Turbulence, mixing and biological feedbacks in the ocean mixed layer and their relevence to the coupled ocean-atmosphere system

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In the Tropics, the sea surface temperature (SST) plays the dominant role in determining the atmospheric circulation. As a result, small changes in SST (of about 1 degree centigrade) can have a significant impact on tropical weather and climate. The temperature profile in the upper ocean (the top hundred metres or so), and hence the SST is strongly influenced by the heat flux through the surface of the ocean, and the amount of turbulent mixing in the ocean. The amount of mixing determines the depth of the ocean column through which the surface fluxes are distributed (the mixed layer depth) and hence the rate of change of SST. The mixing in the ocean is driven primarily by the wind stress at the surface, but there are other influences, including mixing driven by surface waves. Mixing is inhibited by warmer (and therefore less dense) water lying on top of colder water, and enhanced by cold water lying on top of warm water. There are four components of the surface heat flux; heating by absorption of solar radiation, cooling (usually) by infrared radiation, wind driven evaporation and convective heat loss from the surface. Unlike the other components of the surface flux, the solar radiation is absorbed through quite a large depth of the ocean, and small amounts can even penetrate as deep as 100m. The exact profile of this heating is determined by how clear the water is, and can have a strong influence on the amount of mixing and the rate of change of SST. One of the major factors determining the clarity of the water is the amount of phytoplankton in the water. Phytoplankton are small free floating marine plants which, like land plants, absorb solar radiation and carbon dioxide to grow (given sufficient nutrients). The phytoplankton are then consumed by zooplankton (small marine animals). The plankton are the base of the marine food chain. Because the phytoplankton need sunlight to grow, their distribution in the oceans, both horizontally and vertically, depends strongly on the availability of solar radiation as well as the availability of nutrients. However, the vertical profile of the solar radiation in the upper ocean is primarily determined by the amount and vertical profile of the phytoplankton. Because the phytoplankton and the nutrients that they need for growth are free floating, the vertical distribution of plankton and nutrients is strongly determined by the mixing in the upper ocean. These strong interactions between the mixing, phytoplankton and the absorption of the solar radiation lead to the potential for complex feedbacks, which can ultimately drive variations in SST, climate and the biological productivity of the ocean. This project plans to investigate these links and the extent to which they can be reproduced in global climate models, by detailed computer modelling of the oceanic mixed layer including the phytoplankton and zooplankton and the availability of nutrients.