New Guinea Coastal Undercurrent Research Articles

Morphometrics and long-weight relationship Halmahera walking shark (Hemiscyllium halmahera, Allen, 2013) population in Morotai Island Sea, North Maluku, Indonesia

Morotai Island is under the influence of plates, Indonesian Throughflow, Halmahera Eddy, New Guinea Coastal Undercurrent (NGCUC) and is located in the coral reef triangle. Halmahera walking shark (Hemiscyllium halmahera) was found in the Morotai Island Sea. The aim was to study the morphometric and length-weight relationship of H. halmahera on Morotai Island. Sample collection of H. halmahera in South Morotai, North Morotai and South West Morotai. H. halmahera samples were taken using scuba diving at a depth of 3-10 meters. The morphometric measurements were 11 variables TL (total length)- LTL (lower tail length). The diversity of morphometric characteristics found was analyzed using the Principal Component Analysis (PCA) method. Analysis length-weight relationship using linear regression equation. The results found 30 individuals (South Morotai = 8, North Morotai = 9, South West Morotai = 8, and East Morotai = 5) and the frequency of sex presence individuals (male = 16 and female = 14). Morphometric parameters found variation in each individual. PCA analysis found that the distribution of morphometric characteristics had body size variation. Dendogram is divided into Cluster I (North Morotai dan South West Morotai) dan Cluster II (South Morotai dan East Morotai). The length-weight relationship found negative allometric growth.

Currents off the Papua New Guinea Coast During and After the El Niño of 2015–2016

AbstractInterannual variation of New Guinea Coastal Undercurrent (NGCUC) along the Papua New Guinea (PNG) coast was investigated using mooring observations during 2015–2019 combined with eddy‐resolving simulations of the Ocean General Circulation Model for the Earth Simulator (OFES) during 1980–2019. Strong interannual variation appears in the upper 800 m of the currents recorded by the mooring, which is well reproduced by OFES outputs. On the interannual time scale, a relatively larger proportion (1.1 Sv) of the transport signals related to El Niño/Southern Oscillation (ENSO) in the Solomon Sea is transmitted through the Solomon Strait, while about 0.84 Sv of ENSO signal is through the Vitiaz Strait due to the topography constraint. As the NGCUC flows along the PNG coast, the mean transport above 400 m gradually increases while its relationship with ENSO rapidly weakens. This weakening is due to its merging with the westward South Equatorial Current (SEC). The westward SEC weakens (strengthens) during El Niño (La Niña) events, which weakens the interannual signal of NGCUC. The interannual variations of the currents above and below the thermocline off the PNG coast are opposite: a clockwise anomalous circulation straddling the equator appears above the thermocline, while a counterclockwise anomalous circulation appears below the thermocline during El Niño, which is connected to the recharge‐discharge oscillation during ENSO cycle.

Destination of New Guinea Coastal Undercurrent in the western tropical Pacific: Variability and linkages

The New Guinea Coastal Undercurrent (NGCUC) is considered a bottleneck in the western tropical Pacific (WTP), carrying upper-to-intermediate waters from the south to the northwestern Pacific, thereby playing a fundamen tal role in the interhemispheric water mass exchange. However, how the NGCUC links to the circulation in the WTP was insufficiently studied. This work explores the destination of NGCUC waters, its spatiotemporal changes, and possible physical processes linked with the downstream NGCUC using ocean reanalysis for 22 years (1994 – 2015). Lagrangian particle tracking discloses eight major destinations of the NGCUC: The Equatorial Undercurrent (EUC, 35.26%), the North Equatorial Countercurrent (NECC, 12.3%), the North (13.33%) and South (8.85%) Subsurface Countercurrents, the Equatorial Deep Jet (11.49%), the Mindanao Undercurrent (13.24%), and the Indonesian (3.47%) and Halmahera (0.86%) Throughflows. The NGCUC waters are distributed mainly to the east (81.65%) and their dissemination varies markedly with depth. These destinations exhibit significant variations on seasonal and interannual time scales. The NGCUC strengthens (weakens) during summer (winter) and more NGCUC waters are distributed westward and northeastward (eastward). Interannually, the distribution of the NGCUC water is influenced by El Niño-Southern Oscillation, in which most of its eastward-distributed waters shift northward (equatorward) in El Niño (La Niña) phase joining the strengthened NECC (EUC). Changes in the NGCUC water destination can transform the water mass properties in the WTP. The findings of this study also emphasize the fundamental role of eddies in trapping and redistributing the NGCUC waters and linking the currents in the WTP.

