SEASONAL COOLING AND BLOOMING IN TROPICAL OCEANS

The relative importance of tropical pelagic algal blooms in not yet fu lly appreciated and the way they are induced not well understood. The tropical Atlantic supports pelagic blooms together equivalent to the N orth Atlantic spring bloom. These blooms are driven by thermocline lil ting, curl of wind stress and eddy upwelling as the ocean responds to intensified basin-scale winds in boreal summer. The dimensions of the Pacific Ocean are such that seasonal thermocline tilting does not occu r, and nutrient conditions are such that tilting might not induce bloo m, in any case. Divergence at the equator is a separate process that s trengthens the Atlantic bloom, is more prominent in the eastern Pacifi c, and in the Indian Ocean induces a bloom only in the western part of the ocean. Where western jet currents are retroflected from the coast off Somalia and Brazil, eddy upwelling induces prominent blooms. In t he eastward flow of the northern equatorial countercurrents, positive wind curl stress induces Ekman pumping and the induction of algal bloo ms aligned with the currents. Some apparent algal blooms, such as that seen frequently in CZCS images westwards from Senegal, must be due to interference from airborne dust.