Can Satellite-derived Chlorophyll Imagery Be Used to Trace Surface Dynamics in Coastal Zone? A Case Study in the Northwestern Mediterranean Sea

Philippe Forget,Gael André

doi:10.3390/s7060884

Abstract

A comparison of chlorophyll data from SeaWiFS imagery and modeling results from a 3D hydrodynamical model was performed over the northwestern Mediterranean for the entire year of 2001. The study aims at investigating the information content brought by satellite-derived chlorophyll concentration ([Chl]) maps concerning surface dynamics in coastal zone. The study is mainly focused on the Gulf of Lions (GoL) and its outer region, which are mainly influenced by the Rhône River, local winds and the Northern Current (NC) flowing from the East along the continental slope. The physical hydrodynamical model was continuously run and 40 SeaWiFS images, presenting a significant coverage of the studied area, were selected. The comparison between [Chl] and sea surface salinity (SSS) fields on a pixel basis showed no definite correlation trends. Three reasons are given in discussion for that result. However, the comparison emphasized areas close to the coasts which were under the influence of different inputs not considered in the model and also of upwellings. A qualitative analysis of the data performed out of these regions exhibited significant similarities between [Chl] and SSS features. The signature of the Rhône ROFI (Region of Fresh Water Influence) and, in some cases, of the NC, was evidenced on [Chl] maps. We found that the intensity of this signature is seasonally modulated, e.g., it is low in open sea during the summer, oligotrophic, season. In addition, the signature of the Rhône ROFI in the western part of the GoL can be only partial due to local chlorophyll deficits. We conclude that, for the regional case studied, chlorophyll imagery can be used as a tracer of surface dynamics through surface salinity but with limitations, especially near the coasts.

Highlights

Physical modeling is a modern tool to investigate the complex hydrodynamics of coastal zones
The Gulf of Lions (GoL) dynamics is mainly driven by local forcings such as wind and river discharge, but it is dependent on the background general circulation of the northwestern Mediterranean and in particular on the Northern Current (NC)
The overall ability of the model to reproduce the hydrodynamics of the northwestern Mediterranean and especially of the GoL was demonstrated in a previous study by comparing modeled sea surface temperature (SST) and AVHRR satellite temperature measurements [23]

Summary

Introduction

Physical modeling is a modern tool to investigate the complex hydrodynamics of coastal zones. Validation is generally accomplished using in situ sensors such as scalar and vector current-meters, thermo-salinometers, bathythermograghs, drifters etc. Limitations of these devices are well known and the most severe one probably lies in the sampling rate of physical parameters, which is generally not sufficient regarding the high variability in time and space of these variables in coastal environment. That difficulty can be circumvented using assimilation methods. Another route to increase the number of measurements and, in particular, to extent the synopticity and time revisit scales that are required in coastal zone is remote sensing

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Discussion

Conclusion