Interannual variability in zonal flow component with upward phase propagation observed by a mooring in the western tropical North Pacific at (12°50′N, 137°E)
摘要
Long-term mooring observations conducted in the western tropical North Pacific (12°50′N, 137°E) for ~ 8 years (1989–1997) at four depths (500, 700, 2500, and 4500 m) revealed interannual variability in the zonal flow component (U) of the subsurface layer (500 and 700 m) with a period of ~ 4 years. Spectral analysis indicated that coherent variability extended to 2500 m. This variability showed that the phase at 2500 m preceded that at 700 m by 7 months, which suggested upward phase propagation. High-resolution simulation results suggested that the observed interannual variability in U may be due to the movement of zonal jets appearing in the model. The 5-year pressure (P) harmonic, which represents the simulated interannual variabilities, was somewhat longer than the observed period and showed anticlockwise phase propagation in the surface layer along a circuit in the equatorial and tropical North Pacific. The mooring site was located in the latitude band of the circuit with the off-equatorial Rossby waves, where the westward–downward energy translation and westward–upward phase propagation of the 5-year P and U harmonics were seen. The observed upward phase propagation of U may be a manifestation of such vertically propagating waves, as their ray paths and corresponding equi-phase lines were consistent with the amplitude and phase distributions of the 5-year P harmonic. These results suggest that the observed interannual variability may represent an aspect of the ENSO cycle, providing a new insight for the linkage between the ENSO and deeper ocean variability.