The oceanography of winter leads
The oceanography of winter leads
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冬季海洋学引领
DOI：
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发表时间：
1992
期刊：
影响因子：
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通讯作者：
C. Paulson
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作者：
J. Morison;M. Mcphee;T. Curtin;C. Paulson
长期以来，浮冰中的铅一直被认为对极地热力学很重要。冬季铅会影响周围的海洋，因为它是密度源。当表面结冰时，盐会被排斥并形成密度更大的水，沉入铅下。这就建立了一个循环，淡水从靠近地表的侧面流入，浓水从混合层底部的铅流走。如果混合层完全湍流，则可能不会出现这种模式；相反，表面排斥的盐可能简单地混合到表面边界层中。在任何一种情况下，引线表面产生的不稳定性都是不稳定浮力通量的主要来源，因此对混合层产生强烈影响。这里汇集了尽可能多的不同的、几乎是轶事性的铅海洋学观察结果，并与理论论证相结合，以预测冬季铅引起的海洋扰动的形式和规模。实验数据表明与铅对流相关的速度扰动约为 1-5 cm s−1。当穿过导线的冰速度小于约 5 cm s−1 时，这些在混合层表面和底部附近表现为射流。盐度扰动约为 0.01 至 0.05 psu。尺度论证表明，由铅密度扰动产生的地转流也具有 1-5 cm s−1 的量级。当冻结速度快且冰速较低时，扰动最为明显，因为上层海洋的盐度和速度扰动不会被湍流所掩盖。在这种情况下，铅对流的一个极端特征可能是自由对流，其中密度扰动迫使循环。在另一个极端，铅对流的特征可能是强制对流，其中密度扰动与边界层湍流迅速混合。导数 Lo（动量方程中压力项与湍流项之比）和湍流导数 Lot（湍流动能方程中浮力产生量与剪切产生量之比）定义了自由状态和受迫状态之间的边界。当 Lo 和 Lot 小于 1 时，大尺度环流和湍流都是由表面应力产生的。当 Lo 和 Lot 大于 1 时，大尺度环流和湍流都是由浮力通量强迫的。来自其他地方开发的适合自由对流的模型的速度和盐度扰动的大小与我们仅有的少量观察结果一致。这里开发的强制对流模型的结果表明，盐度扰动约为 0.01-0.02 个实际盐度单位，最大值出现在铅表面，并在 5-10 m 以下大幅下降。这种不稳定的梯度是铅对流的独特特征。尽管当冰速较大时盐度扰动可能较小，但引导中的浮力通量对边界层湍流具有重大影响。
Leads in pack ice have long been considered important to the thermodynamics of the polar regions. A winter lead affects the ocean around it because it is a density source. As the surface freezes, salt is rejected and forms more dense water which sinks under the lead. This sets up a circulation with freshwater flowing in from the sides near the surface and dense water flowing away from the lead at the base of the mixed layer. If the mixed layer is fully turbulent, this pattern may not occur; rather, the salt rejected at the surface may simply mix into the surface boundary layer. In either event the instability produced at the surface of leads is the primary source of unstable buoyancy flux and, as such, exerts a strong influence on the mixed layer. Here as many as possible of the disparate and almost anecdotal observations of lead oceanography are assembled and combined with theoretical arguments to predict the form and scale of oceanographic disturbances caused by winter leads. The experimental data suggest the velocity disturbances associated with lead convection are about 1–5 cm s−1. These appear as jets near the surface and the base of the mixed layer when ice velocities across the lead are less than about 5 cm s−1. The salinity disturbances are about 0.01 to 0.05 psu. Scaling arguments suggest that the geostrophic currents set up by the lead density disturbances are also of the order of 1–5 cm s−1. The disturbances are most obvious when freezing is rapid and ice velocity is low because the salinity and velocity disturbances in the upper ocean are not smeared out by turbulence. In this vein, lead convection may be characterized at one extreme as free convection in which the density disturbance forces the circulation. At the other extreme, lead convection may be characterized as forced convection in which the density disturbance is mixed rapidly by boundary layer turbulence. The lead number Lo, which is the ratio of the pressure term to the turbulence term in the momentum equation, and the turbulent lead number Lot, which is the ratio of buoyant production to shear production in the turbulent kinetic energy equation, define the boundary between the free and forced regimes. For Lo and Lot less than one, both the large-scale circulation and the turbulence are forced by surface stress. For Lo and Lot greater than one, both the large-scale circulation and the turbulence are forced by the buoyancy flux. The magnitudes of velocity and salinity disturbances from a model developed elsewhere, suitable to free convection, agree with what few observations we have. The results of a forced convection model, developed here, suggest salinity disturbances of the order of 0.01–0.02 practical salinity units, with the maximum occurring at the surface of the lead and decreasing substantially below 5–10 m. This unstable gradient is a unique characteristic of lead convection. Though the salinity disturbances may be small when ice velocities are large, the buoyancy flux in leads has a major effect on the boundary layer turbulence.