Guinea Coastal Undercurrent Research Articles

Elevated Mixing Estimates and Trapped Near‐Inertial Internal Waves on the Inshore Flank of the New Guinea Coastal Undercurrent From Sustained Glider Observations in the Solomon Sea

AbstractThe Solomon Sea is a major contributor to (a) the volume transport into the Equatorial Undercurrent and (b) the associated heat transport, which has an order one effect on interannual temperature variability on the equator according to previous work. The narrow western boundary current (New Guinea Coastal Undercurrent, NGCU) merges with the broad, shallow North Vanuatu Jet within 100 km of the southern entry to the sea, which implies mixing. Existing estimates from observations suggest mixing is larger than in models with different mixing parameterizations, which produce disparate properties of these exiting waters. Here, we use sustained underwater glider measurements across the Solomon Sea from 2007 to 2020 to examine the spatial variability of mixing estimates and internal waves. We estimate diffusivity via a finescale parameterization using the vertical strain of isopycnal displacements from internal waves. A typical accuracy of this parameterization when compared to turbulence measurements is within a factor of 2–3. Our results and previous observations in this area agree within this factor. Our main results are: (a) vertical diffusivity estimates are about on the inshore, anticyclonic side of the NGCU, which are 10–100 times higher than offshore and (b) elevated near‐inertial internal wave (NIW) amplitudes are also found inshore. Taken together, these results suggest trapping of NIW by the anticyclonic vorticity of the NGCU leads to the elevated mixing within 100 km of the entry to the Solomon Sea.

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.

Mean, Annual, and Interannual Circulation and Volume Transport in the Western Tropical North Pacific From the Western Pacific Ocean State Estimates (WPOSE)

AbstractInterannual volume transport anomalies in the far western tropical North Pacific were calculated using the Western Pacific Ocean State Estimates (WPOSE; 2009–2017; 115°E−170°E, 15°S–27°N) to assess the magnitude, phase, and vertical structure of volume transport, and the relationship of interannual transport to the El Niño‐Southern Oscillation phenomena. The mean velocity and thermohaline structure for the WPOSE were verified using Argo climatology and autonomous glider observations. Transport was calculated for both an upper layer (surface‐26 kg m−3) and a lower layer (26–27.3 kg m−3). Annual volume transport anomalies were small north of 8°N but varied by 50% of the mean volume transport between the equator and 8°N. Interannual anomalies exceeded annual anomalies throughout the region. Volume transports of the North Equatorial Current, the Mindanao Current, and the South Equatorial Current/New Guinea Coastal Undercurrent increased beginning in September 2014, leading to a large increase in transport in the North Equatorial Counter Current before the mature phase of El Niño in 2015/2016. The increase in transport was related to the meridional gradient in thermocline depth centered at 5°N at the southern part of the Mindanao Dome, where extreme shoaling of the thermocline took place. Lower‐layer volume transports were not always in phase with those of the upper‐layer and made considerable contributions to total transport variability.

Effect of the Intensified Sub‐Thermocline Eddy on Strengthening the Mindanao Undercurrent in 2019

AbstractThe northward‐flowing Mindanao Undercurrent (MUC) was directly measured by acoustic Doppler current profilers from a subsurface mooring at about 8°N, 127°E during 2 years (November 2017–December 2019). Its depth covers a range from 400 m to deeper than 1,000 m with its core appearing at around 900 m. The mean velocity of MUC's core was approximately 5.8 cm s−1 with a maximum speed of about 47.6 cm s−1. The MUC was observed as a quasi‐permanent current with strong intraseasonal variability (ISV) with a period of 70–80 days. Further analyses with an eddy‐resolving circulation model output suggest that the ISV is closely related to sub‐thermocline eddies (SEs). In this study, two types of SEs near the Philippine coast are disclosed: the westward propagating SE (SE‐1) and the quasi‐stational SE southeast of Mindanao Island (SE‐2). The SE‐1 has both cyclonic and anticyclonic polarities with the propagation speed of 7–8 cm s−1, while the SE‐2 is an anticyclonic eddy that moves erratically within 4–8°N, 127–130°E with the mean translation speed of about 11 cm s−1. Even though the SE‐1 plays an important role in modulating the MUC, our results show that the observed strong MUC event (May–July 2019) is evidently induced by the intensified SE‐2 that moves northwestward. This study emphasizes that the SE‐2 when intensified, receives more energy from the strengthened New Guinea Coastal Undercurrent and loses the energy northward along the Philippine coast by intensifying the MUC.

Linking Seasonal‐to‐Interannual Variability of Intermediate Currents in the Southwest Tropical Pacific to Wind Forcing and ENSO

AbstractSeasonal‐to‐interannual variability of intermediate circulations in the southwest tropical Pacific is key to interhemisphere and interbasin water mass exchange and is shown to be interconnected with wind forcing and the El Niño–Southern Oscillation (ENSO) based on numerical model results validated by mooring observations. The Lower Equatorial Intermediate Current and the intermediate part of the New Guinea Coastal Undercurrent show clear seasonal variability and a significant interannual response to ENSO at a 7–12‐month lag, both of which are induced by Rossby waves. Baroclinic modes 1 and 2 of the direct wind‐forced Rossby waves contribute mainly to the seasonal variability. In contrast, the Rossby waves from baroclinic modes 2 and 4 and the reflection of Kelvin waves are the main contributors to the variability on ENSO time scales. The presence of the southwest tropical Pacific coastline enhances the intermediate seasonal variance and the interannual relationship between intermediate currents and ENSO.

