Physical oceanography and processed 2 minutes-averaged continuous VM-ADCP profiles during POLARSTERN cruise ANT-XXVIII/3

The physical and biological carbon pumps in the different hydrographic and biogeochemical regimes of the Atlantic Sector of the Southern Ocean are controlled by a series of coupled physical, chemical and biological processes and a project named Eddy-Pump was designed to study them. The Eddy Pump field campaign was carried out during RV Polarstern Cruise ANT-XXVIII/3 between January and March 2012. Particular emphasis was laid on the differences which occur along the axis of the Antarctic Circumpolar Current (ACC) with its associated mesoscale eddy field. The study sites were selected in order to represent (1) the central ACC with its regular separation in different frontal jets, investigated by a meridional transect along 10°E; (2) a large-scale bloom west of the Mid-Atlantic Ridge which lasted several months with conspicuous chlorophyll-poor waters to its immediate east studied by a three-dimensional mesoscale survey centred at 12°40'W; and (3) the Georgia Basin north of the island of South Georgia, which regularly features an extended and dense phytoplankton bloom, was investigated by a mesoscale survey centred at 38°12'W. While Eddy-Pump represents an interdisciplinary project by design, we here focus on describing the variable physical environment within which the different biogeochemical regimes developed. For describing the physical environment we use measurements of temperature, salinity and density, of mixed-layer turbulence parameters, of dynamic heights and horizontal current vectors, and of flow trajectories obtained from surface drifters and submerged floats. This serves as background information for the analyses of biological and chemical processes and of biogeochemical fluxes addressed by other papers in this issue.
The section along 10°E between 44°S and 53°S showed a classical ACC structure with well-known hydrographic fronts, the Subantarctic Front (SAF) at 46.5°S, the Antarctic Polar Front (APF) split in two, at 49.25°S and 50.5°S, and the Southern Polar Front (SPF) at 52.5°S. Each front was associated with strong eastward flows. The West Mid-Atlantic Ridge Survey showed a weak and poorly resolved meander structure between the APF and the SPF. During the first eight days of the survey the oceanographic conditions at the Central Station at 12°40'W remained reasonably constant. However after that, conditions became more variable in the thermocline with conspicuous temperature inversions and interleavings and also a decrease in temperature in the surface layer. At the very end of the period of observation the conditions in the thermocline returned to being similar to those observed during the early part of the period with however the mixed layer temperature raised. The period of enhanced thermohaline variability was accompanied by increased currents. The Georgia Basin Survey showed a very strong zonal jet at its northern edge which connects to a large cyclonic meander that itself joins an anticyclonic eddy in the southeastern quadrant. The water mass contrasts in this survey were stronger than in the West Mid-Atlantic Ridge Survey, but similar to those met along 10°E with the exception that the warm and saline surface water typical of the northern side of the SAF was not covered by the Georgia Basin Survey. Mixed layers found during Eddy-Pump were typically deep, but varied between the three survey areas; the mean depths and standard variations of the mixed layer along the 10°E were 77.2±24.7 m, at the West Mid-Atlantic Ridge 66.7±17.7 m, and in the Georgia Basin 36.8±10.7 m.

南大西洋扇区不同水文与生物地球化学区域的物理碳泵与生物碳泵，受一系列耦合的物理、化学与生物过程共同调控，为此专门设计了名为Eddy-Pump（涡旋泵）的研究项目。该涡旋泵野外科考工作于2012年1月至3月期间，在“极星号”（RV Polarstern）科考航次ANT-XXVIII/3中完成。本研究重点关注南极绕极流（Antarctic Circumpolar Current, ACC）轴线及其伴随的中尺度涡旋场的空间差异，据此选取了三类代表性研究区域：（1）沿10°E经向断面开展调查的南极绕极流核心区，该区域存在多股锋面的常规分离；（2）以12°40'W为中心开展三维中尺度调查的中大西洋海岭西侧大规模水华区，该水华持续数月，其紧邻东侧海域存在显著的低叶绿素水体；（3）以南乔治亚岛北侧乔治亚海盆为对象的研究区，该区域常出现大范围密集浮游植物水华，以38°12'W为中心开展中尺度调查。尽管Eddy-Pump项目本质上属于跨学科研究，本文仅聚焦于描述支撑不同生物地球化学区域形成的可变物理环境。用于描述该物理环境的观测数据包括温度、盐度、密度、混合层湍流参数、动力高度与水平流矢量，以及由表面漂流浮标和水下浮标获得的流场轨迹。这些观测数据将作为本期其他论文中分析生物、化学过程及生物地球化学通量的背景资料。
沿10°E经线、44°S至53°S区间的断面呈现出典型的ACC结构，包含已被广泛认知的水文锋面：南亚热带锋（Subantarctic Front, SAF）位于46.5°S，南极极锋（Antarctic Polar Front, APF）分为两支，分别位于49.25°S与50.5°S，以及南极地锋（Southern Polar Front, SPF）位于52.5°S。每一处锋面均伴随强劲的东向流。中大西洋海岭西侧调查区显示，在APF与SPF之间存在一处弱且分辨率较低的弯曲流结构。在调查的前8天，位于12°40'W的核心站海洋学环境保持相对稳定。但在此之后，温跃层环境变得更为复杂，出现显著的温度逆增与水团叠置现象，同时表层海水温度出现下降。在观测期的末期，温跃层环境恢复至观测初期的状态，但混合层温度有所升高。温盐变率增强的时段伴随流场流速的增加。乔治亚海盆调查区的北部边缘存在一处极强的纬向急流，该急流连接至一个大型气旋式弯曲流结构，后者又与东南象限的反气旋涡旋相连。该调查区的水团对比度强于中大西洋海岭西侧调查区，但与沿10°E断面的观测结果类似，唯独有一处例外：SAF北侧典型的暖盐表层水未被乔治亚海盆调查区覆盖。Eddy-Pump科考项目中观测到的混合层通常较深，但在三个调查区域间存在显著差异：沿10°E断面的混合层平均深度及标准差为77.2±24.7 m，中大西洋海岭西侧区域为66.7±17.7 m，乔治亚海盆区域则为36.8±10.7 m。