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Gérard Eldin

Abstract

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.

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Thierry Delcroix, Gerard Eldin, and Christian Hénin

Abstract

As part of the international TOGA program, the ORSTOM Center in Nouméa (New Caledonia) initiated in January 1984 a series of semi-annual cruises along the 165°E meridian from 20°S to 10°N, across the equatorial current system of the western Pacific. This paper presents an analysis of the first six hydrographic (0-1000 m) and current (0-600 m) sections.

A detailed description of “typical” January 1986 vertical structures of temperature, salinity and zonal measured velocity is offered. Differences are noted with structures previously obtained in the tropical Pacific. Compared to the central and eastern Pacific, the 165°E dataset evidences a much weaker equatorial upwelling and deeper surface isothermal layer and subsurface currents. Compared to the few western Pacific measurements, the two speed cores of the Equatorial Undercurrent (EUC) previously reported at 100 and 200 m are not observed here.

Special attention is given to the eastward equatorial jet (2°S-2°N; 0-75 m) measured in January 1985 when westerly winds were present from the north of New Guinea to 160°E.

For the purpose of volume transport calculations, eastward flows at 165°E are not sufficiently separated to be easily differentiated. A definition based on an isodensity surface (sigma-t=23.5 kg m−3) is thus adopted to discriminate the EUC and the North and South Subsurface Countercurrents (NSCC, SSCC) from the North and South Equatorial Countercurrents (NECC, SECC). The EUC is assumed to lie within 2 degrees of the equator below sigma-t = 23.5 kg m−3. Using these current boundaries, transports of the South Equatorial Current (SEC), EUC and NECC agree within 30% with estimates previously computed in the western, central and eastern Pacific; e.g., the mean NECC transport is 27 ± 13 106 m3 s−1. A noticeable exception is the SECC transport which is two to four times as much as that estimated for the central Pacific. The weaker (stronger) EUC and the farthest northern (southern) NECC were observed during the three January (June-July) cruises.

Large transport variability was observed and calls for a denser time-space sampling rate of observation. Hence, the credibility of dynamic height and geostrophic currents calculated from XBT (0-400 m) and mean temperature-salinity (T-S) curves are investigated. Major limitations, stressed by the semiannual transects, are caused by:

1) notable density variations in the 400–1000 m layer, and

2) the effects of variability of the T-S relation in the 0–400 m layer.

These two points can each result in signals of as much as 6 dyn cm in the surface dynamic height and therefore significant errors in geostrophic velocities calculated from individual cruises. These errors are generally not accounted for when the geostrophic method is applied to XBT data. However, poleward of 2° latitude, a fair agreement is observed between mean geostrophic and measured currents (5 cm s−1 rms difference), after eliminating the errors introduced by the 400 db reference level and mean T-S curves. In the 2°S-2°N band, the agreement is only qualitative (30 cm s−1 rms difference) and better in the EUC than in surface flows.

Deeper temperature sampling and a better knowledge of T-S variability than the present one are particularly recommended to monitor the equatorial current system from XBTs in the western tropical Pacific Ocean.

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Thierry Delcroix, François Masia, and Gérard Eldin

Abstract

Profiling current meter (PCM) measurements under a drifting buoy are compared with concurrent shipboard acoustic Doppler current profiler (ADCP) measurements carried out in the western equatorial Pacific in March 1991, from 10°S to 7°N along the 165°E meridian. The mean (ADCP minus PCM)±rms differences between zonal and meridional velocity components are 5.7±11.2 cm s−1 and 0.0±8.8 cm s−1, respectively, when PCM measurements are relative to 600 m. The mean±rms differences decrease to 2.3±7.8 cm s−1 and 0.0±6.3 cm s−1 when the PCM and ADCP data are both referenced to the same layer (on a mean, 16–240 m). As compared with ADCP, it is found that PCM underestimates velocities of less than 20 cm s−1 by about 25%.

