North Equatorial Current Bifurcation Latitude Research Articles

Seasonal Variability of the Deep Western Boundary Current in the Philippine Sea

The deep western boundary current (DWBC) in the Philippine Sea has been expected to play a crucial role in transporting lower circumpolar deep water and to contribute to regional and global climate regulation. Two-year-long near-bottom current measurements reveal a southward-flowing DWBC with a mean velocity of 5 cm/s and seasonal variations—weaker in summer and stronger in winter. Seasonal variability in the DWBC is hypothesized to be induced by changes in the North Equatorial Current bifurcation latitude (NECBL) and upper pycnocline depth through potential vorticity conservation. Data-assimilated reanalysis model (GLORYS12V1) outputs, which reproduce the seasonal variability of DWBC similarly to the observation, are used for further analysis. During the seasonal period, the NECBL displays significant coherence (>0.9) with the first-mode empirical orthogonal function principal component of the simulated along-slope DWBC. The upper pycnocline depth, varying seasonally within a range of approximately 27 m, induces seasonal variability in a deep anticyclonic eddy trapped by topography. In summer, the intensified deep anticyclonic eddy obstructs the adjacent southward-flowing DWBC, weakening its strength, whereas in winter, the southward flow of the DWBC is enhanced due to the weakening of the deep anticyclonic eddy.

Decreasing trend of kuroshio intrusion and its effect on the chlorophyll-a concentration in the Luzon Strait, South China Sea

ABSTRACT Material exchange between oceans and currents is crucial for biogeochemical processes in marginal seas, such as the Kuroshio intrusion in the Luzon Strait, which contributes to nutrient and phytoplankton growth in the adjacent water. Therefore, it is necessary to understand changes in major current systems and their possible effects on marine ecology. In this study, we applied an edge-detection method to satellite-derived sea-surface temperature (SST) images to identify the SST front as the index of Kuroshio intrusion during 1985–2017. Chlorophyll-a (Chl-a) concentrations and geostrophic currents (determined from satellite observations after 1993) were examined by comparing long-term variations related to the Kuroshio front using empirical-mode decomposition analysis. The Kuroshio intrusion into the Luzon Strait gradually decreased after the 1990s, as did the Chl-a concentrations and current speed in the Strait. In contrast, the speed of the Kuroshio Current in the eastern Philippine tended to increase, indicating that the northward Kuroshio Current from the eastern Philippines became stronger, curtailing the westward intrusion of the branch current into the Luzon Strait. The speed of the Kuroshio Current correlated with the bifurcation of the North Equatorial Current (NEC), which is upstream of the Kuroshio Current starting from the southeast Philippines. Therefore, the NEC bifurcation latitude may have shifted more southward, which strengthened the water mass and current speed of the Kuroshio, resulting in decreased westward motion of the Kuroshio through the Luzon Strait. Consequently, the weakening Kuroshio intrusion may have caused a decreasing trend of biogeochemical processes in the South China Sea.

Intraseasonal Variability of the North Equatorial Current Bifurcation Off the Philippines

AbstractCharacteristics 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° 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–106 m3 s−1) waters to the NBL, and the bifurcation shifts northward with an amplitude of about 8° 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 Niños, 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.

Pacific North Equatorial Current bifurcation latitude and Kuroshio Current shifts since the Last Glacial Maximum inferred from a Sulu Sea thermocline reconstruction

