DUST UP

Oceanic biogeochemcial cycles are an important part of the earth system, these are the set of interlinked physical, chemical and biological processes that control our environment. Biogeochemical cycles are important not only in the ocean (its composition and the life within it) but also to the atmosphere, in particular through greenhouse gases such as carbon dioxide and climatically active gases such as dimethyl sulphide (DMS). Oceanic biologically mediated DMS is the dominant natural source of sulphur to the atmosphere. It has been proposed that the air-sea cycling of sulphur may regulate climate via a feed-back mechanism involving cloud formation which in turn regulates the biological produciton of DMS. If we wish to be able to predict climate change then we must understand whether such a feed-back system exists and if so, how large it is. The chemical precursors of DMS in the upper ocean are a by product of photosynthesis by phytoplankton (microscopic plants). The growth of phytoplankton in the upper ocean is controlled by the availabilty of light and nutrients (nitrogen, phsophorous, silicate and iron). The UK Surface Ocean Lower Atmosphere Study (SOLAS) program has already estabilshed a fieldwork program to investigate the response of DMS production to changes in these contolling factors. One focus of these studies is the role of iron, nitrogen and phosphorus rich Saharan dust, which helps to fertilize the huge plankton blooms that occur in the tropical eastern Atlantic. Somewhere in the region of 500 million tonnes per year is involved; an amount sufficient to affect the climate. By partly absorbing and partly reflecting sunlight, the dust particles heat the air but cool the ocean surface. They also encourage cloud formation, which reflects light back into space. Another major focus is the role of upwelling in driving planktonic production of biogases. Upwelling is a physical process which transports large quantities of cold nutrient rich deep ocean water to the surface. Enhanced planktonic production is a well known consequence of upwelling in coastal regions, such as the NW African coast. We propose to use computor models to simulate the production of DMS in the upper ocean in response to atmospheric inputs of iron, and nutrients from Saharan dust and oceanic upwelling. We will construct a regional model of the NW African coast and the eastern sub tropical Atlantic and use it to assess the significance of impacts of these drivers on DMS fluxes to the atmosphere at climate scales.