Intraseasonal Variability of the North Equatorial Current Bifurcation Off the Philippines

Characteristics of the intraseasonal variability (ISV) of the North Equatorial Current bifurcation latitude (NBL) are analyzed by using both observations and reanalysis data. The maximum amplitude of the ISV of the NBL can reach about 6 degrees of latitude, which almost triples its annual excursion. The NBL shows a broad spectral peak around 50-140 days with the highest peak occurring around 73 days. Both the Madden-Julian oscillation and mesoscale eddies have a significant impact on its ISV. A case study for a cyclonic eddy indicates that it can transport about 1.5 Sv (1 Sv-10(6) m(3) s(-1)) waters to the NBL, and the bifurcation shifts northward with an amplitude of about 8 degrees of latitude in 1 month. Meanwhile, the influence of mesoscale eddies on the ISV of the NBL is highly modulated by the interannual variations of the eddy field. Active eddy activities associated with enhanced barotropic and baroclinic instabilities show a large influence on the NBL. The eddy kinetic energy level around the NBL showed a quite diverse character for the 1997-1998 and the 2015-2016 extreme El Ninos, with the latter being double or triple that of the former due to the much stronger vertical and horizontal shears. Given the significant impact on the NBL by the individual eddies and the unevenly distributed cyclonic and anticyclonic eddies around the NBL, the effect of mesoscale eddies on the bifurcation latitude may not cancel each other on seasonal or interannual time scales.