Interactions between the Somali Current eddies during the summer monsoon: insights from a numerical study

C Q C Akuetevi,B Barnier,A Lecointre,J Verron,J.-M Molines

doi:10.5194/os-12-185-2016

C Q C Akuetevi, B Barnier + Show 3 more

Open Access

https://doi.org/10.5194/os-12-185-2016

Copy DOI

Abstract

Abstract. Three hindcast simulations of the global ocean circulation differing by resolution (1/4 or 1/12°) or parametrization or atmospheric forcing are used to describe the interactions between the large anticyclonic eddies generated by the Somali Current system during the Southwest Monsoon. The present investigation of the Somalian coherent eddy structures allows us to identify the origin and the subsequent development of the cyclones flanked upon the Great Whirl (GW) previously identified by Beal and Donohue (2013) in satellite observations and to establish that similar cyclones are also flanked upon the Southern Gyre (SG). These cyclones are identified as potential actors in mixing water masses within the large eddies and offshore the coast of Somalia. All three simulations bring to light that during the period when the Southwest Monsoon is well established, the SG moves northward along the Somali coast and encounters the GW. The interaction between the SG and the GW is a collision without merging, in a way that has not been described in observations up to now. During the collision the GW is pushed to the east of Socotra Island, sheds several smaller patches of anticyclonic vorticity, and often reforms into the Socotra Eddy, thus proposing a formation mechanism for that eddy. During this process the GW gives up its place to the SG. This process is robust throughout the three simulations.

Highlights

The near-surface circulation of the northwestern Indian Ocean during the summer monsoon is the siege of large and strong anticyclonic eddies produced by recirculation cells of the Somali Current system
The results described illustrated with ORCA025, hold for the two ORCA12 simulations, and are very consistent with the circulation schemes proposed in the literature (e.g. Schott et al, 2009; Beal et al, 2013)
If we look at years when collision does not occur, the PDF indicates that almost no water of spiciness less than 5.0 can be found in the region of the Great Whirl (GW): in those years, the spiciness is most of the time found in the high range (5.5 to 5.6) typical of early GW waters

Summary

Introduction

The near-surface circulation of the northwestern Indian Ocean during the summer monsoon is the siege of large and strong anticyclonic eddies produced by recirculation cells of the Somali Current system. A large branch of the EACC turns offshore after crossing the Equator at about 2 or 3◦ N and forms the so-called Southern Gyre (SG), a large anticyclonic retroflection cell with a well-marked wedge of cold upwelled water attached to its northern flank (the southern cold wedge driven by the upwelling favourable winds). According to Beal et al (2013), this cell re-circulates southward across the Equator to feed into the South Equatorial Counter Current (SECC). Its generation mechanism involves the arrival of remote Rossby waves in spring (see Beal and Donohue, 2013) and an amplification in summer by the monsoon winds via an intensification and a retroflection of the Somali Current.

Objectives

Methods

Findings

Conclusion