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A seasonal dipolar eddy near Ras Al Hamra (Sea of Oman)

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Abstract

Trajectories and hydrological data from two Argo floats indicate that warm and salty water at 200–300-m depths was ejected from the coast of Oman, near Ras al Hamra, in spring 2008, 2011, and 2012. This warm and salty water, Persian Gulf Water (PGW), once ejected from the coast, recirculated cyclonically in the western Sea of Oman, but also flowed eastward along the Iranian and Pakistani coasts. There, it was expelled seaward by mesoscale eddies as shown by other float data. Seasonal maps of salinity were computed from all available Argo floats; they showed that, in spring, PGW is present in the middle and north of the Sea of Oman, contrary to fall, when the salinity maxima lie southeast of Ras al Hadd. The ejection of PGW from Ras al Hamra is related here to the influence of a mesoscale dipolar eddy which often appears near this cape in spring. The time-averaged and empirical orthogonal functions of altimetric maps over 11 years for this season confirm the frequent presence and the persistence of this feature. From surface currents and hydrology, deep currents were computed via thermal wind balance, and the associated shear and strain fields were obtained. This deformation field is intense near Ras al Hamra, with an offshore direction. This flow structure associated with the mesoscale dipole explains PGW ejection from the coast. This observation suggests that PGW distribution in the Northern Arabian Sea can be strongly influenced by seasonal mesoscale eddies.

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Acknowledgments

The authors thank Universite de Bretagne Occidentale and DGA for their support under REI program COMINO and ASTRID program SYNBIOS, during the course of this work. They also thank Dr A. Bentamy (LOS, IFREMER) for providing wind data and CNES/AVISO for the altimetric data. The authors are grateful to the referees for their in-depth analysis of this work and for an essential suggestion which improved the quality of this paper.

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Authors and Affiliations

  1. LPO, Université de Bretagne Occidentale, 6 Av Le Gorgeu, 29200, Brest, France

    Pierre L’Hégaret, Léo Lacour, Xavier Carton & Guillaume Roullet

  2. SHOM, Campus de Meteo France, 42 Av G Coriolis, 31400, Toulouse, France

    Rémy Baraille & Stéphanie Corréard

Authors
  1. Pierre L’Hégaret
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  2. Léo Lacour
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  3. Xavier Carton
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  4. Guillaume Roullet
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  5. Rémy Baraille
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  6. Stéphanie Corréard
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Corresponding author

Correspondence to Pierre L’Hégaret.

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Responsible Editor: Jin-Song von Storch

Appendices

Appendix 1: Validation of methods for horizontal maps of hydrology and altimetry

Firstly, we present two temperature maps, for spring, over years 2002–2012; one obtained with our interpolation method and the other created directly with ODV ((Schlitzer 2005); see Fig. 16). The agreement is good, except west of 57.5° E (see below for the error on hydrology).

Fig. 17
figure 17

Error map for sea surface elevation, on March 12, 2008, in the Sea of Oman

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Secondly, we show the map of significative points for hydrology obtained by interpolation/extrapolation of float data: this corresponds to the weight attributed to each point of the grid, related to the density of measurements in the region (the marked points on Fig. 16 for example). Below 5 points, in particular west of 57.5°, where no float ever profiled, the interpolation is too poor to be trusted. Elsewhere, the number of points is sufficient, more specifically near Ras al Hamra where PGW ejection occurs.

Thirdly, we also show the error map for sea surface elevation (see Fig. 17): this error is maximum near the coasts, but at the location of the eddies that we describe in the text, the error is only ±6 cm. This must be compared with a difference of 20–25 cm in altimetry, between the crest and trough of the anticyclonic and cyclonic eddies considered. Therefore, the altimetric maps that we show have a 20–30 % error for eddies near a coast, and a smaller error in the deep ocean.

Fig. 18
figure 18

Time average of ADT over January (top, left); February (top, right); March (center, left); April (center, right); May (bottom, left); June (bottom, right)

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Appendix 2: Evolution of altimetry over a year

Figures 18 and 19 show the evolution of ADT for each month, time-averaged over 3 years: 2008, 2011, and 2012.

Fig. 19
figure 19

Time average of ADT over July (top, left); August (top, right); September (center, left); October (center, right); November (bottom, left); December (bottom, right)

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L’Hégaret, P., Lacour, L., Carton, X. et al. A seasonal dipolar eddy near Ras Al Hamra (Sea of Oman). Ocean Dynamics 63, 633–659 (2013). https://doi.org/10.1007/s10236-013-0616-2

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  • DOI: https://doi.org/10.1007/s10236-013-0616-2

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