High resolution modelling of particle fluxes in the NW African Upwelling System
The region off Northwest Africa has complex flow dynamics associated with the longshore Canary Current and eddies, filaments, and jets extending offshore. To study the effects of the vigorous and highly variable small-scale features on the transport of particulate material, the Regional Oceanic Modeling System (ROMS) have been implemented in a configuration with two nested grids. The inner part (see Figure 1 for size of the inner domain) has a resolution of only 2.7 km and is embedded in a much larger overall model domain with 8 km resolution. The model realistically reproduces the temporal and spatial variability as well as time-mean features of the flow. In agreement with observations, the strength of the Canary Current varies with season, while filaments, vortices and jets are produced all year around. Eddies associated with the filaments are observed to have typical lifetimes of several tens of days. These features of the flow are to a great extent compatible with the satellite-derived sea surface temperatures and chlorophyll (MODIS, SeaWiFS).
Figure 1. Simulated distribution of shelf-originated particles at 500m depth and particle camera profiling stations.
Application
The model was used to simulate the transport of biologically produced and idealised, non-reactive particles with specified source regions and sinking properties. One application was to test whether shelf erosion could explain the sub-surface particle maxima frequently observed with particle camera profiles. The model was seeded in the bottom layer over the shelf with particles of two size classes with different sinking velocities. It was observed (Fig. 1) that the small-scale dynamics in the region has clear implications for the off-shore transport and the lateral distributions of particles. In the vertical (Fig. 2 and Fig. 3), the model reproduces the observed sub-surface particle abundance maxima, suggesting that these features derive from particle sources on the shelf (Karakas et al., 2006; Fischer et al., 2007).
Figure 2. Comparison of modelled and measured particle profiles at several stations.
Figure 3. A cross shore transect off Cape Blanc showing detachment of particle clouds from the shelf and upper slope.
AWI investigators
External Collaborators
Gerhard Fischer, University of Bremen
Nicholas Nowald, MARUM
Patrick Marchesiello, IRD-Noumea
Meinte Blaas, WL|Delft Hydraulics
Funding
DFG-Research Centre/Excellence Cluster "The Oceans in the Earth System"
References
Karakas G, Nowald N, Blaas M, Marchesiello M, Frickenhaus S & R Schlitzer (2006) High resolution modelling of sediment erosion and particle transport across the NW African shelf. J. Geophys. Res., 111, C06025, doi:10.1029/2005JC003296.
Nowald, N., Karakas, G., Ratmeyer, V., Fischer, G., Schlitzer, R., Davenport, R., Wefer, G.(2006).Distribution and transport processes of marine particulate matter off Cape Blanc (NW-Africa): results from vertical camera profiles, Ocean Science Discussions, 3, 903-938.
Fischer, G., Karakas, G., Blaas, M., Ratmeyer, V., Nowald, N., Schlitzer, R., Helmke, P., Davenport, R., Donner, B., Neuer, S. and G. Wefer (2007). Mineral ballast and particle settling rates in the coastal upwelling system off NW Africa and the South Atlantic. Int. J. Earth Sci., doi 10.1007/s00531-007-0234-7.
Fischer, G., Karakas, G. (2008). Sinking rates of particles in biogenic silica- and carbonate-dominated production systems of the Atlantic Ocean: implications for the organic carbon fluxes to the deep ocean (submitted).
Regional Oceanic Modelling System (ROMS) link: www.brest.ird.fr/Roms_tools/