CoAuthors

Shuiming Chen (University of Hawaii at Manoa, USA); Brian Powell (University of Hawaii at Manoa, USA); Patrick Colin (Coral Reef Research Foundation, Palau); Dan Rudnick (Scripps Institution of Oceanography, USA); Martha Schonau (Scripps Institution of Oceanography, USA)

Due to the presence of a well-defined permanent thermocline, low-frequency upper ocean circulation variability in the tropical western Pacific Ocean is effectively captured by the linear baroclinic Rossby wave dynamics. A careful examination of the circulation variations surrounding Palau using the in-situ temperature measurements, satellite altimetry data, and data-assimilated ECCO2 state estimate reveals that linear dynamics fails to describe the observed large-amplitude upper ocean variations on the monthly time scales. These short-timescale variations are particularly active during the transition period from El Niño to La Niña conditions. As an El Niño event terminates and the tropical trade winds rebound, downwelling baroclinic Rossby waves are generated across the off-equatorial Pacific basin. When these wind-forced Rossby waves propagating into the western North Pacific basin, they deform the pre-existing, horizontally-sheared, North Equatorial Countercurrent (NECC) and subject it to barotropic instability. By breaking down into large-amplitude eddies, the unstable NECC generates rapidly-evolving upper ocean changes with sea level and upper ocean temperature fluctuations exceeding 0.5 m and 10°C, respectively, over a period of 1~2 months around Palau.