Influence of Rossby waves on primary production from a coupled physical-biogeochemical model in the North Atlantic Ocean

G Charria,V Garçon,P Cipollini,I Dadou,M Drévillon

doi:10.5194/os-4-199-2008

Abstract

Abstract. Rossby waves appear to have a clear signature on surface chlorophyll concentrations which can be explained by a combination of vertical and horizontal mechanisms. In this study, we investigate the role of the different physical processes in the north Atlantic to explain the surface chlorophyll signatures and the consequences on primary production, using a 3-D coupled physical/biogeochemical model for the year 1998. The analysis at 20 given latitudes, mainly located in the subtropical gyre, where Rossby waves are strongly correlated with a surface chlorophyll signature, shows the important contribution of horizontal advection and of vertical advection and diffusion of inorganic dissolved nitrogen. The main control mechanism differs according to the biogeochemical background conditions of the area. The surface chlorophyll anomalies, induced by these physical mechanisms, have an impact on primary production. We estimate that Rossby waves induce, locally in space and time, increases (generally associated with the chlorophyll wave crest) and decreases (generally associated with the chlorophyll wave trough) in primary production, ~±20% of the estimated background primary production. This symmetrical situation suggests a net weak effect of Rossby waves on primary production.

Highlights

The detection of westward propagating signals in surface chlorophyll concentrations related to Rossby waves (e.g. Machu et al, 1999; Cipollini et al, 2001; Uz et al, 2001; Kawamiya and Oschlies, 2001) prompted the question of the underlying physical/biogeochemical inter-actions
Three main processes were suggested: (1) the upwelling mechanism associated with nutrient injection (Cipollini et al, 2001; Uz et al, 2001; Siegel, 2001), (2) the uplifting of a deep chlorophyll maximum towards the surface (Cipollini et al, 2001; Kawamiya and Oschlies, 2001; Charria et al, 2003), and (3) the meridional advection of chlorophyll by geostrophic currents associated with baroclinic Rossby waves (Killworth et al, 2004)
The primary production, which is nutrient-limited between 8◦ N and 40◦ N, is function of dissolved inorganic nitrogen and phytoplankton concentrations (Eq 2)

Summary

Introduction

The detection of westward propagating signals in surface chlorophyll concentrations related to Rossby waves (e.g. Machu et al, 1999; Cipollini et al, 2001; Uz et al, 2001; Kawamiya and Oschlies, 2001) prompted the question of the underlying physical/biogeochemical inter-actions. Three main processes were suggested: (1) the upwelling mechanism associated with nutrient injection (Cipollini et al, 2001; Uz et al, 2001; Siegel, 2001), (2) the uplifting of a deep chlorophyll maximum towards the surface (Cipollini et al, 2001; Kawamiya and Oschlies, 2001; Charria et al, 2003), and (3) the meridional advection of chlorophyll by geostrophic currents associated with baroclinic Rossby waves (Killworth et al, 2004) These three processes are described in Killworth et al (2004) using theoretical models and compared to the remotely sensed observations. In the two latter studies, several first-cut approximations were made in the modelling of the biological signal

Results

Discussion

Conclusion