Upwelling spots and vertical velocities in the Gulf of Cádiz: An approach for their diagnose by combining temperature and ocean colour remote sensing

Upward velocities have been diagnosed through the development of an approach based on remotely sensed daily images of both chlorophyll and temperature at the ocean surface in the Gulf of Cádiz. This novel approach allows to the detection of areas with vigorous vertical velocities (w), through the generation of a calculation algorithm built from the simple Lagrangian model of phytoplankton growth that occurs during the shoaling of deep waters to surface. The fast initial evolution of chlorophyll and temperature in surface limits the implementation of the algorithm to high upward velocities. Assuming this limitation, the values of w diagnosed have been found to be coherent with independent estimations based on diffusive and advective inputs of nitrogen into the euphotic zone in the area. The new production resulting from vertical advection of nitrogen during a regular upwelling event in the basin was more than two orders of magnitude greater than that maintained by vertical diffusion. According to our calculations the maintenance of vertical advection for 1 week under upwelling conditions could support new production equivalent to that due to residual vertical mixing during about half a year under conditions of stratification.