Rossby wave and eddy in the North Pacific Subtropical Countercurrent
Qinyu Liu(1) , Lijuan Li(1) , and Wei Liu(1)
Physical Oceanography Lab.,
No.5 Yusgan Road , Ocean University of China,
Since the earlier study of low-frequency large-scale oceanic Rossby waves using 3 years of TOPEX/POSEIDON data by Chelton and Schlax (1996), some new features of the oceanic Rossby wave have been found with more and more available observational data including Sea Surface Height Anomaly (SSHA) , ARGO and other data in the North Pacific Subtropical Countercurrent (STCC) area.
There are two branches of the STCC in the North Pacific: the North STCC located in the Northwestern Pacific (20º~26ºN to the west of 170ºW) and the South STCC extends from 145 ºE to the west of the Hawaii Islands (158ºW) along 19º~20ºN. In both STCC regions, the SSHA exhibits remarkable intraseasonal oscillations with period of 80-120 days, corresponding to westward propagation of free Rossby waves . The amplitude of those intraseasonal Rossby waves appears larger in the west end of the North STCC than that in the east end of the North STCC, seemingly due to the stronger baroclinic instability in the North STCC. As a result, the intraseasonal variation of SSHA appears to be stronger than its annual variation in the west end of the North STCC, and the transport of the Kuroshio in the east of Taiwan also exhibits strong intraseasonal variation with a period of about 100 days.
Eddies in the STCC associated with the intraseasonal Rossby wave move westward at an average speed of approximately 0.098 m/s and with an average radius of about 200 km. These eddies from the interior North Pacific seems not to be able to enter the South China Sea through the Luzon Strait . The transport of the Kuroshio in the east of Taiwan, however, can be affected by these eddies with a high correlation with the SSHA (22-24oN, 121.75-124oE). During the period of Oct. 1992~ Jan. 1998, two northwestward propagating cyclonic SSHA converged to the Kuroshio and caused a substantial reduction of the Kuroshio transport as a contrast to the normal westward propagating cyclonic SSHA. Although the lower Kuroshio transport event can be generated in different ways, the intraseasonal Rossby wave plays a domiant role in the 100-day period oscillation of Kuroshio transport in the east of Taiwan.
In addition to those eddies, there is also a vortex pair with the orbital period of 10-11days and the radius of 58-68km in the west of Hawaii Islands. The average T/P SSHA shows general distribution of the vortices in a region broader than that covered by the trajectories. The T/P SSHA clearly demonstrates two symmetrical arrays of cyclonic and anticyclonic vortices that are similar to the pattern of the vortex street.