Dynamique océanique dans les golfes Persique et d'Oman

The circulation in the Persian Gulf and the Gulf of Oman is a complex and badly known system that depends on the interaction of several forcing mechanisms over a range of different time scales. On a timescale of few hours, the tide plays a significant role. On a seasonal and inter-annual basis, there is a thermohaline circulation, specific of a concentration basin. It is characterized by a cyclonic circulation with strong seasonal variability and by the mixing of two different water masses : a dense outflow water formed in the Persian Gulf (Persian Gulf Water) that flows throught the Gulf of Oman and a overlying lighter water from the Indian Ocean (Indian Ocean Surface Water). The analysis of the data from the GOGP experiment, carried out in autumn 1999, shows the different PGW and IOSW water masses, as well as a large temporal and spatial variability of the hydrologic and velocity structures. The evolution of the PGW outflow features down to the Arabian sea is described. A realistic model of the circulation in both gulfs is designed to understand the different aspects of the oceanic dynamics. The circulation in the Persian Gulfe, the mixing of the PGW and IOSW, and the PGW formation and movement towards the Gulf of Oman are explained and a large variability is highlighted. A process study determines the role of the different forcing mechanisms, and the impact of latent heat fluw parameterization is underlined. Finally, a study of the sensitivity to the variation of some not very well-known forcing (rivers, evaporation) and to the turbulence parameterization is presented.La circulation dans les golfes Persique et d'Oman est un système complexe et mal connu qui résulte de l'interaction de nombreux forçages à différentes échelles de temps. A l'échelle de quelques heures , la marée joue un rôle important. A l'échelle saisonnière et inter-annuelle, une circulation thermohaline, typique d'un bassin de concentration, se met en place. Elle est caractérisée par une circulation cyclonique à forte variabilité saisonnière et par l'interaction de deux masses d'eaux : une eau dense formée dans le Golfe Persique (Persian Gulfe Water) qui s'écoule sous la forme d'une veine dans le Golfe d'Oman et une eau plus légère en provenance des couches de surface de l'Océan Indien (Indian Ocean surface water). L'analyse des données de la campagne GOGP99, réalisée en automne 1999, montre l'existence des deux masses d'eaux PGW et IOSW, ainsi qu'une variabilité temporelle et spatiale importante des structures hydrologiques et courantologiques, l'évolution des caractéristiques de la veine d'eau PGW jusqu'en mer d'Arabie est précisée. Un modèle réaliste de la circulation dans les deux golfes est développé pour comprendre les différents aspects de la dynamique. La circulation dans le golfe persique, les interactions entre les masses d'eau PGW et IOSW, la formation puis le développement de l'eau PGW dans le Golfe d'Oman sont détaillés et une variabilité importante est mise en évidence. Une étude de processus précise la part des différents forçages. L'importance de la paramétrisation des flux de chaleur est mise en évidence. Finalement,une étude de sensibilité à la variation de certains forçages (fleuve, évaporation) et à la paramétrisation de la turbulence est réalisée

Investigation of tropical eel spawning area in the South-Western Indian Ocean: Influence of the oceanic circulation

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Dynamique océanique dans les golfes Persiques et d'Oman

