Mesoscale variability of water masses in the Arabian Sea as revealed by ARGO floats

Abstract. By analysing ARGO float data over the last four years, a few aspects of the mesoscale variability of water masses in the Arabian Sea are described. The Red Sea Outflow Water (RSOW) is concentrated in the Southwestern Gulf of Aden, in particular when a cyclonic gyre predominates in this region. Salinities of 36.5 and temperatures of 16 °C are found in this area at depths between 600 and 1000 m. RSOW is more dilute in the eastern part of the Gulf, where intense and relatively barotropic gyres mix it with Indian ocean Central Water. RSOW is also detected along the northeastern coast of Socotra, and fragments of RSOW are found between one and three degrees of latitude north of this island. In the whole Gulf of Aden, the correlation between the deep motions of the floats and the sea-level anomaly measured by altimetry is strong, at regional scale. The finer scale details of the float trajectories are not sampled by altimetry and are often related to the anomalous water masses that the floats encounter. The Persian Gulf Water (PGW) is found in the float profiles near Ras ash Sharbatat (near 57° E, 18° N), again with 36.5 in salinity and about 18–19 °C in temperature. These observations were achieved in winter when the southwestward monsoon currents can advect PGW along the South Arabian coast. Fragments of PGW were also observed in the Arabian Sea between 18 and 20° N and 63 and 65° E in summer, showing that this water mass can escape the Gulf of Oman southeastward, during that season. Kinetic energy distributions of floats with respect to distance or angle share common features between the two regions (Gulf of Aden and Arabian Sea), in particular peaks at 30, 50 and 150 km scales and along the axis of monsoon currents. Hydrological measurements by floats are also influenced by the seasonal variations of PGW and RSOW in these regions.

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Mesoscale variability of water masses in the Arabian Sea as revealed by ARGO floats

Ocean Science Discussions ◽

10.5194/osd-8-1369-2011 ◽

2011 ◽

Vol 8 (3) ◽

pp. 1369-1402

Author(s):

X. Carton ◽

P. L'Hegaret

Keyword(s):

Red Sea ◽

Arabian Sea ◽

Water Mass ◽

Sea Water ◽

Regional Scale ◽

Water Masses ◽

Northern Coast ◽

Gulf Of Aden ◽

Mesoscale Variability ◽

Water Outflow

Abstract. By analysing ARGO float data over the last four years, some aspects of the mesoscale variability of water masses in the Arabian Sea are described. The Red Sea Water outflow is strong in the Southwestern Gulf of Aden, in particular when a cyclonic gyre predominates in this region. Salinities of 36.5 and temperatures of 16 °C are found there between 600 and 1000 m depths. The Red Sea Water is more dilute in the eastern part of the Gulf, and fragments of this water mass can be advected offshore across the gulf or towards its northern coast by the regional gyres. The Red Sea Water outflow is also detected along the northeastern coast of Socotra, and fragments of RSW are found between one and three degrees of latitude north of this island. In the whole Gulf of Aden, the correlation between the deep motions of the floats and the SSH measured by altimetry is strong, at regional scale. The finer scale details of the float trajectories are more often related to the anomalous water masses that they encounter. The Persian Gulf Water (PGW) is found in the float profiles near Ras ash Sharbatat (near 57° E, 18° N), again with 36.5 in salinity and about 18–19 °C in temperature. These observations were achieved in winter when the southwestward monsoon currents can advect PGW along the South Arabian coast. Fragments of PGW are found in the Arabian Sea between 18 and 20° N and 63 and 65° E, showing that this water mass can escape the Gulf of Oman southeastward, in particular during summer.

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Mesoscale variability in the Arabian Sea from HYCOM model results and observations: impact on the Persian Gulf Water path

Ocean Science ◽

10.5194/os-11-667-2015 ◽

2015 ◽

Vol 11 (5) ◽

pp. 667-693 ◽

Cited By ~ 21

Author(s):

P. L'Hégaret ◽

R. Duarte ◽

X. Carton ◽

C. Vic ◽

D. Ciani ◽

...

