Cold and chlorophyll poor waters are used as a tracer of the cores where the deep waters touch the surface in upwelling areas of the Gulf of Cadiz. A very simple Lagrangian model of phytoplankton growth during the ascending of deep waters to the surface is implemented to diagnose vertical velocities in these cores. The diagnose combines daily images of chlorophyll and temperature to remotely sense upward velocities provided they have, in the Gulf of Cadiz, values above 9 m/d. The approach is valid for other areas subject to intense upwelling as far a local knowledge of nutrient and chlorophyll fields is implemented in the algorithms.
The time-space distribution of chlorophyll concentration in the gulf of Cadiz (SW Spain) is analysed by using remotely sensed data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) between 1998 and 2002. Climatological and monthly averages show more productive waters at the inner shelf, especially during winter-spring and fall blooms. This pattern is further confirmed by the modes obtained with an Empirical Orthogonal Function (EOF) decomposition of weekly chlorophyll composite images for the whole period. The first mode (10%) distinguishes several regions with patterns of variability correlated with oceanographic features in the area. It identifies a coastal zone between Huelva and Cadiz which is strongly coupled to local zonal winds where west and easterlies induce an increase and decrease of the chlorophyll respectively. Chlorophyll concentration of this area is also sensitive to precipitation.