CHLOROPHYLL PATCHINESS CAUSED BY MESOSCALE UPWELLING AT FRONTS

Gyre-scale high-resolution sections sampled by use of a towed undulating vehicle in the open North Atlantic during summer reveal mesoscale patches of high chlorophyll concentration (Chl) within the deep chlorophyll maximum; the chlorophyll concentration in these patches can significantly exceed that occurring during the phytoplankton spring bloom. The existence of a typical mesoscale Chl patch size is documented by horizontal chlorophyll variance spectra displaying a secondary spectral peak at wavelengths between 10 and 20 km, in addition to the low-wavenumber peak at wavelengths between 100 and 300 km associated with the energy-containing eddies. The typical sub eddy-scale patch size coincides with the local internal Rossby-radius of deformation. The input to the Chl variance spectrum at a 10-20 km wavelength is explained by primary production stimulated during mesoscale upwelling at fronts. The mesoscale patches with highest chlorophyll concentrations occur at those sites where the hydrographic conditions are favourable for dynamic upwelling or where the distributions of the hydrographic variables indicate local upwelling events.