La circulation dans les golfes Persique et d'Oman est un système complexe et mal connu qui résulte de l'interaction de nombreux forçages à différentes échelles de temps. A l'échelle de quelques heures , la marée joue un rôle important. A l'échelle saisonnière et inter-annuelle, une circulation thermohaline, typique d'un bassin de concentration, se met en place. Elle est caractérisée par une circulation cyclonique à forte variabilité saisonnière et par l'interaction de deux masses d'eaux : une eau dense formée dans le Golfe Persique (Persian Gulfe Water) qui s'écoule sous la forme d'une veine dans le Golfe d'Oman et une eau plus légère en provenance des couches de surface de l'Océan Indien (Indian Ocean surface water). L'analyse des données de la campagne GOGP99, réalisée en automne 1999, montre l'existence des deux masses d'eaux PGW et IOSW, ainsi qu'une variabilité temporelle et spatiale importante des structures hydrologiques et courantologiques, l'évolution des caractéristiques de la veine d'eau PGW jusqu'en mer d'Arabie est précisée. Un modèle réaliste de la circulation dans les deux golfes est développé pour comprendre les différents aspects de la dynamique. La circulation dans le golfe persique, les interactions entre les masses d'eau PGW et IOSW, la formation puis le développement de l'eau PGW dans le Golfe d'Oman sont détaillés et une variabilité importante est mise en évidence. Une étude de processus précise la part des différents forçages. L'importance de la paramétrisation des flux de chaleur est mise en évidence. Finalement,une étude de sensibilité à la variation de certains forçages (fleuve, évaporation) et à la paramétrisation de la turbulence est réalisée.The circulation in the Persian Gulf and the Gulf of Oman is a complex and badly known system that depends on the interaction of several forcing mechanisms over a range of different time scales. On a timescale of few hours, the tide plays a significant role. On a seasonal and inter-annual basis, there is a thermohaline circulation, specific of a concentration basin. It is characterized by a cyclonic circulation with strong seasonal variability and by the mixing of two different water masses : a dense outflow water formed in the Persian Gulf (Persian Gulf Water) that flows throught the Gulf of Oman and a overlying lighter water from the Indian Ocean (Indian Ocean Surface Water). The analysis of the data from the GOGP experiment, carried out in autumn 1999, shows the different PGW and IOSW water masses, as well as a large temporal and spatial variability of the hydrologic and velocity structures. The evolution of the PGW outflow features down to the Arabian sea is described. A realistic model of the circulation in both gulfs is designed to understand the different aspects of the oceanic dynamics. The circulation in the Persian Gulfe, the mixing of the PGW and IOSW, and the PGW formation and movement towards the Gulf of Oman are explained and a large variability is highlighted. A process study determines the role of the different forcing mechanisms, and the impact of latent heat fluw parameterization is underlined. Finally, a study of the sensitivity to the variation of some not very well-known forcing (rivers, evaporation) and to the turbulence parameterization is presented.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

A model of the general circulation in the Persian Gulf and in the Strait of Hormuz: Intraseasonal to interannual variability

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Hydrology and circulation in the Strait of Hormuz and the Gulf of Oman—Results from the GOGP99 Experiment: 2. Gulf of Oman.

International audienceHydrological, ADCP, and drifting buoy data obtained during the GOGP99 Experiment in October and early November 1999 are analyzed to describe the Persian Gulf Water (PGW) core and the regional circulation in the Gulf of Oman. The warm and salty PGW core flows out of the Strait of Hormuz heading southeastward unto (25°20′N, 57°E), approximately. From there, it cascades down the continental slope, veers southwestward, and joins the Omani coast near (24°50′N, 56°50′E) to form a slope current. This PGW current has then thermohaline maxima on isopycnal σ0 = 26.5, near 220 m depth. Its thermohaline characteristics decrease along its progression to Ra's al Hadd (and then offshore into the Arabian Sea) but maintain a sharp contrast with surrounding waters. Outflow variability at the Strait of Hormuz can be related to downstream fluctuations of the thermohaline maxima in the PGW core at gulf scale and over a 2- to 3-week period. Moreover, several mechanisms (baroclinic instability, flow intermittency, cape effects) are examined to explain the widening of this PGW core upstream and downstream of Ra's al Hamra. In the eastern part of the Gulf of Oman, the regional circulation is a cyclonic gyre. The circulation in the western part of the Gulf is more complex, with the outflow of PGW and southeastward currents in the upper 250 m near the Omani coast, and a recirculation of upwelled waters near Ra's Jagin (on the Iranian coast). The large cyclonic gyre occupies at least the upper 300 m of the water column and undergoes little variation over a month. The PGW outflow in the northern Arabian Sea is southward and located 50–100 km from the coast. It borders a shallower northward current located offshore

Hydrology and circulation in the Strait of Hormuz and the Gulf of Oman—Results from the GOGP99 Experiment: 1. Strait of Hormuz

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A Process Study of the Wind-Induced Circulation in the Persian Gulf

