The Rhone river dilution zone present in the northeastern shelf of the Gulf of Lion in December 2003

Julie Gatti,Anne Petrenko,Jean-Luc Devenon,Yann Leredde,Caroline Ulses

doi:10.1016/j.csr.2006.05.012

Abstract

Acoustic doppler current profiler (ADCP), CTD and thermosalinograph data from the Gulf Of Lion Time Series (GOLTS) cruise of December 2003, and corresponding AVHRR images, show the unusual presence of the Rhone river dilution zone far east from the Rhone river mouth. It is the first time this event is studied with simultaneous hydrological and current data. This dilution zone extends as far as 5.27°E in longitude (45 km from the Rhone river mouth). At longitude 5.13°E (37 km from the Rhone river mouth), the dilution zone is 40 m deep and spreads over 0.075° latitude (8 km). It is due to an eastward current present there throughout the ADCP-detected range (12–120 m). The analysis of moored ADCP time series reveals that such eastward currents occur there about 18% of the time and that diluted waters from the Rhone reach the Station d’Observation Fixe (SOFI) site between 3.9% and 8.4% of the time. This December 2003 event is the consequence of the combined effects of a storm with east winds and the presence of freshwater along the coast.

Highlights

Since the construction of the Aswan dam and the consecutive reduction of the Nile river inflows, the Rhone River has become the most significant source of freshwater in the microtidal Mediterranean Sea (Fig. 1)
The in situ experiments of the Gulf Of Lion Time Series (GOLTS) project include two main components: time series collected at a moored acoustic doppler current profiler (ADCP) and cruises
The GOLTS cruise occurred after quite outstanding events of river discharge and precipitation

Summary

Introduction

Since the construction of the Aswan dam and the consecutive reduction of the Nile river inflows, the Rhone River has become the most significant source of freshwater in the microtidal Mediterranean Sea (Fig. 1). Freshwater input in the Gulf of Lion’s (GoL) continental shelf (Fieux, 1974; Durrieu de Madron et al, 2003). It is a major nutrient source for the GoL, and influences its productivity (Coste, 1974; Morel et al, 1990) and fishery activities. The Rhone river plume presents a Kelvin number greater than one and is submitted to strong Coriolis effects. The particularity of the Rhone plume is that both its Kelvin number and its Froude number (Fr) are greater than one (calculated from observations in ArnouxChiavassa (1998); from modelling results in Marsaleix et al (1998)). Whereas, according to the Garvine’s classification system (Garvine, 1995), low Kelvin numbers are normally associated to supercritical plumes (Fr41; Chao, 1988) and, inversely, high Kelvin numbers to low Fr

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