The formation and dynamics of cold-dome northeast of Taiwan

1. The formation and dynamics of cold-dome northeast of Taiwan Mao-Lin Shen1 E-mail: earnestshen@gmail.com Yu-Heng Tseng1 Sen Jan2 1Atmospheric Sciences, Nation Taiwan University, Taiwan (R.O.C.) 2Institute of Oceanography, Nation Taiwan University, Taiwan (R.O.C.) October 26, 2010

2. Outline • Overview of the documented cold-dome and suggested formation mechanisms • Temporal SST variation during 2008-2009 and Argo data northeast of Taiwan • Numerical model and the associated results • Analysis of formation mechanisms • Conclusion

3. Introduction (1/5) Sea Surface Temperature (SST) November 7 2009

4. Introduction (2/5) An active upwelling and nutrient-rich area The exchange of Kuroshio Water and Continental Water of East China Sea (Isobe, 2008; Matsuno et al., 2009) Fundamental characteristics have been well-documented by Gong et al. (1992), Lin et al. (1992), Tang and Tang (1994), Chen et al. (1995) and Tang et al. (1999), etc. 2014/9/11

5. Kuroshio Surface Water Kuroshio Tropical Water Introduction (3/5) Cheng, Ho et al., 2009, Sensors Chen, 1995

6. Liu et al., 1992 Suggested the year round upwelling should be 5 m/day. Kuroshio Tropical Water Hsueh, Wang, Chern, 1992, JGR. Stated how the baroclinic transport work in this region. Chen et al., 1995 Contribution of Kuroshio Water Introduction (4/5)

7. Introduction (5/5) • What contribute to the formation mechanisms of cold-dome ? • Which one dominates the formation process? • What kind of connections between cold-dome and the surrounding currents, e.g. Kuroshio and Taiwan Strait flow?

8. Observation (1/3)─ MW and IR merged SST Area for meridional averaging Hovmoller plot Jul 30 2008 Typhoon Fung-Wongpassed on Jul 28. May 16 2008 Nov 7 2009 May 2 2009 2014/9/11

9. Time-longitude plot of filtered SST north of Taiwan. the dashed lines denote typhoons from left to right is Kalmaegi, Fung-Wong, Sinlaku and Jangmi, respectively. Of 2009 the dashed line denotes typhoon Morakot. 2014/9/11

10. Observation (3/3) ─ cold-dome in winter Argo float, WMOID 2900797, for (a) the trajectory; (b) MW_IR SST and a marker denotes the Argo data on December 16, 2008. The rests are subsurface comparisons of (c) temperature and (d) salinity. 2014/9/11

11. DUPOM (North Pacific adaptation of TIMCOM)

12. Numerical model (1/2) • 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)

13. Numerical model (2/2) ─ Domain

14. Numerical results (1/4) Z = 6 m Z = 54 m On day 157, Year 37 Model results Z = 98 m Z = 75 m 2014/9/11

15. Numerical results (2/4) ─ Trajectories Kuroshio Tropical Water (KTW), 150-250 m.

16. Mien-Hua Canyon North Mien-Hua Canyon Numerical results (3/4) ─ Trajectory A trajectory shows the route of Kuroshio Tropical Water. The background flow field are model results at z = 159 m.

18. Mechanism Analysis (1/10) • Possible mechanisms • Wind-driven Ekman upwelling • Boundary layer effect • Current-driven Ekman upwelling • Ekman boundary mixing • Dynamic uplift due to geostrophic adjustment • mesoscale eddy • Kuroshio • Topographically controlled upwelling • Vertical mixing

19. Garrett et al. (1993), ARF. Boundary mixing Mechanism Analysis (2/10) • Wind-driven Ekman upwelling • Boundary layer effect • Current-driven Ekman upwelling • Ekman boundary mixing • Dynamic uplift: • mesoscale eddy • Kuroshio • Topographical upwelling • Vertical mixing

20. Mechanism Analysis (3/10) • Wind-driven Ekman upwelling • Boundary layer effect • Current-driven Ekman upwelling • Ekman boundary mixing • Dynamic uplift: • cyclonic eddy • Kuroshio • Topographical upwelling • Vertical mixing

