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Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents - in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow - has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.

A 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, delta = 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.

The equatorial edge of the Western Pacific Warm Pool is operationally identified by one isotherm ranging between 28° and 29 °C, chosen to align with the interannual variability of strong zonal salinity gradients and the convergence of zonal ocean currents. The simulation of this edge is examined in 19 models from the World Climate Research Program Coupled Model Intercomparison Project Phase 5 (CMIP5), over the historical period from 1950 to 2000. The dynamic warm pool edge (DWPE), where the zonal currents converge, is difficult to determine from limited observations and biased models. A new analysis technique is introduced where a proxy for DWPE is determined by the isotherm that most closely correlates with the movements of the strong salinity gradient. It can therefore be a different isotherm in each model. The DWPE is simulated much closer to observations than if a direct temperature-only comparison is made. Aspects of the DWPE remain difficult for coupled models to simulate including the mean longitude, the interannual excursions, and the zonal convergence of ocean currents. Some models have only very weak salinity gradients trapped to the western side of the basin making it difficult to even identify a DWPE. The model’s DWPE are generally 1–2 °C cooler than observed. In line with theory, the magnitude of the zonal migrations of the DWPE are strongly related to the amplitudes of the Nino3.4 SST index. Nevertheless, a better simulation of the mean location of the DWPE does not necessarily improve the amplitude of a model’s ENSO. It is also found that in a few models (CSIROMk3.6, inmcm and inmcm4-esm) the warm pool displacements result from a net heating or cooling rather than a zonal advection of warm water. The simulation of the DWPE has implications for ENSO dynamics when considering ENSO paradigms such as the delayed action oscillator mechanism, the Advective-Reflective oscillator, and the zonal-advective feedback. These are also discussed in the context of the CMIP5 simulations.

Thanks to the agreement found between in situ measurements and TOPEX/POSEIDON data in the western tropical Atlantic Ocean, a realistic picture of the spatial and temporal variability over the 1992–1997 period is obtained. The sea level variability clearly emphasizes three ranges of variability. The intraseasonal variability is associated with propagating features north of the equator, consistent with a first baroclinic Rossby wave characteristic. The annual variability, which represents the largest part of the variability, describes the seasonal cycle of the North Equatorial Countercurrent; but, more interesting, is a clear year-to-year variability in the northernmost part of the area. The surface currents also reveal an intraseasonal tendency with a peak of energy at 62 days at 5°N. Upper layer volume transport across 38°W and between 3°N and 9°N shows a regular seasonal contrast mostly due to the 3°N–6°N region. Year-to-year variations are also clearly evidenced. The eastward transport loses about 35% of its strength in the second half of 1995 compared with 1994. This event is followed by a stronger than usual westward transport in early 1996, and in early 1997, the transport seems abnormally eastward. However, contrary to the seasonal cycle, this interannual variability occurs mainly in the 6°N–9°N band. Interhemispheric transport computed across 7°30′N, between 50°W and 35°W, is northward during the whole period 1992–1995. A seasonal cycle can be detected with maximum transport during boreal winter and minimum in spring. An interesting result is the increasing tendency of the northward transport from April 1993 until boreal fall 1995, when the maximum value for the period is reached.

Isotherm vertical displacements within the thermocline and surface currents were investigated in the tropical Atlantic Ocean from 12°N to 12°S in 1982–1984, the period of the FOCAL-SEQUAL experiment. The study is based on a numerical simulation of an oceanic general circulation model tuned for the study of the equatorial regions, and on the analysis of the large scale thermocline displacements and currents using observed temperature profiles. Ground truth is provided by temperature and currents from moorings, records from inverted echo sounders and tide gauges as well as from drifting buoys. Comparison of the analysis with the ground truth shows that some important aspects of the low frequency variability are “captured” by the analysis when the data base is large enough. On large scales, the simulation generally resembles the analysis. Along the equator, the upwelling signal propagates eastward. The seasonal set-up of the westerly winds is associated with large westward currents, and a following overshoot of the zonal dynamic topography. Otherwise, the zonal dynamic topography is in near-equilibrium with the winds. The North Equatorial Countercurrent is portrayed comparably in the analysis and the simulation, where, after starting as a narrow eastward flow near 5°N, it extends northward through the northern summer. Interannual variations are found both in the analysis and the simulation. In particular, the thermocline flattened early in 1984. However, the simulation differs in significant respects from the real world: the equatorial undercurrent is too weak in the east and the model produces too much variability south of the equator. The 20°C isotherm is too shallow above the core of the thermocline, and the surface layer is too stratified. Because the surface layer is where the wind stress, main forcing of the model is applied, major effort will have to be devoted to parameterizing the near-surface downward mixing of momentum, heat and fresh water.