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.

The Equatorial Undercurrent and Its Origin in the Region Between Mindanao and New Guinea

AbstractHerein, the spatial characteristics of the Equatorial Undercurrent (EUC) in the region between Mindanao and New Guinea and the origins of the EUC and the North Equatorial Countercurrent (NECC) were investigated using a combination of high‐resolution Argo hydrographic data and numerical modeling. The Mindanao Current (MC) results in the formation of a strong EUC‐depth eastward current that splits into two branches near the Halmahera Eddy west of 132°E, with one branch directly flowing eastward without experiencing the Halmahera Eddy rotation effect, and the other branch flowing southwestward and then turning northeastward. The EUC and NECC were difficult to distinguish between 132°E and 136°E, since the former combined with the latter to form a continuous equator‐tilted current. At ~137°E, the EUC axis started turning toward the equator, and an isolated EUC core was observed at 143°E and ~200‐m depth. The Pacific EUC was shown to originate at ~130°E, where it featured an isolated current core with a maximum velocity of 5–10 cm/s at ~1°N and 200‐ to 400‐m depth. The EUC and NECC in the western tropical Pacific were found to have the same water sources, namely, the MC and the New Guinea Coastal Undercurrent, with the shallow parts (centered at ~100‐m depth) of MC and New Guinea Coastal Undercurrent finally reaching the NECC and their deeper parts (centered at ~200‐m depth) finally reaching the EUC.

Complementary Use of Glider Data, Altimetry, and Model for Exploring Mesoscale Eddies in the Tropical Pacific Solomon Sea

AbstractMesoscale activity is an important component of the Solomon Sea circulation that interacts with the energetic low‐latitude western boundary currents of the South Tropical Pacific Ocean carrying waters of subtropical origin before joining the equatorial Pacific. Mixing associated with mesoscale activity could explain water mass transformation observed in the Solomon Sea that likely impacts El Niño Southern Oscillation dynamics. This study makes synergetic use of glider data, altimetry, and high‐resolution model for exploring mesoscale eddies, especially their vertical structures, and their role on the Solomon Sea circulation. The description of individual eddies observed by altimetry and gliders provides the first elements to characterize the 3‐D structure of these tropical eddies, and confirms the usefulness of the model to access a more universal view of such eddies. Mesoscale eddies appear to have a vertical extension limited to the Surface Waters (SW) and the Upper Thermocline Water (UTW), i.e., the first 140–150 m depth. Most of the eddies are nonlinear, meaning that eddies can trap and transport water properties. But they weakly interact with the deep New Guinea Coastal Undercurrent that is a key piece of the equatorial circulation. Anticyclonic eddies are particularly efficient to advect salty and warm SW coming from the intrusion of equatorial Pacific waters at Solomon Strait, and to impact the characteristics of the New Guinea Coastal Current. Cyclonic eddies are particularly efficient to transport South Pacific Tropical Water (SPTW) anomalies from the North Vanuatu Jet and to erode by diapycnal mixing the high SPTW salinity.

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.

Water mass analysis of the Coral Sea through an Optimum Multiparameter method

AbstractA water mass analysis of the Coral Sea thermocline waters provides a description of their distribution, pathways and mixture based on recent oceanographic cruises in this region of strong western boundary currents. The Optimum Multiparameter method is used to determine the relative contribution of core water masses based on their measured temperature, salinity and dissolved oxygen. The thermocline waters, carried by the broad South Equatorial Current (SEC), are essentially composed of four core water masses of different origins. Coming from the south, the South Pacific Tropical Water South (SPTWS, σ = 25.3 kg m−3) and the Western South Pacific Central Water (WSPCW, σ = 26.3 kg m−3) enter the Coral Sea by the channel between the island of New Caledonia and the Vanuatu archipelago. Coming from the north, the South Pacific Tropical Water North (SPTWN, σ = 24.5 kg m−3) and the Pacific Equatorial Water (PEW, σ = 26.3 kg m−3) flow north of Vanuatu. The upper thermocline water that exits the Coral Sea equatorward, is mainly composed of SPTWN carried by the New Guinea Coastal Undercurrent. In contrast, upper thermocline waters exiting the Coral Sea poleward, in the East Australian Current, is dominated by SPTWS. The relative contributions are different in the lower thermocline where WSPCW dominates both western boundary currents. This refined description is consistent with the dynamics of the main currents, with a very strong depth dependence in the partitioning of incoming SEC waters.