Pacific Equatorial Undercurrent: Mean state, sources, and future changes across models

The Equatorial Undercurrent (EUC) stretches across the Pacific, transporting cool waters rich in oxygen and nutrients eastward to one of the most productive regions in the ocean. As an intricate part of the global climate system, EUC dynamics are essential to understanding future climate change but are poorly represented in global coupled climate models. This study examines EUC representation and future changes in the latest generations of the Coupled Model Intercomparison Project (CMIP6 and CMIP5) and an eddy-permitting ocean model. We also examine historical and projected changes in EUC source waters, including the Mindanao Current (MC), New Guinea Coastal Undercurrent (NGCU), and interior thermocline convergence. The circulation features in the models are broadly consistent with observations and ocean reanalyses, but improvements from CMIP5 to CMIP6 are limited. In the future projections, the EUC is enhanced in the western Pacific, with less prominent changes in CMIP6, but more so in the eddy-permitting model. The western Pacific EUC enhancement is likely associated with a wind-driven redirection of waters south of the equator, in which the NGCU boundary flow increases while the interior thermocline convergence decreases. This is superimposed on an overall weakening of the North Pacific subtropical overturning cell, including the MC, interior thermocline convergence, and Ekman divergence. As EUC heat and nutrient composition is linked to its sources, these projected changes have implications for the EUC's role in air–sea feedbacks, nutrient replenishment, and oxygen minimum zone ventilation in the eastern Pacific.

Multi-decadal trends in the tropical Pacific western boundary currents retrieved from historical hydrological observations

As large-scale ocean circulation is a key regulator in the redistribution of oceanic energy, evaluating the multi-decadal trends in the western Pacific Ocean circulation under global warming is essential for not only understanding the basic physical processes but also predicting future climate change in the western Pacific. Employing the hydrological observations of World Ocean Atlas 2018 (WOA18) from 1955 to 2017, this study calculated the geostrophic currents, volume transport and multi-decadal trends for the North Equatorial Current (NEC), the North Equatorial Countercurrent (NECC), the Mindanao Current (MC), the Kuroshio Current (KC) in the origin and the New Guinea Coastal Undercurrent (NGCUC) within tropical western Pacific Ocean over multi-decades. Furthermore, this study examined the contributions of temperature and salinity variations. The results showed significant strengthening trends in NEC, MC and NGCUC over the past six decades, which is mainly contributed by temperature variations and consistent with the tendency in the dynamic height pattern. Zonal wind stress averaged over the western Pacific Ocean in the same latitude of each current represents the decadal variation and multi-decadal trends in corresponding ocean currents, indicating that the trade wind forcing plays an important role in the decadal trend in the tropical western Pacific circulation. Uncertainties in the observed hydrological data and trends in the currents over the tropical western Pacific are also discussed. Given that the WOA18 dataset covers most of the historical hydrological sampling data for the tropical western Pacific, this paper provides important observational information on the multi-decadal trend of the large-scale ocean circulation in the western Pacific.

Seasonal and Interannual Variability of the Currents off the New Guinea Coast From Mooring Measurements

AbstractSeasonal and interannual variability of the New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC) were investigated with 3 years of mooring measurements off the northern coast of New Guinea and outputs from the Ocean General Circulation Model for the Earth Simulator during 1980–2018. Acoustic Doppler Current Profilers mounted on the two moorings captured variations of the currents in the upper 800 m off the New Guinea coast during 2015–2018. NGCC is a seasonally reversing current in the upper 100 m, which flows southeastward in boreal winter with maximum velocity of 63 cm/s near the surface, and flows northwestward in boreal summer with maximum velocity of −55 cm/s at 80 m. NGCUC flows northwestward all year round between 100 and 400 m, and its temporal mean velocity reaches −40 cm/s at 200 m. A seasonally reversing current named New Guinea Coastal Intermediate Current with speed of 10 cm/s is detected below the NGCUC, which is in phase with the NGCC on seasonal time scale. Seasonal variation of the NGCUC is also in phase with that of the NGCC, and it is strong in boreal summer and weak in winter. Such seasonal signal reaches down to the depth of 800 m. Both mooring measurements and model outputs indicate that NGCUC demonstrates significant interannual variations associated with El Nino‐Southern Oscillation (ENSO), with its velocity core shoaling during El Nino and deepening during La Nina, but the net transport of NGCUC exhibits no significant relationship with ENSO.