Tropical instability waves and tropical instability vortices (TIVs) exert major controls on ocean dynamics, thermodynamics, and biology on intraseasonal to seasonal time scales. To understand the fundamental mechanisms at play, a Lagrangian analysis of the 3D circulation of westward-propagating TIVs was performed in a high-resolution Atlantic Ocean simulation. The model reproduces the main temperature and velocity features of the tropical Atlantic mean state and the TIVs. Lagrangian diagnostics were used to track the water masses transported in vortices and exchanged with surrounding waters. The 3D circulation within vortices is consistent with previous observations and dominated by anticyclonic rotation with downwelling and upwelling near the leading and trailing edges of the vortex, respectively. This convergent flow creates sharp gradients at the TIV southwestern edge, where vertical mixing is most efficient. While TIVs remain highly dynamically coherent throughout their lifetime, significant exchanges occur with their surroundings, with 50% of their water being renewed over one rotation cycle. A detailed investigation of the eddies’ sources and sinks reveals that they mostly transport southern water zonally, while northern waters are mostly passing through or fluxed southward in their lee. A notable source of entrained water is the Equatorial Undercurrent.

The total of 13 existing cross-equatorial shipboard current profiling sections taken during the WOCE period between 1990 and 2002 along 35°W are used to determine the mean meridional structure of the zonal top-to-bottom circulation between the Brazilian coast, near 5°S, and 5°N and to estimate mean transports of the individual identified shallow, intermediate and deep current branches. One of the results is that, on the equator, a mean westward Equatorial Intermediate Current below the Equatorial Undercurrent exists.

The sensitivity of tropical ocean dynamics to the ocean lateral mixing orientation is explored using a z-coordinate climate-type ocean general circulation model. Compared to a simulation using horizontal mixing on both tracers and dynamics (HOR), a rotation of the tracer tensor in which mixing occurs along isopycnals (ISOT) improves the structure of the off-equatorial density field, which consequently enhances the off-equatorial circulation through geostrophy. However, the dynamics at the equator in ISOT are degraded compared to observations, as the upper part of the Equatorial Undercurrent (EUC) is too deep and the South Equatorial Current (SEC) is too intense. Next, isopycnal diffusion on momentum is implemented (ISOMT). An examination of the momentum balance at the equator shows that this change in lateral diffusion direction significantly reduces the meridional diffusive flux of momentum at the top of the EUC. This intensifies the EUC, which, in turn, weakens the SEC along the equator through vertical diffusion. The equatorial degradations observed in ISOT disappear. Compared to ISOT and HOR, the separation of the two SEC branches and the equatorial current magnitude in the surface layers are much better reproduced. Moreover, in contrast to the results from ISOT and HOR, isopycnal momentum mixing allows the existence of both horizontal up-gradient and down-gradient eddy momentum fluxes, notably improving the vertical profile of the lateral viscosity term. The ISOMT experiment is therefore shown to be the closest to observations. These results suggest that isopycnal mixing should be used on both tracers and momentum to better simulate the observed eddy effects and the tropical circulation in climate models.

The atolls of the Tuamotu archipelago (South Tropical Central Pacific) are located in the central part of the great anticyclonic gyre where air subsidence leads to an arid climate with high evaporation. The bi-modal system of trade winds created by the existence of dual subtropical high pressure centres (Easter Island and Kermadec Island) drives a global westward drift, the South Equatorial Current, with a speed of 10 cm.s(-1). The zone where the two south trade winds converge (SPCZ) has a strong seasonal variability in position and intensity and controls the eastward counter circulation. During abnormal ENSO events, the counter-circulation is dominant between the equator and 15 degrees S, and the elevated heat content favours active cyclogenesis in French Polynesia. The average thermohaline field is characteristic of a two-layered system : (i) a warm surface mixing layer (0-150 m), with a high salinity core, (> 36.3), South Tropical Water (STW); and (ii) Antarctic Intermediate Water (AIW) with a salinity minimum (34.5). They are separated by a permanent pycnocline. In the central convergent part of the gyre (12 degrees S-30 degrees S), the surface mixing layer is depleted in nutrients; they occur below 200 m with strong vertical gradients in the vicinity of the nutrient-rich AIW. The strong oligotrophy of the euphotic layer is preserved even close to atolls, due to the great depth of the pycnocline-nutricline barriers, which impedes any diapycnal exchange from AIW toward surface layers. Oceanic waters surrounding the Tuamotu atolls constitute a buffered system with very low interannual variability and extreme oligotrophy and clarity.