The meridional migration of the bifurcation latitude of the Pacific North Equatorial Current (NEC) in the western boundary of the tropical Pacific modulates the strength of the Kuroshio Current. Using salinity reanalysis data, we show the NEC bifurcation latitude also acts as the dominant control on thermocline salinity of the Sulu Sea, just west of the Philippine archipelago, by regulating influx of western Pacific thermocline water via the Luzon Strait. We used oxygen isotopes (δ18O) and Mg/Ca in the thermocline-dwelling foraminifera Globorotalia tumida from Sulu Sea sediment core MD97-2141 to determine past thermocline δ18Ow and salinity variability spanning ∼20–5 ka with an average sampling interval of ∼50 years and infer past changes in the NEC bifurcation latitude. Our Sulu Sea thermocline reconstruction reveals high salinity from ∼18.8–15.5 ka, ∼12.2–11.5 ka, and from ∼9.5–8.5 ka indicating the NEC bifurcation latitude was shifted north and the Kuroshio was weak at those times. Low Sulu Sea thermocline salinity from ∼13.0–12.4 ka, ∼11.5–10.9 ka, and from ∼8.5 ka until the end of the record at ∼5.6 ka indicates the NEC bifurcation latitude was shifted south and the Kuroshio Current was relatively strong. Comparison to other paleoclimate records suggests the observed northward (southward) shifts of the NEC bifurcation latitude were driven by southward (northward) shifts of the Indo-Pacific ITCZ, consistent with modern mechanisms controlling interannual NEC bifurcation variability. The NEC bifurcation latitude shifts likely modulated northward energy transport via the Kuroshio Current and the mean temperature and salinity of the Indonesian Throughflow.

Can Tropical Pacific Winds Enhance the Footprint of the Interdecadal Pacific Oscillation on the Upper-Ocean Heat Content in the South China Sea?

AbstractIn this study, an enhanced footprint of the interdecadal Pacific oscillation (IPO) on the upper-ocean heat content (OHC) in the South China Sea (SCS) since the 1990s is revealed. The negative OHC–IPO correlation is significant (r= −0.71) during 1990–2010 [period 2 (P2)], whereas it is statistically insignificant during 1960–80 [period 1 (P1)]. Analyses show that the scope of the equatorial Pacific wind anomalies is wider during P2 compared with that during P1 due to a larger east–west SST gradient and enhanced tropical warming in the Indian Ocean. When the IPO is negative during P2, a wider scope of the wind stress anomalies associated with the IPO could lead to 1) the southward migration of the North Equatorial Current bifurcation latitude (NECBL) by affecting the wind stress curl over the key region where it is near the climatological NECBL and 2) an increase in the interbasin pressure gradient (sea surface height difference) between the western Pacific and the SCS; these two processes strengthen the Kuroshio and weaken the Luzon Strait transport (LST) or SCS throughflow into the SCS. Also, 3) the equatorial Pacific wind anomalies are wide enough to directly weaken the LST in the SCS through the “island rule.” These three pathways finally change the oceanic gyre in the SCS and increase the OHC. Our results suggest that the scope of the tropical wind stress is the crucial factor when we consider the relationship between the upper ocean thermal conditions in the SCS and the Pacific variability.

Seasonal variation of the North Equatorial Current bifurcation in regional model: Role of open boundary conditions

The role of open boundary conditions in modelling the seasonal variation of the North Equatorial Current Bifurcation latitude (NBL) is studied based on a linear Rossby wave model, 1.5-layer reduced-gravity model and Hybrid Coordinate Ocean Model (HYCOM). For a regional ocean model that covers the western Pacific, it is found that the seasonal variation of the NBL can be well reproduced as long as the data at the open boundary is provided by the corresponding basin experiment, no matter where the location is. However, the south–north migration of NBL as well as its peak seasons is not properly simulated if we apply other types of data at the open boundary as its location is close to the west. Sensitivity experiments indicate that the bad performance is attributed to the over-specification error and incomplete adjustment of interaction between wind forcing and westward propagating signals from incompatible open boundary data. The performance gets better as we set the open boundary at proper location and choose suitable open boundary condition. These conclusions can be extended to the simulations of low-latitude western boundary currents.

