1. Long-term variability of the Kuroshio Transport East of Taiwan and the climate it conveys Mao-Lin Shen 1 E-mail: earnestshen@gmail.com Yu-Heng Tseng 1, Sen Jan 2, Chih-Chieh Young 1, Ming-Da Chiou 2 1 Atmospheric Sciences, Nation Taiwan University, Taiwan (R.O.C.) 2 Institute of Oceanography, Nation Taiwan University, Taiwan (R.O.C.) December 16, 2015 AOGS 2011 Taipei International Convention Center

2. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 2 Heat Exchange between Tropics and Extratropics [Daifang and Philander, 1994, Science] Kuroshio

3. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 3 North Equatorial Current-Mindanao Current-Kuroshio (NMK) Current System (b) 23 April, 1988 (a) 20 Sep, 1987 Fig. NEC bifurcation near the Philippine Coast. NEC flow poleward of this latitude turned north as the Kuroshio while flow to the south fed the Mindanao Current. [Toole et al., 1990, JPO]

4. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 4 ENSO and NEC [Qiu and Chen, 2010b, JPO]

5. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 5 2015/12/16 [Qiu and Chen, 2010b, JPO] North Equatorial Current bifurcation (NECB) Zonally distributed wind stress curl

6. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 6 Things we already known Why does the Kuroshio Transport on Luzon Strait and northeast of Taiwan present different patterns [Hwang and Kao, 2002, GJI] ?

7. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 7 AVISO SLP + MDT (cm) (1996~2001) NEC STCC

8. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 8 Possible Source of the KT [Vianna and Menezes, 2010, JGR] Subtropical Tropical ENSO West Pacific pattern STCC NEC

9. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 9 Questions will be answered How to identify and estimate the Kuroshio Transport reasonably? What is the origin/source of Kuroshio if Section PCM-1 is focused? (suggesting the possible climate variability conveyed by Kuroshio Transport) Is our hypothesis correct?

10. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 10 2015/12/16 Methods for obtaining the KT Geostrophic Relationship (GR) Considering the geostrophic balance is prevailing. Empirical Relationship (ER) The relation built by linear curve fitting. Transfer Function (TF) A spectral relationship of the KT and the associated SSH different.

11. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 11 Model Results The frequency ‐ dependent “transfer function” [Åström and Murray, 2008] estimates the linear causal relationship between any pair of variables. Geostrophic Balance Modeled SSH on day 180, Year 37 Taiwan Multi-scale Community Ocean Model (TIMCOM) (c)

12. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 12 Observation KT on PCM-1 Section (Johns et al., 2001) Geostrophic Balance AVISO SSH (c) (d)

13. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 13 Re-constructed KT Seasonal variation Interannual variation

14. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 14 KT vs. -WP KT vs. ENSO

15. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 15 Possible Source of the KT [Vianna and Menezes, 2010, JGR] Subtropical Tropical ENSO West Pacific pattern STCC NEC

16. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 16 Different KT Phases Huge NEC transport Limit NEC transport mean std

17. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 17 Conclusions The re-constructed KT obtained by GR and TF shows good agreement with the modeled ten-year KT. Using a longer period of observation data, the TF can provide a very good view on the relationship between the KT and the associated SSH difference and minimize uncertainty. The GR can be applied only when the focused period is longer than 4 months (low frequency) and is good to obtain the long-term KT variability.

18. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 18 Conclusions The annual-averaged KT on Section PCM-1 was controlled by the northern branch of the NEC and mesoscale eddies generated from the STCC while the NEC is strongly influenced by the ENSO events, and the mesoscale eddies are related with the WP pattern. Future works The heat transport from tropical to extratropical via KT have to be determined. (The knowledge of thermocline variation is necessary.)

19. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 19 2015/12/16 Thank you for your attention. Questions?

20. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 20 TF of modeled ten-years KT

21. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 21 Re-constructed KT

22. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 22 2015/12/16 STCC STCC, subtropical countercurrent Ekman convergence forcing, which associated with meridional temperature gradient and zonal geostrophic velocity [Qiu and Chen, 2010a, JPO]

23. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 23 2015/12/16 Fig. Comparison between the interannually-varying Ekman forcing term and the interannually-varying EKE time series in the STCC band. [Qiu and Chen, 2010a, JPO]

24. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 24 Can sea surface height be used to estimate oceanic transport variability? Ivchenko et al. (2011), GRL, 38, L11601. The relation between the sea surface height and the meridional transport across a zonal section at 26.5°N in the North Atlantic

25. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 25 DUPOM (North Pacific adaptation of TIMCOM)

26. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 26 2015/12/16 Numerical model Surface sources of heat and fresh water Levitus94 seasonal climatology Bathymetry unfiltered ETOPO-2 depth data supplemented with the Taiwan’s NCOR 1-minute high accuracy depth archive in the Asian Seas Winds stress monthly Hellerman and Rosenstein winds stress Vertical mixing Modified Richardson number dependent formula based on Pacanowski and Philander (1981)

27. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 27 2015/12/16 Model validation Fig. Comparison of SSH and the cross-section velocity on Section PCM-1. [Tseng et al., 2011, PO]

28. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 28 Fig. TIMCOM output and the schematic of the western North Pacific current system. Annual mean has been taken.

29. Dr. Mao-Lin Shen, Dept. of Atmospheric Sciences, NTU Page 29 PCM-1 NEC contributed Streamfunction of annual- averaged model results (Sv) (0-800 m) Ratio: NEC:PCM-1 1:2.1978