东地中海小型海底斜坡上方的侵入现象与湍流混合
Intrusions and turbulent mixing above a small Eastern Mediterranean seafloor-slope
AI总结：
本研究利用东地中海小型海底斜坡上方的高分辨率温度观测，发现该弱潮汐海域的湍流以局部浮力驱动的对流-湍流为主，其强度介于陡峭地形区与开阔大洋内部之间，明确了缓坡地形上方的湍流混合特征。
摘要：
观测和数值模拟中越来越多的证据表明，陡峭海底地形对导致悬浮物和营养物质再分布的湍流跨密度面混合至关重要，在内部潮汐丰富的水域中尤其如此。尚存的问题之一是，近乎平坦地形之外及上方的湍流混合范围有多大，本文对此展开研究。本文评估了在东地中海约1200米水深处的一个小型海底斜坡上方，通过为期一周的机会性锚定高分辨率温度传感器获得的观测数据。该海域潮汐较弱，因此近惯性运动和剪切主导着内波。垂直位移形态表明，不稳定性代表的是局部产生的湍流翻转，而非从斜坡附近湍流沿等密度面扩散的部分盐度补偿侵入体。这一结论得到了不稳定性持续时间的支持：所有单个翻转的持续时间都短于平均浮力周期，且翻转序列的持续时间短于当地惯性周期。与剪切驱动湍流的强分层水域中观测到的情况相比，这里的位移形态更不规则，更符合以浮力驱动为主的对流-湍流特征。这种对流-湍流得到了光谱信息的证实，通常主要发生在靠近海底的区域，且仅出现在海底上方较远的弱分层水层中。平均湍流值比陡峭海洋地形上方测得的小10到100倍，但比开阔大洋内部的测得值大10倍。
英文摘要：
Growing evidence is found in observations and numerical modelling of the importance of steep seafloor topography for turbulent diapycnal mixing leading to redistribution of suspended matter and nutrients, especially in waters with abundant internal tides. One of the remaining questions is the extent of turbulent mixing away from and above nearly flat topography, which is addressed in this paper. Evaluated are observations from an opportunistic, week-long mooring of high-resolution temperature sensors above a small seafloor slope in about 1200 m water depth of the Eastern Mediterranean. The environment has weak tides, so that near-inertial motions and -shear dominate internal waves. Vertical displacement shapes suggest instabilities to represent locally generated turbulent overturns, rather than partial salinity-compensated intrusions dispersed isopycnally from turbulence near the slope. This conclusion is supported by the duration of instabilities, as all individual overturns last shorter than the mean buoyancy period and sequences of overturns last shorter than the local inertial period. The displacement shapes are more erratic than observed in stronger stratified waters in which shear drives turbulence, and better correspond with predominantly buoyancy-driven convection-turbulence. This convection-turbulence is confirmed from spectral information, generally occurring dominant close to the seafloor and only in weakly stratified layers well above it. Mean turbulence values are 10-100 times smaller than found above steep ocean topography, but 10 times larger than found in the open-ocean interior.
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