Keyword(s):

Persian Gulf ◽

Arabian Sea ◽

Meteorological Data ◽

Mesoscale Eddies ◽

Ocean Model ◽

Water Masses ◽

Dimensional Structure ◽

Persian Gulf Water ◽

The Persian Gulf ◽

Sea Of Oman

Abstract. The Arabian Sea and Sea of Oman circulation and water masses, subject to monsoon forcing, reveal a strong seasonal variability and intense mesoscale features. We describe and analyze this variability and these features, using both meteorological data (from ECMWF reanalyses), in situ observations (from the ARGO float program and the GDEM – Generalized Digital Environmental mode – climatology), satellite altimetry (from AVISO) and a regional simulation with a primitive equation model (HYCOM – the Hybrid Coordinate Ocean Model). The model and observations display comparable variability, and the model is then used to analyze the three-dimensional structure of eddies and water masses with higher temporal and spatial resolutions than the available observations. The mesoscale features are highly seasonal, with the formation of coastal currents, destabilizing into eddies, or the radiation of Rossby waves from the Indian coast. The mesoscale eddies have a deep dynamical influence and strongly drive the water masses at depth. In particular, in the Sea of Oman, the Persian Gulf Water presents several offshore ejection sites and a complex recirculation, depending on the mesoscale eddies. The associated mechanisms range from coastal ejection via dipoles, alongshore pulses due to a cyclonic eddy, to the formation of lee eddies downstream of Ra's Al Hamra. This water mass is also captured inside the eddies via several mechanisms, keeping high thermohaline characteristics in the Arabian Sea. The variations of the outflow characteristics near the Strait of Hormuz are compared with variations downstream.

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Mesoscale variability in the Arabian Sea from HYCOM model results and observations: impact on the Persian Gulf Water path

Ocean Science Discussions ◽

10.5194/osd-12-493-2015 ◽

2015 ◽

Vol 12 (2) ◽

pp. 493-550 ◽

Cited By ~ 1

Author(s):

P. L'Hégaret ◽

R. Duarte ◽

X. Carton ◽

C. Vic ◽

D. Ciani ◽

...

Keyword(s):

Persian Gulf ◽

Seasonal Variability ◽

Arabian Sea ◽

Meteorological Data ◽

Mesoscale Eddies ◽

Water Masses ◽

Dimensional Structure ◽

Persian Gulf Water ◽

The Persian Gulf ◽

Sea Of Oman

Abstract. The Arabian Sea and Sea of Oman circulation and water masses, subject to the monsoon forcing, reveal a strong seasonal variability and intense mesoscale features. We describe and analyse this variability and these features, using both meteorological data (from ECMWF reanalyses), in-situ observations (from the ARGO float program and the GDEM climatology), satellite altimetry (from AVISO) and a regional simulation with a primitive equation model (HYCOM). The EOFs of the seasonal variability of the water masses quantify their main changes in thermohaline characteristics and in position. The model and observations display comparable variability, and the model is then used to analyse the three-dimensional structure of eddies and water masses with a higher resolution. The mesoscale eddies have a deep dynamical influence and strongly drive the water masses at depth. In particular, in the Sea of Oman, the Persian Gulf Water presents several offshore ejection sites and a complex recirculation, depending on the mesoscale eddies. This water mass is also captured inside the eddies via several mechanisms, keeping high thermohaline characteristics in the Arabian Sea. These characteristics are validated on the GOGP99 cruise data.

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Mesoscale eddies and submesoscale structures of Persian Gulf Water off the Omani coast in spring 2011

Ocean Science ◽

10.5194/os-12-687-2016 ◽

2016 ◽

Vol 12 (3) ◽

pp. 687-701 ◽

Cited By ~ 18

Author(s):

Pierre L'Hégaret ◽

Xavier Carton ◽

Stephanie Louazel ◽

Guillaume Boutin

Keyword(s):

