Nutrient pumping by submesoscale circulations in the mauritanian upwelling system
Nutrient pumping by submesoscale circulations in the mauritanian upwelling system
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DOI：
10.1016/j.pocean.2017.10.004
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发表时间：
2017-12
影响因子：
4.1
通讯作者：
P. Hosegood;P. Nightingale;A. Rees;C. Widdicombe;E. Woodward;D. Clark;R. Torres
中科院分区：
文献类型：
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作者：
P. Hosegood;P. Nightingale;A. Rees;C. Widdicombe;E. Woodward;D. Clark;R. Torres
在一个冷丝内的Dakanian上升流系统的观测表明，强烈的亚中尺度环流在外围边缘的细丝可能是负责通过补充营养物质的真光层观察到的初级生产力的高水平。在最近涌上来的水的大陆架上进行的测量表明，8.2 gC/m− 2 day − 1的初级生产力（PP）得到了8 mmol m−3的硝酸盐浓度（NC）的支持。当上升的水被输送到近海时，在限定包含上升水的细丝边缘的前部，PP下降到1.6 gC m− 2 day − 1，而NC下降到5.5 mmol m−3。因此，虽然在货架上观察到的营养物质占90%的新生产，这个值下降到近60%的长丝的前面占垂直湍流通量和埃克曼泵。我们证明，N15很可能是由亚中尺度环流在锋面提供的，这些环流是由穿过亚中尺度表面温度锋面的漂移声学多普勒海流剖面仪直接测量到的强烈垂直速度为每天100米。与此同时，最近释放的示踪剂被俯冲出的混合层内24小时的释放，提供直接的证据表明，锋面环流能够访问水库的营养密度跃层下方。在1-10 km水平尺度上，用O（1）Rossby数证明了暗条边缘对亚中尺度不稳定性的敏感性。锋面环流是一致的不稳定性所产生的风驱动的非线性Ekman浮力通量产生的持续偏北风应力，有一个向下的前组件在北方边缘的近岸部分的细丝。亚中尺度不稳定性及其相关的垂直环流的流行被认为是一个关键的机制，在次网格尺度上运作，并维持整个上升流系统的新生产。
Observations made within a cold filament in the Mauritanian upwelling system demonstrate that intense submesoscale circulations at the peripheral edges of the filament are likely responsible for anomalously high levels of observed primary productivity by resupplying nutrients to the euphotic zone. Measurements made on the shelf within the recently upwelled water reveal that primary production (PP) of 8.2 gC/m−2day−1was supported by nitrate concentrations (NC) of 8 mmol m−3. Towards the front that defined the edge of the filament containing the upwelled water as it was transported offshore, PP dropped to 1.6 gC m−2day−1whilst NC dropped to 5.5 mmol m−3. Thus, whilst the observed nutrients on the shelf accounted for 90% of new production, this value dropped to ∼60% near the filament’s front after accounting for vertical turbulent fluxes and Ekman pumping. We demonstrate that the N15was likely to have been supplied at the front by submesoscale circulations that were directly measured as intense vertical velocities ⩾100 m day−1by a drifting acoustic Doppler current profiler that crossed a submesoscale surface temperature front. At the same time, a recently released tracer was subducted out of the mixed layer within 24 h of release, providing direct evidence that the frontal circulations were capable of accessing the reservoir of nutrients beneath the pycnocline. The susceptibility of the filament edge to submesoscale instabilities was demonstrated byO(1) Rossby numbers at horizontal scales of 1–10 km. The frontal circulations are consistent with instabilities arising from a wind-driven nonlinear Ekman buoyancy flux generated by the persistent northerly wind stress that has a down-front component at the northern edge of the inshore section of the filament. The prevalence of submesoscale instabilities and their associated vertical circulations are proposed to be a key mechanism operating at sub-grid scales and sustaining new production throughout the upwelling system.