Dinamika Massa Air di Sekitar Pulau-Pulau Kecil Terluar (PPKT) Perairan Utara Papua

<p class="Papertext"><strong>Water Mass Dynamics Around the Indonesia’s Outer Small Islands at <br /> North Papua Waters. </strong>Coastal area of Indonesia’s Outer Islands (PPKT) at North Papua Waters have a huge potential of marine resources which needs to be managed in sustainable ways. In order to make the processes of protection, conservation, rehabilitation, utilization, and enrichment of coastal resources work well, the study about environmental carrying capacity which includes ocean dynamics analysis around PPKT area is needed. Hereby, the study area were including coastal and off shore area of Jiew, Budd, Fani, Bras and Fanildo Island. The measurement of water mass and ocean current was conducted to analyze the impact of regional ocean dynamics to coastal waters of PPKT. This study found the domination of New Guinea Coastal Current (NGCC) at surface layer, the effect of Indonesian throughflow at subsurface layer (100 – 150m depth), and counter current towards northeast which is believed as a part of New Guinea Coastal Under Current (NGCUC) and Equatorial Under Current (EUC). This ocean current system carried South Pacific Subtropical Water (SPSW) noticed by maximum salinity of 35.4 PSU at 150m depth. There are spatial variability of mixed layer depth and slope of T-S curve at the western and eastern part of study area which might be caused by isopycnal mixing. However, the spatial and temporal variability of Sea Surface Temperature at this area is relatively small due to the presence of Western Paicific Warm Water Pool. With the small annual SST variability, strong ocean current and water mass mixing, this area has a potential for the development of marine ecosystem.</p>

Iron sources and pathways into the Pacific Equatorial Undercurrent

AbstractUsing a novel observationally constrained Lagrangian iron model forced by outputs from an eddy‐resolving biogeochemical ocean model, we examine the sensitivity of the Equatorial Undercurrent (EUC) iron distribution to EUC source region iron concentrations. We find that elevated iron concentrations derived from New Guinea Coastal Undercurrent (NGCU) alone is insufficient to explain the high concentrations observed in the EUC. In addition, due to the spread in transit times, interannual NGCU iron pulses are scavenged, diluted, or eroded, before reaching the eastern equatorial Pacific. With an additional iron source from the nearby New Ireland Coastal Undercurrent, EUC iron concentrations become consistent with observations. Furthermore, as both the New Guinea and New Ireland Coastal Undercurrents strengthen during El Niño, increased iron input into the EUC can enhance the iron supply into the eastern equatorial Pacific. Notably, during the 1997/1998 El Niño, this causes a simulated 30% iron increase at a 13 month lag.

Provenance and supply of Fe-enriched terrigenous sediments in the western equatorial Pacific and their relation to precipitation variations during the late Quaternary

Iron (Fe) deposition in the equatorial Pacific has important implications for the global carbon cycle, while the provenance of Fe supply and its change remain highly debated. Here, we geochemically characterize the provenance of terrigenous sediments deposited on the pathways of the Equatorial Undercurrent (EUC) and the New Guinea Coastal Undercurrent (NGCUC). The Fe-enriched sediments in the western equatorial Pacific are mostly derived from fluvial inputs of Papua New Guinea (PNG), while nearly negligible impact from eolian dust could be detected. Variability of the terrigenous Fe-enriched deposition (7.4–13.4%) for core KX21-2 in the western equatorial Pacific over the past 380ka shows dominant precession periods, superimposed on a clear glacial–interglacial trend with higher input during glacials. The precession periods are correlated with the precipitation over PNG, in response to the local summer insolation (5°S, March) and meridional migration of the Intertropical Convergence Zone (ITCZ). The glacial–interglacial trend is induced by sea level fluctuations that significantly influence the fluvial input from southern PNG. The different expressions of precession periods between glacials and interglacials in core KX21-2 are tightly associated with the undercurrent. The subdued precession periods during interglacials can be attributed to the weakness of the NGCUC, which may link to La Niña-like conditions. The enhanced precession periods during glacials should result from increased input from southern PNG on one hand, and an intensified NGCUC on the other hand, due to the El Niño-like conditions. Compared to Fe, the proxy ln (Ti/Total) (XRF log-ratio of Ti/Total counts) for core KX21-2 preferentially indicates the northern PNG input, and therefore could be used to reflect the glacial changes in the NGCUC. Our records imply that the NGCUC was particularly stronger in MIS 6 and 10, and weaker in MIS 8.