Persian Gulf ◽

Arabian Sea ◽

Cold Water ◽

Mesoscale Eddies ◽

High Salinity Water ◽

Strait Of Hormuz ◽

Persian Gulf Water ◽

Salinity Water ◽

The Persian Gulf ◽

Sea Of Oman

Abstract. The Persian Gulf produces high-salinity water (Persian Gulf Water, PGW hereafter), which flows into the Sea of Oman via the Strait of Hormuz. Beyond the Strait of Hormuz, the PGW cascades down the continental slope and spreads in the Sea of Oman under the influence of the energetic mesoscale eddies. The PGW outflow has different thermohaline characteristics and pathways, depending on the season. In spring 2011, the Phys-Indien experiment was carried out in the Arabian Sea and in the Sea of Oman. The Phys-Indien 2011 measurements, as well as satellite observations, are used here to characterize the circulation induced by the eddy field and its impact on the PGW pathway and evolution. During the spring intermonsoon, an anticyclonic eddy is often observed at the mouth of the Sea of Oman. It creates a front between the eastern and western parts of the basin. This structure was observed in 2011 during the Phys-Indien experiment. Two energetic eddies were also present along the southern Omani coast in the Arabian Sea. At their peripheries, ribbons of freshwater and cold water were found due to the stirring created by the eddies. The PGW characteristics are strongly influenced by these eddies. In the western Sea of Oman, in 2011, the PGW was fragmented into filaments and submesoscale eddies. It also recirculated locally, thus creating salty layers with different densities. In the Arabian Sea, a highly saline submesoscale lens was recorded offshore. Its characteristics are analyzed here and possible origins are proposed. The recurrence of such lenses in the Arabian Sea is also briefly examined.

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Challenges in modeling spatiotemporally varying phytoplankton blooms in the Northwestern Arabian Sea and Gulf of Oman

Biogeosciences ◽

10.5194/bg-13-1049-2016 ◽

2016 ◽

Vol 13 (4) ◽

pp. 1049-1069 ◽

Cited By ~ 5

Author(s):

S. Sedigh Marvasti ◽

A. Gnanadesikan ◽

A. A. Bidokhti ◽

J. P. Dunne ◽

S. Ghader

Keyword(s):

Interannual Variability ◽

Annual Cycle ◽

Harmful Algal Blooms ◽

Algal Blooms ◽

Arabian Sea ◽

Regional Scale ◽

Phytoplankton Productivity ◽

Gulf Of Oman ◽

The North ◽

The Persian Gulf

Abstract. Recent years have shown an increase in harmful algal blooms in the Northwest Arabian Sea and Gulf of Oman, raising the question of whether climate change will accelerate this trend. This has led us to examine whether the Earth System Models used to simulate phytoplankton productivity accurately capture bloom dynamics in this region – both in terms of the annual cycle and interannual variability. Satellite data (SeaWIFS ocean color) show two climatological blooms in this region, a wintertime bloom peaking in February and a summertime bloom peaking in September. On a regional scale, interannual variability of the wintertime bloom is dominated by cyclonic eddies which vary in location from one year to another. Two coarse (1°) models with the relatively complex biogeochemistry (TOPAZ) capture the annual cycle but neither eddies nor the interannual variability. An eddy-resolving model (GFDL CM2.6) with a simpler biogeochemistry (miniBLING) displays larger interannual variability, but overestimates the wintertime bloom and captures eddy-bloom coupling in the south but not in the north. The models fail to capture both the magnitude of the wintertime bloom and its modulation by eddies in part because of their failure to capture the observed sharp thermocline and/or nutricline in this region. When CM2.6 is able to capture such features in the Southern part of the basin, eddies modulate diffusive nutrient supply to the surface (a mechanism not previously emphasized in the literature). For the model to simulate the observed wintertime blooms within cyclones, it will be necessary to represent this relatively unusual nutrient structure as well as the cyclonic eddies. This is a challenge in the Northern Arabian Sea as it requires capturing the details of the outflow from the Persian Gulf – something that is poorly done in global models.

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Thermocline climatology of the Arabian Sea - a review

Marine and Freshwater Research ◽

10.1071/mf96123 ◽

1997 ◽

Vol 48 (6) ◽

pp. 465

Author(s):

K. G. Radhakrishnan ◽

Basil Mathew ◽

P. V. Hareesh Kumar ◽

N. Mohan Kumar

Keyword(s):

Arabian Sea ◽

Water Masses ◽

Reverse Direction ◽

Monsoon Period ◽

Equatorial Undercurrent ◽

Large Variability ◽

Northern Arabian Sea ◽

South West Monsoon ◽

Persian Gulf Water ◽

Strong Surface

The monthly evolution of the thermocline characteristics for the Arabian Sea based on temperature profiles averaged over 2° × 2° grid cells is presented. During the pre-monsoon period, the entire Arabian Sea is characterized by a shallow thermocline (30–40 m) due to strong surface heating. Upwelling in the coastal regions and downwelling in the central Arabian Sea result in large variations in the thermocline topography during the south-west monsoon. This pattern reverses during winter when the surface currents reverse direction. The thermocline gradient is between 0.04°C m-1 and 0.14°C m-1 . An interesting result is the large variability in thermocline gradient (from 0.04°C m-1 near the coasts to 0.12°C m-1 in the central Arabian Sea) during July–September throughout the Arabian Sea. Weak gradients noticed in the northern Arabian Sea are due to the interaction of two different warm-water masses (the Arabian Sea and Persian Gulf water masses). The decrease in gradient in the equatorial Arabian Sea from January to June is attributed to the equatorial undercurrent which causes spreading of isotherms in the thermocline.

