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Bruno Blanke and Stéphane Raynaud

Abstract

Three-dimensional monthly velocity fields from an ocean general circulation model are used to study the annual mean mass balance of the Pacific Equatorial Undercurrent (EUC). Eulerian diagnostics are used to evaluate the various meridional, vertical, and zonal mass fluxes related to the EUC. There are several distinct regimes along the equator, showing clear asymmetries between the western and eastern parts of the basin, and between the northern and southern edges of the EUC. Meridional fluxes are decomposed into pure Ekman divergence and geostrophic convergence, and it is shown that the asymmetries are mainly related to the spatial structure of the Ekman divergence, and thus to that of the trade winds. Lagrangian calculations are used to evaluate accurately the mass transfers between various sections of the EUC and between the EUC domain and the Tropics. The authors show that geostrophic convergence only ventilates the upper layers of the EUC and that the EUC really is a tongue of water flowing from the western Pacific to the Galapagos Islands and beyond. Finally, Lagrangian integrations extended to extratropical regions show that the EUC contributes to an exchange of water between the southern and northern Pacific (and the Indian Ocean through the Indonesian Throughflow): The equatorial zonal pressure gradient draws water from the western boundary currents that originate mostly in the south subtropical gyre. The poleward Ekman divergence associated with the equatorial upwelling distributes EUC water over the surface, with significant recirculation within the EUC (more than 15% of the total transport at 150°W).

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Bruno Blanke and Pascale Delecluse

Abstract

The embedment of a 1.5 turbulence closure model in an ocean general circulation model of the equatorial Atlantic is presented. The eddy viscosity and diffusivity involved in the vertical mixing are defined as the product of a characteristic turbulent velocity—the root square of the turbulent kinetic energy—and a characteristic mixing length. The turbulent kinetic energy is defined through a prognostic equation while the turbulent length scales are defined by a diagnostic formulation. The results of an experiment that includes this closure scheme are compared to the results issued from another experiment that includes a Richardson number-dependent parameterization of the mixing coefficients. The two simulations were performed over the tropical Atlantic during the 1982–1984 period, which allows direct comparisons with data from the FOCAL and SEQUAL experiments. Obvious contrasts between the two experiments on the sea surface temperature and on the dynamics indicate that the turbulent vertical diffusion plays a major role in the surface processes simulated by the model. Comparisons with available observations show that the introduction of the 1.5 closure scheme improves the ability of the general circulation model to represent the sea surface temperature, the vertical mixed-layer structure, the equatorial meridional circulation cell, as well as the equatorial undercurrent, which becomes more energetic. Despite strong assumptions in the turbulent vertical mixing scheme, the turbulent fields provided by the turbulent kinetic parameterization allow a comparison with direct measurements of turbulence performed in the tropical oceans and highlight the complex behavior of turbulent mixing in the ocean.

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Bruno Blanke, J. David Neelin, and David Gutzler

Abstract

One open question in El Niño–Southern Oscillation (ENSO) simulation and predictability is the role of random forcing by atmospheric variability with short correlation times, on coupled variability with interannual timescales. The discussion of this question requires a quantitative assessment of the stochastic component of the wind stress forcing. Self-consistent estimates of this noise (the stochastic forcing) can be made quite naturally in an empirical atmospheric model that uses a statistical estimate of the relationship between sea surface temperature (SST) and wind stress anomaly patterns as the deterministic feedback between the ocean and the atmosphere. The authors use such an empirical model as the atmospheric component of a hybrid coupled model, coupled to the GFDL ocean general circulation model. The authors define as residual the fraction of the Florida State University wind stress not explained by the empirical atmosphere run from observed SST, and a noise product is constructed by random picks among monthly maps of this residual.

The impact of included or excluded noise is assessed with several ensembles of simulations. The model is run in coupled regimes where, in the absence of noise, it is perfectly periodic: in the presence of prescribed seasonal variability, the model is strongly frequency locked on a 2-yr period; in annual average conditions it has a somewhat longer inherent ENSO period (30 months). Addition of noise brings an irregular behavior that is considerably richer in spatial patterns as well as in temporal structures. The broadening of the model ENSO spectral peak is roughly comparable to observed. The tendency to frequency lock to subharmonic resonances of the seasonal cycle tends to increase the broadening and to emphasize lower frequencies. An inclination to phase lock to preferred seasons persists even in the presence of noise-induced irregularity. Natural uncoupled atmospheric variability is thus a strong candidate for explaining the observed aperiodicity in ENSO time series. Model–model hindcast experiments also suggest the importance of atmospheric noise in setting limits to ENSO predictability.

