Modulation of Rossby waves on the Pacific North Equatorial Current bifurcation associated with the 1976 climate regime shift

Simulated current velocity and long-term reanalysis wind data are used to investigate interannual variations in the bifurcation of the Pacific North Equatorial Current (NEC) after the 1976 climate regime shift. Wind stress curl anomaly (WSCA) in the region of 10N-15N and 160E-170E generates Rossby waves and affects the NEC bifurcation along the Philippine coast. From 1976 to 1992, following a regime shift to the positive Pacific Decadal Oscillation (PDO) phase, PDO and El Ni~no-Southern Oscillation (ENSO) match each other in strength and have a neutralized effect on the WSCA. From 1993 to 2009, WSCA matches PDO well, and its correlation with ENSO is lower. Using a linear regression model, we show that the influence of PDO has nearly 13 times weight over that of ENSO. Prior to the 1976 regime shift, WSCA is closely related to ENSO from 1961 to 1975, and it does not correlate significantly with PDO. Our analysis results show that Rossby waves are preferentially generated in either the negative PDO phase when the ENSO signal dominates, or in the positive PDO phase when the ENSO signal is overshadowed. In the phase when the positive PDO counteracts with the ENSO signal, neither ENSO nor PDO has a significant influence on Rossby wave generations through the WSCA.