Multiscale dynamical analysis of a high‐resolution numerical model simulation of the Solomon Sea circulation

AbstractA high 1/36° resolution numerical model is used to study the ocean circulation in the Solomon Sea. An evaluation of the model with (the few) available observation shows that the 1/36° resolution model realistically simulates the Solomon Sea circulations. The model notably reproduces the high levels of mesoscale eddy activity observed in the Solomon Sea. With regard to previous simulations at 1/12° resolution, the average eddy kinetic energy levels are increased by up to ∼30–40% in the present 1/36° simulation, and the enhancement extends at depth. At the surface, the eddy kinetic energy level is maximum in March‐April‐May and is minimum in December‐January‐February. The high subsurface variability is related to the variability of the western boundary current (New Guinea Coastal Undercurrent). Moreover, the emergence of submesoscales is clearly apparent in the present simulations. A spectral analysis is conducted in order to evidence and characterize the modeled submesoscale dynamics and to provide a spectral view of scales interactions. The corresponding spectral slopes show a strong consistency with the Surface Quasi‐Geostrophic turbulence theory.

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.

Detection of Pacific tropical water variability by taut-line moorings in the western equatorial Pacific

This study uses temperature and salinity time series acquired with taut-line moorings in the western equatorial Pacific to investigate water mass behavior on the thermocline layer. Basically, it is insufficient to trace water mass variation by the original discrete depth coordinate data because of relatively high variability of density at fixed depth near the thermocline. A reconstruction method based on the density surface motion caused by tidal forcing was used to derive continuous profiles of temperature and salinity from vertically discrete measurements at fixed depths. This method can represent detailed vertical salinity structures and their variation, especially along the potential density surface of 24.8σθ, where the salinity maximum of South Pacific tropical water (SPTW) appeared. Variability around the 24.8σθ surface at each site was as large as that observed at the surface, which suggests a strong influence of SPTW behavior. High salinity along the 24.8σθ surface within the equatorial band of the western Pacific appeared during boreal fall-winter at sites far from New Guinea. In contrast, high salinity appeared near New Guinea during the boreal spring-summer. These features suggest the influence of the New Guinea Coastal Undercurrent. Over longer time scales, several higher salinity events were observed. The most pronounced salinity event occurred during 2007–2008. Interannual variation of the salinity anomaly along the 24.8σθ surface was negatively correlated with the Nino 3.4 sea surface temperature anomaly. A long-term salinity anomaly shift from negative to positive occurred around the end of 2002. The relationship with decadal variation in subtropical cell transport is also discussed.

Solomon Sea circulation and water mass modifications: response at ENSO timescales

The South Pacific low latitude western boundary currents (LLWBCs) carry waters of subtropical origin through the Solomon Sea before joining the equatorial Pacific. Changes in their properties or transport are assumed to impact El Nino Southern Oscillation (ENSO) dynamics. At ENSO timescales, the LLWBCs transport tends to counterbalance the interior geostrophic one. When transiting through the complex geography of the Solomon Sea, the main LLWBC, the New Guinea Coastal Undercurrent, cannot follow a unique simple route to the equator. Instead, its routes and water mass properties are influenced by the circulation occurring in the Solomon Sea. In this study, the response of the Solomon Sea circulation to ENSO is investigated based on a numerical simulation. The transport anomalies entering the Solomon Sea from the south are confined to the top 250 m of the water column, where they represent 7.5 Sv (based on ENSO composites) for a mean transport of 10 Sv. The induced circulation anomalies in the Solomon Sea are not symmetric between the two ENSO states because of (1) a bathymetric control at Vitiaz Strait, which plays a stronger role during El Nino, and (2) an additional inflow through Solomon Strait during La Nina events. In terms of temperature and salinity, modifications are particularly notable for the thermocline water during El Nino conditions, with cooler and fresher waters compared to the climatological mean. The surface water at Vitiaz Strait and the upper thermocline water at Solomon Strait, feeding respectively the equatorial Pacific warm pool and the Equatorial Undercurrent, particularly affect the heat and salt fluxes. These fluxes can change by up to a factor of 2 between extreme El Nino and La Nina conditions.

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.

Oceanic transports through the Solomon Sea: The bend of the New Guinea Coastal Undercurrent

Thermocline waters of the tropical southwest Pacific can be traced back to the center of the South Pacific basin and have a potential influence on equatorial surface conditions and on the characteristics of the El Niño Southern Oscillation on decadal timescales. The Solomon Sea is traversed by this influential flow, and therefore is an optimal place for exploring this oceanic connection to the equator. From a high‐resolution hydrographic survey at which we applied an inverse box model, we describe the main pathways at the entrance of the Solomon Sea, and more particularly the extremely sharp bend of the western boundary current around the south‐east tip of Papua New Guinea. Of the 30 Sv subtropical waters transported into the Coral Sea from the east, above 1300 m, 29 ± 5 Sv makes its way through the Solomon Sea with a large part transported in a boundary current, at the entrance of the Solomon Sea. Around the south‐east tip of Papua New Guinea, the Gulf of Papua Current turns abruptly to the north, in a very sharp bend as it merges into the New Guinea Coastal Undercurrent, on its way, toward the equator. The warm currents transport large amounts of internal energy, with a total of 1.0 ± 0.3 1015W entering the Solomon Sea from the south.