International audienceA shallow-water model, coupled with a three dimensional, hydrostatic ocean model, is used to study the wind induced circulation, and the Shatt-al-Arab river plume expansion, in the Persian Gulf. The models are used in an idealized configuration. The following results are obtained: 1) with northwesterly winds, a double gyre is formed: this gyre is cyc-lonic in the south and anticyclonic in the north. Southeastward currents flow along the Iranian and Arabian coast where the wind stress at the surface dominates the pressure gradient related to the free surface slope, and conversely in the deeper region of the Gulf; 2) in the eastern part of the Gulf, the cyclonic gyre intensifies, as observed and reported in the literature; 3) for northwesterly winds, the plume from Shatt-al-Arab first heads towards the Iranian coast and then spreads southeastward along the Arabian coast; for northerly and northeasterly winds, the plume directly follows the Kuwaiti coast and then the Arabian coast. This sensitivity of the orientation can be related to the double gyre flow structure; 4) a southeasterly wind confines the plume in the northern end of the Gulf as does a pure tidal flow

A Process Study of the Tidal Circulation in the Persian Gulf

International audienceA homogeneous shallow-water model with free surface is used to model the tidal circulation in the Persian Gulf. The numerical finite-difference model includes harmonic diffusion of horizontal momentum and quadratic bottom friction, it has a 9 km mesh size and it is forced by 7 tidal components at its southern boundary. High precision bathymetric data are used to obtain the bottom topography. The numerical model is run for more than a year. The results are the following: 1) The model accurately reproduces the tidal phase and amplitude observed at 42 tidal gauges in the region. This accuracy is attributed to the presence of the 7 components which are able to interact nonlinearly; 2) The amphidromic points are also well positioned by the model due to a proper choice of bathymetry. This was checked also with a simpler geometry of the domain; 3) The tidal currents can be strong in the Straits of Hormuz and in shallow areas; thus they will have an effect of the hydrology of the region. The residual currents are weak so that they will be negligible for the large-scale circulation on long periods; 4) Finally, the sea-surface elevation forecast by the model is in close agreement with in-situ measurements of pressure in the Straits, performed during the GOGP99 experiment

Cyclogeostrophic balance in the Mozambique Channel

International audienceThree methods are proposed for the inclusion of inertia when deriving currents from sea surface height (SSH) in the Mozambique Channel: gradient wind, perturbation expansion, and an iterative method. They are tested in a model and applied to satellite altimetry. For an eddy of 25 cm amplitude and 100 km radius, typical of Mozambique Channel rings at 18°S, the error made with geostrophy is 40% for the anticyclones and 20% for the cyclones. Inertia could reach one third of the pressure gradient. Geostrophy underestimates subsurface currents by up to 50 cm s−1, resulting in errors of 30–40%. The iterative method results in errors of 50% in Mozambique Channel rings. Geostrophic EKE reaches 1400 cm2 s−2, while it attains 1800 cm2 s−2 when inertia is added. Applied to the Gulf Stream, these methods confirm the hypothesis of Maximenko and Niiler [2006] that centrifugal accelerations should be the main cause for the difference observed between geostrophic and drifter EKE. This methodology should result in a net improvement for operational surface ocean currents

Circulation around La Réunion and Mauritius islands in the south-western Indian Ocean: A modeling perspective

International audienceThe objective of this study is to document the circulation in the vicinity of La Réunion and Mauritius islands, i.e., within 500 km offshore, on the intraseasonal time scale, using a high-resolution realistic modeling strategy. The simulated sea level anomalies, water mass properties, and large-scale circulation compare favorably with satellite and in situ observations. Our high-resolution simulation suggests that the currents around the islands are maximal locally, oriented southwestward, to the southeast of both islands which is not visible in low-resolution satellite observations. It also highlights the high degree of variability of the circulation, which is dominated by westward propagating features. The predominant time scale of variability is 60 days. This coincides with the period of a barotropic mode of variability confined to the Mascarene Basin. The characteristics of the westward propagating anomalies are related to baroclinic Rossby waves crossing the Indian Ocean but only in the long-wave resting ocean limit. Tracking those anomalies as eddies shows that they also have a meridional tendency in their trajectory, northward for cyclones and southward for anticyclones, which is consistent with previous studies. Sensitivity experiments suggest that they are predominantly advected from the east, but there is also local generation in the lee of the islands, due to interaction between the circulation and topography