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Yann Friocourt, Sybren Drijfhout, and Bruno Blanke

Abstract

The dynamics of the baroclinic slope current system along the western European margin in the Bay of Biscay and along the northern Iberian Peninsula are investigated in two different models, one analytical and one numerical. Investigated here is the hypothesis that the steady-state slope current system is driven by the large-scale meridional density gradients. An analysis of the observed density fields evidences a four-layer structure with meridional gradients of alternate signs, which is also found in the numerical model. The linear analytical model of the continental margin shows that such a density structure is enough to obtain a steady-state four-layer slope current system comparable to the observed annual mean circulation. The slope currents result from a balance between bottom friction and meridional density gradients. The numerical simulation with an ocean general circulation model forced only by the large-scale density gradients at the lateral boundaries presents a four-layer slope current system similar to the circulation obtained in the analytical model. The study confirms that the large-scale meridional density gradients are the main driving mechanism for the steady-state slope current system; the large seasonality of these currents, however, requires a more extended model, which is discussed in a companion paper (Part II).

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Daniele Iudicone, Gurvan Madec, Bruno Blanke, and Sabrina Speich

Abstract

Despite the renewed interest in the Southern Ocean, there are yet many unknowns because of the scarcity of measurements and the complexity of the thermohaline circulation. Hence the authors present here the analysis of the thermohaline circulation of the Southern Ocean of a steady-state simulation of a coupled ice–ocean model. The study aims to clarify the roles of surface fluxes and internal mixing, with focus on the mechanisms of the upper branch of the overturning. A quantitative dynamical analysis of the water-mass transformation has been performed using a new method. Surface fluxes, including the effect of the penetrative solar radiation, produce almost 40 Sv (1 Sv ≡ 106 m3 s−1) of Subantarctic Mode Water while about 5 Sv of the densest water masses (γ > 28.2) are formed by brine rejection on the shelves of Antarctica and in the Weddell Sea. Mixing transforms one-half of the Subantarctic Mode Water into intermediate water and Upper Circumpolar Deep Water while bottom water is produced by Lower Circumpolar Deep Water and North Atlantic Deep Water mixing with shelf water. The upwelling of part of the North Atlantic Deep Water inflow is due to internal processes, mainly downward propagation of the surface freshwater excess via vertical mixing at the base of the mixed layer. A complementary Lagrangian analysis of the thermohaline circulation will be presented in a companion paper.

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Bruno Blanke, Michel Arhan, Gurvan Madec, and Sophie Roche

Abstract

Monthly mean velocity fields from a global ocean general circulation model are used to study the main circulation patterns within the upper 1200 m of the equatorial Atlantic. Some recently developed Lagrangian techniques are used to picture and quantify the routes followed in the model by distinct water mass classes, defined by their initial temperature on model transatlantic sections at 10°S and 10°N. The qualitative description in terms of equatorial pathways of this warm component of the so-called global “conveyor belt” is found coherent with the most recent circulation schemes inferred from direct measurements. Diagnostics emphasize the crucial role of the western boundary current system and that of the equatorial subsurface jets in distributing the flow in the equatorial domain, both for northward-flowing and southward-recirculating warm water masses. As the model tracer fields are constrained to remain close to the observed climatology outside the equatorial strip, the circulation calculated by the model is also shown to fairly reproduce the intensity of the net northward interhemispheric warm water exchange as inferred from direct measurements, as well as known conversions of warm water masses within the tropical Atlantic.

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Daniele Iudicone, Sabrina Speich, Gurvan Madec, and Bruno Blanke