Poleward Shift of the Pacific North Equatorial Current Bifurcation

AbstractThe dynamics of the poleward shift of the Pacific North Equatorial Current bifurcation latitude (NBL) is studied using a 5.5‐layer reduced gravity model. It is found that the poleward shift of the NBL is associated with the asymmetric intensity of the wind stress curl input to the Pacific tropical and subtropical gyres. Stronger wind stress curl in the subtropical gyre leads to equatorward transport in the interior upper ocean across the boundary between the two gyres, causing a poleward transport compensation at the western boundary. In the lower layer ocean, in turn, there is poleward (equatorward) transport at the interior (western boundary) due to Sverdrup balance which requires zero transport at the gyre boundary where zonally integrated wind stress curl is zero. Therefore, the NBL exhibits a titling feature, with its position being more equatorward in the upper layer and more poleward in the lower layer. The equatorial currents bifurcations in other basins are also characterized by the poleward titling vertical structure. The wind stress curl over the subtropical gyre is generally stronger than that over the tropical gyre, resulting in the bifurcations shifting poleward with increasing depth.

Decadal variations of the transport and bifurcation of the Pacific North Equatorial Current

Decadal variations of the transport and bifurcation latitude of the North Equatorial Current (NEC) in the northwestern tropical Pacific Ocean over 1959–2011 are investigated using outputs of the Ocean Analysis/Reanalysis System 3 prepared by the European Centre for Medium-Range Weather Forecasts. The results indicate that the NEC transports at different longitudes have different decadal fluctuations, which are strongest around 139°E. The NEC bifurcation latitude (NBL) has its largest decadal variations around 150 m. Extremes of the decadal NEC transport and NBL before 1975 correspond to different circulation anomalies from those after 1975. The regression map against decadal NBL exhibits negative sea surface height (SSH) anomalies and a cyclonic gyre anomaly over the northwestern tropical Pacific Ocean, while that against the decadal NEC transport exhibits a dipole structure, with positive/negative SSH anomalies to the north/south of about 13°N. Furthermore, decadal variations of the NEC transport and NBL over the whole period have different correlations with Pacific Decadal Oscillation (PDO) and Tropical Pacific Decadal Variability (TPDV). Generally, the decadal NEC transport shows higher correlations with PDO than with TPDV, while the NBL has higher correlations with TPDV than with PDO. The high correlation of decadal NEC transport with PDO mainly comes from that of its northern branch with PDO, while its southern branch shows higher correlation with TPDV.

Projected changes of the low‐latitude north‐western Pacific wind‐driven circulation under global warming

AbstractBased on the outputs of 25 models participating in the Coupled Model Intercomparison Project Phase 5, the projected changes of the wind‐driven circulation in the low‐latitude north‐western Pacific are evaluated. Results demonstrate that there will be a decrease in the mean transport of the North Equatorial Current (NEC), Mindanao Current, and Kuroshio Current in the east of the Philippines, accompanied by a northward shift of the NEC bifurcation Latitude (NBL) off the Philippine coast with over 30% increase in its seasonal south‐north migration amplitude. Numerical simulations using a 1.5‐layer nonlinear reduced‐gravity ocean model show that the projected changes of the upper ocean circulation are predominantly determined by the robust weakening of the north‐easterly trade winds and the associated wind stress curl under the El Niño‐like warming pattern. The changes in the wind forcing and intensified upper ocean stratification are found equally important in amplifying the seasonal migration of the NBL.

Multidecadal oceanographic changes in the western Pacific detected through high‐resolution bomb‐derived radiocarbon measurements on corals

AbstractHigh‐resolution measurements of radiocarbon (14C) in corals can be used to reconstruct past variability in ocean conditions. Here we report seasonal Δ14C changes in coral from Ishigaki Island, Japan, and compare with previously reported data from Palau and Guam. Our data clearly indicate a significant increase in Δ14C from 1947 to 1998 related to atmospheric nuclear bomb testing. The three early Δ14C spikes related to the atmospheric nuclear bomb tests in the US Proving Grounds at Bikini and Enewerak atoll conducted in 1954, 1956, and 1958 were detected from the Ishigaki coral. After 1976, variability in the Mindanao Dome region related to North Equatorial Current (NEC) bifurcation latitude migration affected the Δ14C difference between Palau and Guam, whereas the difference between Ishigaki and Guam was not correlated with the bifurcation latitude. The Δ14C difference between Ishigaki and Guam may be due to mesoscale eddies in the Kuroshio area. On the decadal scale, the northward shift of NEC bifurcation latitude after 1976, the year as known as Pacific Decadal Oscillation regime shift from negative to positive, was concurrent with the abundant westward‐propagating mesoscale eddies in the Subtropical Countercurrent region and stronger Kuroshio transport off the east Taiwan, which may be represented by a smaller Δ14C difference between Ishigaki and Guam after 1976.

