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Bach Lien Hua
,
Frédéric Marin
, and
Richard Schopp

Abstract

A fully three-dimensional primitive equation simulation is performed to “reunite” the local equatorial dynamics of the subsurface countercurrents (SCCs) and thermostad with the large-scale tropical ventilated ocean dynamics. It captures (i) the main characteristics of the equatorial thermostad, the SCCs' location and their eastward evolution, and the potential vorticity budget with its equatorial homogenization to zero values and (ii) the large-scale meridional shoaling of the thermocline equatorward. It supports the idea that the two-dimensional Hadley cell mechanism proposed by Marin et al. is a candidate able to operate in a fully three-dimensional ocean. The main difference between the 2D Hadley cell mechanism and the oceanic 3D case is that for the 3D case the large-scale meridional velocity at zeroth order is geostrophic, while the cell mechanism is a next-order, small-scale mechanism. A detailed budget of the zonal momentum equation is provided for the ageostrophic dynamics at work in the SCCs. The mean meridional advection and the Coriolis term dominate, discounting the possibility that lateral eddies play a major role for the SCCs' creation. A 3½-layer idealized ventilation model, calibrated to the three-dimensional simulation parameters, is able not only to capture the tropical density structure, but also to isolate the main controlling factors leading to the triggering of the equatorial secondary cells with its associated jet and thermostad, namely, the shoaling of the equatorial thermocline because of low potential vorticity injection at distant subduction latitudes. It is also shown that equatorial recirculation gyres play a quantitative role that may be of the same order of magnitude as ventilation from higher latitudes.

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Frédéric Marin
,
Richard Schopp
, and
Bach Lien Hua

Abstract

Sensitivity tests are performed to assess the respective influences of the large-scale ventilation and of the near-equatorial winds on the dynamics of the the subsurface countercurrents (SCCs) and thermostad. They show that the intensity of the inertial jets is a function of the potential vorticity (PV) values at subduction and that stronger jets are favored by low PV injection, forced in the authors' framework either by a deep mixed layer at subduction and/or by an injection of PV at lower latitudes. Such circumstances lead to a strong meridional shoaling of the thermocline near the equator. The resulting inertial jets occur at about 3°N in the western part of the basin and are the poleward limit of a near-0 PV region and of an equatorial thermostad. A necessary condition for the existence of inertial jets is that the equatorial wind fetch is large enough, otherwise only weak time-mean eastward currents are produced by a nonlinear rectification of instability waves farther away from the equator. The presence of a North Equatorial Countercurrent does not constitute a barrier for equatorward motions within the lower thermocline, and inertial jets are still controlled by the meridional slope of the SSCs' layer setup through the establishment of tropical PV pools predicted by ventilation theory.

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Frédéric Marin
,
Bach Lien Hua
, and
Richard Schopp

Abstract

From a numerical simulation of the Atlantic Ocean, Jochum and Malanotte-Rizzoli provide evidence that the equatorial subsurface countercurrents can be triggered by tropical instability waves through eddy–mean flow interactions in a low-Rossby-number regime. Adapting the transformed Eulerian mean formalism to a shoaling jet, they propose eddy heat fluxes to be the driving mechanism for the subsurface countercurrents. Here it is shown that such a formalism relying on the existence of a residual meridional streamfunction cannot be applied to a shoaling jet, so that the eddy heat fluxes term in the zonal momentum equation cannot be rigorously justified. Moreover, the role of the zonal pressure gradient that was dropped in their study needs to be reassessed. Despite this mathematical questioning of Jochum and Malanotte-Rizzoli’s framework, the authors agree with them that eddy heat fluxes may contribute to the dynamics of the subsurface countercurrents.

