DUST UP

Lead Research Organisation: Plymouth Marine Laboratory
Department Name: Plymouth Marine Lab

Abstract

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.
 
Description The aim of DUST-UP was to conduct a modelling comparison of the consequences of elevated nutrient availability to dimethylsulfide (DMS) sea-to-air flux in two contrasting, natural, nutrient-addition scenarios in the NE Atlantic, these being Fe from dust deposition and nutrients from upwelling. The major goal is to use a coupled 3D hydrodynamic DMS model to simulate the SOLAS cruise period, validating the simulation with cruise data thus allowing the quantification of the potential impacts of major drivers, both individually and in combination at spatial and temporal scales which are significant for climate. We will test the hypotheses that both Saharan dust deposition and coastal upwelling are significant contributors to the global DMS budget. The work program had 4 main components; DMS process model development, regional modelling, air sea gas exchange parameterisations and hypothesis testing. DMS Model development: The ERSEM DMS model has been refined to provide and explanation for the 'summer paradox', i.e. the observed decoupling of, maximum DMS from its precursor, dimethylsulfoniopropionate (DMSP). New processes have been added. In the new model, light stress and phytoplankton succession increase the DMSP to chlorophyll ratio, decoupling Chl-a, from DMSP. Bacterial nutrient limitation increases DMS yield from DMSP which decouples DMSP and DMS. This underlines the major role that bacteria potentially play in DMS production and fate. A paper is in preparation. Regional Model: Based on the GCOMS system a high resolution (eddy-resolving) regional coupled hydrodynamic ecosystem model was set up for the SOLAS fieldwork area off the African Coast (10N-30N, 10W-40W). The physical model configuration work focused on the treatment of steep topography, open-boundaries and the parameterisation of un-resolved horizontal mixing. The new DMS production process model was implemented in the ecosystem model and the period 2005-2008 simulated (the period of the SOLAS field studies in the region) in a series of model sensitivity experiments. Model outputs were compared with satellite SST and chlorophyll showing reasonable correspondence and reproduced observed concentrations of DMSP but underestimated those of DMS. The sensitivity of DMS production via phytoplankton exudation is being explored to address this issue. Air Sea gas exchange: In collaboration with NOAA the COARE air sea flux parameterisation has been included into a DMS-ecosystem model for the first time. The COARE model is backed-up by / informed by direct estimates of DMS flux and transfer velocities that were generated / validated in part through our collaborations with Huebert (U-of-Hawaii) on the DOGEE cruise. Hypothesis testing: We have used the computer 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. Simulation experiments were designed to explore the impacts of dust events, upwelling events and the combination of the two. Sensitivity analysis shows the production of DMSP is found to be locally enhanced by dust events (up to 15%) and upwelling (up to 10%), but these impacts don't appear to impact significantly over a wider area. |The production of DMS is less sensitive because the increased production of DMSP is offset by carbon limitation of bacteria which restricts the conversion of DMSP to DMS in the model. The combination of the dust event and enhanced upwelling increases PP and DMSP production relative to the baseline. Enhanced upwelling reduces the localised impact of the dust event relative to the dust event-only, but the area affected is larger, with a narrow filament (of both PP and DMSP) going toward the open ocean. The impact of shading by dust clouds is negligible because the model plankton rapidly adapted to the reduction in PAR. In summary the impacts of events such as dust deposition and upwelling on DMSP production are largely confined to the immediate area of impact indicating production is sensitive to the spatial scale and magnitude of the event.
Exploitation Route The finding on air seas gas exchange parameterisations have informed the choice for model applications of ERSEM to investigate climate change, ocean acidification and Carbon capture and storage.
Sectors Environment

 
Description Testing of air sea gas exchange parameterisation for DMS and Co2 has informed the choice of such parameterisation used in applications of the ERSEM models for climate and policy, including those to be applied in UKESM
First Year Of Impact 2010
Sector Environment
Impact Types Policy & public services

 
Title DMS module for the European Regional Seas Ecosysytem Model (ERSEM) 
Description The DMS process model developed during DUST UP has been included in the standard release of the ERSEM model. 
Type Of Material Computer model/algorithm 
Provided To Others? No  
Impact Contribution to the debate on how the represent marine DMS production in earth system models.