Decadal variations of the North Equatorial Current in the Pacific at 137°E
Hydrographic observations, ocean state estimates, and ocean objective analyses are combined to investigate the decadal variations of the North Equatorial Current (NEC) in the Pacific at 137 degrees E during the last three decades (1975-2005). Observations show that the decadal NEC transport has three maxima around 1980/1981, 1994/1995, and 2004/2005, and two minima around 1989/1990 and 1999/2000, respectively. Associated with these maxima/minima, the sea surface height (SSH) falls/rises and the subsurface isopycnals shoal/deepen in the southern part of NEC, resulting in westward/eastward zonal velocity anomalies. Results from the ocean state estimates and ocean objective analyses show good agreement with observations. Further analysis indicates that the observed zonal velocity anomalies at 137 degrees E are part of the cyclonic/anticyclonic gyre anomalies formed in the tropical northwestern Pacific east of the Philippines, coinciding with the tropical gyre. Results from a 11/2 layer reduced gravity model suggest that these oceanic variations are mainly controlled by the decadal wind forcing in the tropical western Pacific and can be attributed to both local Ekman dynamics and baroclinic Rossby wave propagation.