Variation of the North Equatorial Current, Mindanao Current, and Kuroshio Current in a High-Resolution Data Assimilation during 2008–2012

Outputs from a high-resolution data assimilation system,the global Hybrid Coordinate Ocean Model and Navy Coupled Ocean Data Assimilation （HYCOM＋NCODA） 1/12° analysis,were analyzed for the period September 2008 to February 2012.The objectives were to evaluate the performance of the system in simulating ocean circulation in the tropical northwestern Pacific and to examine the seasonal to interannual variations of the western boundary currents.The HYCOM assimilation compares well with altimetry observations and mooring current measurements.The mean structures and standard deviations of velocities of the North Equatorial Current （NEC）,Mindanao Current （MC） and Kuroshio Current （KC） also compare well with previous observations.Seasonal to interannual variations of the NEC transport volume are closely correlated with the MC transport volume,instead of that of the KC.The NEC and MC transport volumes mainly show well-defined annual cycles,with their maxima in spring and minima in fall,and are closely related to the circulation changes in the Mindanao Dome （MD） region.In seasons of transport maxima,the MD region experiences negative SSH anomalies and a cyclonic gyre anomaly,and in seasons of transport minima the situation is reversed.The sea surface NEC bifurcation latitude （NBL） in the HYCOM assimilation also agrees well with altimetry observations.In 2009,the NBL shows an annual cycle similar to previous studies,reaching its southernmost position in summer and its northernmost position in winter.In 2010 and 2011,the NBL variations are dominantly influenced by La Ni（n）a events.The dynamics responsible for the seasonal to interannual variations of the NEC-MC-KC current system are also discussed.

Numerical Study on the Bifurcation of the North Equatorial Current

A 1.5-layer reduced-gravity model forced by wind stress is used to study the bifurcations of the North Equatorial Current(NEC).The authors found that after removing the Ekman drift,the modelled circulations can serve well as a proxy of the SODA circulations on the σθ=25.0 kg m～-3 potential density surface based on available long-term reanalysis wind stress data.The modelled results show that the location of the western boundary bifurcation of the NEC depends on both zonal averaged and local zero wind stress curl latitude.The effects of the anomalous wind stress curl added in different areas are also investigated and it is found that they can change the strength of the Mindanao Eddy(ME),and then influence the interior pathway.

Low-frequency variability of the North Equatorial Current bifurcation in the past 40 years from SODA

The low-frequency variability of the North Equatorial Current （NEC） bifurcation during 1958 to 2001 was investigated with the Simple Ocean Data Assimilation （SODA） 2.0.2 dataset. In agreement with recent observations, the NEC bifurcation latitude （NBL） shifted northward as depth increases, from about 12.7°N near the surface to about 17.1°N at depths around 500 m for the annual average. This study reveals that the interannual variations of NBL, with five years period, mainly focused on the upper 500 m with amplitude increasing as depth increased. The NBL shifted southward in the past 40 years, which was more significant in the subsurface at more than -0.02°/a. The NBL manifests itself in the transports of NMK （NEC-Mindanao Current （MC）-Kuroshio） system in strong relationship with MC （0.7） and Kuroshio （-0.7）. The EOF analysis of meridional velocity off the Philippine coast shows that the first mode, explaining 62% of variance and 5 years period, was highly correlated with the southward shift of NBL with coefficient at about 0.75. The southward shift of NBL consists with the weakening of MC and strengthening of Kuroshio, which exhibited linear trends at -0.24Sv/a and 0.11Sv/a. Both interannual variation and trend of NBL were closely related to the variation of NMK system.

Correlation analysis of the North Equatorial Current bifurcation and the Indonesian Throughflow

Based on monthly mean Simple Ocean Data Assimilation （SODA） products from 1958 to 2007, this study analyzes the seasonal and interannual variability of the North Equatorial Current （NEC） bifurcation latitude and the Indonesian Throughflow （ITF） volume transport. Further, Empirical Mode Decomposition （EMD） method and lag-correlation analysis are employed to reveal the relationships between the NEC bifurcation location, NEC and ITF volume transport and ENSO events. The analysis results of the seasonal variability show that the annual mean location of NEC bifurcation in upper layer occurs at 14.33°N and ITF volume transport has a maximum value in summer, a minimum value in winter and an annual mean transport of 7.75×10^6 m^3/s. The interannual variability analysis indicates that the variability of NEC bifurcation location can be treated as a precursor of El Nino. The correlation coefficient between the two reaches the maximum of 0.53 with a time lag of 2 months. The ITF volume transport is positively related with E1 Nifio events with a maximum coefficient of 0.60 by 3 months. The NEC bifurcation location is positively correlated with the ITF volume transport with a correlation coefficient of 0.43.