Particulate iron, aluminum, and manganese in the Pacific equatorial undercurrent and low latitude western boundary current sources

Biological production and ecosystem structure in the central equatorial Pacific are limited by iron. The primary source of the iron appears to be from western island sources transported eastward by the equatorial undercurrent (EUC). In order to assess this hypothesis water column samples from 15m to 1000m from the central and western equatorial Pacific and from the northeastern coastal margin of New Guinea and New Ireland were analyzed for total particulate Fe, Al and Mn. These new data were compared with previously published data for dissolved and total dissolvable Fe, Al and Mn. Leachable and refractory Fe, Al and Mn were calculated. There were large zonal gradients for all three particulate metals near the depth of the Pacific EUC. The subsurface maxima were highest in the west (145°E) and decreased significantly to 140°W for all species except PMn. The maxima in all three particulate metals were at the same depth along the equatorial transect. The PFe and PAl concentrations were much higher along the NE coast of Papua New Guinea and New Ireland but there were no maxima associated with the New Guinea Coastal Undercurrent (NGCU), although PFe appeared to have a maximum in the New Ireland Coastal Undercurrent (NICU). The distributions of PMn in the western source region differed from the other two metals and the equatorial PMn maxima west of the dateline had higher concentrations than in the NGCU. There were significant correlations between PFe and PAl in both the coastal and equatorial stations. The correlations with PMn were variable suggesting that PMn may have different sources. In all samples the leachable particulate fraction increased from Al<Fe<Mn. There were maxima in nonleachable, refractory PFeref and PAlref in the NGCU and NICU not seen in the PFe and PAl data. The molar ratios of PFe:PAl (0.41), and especially Mn:Al (0.008), were slightly higher than ratios for suspended matter from the Sepik River (0.34–0.39 for Fe:Al and 0.005 for Mn:Al) suggesting additional continental margin sources of Fe and Mn. The primary source of PFe and PAl appeared to be material of riverine origin from the continental margin of Papua New Guinea and New Ireland. To understand how a particulate iron maximum could be maintained at a constant density across the equatorial Pacific we applied a simple model where the zonal decrease in leachable particulate iron (LPFe) was due to iron loss due to remineralization and loss due to particle sinking. Using a sinking rate for 1-μm sized particles of 0.086md−1, an extremely low remineralization rate of 0.006d−1 would be necessary to produce the observed gradient.

Drivers of the projected changes to the Pacific Ocean equatorial circulation

Climate models participating in the third Coupled Model Inter Comparison Project (CMIP3) suggest a significant increase in the transport of the New Guinea Coastal Undercurrent (NGCU) and the Equatorial Undercurrent (EUC, in the central and western Pacific) and a decrease in the Mindanao current and the Indonesian Throughflow. Most models also project a reduction in the strength of the equatorial Trade winds. Typically, on ENSO time scales, a weakening of the equatorial easterlies would lead to a reduction in EUC strength in the central Pacific. The strengthening of the EUC projected for longer timescales, can be explained by a robust projected intensification of the south‐easterly trade winds and an associated off‐equatorial wind‐stress curl change in the Southern Hemisphere. This drives the intensification of the NGCU and greater water input to the EUC in the west. A 1½‐layer shallow water model, driven by projected wind stress trends from the CMIP3 models demonstrates that the projected circulation changes are consistent with a purely wind driven response. While the equatorial winds weaken for both El Niño events and in the projections, the ocean response and the mechanisms driving the projected wind changes are distinct from those operating on interannual timescales.

From the western boundary currents to the Pacific Equatorial Undercurrent: Modeled pathways and water mass evolutions

