On the origin of the warm inflow to the Nordic Seas
On the origin of the warm inflow to the Nordic Seas
复制标题
北欧海域暖流的起源
DOI：
10.1016/s0079-6611(01)00084-2
复制
发表时间：
2001
影响因子：
4.1
通讯作者：
C. Mauritzen
中科院分区：
文献类型：
--
作者：
M. Mccartney;C. Mauritzen
暖水和咸水从南部进入北欧海，这是北大西洋北部温盐环流从暖水到冷水转变的一部分。对北欧海流入的起源的一种解释是“浅源假说”，根据该假说，流入水是上层海洋亚热带水的变化。来自副热带环流的暖水被北大西洋暖流带到北大西洋东部，并向北分支，与沿大陆上坡向极地的上温跃层流相连，形成北欧海流入。沿着这条路径，上层水柱逐渐被冬季对流冷却和新鲜，这是亚极模态水转化过程，这决定了流入特征。“深源假说”为北欧海流入的特征和向流入输送水的途径提供了另一种解释。根据这一假说，流入水起源于地中海溢流的核心，位于加的斯湾，在副热带的东部边界潜流中向北移动，沿着东部边界继续进入亚极地环流，从罗卡尔海槽的1200米深处上升到600米以下，穿过威维尔-汤姆森海脊进入法罗-设得兰海峡，再从那里进入北欧海。深源假说的重点是北欧海流入的基台深度以下的下温跃层源水，而浅源假说的重点是基台深度以上的上温跃层水的转化。基于区域水团分布、地转切变和直流测量，我们摒弃了深源假说，支持浅源假说。而不是沿着东部边界的深地中海溢出水从深度上升到北欧海的流动，流入是由与流入相同深度范围的北大西洋洋流直接提供的。地中海溢出水是流入的一个组成部分，但只是通过它对定义亚热带环流内部的温度-盐度关系的贡献，北大西洋流从中吸取水，而不是通过地中海流出水通过东部边界潜流从亚热带进入亚极地区直接向北平流。相反，东边界潜流将地中海溢流全部驱逐到副热带环流的东边缘，形成向西延伸的副热带盐度羽流。对于地中海溢流水核心的下温跃层等压线范围，测量表明，在Wyville-Thomson Ridge，水流是向南下降的（而不是向北上升的），这表明来自法罗海峡的冷溢出带着温暖的上层海水，溢出羽流下降并变暖进入罗卡尔海槽北部。在54°N以北的罗卡尔海槽（豪猪滩）没有发现地中海溢流通过东部边界潜流延伸直接平流的证据，海槽内非常稀薄的地中海溢流反映了Wyville-Thomson Ridge羽流从北部降至海槽，以及下温跃层水从南部输送到海槽的相互作用。
Warm and saline waters enter the Nordic Seas from the south as part of the warm-to-cold water transformation of the thermohaline circulation of the northern North Atlantic. One explanation for the origin of the Nordic Seas Inflow is a “shallow source hypothesis” under which the Inflow waters are a modification of upper ocean subtropical waters. Warm waters from the subtropical gyre are carried to the eastern North Atlantic by the North Atlantic Current and branch northwards, joined by poleward upper thermocline flow along the upper continental slope, to provide the Nordic Seas Inflow. Along this pathway the upper water column is progressively cooled and freshened by winter convection, the subpolar mode water transformation process, and this sets the Inflow characteristics. A “deep source hypothesis” provides an alternative explanation for the characteristics of the Nordic Seas Inflow and the pathway delivering the waters to the Inflow. Under this hypothesis Inflow waters originate from the core of the Mediterranean Overflow Waters in the Gulf of Cadiz carried northward at mid-depth by the eastern boundary undercurrent in the subtropics, continuing into the subpolar gyre along the eastern boundary, and rising from depths near 1200m in the Rockall Trough to less than 600m to cross the Wyville-Thomson Ridge into the Faroe-Shetland Channel and thence to the Nordic Seas. The deep source hypothesis focus is on lower thermocline source waters beneath the sill depth for the Nordic Seas Inflow, in contrast to the shallow source hypothesis focus of transformation of upper thermocline waters above the sill depth. On the basis of regional water mass distributions, geostrophic shear, and direct current measurements, we reject the deep source hypothesis in favor of the shallow source hypothesis. Rather than a flow of deep Mediterranean Overflow Water along the eastern boundary rising from depth to feed the Nordic Seas, the Inflow is supplied directly by transformed North Atlantic Current waters from the same depth range as the Inflow. Mediterranean Overflow Water is a constituent of the Inflow, but only through its contribution to defining the temperature-salinity relationship of the interior of the subtropical gyre from which the North Atlantic Current draws its water, rather than by direct northward advection of Mediterranean Outflow Water via the eastern boundary undercurrent crossing from the subtropics into the subpolar domain. Instead, the eastern boundary undercurrent wholly expels its transport of Mediterranean Overflow Water into the eastern edge of the subtropical gyre, creating the mid-depth westward extending subtropical salinity plume. For the lower thermocline isopycnal range of the core of the Mediterranean Overflow Water, measurements show that at the Wyville-Thomson Ridge the flow is southward and descending (rather than northward and rising), representing cold spillover from the Faroese Channels entraining warm upper ocean waters as the spillover plume descends and warms into the northern Rockall Trough. No evidence is found in Rockall Trough northward of 54°N (Porcupine Bank) for direct advection of Mediterranean Overflow Waters via an eastern boundary undercurrent extension, and the very dilute Mediterranean Overflow Waters in the Trough reflect the interaction of the Wyville-Thomson Ridge plume descending into the Trough from the North, and lower thermocline waters delivered to the Trough from the south.