DOI: 10.1594/pangaea.844814
title: Area-averaged mean seasonal cycles of chlorophyll-a concentration, sea surface temperature, alongshore wind stress, westward wind speed and dust component of the aerosol optical depth at 550 nm

description: Nutrient supply in the area off Northwest Africa is mainly regulated by two processes, coastal upwelling and deposition of Saharan dust. In the present study, both processes were analyzed and evaluated by different methods, including cross-correlation, multiple correlation, and event statistics, using remotely sensed proxies of the period from 2000 to 2008 to investigate their influence on the marine environment. The remotely sensed chlorophyll-a concentration was used as a proxy for the phytoplankton biomass stimulated by nutrient supply into the euphotic zone from deeper water layers and from the atmosphere. Satellite-derived alongshore wind stress and sea-surface temperature were applied as proxies for the strength and reflection of coastal upwelling processes. The westward wind and the dust component of the aerosol optical depth describe the transport direction of atmospheric dust and the atmospheric dust column load. Alongshore wind stress and induced upwelling processes were most significantly responsible for the surface chlorophyll-a variability, accounting for about 24% of the total variance, mainly in the winter and spring due to the strong north-easterly trade winds. The remotely sensed proxies allowed determination of time lags between biological response and its forcing processes. A delay of up to 16 days in the surface chlorophyll-a concentration due to the alongshore wind stress was determined in the northern winter and spring. Although input of atmospheric iron by dust storms can stimulate new phytoplankton production in the study area, only 5% of the surface chlorophyll-a variability could be ascribed to the dust component in the aerosol optical depth. All strong desert storms were identified by an event statistics in the time period from 2000 to 2008. The 57 strong storms were studied in relation to their biological response. Six events were clearly detected in which an increase of chlorophyll-a was caused by Saharan dust input and not by coastal upwelling processes. Time lags of &lt;8 days, 8 days, and 16 days were determined. An increase in surface chlorophyll-a concentration of up to 2.4 mg m**3 after dust storms in which the dust component of the aerosol optical depth was up to 0.9 was observed.
description: The mean seasonal cycles were determined from satellite data of years 2000 to 2008. The mean seasonal cycles were area-averaged between 10° W to 30° W and 9° N to 19° N. The satellite data based on NASA's Giovanni, an online data visualization and analysis tool (MODIS and SeaWiFS data) maintained by the Goddard Earth Sciences (GES) Data and Information Services Center (DISC), a section of the NASA Earth-Sun System Division (giovanni.gsfc.nasa.gov). QuikScat and TMI data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team as well as by the NASA Earth Science REASON DISCOVER Project. The data are available at: www.remss.com.
description: Supplement to: Ohde, Thomas; Siegel, Herbert (2010): Biological response to coastal upwelling and dust deposition in the area off Northwest Africa. Continental Shelf Research, 30(9), 1108-1119