The North Equatorial Current/Undercurrent volume transport and its 40-day variability from a mooring array along 130°E

Traditionally,the estimated volume transport of the North Equatorial Current/Undercurrent(NEC/NEUC)is based on geostrophic equations and/or model results;however,direct observational evidence has not been acquired.We focused on one-year mooring observation data collected along 130°E and calculated the NEC/NEUC volume transport and explore its variability.Results show that the mean NEC and NEUC volume transports calculated from the mean velocity structures in the upper 950 m are 39 Sv and 6 Sv,respectively.Analysis of daily mooring data indicated that the volume transport of the NEC is approximately 52(±14)Sv and the volume transport of the NEUC is approximately 18(±13)Sv.A significant 40-day variation existed for the volume transport of both the NEC and NEUC.Overall,the intraseasonal variability of the NEC is vertically coherent with that of the NEUC.Observations indicated that the NEUC has three cores centered at approximately 8.5°N(~500 m),12.5°N(~700 m),and 17.5°N(~900 m),of which the middle core(12.5°N)is the strongest.The 40-day variability of the NEC and NEUC is related to the variability of local wind stress curl anomalies among various Madden-Julian Oscillation phases.When local wind field generates a negative(positive)wind stress curl anomaly,a weaker NEC(NEUC)and stronger NEUC(NEC)would occur.

Instability in boundary layer between the North Equatorial Current and underlying zonal jets based on mooring observations

Instability/stability in the North Equatorial Current(NEC)basin is studied based on data obtained from nine moorings deployed at 8.5°N,10.5°N,11.0°N,12.5°N,13.0°N,15.0°N,15.5°N,17.5°N,and 18.0°N along 130.0°E during cruises in 2015–2017.In low latitudes,the Coriolis parameter and stratifi cation ratio play important roles in NEC stability,whereas velocity shear and the layer depth ratio are important for NEC stability in high latitudes.Beneath the westward NEC,eastward zonal jets occur intermittently centered around 8.5°N,12.5°N,and 17.5°N along 130.0°E.Similar to the NEC,the main body of these zonal jets also deepens with latitude.In the boundary layer comprising the bottom NEC and upper zonal jets,the growth rate of the NEC is attributed not only to velocity shear but also to zonal jet velocity based on the longwave assumption.Based on the shortwave assumption,the growth rate is proportional to zonal jet velocity but has no relationship with velocity shear.Climatologically,the growth rate in the boundary layer is not zero at 8.5°N,12.5°N,and 13.0°N,where the velocity shear and zonal jets are larger than at other stations.The instability also occurs at the time node when the zonal jets are strong enough,although the mean zonal jets may disappear at this station.

BIFURCATION OF THE NORTH EQUATORIAL CURRENT DERIVED FROM ALTIMETRY IN THE PACIFIC OCEAN

The gridded （1/3°＊1/3°） altimetry data from October 1992 through December 2004 were analyzed to study the seasonal and interannual variabilities of the bifurcation of the North Equatorial Current （NEC） at the surface in the western North Pacific Ocean. Calculations show that on annual average the bifurcation occurs at about 13.4°N at the surface. The geostrophic flow derived from Sea Surface Height （SSH） data shows that the southernmost latitude of the NEC bifurcation at the surface is about 12.9°N in June and the northernmost latitude is about 14.1°N in December. Correlation analyses between the bifurcation latitude and the Southern Oscillation Index （SOl） suggest that the bifurcation latitude is highly correlated with the E1 Nino/Southern Oscillation （ENSO） events. During the E1 Nino years the bifurcation of the NEC takes place at higher latitudes and vice versa.

Seasonal variability of the bifurcation of the North Equatorial Current

Seasonal variability of the bifurcation of the North Equatorial Current （NEC） is studied by constructing the analytic solu- tion for the time-dependent horizontal linear shallow water quasi-geostrophic equations. Using the Florida State University wind data from 1961 through 1992, we find that the bifurcation latitude of the NEC changes with seasons. Furthermore, it is shown that the NEC bifurcation is at its southernmost latitude （12.7°N） in June and the northernmost latitude （14.4~ N） in November.

Low-frequency variability of the shallow meridional overturning circulation in the South China Sea

The low-frequency variability of the shallow meridional overturning circulation（MOC） in the South China Sea（SCS） is investigated using a Simple Ocean Data Assimilation（SODA） product for the period of 1900-2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells： a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport（LST） that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current（NEC） bifurcation and thus a stronger transport of the Kuroshio east of Luzon.