Submesoscale dynamics in presence of freshwater forcing

沿海海洋是动态的区域，河流的淡水输入、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循环和相关的物质运输。次中尺度流动(约一公里)与小尺度洋流、旋涡和锋面一样，直接受到这些因素的影响，可以加强或削弱沿海流动。在有淡水水源的情况下，可能会产生两种相互竞争的影响：一方面，淡水水源可以增强密度梯度，从而产生海洋锋，进而加深混合层；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，风或潮汐的物理强迫可以加强或破坏次中尺度水平的气流，使得研究它们变得困难。该项目旨在利用区域海洋模式了解淡水通量对次中尺度流动的贡献，以及它们对输送和混合的联合影响。作为一个案例研究，将对越南和湄公河三角洲附近的沿海水域进行调查，那里的季风风是主导力量，有大量的淡水输入。该研究区具有重要的生态、社会和经济意义。这位研究人员从墨西哥湾之前的一项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，物理过程可能会产生重要的、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再到海洋或从一公里到盆地范围的跨尺度联系。通过该项目支持的一名研究生将在使用以理想化和现实化配置配置的区域海洋模型、现场数据和数据分析工具方面获得宝贵的培训。至少有一名本科生将被介绍参加研究，作为本科生研究体验(REU)奖学金的一部分。通过与佐治亚理工学院综合科学、数学和计算教育中心的合作，将为6-8年级课程开发一个讲座模块，以促进对与浮动污染有关的近岸海洋运输的了解，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。该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、地形和潮汐之间的相互作用，以及表层亚中尺度动力学、水平和垂直混合以及混合层深度之间的相互作用。其主要目标是核实是否存在一个通用框架，将次中尺度动力归因于沿海地区的不同物理强迫。将使用运行在0.5公里水平分辨率的区域海洋模式来研究次中尺度环流的形成、分布和影响。重点将放在中国南部的西南部和越南沿海地区，那里的环流特征是季风强迫的强烈季节性上升流，以及由于季风周期期间强烈的降水脉冲和湄公河季节性变化的流入而产生的大量淡水输入。敏感性模拟将有助于阐明各种强迫(风、热通量、淡水通量、水深测量、潮汐)对混合层中输送和混合的贡献。通过河流或降雨输入向外部提供横向密度梯度的区域是次中尺度活动的“热点”。在这些地区，亚中尺度过程，特别是锋生过程，不仅由混合层中储存的可用势能推动，而且还受到淡水通量引起的横向密度梯度的推动。因此，淡水通量有助于确定水平和垂直混合性质，以及至少部分确定亚中尺度环流的季节周期。然而，所有可能的强迫作用和次中尺度环流之间的相互作用还没有被很好地理解或描述。作为该项目的结果，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，现有的参数可以推广，使其适用于拥有大型淡水水源的其他沿海地区。

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.