PHOTOCHEMISTRY, MIXING AND DIURNAL CYCLES IN THE UPPER OCEAN

The interplay between ocean photochemistry and surface boundary-layer physics is explored in a range of analytical and numerical process models. For simple systems, key attributes of the photochemical distribution-diurnal cycle, surface concentration, and the bulk concentration difference across the ''mixed layer'' - can be expressed in terms of a small number of physical (vertical diffusivity) and photochemical (turnover timescale and production depth scale) scaling factors. A coupled, 1-D photochemical/physical model is used to examine the more general case with finite mixing rates, variable photochemical production and evolving boundary layer depth. Finite boundary layer mixing rates act to increase both the diurnal cycle and mean concentration at the surface. The diurnal cycle and mean surface concentration are further amplified by coupling between photochemistry and diurnal physics. The daily heating/cooling cycle of the upper ocean can lead to a significant reduction in mixing and boundary-layer depth during the day when photochemical production is at a maximum. Accounting for these effects results in additional surface trapping of photochemically produced species and significant enhancements of the surface diurnal cycle and daily mean. The implications of our model results for field data interpretation and global air-sea flux calculations are also discussed.