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Remote Sensing of the Northwest African Upwelling and Its Production

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Remote Sensing of the Northwest African Upwelling and Its Production Remote sensing of the Northwest African upwelling and its production dynamics Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften am Fachbereich 5 – Geowissenschaften der Universität Bremen Gutachter: Herr Prof. Dr. Gerold Wefer Herr Prof. Dr. Venugopalan Ittekkot vorgelegt von Peer Helmke Bremen, Dezember 2003 Tag des Kolloquiums: 2 Februar 2004 Gutachter: Herr Prof. Dr. Gerold Wefer Herr Prof. Dr. Venugopalan Ittekkot Prüfer: Herr Prof. Dr. Helmut Willems Frau Prof. Dr. Katrin Huhn Inhaltsverzeichnis Abstract........................................................................................................................... 3 Zusammenfassung ......................................................................................................... 5 Introduction.................................................................................................................... 8 The global carbon cycle ......................................................................................... 8 Main objectives .................................................................................................... 10 Study area............................................................................................................. 12 Topography................................................................................................. 12 Wind-system ............................................................................................... 13 Oceanography ............................................................................................. 13 Material and Methods ................................................................................................. 14 Sea surface temperature data................................................................................ 14 Chlorophyll-a -measurements and processing .................................................... 17 Trap data............................................................................................................... 20 Overview of own research........................................................................................... 20 Manuscript 1: Synoptic view on the Cape Blanc upwelling area on the basis of 12-years remotely sensed sea surface temperature (Peer Helmke, Robert Davenport) ........................................................................ 21 Manuscript 2: Wind stress-related filament structures off Cape Ghir, NW Africa (Peer Helmke, Robert Davenport, Holger Kuhlmann) ......................................... 34 Manuscript 3: Northwest African upwelling and its effect on off-shore organic carbon export to the deep-sea (Peer Helmke, Oscar Romero, Gerhard Fischer)................................................. 51 Conclusion .................................................................................................................... 74 Outlook ......................................................................................................................... 76 References..................................................................................................................... 77 1 Appendix: further publications.................................................................................. 83 Manuscript 1: Primary productivity in the northern Canary Islands region as inferred from SeaWiFS imagery (Robert Davenport, Susanne Neuer, Peer Helmke, Javier Perez-Marrero, and Octavio Llinas)............................................................................................... 84 Manuscript 2: Reconstruction of paleoceanography off NW Africa during the last 40,000 years: influence of local and regional factors on sediment accumulation (Kuhlmann, H., T. Freudenthal, P. Helmke, and H. Meggers)........................... 107 Manuscript 3: Assessment of geochemical and micropaleontological sedimentary parameters as proxies of surface water properties in the Canary Islands region (Meggers, H., T. Freudenthal, S. Nave, J. Targarona, F. Abrantes, and P. Helmke) ............................................................................................................... 131 Manuscript 4: Provenance of present-day eolian dust collected off NW Africa (Jan-Berend Stuut, Mattias Zabel, Volker Ratmeyer, Peer Helmke, Enno Schefuß, Gaute Lavik & Ralph Schneider) ......................................................... 163 Danksagung................................................................................................................ 165 2 Abstract Abstract The North Atlantic off NW Africa is characterized by the trade wind induced upwelling of cold and nutrient rich waters. As one of most strongest eastern boundary upwelling areas it is marked by low sea surface temperature and high bio-production, both subject to strong seasonal and interannual variations. Satellite measurements of sea surface temperature (SST) and chlorophyll-a (Chl-a) in the upper water-column were used to (a) recognize the upwelling of deep water and its distribution and mixing in the surface, (b) identify the development of phytoplankton blooms and (c) provide further insights in the characteristics of its variations. The spatial structure of the Cape Ghir filament – a recurring patterns of westward transported high Chl-a concentration – was classified and correlated to distinct pattern of wind forcing. This was done to evaluate the usability of the Cape Ghir filament as a proxy parameter for variations in wind forcing and climate. Measurements of surface Chl-a were compared to deep-sea fluxes of organic carbon in order to determine the relationship between both parameters and to further calculate the export of Corg and the export variation strength. The upwelling intensity and variation were studied from the oscillation of SST and Chl-a. Each parameter was used with its own allocated definition-set for 'upwelling'. In the SST-based study, upwelling was defined as the area where the near-coast SST was 3.5 K below an off-shore reference temperature at the corresponding latitude. When using chlorophyll-a as a proxy for upwelling, the area with a Chl-a concentration > 1mg m-3 was defined as a high chlorophyll-a zone (HCZ). To find the link between surface and deep ocean the source area was determined where the concentration of Chl-a in the surface ocean has the highest correlation with Corg-flux in the deep sea as measured by the sediment trap. The regression between the Chl-a-signal in the area of highest correlation, assumed to be an approximation to the source region of marine particles sampled with the sediment trap, and the trapped Corg-concentration served as an empirical export model for organic carbon. The export model was applied to the HCZ off Northwest Africa to calculate upwelling-driven export of organic carbon. We used 9 and 1 km-resolution satellite data in order to categorize mesoscale patterns and processes such as the Cape Ghir upwelling filament. From this data-base five major groups of filament structures were established on the basis of filament extension, filament axis and types of vortices. 3 Abstract The off-shore extension of SST-defined upwelling from 1988 through 1999 between 18° and 25°N shows maxima in January and May/June with an average area of 140,000 km2 (standard deviation of 20,000 km2). Minima in August exhibit an extension of 43,000 km2 (15,000 km2 standard deviation). A correlation between SST and North Atlantic Oscillation (NAO), previously found in global studies from the North Atlantic, was not discovered in this local-to-regional investigation. On a small-scale-view the upwelling development exhibits a highly variable nature. Local patches of low upwelling intensity exist even in above average years of upwelling. The time period fall 1998 - summer 1999 stand out because of unparalleled strong upwelling not correlated with strong positive NAO, although the opposite should be expected. An intense El Niño-event followed in the Pacific Ocean whose long-distance effects are assumed to have influence, for instance, on the SST in the Atlantic. The period of strong upwelling in 1998-99 is recognized in the SST record and Chl-a-concentration, and also calculated with the Corg-export model. The yearly mean value of Corg exported from the HCZ is ca. 1.5 Tg while increased to ca. 2.62 Tg from August 1998 to July 1999. The export increase was caused by an expansion of the HCZ rather than by an increase of the mean Chl-a- concentration in the surface water. During this outstanding period, the derived Corg-export from the HCZ to a 1000 m-depth level did not significantly differ from the four year mean (20.6 mg m-2 d-1). The correlation analysis between Corg-export and its relationship to the sea-surface Chl-a reveals a region which more accurately represents variations in Corg-flux (r = 0.74) than rectangular boxes in the vicinity of trap locations elsewhere do (r = 0.52). The enhanced relation increased the export-model quality. Different structure-types of the Cape Ghir filament were assigned to distinct forcing situations. It was recognized that the main axis of the filament moved from southwest westward during enhancing along-shore wind-stress. The spatial filament distribution agrees with the enhanced abundance of the upwelling-indicator foraminifer Globigerina bulloides in underlying surface sediments. This indicates that the actual upwelling spatial dynamics resembles that of the last 500 – 1000 years. The spatial upwelling pattern reacts sensitively to changes in forcing
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