1. Bimodal Behavior of the Seasonal Upwelling off the northeastern coast of Taiwan Yu-Lin Eda Chang Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan Chau-Ron Wu and Lie-Yauw Oey

2. Goal Explain differences in the seasonal upwelling in the water column NE Taiwan. Depth(m) 0 100 30 Time winter summer winter strong weak

3. Outline of presentation Introduction POM, data and results conclusions

4. Study Area East China Sea Taiwan China kuroshio Pacific Ocean

5. Introduction (A) Satellite sea surface temperature (B) Nitrate concentration (Gong et al. (1995)) Current velocity from Sb-ADCP with SST (Tang et al. (1999)) AUG/12/1994 The Kuroshio subsurface water intrude onto the shelf, bringing the nutrient rich water.

6. Previous study East China Sea Taiwan China kuroshio Gong et al. (1997) year-round Shift with Kuroshio Liu et al. (1992) year-round Tang et al. (2000) :only in summer, but not in winter wintersummer

7. observed nitrate+nitrite concentration Appears to contradict to the early finding…… summer winter and spring Hsu (2005) found that the upwelling intensified during winter and early spring base on nitrate and phosphate concentration of the euphotic zone Top 30m Station: 9, 10, 11

8. Hypothesis Upwelling near the surface is wind driven, and in the deeper layer the upwelling is forced by Kuroshio

9. Model Description- nesting model system East Asia Marginal Seas Model Resolution: 1/8 º ×1/8 º ×26 sigma levels Forcing: 6-hourly NCEP/QSCAT blend Wind Seas Around Taiwan Model Resolution: 1/20 º ×1/20 º ×26 sigma levels Forcing: 6-hourly NCEP/QSCAT blend Wind IC and BC: EAMS output Wu et al., (2008) Wu et al., (2005) Base on POM, (Mellor, 2004) EAMS SAT EAMSSAT 1980 200519992004 Time

10. Nutrient data with model 30 day-running mean vertical velocity The nitrogen concentration is higher during winter and early spring when the model shows intensified upwelling. Summer is low in NOx coincident with the modeled weakened upwelling. Different tendency may cause by wind Station: 9, 10, 11 Top 30m model NOx summer winter and spring

11. 30day-running mean modeled Vertical velocity Averaged over top 30m This seasonal tendency is consistent with the NOx data (Hsu, 2005) Model seems to support the wind driven hypothesis winter summer

12. Isolate wind effect, and extract only the important element Simple Ekman model: Where The continuity equation then gives the vertical velocity: (1) (2) (3) wind stress divergence wind stress curl

13. Strong negative in winter Positive in winter, but negative in summer. Negative in summer. Negative near surface and generate upwelling below 20m. summer winter Wind stress curl Summer winter Wind stress divergence

14. Liu et al. (1992) who pointed out that the upwelling is year- round, so wind driven alone cannot explain…… Gong et al. (1997) found that as the Kuroshio moves shoreward in winter, the center of upwelling also moves shoreward. In summer, the center of upwelling shifts seaward as the Kuroshio moves seaward. Wu et al. (2008) summer winter

15. Upwelling: year-round Top layer: stronger in winter, weaker in summer Deeper layer: stronger in summer, weaker in winter summer winter summer winter water depth (m) Bimodal behavior of different depths

16. Summary Observations suggest that in the surface 30m the upwelling is stronger in winter and early spring, but weaker in summer – in contrast to what was previously believed I use numerical and simple Ekman models to understand this behavior I conclude that there is a bimodal behavior with opposite seasonal tendencies of upwelling at different depths: (a) The near surface layer is forced by the wind, showing stronger upwelling in winter, but weaker in summer (b) At deeper levels(~100m), the upwelling is forced by the Kuroshio and shows all year round upwelling, but stronger in summer, and weaker in winter

17. Reference Gong, G.-C., Liu, K.-K., Pai, S.-C., 1995. Prediction of nitrate concentration from two end member mixing in the southern East China Sea. Continental Shelf Research, 15, 827-842. Gong, G. C., F. K., Shiah, K. K., Liu, and et al. (1997): Effect of Kuroshio intrusion on the chlorophyll distribution in the southern East China Sea north of Taiwan during Spring, 1993, Continental Shelf Res., 17, 79-94. Hsu, D. Y. (2005): The temporal-spatial variation of bacterioplankton in the upwelling region, East China Sea, M. S. dissertation, National Taiwan University, 50 pp. Liu, K. K., G. C., Gong, S. W., Lin, and et al. (1992): The year-round upwelling at the shelf break near the northern tip of Taiwan as evidenced by chemical hydrography. Terrestrial, Atmospheric and Oceanic Sciences, 3, 243-276 Tang, T.Y., Hsueh, Y., Yang, Y.J., Ma, J.C., 1999. Continental slope #ow northeast of Taiwan. Journal of Physical Oceanography 29, 1353}1362. Tang, T. Y., J. H. Tai, and Y. J., Yang (2000): The Flow Pattern north of Taiwan and the migration of the Kuroshio. Continental Shelf Res., 20, 349-371. Wu, C. -R. and Y. C. Hsin (2005): Volume transport through the Taiwan Strait : a numerical study. Terrestrial, Atmospheric and Oceanic Sciences, Vol. 16, No. 2 Wu, C. -R., H. -F. Lu, and S. -Y. Chao (2008): A numerical study on the formation of upwelling off northeast Taiwan, J. Geophy. Res., 113, C08025, doi:10.1029/ 2007JC004697.

18. Thank you very much

19. w wind w kuroshio Six year averaged over top 100m I assume that