Körner Mareike, Brandt Peter, Illig Serena, Dengler Marcus, Subramaniam Ajit, Bachèlery Marie-Lou, Krahmann Gerd
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany.
Faculty of Mathematics and Natural Sciences, Kiel University, Kiel, Germany.
Sci Adv. 2024 Jan 26;10(4):eadj6686. doi: 10.1126/sciadv.adj6686.
Eastern boundary upwelling systems are hotspots of marine life and primary production. The strength and seasonality of upwelling in these systems are usually related to local wind forcing. However, in some tropical upwelling systems, seasonal maxima of productivity occur when upwelling favorable winds are weak. Here, we show that in the tropical Angolan upwelling system (tAUS), the seasonal productivity maximum is due to the combined effect of coastal trapped waves (CTWs) and elevated tidal mixing on the shelf. During austral winter, the passage of an upwelling CTW displaces the nitracline upward by more than 50 m. Thereby, nitrate-rich waters spread onto the shelf, where elevated vertical mixing causes a nitrate flux into the surface mixed layer. Interannual variability of the productivity maximum is strongly correlated to the amplitude of the upwelling CTW as seen in sea level data. Given that CTWs are connected to equatorial forcing, a predictability of the strength of the productivity maximum is suggested.
东边界上升流系统是海洋生物和初级生产力的热点区域。这些系统中上升流的强度和季节性通常与当地风应力有关。然而，在一些热带上升流系统中，当上升流有利风较弱时，生产力会出现季节性最大值。在此，我们表明，在热带安哥拉上升流系统（tAUS）中，季节性生产力最大值是由于沿岸捕获波（CTW）和陆架上增强的潮汐混合的共同作用。在南半球冬季，一个上升流CTW的通过使硝酸盐跃层向上移动超过50米。由此，富含硝酸盐的水体扩散到陆架上，在那里增强的垂直混合导致硝酸盐通量进入表层混合层。如海平面数据所示，生产力最大值的年际变化与上升流CTW的振幅密切相关。鉴于CTW与赤道强迫有关，这表明生产力最大值的强度具有可预测性。
