Variation of the Southern Subtropical Countercurrent related to Sea Surface Height and Eddies in the Northwest Tropical Pacific

Abstract

The ADCP records obtained at about 18 degrees N, 135 degrees E show the southern branch of the Subtropical Countercurrent (STCC). The sea surface heights (SSH) show that there is a tendency to increase and decrease in the south/north of STCC, respectively. So the variability of SSH ultimately contributes to the strengthening of STCC through geostrophic balance. The southern STCC branch distinctly persists from winter to spring. Since 2005, the southern STCC exists almost throughout the year, and the STCC is clearly stronger to the east of 145 degrees E. Anticyclonic and cyclonic eddies exist seemingly as bands around 17.5 degrees N and 20.5 degrees N, respectively. The STCC flowing eastward, which is formed by the geostrophic balance, is maintained with the interaction between geostrophic currents and anticyclonic-cyclonic eddies. The rotating eddies exert an additional driving force to maintain the eastward flow of STCC, and then the STCC reveals a meandering movement due to the interaction with the eddies. The trajectories of surface drifters together with the altimeter data analysis in June 2009 dictate the variability of the STCC induced by the interaction between eddies and the eastward flow. These results suggest that the southern STCC slowly changes from an intra-seasonal event an annual one with time duration of over 21 months.