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

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.

非洲西北外海区域的营养盐供给主要受两大过程调控：沿岸上升流与撒哈拉沙尘沉降。本研究针对2000年至2008年时段的遥感替代指标（remotely sensed proxies），采用互相关、多重相关及事件统计法等多种分析方法，对这两个过程展开系统分析与评估，以探究其对海洋环境的影响。研究以遥感反演的叶绿素a浓度（chlorophyll-a concentration）作为浮游植物生物量的替代指标——该生物量由深层水层与大气向真光层（euphotic zone）输入营养盐所促生。卫星反演的沿岸风应力（alongshore wind stress）与海表温度（sea-surface temperature）被用作沿岸上升流过程强度与响应的替代指标。西风与气溶胶光学厚度（aerosol optical depth）中的沙尘组分，分别表征大气沙尘的输送方向与大气沙尘柱载荷。沿岸风应力及其诱导的上升流过程是表层叶绿素a浓度变异的最主要驱动因子，其贡献率约占总变异的24%，该现象主要出现在冬春两季，得益于强劲的东北信风（trade winds）。借助遥感替代指标，可确定生物响应与其强迫过程之间的时间滞后效应：在北部海域的冬春季节，表层叶绿素a浓度相较于沿岸风应力存在最长达16天的滞后。尽管沙尘风暴输入的大气铁可刺激研究区浮游植物的新生产，但气溶胶光学厚度中的沙尘组分仅能解释约5%的表层叶绿素a浓度变异。本研究通过事件统计法识别了2000至2008年间的所有强沙尘暴事件，并针对57次强风暴事件的生物响应展开分析。其中6次事件明确由撒哈拉沙尘输入而非沿岸上升流引发叶绿素a浓度升高，其时间滞后分别为<8天、8天及16天。在气溶胶光学厚度沙尘组分最高达0.9的沙尘暴过后，观测到表层叶绿素a浓度最高升高2.4 mg·m⁻³。