Physical oceanography, current meter data, and microstructure measurements 180 km downstream of Denmark Strait sill during Maria S. Merian cruise MSM21/1b@en

The Denmark Strait Overflow (DSO) contributes roughly half to the total volume transport of the Nordic overflows. The overflow increases its volume by entraining ambient water as it descends into the subpolar North Atlantic, feeding into the deep branch of the Atlantic Meridional Overturning Circulation. In June 2012, a multiplatform experiment was carried out in the DSO plume on the continental slope off Greenland (180 km downstream of the sill in Denmark Strait), to observe the variability associated with the entrainment of ambient waters into the DSO plume. In this study, we report on two high-dissipation events captured by an autonomous underwater vehicle (AUV) by horizontal profiling in the interfacial layer between the DSO plume and the ambient water. Strong dissipation of turbulent kinetic energy of O(10**-6) W/kg was associated with enhanced small-scale temperature variance at wavelengths between 0.05 and 500 m as deduced from a fast-response thermistor. Isotherm displacement slope spectra reveal a wave number-dependence characteristic of turbulence in the inertial-convective subrange ( math formula) at wavelengths between 0.14 and 100 m. The first event captured by the AUV was transient, and occurred near the edge of a bottom-intensified energetic eddy. Our observations imply that both horizontal advection of warm water and vertical mixing of it into the plume are eddy-driven and go hand in hand in entraining ambient water into the DSO plume. The second event was found to be a stationary feature on the upstream side of a topographic elevation located in the plume pathway. Flow-topography interaction is suggested to drive the intense mixing at this site.丹麦海峡溢流（Denmark Strait Overflow, DSO）对北欧溢流（Nordic overflows）总体积输运的贡献约占一半。该溢流在向下沉降进入亚极地北大西洋的过程中，会通过卷吸周围水体增大自身体积，并最终汇入大西洋经向翻转环流（Atlantic Meridional Overturning Circulation）的深层分支。2012年6月，科研团队在格陵兰岛外大陆坡（丹麦海峡海槛下游180公里处）的DSO羽流区域开展了多平台实验，旨在观测伴随周围水体卷吸进入DSO羽流的各类变化过程。本研究报道了自主水下航行器（autonomous underwater vehicle, AUV）通过在DSO羽流与周围水体的界面层开展水平剖面测量所捕获的两起高耗散事件。基于快速响应热敏电阻（fast-response thermistor）的反演结果，湍流动能的强耗散（量级为O(10⁻⁶) W/kg）与波长0.05至500米区间内的小尺度温度方差增强现象密切相关。等温面位移斜率谱显示，在波长0.14至100米范围内，其波数依赖特征符合惯性对流子区间的湍流特性（math formula）。自主水下航行器观测到的第一起事件为瞬态过程，发生在底部强化型强涡旋的边缘区域。本次观测结果表明，暖水的水平平流与暖水垂直混合进入羽流这两个过程均由涡旋驱动，二者协同完成了周围水体向DSO羽流的卷吸过程。第二起事件则表现为定常特征，位于羽流路径上一处地形隆起的上游侧。研究推测，地形与流场的相互作用是该区域强混合现象的驱动机制。