21. Mechanism Analysis ─ Wind-driven upwelling Wind-driven Ekman upwelling , : the thickness of Ekman layer Chang, Wu and Oey, 2009 Mean: -0.3 m/day Max: 0.7 m/day (7) • Needs 4~5 months to uplift 100 m with maximum upwelling velocities Wind-driven Ekman upwelling 2014/9/11

22. Mechanism Analysis ─ Boundary layer effects Boundary layer effect Current-driven Ekman upwelling Max: only about 0.00002 m/day Ekman boundary mixing W (m/day) Garrett et al. (1993), ARF. Boundary mixing Inverse currents introduced little dowelling transport. Garrett et al. (1993), ARF. Boundary mixing Meridional current velocity distribution (Tang et al., 2000). 2014/9/11

23. Isotherm redistribution due to upwelling. Buoyancy instability Mixing Diffusion, little H. Advection Temperature increasing Upwelling Zonal temperature profile at 25.6°N Mechanism Analysis (6/10) • Wind-driven Ekman upwelling: -0.3 m/day • Boundary layer effect • Current-driven Ekman upwelling: 0.00002 m/day • Ekman boundary mixing: inverse flow • Dynamic uplift • mesoscale eddy • Kuroshio • Topographically controlled upwelling • Vertical mixing

24. Mechanism Analysis (7/10) • Isothermal plain have lower depth east of Kuroshio and higher depth on CDFR. • The isothermal plain on CDFR can be shallower than 50 m deep. Isothermal plain at 21℃ calculated by model output.

25. Mechanism Analysis (8/10) ─ Isotherm uplift Comparison of uplift height introduced by different mechanism. Topographical upwelling, eddy-introduced dynamic uplift and other minor effects.

26. Mechanism Analysis (9/10) ─ Topographic effects • Only the realistic bathymetry can constrain sufficient cold water source for surface cold-dome formation. (a) Realistic bathymetry (b) Deepened Case (c) Shallowed Case Flow field and temperature at 50 m numerical experiments.

27. Mechanism Analysis (10/10) ─ vertical mixing (a) Zonal Temperature (℃) profile (b) Vertical eddy diffusivities (cm2/s) Instantaneous zonal profiles of temperature and eddy diffusivities at 25.6°N. The vertical temperature gradient near surface coupled with the high surface eddy diffusivities suggested energetic vertical heat transfer in surface cold-dome.

28. Conclusion (1/2) • Different cold-dome patterns from the observations. • Evident cold-domes can be found due to the typhoons. • Dynamic uplift introduced by Kuroshio dominates the fundamental pattern of cold-dome. • Bathymetry not only suggests topographically controlled upwelling, but also constrains cold water in deep sea northeast of Taiwan.

29. Conclusion (2/2) • Mesoscale eddy contributes few dynamic uplift but can reduces horizontal advection for cold-dome. • Dynamic due to the surface and bottom boundary layers is too weak for the observed cold-dome formation. • Vertical mixing plays an important role for surface cold-dome formation.

30. Thank you for your attention. • Questions?

31. Observation─ Argo floats (b) (a) (c) Fig. 5. The Argo data, marked as red solid circles, since 3 August 2001 to 6 September 2009 in the study region, only 2047 data are available. Argo data gathered on Kuroshio main stream totally 21 profiles in from May to October, stand for summer pattern (b), and 12 profiles in from November to April, stand for winter pattern (c).

32. Mechanism Analysis ─ Uplift height • Take the depth of isotherm 21℃ at 122.8°E and 24.4°N as reference. • Large uplifted height in summer. Fig. 15. Contour of Uplift height.

34. Observation─ cold-dome in summer Fig. 7. Argo float, WMOID 2900819, for (a) trajectory of the float; (b) MW_IR SST and the a marker denotes the Argo data on 17 July 2008. The rest figures are subsurface comparisons of (c) temperature, (d) salinity, and (e) T-S profiles of the four measures. Typhoon Kalmaegi passed this region on 17-18 July 2008. The path of Typhoon Kalmaegi are marked as hollow circles in (a). 2014/9/11