The hydroclimatic pattern of the French Polynesian zone is determined mainly by the pressure difference between the Easter Island High and the Western Equatorial Low. Trade wind activity is correlated with this gradient and drives surface oceanic water westward to produce the South Equatorial Current, which constitutes the northern part of a vast anticyclonic gyre in the South Pacific. High evaporation from the centre of the anticyclonic high results in the formation of hypersaline surface water, the Southern Tropical Water, which is dense enough to sink. This downwelling process removes plankton and particles from the euphotic zone and contributes to the strong oligotrophy of the gyre. Thermohaline pattern underlines the crucial role of vertical mixing processes in the upper mixed layer, which is isolated from the deeper Antarctic Intermediate Water by a stable pycnocline. Oceanic circulation measured in the band 140–145°W between latitudes 8–28°S during six HYDROPOL cruises (1986–1989) showed a weak (5–10 cm s −1 ) shallow (0–300 m) westward flow, South Equatorial Current, that was often dissected by small eastward countercurrents. The eastward component was increased considerably during the ENSO event of 1987 indicating an aperiodic forcing superimposed on the dynamic instability of the zone. Data gathered in the vicinity of barrier and atoll reefs showed that oceanic conditions (that is oligotrophic mixed layer) prevailed around these structures in spite of periodic thermal oscillations related to the tide regime. The Polynesian oceanic province being mainly in low dynamical conditions, vortex and eddies created around islands are insufficient to modify the vertical stratification or to pump up nutrients from the nutricline, that explains the absence of local upwelling or ‘island mass effect’. The permanence of a clear oligotrophic, euphotic layer then allows coral and reef communities to thrive and reach the atoll stage.

Dans cet article sont présentés les résultats de mesures courantométriques effectuées le long d'une ligne de mouillage, disposée le 31 mars 1990 au 15 septembre 1991 en deux fois, en un point situé vers 6°N et à mi-pente du plateau continental de la Guyane Française. Six courantomètres étaient placés entre 200 m et 2700 m de profondeur. Dans les couches de surface, les observations montrent la présence du courant nord brésilien (CNB) orienté vers le nord-ouest, avec des vitesses moyennes décroissant de 10 cm/s dans la couche 200-250 m à des valeurs presque nulles vers 800 m de profondeur, niveau de l'eau Antarctique intermédiaire. Au printemps, dans la couche 500-800 m, le courant s'inverse : apparaît alors le sous-courant de bord Ouest (SCBO), de vitesse moyenne d'environ 2 cm/s entre février et juin 1991. Les mesures en profondeur effectuées le long du plateau de la pente continentale indiquent la présence d'un courant profond de bord ouest (CPBO) intense, orienté vers le sud-est et s'étendant d'environ 1 000 m de profondeur jusqu'au fond (3 000 m dans le cas de notre mouillage) avec des vitesses maximales moyennes de 20 cm/s vers 2 000/2 070 m de profondeur. (D'après résumé d'auteur)

Pegasus current measurements carried out along a section located off French Guiana are presented; the section was repeated five times from september 1989 to june 1991 during the NOE (région Nord-Ouest équatoriale)/STACS (Sub-Tropical Atlantic Climate Studies) cruises on board the NOAA (National Oceanic and Atmospheric Administration) research vessels. The data (vertical and horizontal distributions) show, as suggested by earliest observations, the presence of different currents. At the surface, and in winter, the North Brazil Current (NBC) flows northwestward (positive), is strong (120 cm/s), confined in the first 250 m, constally trapped (within 250 km) and fed by offshore waters; in summer the NBC strengthens (the velocity increases to 150 cm/s), vertically extends (down to 800 m) and veers offshore. (D'après résumé d'auteur)

A migrating Sound Scattering Layer (SSL) was observed in the Eastern Atlantic equatorial system during warm and cool seasons. Averaged column scattering strength was nearly the same during daytime at 350-400 m depth and during night-time close to the surface. The depth of the nocturnal SSL was correlated with the depth of the thermocline. In the warm stable season, the maximum column scattering strength was found south of the equator in a dome shaped structure. In the cool season, the maximum was found at the northern convergence where the thermocline was well-marked, whereas upwelled waters were acoustically poor. For the warm season, this pattern is consistent with the distribution of chlorophyll, zooplankton and micronekton. For the cool season, it fits also rather well, except for the net-sampled micronekton : this discrepancy could be related to the poor efficiency of the net in sampling adequately a thin layer. Finally, SSL strength coincides well with tuna catch distribution in this area, suggesting a trophic relationship.