Variability of the Pacific North Equatorial Current and its implications on Japanese eel (Anguilla japonica) larval migration

AbstractThe zonal velocity produced by a Regional Ocean Modeling System (ROMS)‐based Pacific Ocean circulation model was validated against in situ measurements along the 137°E longitude. The Pacific model successfully reproduced the position and the shape of the North Equatorial Current (NEC) as well as the latitude of maximum surface velocity in the NEC region (8–17°N). The flow field produced by the Pacific model was then used to conduct numerical Lagrangian experiments, in which passive particles were released along a transect (142.5°E, 12.5–17°N) that traverses the known Japanese eel spawning area, and the effects of NEC strength and bifurcation latitude on the particle advection in the northwest Pacific were studied. Our results suggest that, in the 20‐yr period (1993–2012), the variability of the currents alone can cause interannual variability of one order of magnitude in the Kuroshio Entrance (KE), the percentage of particles entering the Kuroshio, the range of which varies from 43% in 1997 to 6% in 2012. The yearly‐averaged KE is not sensitive to the NEC bifurcation latitude. Instead, it is controlled by the average zonal velocity of a fixed domain (125–143°E, 13.5–17°N) and related to a recently‐developed climate index, the Philippines–Taiwan Oscillation (PTO). During the positive phase of the PTO, the zonal velocity in the domain, hence the yearly‐averaged KE, increases, and the opposite is true in the negative phase of the PTO. Considering only the trajectories, diel vertical migrations (DVM) in the top 400 m do not significantly affect Japanese eel larval transport, as incorporating DVM schemes does not increase the KE.

Weakening of the Kuroshio Intrusion Into the South China Sea Under the Global Warming Hiatus

We analyze a 22-year (1993–2014) record of the Kuroshio intrusion (KI) index defined from maps of mean absolute dynamic topography west of Luzon Island in the Philippines to investigate the long-term trend of the KI into the South China Sea (SCS). The monotonically increasing trend of the KI index extracted by empirical mode decomposition implies a slight decreasing trend of the meridional pressure gradient across the Luzon Strait, suggesting a weakening of the KI into the SCS based on the geostrophy. This weakening is largely due to an intensification of the upstream Kuroshio east of Luzon, which is closely correlated with the latitude of the North Equatorial Current (NEC) bifurcation off the Philippines. We also find that the latitude of the NEC bifurcation has tended to migrate southward in recent years, coincident with the strengthening of the Kuroshio east of Luzon. The forcing of the trends of low-latitude circulation is attributable to changing surface wind fields in the tropical Pacific. The trade winds have intensified in response to the recent global warming hiatus, and an anomalous cyclonic wind field has appeared over the Philippine Sea. Both of these phenomena have contributed to the southward migration of the NEC bifurcation latitude in recent years. This has led to enhance the Kuroshio transport east of Luzon, and the Kuroshio has tended to bypass the Luzon Strait without significant westward encroachment.

