ALBERT

Description: Highlights: • The distribution of particulate matter was studied using Argo float measurements. • Its spatio-temporal properties were analyzed in the eastern tropical North Atlantic. • Surface, subsurface, intermediate and bottom nepheloid layers were considered. • High correlations between particulate matter and phytoplankton were verified. • The depth of subsurface particle maxima correlated to the distance to shore. Abstract: The spatial and temporal distribution of particulate matter in the water column of the eastern tropical North Atlantic between 16.9–22.9°N and 16.6–29.3°W was investigated using optical measurements from transmissometers mounted on Argo floats. The corresponding profiles of beam attenuation coefficients measured from February 2008 to May 2009 were used to study particulate matter in different layers such as the surface nepheloid layer (SNL), subsurface nepheloid layer (SSNL), intermediate nepheloid layer (INL) and bottom nepheloid layer (BNL) as well as to investigate sinking particles (SP). The SNL were down to about 60 m water depth at thicknesses between 20 and 60 m. Our analyses verified high correlation between particulate matter and phytoplankton in the SNL. High offshore SNL extension of up to 750 km was found in the area of Cape Blanc filaments in January 2009. Their typical widths ranged from 11 to 72 km. Furthermore, float-borne observations even resolved atmospheric dust deposition into the surface water layer during a strong Saharan dust event in October 2008. The observed dust concentration in the mixed water layer was found to vary between 0.0021 and 0.0168 g m−3 depending on applied assumptions. An abrupt change from a SNL to a SSNL regime over distances of only 80 to 90 km was observed. The particulate matter in the SSNL showed lateral extensions from 420 to 1020 km offshore. A statistically significant correlation between the depth of subsurface particle maxima and the distance to shore was found. An averaged diameter of 30 km was determined for the sharply isolated patches of INL which was consistent with model simulations of other studies. The lateral transport of particulate matter in these INL features in the area of the giant Cape Blanc filaments was found to be more pronounced than reported in earlier studies. The distribution of particulate matter within the INL filaments reached up to 610 km off the shelf edge. The frequency of INL decreased with increasing distance to shore. The sinking velocity of particulate matter of one long-term observed INL was approximately 1.3 m day−1. Highly concentrated BNLs with beam attenuation coefficients of up to 4.530 m−1 were observed in the continental slope region. INLs appeared more frequently than SP events which lead to the conclusion that the lateral transport of particulate matter in INL features in the study area was more important than their passive vertical sinking.

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 〈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: Measurements of downward irradiance at sea level in the area off Northwest Africa in February 2008 were analyzed to determine the spectral effects of atmospheric dust and clouds on solar irradiance and photosynthetically available radiation. For the first time not only the pure spectral effects of dust and clouds were considered but also the spectral modifications by sky conditions with dust and clouds together in the atmosphere. The influence of dust on spectral distribution of downward irradiance is smaller than that of clouds reducing the incoming radiation. The spectral effect of pure dust causes deviations of up to 6% at 400 nm compared to the clear sky case. In contrast, the deviations in the case of clouds are up to 31%. Furthermore, atmospheric dust modifies the spectral effect of clouds. In the case of clouds reducing the incoming radiation, the spectral effect depends mainly on the ratio between clouds and dust in the atmosphere. The spectral dependence changes if the optical properties of clouds or dust predominate. Dust increases the spectral effect of clouds mainly in the blue spectral range in the case of clouds enhancing the incoming radiation. An important result was the parameterization of the spectral effects of different atmospheric conditions by power functions. The functions depend on the wavelength and the introduced normalization factors describing the effect of clouds and atmospheric dust on the magnitude of downward irradiance. The parameterization was used to investigate the influence of the spectral effects on photosynthetically available radiation. Their correct knowledge is important for many applications such as biology experiments and ecological modeling. The influence of spectral effects compared to clear sky case is in the order of few percents for all considered atmospheric cases. The spectral effects of different types of clouds reduce or enhance the photosynthetically available radiation up to 6.1% or 1.9%, respectively. Atmospheric dust modifies the influence of spectral effects of clouds on photosynthetically available radiation in dependence on the ratio of clouds to dust.