Phytoplankton and thermal structure in the tropical ocean

Variation in the absorption of solar irradiance in the upper ocean is strongly influenced by variation in the concentration of phytoplankton. In optically clear tropical regions, where the mixed layer depth is shallow relative to the depth of significant penetration of irradiance ( lambda < 700 nm) all year, such variation is particularly important for the thermal structure and dynamics of the upper ocean. The nonuniformities in vertical chlorophyll distribution associated with "typical tropical structure" can give rise to local heating rates that increase with depth; the vertical motions that may result are discussed. The penetration of irradiance through the mixed layer is a downward energy flux which is shown to be of the same order as that thought to be transported vertically by turbulent fluid motions