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Alice Pietri, Pierre Testor, Vincent Echevin, Alexis Chaigneau, Laurent Mortier, Gerard Eldin, and Carmen Grados

Abstract

The upwelling system off southern Peru has been observed using an autonomous underwater vehicle (a Slocum glider) during October–November 2008. Nine cross-front sections have been carried out across an intense upwelling cell near 14°S. During almost two months, profiles of temperature, salinity, and fluorescence were collected at less than 1-km resolution, between the surface and 200-m depth. Estimates of alongshore absolute geostrophic velocities were inferred from the density field and the glider drift between two surfacings. In the frontal region, salinity and biogeochemical fields displayed cross-shore submesoscale filamentary structures throughout the mission. Those features presented a width of 10–20 km, a vertical extent of ~150 m, and appeared to propagate toward the shore. They were steeper than isopycnals and kept an aspect ratio close to f/N, the inverse of the Prandtl ratio. These filamentary structures may be interpreted mainly as a manifestation of submesoscale turbulence through stirring of the salinity gradients by the mesoscale eddy field. However, meandering of the front or cross-frontal wind-driven instabilities could also play a role in inducing vertical velocities.

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Frédéric Marin, Elodie Kestenare, Thierry Delcroix, Fabien Durand, Sophie Cravatte, Gérard Eldin, and Romain Bourdallé-Badie

Abstract

A large reversal of zonal transport below the thermocline was observed over a period of 6 months in the western Pacific Ocean between 2°S and the equator [from 26.2 Sv (1 Sv ≡ 106 m3 s−1) eastward in October 1999 to 28.6 Sv westward in April 2000]. To document this reversal and assess its origin, an unprecedented collection of ADCP observations of zonal currents (2004–06), together with a realistic OGCM simulation of the tropical Pacific, was analyzed. The results of this study indicate that this reversal is the signature of intense annual variability in the subsurface zonal circulation at the equator, at the level of the Equatorial Intermediate Current (EIC) and the Lower Equatorial Intermediate Current (L-EIC). In this study, the EIC and the L-EIC are both shown to reverse seasonally to eastward currents in boreal spring (and winter for the L-EIC) over a large depth range extending from 300 m to at least 1200 m. The peak-to-peak amplitude of the annual cycle of subthermocline zonal currents at 165°E in the model is ∼30 cm s−1 at the depth of the EIC, and ∼20 cm s−1 at the depth of the L-EIC, corresponding to a mass transport change as large as ∼100 Sv for the annual cycle of near-equatorial zonal transport integrated between 2°S and 2°N and between 410- and 1340-m depths. Zonal circulations on both sides of the equator (roughly within 2° and 5.5° in latitude) partially compensate for the large transport variability. The main characteristics of the annual variability of middepth modeled currents and subsurface temperature (e.g., zonal and vertical phase velocities, meridional structure) are consistent, in the OGCM simulation, with the presence, beneath the thermocline, of a vertically propagating equatorial Rossby wave forced by the westward-propagating component of the annual equatorial zonal wind stress. Interannual modulation of the annual variability in subthermocline equatorial transport is discussed.

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Cyril Germineaud, Alexandre Ganachaud, Janet Sprintall, Sophie Cravatte, Gérard Eldin, Marion S. Alberty, and Emilien Privat

Abstract

The semienclosed Solomon Sea is the final passage in the equatorward transit of the South Pacific western boundary currents (WBCs) that play a key role in heat and mass budgets of the equatorial Pacific. The Solomon WBCs and their associated water properties are examined using data from two oceanographic cruises undertaken during the contrasting trade wind seasons: July 2012 and March 2014. The mean circulation and associated transports with uncertainties is determined from the cruise data using a unique configuration of an inverse box model formulated based on measured shipboard acoustic Doppler current profiler velocities. An intense inflow of 36 Sv is found entering the Solomon Sea in July–August 2012 that falls by 70% to 11 Sv in March 2014. Large differences are also found in the total transport partitioning through each of the major exit passages during each season. Different water masses are found in the WBC stream northeast of the Solomon Islands that are likely related to a northern stream of the South Equatorial Current. Within the Solomon Sea, isopycnal salinity gradients are gradually stronger than within the subtropical Pacific, likely induced by stronger diapycnal mixing processes. WBC pathways exhibit distinct water mass signatures in salinity, oxygen, and nutrients that can be traced across the Solomon Sea, associated with significant water mass modifications at the northern exit straits and south of the Woodlark Island.

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