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Sophie Cravatte
,
William S. Kessler
, and
Frédéric Marin

Abstract

Argo float data in the tropical Pacific Ocean during January 2003–August 2011 are analyzed to obtain Lagrangian subsurface velocities at their parking depths. Maps of mean zonal velocities at 1000 and 1500 m are presented. At both depths, a series of alternating westward and eastward zonal jets with a meridional scale of 1.5° is seen at the basin scale from 10°S to 10°N. These alternating jets, with mean speeds about 5 cm s−1, are clearly present in the western and central parts of the basin but weaken and disappear approaching the eastern coast. They are stronger in the Southern Hemisphere. Along the equator at both 1000 and 1500 m, a westward jet is seen. The jets closer to the equator are remarkably zonally coherent across the basin, but the jets farther poleward appear broken in several segments. In the western half of the basin, the 1000-m zonal jets appear to slant slightly poleward from east to west. At the western boundary in the south (east of Solomon Islands and Papua New Guinea), the alternating jets appear to connect in narrow boundary currents. Seasonal zonal velocity anomalies at 1000 and 1500 m are observed to propagate westward across the basin; they are consistent with annual vertically propagating Rossby waves superimposed on the mean zonal jets. Their meridional structure suggests that more than one meridional mode is present.

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Julien Jouanno
,
Frédéric Marin
,
Yves du Penhoat
, and
Jean-Marc Molines

Abstract

A regional numerical model of the tropical Atlantic Ocean and observations are analyzed to investigate the intraseasonal fluctuations of the sea surface temperature at the equator in the Gulf of Guinea. Results indicate that the seasonal cooling in this region is significantly shaped by short-duration cooling events caused by wind-forced equatorial waves: mixed Rossby–gravity waves within the 12–20-day period band, inertia–gravity waves with periods below 11 days, and equatorially trapped Kelvin waves with periods between 25 and 40 days. In these different ranges of frequencies, it is shown that the wave-induced horizontal oscillations of the northern front of the mean cold tongue dominate the variations of mixed layer temperature near the equator. But the model mixed layer heat budget also shows that the equatorial waves make a significant contribution to the mixed layer heat budget through modulation of the turbulent cooling, especially above the core of the Equatorial Undercurrent (EUC). The turbulent cooling variability is found to be mainly controlled by the intraseasonal modulation of the vertical shear in the upper ocean. This mechanism is maximum during periods of seasonal cooling, especially in boreal summer, when the surface South Equatorial Current is strongest and between 2°S and the equator, where the presence of the EUC provides a background vertical shear in the upper ocean. It applies for the three types of intraseasonal waves. Inertia–gravity waves also modulate the turbulent heat flux at the equator through vertical displacement of the core of the EUC in response to equatorial divergence and convergence.

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S. Cravatte
,
Elodie Kestenare
,
Frédéric Marin
,
Pierre Dutrieux
, and
Eric Firing

Abstract

The mean subthermocline and intermediate zonal circulation in the tropical Pacific is investigated using a compilation of shipboard ADCP measurements and absolute geostrophic velocities constructed from a high-resolution 0–2000-m Argo climatology referenced to a 1000-m velocity field derived from Argo float drifts. This reference field is dominated by basinwide alternating zonal jets with a meridional wavelength of about 3°. In regions where the sampling of SADCP data is sufficient, the consistency between the two independent datasets is striking; using the Argo drift reference is crucial to capture the current structures. Two apparently distinct systems of alternating westward and eastward zonal jets are seen in both datasets equatorward of 10°: a series of low-latitude subthermocline currents (LLSCs) below the thermocline, extending from about 200 to 800 m, including the eastward Tsuchiya jets; and a series of low-latitude intermediate currents (LLICs), extending from about 700 to at least 2000 m. These systems seem to merge poleward of 10°. Both series shoal to lighter densities eastward. The subthermocline currents and their associated potential vorticity structures undergo a major shift near 155°W, suggesting some difference in the dynamic regime between the regions west and east of this longitude. Differing behaviors (the LLSCs tend to angle poleward to the east, whereas the LLICs angle slightly equatorward) suggest that these jets may be dynamically distinct, with different forcing mechanisms.

