Lovecchio Elisa, Henson Stephanie, Carvalho Filipa, Briggs Nathan
National Oceanography Centre European Way Southampton UK.
J Geophys Res Oceans. 2022 Nov;127(11):e2022JC019063. doi: 10.1029/2022JC019063. Epub 2022 Nov 5.
Despite their role in modulating the marine ecosystem, variability and drivers of low-oxygen events in the offshore northern Benguela Upwelling System (BenUS) have been rarely investigated due to the events' episodicity which is difficult to resolve using shipboard measurements. We address this issue using 4 months of high-resolution glider data collected between February and June 2018, 100 km offshore at 18°S. We find that oxygen (O) concentrations in the offshore northern Benguela are determined by the subsurface alternation of low-oxygen Angola-derived water and oxygenated water from the south at 100-500 m depth. We observe intermittent hypoxia (O < 60 μmol kg) which occurs on average for ∼30% of the 4 months deployment and is driven by the time-varying subsurface pulses of Angola-derived tropical water. Hypoxic events are rather persistent at depths of 300-450 m, while they are more sporadic and have weekly duration at shallower depths (100-300 m). We find extreme values of hypoxia, with O minima of 16 μmol kg, associated with an anticyclonic eddy spinning from the undercurrent flowing on the BenUS shelf and showing no surface signature. Fine-scale patchiness and water mass mixing are associated with cross-frontal stirring by a large anticyclone recirculating tropical water into the northern BenUS. The dominance of physical drivers and their high variability on short time scales reveal a dynamic coupling between Angola and Benguela, calling for long-term and high-resolution measurements and studies focusing on future changes of both tropical O minima and lateral fluxes in this region.
尽管低氧事件在调节海洋生态系统中发挥着作用，但由于其突发性，利用船上测量难以解决，因此很少有人对本格拉上升流系统（BenUS）北部近海低氧事件的变异性和驱动因素进行研究。我们利用2018年2月至6月在南纬18°离岸100公里处收集的4个月高分辨率滑翔器数据来解决这一问题。我们发现，本格拉北部近海的氧（O）浓度取决于100 - 500米深度处来自安哥拉的低氧水和来自南方的含氧水在次表层的交替。我们观测到间歇性缺氧（O < 60 μmol kg），在4个月的部署期内平均约30%的时间出现，其由来自安哥拉的热带水随时间变化的次表层脉冲驱动。缺氧事件在300 - 450米深度较为持久，而在较浅深度（100 - 300米）则更零散且持续时间为一周。我们发现了极端缺氧值，O最小值为16 μmol kg，与一个从BenUS陆架上的潜流旋转而来的反气旋涡旋有关，且没有表面特征。精细尺度的斑块状分布和水体混合与一个大型反气旋将热带水再循环到BenUS北部的跨锋面搅动有关。物理驱动因素的主导地位及其在短时间尺度上的高变异性揭示了安哥拉和本格拉之间的动态耦合，这就需要进行长期和高分辨率的测量以及专注于该区域热带O最小值和横向通量未来变化的研究。
