On the 3D structure of eddies in the Arabian Sea

Abstract

In the Arabian Sea, mesoscale eddies are prominent features of the circulation. They influence biological activities, tracer transport, and physical and chemical properties of the seawater. In particular, the pathways of salty water masses outflowing from the marginal seas are strongly impacted by the mesoscale eddies that evolve in the western part of the basin. The impact of mesoscale eddies on the different salty water masses has been investigated in previous studies with altimetric data and in situ data for short durations. In this study, we extend this analysis to most of the mesoscale eddies detected from altimetry data for the period 2000–2015. We use Argo floats colocalized with eddies to describe the 3D impact of mesoscale eddies on the dynamical and thermohaline properties of water masses in the Arabian Sea. The ∼30,000 Argo stations used show a well characterized distribution of water masses in the different sub-basins of the Arabian Sea, with the Persian Gulf Water (PGW) flowing from the north and the Red Sea Outflow Water (RSOW) from the northwest. In addition, colocalizing Argo profiles in or out of these eddies allows for the extraction of temperature and salinity anomalies related to mesoscale eddies. Then, we present the vertical structure of density associated with these eddies. These results allow us to divide the Arabian Sea into areas depending on their hydrological and dynamical properties. In the Gulf of Oman and the Gulf of Aden, the water in the core of eddies is fresher than at their edge. Thus, most of eddies residing in these gulfs were generated in the Arabian Sea. In these eddies, the salinity anomalies corresponding to PGW and RSOW are on average stronger at their edge than in their core; this indicates that eddies in these gulfs impact outflow waters via advecting processes on their rim. Finally, we select the northern part of the Arabian Sea (including the Gulf of Oman), where most Argo profiles were collected. We calculate the composite 3D profiles of angular velocity and salinity anomaly for surface-intensified cyclonic eddies and subsurface anticyclonic eddies, with radii of about 100 km. The velocity profiles computed via the thermal wind equations are validated using surface velocities from altimetry and surface drifter measurements. On average, eddies in the northern Arabian Sea exhibit a clear pattern of PGW in their core. The comparison of these 3D composite structures with a particular cyclone near the Strait of Hormuz and a dipole sampled with a SeaSoar along the Omani Coast shows that they are representative of mesoscale eddies often observed in the area.