Understanding the influence of Rossby waves on surface chlorophyll concentrations in the North Atlantic ocean
Résumé
The variability (in space and time) of westward propagating Rossby waves is analyzed with a wavelet method between 10N and 40N in the North Atlantic Ocean using two remotely sensed data sets (Sea Level Anomalies - SLA and surface chlorophyll-a concentrations) in order to better understand the waves' characteristics and their impacts on the chlorophyll distribution. Signals with wavelengths between ∼ 500 km and ∼ 1000 km with ∼ 4- to ∼ 24-month periods were detected and identified as the first baroclinic mode of Rossby waves. The spatial and temporal information has also highlighted a particular situation in 1998 at 34N, with the simultaneous existence of two distinct wave components corresponding to wavelengths 500 km and 1000 km. Signatures of the waves in ocean color prompt the question of how Rossby waves influence surface chlorophyll concentrations. Several physical/biological processes have been suggested: the eddy pumping mechanism associated with nutrient injection, the uplifting of a deep chlorophyll maximum toward the surface, and the meridional advection of horizontal chlorophyll gradients by geostrophic currents associated with baroclinic Rossby waves. A statistical decomposition of the observed signal into the different processes modeled by Killworth et al. (2004) confirms a main contribution of the north-south advection of the surface chlorophyll-a gradients south of 28N. In this part of the basin, more than ∼ 70% of the signal is explained by this horizontal process. North of 28N, Rossby wave signatures seem to be due to the horizontal advection as well as the vertical nutrient injection (∼ 50% of the observed amplitude). This vertical mechanism may have an impact on the primary production in this part of the basin.