental slopes depends on [Baines, 1982] α bottom slope characteristic ray path of internal waves s2-f2 [g =] γ N2-s2 α/γ ~ 0.1 in Nanwan Bay: indicates a sub-critical condition of internal tide generation. α/γ ~1 in the Heng-chun Ridge zone: internal tides produced outside the bay must be stronger than those in the bay.

22. Nitrogen and phosphate fluxes in each internal tide-induced upwelling phase NO3- flux is 2.4–22.9 mmol N m-2h-1 PO43- flux is 0.4–1.75 mmol P m-2h-1 Total upward amount of nitrogen or phosphate in 6 h is estimated as Q = flux × Area × 6 h radius: 2 km vertical excursion:30 m from 50-80 m depths duration ~6 h which are as much as ~40 times larger than inputs to near-shore waters from waste water and storm water runoff [Leichter et al., 2003].

23. Concluding remarks • Regular temperature drops ranging between 2–10°C in a few hours are normally observed in Nanwan Bay, helping maintain the annual mean temperature at around 26°C. • The associated upwelling may pump significant amounts of nutrient at rates of 2.4–22.9 mmol N m–2 h–1 for NO3− and 0.4 – 1.75 mmol P m–2 h–1 for PO43− during the upward phase in an internal tide, which are ~40 times greater than those discharged into coastal waters from storm-rain runoff and wastewater. • The observed regular temperature drops and nutrients enrichment are primarily caused by internal tide-induced upwelling. • The internal tides propagating to the coast are remotely generated on the eastern flank of Heng-Chun Ridge rather than locally in the bay. • The seasonal variation of the pycnocline in the surrounding waters, which is stronger and shallower during summer than winter, results in relatively larger temperature decreases in the summer.

24. Coupled Physical & Biogeochemical Model u, v, w, t, s, Kz (Liu, Chen, Tseng, Lin and Snidvongs, 2007. DSR II)

25. Nutrient enrichment in shallow and deep seas Around Dongsha Atoll Deep region Continental shelf & island reef topography may be important to the increase of primary production in the SCS. Mean: 87.92 mg C/m2-day Mean: 17.07 mg C/m2-day