English

English

Correlation between subsurface high-salinity water in the northern South China Sea and the North Equatorial Current–Kuroshio circulation system from HYCOM simulations

Abstract. The North Pacific Tropical Water (NPTW), characterized by subsurface high salinity, is observed in the South China Sea (SCS) and is often used as an indicator of the water intrusion from the northwestern Pacific into the SCS. Based on the assimilation product from a global high-resolution Hybrid Coordinate Ocean Model (HYCOM) from 2008 through 2013, this study investigates the seasonal variability of subsurface high-salinity water (SHSW) in the northern SCS and its relationship with the North Equatorial Current–Kuroshio circulation system. Results show that the obvious seasonal variability of the SHSW appears at about 100–200 m in depth. It extends as far west as southeast of Hainan, reaching its volume maximum (minimum) in January (May). The seasonal variance contribution (seasonal variance accounting for the entire variance) is 0.38 in the period we considered, albeit with significant annual variance in other years. Further analysis shows that the changes in high-salinity water volume are highly correlated with the shift in the North Equatorial Current bifurcation latitude (NECBL), which reaches its northernmost point in December and its southernmost point in May. Due to the large-scale wind changes in the Pacific, the Luzon Strait transport (LST) weakens (strengthens) when the NECBL shifts to the south (north) during summer (winter), which results in the reduced (enhanced) SHSW intrusion from the northwestern Pacific into the northern SCS. It is also found that, on a seasonal timescale, the Kuroshio transport (KT) does not vary in phase with NECBL, LST and SHSW, indicating that the KT changes are probably not the governing factor for the seasonal variability of SHSW in the northern SCS.

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 Nina events. The dynamics responsible for the seasonal to interannual variations of the NEC-MC-KC current system are also discussed.

Role of the western tropical Pacific in the North Pacific regime shift in the winter of 1998/1999

AbstractIn this study, we examine the role of the western tropical Pacific in the North Pacific regime shift during the winter (December–January–February) of 1998/1999. The North Pacific regime shift in the winter of 1998/1999 is characterized by a dipole‐like structure along 40°N where significant warming is prominent in the western and central North Pacific. This shift might have been associated with an abrupt warming in the western tropical Pacific after 1998/1999, in the North Equatorial Current (NEC) bifurcation region (8°N–18°N, 125°E–160°E) in particular. The NEC bifurcation region experiences a regime shift from cooling to warming during the 1998/1999. Consequently, the Kuroshio current, which originates in the NEC bifurcation region, could transport the anomalous warm water into the North Pacific. In addition, a sudden shift in the NEC bifurcation latitude to the south also contributes to the 1998/1999 regime shift in the North Pacific. We suggest a possible mechanism that could have caused the regime shift in the NEC bifurcation region during the 1998/1999. An abrupt deepening of the thermocline depth in the NEC bifurcation region from 1997/1998 to 1998/1999, which is associated with the biggest El Niño in 1997/1998 and a subsequent La Niña event in 1998/1999, plays a role in initiating the shift in the NEC bifurcation region. Subsequently, the frequent occurrence of the La Niña events after 1998/1999 plays a role in maintaining the regime shift toward warming by sustaining a deepening of the thermocline depth.

Long-term trend of Pacific South Equatorial Current bifurcation over 1950-2010

This study investigates the long-term change of the Pacific South Equatorial Current (SEC) bifurcation latitude (SBL) over 1950–2010 with Simple Ocean Data Assimilation version 2.2.4. Results indicate that the SBL averaged within upper 200 m has migrated southward at 0.020°S yr−1, comparable in magnitude with −0.024°N yr−1 for the North Equatorial Current bifurcation latitude (NBL). The SEC transport into the Coral Sea has increased. Due to the southward SBL migration, most of the increased SEC water was transported equatorward, contributing to the Equatorial Undercurrent intensification. Experiments with a nonlinear 1.5 layer reduced gravity model indicate that the southward migration of SBL is mainly caused by positive Ekman flux divergence trend in the eastern tropical South Pacific, while that of NBL is caused by negative Ekman flux divergence trend in the western tropical North Pacific.

