Submesoscale dynamics in presence of freshwater forcing

沿海海洋是动态区域，其中包括河流、风、地形、潮汐、密度梯度以及水平和垂直混合等多种因素影响其环流和相关的物质运输。亚中尺度流（约一公里），如小尺度流、涡流和锋面，直接受到这些因素的影响，可以加强或削弱沿海流。在淡水源存在的情况下，会产生两种相互竞争的影响：一方面，淡水源可以增强密度梯度，从而产生海洋锋，这反过来可能加深混合层；另一方面，它可以使表层海洋分层，使混合层变浅。在某些情况下，风或潮的物理强迫可以加强或破坏亚中尺度水平的流动，使它们难以研究。本项目旨在利用区域海洋模式了解淡水通量对亚中尺度流动的贡献，以及它们对运输和混合的共同影响。作为一个案例研究，越南和湄公河三角洲的沿海水域将被调查，在那里季风是一个主导力量，有大量的淡水输入。该研究区域具有重要的生态、社会和经济意义。该研究人员从墨西哥湾先前的研究中获得的见解将应用于湄公河三角洲地区，探索将其推广到其他热带和亚热带系统的可能性。此外，该项目将提高人们对沿海亚中尺度地区存在的认识，在这些地区，物理过程可能具有重要的、但大多未被探索的生物地球化学影响。该项目将有助于更好地了解从大气到陆地到海洋或从一公里到整个盆地范围的跨尺度联系。通过该项目支持的研究生将获得使用理想化和现实配置的区域海洋模型、现场数据和数据分析工具的宝贵培训。作为本科生研究经验奖学金的一部分，至少有一名本科生将被引入研究。通过与佐治亚理工学院科学、数学和计算综合教育中心的合作，将为6-8年级的课程开发一个讲座模块，以促进对近海海洋运输与漂浮污染之间关系的理解，其中可以介绍汇聚、斑块和垂直混合的概念，以及使用模型来解释观测结果。本项目是一项以过程为导向的研究，旨在数值研究受大淡水通量影响的沿海地区，淡水通量、大气强迫、地形和潮汐与地面亚中尺度动力学、水平和垂直混合以及混合层深度之间的相互作用。主要目的是验证存在一个将亚中尺度动力学归因于沿海地区不同物理强迫的通用框架。亚中尺度环流的形成、分布和影响将利用0.5公里水平分辨率的区域海洋模式进行研究。重点将放在华南西南部和越南沿海地区，那里的环流特点是受季风驱动的强烈季节性上升流，以及季风周期期间强烈降水脉冲和湄公河季节性变化的流入造成的大量淡水输入。敏感性模拟将有助于阐明每种强迫（风、热通量、淡水通量、测深、潮汐）对混合层中输送和混合的贡献。横向密度梯度由河流或降雨输入外部供给的地区是亚中尺度活动的“热点”。在这些地区，一般的亚中尺度过程，特别是锋生，并不完全由储存在混合层中的可用势能推动，而且还由淡水通量引起的横向密度梯度推动。因此，淡水通量有助于确定水平和垂直混合特性，至少在一定程度上有助于确定亚中尺度环流的季节循环。然而，所有可能的强迫与亚中尺度环流之间的相互作用还没有得到很好的理解或描述。由于这个项目，人们将更好地了解淡水通量对亚中尺度环流的贡献及其在不同大气强迫条件下对混合层输送和混合的联合影响。此外，现有的参数化可以普遍化，使它们适用于受大型淡水资源影响的其他沿海区域。

Coastal oceans are dynamic regions where multiple factors including freshwater input from rivers, wind, topography, tides, density gradients, as well as horizontal and vertical mixing influence their circulation and the associated transport of material. Submesoscale flows (of order one kilometer), like small-scale currents, eddies and fronts, are directly influenced by these factors and can strengthen or weaken coastal flows. In the presence of freshwater sources two competing effects can be at play: A freshwater source can, on one hand, enhance density gradients and therefore the generation of ocean fronts, which in turn may deepen the mixed layer, and, on the other, it can stratify the surface ocean and shoal the mixed layer. In some cases, the physical forcing of the wind or tide can reinforce or destroy flow at the submesoscale level, making them difficult to study. This project aims to understand the contribution of freshwater fluxes to submesoscale flow, and of their joint impact on transport and mixing using a regional ocean model. As a case study, coastal waters off Vietnam and the Mekong Delta, where the monsoonal winds are a dominant force and there are large freshwater inputs, will be investigated. This study area has key ecological, social and economic importance. Insights gained from a previous study by the researcher in the Gulf of Mexico will be applied to the Mekong Delta region, exploring possible generalization to other tropical and sub-tropical systems. Furthermore, this project will raise awareness of the existence of coastal submesoscale areas where physical processes are likely to have important, but mostly unexplored, biogeochemical impacts. This project will contribute to a better understanding of cross-scale linkages, from the atmosphere to the land to the ocean or from a kilometer to basin-wide scale. A graduate student supported through this project will gain valuable training in using a regional ocean model configured in idealized and realistic configurations, in-situ data and data-analysis tools. At least one undergraduate student will be introduced to research as part of a Research Experience for Undergraduates (REU) fellowship. Through a collaboration with the Center for Education Integrating Science, Mathematics and Computing at Georgia Tech, a lecture module for grades 6-8 curricula will be developed to promote understanding of near coastal ocean transport in relation to floating pollution, where the concepts of convergence, patchiness and vertical mixing, and the use of models to interpret observations can be introduced. This project is a process-oriented study to investigate numerically the interplay between freshwater fluxes, atmospheric forcing, topography and tides on one hand, and surface submesoscale dynamics, horizontal and vertical mixing, and mixed layer depth on the other, in coastal areas impacted by large freshwater fluxes. The main objective is to verify the existence of a generic framework to attribute submesoscale dynamics to different physical forcings in coastal areas. Formation, distribution and impact of submesoscale circulations will be investigated using a regional ocean model run at 0.5 km horizontal resolution. The focus will be on the southwestern South China and the Vietnam coastal region, where the circulation is characterized by strong seasonal upwelling forced by the monsoonal winds, and by large freshwater inputs due to both intense precipitation pulses during the monsoonal cycle and to the seasonally varying inflow of the Mekong River. Sensitivity simulations will help elucidate the contribution of each forcing (wind, heat fluxes, freshwater fluxes, bathymetry, tides) to transport and mixing in the mixed-layer. Regions where lateral density gradients are externally supplied through riverine or rainfall inputs are "hotspots" of submesoscale activities. In these areas submesoscale processes in general and frontogenesis in particular are not fueled exclusively by the available potential energy stored in the mixed layer, but also by lateral density gradients induced by the freshwater fluxes. Therefore the freshwater fluxes contribute to defining the horizontal and vertical mixing properties, and, at least in part, the seasonal cycle of the submesoscale circulations. The interplay between all possible contributing forcings and the submesoscale circulations, however, is not well understood or characterized. As a result of this project, the contribution of freshwater fluxes to submesoscale circulations and of their joint impact on mixed-layer transport and mixing under varying atmospheric forcing conditions will be better understood. Furthermore, existing parameterizations may be generalized, making them relevant to other coastal regions subjected to large freshwater sources.