Rossby wave and eddy in the North Pacific Subtropical Countercurrent (STCC)

1. Rossby wave and eddy in the North Pacific Subtropical Countercurrent (STCC) Qinyu Liu, Lijuan Li and Wei Liu Physical Oceanography Lab. & Ocean-Atmosphere Interaction and Climate Lab, Ocean University of China, Qingdao, 266003, China)

2. Outline • Motivation • Amplification features of the Rossby wave in STCC ; • Eddies in the STCC and Their Possible Effects on the South China Sea • The “Vortex Street ” to the west of the Big Island • Summary

3. Motivation Chelton and Schlax (1996) speculated that the topography may be the source of the long Rossby wave amplification. 21N The location of Rossby wave amplification is variation with time

4. Zonal current Northwest STCC STCC to west of Hawaii Islands Vertical Shear Two branches ( Kobashi andKawamura 2003)

5. Problems • How about the amplification features of the Rossby wave in STCC area? • Whether those Rossby wave or some eddy in the STCC can enter the Luzon strait? • Is there ocean vortex Street or not, when North Equator Current overpass the Hawaii Islands ?

6. Data • Sea Surface Height Anomaly :The Maps of Sea Level Anomaly (MSLA), altimeter products by the CLS Space Oceanography Division • Drifter data :Global Drifter Center (GDC) and the Marine Environmental Data Service (MEDS)

7. Amplification features of the Rossby wave (80-210 day) when westward propagation Low PV

8. Stability analysis The necessary condition for longer Rossby wave to be unstable: ， ， This condition is easy to be fulfilled when there is Mode water in Northwest STCC

9. Conclusion(1)Amplification features of the Rossby wave • In both STCC branches, the SSHA exhibits remarkable oscillations with period of 80-210 days, corresponding to westward propagation of free Rossby waves. • The amplitude of those Rossby waves appears larger in the west end of the Northwest STCC than that in the east end, seemingly due to the stronger baroclinic instability in the North STCC, because mode water exists in subsurface layer.

10. Eddy radii vs. longitude in the area between 5N-25N, Eddy: according single drifter path and SSHA Eddy radii vs. longitude in the area between 5N-25N, Estimate: radius :98 km~298km ; westward eddy speed approximately 0.098 m/s in STCC

11. Approximately 200 drifters in the area around the Luzon Strait (18N-22N and 121E-125E), only 4 of the total (2%) actually entered the SCS through the Luzon Strait, and all the four drifters entered the SCS in the winter.(1979-1998)

12. Dec~Feb. Sep.-Nov. 15/323=>5%winter(1979~2004)

13. Conclusion(2) • Eddies in the STCC, corresponding to the Rossby wave, move westward at an average speed of approximately 0.098 m/s and their average radius is about 200 km (wavelengh~800km). • These westward eddies from the North Pacific interior are unlikely to enter the South China Sea through the Luzon Strait.

14. Vortex Street to the west of the Big Island (Aug~Dec 1995). Drifter SSHA

15. Drifter 1989 to 1998 EOF first mode (21.2%) SSHA 1995~2001 two symmetrical arrays of cyclonic and anticyclonic vortices similar to the pattern of the vortex Street and often apperars

16. Conclusion (3) • there is the vortex pair with the orbital period of 10-11days and the radius of 58-68 km to the west of Hawaii Islands. • The obvious feature of the SSHA field is that there are two symmetrical arrays of cyclonic and anticyclonic vortices similar to the pattern of the vortex Street .

17. Thank you!