The hydrological and geochemical properties of the waters constituting the Pacific Equatorial Undercurrent (EUC) determine the properties of the equatorial cold tongue. Understanding and quantifying the various EUC origins is therefore of prime importance. For this purpose, a high-resolution (1/4°) interannual oceanic simulation was analyzed from the western tropical Pacific boundaries to 140°W, using a Lagrangian framework. Waters from the Low-Latitude Western Boundary Currents (LLWBCs) transiting from Vitiaz Strait (the main contributor), from Solomon Strait, and via the Mindanao Current were identified as the principal sources to the EUC. Waters conveyed by the interior ocean off equator are negligible till 180°E. The LLWBCs' waters represent 87% of the EUC transport at 156°E out of which the New Guinea Coastal Undercurrent (NGCU) is as large as 47%. The EUC meridional distribution suggests that the waters originating from Solomon Strait and Mindanao Current mostly remain in the hemisphere from which they originate. Contrastingly, Vitiaz Strait waters are found in both hemispheres. The vertical EUC distribution shows that the lower layer of the EUC is mainly composed of Vitiaz Strait waters. Finally, the source transport distributions were characterized, at their origin and within the EUC, as a function of density. These distributions showed that waters flowing through Vitiaz Strait at densities higher than those of the EUC (down to sigma = 27.2 kg m -3 ) undergo a diapycnal mixing and lighten during their journey to join the EUC. This lightening supports the suggestion that the NGCU is a major source for the EUC geochemical enrichment.

Western Pacific coastal sources of iron, manganese, and aluminum to the Equatorial Undercurrent

We present results from the first zonal transect of iron, aluminum, and manganese conducted from the western source region of the Equatorial Undercurrent (EUC) to the central equatorial Pacific. Trace metals were elevated along the slope of Papua New Guinea and within the New Guinea Coastal Undercurrent (NGCU), which is the primary Southern Hemisphere entry path of water to the EUC. Subsurface maxima in total acid‐soluble iron, aluminum, and manganese were evident in the EUC. These maxima were generally greatest in the western equatorial Pacific and decreased in magnitude eastward. Maxima in iron and aluminum persisted to 140°W; maxima in manganese extended to 175°W. Iron and manganese maxima were deeper (25–75 m) than aluminum maxima and located in the lower EUC, which undergoes less interior ocean mixing than shallower waters. The depth of the aluminum subsurface maxima correlated strongly (r = 0.88) with the depth of the EUC velocity maximum. Surface waters were enriched in aluminum and manganese offshore of Papua New Guinea. Surface metal concentrations decreased eastward throughout the western warm pool up to the longitude (∼180°W) of the salinity front. Detrital sediment input from either direct riverine input or sediment resuspension appeared to be the primary mechanism of supplying metals to the NGCU. We estimated eastward fluxes of metals in the EUC and found greatest fluxes in the western equatorial Pacific between 160°E and 165°E, except for aluminum. Fluxes of aluminum and, to a lesser extent, manganese increased concurrently with water volume transport in the central equatorial Pacific. Iron transport in the EUC remained constant east of the dateline, apparently due to the combined effects of dilution by meridional entrainment and scavenging. Iron was mobilized in a highly active western boundary current region and transported eastward in the lower EUC.

Revisiting the La Niña 1998 phytoplankton blooms in the equatorial Pacific

A biogeochemical model of the tropical Pacific has been used to assess the impact of interannual variability in a western Pacific iron source on the iron-limited ecosystem of the central and eastern Pacific during the 1997–1998 El Niño. A reference simulation and two simulations with an iron source in the western Pacific have been performed. The two “source” simulations differed only in the temporal variability of the iron source. In the variable source simulation, the iron concentration in the source region was proportional to the velocity of the New Guinea Coastal Undercurrent (NGCUC). In the constant source simulation, the same time-averaged concentration of iron was imposed with no temporal variability. The variable source was designed to mimic variations of iron flux from the northeast slope of New Guinea to the NGCUC due to modulation of sedimentary iron resuspension as previously hypothesized. Through the comparison of these simulations, it appeared that: (i) an iron source in the NGCUC, regardless of its source variability, increases biomass in the eastern equatorial Pacific because of the greater eastward iron flux by the Equatorial Undercurrent and (ii) a variable NGCUC iron source does not change the temporal variability of eastern Pacific chlorophyll, and in particular the timing and intensity of the June 1998 bloom. To explain eastern Pacific biological variability, local rather than remote processes are needed, such as wind-driven upwelling, the local depth of the thermocline, tropical instability waves and biological processes such as high grazing pressure. Therefore, while the western Pacific sources of dissolved iron are important in our model to sustain annually integrated equatorial Pacific production, they are unlikely to strongly constrain the timing of blooms in the central and eastern Pacific such as during the 1998 La Niña.

