Study On The Multi Time Scale Variability And Dynamics Of The North Equatorial Current Bifurcation In The Pacific

The Tropical Northwestern Pacific consists of complex ocean current system.The Kuroshio and the Mindanao Current, which are two of the strongest westernboundary currents in the world, originate here. The North Equatorial Current (NEC)which is driven by the trade wind bifurcates as it encounters the Philippine coast,separating into the northward flowing Kuroshio and southward flowing MindanaoCurrent. The bifurcation of the NEC is of vital importance, not only in determiningthe meridional heat and mass transport of the low-latitude western boundary currents,but also in regulating local climate. Studying on the variability and its relevantdynamics is, therefore, of great instructive significance on the role of the oceancirculation in the climate change. Over recent years, the main focus has been on thefeatures of its seasonal and inter-annual variability. However, there have been fewcomprehensive and systematic studies addressing its multi time scale variability andrelevant controlling mechanisms of the NEC bifurcation.In this study, we focus on the multi time scale variability of the NEC bifurcationand its dynamics as a key scientific issue. The multi time scale variability as well asits controlling mechanism is studied by analyzing observations and newly developedhigh-quality ocean assimilation datasets together with a1.5-layer nonlinear reducedgravity model simulation and a linear vorticity equation under long waveapproximation.It is found that both observational and model results indicate that the NECbifurcation latitude (NBL) reaches its southernmost position in summer andnorthernmost position in winter with south to north migration of1°-2°. We highlightthe role of extra-tropical wind forcing on the seasonal amplitude of the NBL throughcoastal Kelvin waves (CKWs) along the western boundary. Existence of the CKWs could regulate the zonal pressure gradient of the western boundary current near thePhilippine coast, thus favoring the amplification of the NBL south to north migration.We highlight the role of the zero curl line of the tropical wind stress and baroclinicadjustment in regulating the NBL’s south to north migration. By adopting a linearvorticity model, it is found that a fast (slow) adjustment leads to less (more)cancellation between the local wind forcing and the remote forcing carried by theRossby waves, thus favoring a large (small) seasonal amplitude of the NECbifurcation. Sensitivity experiments indicate that the peak seasons of the NBL isdominated by the wind forcing in the western Pacific (local wind forcing) andbaroclinic adjustment does not exert great impacts on the phase of the NBL.Significant inter-annual variability of the NEC bifurcation can be found by bothobservational and modeling approaches. Composite analysis indicate that thenorthward (southward) migration of the NBL at inter-annual time scale corresponds tothe negative (positive) sea surface height (SSH), the lifting (falling) of thethermocline and the El Ni o-(La Ni a-) like sea surface temperature (SST) anomalyin the tropical western Pacific. By adopting a1.5-layer nonlinear reduced gravitymodel, it is found that the inter-annual variability of the NBL corresponds well withinter-annual wind forcing in the western Pacific especially west of the dateline. Thewind forcing in this region accounts for over50%of the total variance of the NBLinter-annual variations. Model simulations forced by the wind stress which isregressed on Ni o3.4index (ENSO-wind) indicates that this type of ENSO-windcould only account for less than15%of the NBL variations, while the modeled NBLforced by the wind stress which is regressed on the wind stress curl in the key regionaccounts for over50%. This clarifies again the crucial role of the western Pacific windforcing around the bifurcation latitude on inter-annual time scale.The long-term change of the North Equatorial Current (NEC) bifurcation in thePacific is assessed based on recently developed Simple Ocean Data Assimilation(SODA version2.2.4). It is found that the bifurcation latitude has shifted southwardover the last60years with significant multi-decadal variability. This southward shift of the bifurcation latitude is associated with changes in the wind stress curl over thetropical Pacific. It is demonstrated that the long-term change of the seasonal south tonorth migration of the bifurcation is modulated by the southward shift of the meanposition. Over the last six decades, the seasonal amplitude of the NBL has beenincreased (decreased) in both SODA and model simulations. This is due to the factthat the phase speed of first-mode baroclinic Rossby waves (CR) at the latitude of thebifurcation increases in association with the1.5°equatorward shift of the meanposition. Using a linear vorticity model, it is found that the increasing/decreasingpattern can be derived by increasing CRlinearly while the wind stress curl is setmonthly climatology. It is expected that the seasonal amplitude will decreasemoderately in the following decades if the ocean continues warming.