Large-scale forcing of the European Slope Current and associated inflows to the North Sea
Large-scale forcing of the European Slope Current and associated inflows to the North Sea
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DOI：
10.5194/os-13-315-2017
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发表时间：
2016-08
期刊：
影响因子：
3.2
通讯作者：
R. Marsh;I. Haigh;S. Cunningham;M. Inall;M. Porter;B. Moat
中科院分区：
文献类型：
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作者：
R. Marsh;I. Haigh;S. Cunningham;M. Inall;M. Porter;B. Moat
抽象的。欧洲坡流为大西洋水提供了陆架边缘管道，其中很大一部分流向北海北部，对区域水文学和生态系统产生影响。在赫布里底大陆架断裂处欧洲坡流中 50 m 处漂流的漂流者遵循多种路径，表明大西洋流入北海的流量变化很大。 1988 年至 2007 年期间的坡流路径、时间尺度和传输在涡旋解析海洋模型后报中得到进一步量化。用模型水流计算的粒子轨迹表明，坡流水主要来自北大西洋副极地东部。对绝对动态地形和气候密度的观测支持了理论预期，即斜坡洋流输送是与东部副极地环流中的经向密度梯度相关的一阶，这支持了朝向斜坡的地转流入。在模型后报中，坡流输运变化主要是 1996 年至 1998 年期间这些密度梯度突然减少 25-50%。以密度梯度表示的风力强迫的同时变化，以同样的方式减少坡流输送。这表明浮力和风力的协调区域变化共同作用减少了 20 世纪 90 年代的坡流输送。粒子轨迹进一步表明，10-40% 的坡流水在流向苏格兰西部后的 6 个月内将流向北海北部，而 1988-2007 年这一比例总体下降。海洋分析数据证明，斜坡流中的盐度相应下降。再往北，在由斜坡洋流通过法罗-设得兰海峡 (FSC) 输送的大西洋水中，观察到盐度在此期间增加，而在后报中下降。观察到的趋势可能在很大程度上补偿了大西洋流入量的下降，限制了这一时期北海北部的盐度变化。坡流输送和大西洋流入北海的替代指标是通过 FSC 以及设得兰群岛和苏格兰大陆（威克）之间的海平面高度差来寻找的。在 1957 年的验潮仪记录和 1958 年至 2012 年较长的模型后报中，从季节性到数十年的各种时间尺度上的坡流输送变化都隐含在勒威克（设得兰群岛）和托尔斯港（法罗群岛）之间的海平面差异中。 1965 年验潮仪记录中的威克-勒威克海平面差异表明，费尔岛洋流输送存在相当大的年代际变化，该洋流主导大西洋流入北海西北部，而后报中的海平面差异则由强烈的季节变化主导。威克潮汐测量仪记录的不确定性限制了对该代理的信心。
Abstract. The European Slope Current provides a shelf-edge conduit for Atlantic Water, a substantial fraction of which is destined for the northern North Sea, with implications for regional hydrography and ecosystems. Drifters drogued at 50 m in the European Slope Current at the Hebridean shelf break follow a wide range of pathways, indicating highly variable Atlantic inflow to the North Sea. Slope Current pathways, timescales and transports over 1988–2007 are further quantified in an eddy-resolving ocean model hindcast. Particle trajectories calculated with model currents indicate that Slope Current water is largely recruited from the eastern subpolar North Atlantic. Observations of absolute dynamic topography and climatological density support theoretical expectations that Slope Current transport is to first order associated with meridional density gradients in the eastern subpolar gyre, which support a geostrophic inflow towards the slope. In the model hindcast, Slope Current transport variability is dominated by abrupt 25–50 % reductions of these density gradients over 1996–1998. Concurrent changes in wind forcing, expressed in terms of density gradients, act in the same sense to reduce Slope Current transport. This indicates that coordinated regional changes of buoyancy and wind forcing acted together to reduce Slope Current transport during the 1990s. Particle trajectories further show that 10–40 % of Slope Current water is destined for the northern North Sea within 6 months of passing to the west of Scotland, with a general decline in this percentage over 1988–2007. Salinities in the Slope Current correspondingly decreased, evidenced in ocean analysis data. Further to the north, in the Atlantic Water conveyed by the Slope Current through the Faroe–Shetland Channel (FSC), salinity is observed to increase over this period while declining in the hindcast. The observed trend may have broadly compensated for a decline in the Atlantic inflow, limiting salinity changes in the northern North Sea during this period. Proxies for both Slope Current transport and Atlantic inflow to the North Sea are sought in sea level height differences across the FSC and between Shetland and the Scottish mainland (Wick). Variability of Slope Current transport on a wide range of timescales, from seasonal to multi-decadal, is implicit in sea level differences between Lerwick (Shetland) and Torshavn (Faroes), in both tide gauge records from 1957 and a longer model hindcast spanning 1958–2012. Wick–Lerwick sea level differences in tide gauge records from 1965 indicate considerable decadal variability in the Fair Isle Current transport that dominates Atlantic inflow to the northwest North Sea, while sea level differences in the hindcast are dominated by strong seasonal variability. Uncertainties in the Wick tide gauge record limit confidence in this proxy.