Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling
Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling
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DOI：
10.5194/bg-14-2167-2017
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发表时间：
2016-03
期刊：
影响因子：
4.9
通讯作者：
J. Karstensen;F. Schütte;A. Pietri;G. Krahmann;B. Fiedler;D. Grundle;H. Hauss;A. Körtzinger;C. Löscher;P. Testor;N. Vieira;M. Visbeck
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文献类型：
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作者：
J. Karstensen;F. Schütte;A. Pietri;G. Krahmann;B. Fiedler;D. Grundle;H. Hauss;A. Körtzinger;C. Löscher;P. Testor;N. Vieira;M. Visbeck
利用高分辨率的滑翔机和船舶资料，研究了一个贫氧反气旋模式水涡的物理和生物地球化学结构的时间演变。在浅深度的弱分层涡核（浮力频率平方N2 = 0.1 × 10 −4 s −2）被确定为水平延伸约70 km，并以N2的最大值为界。上部N2最大值（3-5 × 10 - 4s-2）与混合层底部一致，下部N2最大值（0.4 × 10 - 4s-2）出现在涡旋中心约200 m深度处。涡核的温度/盐度（T /S）的特性在2个月内表现出恒定的斜率，但核心的侵蚀逐渐缩小T /S的范围。涡旋最小氧浓度在2个月内下降了约5 µmol kg −1，证实了早期对这些涡旋中氧消耗率的估计。分离中尺度和扰动流分量揭示了涡旋下方及其侧翼的振荡速度精细结构（约0.1 m s −1）。速度精细结构是按层组织的，这些层与属性（盐度，温度）层对齐，但大多数穿过恒定密度的表面。速度精细结构的最大量级出现在表面和最大中尺度流外部140 m之间，而且也出现在涡流中心下方250 m至450 m之间的层中。对于这两个地区的气旋旋转的速度finestructure与深度表明近惯性波（NIW）能量的垂直传播。反气旋（涡核）和气旋（涡外围）相对涡度对行星涡度的修改最有可能影响NIW能量传播。在低氧核的下方，发现了盐指型双扩散层，它们与速度精细结构一致。涡核中的表观氧利用率（AOU）与溶解无机硝酸盐（NO − 3）的比率（16）大约是周围沃茨（8.1）的两倍。确定了4至6 µmol kg −1的大量NO − 3赤字，使反硝化作用成为一个不太可能的解释。这里假设，作为涡动动力学的结果，氮和氧的局部再循环的差异导致AOU：NO sup>-3比率的变化。高NO − 3和低氧沃茨通过从涡核混合而被侵蚀，并夹带到混合层中。氮通过颗粒物质的重力沉降从真光层重新引入到核心中。低氧水在混合层中通过海气交换达到平衡，不参与重力下沉。最后，我们提出了一个中尺度-亚中尺度相互作用的概念，风能，通过NIWs介导，驱动营养物质供应到透光层，并驱动反气旋模式的水涡旋异常盛开。
The temporal evolution of the physical and bio-geochemical structure of an oxygen-depleted anticyclonic modewater eddy is investigated over a 2-month period using high-resolution glider and ship data. A weakly stratified eddy core (squared buoyancy frequency N 2 ∼ 0.1 × 10 −4 s −2 ) at shallow depth is identified with a horizontal extent of about 70 km and bounded by maxima in N 2 . The upper N 2 maximum (3–5 × 10 −4 s −2 ) coincides with the mixed layer base and the lower N 2 maximum (0.4 × 10 −4 s −2 ) is found at about 200 m depth in the eddy centre. The eddy core shows a constant slope in temperature/salinity (T /S) characteristic over the 2 months, but an erosion of the core progressively narrows down the T /S range. The eddy minimal oxygen concentrations decreased by about 5 µmol kg −1 in 2 months, confirming earlier estimates of oxygen consumption rates in these eddies. Separating the mesoscale and perturbation flow components reveals oscillating velocity finestructure (∼ 0.1 m s −1 ) underneath the eddy and at its flanks. The velocity finestrucure is organized in layers that align with layers in properties (salinity, temperature) but mostly cross through surfaces of constant density. The largest magnitude in velocity finestructure is seen between the surface and 140 m just outside the maximum mesoscale flow but also in a layer underneath the eddy centre, between 250 and 450 m. For both regions a cyclonic rotation of the velocity finestructure with depth suggests the vertical propagation of near-inertial wave (NIW) energy. Modification of the planetary vorticity by anticyclonic (eddy core) and cyclonic (eddy periphery) relative vorticity is most likely impacting the NIW energy propagation. Below the low oxygen core salt-finger type double diffusive layers are found that align with the velocity finestructure. Apparent oxygen utilization (AOU) versus dissolved inorganic nitrate (NO − 3 ) ratios are about twice as high (16) in the eddy core compared to surrounding waters (8.1). A large NO − 3 deficit of 4 to 6 µmol kg −1 is determined, rendering denitrification an unlikely explanation. Here it is hypothesized that the differences in local recycling of nitrogen and oxygen, as a result of the eddy dynamics, cause the shift in the AOU : NO sup>− 3 ratio. High NO − 3 and low oxygen waters are eroded by mixing from the eddy core and entrain into the mixed layer. The nitrogen is reintroduced into the core by gravitational settling of particulate matter out of the euphotic zone. The low oxygen water equilibrates in the mixed layer by air–sea gas exchange and does not participate in the gravitational sinking. Finally we propose a mesoscale–submesoscale interaction concept where wind energy, mediated via NIWs, drives nutrient supply to the euphotic zone and drives extraordinary blooms in anticyclonic mode-water eddies.