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海洋科学学术沙龙第73期(2018年第8期):Open boundary conditions for tidally and subtidally forced circulation in a limited-area coastal model using the Regional Ocean Modeling System (ROMS)

   浏览数:30   日期:2018年06月22日

时   间: 2018年6月24日(星期日) 14:30 - 17:30

地   点中山大学珠海校区教学楼A404会议室

主持人:靳光震 研究员(中山大学海洋科学学院)

报告1:On the Formation Dynamics of Layered-Circulation and Long-Lived Eddy-Train in the South China Sea: Modeling Study

主讲人:蔡忠亚 博士后(香港科技大学数学系)

报告2:Open boundary conditions for tidally and subtidally forced circulation in a limited-area coastal model using the Regional Ocean Modeling System (ROMS)

主讲人:刘志强 研究助理教授(香港科技大学环境和可持续发展学部)

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主讲人简介(按报告顺序)

蔡忠亚,香港科技大学博士后。2006年进入中国海洋大学环境科学系学习,2010年和2013年自中国海洋大学获得学士和硕士学位,期间研究方向为近海动力过程,基于多船同步观测数据和诊断模型,研究温盐潮内变化对密度流影响及潮流空间不对称现象。2013年至2017年于香港科技大学海洋环境专业攻读博士学位,目前在香港科技大学从事博士后研究工作。主要研究兴趣为南海海洋环流、涡旋和海洋数值模拟。结合地球物理流体动力学与理想化数值模型,关注南海多层环流对外部强迫的响应过程和动力机制。发现南海多层环流的生成和维持受通过海峡的外部作用力和内部过程的共同影响。内部跨陆坡运动可重新分配水体和涡度通量并在各层环流间建立动力联系。该研究促进了对南海环流过程动力机制的认识。南海环流内部,观测中发现夏季可生成的长时间稳定存在的涡链,为此建立了三维理想化过程导模型模拟涡链的生成和演化过程,并基于涡度平衡提出了新的解释,认为大尺度环流和边界流分离控制了涡链的生成。相关研究成果已发表在海洋学TOP期刊Journal of Geophysical Research和Journal of Physical Oceanography。

刘志强,香港科技大学跨学科课程事务处,环境及可持续发展学部博士后,研究助理教授。2002年到2006年就读于中国海洋大学海洋环境学院,获理学学士学位。2009年毕业于国家海洋局第二海洋研究所,获物理海洋学硕士学位。之后考入香港科技大学,并于2013年获该校海洋环境学博士学位。刘志强博士主要从事西北太平洋-中国海中多尺度物理海洋动力过程、变异及其调控机理的观测研究和数值模拟。2013年起在物理海洋学领域主要杂志,包括JGR,JPO等,以主要作者发表论文7篇(第一作者5篇),引用超过 60次。多次以首席科学家身份组织厦门大学、香港科技大学南海北部航次,并得到了很好的观测结果。

报告内容:

This research conducts process-oriented numerical study to investigate (1) responses of the layered circulation to the external forcing of inflow/outflow through straits around the South China Sea (SCS) and the internal vertical transport linking and sustaining the circulations among the layers; and (2) the formation of long-lived eddy-train associated with boundary current separation. Vertical inflow-outflow-inflow through Luzon Strait provides a planetary vorticity flux for the layered-circulation, in which the flux is transported by the three-dimensional slope current and balanced by the vorticity induced by the flow-slope interaction and nonlinear advection inside the SCS. During the development of the layered circulation, the downward/upward transport in the upper/lower layer is established over the slope, which acts as the source/sink of water mass and redistributes the intruding vorticity in the water column. The vertical transport is largely associated with the cross-slope motion induced by bottom friction, nonlinear advection and pressure gradient force (PGF). Driven by the separated jet current, the eddy-train with three anti-cyclonic eddies is generated along jet’s path. The inherent stratification regulates the three-dimensional scale of eddies, and constrains their intensities and vertical extensions. The eddy-train’s negative vorticity is originated from the beta effect of the northward-flowing western boundary current, and from the subsequent downstream vorticity advection along the jet. The jet separation is a necessary condition for the formation of eddy-train, and the enhanced stratification, increased wind forcing are favorable conditions for the formation of eddies.

In limited-area ocean models, open boundary conditions (OBCs) often create dynamic inconsistencies and perform poorly in resolving tidal or subtidal flow when both forces exist. Orlanski-type radiation OBCs are reasonably efficient at treating the subtidally forced flow, and Flather-type OBCs are commonly adapted for the tidally forced flow. However, neither of them performs well when tidal and subtidal forces simultaneously drive the flows. We have developed a novel OBC that integrates the active OBC in Gan and Allen (2005) and a Flather-type OBC. This new OBC accommodates the concurrent Tidal and Subtidal (TST) forcing, and the respective tidal or subtidal forcing, at the open boundary of a limited-area model. This new TST-OBC treats the tidal component with a Flather-type OBC, and it separates subtidal barotropic and baroclinic components into local (forced) and global (unforced) components. Then an unforced Orlanski-type OBC can be applied to the global part. We applied the TST-OBC to all model variables to reduce dynamic inconsistence. Using the Regional Ocean Modeling System, we applied the TST-OBC to the shallow East China Sea shelf where strong tidal and subtidal forces over complex topography govern the circulation. Our numerical experiments and analyses suggest that the TST-OBC was robust for both concurrent tidal-subtidal forcing and solely tidal or subtidal forcing at the open boundary. It reduced spurious energy reflection, and, overall, it performed better than an Orlanski-type or Flather-type OBC in reproducing realistic tidal and subtidal shelf circulation.