Decadal variations of Pacific North Equatorial Current bifurcation from multiple ocean products

AbstractIn this study, we examine the decadal variations of the Pacific North Equatorial Current (NEC) bifurcation latitude (NBL) averaged over upper 100 m and underlying dynamics over the past six decades using 11 ocean products, including seven kinds of ocean reanalyzes based on ocean data assimilation systems, two kinds of numerical simulations without assimilating observations and two kinds of objective analyzes based on in situ observations only. During the period of 1954–2007, the multiproduct mean of decadal NBL anomalies shows maxima around 1965/1966, 1980/1981, 1995/1996, and 2003/2004, and minima around 1958, 1971/1972, 1986/1987, and 2000/2001, respectively. The NBL decadal variations are related to the first Empirical Orthogonal Function mode of decadal anomalies of sea surface height (SSH) in the northwestern tropical Pacific Ocean, which shows spatially coherent variation over the whole region and explains most of the total variance. Further regression and composite analyzes indicate that northerly/southerly NBL corresponds to negative/positive SSH anomalies and cyclonic/anticyclonic gyre anomalies in the northwestern tropical Pacific Ocean. These decadal circulation variations and thus the decadal NBL variations are governed mostly by the first two vertical modes and attribute the most to the first baroclinic mode. The NBL decadal variation is highly positively correlated with the tropical Pacific decadal variability (TPDV) around the zero time lag. With a lead of about half the decadal cycle the NBL displays closer but negative relationship to TPDV in four ocean products, possibly manifesting the dynamical role of the circulation in the northwestern tropical Pacific in the phase‐shifting of TPDV.

On the dynamics of the seasonal variation in the South China Sea throughflow transport

The Luzon Strait transport (LST) of water mass from the Pacific Ocean to the South China Sea (SCS) varies significantly with seasons. The mechanisms for this large variability are still not well understood. The steady‐state island rule, which is derived from a steady‐state model, is not applicable to seasonal time scale variations in a large basin like the Pacific Ocean. In this paper, we will use a theoretical model that is based on the circulation integral around the Philippines. The model relates the LST variability to changes in the boundary currents along the east coast of the Philippines, including the North Equatorial Current (NEC) Bifurcation Latitude (NECBL), the transports of Kuroshio and Mindanao Currents (KC and MC), and to the local wind‐stress forcing. Our result shows that a northward shift of the NECBL, a weakening of the KC or a strengthening of the MC would enhance the LST into the SCS. This relationship between the LST and the NEC‐KC‐MC is consistent with observations. The analytical result is tested by a set of idealized numerical simulations.

Low‐frequency western Pacific Ocean sea level and circulation changes due to the connectivity of the Philippine Archipelago

Interannual‐to‐decadal sea level and circulation changes associated with the oceanic connectivity around the Philippine Archipelago are studied using satellite altimeter sea surface height (SSH) data and a reduced gravity ocean model. SSHs in the tropical North Pacific, the Sulu Sea and the eastern South China Sea (ESCS) display very similar low‐frequency oscillations that are highly correlated with El Niño and Southern Oscillation. Model experiments reveal that these variations are mainly forced by the low‐frequency winds over the North Pacific tropical gyre and affected little by the winds over the marginal seas and the North Pacific subtropical gyre. The wind‐driven baroclinic Rossby waves impinge on the eastern Philippine coast and excite coastal Kelvin waves, conveying the SSH signals through the Sibutu Passage‐Mindoro Strait pathway into the Sulu Sea and the ESCS. Closures of the Luzon Strait, Karimata Strait, and ITF passages have little impacts on the low‐frequency sea level changes in the Sulu Sea and the ESCS. The oceanic pathway west of the Philippine Archipelago modulates the western boundary current system in the tropical North Pacific. Opening of this pathway weakens the time‐varying amplitudes of the North Equatorial Current bifurcation latitude and Kuroshio transport. Changes of the amplitudes can be explained by the conceptual framework of island rule that allows for baroclinic adjustment. Although it fails to capture the interannual changes in the strongly nonlinear Mindanao Current, the time‐dependent island rule is nevertheless helpful in clarifying the role of the archipelago in regulating its multidecadal variations.