Ce rapport présente les résultats des mesures au profileur de courant acoustique à effet Doppler (ADCP) effectuées au cours de la traversée Brest-Nouméa du navire océanographique LE NOROIT de décembre 1990 à mars 1991. Les données ont été recueillies durant le transit Atlantique des Açores à Panama, et pendant la campagne ALIZE 2 le long de l'équateur dans le Pacifique. Le principe de fonctionnement de l'ADCP est rappelé brièvement, et l'installation d'un modèle RDVM-150 fabriqué par RD Instruments sur LE NOROIT est décrite. Les méthodes d'acquisition et de traitement des données sont détaillés. Les résultats sont exposés sous forme de coupes verticales des composantes zonale et méridienne des courants, pour chaque partie de la traversée. (Résumé d'auteur)

Dans le Pacifique tropical, la concentration de données du réseau XBT (expendable bathythermograph) ORSTOM-SIO (Scripps Institutio of Oceanography) autour de trois rails de navigation a permis de construire des champs de températures et de courants géostrophiques. Afin d'obtenir des champs de températures les plus précis possible, le fichier original ORSTOM-SIO a été tout d'abord triplé par la récupération d'un maximum de données XBT et de quelques données hydrologiques et de sondes CTD (conductivité, température, profondeur) auprès de différents services océanographiques. Les champs de températures à trois dimensions (latitude, profondeur, temps) ont été obtenu en regroupant, indépendamment de la longitude, les données de profils thermiques autour de trois rails moyens situés dans les zones ouest, centre et est du Pacifique tropical. Ces champs s'étendent en latitude de 20°S à 20°N pour les zones ouest et centre et de 20°S à 7°N pour la zone est, en profondeur de 0 à 4OO m et en temps de janvier 1979 à décembre 1985

The zonal pressure gradient (ZPG) along the Atlantic equator and the Equatorial Undercurrent (EUC) transport are discussed for four cruises representative of each season. A very clear sea surface slope reversal occurred in the eastern area during autumn as far west as 14°W. An early onset of the equatorial thermocline rising was evident during spring 1983. An eastward equatorial surface jet clearly distinct from the EUC was observed at 35°W and 29°W. The greatest ZPG but the lowest EUC transport were observed during summer 1983. A nearly total absence of the ZPG and a large surfacing of the EUC as far as 10°W characterized the 1984 winter.

In the Atlantic Ocean the summer equatorial nutrient enrichment seems due to the increased vertical mixing induced by the vertical shear between the westward South Equatorial Current and the eastward Equatorial Undercurrent. The high salinity core of the undercurrent is a good indicator of the intensity of vertical mixing: its salinity decreases as vertical mixing increases. There is a linear relationship between the salinity of the core and the nitrate enrichment of the surface layer, and at times when nitrate is limiting, the salinity maximum of the Equatorial Undercurrent can be used as a measure of the primary production.

Data obtained during the Hawaii-to-Tahiti Shuttle Experiment in the central Pacific from the Equator to 17°S are used to study the variability of the two eastward flowing currents in the area: the South Equatorial Countercurrent (SECC), and the South Subsurface Countercurrent (SSCC). The meridional position of the SECC varies between 7 and 14°S, and its transport is affected by the wind stress west of 160°W. In contrast to observations in the eastern Pacific, the SSCC shows seasonal variations and extends as far south as 10°S in austral winter.