Dispersion and cycling of organic matter from the Sepik River outflow to the Papua New Guinea coast as determined from biomarkers

The Sepik River is a major contributor of water, sediment and associated organic loads to the coastal waters of northern New Guinea. With a narrow continental shelf and strong coastal currents, much of this exported material is available for long distance transport into the Bismarck Sea and beyond. CTD casts and associated instrument data showed that the river signature was visible in optical measurements of deep profiles from the Sepik Canyon. Discrete water samples were collected in clean Niskin bottles for organic analysis. Additional high volume samples for lipid classes were collected with Infiltrex samplers deployed on a floating mooring, along with two sediment trap arrays set at 100 and 260 m depth. The Infiltrex samplers were set at 55, 180, 200 and 220 m depth in an effort to target the surface layers and those of westward flowing water in the New Guinea Coastal Undercurrent (NGCUC). The samplers allowed analysis of lipid classes in both dissolved and particulate phases. Analyses of lignin phenols, hydrocarbons, fatty acids, sterols, and n-alcohols in the samples were used to estimate the cycling of the organic input from the river system to the coastal waters and possible entrainment in the NGCUC. The molecular biomarkers confirmed the Sepik River as a significant source of reduced carbon in the near coastal zone influenced by the plume. They indicated that ca. 90% of the organic carbon delivered to the coastal zone is dispersed or degraded, with only about 10% of terrestrial carbon being deposited on to the narrow shelf of coastal sediments. The dissolved lignin phenols provided evidence for desorption of organics from particles with increasing depth. However, this desorption process did not change the C/N ratio of the dissolved organic carbon (DOC) in the deep water of the Sepik Canyon. The hydrocarbons, sterols and fatty acids showed the change from lipid content dominated by phytoplankton in surface layers to zooplankton and bacterial biomarkers in deeper waters. The organic biomarkers provided evidence that some of the dissolved organic input from the Sepik River was injected into fast moving undercurrents. However, concentrations of terrestrial biomarkers were diluted and/or degraded to non-detectable levels within 100 km of the source.

Variability and Linkages of New Guinea Coastal Undercurrent and Lower Equatorial Intermediate Current

Abstract Velocity at depths of 700–800 m was measured between September 1998 and October 2002 at 2.5°S, 142°E off the New Guinea coast and at 0°, 138°E to examine the New Guinea Coastal Undercurrent (NGCUC) and the current on the equator carrying Antarctic Intermediate Water (AAIW). Velocity characteristics before November 1999 were markedly different from those after November 1999. The typical state occurred during the second period: the intermediate NGCUC and the Lower Equatorial Intermediate Current (LEIC) varied markedly with an annual cycle in opposite phases. In austral winter, the NGCUC flowed west-northwestward strongly (14 cm s−1, 285°T), especially in May–July during which the LEIC disappeared and eddylike equatorial variations with periods of 20–60 days were significant. In austral summer, the LEIC flowed westward strongly (12 cm s−1, 270°T), especially in October–December, whereas the NGCUC reversed its direction repeatedly to flow east-southeastward in November–February. Thus, the intermediate NGCUC and LEIC are present stably in austral winter and summer, respectively. These variations of the currents must change the pathway of AAIW seasonally. The state during the first period was atypical: the current on the equator flowed eastward strongly (13.0 cm s−1, 81°T), that is, no LEIC was present, and the NGCUC flowed west-northwestward strongly (14.8 cm s−1, 280°T) without changing direction. The atypical state may be related to the 1998–99 La Niña. In addition, power spectral peaks at periods of 14–35 days of meridional velocity at the equator suggest that intermediate tropical instability waves are generated in October–December in the typical state.

A ribbon of dark water: phytoplankton blooms in the meanders of the Pacific North Equatorial Countercurrent

Abstract In the far southwestern corner of the North Pacific Ocean, the Mindanao Current retroflects and flows northeast into the nascent North Equatorial Countercurrent (NECC). Ocean-color images show that phytoplankton blooms occur both in the nearly closed cyclonic loop of the retroflection, and in the meandering NECC, which were particularly strong during the El Nino event of 1997–98. In the winter of 1997–98, the bloom persisted for about 4 months, and, at its peak, extended for several thousand kilometers along the current stream. The NECC meanders are observed in SeaWiFS and OCTS images at various times throughout the nearly 5 years of data examined, but regionally averaged chlorophyll concentrations during the 1997–98 El Nino were higher than at any time since. The current meanders are sometimes visible in SeaWiFS images during winter, spring and summer, but not in autumn, consistent with the seasonal cycle of formation and decay of the Mindanao Dome (MD). Although the MD has a cyclonic circulation pattern, vertical nutrient flux appears to be maximal in the NECC, which forms its southern boundary, rather than in the MD itself. The appearance of phytoplankton blooms along the stream of the NECC is most plausibly attributed to upwelling associated with current meandering, although El Nino-induced shoaling of the thermocline and seasonal Ekman pumping also may contribute. In addition, transport of nutrient-rich water from the South Pacific by the New Guinea Coastal Undercurrent (NGCUC), which is maximal in boreal winter, creates a gradient of nutrient concentration across the NECC stream with much greater concentrations to the south at a given density (J. Geophys. Res. 103 (1998) 12959). In 1997-98 a confluence of all of these factors occurred. Shoaling of the thermocline and an intensified NECC due to the El Nino event coincided with the seasonal formation of the MD and increased westward transport of nutrient-rich water by the NGCUC to produce the highest chlorophyll concentrations observed in 5 years of ocean-color data. Similar though weaker blooms were observed in the spring of 2003 in association with the very weak 2002–03 El Nino.