Abstract

Recent studies have proposed the Southern Ocean as the site of large water-mass transformations; other studies propose that this basin is among the main drivers for North Atlantic Deep Water (NADW) circulation. A modeling contribution toward understanding the role of this basin in the global thermohaline circulation can thus be of interest. In particular, key pathways and transformations associated with the thermohaline circulation in the Southern Ocean of an ice–ocean coupled model have been identified here through the extensive use of quantitative Lagrangian diagnostics. The model Southern Ocean is characterized by a shallow overturning circulation transforming 20 Sv (1 Sv ≡ 106 m3 s−1) of thermocline waters into mode waters and a deep overturning related to the formation of Antarctic Bottom Water. Mode and intermediate waters contribute to 80% of the upper branch of the overturning in the Atlantic Ocean north of 30°S. A net upwelling of 11.5 Sv of Circumpolar Deep Waters is simulated in the Southern Ocean. Antarctic Bottom Water upwells into deep layers in the Pacific basin, forming Circumpolar Deep Water and subsurface thermocline water. The Southern Ocean is a powerful consumer of NADW: about 40% of NADW net export was found to upwell in the Southern Ocean, and 40% is transformed into Antarctic Bottom Water. The upwelling occurs south of the Polar Front and mainly in the Indian and Pacific Ocean sectors. The transformation of NADW to lighter water occurs in two steps: vertical mixing at the base of the mixed layer first decreases the salinity of the deep water upwelling south of the Antarctic Circumpolar Current, followed by heat input by air–sea and diffusive fluxes to complete the transformation to mode and intermediate waters.

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Yann Friocourt, Bruno Blanke, Sybren Drijfhout, and Sabrina Speich

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The seasonality of the baroclinic slope current system along the western European margin in the Bay of Biscay and along the northern Iberian Peninsula is investigated in a joint analysis of an analytical model and numerical simulations with various forcings. A distinction is made between local winds and basin-scale winds, in which the effect of the latter is indirectly apparent through the basin-scale density gradients. The slope currents are mainly forced by the large-scale structure of the density field. The analysis indicates significant differences in the behavior of the uppermost slope current and of the deeper currents. At all depths, seasonal variations in the large-scale density structure of the ocean alter the strength of the slope currents but are not able to cause robust, long-lasting reversals. Reversals of the uppermost slope current appear to be caused by changes in the alongshore component of the local wind stress, provided that the opposing forcing from the density structure is weak enough. However, the deeper slope currents are not very much affected by the wind stress, so that flow reversals can be explained neither by the wind nor by seasonal changes in the density structure. A numerical simulation suggests that the reversals of the deeper slope currents are at least partly forced by seasonal changes in the flow upstream of the slope current system. The authors demonstrate that the larger part of these seasonal changes is associated with annual baroclinic Rossby waves caused by the seasonal cycle of the large-scale wind stress over the whole basin.

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Yann Friocourt, Sybren Drijfhout, Bruno Blanke, and Sabrina Speich

Abstract

The northward export of intermediate water from Drake Passage is investigated in two global ocean general circulation models (GCMs) by means of quantitative particle tracing diagnostics. This study shows that a total of about 23 Sv (Sv ≡ 106 m3 s−1) is exported from Drake Passage to the equator. The Atlantic and Pacific Oceans are the main catchment basins with 7 and 15 Sv, respectively. Only 1–2 Sv of the water exported to the Atlantic equator follow the direct cold route from Drake Passage without entering the Indian Ocean. The remainder loops first into the Indian Ocean subtropical gyre and flows eventually into the Atlantic Ocean by Agulhas leakage. The authors assess the robustness of a theory that relates the export from Drake Passage to the equator to the wind stress over the Southern Ocean. Our GCM results are in reasonable agreement with the theory that predicts the total export. However, the theory cannot be applied to individual basins because of interocean exchanges through the “supergyre” mechanism and other nonlinear processes such as the Agulhas rings. The export of water from Drake Passage starts mainly as an Ekman flow just northward of the latitude band of the Antarctic Circumpolar Current south of South America. Waters quickly subduct and are transferred to the ocean interior as they travel equatorward. They flow along the eastern boundaries in the Sverdrup interior and cross the southern basins northwestward to reach the equator within the western boundary current systems.

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Bruno Blanke, Michel Arhan, Sabrina Speich, and Karine Pailler

Abstract

The monthly mean velocity, salinity, and temperature fields of a numerical simulation of the World Ocean climatological circulation are used to study the intensity and pathways associated with the meridional overturning in the North Atlantic. Lagrangian diagnostics based on the computation of several hundreds of thousands of individual three-dimensional trajectories are combined with an appropriate study of water mass potential densities in order to describe the warm and cold limbs of the so-called conveyor belt. Circulation schemes are established for both limbs of the overturning, and can be easily compared with schemes or transport estimates deduced from direct measurements, as the model temperature and salinity fields are constrained to remain close to the observed climatology. Diagnostics emphasize most typical pathways as well as main mass transfers that lead to the establishment of such numerical circulation schemes.

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