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Challenges in modelling spatiotemporally varying phytoplankton blooms in the Northwestern Arabian Sea and Gulf of Oman

Biogeosciences Discussions ◽

10.5194/bgd-12-9651-2015 ◽

2015 ◽

Vol 12 (13) ◽

pp. 9651-9693 ◽

Cited By ~ 2

Author(s):

S. Sedigh Marvasti ◽

A. Gnanadesikan ◽

A. A. Bidokhti ◽

J. P. Dunne ◽

S. Ghader

Keyword(s):

Interannual Variability ◽

Arabian Sea ◽

Regional Scale ◽

Height Anomaly ◽

Phytoplankton Blooms ◽

Gulf Of Oman ◽

The North ◽

Sea Surface Height Anomaly ◽

One Year ◽

The Persian Gulf

Abstract. We examine interannual variability of phytoplankton blooms in northwestern Arabian Sea and Gulf of Oman. Satellite data (SeaWIFS ocean color) shows two climatological blooms in this region, a wintertime bloom peaking in February and a summertime bloom peaking in September. A pronounced anti-correlation between the AVISO sea surface height anomaly (SSHA) and chlorophyll is found during the wintertime bloom. On a regional scale, interannual variability of the wintertime bloom is thus dominated by cyclonic eddies which vary in location from one year to another. These results were compared against the outputs from three different 3-D Earth System models. We show that two coarse (1°) models with the relatively complex biogeochemistry (TOPAZ) capture the annual cycle but neither eddies nor the interannual variability. An eddy-resolving model (GFDL CM2.6) with a simpler biogeochemistry (miniBLING) displays larger interannual variability, but overestimates the wintertime bloom and captures eddy-bloom coupling in the south but not in the north. The southern part of the domain is a region with a much sharper thermocline and nutricline relatively close to the surface, in which eddies modulate diffusive nutrient supply to the surface (a mechanism not previously emphasized in the literature). We suggest that for the model to simulate the observed wintertime blooms within cyclones, it will be necessary to represent this relatively unusual nutrient structure as well as the cyclonic eddies. This is a challenge in the Northern Arabian Sea as it requires capturing the details of the outflow from the Persian Gulf.

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Mesoscale eddies and submesoscale structures of Persian Gulf Water off the Omani coast in Spring 2011

Ocean Science Discussions ◽

10.5194/osd-12-2743-2015 ◽

2015 ◽

Vol 12 (6) ◽

pp. 2743-2782

Author(s):

P. L'Hégaret ◽

X. Carton ◽

S. Louazel ◽

G. Boutin

Keyword(s):

Persian Gulf ◽

Arabian Sea ◽

High Salinity ◽

Cold Water ◽

Mesoscale Eddies ◽

High Salinity Water ◽

Persian Gulf Water ◽

Salinity Water ◽

The Persian Gulf ◽

Sea Of Oman

Abstract. The Persian Gulf produces a high salinity water (Persian Gulf Water, PGW hereafter) flowing into the Sea of Oman, in the northwestern Indian Ocean. Past the Strait of Hormuz, the PGW cascades down the continental slope and spreads in the Sea of Oman under the influence of the energetic mesoscale eddies with different thermohaline signatures and pathways depending of the season. In spring 2011, the Phys-Indien experiment was carried out in the Arabian Sea an in the Sea of Oman. This study uses the results from the measurements to characterize the water masses, their thermohaline and dynamical signatures. During the spring intermonsoon, an anticyclonic eddy is often observed at the mouth of the Sea of Oman. This structure was present in 2011 and created a front between the eastern and western part of the basin. As well two energetic gyres were present along the Omani coast in the Arabian Sea. At their peripheries, injections of fresh and cold water are found in relation with the stirring of the eddies. The PGW observed below or between these eddies have a different dilution depending of the position and formation periods of the gyres. Furthermore, in the western Sea of Oman, the PGW is fragmented in filaments and submesoscale eddies. As well, recirculation of the PGW is observed, thus having the presence of salty nearby patches with two densities. Offshore, in the Arabian Sea, a submesoscale lens was recorded. The different mechanisms leading to its formation and presence are assessed here.

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