Surface Mixed Layer at Submesoscales (SMILES)

我们目前对地球气候的理解很大程度上是基于数值模型的预测，这些模型模拟了大气和海洋的行为和相互作用。然而，这些模型的分辨率受到严重限制，以至于海洋中水平尺度小于约10公里的过程无法明确表示，需要对其影响进行参数化，才能将其纳入模型。该项目名为“次中尺度表面混合演变”，目的是查明一种称为次中尺度的未解决过程在影响上层海洋的结构和性质，从而影响南大洋内表层水团的转变方面可能发挥的关键作用。次中尺度是空间尺度为1-10公里的气流，发生在海洋和大气之间进行通信和交换的上层海洋中。以前被认为对气候尺度研究不重要，因为它们的尺度小，而且假定它们的动力学不重要，现在出现了来自高分辨率区域模式和观测研究的最新证据，这些证据表明，次中尺度实际上广泛存在于整个上层海洋，并由于它们能够迅速使上层海洋分层并减少从海洋到大气的浮力损失，在气候动力学中发挥关键作用。这种过程的影响对于亚北极模式水(SAMW)等水团的表面变化尤为重要，它是子午线翻转环流(MOC)的重要组成部分，在全球范围内重新分配热量、淡水和示踪剂。在MOC内部，像SAMW这样的致密水团在被俯冲到海洋内部之前，通过表面过程在高纬度形成和转化。俯冲水团及其内部的示踪剂和二氧化碳等溶解气体的性质对全球气候和地球化学循环至关重要，因为这些水团在十年至百年的时间尺度上与地表保持不接触。鉴于最近关于次中尺度对上层海洋性质产生重大影响的能力的发现，我们将直接研究次中尺度对斯科舍海内SAMW性质的影响。使用一种综合方法，我们将在一系列空间和时间尺度上观察和模拟上层海洋中的亚中尺度，从湍流到模式水形成。这项研究的主要目标是诊断亚中尺度在水团转化中所起的作用，以便我们能够准确地将这些影响纳入无法明确解决它们的气候尺度模式中。我们的方法将需要在福克兰群岛以南约200英里处的南极洲前线(SAF)巡航，在其以北改造SAMW，并使用最先进的全球环流模式进行同步模拟研究。在巡航期间，我们将使用拖曳仪器测量整个海洋300米以上的温度、盐度和相关场的长度变化尺度。这些数据将使我们能够确定次中尺度在SAF附近的强度和分布，并确定产生它们的强迫机制。我们将结合项目的模拟部分，包括利用MITgCM和大涡模拟(LES)进行的高分辨率和粗尺度模拟，评估次中尺度最终如何影响区域内的SAMW特性，以及这对SAMW形成的最终影响。

Our current understanding of the Earth's climate is largely based on the predictions of numerical models that simulate the behaviour of, and interaction between, the atmosphere and the ocean. These models are crucially limited in their resolution, however, such that processes within the ocean that have horizontal scales of less than approximately 10 km cannot be explicitly represented and need to be parameterised for their effects to be included within the models. The purpose of this project, Surface Mixed Evolution at Submesoscales (SMILES), is to identify the potentially crucial role played by one variety of these unresolved processes, referred to as submesoscales, in influencing the structure and properties of the upper ocean, and thereby the transformation of surface water masses, within the Southern Ocean. Submesoscales are flows with spatial scales of 1-10 km that occur within the upper ocean where communication and exchange between the ocean and the atmosphere occurs. Previously considered unimportant to climate-scale studies due to their small scale and the presumed insignificance of their dynamics, recent evidence from high resolution regional models and observational studies is now emerging which suggests that submesoscales are actually widespread throughout the upper ocean and play a key role within climate dynamics due to their ability to rapidly restratify the upper ocean and reduce buoyancy loss from the ocean to the atmosphere. The impact of such a process is particularly important to the surface transformation of water masses such as Subantarctic Mode Water (SAMW), which is an important component of the Meridional Overturning Circulation (MOC) that redistributes heat, freshwater and tracers around the globe. Within the MOC, dense water masses such as SAMW are formed and transformed at high latitudes by surface processes before being subducted into the ocean interior. The properties of the subducted water masses and the tracers and dissolved gases such as carbon dioxide contained within them are vitally important to the global climate and geochemical cycles as these water masses remain out of contact with the surface over decennial to centennial timescales. In the light of the recent discoveries concerning the ability of submesoscales to substantially influence the properties of the upper ocean, we will directly study the impacts of submesoscales on SAMW properties within the Scotia Sea. Using an integrated approach, we will both observe and simulate submesoscales within the upper ocean at a range of spatial and temporal scales, spanning from turbulence up to mode water formation. The principal goal of the study is the diagnosis of the role played by submesoscales in water mass transformation so that we can accurately incorporate these effects into climate-scale models which cannot explicitly resolve them. Our methods will entail a cruise approximately 200 miles south of the Falklands Islands at the Subantarctic Front (SAF), to the north of which SAMW is transformed, and a concurrent modelling study using a state-of-the-art global circulation model. During the cruise, we will use towed instruments to measure the length scales of variability in the temperature, salinity and related fields throughout the upper 300 m of the ocean. The data will enable us to identify the intensity and distribution of submesoscales within the vicinity of the SAF, and to ascertain the forcing mechanisms that generate them. In conjunction with the modelling component of the project, which will include both high resolution and coarse-scale simulations with the MITgcm and large eddy simulations (LES), we will assess how submesoscales ultimately impact on the properties of SAMW within the region and the ultimate effect this has on the formation of SAMW.