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Audrey Delpech
,
Sophie Cravatte
,
Frédéric Marin
,
Yves Morel
,
Enzo Gronchi
, and
Elodie Kestenare

Abstract

The middepth ocean circulation in the tropical Pacific is dominated by sets of alternating eastward and westward jets. The origin and transport properties of these flow features remain in many ways an open question, all the more crucial since their usual underestimation in ocean global circulation models has been identified as a potential bias for the misrepresentation of the oxygen minimum zones. In this study, we analyze the water mass properties associated with these systems of jets using velocity and hydrographic sections. Data acquired during a dedicated cruise carried out in the western part of the basin and supplemented by cross-equatorial sections from historical cruises in the central and eastern parts are analyzed. While it is confirmed that the near-equatorial jets carry oxygen anomalies, contributing to the ventilation of the eastern tropical Pacific, the data also revealed unexpected features. Tracer distributions (oxygen, salinity, and potential vorticity) show the presence of fronts extending from 500 to 3000 m and flanked by homogeneous regions. These structures define meridional staircase profiles that coincide with the alternating velocity profiles. Historical data confirm their presence in the off-equatorial deep tropical ocean with a zonal and temporal coherence throughout the basin. These observations support existing theoretical studies involving homogenization by isopycnic turbulent mixing in the formation of staircase profiles and maintenance of zonal jets. The effect of other processes on the equilibration of tracer structures is also discussed.

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Audrey Delpech
,
Claire Ménesguen
,
Yves Morel
,
Leif N. Thomas
,
Frédéric Marin
,
Sophie Cravatte
, and
Sylvie Le Gentil

Abstract

At low latitudes in the ocean, the deep currents are shaped into narrow jets flowing eastward and westward, reversing periodically with latitude between 15°S and 15°N. These jets are present from the thermocline to the bottom. The energy sources and the physical mechanisms responsible for their formation are still debated and poorly understood. This study explores the role of the destabilization of intra-annual equatorial waves in the jets’ formation process, as these waves are known to be an important energy source at low latitudes. The study focuses particularly on the role of barotropic Rossby waves as a first step toward understanding the relevant physical mechanisms. It is shown from a set of idealized numerical simulations and analytical solutions that nonlinear triad interactions (NLTIs) play a crucial role in the transfer of energy toward jet-like structures (long waves with short meridional wavelengths) that induce a zonal residual mean circulation. The sensitivity of the instability emergence and the scale selection of the jet-like secondary wave to the forced primary wave are analyzed. For realistic amplitudes around 5–20 cm s−1, the primary waves that produce the most realistic jet-like structures are zonally propagating intra-annual waves with periods between 60 and 130 days and wavelengths between 200 and 300 km. The NLTI mechanism is a first step toward the generation of a permanent jet-structured circulation and is discussed in the context of turbulent cascade theories.

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Julien Jouanno
,
Frédéric Marin
,
Yves du Penhoat
,
Jean Marc Molines
, and
Julio Sheinbaum

Abstract

A numerical simulation of the tropical Atlantic Ocean indicates that surface cooling in upwelling zones of the Gulf of Guinea is mostly due to vertical mixing. At the seasonal scale, the spatial structure and the time variability of the northern and southern branches of the South Equatorial Current (SEC), and of the Guinea Current, are correlated with the timing and distribution of turbulent heat fluxes in the Gulf of Guinea. Through modulation of the velocity shear at the subsurface, these surface currents control the vertical turbulent exchanges, bringing cold and nutrient-rich waters to the surface. This mechanism explains the seasonality and spatial distribution of surface chlorophyll concentrations better than the generally accepted hypothesis that thermocline movements control the nutrient flux. The position of the southern SEC explains why the cold tongue and high chlorophyll concentrations extend from the equator to 4°S in the southeastern part of the basin.

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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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