Observation of current variations off the New Guinea coast including the 1997–1998 El Niño period and their relationship with Sverdrup transport

Seasonal and interannual variations of the New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC) were investigated by examining the 5 years' data from acoustic Doppler current profiler moorings at two sites (2°S 142°E, 2.5°S 142°E) off the New Guinea coast. The NGCC flowed northwestward as is usual and intensified during the boreal summer, then weakened or even reversed direction to southeastward during the boreal winter. This seasonal change correlated to the monsoonal wind variation. However, during the 1997–1998 El Niño, the southeastward NGCC during the boreal winter was not observed, and northwestward flow was dominant throughout the year. On the other hand, the NGCUC flowed steadily northwestward all year‐round and intensified during the boreal summer. During the growing phase of the El Niño, the NGCUC intensified, and its northwestward flow reached from the surface to a depth of 250 m. Comparison between the volume transport of these currents and the Sverdrup transport along 2°S in the ocean interior indicated a mean difference of 13 × 106 m3 s−1 northward. The relationship between variations of these two transports showed a negative correlation on seasonal timescales except during the El Niño. During the mature phase of the El Niño, northward Sverdrup transport was enhanced significantly, furthermore the transport of these currents was also northward. The result demonstrates a process by which anomalous water volumes can flow into the equatorial region due to an imbalance between the volume transport in the ocean interior and the western boundary.

A Climatological Interpretation of the Circulation in the Western South Pacific*

Abstract Time-averaged circulation is examined using historical hydrographic data near the Australia and Papua New Guinea coast in the Pacific. By averaging the data along isopycnal surfaces in a 0.5° × 0.5° grid, the authors are able to show many detailed phenomena associated with the narrow western boundary currents, including the vertical structure of the bifurcation latitude of the South Equatorial Current (SEC) and the connection between the Solomon and Coral Seas. The bifurcation latitude of the SEC is found to move southward from about 15°S near the surface to south of 22°S in the intermediate layers. The origin of the Great Barrier Reef Undercurrent (GBRUC) is identified to be at about 22°S. Farther to the north, the GBRUC intensifies underlying the surface East Australian Current, and merges with the North Queensland Current (NQC) at about 15°S. The NQC turns eastward to flow along the Papua New Guinea coast and feeds into the New Guinea Coastal Undercurrent (NGCUC) through the Louisiade Archipelago. Further analysis shows that there is a strong water property connection between the Coral and Solomon Seas, confirming the earlier speculation on the water mass origins of the NGCUC.

A Climatological Interpretation of the Circulation in the Western South Pacific*

Abstract Time-averaged circulation is examined using historical hydrographic data near the Australia and Papua New Guinea coast in the Pacific. By averaging the data along isopycnal surfaces in a 0.5° × 0.5° grid, the authors are able to show many detailed phenomena associated with the narrow western boundary currents, including the vertical structure of the bifurcation latitude of the South Equatorial Current (SEC) and the connection between the Solomon and Coral Seas. The bifurcation latitude of the SEC is found to move southward from about 15°S near the surface to south of 22°S in the intermediate layers. The origin of the Great Barrier Reef Undercurrent (GBRUC) is identified to be at about 22°S. Farther to the north, the GBRUC intensifies underlying the surface East Australian Current, and merges with the North Queensland Current (NQC) at about 15°S. The NQC turns eastward to flow along the Papua New Guinea coast and feeds into the New Guinea Coastal Undercurrent (NGCUC) through the Louisiade Archipel...