Structure and Dynamics of the Ras al Hadd Oceanic Dipole in the Arabian Sea

Charly De Marez,Pierre L’Hegaret,Xavier Carton,Alexandre Stegner,Briac Le Vu,Adam Ayouche,Mathieu Morvan

doi:10.3390/oceans2010007

Abstract

The Ras al Hadd oceanic dipole is a recurrent association of a cyclone (to the northeast) and of an anticyclone (to the southwest), which forms in summer and breaks up at the end of autumn. It lies near the Ras al Hadd cape, southeast of the Arabian peninsula. Its size is on the order of 100 km. Along the axis of this dipole flows an intense jet, the Ras al Had jet. Using altimetric data and an eddy detection and tracking algorithm (AMEDA: Angular Momentum Eddy Detection and tracking Algorithm), we describe the life cycle of this oceanic dipole over a year (2014–2015). We also use the results of a numerical model (HYCOM, the HYbrid Coordinate Ocean Model) simulation, and hydrological data from ARGO profilers, to characterize the vertical structure of the two eddies composing the dipole, and their variability over a 15 year period. We show that (1) before the dipole is formed, the two eddies that will compose it, come from different locations to join near Ras al Hadd, (2) the dipole remains near Ras al Hadd during summer and fall while the wind stress (due to the summer monsoon wind) intensifies the cyclone, (3) both the anticyclone and the cyclone reach the depth of the Persian Gulf Water outflow, and (4) their horizontal radial velocity profile is often close to Gaussian but it can vary as the dipole interacts with neighboring eddies. As a conclusion, further work with a process model is recommended to quantify the interaction of this dipole with surrounding eddies and with the atmosphere.

Highlights

Over the last three decades, there has been growing interest in the Arabian Sea and in its marginal gulfs and seas, for geopolitical, economic, and scientific reasons
The AMEDA algorithm applied to the altimetric data allows the tracking in time of the eddies detected
We study the dipole only when it is located at the Ras al Hadd cape at the entrance of the Gulf of Oman (GO), which is not the case for these years

Summary

Introduction

Over the last three decades, there has been growing interest in the Arabian Sea and in its marginal gulfs and seas, for geopolitical, economic, and scientific reasons These seas form a complex region in terms of oceanographic variability as they are influenced by the North Equatorial Current and by the Southwest Monsoon Current. The monsoon winds, blowing over this region, are highly variable with the seasons, coming strong and northeastward from May to September, weaker and southwestward from December to February, and quite weak and variable in direction during the two intermonsoon periods (March–April and October–November) These winds drive coastal and regional ocean currents, upwellings, and downwellings near the coast.

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