Current measurements taken in the equatorial zone at 170°E aboard the R.V. Coriolis reveal equatorial dynamics peculiar to the western Pacific. The west equatorial surface current has several speed cores on either side of the equator; the equatorial undercurrent has one speed core at 100 m and another at 200m; the North Equatorial Countercurrent is connected with the equatorial indercurrent, which has two deep extensions on either side of the equator; beneath the equatorial undercurrent lies a weak west current which appears to reach a depth below 1000m. The flows of these currents are very variable : the equatorial undercurrent may double its flow in 20 days. (Résumé d'auteur)

Resume Description de courants profonds rencontres en aout 1967, entre 1000 m et 1500 m, sur le 170°E, entre 4°N et 4°S. En particulier mise en evidence de 2 courants principaux: un courant portant a l'ouest situe exactement en-dessous du courant de Cromwell, et une extension profonde d'un courant subsuperficiel portant a l'est, centre sur 2°S.

A cruise of the R. V. Capricorne in May 1973, in inner part of the gulf of Guinea, allowed the authors to identify the main part of the Atlantic circulation at the longitude of 5 degrees E, between 4 degrees N and 4 degrees S. It gave new data on the termination of the equatorial undercurrent. At the equator, under the westward south equatorial current flows the Atlantic equatorial undercurrent with a maximum eastward velocity of 90 cm/sec at 30 m depth linked to a salinity maximum higher than 36.20 ppt. Below the equatorial undercurrent, about 80-100 m depth, flows a westward current with a velocity as high as 30 cm/sec. At 4 degrees S, the south equatorial countercurrent is well delineated by a high salinity core (more than 36.10 ppt) at 30 m depth with an eastward velocity core of 40 cm/sec. On the contrary, near 3 degrees 30N, a high salinity core (36.10 ppt) flows westwards with a speed of 40 cm/sec at 40 m depth: it is the "return flow" of the undercurrent (Hisard and Moliere 1974). At 4 degrees N the Guinea current carries eastwards surface salinities of 34.50 ppt at 40 cm/sec. Off Cape Lopez (0 degrees 35'S-8 degrees 42'E) the high salinity core of the undercurrent becomes wider near the shore. It is 25m wide offshore, and 70 m wide near the cape. A part of undercurrent water extends northwards, then flows westwards with the subsurface westward circulation in the inner part of the Gulf of Guinea. Another part flows south-southwestwards in a high salinity tongue along the African coast to 4 degrees S. South-west of Cape Lopez, the trades divergence contributes to an upwelling of cold and high salinity water; this water increases at the Cape Lopez front.

Oceanographic observations were made during the first seven cruises of the FOCAL program along 23°W from July 1982 to January 1984. Using both direct current measurements and the dynamical method the equatorial current system is described. The seasonal variations of the North Equatorial Countercurrent and South Equatorial Current are related to the wind field. A weak eastward surface current near 2°-3°S seems to be a permanent feature during the period of observation.

Observations from "Programme Français Océan Climat dans l'Atlantique Equatorial" (FOCAL) cruises in 1982 and 1984 have confirmed that in the Central Atlantic ocean the North Equatorial countercurrents (NECC) has a well-defined seasonal regime with a very reduced flux during spring. The main core of the NECC always lies between 4°N and 6°N. Focal observations have also shown that a long-term change of NECC may occur and contribute to the development of an "El Nino" type event in the Eastern Atlantic ocean. (Résumé d'auteur)

CO2 evasion within the Atlantic equatorial belt (5°N-5°S) increases from the East to the West (Andrié et al., 1986). Many factors contribute the variations of pCO2 in the equatorial surface waters. To assess their relative importance, a kinematic box model is developed. A 2° x 2° box whose depth is defined by the 24.90 °/°° isopcynal level flows westward from 4°W to 38°W within the Equator-2°S band with the south equatorial current. Time (zonal) evolution of nitrate, total CO2, total alkalinity and mass, and of the corresponding water pCO2, are simulated taking into account advection, meridional divergence, diffusion, biological activity and gas exchange. Initial and boundary conditions are taken from the FOCAL 4 (July-August 1983) data set

All the high-productive systems of the Eastern tropical Atlantic between 20°N and 15°S - coastal upwelling, equatorial upwelling, and thermal domes-are partly controlled by the subsuperficial equatorial countercurrent system, which feeds all of them with Southern Atlantic Central Water. However, the enrichment processes in the systems are quite different, and thus the structures differ from one another. Two types have to be considered : 1) the quasi geostrophic ones which are two-layered systems belonging to the "typical tropical situations", where the primary production is controlled by the depth of the nitraciline; 2) the systems including coastal upwelling and equatorial divergence in summer, where the warm nitrate-depleted mixed layer is absent. (D'après résumé d'auteur)