MEOP-CTD in-situ data collection: a Southern ocean Marine-mammals calibrated sea water temperatures and salinities observations

The Southern Ocean plays a fundamental role in regulating the global climate. This ocean also contains a rich and highly productive ecosystem, potentially vulnerable to climate change. Very large national and international efforts are directed towards the modeling of physical oceanographic processes to predict the response of the Southern Ocean to global climate change and the role played by the large-scale ocean climate processes. However, these modeling efforts are greatly limited by the lack of in situ measurements, especially at high latitudes and during winter months. The standard data that are needed to study ocean circulation are vertical profiles of temperature and salinity, from which we can deduce the density of seawater. These are collected with CTD (Conductivity-Temperature-Depth) sensors that are usually deployed on research vessels or, more recently, on autonomous Argo profilers. The use of conventional research vessels to collect these data is very expensive, and does not guarantee access to areas where sea ice is found at the surface of the ocean during the winter months. A recent alternative is the use of autonomous Argo floats. However, this technology is not easy to use in glaciated areas. In this context, the collection of hydrographic profiles from CTDs mounted on marine mammals is very advantageous. The choice of species, gender or age can be done to selectively obtain data in particularly under-sampled areas such as under the sea ice or on continental shelves. Among marine mammals, elephant seals are particularly interesting. Indeed, they have the particularity to continuously dive to great depths (590 ± 200 m, with maxima around 2000 m) for long durations (average length of a dive 25 ± 15 min, maximum 80 min). A Conductivity-Temperature-Depth Satellite Relay Data Logger (CTD-SRDLs) has been developed in the early 2000s to sample temperature and salinity vertical profiles during marine mammal dives (Boehme et al. 2009, Fedak 2013). The CTD-SRDL is attached to the seal on land, then it records hydrographic profiles during its foraging trips, sending the data by satellite ARGOS whenever the seal goes back to the surface.While the principle intent of seal instrumentation was to improve understanding of seal foraging strategies (Biuw et al., 2007), it has also provided as a by-product a viable and cost-effective method of sampling hydrographic properties in many regions of the Southern Ocean (Charrassin et al., 2008; Roquet et al., 2013).

南大洋（Southern Ocean）在全球气候调控中发挥着基础性作用。该海域同时拥有丰富且生产力极高的生态系统，且极易受到气候变化的影响。当前，各国及国际层面投入了大量资源，用于物理海洋过程的建模，以预测南大洋对全球气候变化的响应，以及大型海洋气候过程所扮演的角色。然而，此类建模工作因缺乏现场实测数据而受到极大限制，在高纬度海域与冬季月份尤为如此。研究海洋环流所需的标准数据为温度与盐度的垂直剖面，借此可推算海水密度。此类数据通常由温盐深（Conductivity-Temperature-Depth, CTD）传感器采集，这类传感器以往多部署于科考船，近年来也可搭载于自主式Argo剖面浮标。使用传统科考船采集此类数据成本高昂，且无法保证在冬季海冰覆盖的洋面区域开展作业。近年来的替代方案是使用自主式Argo浮标，但该技术在冰川覆盖区域的应用难度较高。在此背景下，搭载于海洋哺乳动物身上的CTD传感器所采集的水文剖面数据便具备显著优势。通过选择特定物种、性别或年龄的个体，可针对性地在海冰下方、大陆架等采样严重不足的区域获取数据。在海洋哺乳动物中，象海豹尤为值得关注。事实上，它们具备持续深潜的特性：平均潜深为590±200米，最大潜深可达约2000米，单次潜水时长平均为25±15分钟，最长可达80分钟。21世纪初，科研人员开发出了温盐深卫星中继数据记录仪（Conductivity-Temperature-Depth Satellite Relay Data Logger, CTD-SRDL），可在海洋哺乳动物潜水期间采集温度与盐度的垂直剖面数据（Boehme et al. 2009, Fedak 2013）。CTD-SRDL在陆地阶段被安装至海豹身上，随后在其觅食迁徙过程中记录水文剖面数据，当海豹返回海面时，通过ARGOS卫星传输采集到的数据。尽管为海豹搭载这类设备的初衷是为了加深对海豹觅食策略的理解（Biuw et al., 2007），但该技术也附带产生了一种可行且高性价比的采样方法，可在南大洋诸多区域采集水文属性数据（Charrassin et al., 2008; Roquet et al., 2013）。